Development of High Strength Composite Toecaps ...

3 downloads 0 Views 8MB Size Report
Sitthiracha, Eva Bartok, Xinyue Zhang, Chung-Yueh Lin and all other friends and fellows from CACM and the Department of Mechanical Engineering for their ...
Centre for Advanced Composite Materials Department of Mechanical Engineering The University of Auckland

Development of High Strength Composite Toecaps Using LS-DYNA

Cheng-Chou Yang February, 2010

Supervisors: Dr. Richard J T Lin Dr. Miro Duhovic Prof. Debes Bhattacharyya A THESIS SUBMITTED IN FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ENGINEERING, THE UNIVERSITY OF AUCKLAND, 2010.

Abstract The advancement of technology in composite materials over the past decade has attracted significant attention from the industry for replacing existing metallic products with composite products. To develop critical structural members using composite materials, intensive research is required to determine the best combination of factors that lead to the optimum structural strength of the final product. This thesis presents preliminary research in each of the development stages for a composite safety toecap which includes: material selection, finiteelement modelling, manufacturing process, and structural optimisation.

Toecaps made from composite materials have been identified as having several potential advantages over those traditionally made from metallic materials. However the complex behaviour of composite materials makes it difficult and complex to perform a valid structural analysis. This research aims at developing an analytical strategy for manufacturing composite toecaps with natural fibres that satisfy the current industrial safety standard.

The geometry of the toecap was first scanned and imported into a FEA software package to create a workable testing standard simulation model. The method was then validated by comparing the FE model with experimental results of steel toecaps. Modelling, manufacturing and testing of custom-made flax-PLA composite toecap samples were then carried out. Modelling outputs of composite toecaps were then compared with compression test data for validation. Stress distribution and deformation of the toecaps were also investigated. The structural strength of the developed model was then analyzed using the Taguchi and Layer Tournament approach by varying the fibre orientation of the laminates to achieve structural optimisation. It was found that the strength of composite parts can vary up to 4 times by changing the fibre direction inside the laminates. The developed methods were eventually employed to design the optimum composite products that approach the industrial strength and stiffness requirements. The final product achieved up to 50% weight reduction compared to traditional metallic products.

Modelling of the composite toecaps was performed using LS-DYNA R971 Solver and PrePost developed by LSTC. All FE modelling and analyses were conducted with reference to affiliated European Standards EN12568:1998.

-i-

-ii-

Acknowledgements First of all, I would like to acknowledge and thank my supervisor, Dr. Richard Lin for introducing me to the Centre for Advanced Composite Materials (CACM) at the University of Auckland, for providing the project topic and support for this research. His wisdom, knowledge in composite mechanics, valuable advice and words of encouragement has led me through difficult times and achieved valuable results in this research.

I would like to show my deepest appreciation to my supervisor Dr. Miro Duhovic for leading me into composite structural modelling. His great enthusiasm, patience, support, guidance and board knowledge in composite mechanics and FE modelling has guided me through various technical hurdles and helped me gain precious knowledge and experiences from this research.

I would also like to thank Professor Debes Bhattacharyya for offering me with this precious opportunity to engage in the research of composite modelling, as well as his valuable advice, patience and support over the past twelve months. Together, the help and support I have received from the three supervisors have contributed greatly to the success of the work carried out in this thesis.

Special thanks to the CACM technicians Mr. Callum Turnbull and Mr. Jos Gurts for providing precious suggestions and technical assistance, especially for the enormous help during the sample manufacturing and testing stage.

I would like to thank fellow PhD student Oliver McGregor and Sanjeev Rao for their support, and the permission to reference and use the illustrations and figures from their work.

Finally, I would also like to thank my past and present fellow students: Christoph Fisher, Alan Yang, Erwan Grossmann, Gwendal Huet, Nam-Kyun Kim, Shyam Moham, Manatchanok Sitthiracha, Eva Bartok, Xinyue Zhang, Chung-Yueh Lin and all other friends and fellows from CACM and the Department of Mechanical Engineering for their assistance and company, that has made the past 12 months both enjoyable and unforgettable.

-iii-

-iv-

Table of Contents ABSTRACT ............................................................................................................................................................I ACKNOWLEDGEMENTS ............................................................................................................................... III NOMENCLATURE .......................................................................................................................................... XV ABBREVIATIONS ........................................................................................................................................ XVII 1. INTRODUCTION ............................................................................................................................................. 1 1.1 BACKGROUND AND EVOLUTION OF TOECAP DESIGN .................................................................................... 2 1.2 STRENGTH AND MATERIALS ISSUES FOR MODERN COMPOSITE TOECAPS .................................................... 3 1.3 RESEARCH OBJECTIVES AND SCOPE OF THESIS ............................................................................................ 4 1.4 THESIS OUTLINE ........................................................................................................................................... 5 2. LITERATURE SURVEY ................................................................................................................................. 7 2.1 BIODEGRADABILITY OF THERMOPLASTIC POLYMERS ................................................................................... 7 2.2 NATURAL FIBRES.......................................................................................................................................... 9 2.3 APPLICATION OF NUMERICAL METHODS FOR LARGE DEFORMATION SITUATIONS ..................................... 10 2.3.1 Relevant Researches in Large-Deformation Explicit Modelling........................................................ 10 2.3.2 LSTC LS-DYNA Solver ...................................................................................................................... 12 2.3.3 Modelling of Composite Toecaps ....................................................................................................... 13 2.4 SYSTEMATIC APPROACH FOR OPTIMISATION .............................................................................................. 14 2.5 PREVIOUS WORK AT CENTRE FOR ADVANCE COMPOSITE MATERIALS (CACM) ....................................... 15 2.5.1 Investigation and Comparison of Suitable Materials ........................................................................ 16 2.5.2 Investigation into Manufacturing Process ......................................................................................... 18 3. TESTING STANDARDS AND METHODS ................................................................................................. 23 3.1 EUROPEAN EN12568:1998 STANDARD ...................................................................................................... 23 3.2 GEOMETRY AND DIMENSIONS .................................................................................................................... 24 3.3 COMPRESSION RESISTANCE ........................................................................................................................ 25 3.4 INSTRUMENTATION ..................................................................................................................................... 26 4. COMPOSITE MATERIAL PREPREG ........................................................................................................ 27 4.1 INTRODUCTION ........................................................................................................................................... 27 4.2 SISAL-POLYPROPYLENE (PP) COMPOSITE PREPREG ................................................................................... 28 4.3 FLAX-POLYLACTIC ACID (PLA) (HOT-PRESSED) COMPOSITE PREPREG ..................................................... 31 4.3.1 PLA Matrix Sheets ............................................................................................................................. 32 4.3.2 Flax-PLA Composite Winding and Compression .............................................................................. 34 4.3.3 Prepreg Properties............................................................................................................................. 36 4.4 FLAX-PLA (IMPREGNATED) COMPOSITE PREPREG ..................................................................................... 37 4.4.1 PLA Coating Process ......................................................................................................................... 37 4.4.2 Flax-PLA Winding and Pressing ....................................................................................................... 38 4.4.3 Prepreg Properties............................................................................................................................. 39

-v-

5. FINITE ELEMENT MODELLING & ANALYSIS ..................................................................................... 43 5.1 TOECAP GEOMETRY AND MESH GENERATION ........................................................................................... 43 5.2 MODELLING OF MANUFACTURING PROCESS AND FIBRE ORIENTATION ..................................................... 44 5.2.1 Production Modelling Method .......................................................................................................... 45 5.2.2 Manual Assigning Method ................................................................................................................ 51 5.3 COMPRESSION TEST MODELLING............................................................................................................... 55 5.3.1 Model Setup for Compression Test ................................................................................................... 56 5.3.2 Fleck Steel Toecap Model ................................................................................................................. 58 5.3.3 Sisal-PP Toecap Model ..................................................................................................................... 60 5.3.4 Flax-PLA (Hot-Pressed) Model ........................................................................................................ 61 5.3.5 Flax-PLA (Impregnated) Model ........................................................................................................ 63 5.4 MODELLING RESULTS ................................................................................................................................ 64 5.4.1 Sisal-PP Toecap Results.................................................................................................................... 64 5.4.2 Flax-PLA (Hot-Pressed) Toecap Results .......................................................................................... 66 5.4.3 Flax-PLA (Impregnated) Toecap Results .......................................................................................... 68 5.5 EVALUATION OF RESULTS.......................................................................................................................... 68 6. DESIGN OPTIMISATION ............................................................................................................................. 71 6.1 MODELLING OF PREPREG LAYERS & FIBRE ORIENTATION ........................................................................ 71 6.2 OPTIMISATION AND EVALUATION APPROACHES ........................................................................................ 74 6.2.1 Complete Modelling Matrix .............................................................................................................. 74 6.2.2 Layer-Tournament Approach ............................................................................................................ 75 6.2.3 Taguchi Approach ............................................................................................................................. 78 6.3 ANALYSIS PROCEDURES AND RESULTS ...................................................................................................... 81 6.3.1 Flax-PLA (hot-pressed 4 layers) Toecaps ......................................................................................... 82 6.3.2 Flax-PLA (hot-pressed 6 layers) Toecaps ......................................................................................... 86 6.3.3 Flax-PLA (impregnated 4 prepregs) Toecaps ................................................................................... 93 6.3.4 Comparison of Approaches ............................................................................................................... 97 7. MANUFACTURE AND EVALUATION OF COMPOSITE TOECAP SAMPLES ................................. 98 7.1 SISAL-PP SAMPLES .................................................................................................................................... 98 7.1.1 Production Process ........................................................................................................................... 99 7.1.2 Testing Results and Evaluation of Samples ..................................................................................... 101 7.2 FLAX-PLA(IMPREGNATED) SAMPLES ...................................................................................................... 103 7.2.1 Production Process ......................................................................................................................... 104 7.2.2 Testing Results and Evaluation of Samples ..................................................................................... 107 7.3 FLAX-PLA(HOT-PRESSED) SAMPLES ...................................................................................................... 108 7.3.1 Production Process ......................................................................................................................... 109 7.3.2 Testing Results and Evaluation of Samples ..................................................................................... 112 7.4 RESULT EVALUATION .............................................................................................................................. 119 8. CONCLUSION .............................................................................................................................................. 122 8.1 MATERIAL SELECTION ............................................................................................................................. 122

-vi-

8.2 FE STRUCTURAL MODELLING .................................................................................................................. 123 8.3 STRUCTURAL OPTIMISATION .................................................................................................................... 123 8.4 PRODUCT MANUFACTURING ..................................................................................................................... 123 9. FUTURE WORK .......................................................................................................................................... 126 REFERENCES .................................................................................................................................................. 128 APPENDIX ........................................................................................................................................................ 134 A.

TOECAP COMPRESSIVE RESISTANCE TEST RESULTS: 6 LAYERS HOT-PRESSED FLAX-PLA .................. 134

B.

TOECAP COMPRESSIVE RESISTANCE TEST RESULTS: 4 PREPREGS IMPREGNATED FLAX-PLA .............. 135

C.

TOECAP COMPRESSIVE RESISTANCE TEST RESULTS: 4 LAYER HOT-PRESSED FLAX-PLA .................... 136

D.

IRID CALCULATION PROGRAM ............................................................................................................ 137

E.

TAGUCHI OPTIMISATION ANALYSIS DATA MATRICES: 6 LAYER HOT-PRESSED FLAX-PLA ................ 138

-vii-

-viii-

List of Figures Figure 1-1: Toecap geometry and application [1] ..................................................................... 2 Figure 2-1: Young’s modulus plotted as function of displacement for flax fibres.................. 10 Figure 2-2: Consistent use of unit systems from the user when modelling in LS-DYNA [1] 13 Figure 2-3: Toecap deformation and stress contour after impact /compression test [38] ....... 14 Figure 2-4: Comparison of chord modulus [46] ...................................................................... 16 Figure 2-5: Comparison of tensile strength [46] ..................................................................... 16 Figure 2-6: Compression resistance testing of toecap samples [46] ....................................... 17 Figure 2-7: Process of designing and manufacturing of hot-press mould [46] ....................... 19 Figure 2-8: Comparison of tensile strength [46] ..................................................................... 19 Figure 2-9: Complete assembly of pneumatic mould [46] press [2] ....................................... 20 Figure 2-10: Different layups for Twintex® woven, plytron preform, carbon fibre [46] ...... 21 Figure 2-11: Twintex toecap samples produced [46] ............................................................. 21 Figure 3-1: Toecap geometry: a) indication of e (left) b) illustrations of flanges (right) [1] . 24 Figure 3-2: Toecap central axis: a) plotting method(left) b) illustrations(right) [1] .............. 24 Figure 3-3: Compression test: setup (left) and illustrations (right) [1]................................... 25 Figure 3-4: Instron 1185 machine .......................................................................................... 26 Figure 3-5: Bluehill material testing software ........................................................................ 26 Figure 4-1: Manufacturing illustrations: schematic diagram and equipment photo .............. 29 Figure 4-2: Manufactured sisal-PP prepreg: roll (up) and trimmed sheet (bottom) [47] ...... 30 Figure 4-3: Steel frame assembly for moulding of PLA sheets ............................................. 32 Figure 4-4: Aluminium mould with distributed PLA palettes ................................................ 33 Figure 4-5: Aluminium mould compressed with manual hot press........................................ 33 Figure 4-6: Pre-consolidated PLA sheets ............................................................................... 34 Figure 4-7: Winding of PLA sheets with motor-driven lathe ................................................ 35 Figure 4-8: PLA sheets with wound flax fibre ...................................................................... 36 Figure 4-9: Flax-PLA prepreg manufactured ........................................................................ 36 Figure 4-10: a) Pre-impregnating process diagram b) Ceramic tunnel ................................. 38 Figure 4-11: Composite winded impregnated flax-PLA yarns .............................................. 39 Figure 4-12: Impregnated flax-PLA Composite Prepreg ....................................................... 39 Figure 4-13: 90 degree offset heat-pressed prepreg ............................................................... 40 Figure 5-1: Evolution of digitized toecap geometry scanned surface, fixed, repaired ........... 44 Figure 5-2: Production modelling setup: male mould, female mould, prepreg ...................... 46 -ix-

Figure 5-3: Simulation of deformed prepreg after compression moulding. .......................... 46 Figure 5-4: Trimming of prepreg surface to eliminate surface defects................................. 47 Figure 5-5: Final mushroom-shape prepreg ........................................................................... 48 Figure 5-6: Compression moulds with mushroom-shape prepreg ......................................... 48 Figure 5-7: Assigned fibre direction for mushroom-shape prepreg ....................................... 49 Figure 5-8: Compressed toecap model with simulated fibre layups. ..................................... 49 Figure 5-9: Compression moulding simulation: evolution of the prepreg wrap-folding ....... 50 Figure 5-10: Assignment of material primary direction using AOPT=3 ............................... 51 Figure 5-11: Illustration of assigning primary material direction using AOPT=3 ................ 52 Figure 5-12: Mushroom shape paper cut-out with X direction mark lines. ........................... 52 Figure 5-13: Toecap geometry formed from mushroom paper cut-out with virtual axis ...... 53 Figure 5-14: Mushroom shaped paper cut-out with virtual axis. ........................................... 53 Figure 5-15: Vector component of v’ ..................................................................................... 54 Figure 5-16: Toecap fibre direction for scanned geometry using assignment method. ......... 54 Figure 5-17: Toecap fibre direction from X-direction prepreg Layup.................................. 55 Figure 5-18: Compression test model of the scanned-geometry toecap [47] ........................ 56 Figure 5-19: Compression test model of the production model toecap [47] ......................... 56 Figure 5-20: Force and deformation of simulated FleckSteel© toecap [47] ......................... 59 Figure 5-21: Comparison of Force-displacement curve for FleckSteel© toecap [47] ........... 59 Figure 5-22: Result of stress distribution and failure modes for FleckSteel© toecap [47] ... 60 Figure 5-23: Results of sisal-PP compression testing simulation [47] .................................. 64 Figure 5-24: Result of 2mm mesh sisal-PP toecap models [47] ............................................ 65 Figure 5-25: Comparison laser-scanned & compression-simulated geometry. ..................... 66 Figure 5-26: Comparison between laser-scanned and compression-simulated geometry ..... 67 Figure 5-27: Comparison between hot-pressed and impregnated toecap simulation results . 68 Figure 5-28: Comparison between impregnated and hot-pressed deformation results .......... 69 Figure 5-29: Comparison of toecap simulation results obtaine ............................................. 70 Figure 6-1: Assignment of fibre layout for 0, 90, 45, -45 degrees......................................... 72 Figure 6-2: Assignment of fibre orientations for 0, 90, 45, -45 degrees layup sequences.... 72 Figure 6-3: *INTEGRATION_SHELL card in LS-DYNA Prepost ...................................... 73 Figure 6-4: Cross-section view of 4 layer composite laminates ............................................ 75 Figure 6-5: Four dummy models created for level one analysis ............................................ 76 Figure 6-6: Four models created for level two analysis ......................................................... 76 Figure 6-7: Efficiency vs number of layers curve for the Layer-Tournament approach ....... 77

-x-

Figure 6-8: Efficiency vs number of layer curve for Taguchi approach ................................. 81 Figure 6-9: Top-dome failure mode of the compression test model ....................................... 83 Figure 6-10: Side-wing failure mode of the compression test model ..................................... 84 Figure 6-11: Twist collapse mode of the compression test model .......................................... 84 Figure 6-12: Force-displacement curve of 4 layers flax-PLA toecap models ......................... 85 Figure 6-13: Top-dome failure mode experienced by 900 arrangement (left to right) ........... 90 Figure 6-14: Top-dome failure mode experienced by 555 arrangement (left to right) ........... 90 Figure 6-15: Force-displacement curve of 4 layers flax-PLA toecap models ......................... 92 Figure 6-16: Force-displacement curve of the 6-layer hot-pressed flax-PLA toecaps ............ 93 Figure 6-17: Force-displacement curve of the 4-prepreg impregnated flax-PLA toecaps ...... 95 Figure 6-18: Top-dome failure mode experienced by e arrangement (left to right)................ 96 Figure 6-19: Top-dome failure mode experienced by p arrangement (left to right) ............... 96 Figure 7-1: Sisal-PP rectangular prepreg with folding cuts .................................................... 99 Figure 7-2: Trimmed sisal-PP prepregs soften using baking oven .......................................... 99 Figure 7-3: Sisal-PP prepreg pressed in the aluminium mould ............................................. 100 Figure 7-4: Manufacture 18 sisal-PP toecap samples ............................................................ 101 Figure 7-5: Faulty sisal-PP samples with excessive porosities and uneven thickness .......... 101 Figure 7-6: Comparison of sisal-PP toecap testing data vs modelling results....................... 102 Figure 7-7: Pre-existed cracks in sisal-PP toecap before and after compression test............ 103 Figure 7-8: Mass produced 24 impregnated flax-PLA prepregs ........................................... 104 Figure 7-9: Impregnated flax-PLA prepreg trimmed into mushroom-like shape ................. 104 Figure 7-10: Stepping strips used .......................................................................................... 105 Figure 7-11: Toecap mould inside the hydraulic press…………………………………..... 102 Figure 7-12: The de-molded impregnated flax-PLA product ................................................ 106 Figure 7-13: The completed impregnated flax-PLA composite toecap sample .................... 106 Figure 7-14: Total of 6 impregnated flax-PLA samples produced ........................................ 106 Figure 7-15: Comparison of impregnated toecap testing data vs modelling results ............. 107 Figure 7-16: De-lamination of the top dome from impregnated flax-PLA toecaps .............. 108 Figure 7-17: Hot-pressed flax-PLA prepregs trimmed into mushroom-shape cut-outs ........ 109 Figure 7-18: Hot-pressed flax-PLA precast........................................................................... 109 Figure 7-19: Precast been hand-laid into the mould .............................................................. 110 Figure 7-20: Product demoulding……………………………………………………….... 107 Figure 7-21: Hot-pressed 4 layer flax-PLA sample............................................................... 110 Figure 7-22: Eight samples of the 4 layer hot-pressed flax-PLA toecap............................... 111

-xi-

Figure 7-23: Hot-pressed 6 layers flax-PLA sample ............................................................ 111 Figure 7-24: Three samples of the 6 layer hot-pressed flax-PLA toecap ............................. 112 Figure 7-25: Force-displacement of arrangement L showing the compression stages ......... 113 Figure 7-26: The three compression stages: a) Stage I b) Stage II c) Stage III ................... 113 Figure 7-27: Difference in laminate failure mechanism FE model, actual toecap sample ... 114 Figure 7-28: Compression results of the hot-pressed composite toecap samples. ................ 116 Figure 7-29: Bottom flanges and the side wings where thickness less than expected .......... 116 Figure 7-30: Seven probe points,3.5mm clearance, 5.25mm clearance ............................... 117 Figure 7-31: Misalignment of the toecap mould due to shifting........................................... 118 Figure 7-32: Comparison of failure modes ( top to bottom ): arrangement 900 , L, B, P .... 120

-xii-

List of Tables Table 2-1: Sample weight comparison [46] ............................................................................. 18 Table 3-1: Minimum clearance after testing [1] ....................................................................... 25 Table 4-1: Sisal-PP prepreg material properties [47] ............................................................... 31 Table 4-2: Properties of compression-moulded flax-PLA sheets [48,49] ................................ 36 Table 4-3: Properties of Impregnated flax-PLA prepreg sheets [48,50] .................................. 40 Table 5-1: Manually defined vector axis: AOPT and position data in LS-DYNA[2].............. 54 Table 5-2: FleckSteel© properties defined .............................................................................. 58 Table 5-3: Sisal-PP toecap properties defined .......................................................................... 60 Table 5-4: Flax-PLA (hot-pressed) toecap properties defined ................................................. 61 Table 5-5: Flax-PLA total number of layer-direction arrangements ........................................ 62 Table 5-6: Flax-PLA (impregnated) toecap properties defined ............................................... 63 Table 6-1: Layout of the complete modelling matrix ............................................................... 74 Table 6-2: Comparison of required number of model: L-T approach vs complete matrix ...... 76 Table 6-3: Assignment of factors and levels for Taguchi Analysis ......................................... 78 Table 6-4: Taguchi modified L-16’array .................................................................................. 79 Table 6-5: Taguchi S/N ratio calculation from modelling results ............................................ 79 Table 6-6: Taguchi S/N ratio calculation from modelling results ............................................ 80 Table 6-7: Comparison of number of model from Taguchi approach vs complete matrix ...... 80 Table 6-8: Complete modelling matrix for 4 layerhot-pressed flax-PLA toecap .................... 82 Table 6-9: Ranking matrix for 4 layer hot-pressed flax-PLA toecap modelling results .......... 82 Table 6-10: L-T approach optimisation results for 4-layer hot pressed flax-PLA toecap ........ 85 Table 6-11: Fibre orientation call-signs for 6 layer flax-PLA toecaps ..................................... 87 Table 6-12: Complete model matrix with 64 arrangements ..................................................... 87 Table 6-13: Complete modelling matrix results for 6 layer hot-pressed flax-PLA toecap ...... 89 Table 6-14: Complete modelling matrix results for 6 layer hot-pressed flax-PLA toecap ...... 89 Table 6-15: L-T approach optimisation results for 6-layer hot-pressed flax-PLA toecap ....... 90 Table 6-16: S/N response table for 6-layer hot-pressed flax-PLA toecap ................................ 91 Table 6-17: ANOVA for 6-layer hot-pressed flax-PLA toecap .............................................. 92 Table 6-18: Complete modelling matrix for 4 prepreg impregnated flax-PLA toecap ............ 94 Table 6-19: Raking matrix for 4 prepreg impregnated flax-PLA toecap ................................ 95 Table 6-20: L-T approach optimisation results for 4 prepreg impregnated flax-PLA toecap .. 96 Table 7-1: Clearance from toecap mould models with 3.5mm and 5.25mm clearance ......... 118 -xiii-

-xiv-

Nomenclature Matrix Notations v

vector

m

matrix

Symbols

x

symmetric layer pair

f

number of options for fibre orientation

l

number of laminate layers (even numbers)

A

total number of possible arrangements

T

number of models required from Taguchi Analysis

L

number of models required from Layer-Tournament Analysis

E

Analysis Index

-xv-

Abbreviations BDP

Biodegradable bipolymers

CACM

Centre for Advanced Composite Materials

DOF

Degree of freedom

FEA

Finite element Analysis

FRP

Fibre reinforced plastic

GA

Genetic Algorithms

L-T

Layer-Torunament

NIP

Number of Integrated points

PBSA

Poly(butylene succinate adipate)

PCL

Polycaprolactone

PGA

Polyglycolic acid

PLA

Polylactic acid

PP

Polypropylene

PU

Polyurethane

PVA

Polyvinyl alcohol

QTS4

Quadrilateral thick shell elements

WF

Weight Fraction

-xvii-

1 1. Introduction The advancement of composite materials over the past two decades has today resulted in its widespread use across different industry sectors. The intensive research driven by high-tech industries including aeronautical, automotive and marine during the late 20th century has led to a greater understanding of composites. The advantageous properties of composites initiated the expansion of their use to other lower tech applications such as sports, entertainment and even fashion. The benefits of composites over traditional materials, such as weight-reduction, low thermal conductivity, high strength, possible bio-degradable properties and being more commercially appealing, have led to a popular trend of the re-design of common commercial products using composite materials.

Examples of common consumer products using composites that are commercially available today include: fishing rods, baseball bats, water storage tank, roofing tiles and safety toecaps. Traditionally, finite element (FE) analysis has been widely used as a cost-effective method for designing and predicting product behaviour under different service conditions, this was also the case for common consumer products. The switch from the use of traditional to composite materials in products has now led to the transformation of FE modelling procedures for the industry. Although the design of common products using composites may not require highly advanced FE analysis capabilities, companies nevertheless experience difficulties in design, manufacture and analysis phases.

The main reason lies in the fundamental difference between traditional metallic/plastic and composite material properties: traditional materials that possess isotropic characteristic offer equal strength in all directions; while the strength properties of composite materials greatly …. -1-

1. Introduction depends on its principle fibre directions [1]. Therefore, the FE analysis requires changes in the setup of material models and failure modes. This research involved a systematic analysis of the re-design of products using composites over traditional metallic parts, with a safety toecap chosen as the target product. In this research, the FE modelling, analysis, optimisation and manufacturing issues of the development procedure for common composite products were investigated.

1.1 Background and Evolution of Toecap Design “Safety Toecap” by definition is a protection cover over the toe of a boot or shoe as shown in Figure 1-1. They are widely used as protective elements in safety shoes and boots to work against potential hazards to human such as heavy falling objects, impacts, cuts, penetrations, compressions and other potential risks from chemical or thermal hazards. Although toecaps made from various materials can offer protection against different range of hazards, toecaps are predominantly manufactured from steel or composites [1].

The earliest design of steel toecaps dates back to WWII, when Dr. Klaus Maertens developed a new generation of military boots for the German army in the 1920s after injuring his ankle while wearing old boots [1]. The toecap boots appeared to be a simple and effective solution for offering better protection to the user, which started to draw the attention of businesses. It’s simple and low technology-base nature also made the design more common in consumer level footwear [1].

Figure 1-1: Toecap geometry and application [1]

With the end of the war, Dr. Maertens initiated its business in selling boots that made the design more fashionable and comfortable, by 1960s became one of the most well-known -2-

1. Introduction brand for fashion and safety boots in Britain: Dr. Martins® [1]. Today the use of steel toecap can be commonly seen in military footwear, engineering safety boots to even fashionable and entertainment use. While the public may not notice, its original design purpose of enhancing human safety has now been adopted for over 80 years [1]. Despite being a common and matured commercial product, international research interests in toecap designs remain active. Being part of the protective footwear for different activities, there are constant demands for improvements in its strength, durability, weight, failure recovery and reshaping properties.

There are several well known issues for metallic toecaps since their first development. One of the main problems is the heavy weight of more than 100g for each toecap that causes discomfort for the user. The use of steel for its strength also results in discomfort due to high thermal conductivity that converts to inappropriate temperatures when used in extreme environments, such as deserts or snowfields. The damaged toecaps result in permanent deformation which is also difficult for the removal of the crushed toes, and often cause more severe injuries to the user [1]. Moreover, with increasing security measures internationally, steel toecaps would inevitably trigger the siren at security checkpoints, its magnetic conductive nature can potentially waste time and cause inconvenience for the user.

In recent years, the uses of composite materials for toecaps with thermoplastic matrices have been released to the commercial market. While the new composite toecaps offer several advantages over traditional ones, when comparing to steel toecaps, the strength and material properties of these newly released products have not yet achieved a satisfactory level in both strength and stiffness.

1.2 Strength and Materials Issues for Modern Composite Toecaps The composite toecaps currently available on the market are commonly branded with better thermal insulation, non-magnetic properties and weight reduction of up to 40% [2], however most of the commercial products reveal their weakness in structural strength when verifying with international safety standards such as EN or ASTM. A large proportion of the products only manage to pass specific sections rather than the complete standards. Even for products that fulfils the standard, in the majority of cases the products barely pass the minimum specifications defined by the standards [1]. The reason lies in the properties of composite materials that are difficult to design and predict using traditional analysis methods. Shoe -3-

1. Introduction manufacturers tried to overcome these by adopting various techniques to their composite toecaps, some of which are: 

Increasing the density of the sole so that the downward movement of the toe capturing impact & compression can be restricted.



Using inserts inside the toecap cavity as structural supports.



Increasing the height of the toe cap to create a sacrificial area.



Increasing the front and back depth of the toe cap to gain more height and strength.



Making a cut in the side of toe cap for shock absorption.

These techniques were done mainly using trial and error methods, which often creates fluctuations in strength over toecaps of different size. The problem greatly restricted the development and application of composite toecaps and in some cases has the potential to cause severe injuries to its users. The product’s ability to be recycled can be an important benefit for both the manufacturers and society. Traditional metallic toecaps were made from thin steel sheets stamped into the final cavity shape, used and disposed metallic toecaps can be collected, recycled, melted and remake into new products by the manufacturers. However with the available composite toecaps usually made from fiberglass and polypropylene (PP) or polyurethane (PU) plastics, large quantity of disposed product cannot be recycled and is often disposed as normal waste, which causes significant problems to the environment [1]. With the above factors in mind, it is imperative to develop an accurate method for designing and analysing composite toecap structures, as well as the need to research bio-degradable capabilities that will be sufficient in strength and stiffness for application in safety toecaps [1].

1.3 Research Objectives and Scope of Thesis The motivation of this thesis in the development of composite toecaps using numerical finite element (FE) modelling techniques was to use safety toecaps as a demonstration product to establish a body of knowledge for the development of other composite products. In the near future, these results can be beneficial for New Zealand industries, which enable them to use -4-

1. Introduction composite materials and design new products with better quality and performance. The research for the development process has four main focal points which comprise of: selection of composite materials; FE structural modelling; structural optimisation; product manufacturing. 

Selection of Composite Materials Investigate suitable fibre and matrix materials and the combined prepreg manufacturing process which can offer high production rate, high strength and stiffness, weight reduction, biodegradability and be economically viable.



FE Structural Modelling Research and construct FE models of composite toecaps affiliated with suitable material models which can provide accurate prediction in toecap strength, deformations and failure modes. The model shall also posses the ability to predict toecap structural strength under different layups and fibre orientations.



Structural Optimisation Conduct studies and research in different optimisation methods aimed at suggesting optimum arrangement in fibre orientation and number of layers, in order to achieve the toecap design with optimum structural strength to weight ratio.



Product Manufacturing Experiment the manufacturing of toecap samples using a compression moulding process previously studied, in order to obtain suitable parameters and conditions for the chosen material.

1.4 Thesis Outline This thesis chronologically described the systematic development of the high strength biodegradable toecap prototype through the following chapters:

-5-

1. Introduction Commencing with detailed literature survey that covers all technical aspects of the development process in Chapter 2, the content defines the direction of this research by suggesting the suitable actions and methods for each development stage based on the information and other related research reviewed.

Chapter 3 describes the industrial testing standards for safety toecaps in detail, which covers the verification of toecap geometry, different tests and requirements, and also the testing equipments used in this research for carrying out the regulated tests.

Based on the development process and the needs of the final products, Chapter 4 discusses the three proposed composite prepreg materials, their production process and properties. Biodegradability of the materials is also considered as one of the important features this chapter. Chapter 5 focuses mainly on the development process of FE models for the composite toecaps under standardised compression test. The content covers from geometry digitizing, construction of structural mesh, assignment of material properties to final failure simulation with large deformation. Results and observations from the numerical models were also discussed and evaluated. Chapter 6 discusses the concepts of the two optimisation methods employed, and their optimisation analyses performed for the toecap design based on the results obtained from the numerical models. The final design configuration was then applied in the manufacturing stage to produce the proposed optimal samples.

Chapter 7 combines the analytical results obtained in the previous chapters into one final design, and summarises manufacturing and testing results to verify the FE model and discusses possible cause of difference between the expectation and actual results.

Finally, in Chapters 8 and 9, conclusions and recommendations for future work are drawn from this research to summarise the knowledge developed from this master thesis.

-6-

2 2. Literature Survey This chapter provides a review for the research areas conducted in this thesis that covers: high strength bio-degradable materials, large deformation FE modelling, optimisation analysis for composite laminates, manufacturing of thin composite laminate products, and testing standards for safety toecaps.

2.1 Biodegradability of Thermoplastic Polymers

Flieger et al. [3] outlines a series of potential materials capable for the development of biodegradable biopolymers (BDP). The study shows that the use of BDP has been regarded as an alternative to the traditional petroleum base polymers, where most of the available materials can be produced from natural raw materials such as starch, sugar, cellulose and also fossil fuels [4]. Degradation of BDP was often caused by the action of natural microorganisms including fungi, bacteria and algae, which also offer the ability of controlling the speed of degradation ranging from two weeks up to a several month period depending on the methods applied. Investigation in manufacturing properties such as thermo stability and viscosity also shows high adaptation to conventional technologies and machines without significant modifications [5]. Introduction of BDP in the future engineering sectors is therefore expected to increase the value of the renewable raw materials that will eventually lead to the stimulation of new industrial activities [6]. The study summarises the characteristics including biodegradability, commercial reliability and production process from renewable resources obtained by chemical synthesis process such as polyglycolic acid (PGA), …. -7-

1. Literature Survey polylactic acid (PLA), polycaprolactone (PCL) and polyvinyl alcohol (PVA). Among them, PLA was considered as one of the BDPs with high material strength possessing its Young’s modulus ranging from 350 to 2800MPa, melting temperature of 150℃ and the degradation time between 18~24 months. Composting of PLA under designed temperature and pH was also proven to be an effective method for biodegradation [7].

Hoshino and Isono [8] conducted the experiments to examine the degradation efficiency of PLA immersed in lipases solution. PLA films were immersed in the solution at 37℃ at pH 7.0. The result shows a high efficiency of degradation from PLA as the film degraded completely within 20 days of immersion. Further studies in waste treatment of PLA were also performed concerning the catalysis mechanisms of lipase which provided better understanding in developing an eco-friendly cycle for PLA products [9].

Perego et al. [10] examines the changes in mechanical properties after crystallizing the PLA material of different molecular weight. The study shows significant improvements in tensional and flexural modulus as well as impact stress and heat resistance. Samples with molecular weight of 67,000 u possess the maximum yield strength of 70MPa and elastic modulus of 3750MPa, which demonstrates higher strength compared to other BDP materials.

Tokiwa and Suzuki [11] investigated and pointed out the melting temperature of PLA being one of the highest among the aliphatic material group, which ranged around 160~180℃. The high melting temperature far beyond room temperature made PLA a suitable material to be used for structural purposes.

Lee and Lee [12] investigated the mechanical properties of a binary blend material with PLA and poly(butylene succinate adipate) (PBSA). The study shows a significant increase in impact strength from the blend material compared to the raw PLA, with the PBSA weight proportion of 80% observed to offer the optimum strength in the blend material [11]. Liu et al. [13] studied the thermal and mechanical properties from the blend of PLA with poly(ethylene/butylene succinate) (Bionolle). The result shows addition of bionolle can aid the crystallization of PLA specimens. Samples of the biaxially orientated films also demonstrated an significant increase in tensile strength, modulus, and strain-at-break.

-8-

2. Literature Survey There have been many other investigations into enhancing the properties of PLA and also the method of degradation. These studies have revealed the great potential in the application of PLA material due to its high strength, bio-degradable characteristics and high melting temperature. It was therefore chosen as the targeted composite matrix for this research to construct a high strength solid structure for the desired composite toecap.

2.2 Natural Fibres

Natural fibres are elongated substances obtained from plants or animals which can be spun into filaments, thread or rope. Commercial natural fibres used for high strength composites are mainly made from plants such as Flax, Hemp, Ramie and Sisal fibres [14].

Joseph et al. [15] investigated the mechanical properties of composite sheets made from polypropylene(PP) reinforced by short sisal fibers. Comparison of results between meltmixing and solution mixing produced composites showing better mechanical properties from met-mixing sisal-PP samples with an optimum fibre length of 2mm. Rong et al. [16] studies the influence of fibre treatments towards the mechanical properties of sisal reinforced epoxy composites. Results from the study show significant improvements in tensile strength due to better adhesion between the fibres and the associated matrix, this was done by the partial removal of the lignin and hemicelluloses that offers higher extensibility for the fibres. The improvements in fibre–matrix bonding have been further expanded to sisal-low density polyethylene (LDPE) and sisal-glass fibre composites, which also possess higher tensile strength compare to the original properties [17].

In regards to flax fibres, it has long been considered as an environmental friendly alternative of synthetic fibres for fibre reinforced plastics (FRP) composites [18]. Similar strengthening approaches for sisal fibres discussed above were also applied to flax fibres aiming at enhancing the strength and thermo properties of the fibre composites. Baiardo et al. [19] presents the influence of fibre length distribution, content and processing conditions towards the composite mechanical properties. The results show 30% increase in tensile modulus when having 25% (vol) natural fibres substituted by fibres containing acetate groups, the acetylated flax fibres were also found to be readily attacked by cellulolytic bacteria, which displayed good biodegradability in soil [19]. Stamboulis et al. [20] investigated the relationship between environmental degradation behaviour and the mechanical properties for flax fibres. It was

-9-

1. Literature Survey found that the Duralin treatment upgraded flax fibres were able to retain their mechanical properties with moisture absorption. [20].

Apart from the fibres themselves, modifications in matrix of the flax fibre composites were also proven to give improvements in mechanical properties. Research conducted by Arbelaiz et al. [21] indicated that silanes or a new MA grafted PP matrix led to better performance than the traditional MAPP modification, with maximum tensile stress reaching 48.3MPa and Young’s modulus of 2178MPa for flax/MA composites with 30 wt% fibres. For strength prediction and modelling, Baley C. [22] conducted investigations in predicting the mechanical properties of flax fibres using micro-mechanical equations. The results show good agreement between the predicted and actual data over 80,000Mpa as shown in Figure 2-1.

Figure 2-1: Young’s modulus plotted as function of displacement for flax fibres. (experiment: diamond , Vf=64%: continuous line, Vf>= 74%: dotted line. [22]

The studies of flax and sisal fibres in degradation, strength enhancement, product consistency and predictability discussed above demonstrates the potential of using natural fibres for producing composite parts with strong mechanical properties. A composition of the targeted high strength toecap products was therefore proposed by combining the PLA matrix with the natural fibres mentioned above to form a high strength bio-degradable composite.

2.3 Application of Numerical Methods for Large Deformation Situations

2.3.1 Relevant Researches in Large-Deformation Explicit Modelling Maker and Zhu [23,24] outlines a serious of considerations for constructing a springback simulation model of metal strips using LS-DYNA. As the residual stresses generated inside the material during production plays an important role in the springback behaviour. The construction setups can be divided into three main aspects: forming simulations, development -10-

2. Literature Survey of springback model, trouble shooting and general issues of solving the model. The discussion of simulation development begins from the setup of FE model that outlines the use of *SECTION_SHELL card for defining the material behaviour and integration points NIP within the thickness. Different material models including number 18, 24, 36 and 37 were explained to define the demanded properties. Contacts between the rigid and elastic-plastic were then developed using *CONTROL_CONTACT card that governs the interaction between nodes over rigid and elastic surfaces during springback and forming process. Finally, loading, mesh refinement and explicit setups was also covered to fully define the modelling condition.

Shipway and Hutchings [25] derived the numerical values of elastic stress fields from a FE model simulating an elastic sphere under quasi-static compression using spherical shell elements. The simulation shows the deformation of the sphere geometry over the computation timestep and presented the distribution of stresses circumferentially on the surface of the sphere. Civgin F [26] analyses the torsional deflections and stresses of the composite bars subject to torsion. Numerical modelling of the composite bars was performed using ANSYS 5.4, with the use of the twenty noded straight pipe elements simulating the circular shape of the bar with a uniform defined thickness for the elements. The simulated results from the two models explained above show good agreement with the experimental data and indicated the high accuracy from the modelling output, which demonstrates the high accuracy results from thickness defined shell elements for modelling solid parts. Similar models have also been constructed out by other researchers to evaluate the stress distribution and deformation of the thin-walled structures [27,28,29]

Lamers et al. [30] constructed an FE model of large deformation simulating the forming of woven fabric reinforced composite product using DIEKA. The thermal mechanical behaviour was modelling by firstly predicting the fibre orientations of the product from the forming simulation; a Classic Laminate Theory based model was then was then applied to obtain the local changed thermo-mechanical properties of the composite laminates. The results has demonstrated out-of plane distortions resulted from the mis-match in thermal expansion of the simulated elements, shape distortions of the woven fabric reinforced products was therefore existed in the modelled geometry. Sidhu et al. [31] developed a new finite element approach for plain-weave textile composite perform for modelling its deformation during stamping using LD-DYNA. The woven perform was constructed using the interactions between 3-D

-11-

1. Literature Survey truss elements and 3-D shell elements that models the effect of scissoring and fibre angle jamming which better predicts the deformation of the woven fibres under tensile stress. The results show good correlations with the experimental data and provide the visual presentation of the deforming textiles under tensile stress. The two models explained offers the knowledge for the construction of manufacturing simulations for prepreg forming/pressing, which further enables the prediction of fibre orientations in the 3-D irregular shape.

Cui et al. [32,33] constructed a series of models simulating the buckling behaviour of transversely loaded composite shell structures. Models simulating curve strips with different number of plies and fibre orientations compressed between two rigid flat plates were constructed in LUSAS ®. The strip geometry was modelled using four-nodes quadrilateral thick shell elements (QTS4), while the rigid surface elements were adopted to simulate the rigid platen. From the modelling results, the model shows limited capability for modelling delamination effect with the applied shell elements, and requires the use of a more complex element models to accurately represent the buckling and de-lamination effects.

Approaches from other studies [34,35] also demonstrate the benefits and drawbacks of modelling composite laminates using shell elements. From the survey discussed above, the method of applying shell elements for draping analysis has been proposed in order to predict the fibre orientations in the toecap laminates. Modelling using shell elements with defined thickness has also been considered as one of the suitable approaches for this research.

2.3.2 LSTC LS-DYNA Solver

LS-DYNA is an advanced general-purpose multi-physics simulation software package that is actively developed by the Livermore Software Technology Corporation (LSTC). The programming keyword file consists of a single executable file and is entirely command line driven, all input files are formatted in simple ASCII format and can be opened, viewed and edited using a simple text viewer [36,37]. The software package is widely used for modelling 2D / 3D problems of nonlinear or rigid body dynamics, thermal analysis, fluid analysis, failure analysis, structural-thermal coupling and many other applications. The software is favored by research personnel due to its simple structure and specifically designed features in crash modelling [36,37].

-12-

2. Literature Survey An important consideration when using LS-DYNA is the consistency of units. As the software was written in ASCII format it does not have a default unit system, and requires the user to enter consistent units to generate accurate results. This can sometimes be confusing and therefore extra care should be taken for entering the values. A unit reference system was published by LSTC to offer suggestions on inputs of units [36,37] as shown in Figure 2-2.

Figure 2-2: Consistent use of unit systems required from the user when modelling in LS-DYNA [1]

2.3.3 Modelling of Composite Toecaps From investigation, there is currently limited information available for modelling and stress analysis for toecaps. One of the most related researches focusing on toecap analysis is “Damage Tolerance of Composite Toecaps” by Seung et al. [38], who constructed impact and compression modelling on steel and composite toecaps using ABAQUS. Their modelling results show a high stress concentration located at the front crest edge of the impacted dent for the impact model, and a high stress concentration at front surface of the toecaps for compression model as shown in Figure 2-3 [38]. The research conducted testing on custom made E-glass composite toecaps and concluded that with 3.4mm of thickness, the strength exceeds their steel toecaps of thickness 1.7mm. However no progress on optimisation of fiber orientation was made throughout their analysis [38].

Other research by Kuhn et.al [2] focusing on the strength of FRP toecaps show good results on compression and impact for the custom made toecaps. The modelling was conducted in ANSYS Classic with an analysis carried out in the weft/warp fiber direction, however details on stress distribution inside the geometry was not included.

-13-

1. Literature Survey

Figure 2-3: Toecap deformation and stress contour after impact /compression test [38]

2.4 Systematic Approach for Optimisation

Due to the orthotropic material characteristic and the layers structure of composite materials, optimisation analysis was conducted to achieve the highest material strength with the lowest weight. This process can be made possible by organizing the fibre orientations inside each layer to reach an optimum arrangement for the maximum strength output. Chatiri et al. [39] constructed a thick wall solid element model simulating a hydrogen storage tank in LSDYNA. The structural strength of material laminates investigated using laminate configuration tests demonstrated significant variations of up to 7.4 times between the outputs of different fibre layup arrangements.

Tsau et al. [40] investigated the optimal stacking sequence for fibre reinforced composites by constructing an analytical model with the Hashin failure criterion. The simplex approach using a direct searching method was employed to find the minimum failure function value, so as to derive the optimal stacking sequence. The result shows a significant increase in strength with the optimum design acquired, while the errors of the simplex approach began to rise significantly once the layers exceeds the number of 10. The FE modelling incorporating the -14-

2. Literature Survey simplex method depends greatly on the imported results, and therefore requires large amount of numerical data from the FE models of different layup arrangements.

Park et al. [41] employed genetic algorithms (GA) for searching the optimum design of symmetric composite laminates under various loading and boundary conditions. GAs are a subset of evolutionary algorithms derived from the study of biology, which imitates the gene selection mechanism performed by nature to preserve the optimal arrangements in the data pool. By combing with the tournament selection method for choosing random sets of individuals, the optimum designs were obtained with the precise angles suggested for every layer in the laminates.

There have been many other studies investigating the use of GAs, simplex approach and other statistic methods such as gradient projection algorithms [42,43]. However with the explicit modelling setup proposed for the toecap analysis, the large number of data demanded from the above methods would require unreasonable resources including computation power and time to achieve the targeted results, and would therefore be uneconomical. Methods that are more statistical and less modelling-result dependent were therefore investigated. Miravete A. [44] studies the optimisation of composite plates in bending and buckling by analyzing the fibre orientation of layers with a limited number of orientation options. Results from each layer possessing the orientation options were then compared to observe their strength output before advancing with another layer.

Concepts from the Taguchi method employed for design of experiments, quality control and product design explained by Ranjit K [45] evaluates the optimum criteria and contribution of each factor from the set number of experimental data imported. Subject experiments can be structuralized by isolating the influencing factors and levels of such influence, and conducting comparison analyses through detailed analysis procedures to reveal the optimum combination of each factors based on the data imported for each level. The method therefore possesses potential for analyzing the modelling of composite layers influenced by fibre orientations at different layers.

2.5 Previous Work at Centre for Advance Composite Materials (CACM) Previous research conducted at Centre for Advanced Composite Materials (CACM), at the University of Auckland has experimented with various fibre and matrix materials for toecap -15-

1. Literature Survey applications [46]. In these experiments, the highest structural strength was achieved using glass fibre reinforced PP Twintex® PET materials, while other materials such as flax fibre/ PP or carbon/ fibre epoxy has also demonstrated the potential of achieving satisfactory toecap strength [46].

2.5.1 Investigation and Comparison of Suitable Materials Previous projects aimed at examining the viability of using different types of composite materials yielded valuable results, for the production of toecaps used in footwear protection systems. The materials investigated included fibre reinforced composites with Twintex®, Flax-MaPP woven fabric, Plytron, carbon fibre and Polypropylene (PP) H380F polymers [46]. After initial investigation, further tests for specimens regarded with potential such as PP H380F, unidirectional Twintex®, woven Twintex® and Flax-MaPP Woven were carried out to obtain the chord modulus and ultimate tensile test values.

Figure 2-4: Comparison of chord modulus [46]

Figure 2-5: Comparison of tensile strength [46]

Shown in Figure 2-4 and 2-5 [46], both Twintex® unidirectional and woven tensile specimens of 3 mm thick have significantly higher values in chord modulus and tensile strength, when compared to the PP results. Meanwhile, the tensile strength result for unidirectional Twintex® -16-

2. Literature Survey specimen exceeds more than twice the value of the woven material. The Flax-MaPP specimen on the other hand demonstrated very weak properties in both chord modulus and tensile strength. With the strength properties of Twintex® specimens available, several toecap samples made of one, two and three Twintex® woven composite sheets were produced using a custom press-moulding process. The produced toecap samples along with traditional steel toecaps (FK50) were then compression tested between two flat plates and loaded up to 15kN to observe their difference in structural strength and evolution of failure modes. Cylindrical soft clay was also inserted into the centre cavity to observe the remaining clearance of the samples after the test [46].

Figure 2-6: Compression resistance testing of toecap samples [46]

Shown in Figure 2-6, the reference steel toecap managed to reach a maximum force of 15kN along with the 3 layer Twintex® samples at similar values. The 2 layer Twintex® however possessed significantly lower strength and could only reach 4.5kN, while the 1 layer Twintex® sample failed early just after reaching 1kN [46]. The reduction in weight was also another major focus for the research. Shown in Table 2-1 the 3 layer Twintex® samples weighs only 48% of the traditional steel sample while having the same ability of reaching 15kN. Other samples also reflect a greater reduction in weight however, the strength and stiffness is far weaker than the requirements [46].

-17-

1. Literature Survey Table 2-1: Sample weight comparison [46] Material

Manufacturing

Conventional Steel (FK50)

Weight (g)

Percentage

92.5

100%

Twintex® 3 layer_woven fabric

Mould-pressing

44.7

48%

Twintex® 2 layer_woven fabric

Mould-pressing

28

30%

Twintex® 1 layer_woven fabric

Mould-pressing

14.5

16%

The results above suggest the potential of constructing composite toecaps with light weight and great strength capable of reaching similar values to the steel samples. This early investigation also demonstrated the great difference in structural strength with different fibre orientation as shown by comparing unidirectional material with the woven (bidirectional) Twintex® samples.

2.5.2 Investigation into Manufacturing Process Previous research also conducted a careful investigation into possible composite toecap manufacturing processes. After consulting the manufacturing methods used by current composite toecap manufacturers, the hot press-moulding process was chosen for making the in-house custom composite samples.

Hot press-moulding uses two matched metal moulds placed between the platens of a hydraulic or pneumatic press and heated to cure temperature. To produce composite material toecaps with realistic and consistent toecap geometry, a steel-mould press set has been manufactured for the production of specimens. To ensure the product is of identical geometry, the shape of the steel toecaps was first digitized and then imported into commercial computer aided design (CAD) software using 3D laser scanning equipment [46]. As the scanning method is likely to cause overlapping surfaces and edges, the digitized surface went through further manual repairing and checking before it was processed for the steel mould design. After the toecap surface had been digitized and manually repaired, the geometry of the top and base mould was then created using the CAD software package Pro/ENGINEER Wildfire 4.0. Since the toecap is designed as a protective part for boots, it is reasonable to produce specimens for both left and right pieces. This also increased the production rate and enabled the rectangular mould to be used more efficiently. The designed top and base mould were then imported into Pro/NC for manufacturing validation, where the program generated a simulation file to check for manufacturing defects and the production G code for computer numerical control (CNC) manufacturing. Finally, the two moulds were produced on a CNC machine and assembled -18-

2. Literature Survey onto a pneumatic press with heating unit attached to form the manufacturing set as shown in Figure 2-7 [46].

Figure 2-7: Process of designing and manufacturing of the hot-press mould [46]

Figure 2-8: Comparison of tensile strength [46]

The male matched die part was attached to the jig using bolts which were screwed into four tapped holes. The proper build up of the connection is shown in Figure 2-8. Two thin and one thicker ceramic plate, for which some holes had to be drilled, were used to isolate the conducting heat. The thin plates were utilized to hide the four nuts from the connection bolts. Furthermore, four long through holes were drilled near the edge of the female mould to insert -19-

1. Literature Survey the heating rods for temperature control, therefore only the female part of the mould was directly heated. The gap between the male and female mould was chosen to be 3mm [46]. The complete assembly of the pneumatic mould press ready for the production of toecaps is shown in Figure 2-9.

Figure 2-9: Complete assembly of pneumatic mould press [46]

To manufacture the toecap samples using different materials, most of the materials were first made into fibre-reinforced preforms before further processing into the final toecap samples. Different layups were used depending on the characteristics of the materials as shown in Figure 2-10. Woven Twintex® and Plytron preforms needed to be trimmed or folded into a specific shape before moulding, while carbon fibre was draped directly into the female mould. Pressing of the mould began by coating with silicone spray onto the mould surface and heating up to 150~200℃ depending on the materials. After laying down the material, the mould was closed for approximately 20 minutes allowing heat to fully melt the material and mould it into the final shape. After curing the mould was then cooled down to room

-20-

2. Literature Survey temperature for demoulding to obtain the finished product shown in Figure 2-11. During the investigation it was also found that the manufacturing process depends greatly on layup experience and manual trial and error methods, therefore several trials were needed in order to find the optimum temperature condition.

a

c

b

Figure 2-10: Different layups for a) Twintex® woven b) plytron preform c) carbon fibre thermoset sheet moulding compound[46]

Figure 2-11: Twintex toecap samples produced [46]

With the preliminary knowledge in manufacturing and material selections, the research emphasises the necessity for constructing computer simulated FE models, which will enable -21-

1. Literature Survey further analysis in stress concentration of toecaps under loading. The knowledge in toecap stress distribution can potentially aid in the selections of fibre orientations of composite materials. The FE models can therefore be used as an analytical tool for optimising the toecap design and eventually achieving the maximum possible strength for the material.

-22-

3 3. Testing Standards and Methods Testing methods for evaluating the structural strength of safety toecaps were designed mainly to imitate the hazardous environments that the toecap products may encounter when in service. Among all testing categories, compression and impact testing are widely used by several international standards to determine both the safety level and structural strength of the toecap to its users. Testing standards for toecap strength that are currently employed by the footwear industry include the European Standard EN 12568:1998 “Foot and leg protectors – Requirements and test methods for toecaps and metal penetration resistant inserts”, and the American Standard ASTM F2412-05 “Standard Test Methods for Foot Protection”. For this research, the ASTM standards were considered inappropriate for testing a bare toecap as the standard requires the inclusion of other corresponding shoe parts and materials for testing. The European Standard was considered suitable for the existing specimens due to its specific focus on toecap testing, and the simplicity of requiring only the testing of bare toecaps [1]. The following sections describe the testing procedures that were either used to aid the development of actual testing of samples, or the modelling setup in the numerical simulations.

3.1 European EN12568:1998 Standard The European EN 12568:1998 Standard is utilized by the industry mainly to prove the suitability of the manufactured product. The standard regulates the geometry and dimension, surface finish, internal length, impact and compression resistance, corrosion, penetration and flexing resistance of the toecap products. As structural strength is the primary focus for this …. -23-

1. Testing Standards and Methods research, only compression and impact testing were considered in this study. However, the actual impact test mentioned above could not be performed in this research due to the absence of impact testing facility. This study was, therefore, focused on evaluating toecap strength by referencing only the compression testing part of the standard, and should not be seen as a complete fulfillment of the standard itself [47]. 3.2 Geometry and Dimensions The common geometry of the toecap consists of an outer protective shell with the main inner cavity wide enough to accommodate human toes. The bottom edges of the shell are extended with flanges to form the base rim of the toecap. According to the standard, the extended width e of the bottom flange shall not be greater than 10 mm as shown in Figure 3-1 [1], in this research this was done by measuring 4 different points across the width for quality control.

Figure 3-1: Toecap geometry: a) indication of e (left) b) illustrations of flanges (right) [1]

To aid the measurement and structural testing, a central axis of the toecap is to be determined by overlapping the outer rim of left and right toecap samples as shown in Figure 3-2a. Minimum internal length can therefore be obtained by measuring the distance between the front and rear edge of the material along the central axis as shown in Figure 3-2b, this measurement is taken in a distance of 3-10 mm to the bottom surface were the toecap rests. For a shoe size of UK 9, the space between internal edges needs to be greater than 42mm [1].

Figure 3-2: Toecap central axis: a) plotting method(left) b) illustrations(right) [1]

-24-

Testing standards and Methods 3.3 Compression Resistance For compression test authenticated in EN12568:1998 standard, a load of 15kN is applied onto the top surface of the toecap using a rigid flat plate with an overhead speed of 5mm/min ±2mm/min as shown in Figure 3-5 [1]. To measure the internal clearance after the test, the identical cylinder clay defined in the previous section is positioned inside the toecap cavity to the rear and along the central axis. The same minimum clearance of 21.5mm is required in order to satisfy the standard. The actual compression test will be performed on all manufactured samples, while the testing setup is simulated using LS-DYNA to obtain numerical modelling output.

Figure 3-3: Compression test: setup (left) and illustrations (right) [1]

During testing, a clay cylinder dia.25 mm ±2mm and height 25 mm ±2mm is placed along the centre axis, which touches the back edge of the toecap flange as shown in Figure 3-5 to measure the clearance under the specimen after the test. For UK size 9 toecap samples, internal clearance after both compression and impact testing needs to be greater than 21.5 mm as indicated in Table 3-1. Table 3-1: Minimum clearance after testing [1] Toecap Sizes

Up to and including 5

6

7

8

9

10 and over

Minimum internal clearance (mm)

19.5

20

20.5

21

21.5

22

Minimum external clearance (mm)

24.5

25

25.5

26

26.5

27

-25-

1. Testing Standards and Methods 3.4 Instrumentation The testing equipment used to perform the compression tests was an Instron 1185 Testing Machine as shown in Figure 3-4. Instron 1185 is a screw-driven universal testing machine with 100kN maximum capacity load cell embedded; the machine therefore possesses the capability to conduct both tensile and compressive testing. The equipment consists of 3 main elements: The Instron 1185 Testing Machine, Data Processor and a Computer module. Operation of the machine is controlled by “Bluehill Material Testing Software” installed in the computer module as shown in Figure 3-5, the software allows the user to setup the testing condition prior to the experiments, which enables automatic operation once the test is executed. Configurations of EN12568:1998 Standard setup inside the software allows fullyautomatic operation and ensures the accuracy of the tests.

Figure 3-4: Instron 1185 machine

Figure 3-5: Bluehill material testing software

-26-

4 4. Composite Material Prepreg The selection of the composite prepreg materials for toecaps samples in this study concentrated on two vital properties, structural strength and bio-degradable capability as introduced in section 1.2. A literature review of this study shown that prepreg materials made from bio-degradable PLA matrix and natural Flax fibre demonstrated significant higher results in maximum tensile strength when compared to other prepregs made of natural composites. Therefore, this has been considered as the material with the greatest potential for producing high-strength toecaps.

The main focus of this chapter is to investigate the

manufacturing process of the referenced composite prepregs and the technical complexity involved. Properties of the manufactured prepregs were later imported into the FE models to aid in the modelling of the compression test. 4.1 Introduction In order to produce high strength bio-degradable composite toecaps, flax-PLA composite prepreg has been chosen due to its outstanding strength in comparison to other bio-degradable materials surveyed as outlined in Chapter 2, and also its structured long fibres where manual layup can be performed for optimisation. flax-PLA prepregs made from two different processes have been trialled in this research: The hot-pressed flax-PLA prepreg sheets, and the impregnated flax-PLA prepreg sheets. While both of the material prepregs demonstrated high tensile stress of over 250Mpa during tensile testing, the impregnated sheets in particular offered higher fibre volume fraction of 65%, and were able to achieve a higher value in tensile …. -27-

1. Composite Material Prepreg strength and stiffness when compared to the hot-pressed prepreg sheets. However, breakages from fragile surfaces and de-lamination problems were observed from the manufactured specimens that added uncertainty to the strength outcome of the proposed toecap samples. The hot-pressed sheets on the other hand have a lower fibre volume fraction of 35%, and thus required lower forming pressures during the hot-pressing stage and possess lower tensile strength. Nevertheless, the composition with richer matrix offered better surface finish, stronger bounding between layers and further weight reduction can potentially provide greater benefits to the performance of the final products.

Knowing that the flax-PLA composite model will later be used for design optimisation, ensuring accurate modelling of the composite material behaviour is therefore an important step prior to the full construction of the FE models. Foreseeing this process, a preliminary verification of the material model was performed using simplified composite toecaps made from random fibre orientation composites. The material chosen was the Sisal–Polypropylene (PP) composite sheets due to its abundance and easy access for this research. Despite the lower strength of the material compared to flax-PLA, the process provided a simple indication of the force-displacement relationship of the toecaps under compression testing, and therefore was used as a checkpoint for the construction of FE models before proceeding to further complex settings and optimisation.

4.2 Sisal-Polypropylene (PP) Composite Prepreg sisal-PP fibre prepreg is a custom-made experimental composite material at CACM, The University of Auckland. The material combines wood flour, natural sisal fibres and different grade of recycled polypropylene into a composite prepreg. The material prepreg is comprised of short sisal fibres 4~6mm in length with a mass fraction of approximately 12~15%. Having the matrix made from polypropylene and wood flour, the material possesses the glass transition temperature of 150°C and the melting temperature of 175°C [47]. The material prepreg sheets measured 2mm in thickness. Manufactured from a plastic extruder, the prepreg has 70% of its fibre oriented in longitudinal direction, with the remaining 30% oriented randomly [47]. With the melting of PP that leads to the re-orienting of fibres during moulding, the material is expected to have its properties similar to the 2D isotropic materials. Figure 4-1 illustrates the production process layout of the sisal-PP prepreg. The prepreg was manufactured using a conical twin screw extruder made by Cincinnati Milacron. The mechanism of the conical twin screw used for mixing PP palettes and short sisal fibres -28-

Composite Material Prepreg optimises the homogeneity of the composite prepregs, and offers better uniformity through the thickness of the product.

Figure 4-1: Manufacturing illustrations: schematic diagram (up) and equipment photo (bottom)

-29-

1. Composite Material Prepreg

Figure 4-2: Manufactured sisal-PP prepreg: a)roll (up) and b)trimmed sheet (bottom) [47]

-30-

Composite Material Prepreg The manufactured material prepregs shown in Figure 4-2a have the density of 1250 kg/m3. The random orientation of the embedded sisal fibres shown in Figure 4-2b performed as structural reinforcements that contributed to the ultimate tensile strength of 33MPa, which is up to 40% higher compared to the 20MPa original tensile strength of PP [47]. Detailed properties of the sisal-PP prepreg are listed in Table 4-1 below. This composite prepreg was later used to verify the composite behaviour of the FE models before proceeding into further developments. Table 4-1: Sisal-PP prepreg material properties [47]

Properties

Value (units)

Modulus (chord~0.25%) Modulus (segment~0.25%) Ultimate Tensile Strength Yield Strength Tensile Strain at Yield Density

1.96 1.98 33.0 33.0 3.77 1250

Gpa Gpa Mpa Mpa % kg/m3

4.3 Flax-Polylactic acid (PLA) (Hot-Pressed) Composite Prepreg The hot-pressed flax-PLA composite prepreg with a fibre volume fraction of 35% is a high strength bio-degradable long fibre composite prepreg [48]. It is combined with natural flax fibres and aliphatic polyester PLA derived from renewable resources to provide high tensile strength, yet environmental friendly properties as discussed in Chapter 2. The flax fibre chosen for this study was a Belgian variety yarn with size of 82.7 tex and a specific gravity of 1.49g/ cm3, the product was manufactured by Jayashree Textiles in India. This chosen flax fibre possesses a specific strength of 0.29-0.33 N/tex. The PLA thermoplastic resin used as composite matrix was an extrusion/thermoforming grade of NatureWorks® PLA Polymer (2002D), which has recently been rebranded as PLA (4042D) by the company while maintaining identical material composition and properties.

Manufacturing of the prepreg was carried out using compression moulding that consists of the following steps: forming of the PLA matrix sheets, flax fibre winding and hot-pressing. Matrix sheets were firstly pre-consolidated using commercial resin palettes, natural flax fibres in the form of a yarn were then wound onto the sheets with a motor driven lathe. Finally the wound sheets were trimmed and pressed in the hot press to form the desired prepreg [49].

-31-

1. Composite Material Prepreg 4.3.1 PLA Matrix Sheets To produce the matrix sheets for winding, 4042D PLA palettes were collected and left in the vacuum oven at the temperature of 70℃ overnight for moisture removal. Two aluminium plates of size 140mm  140mm  4mm were cleaned using acetone and covered with Teflon® coated sheets. ROCOL® Silicon FMG Spray was then applied onto the Teflon® sheets to prevent voids from appearing during pressing. A steel frame with thickness of 0.9mm and a rectangular internal opening of 120mm  120mm was then sandwiched between the two Teflon®-covered plates to form the mould as shown in Figure 4-3 below.

Figure 4-3: Steel frame assembly for moulding of PLA sheets

Figure 4-4 shows a measured amount of 24g dried PLA palettes distributed equally inside the steel frame with the silicon spray applied onto the inner edges of the frame. The PLA palettes sandwiched between the aluminium plates were then placed into a manual hot press of 185℃ and remained for 10 seconds in order for the polymer to reach its melting temperature, before been compressed with a load of 1 tonne for 30 seconds as shown in Figure 4-5 [49]. The preconsolidated PLA sheets were then cooled rapidly with an air fan, and finally trimmed into a 120mm  120mm rectangular shape as shown in Figure 4-6.

-32-

Composite Material Prepreg

Figure 4-4: Aluminium mould with distributed PLA palettes

Figure 4-5: Aluminium mould compressed with manual hot press

-33-

1. Composite Material Prepreg

Figure 4-6: Pre-consolidated PLA sheets

From this process, it was found that applying ROCOL® Silicon FMG spray onto the Teflon® sheets can effectively eliminate the existence of voids, while providing easier separation of pre-consolidated PLA sheets from the steel frame. Also, by ensuring the melting of PLA palettes before applying the compression force, significant improvements in surface quality and elimination of voids was also observed. The pre-consolidated PLA sheets have an average weight of 21g, and thickness range between 0.8mm~0.9mm.

4.3.2 Flax-PLA Composite Winding and Compression Composite winding of the flax fibres onto the PLA sheets was performed in order to obtain uniform fibre orientation throughout the prepreg. To control the winding speed and the fibre volume fractions in the prepreg, a motor driven lathe with feed speed controller was therefore utilized. Feed rates of 0.45mm/rev were used at a chuck speed of 45rpm to achieve good alignment and reasonable winding speed, which on average required 6 ~7 mins for the winding of each PLA sheet. The setup was achieved by clamping the PLA sheets onto the headstock chuck and supported by thin steel strips between the chuck jaws; an extra jaw was installed onto the tailstock to use as a support from the opposite end, a spring-tensioned yarn

-34-

Composite Material Prepreg feeder was also used in the position of the cutting tool as shown in Figure 4-7.

Figure 4-7: Winding of PLA sheets with motor-driven lathe

-35-

1. Composite Material Prepreg Figure 4-8: PLA Sheets with wound flax fibre

Figure 4-8 shows the flax winded PLA sheets with an uniformly orientated fibre direction. The wound composite sheets were once again sandwiched by Teflon covered aluminium plates and compressed in the manual press at 185℃ under 230kPa for 30 seconds before cooling down to room temperature. The weight of the finished prepreg averages between of 25~27g, with the average thickness of 0.875mm as shown in Figure 4-9.

Figure 4-9: Flax-PLA prepreg manufactured

4.3.3 Prepreg Properties The finished prepreg was tested by stacking up 4 unidirectional sheets and moulded into 3.2mm composite plate. The plate was then trimmed into 148mm by 15mm tensile specimens for various mechanical testing. The results listed in Table 4-2 demonstrated an increase in tensile strength of 4 times compared to the PLA matrix. The tensile modulus reached a value of 18Gpa, which is much higher than the common bio-degradable materials surveyed in Chapter 2. The high volume fraction of PLA also provided the prepreg with very good formability for 3D geometries. These outcomes have therefore matched the characteristics required for compression-moulding of the composite toecaps. Table 4-2: Properties of compression-moulded flax-PLA sheets [48,49] Product

E_Tensile(GPa)

Vf%

Density (kg/m3)

σ_Tensile(MPa)

flax-PLA Sheets

18

35

1330

250

PLA (4042D)

3.5

nil

1240

60

Product flax-PLA Sheets

-36-

E_Shear(GPa) 1.7

Failure Strain 8%

Melt Temp℃ 165

Glass Temp℃ 65

Composite Material Prepreg PLA (4042D)

1.7

nil

210

60

4.4 fla

x-PLA (Impregnated) Composite Prepreg The second prepreg material chosen was the pre-impregnated flax-PLA prepreg sheets with a fibre volume fraction of 65% [50]. The prepreg uses identical materials as the hot-pressed prepreg previously discussed, while possessing even higher tensile strength.

The main

difference between the two materials lies in their manufacturing process: Rather than hotcompressing the fibres into the matix directly, the PLA resin was dissolved using THF solvent and soak-coated onto the flax yarn before pre-consolidation take place. The process was proven for strengthening the bonding between flax fibres and PLA matrix, and also allows the use of higher fibre volume fraction. The combined effect of the two features significantly increased the tensile strength of the material [50].

Manufacturing of the material consists of the following processes: PLA solution impregnation, solvent evaporation, yarn collection, composite winding and hot-press preconsolidating. The manufacturing processes are labour intensive, hence time consuming.

4.4.1 PLA Coating Process The coating process illustrated in Figure 4-10a is initiated by dissolving 4042D PLA palettes in Tetrahydrofuran (THF) solution with the mass ratio of 1:9 at 38℃ for 48 hours, while the flax fibre were dried in the vacuum oven at 70℃. Yarn impregnation was then carried out by passing the flax yarn through an custom-made bath containing the dissolved 10% THF/PLA solution as shown in Figure 4-10b. The bath contains a series of rollers and pins which compacts the yarn and remove voids between fibres. This also forces the immersing and coating of PLA/THF solution into the fibre yarns [50]. The coated flax yarns were then passed through a 3 meter ceramic heating tunnel at 70℃ to evaporate the remaining THF solution inside the yarn, the fully dried yarn was collected into a spool after the heating tunnel as shown in Figure 4-10c [50].

The production was made into a continuous process. With the coating speed of approximately 30 m/hour, constant monitoring at the coating bath is required as the dried PLA can clog up the rollers and cause disruptions to the process. Ventilation systems in the manufacturing area

-37-

1. Composite Material Prepreg is crucial as inhaling the evaporating THF solvent can cause serious irritation to humans, and its inflammable properties is also regarded as a potential hazard in the laboratory.

Figure 4-10: a) Pre-impregnating process diagram (above) b) Ceramic tunnel (bottom-left) c) Impregnating bath (bottom-right) [50]

4.4.2 Flax-PLA Winding and Pressing Composite winding of the impregnated flax-PLA yarns was carried out using the method similar to the process conducted for the hot-pressed prepreg. Rather than using the PLA sheets, the impregnated yarns were wound onto 120mm  120mm Teflon® covered steel plate with identical settings to yield the wound plate as shown in Figure 4-11. The wound plate was then sandwiched between another two Teflon® covered plates, and heat-pressed in the manual press at 185℃ under 230kpa for 20 seconds before cooling to form the composite -38-

Composite Material Prepreg prepreg [50]. The prepreg was later carefully removed from the steel plate and dried in the vacuum oven at 37℃ to remove moisture. The finished prepreg weighs between 15~17g with thickness of 0.5mm ~ 0.8mm as shown in Figure 4-12.

Figure 4-11: Composite winded impregnated flax-PLA yarns

Figure 4-12: Impregnated flax-PLA Composite Prepreg

4.4.3 Prepreg Properties Identical tests for the hot-pressed samples were carried out for the impregnated prepreg to obtain the material properties. Results in Table 4-3 show a significant increase in both tensile

-39-

1. Composite Material Prepreg strength and modulus for the impregnated prepreg. The tensile strength has increased more than 20% while the tensile modulus doubled. The material therefore demonstrates great potential of producing high-strength composite toecaps that reaches the industrial standard. Table 4-3: Properties of Impregnated flax-PLA prepreg sheets [48,50] Product

E_Tensile(GPa)

Vf%

Density (kg/m3)

σ_Tensile(MPa)

Impregnated Prepreg

39.9

65

1270

308

Hot-pressed Prepreg

18.0

35

1330

250

Product

E_Shear(GPa)

Failure Strain

Melt Temp℃

Glass Temp℃

Impregnated Prepreg

nil

8%

165

65

Hot-pressed Prepreg

1.7

8%

165

65

Despite the impregnated prepreg possessing a high fibre volume fraction of 65%, when experiencing heat, the weak structural connections with small amount of matrix bounding the yarns can easily fall apart and cause difficulties to the moulding process. This can also cause the fibres to shift and re-orientate during moulding and make the fibre direction harder to control. To strengthen the structure, the prepreg was further trimmed into two halves (top and bottom faces) and heat-pressed again at a 90 degree offset angle to form a stronger structural support. The final prepreg comprises of two fibre layers, orientated at horizontal and vertical directions respectively with the thickness of 0.7mm.

Figure 4-13: 90 degree offset heat-pressed prepreg

-40-

Composite Material Prepreg The re-compressed 90 degree offset prepreg shown in Figure 4-13 demonstrates improved connecting strength between the yarns and is unlikely to fall apart during the compression moulding of 3D composite parts.

-41-

5 5. Finite Element Modelling & Analysis Structural analysis using FE modelling was carried out in this study in order to gain a detailed understanding of the stress distribution, material behaviour, composite failure modes as well as the effect of fibre orientation during the impact and compression testing of composite toecaps. With a limited number of related research results available for benchmarking and comparison, the numerical modelling in this research was conducted in three successive steps to ensure the accuracy of the simulation: Firstly, a simplified FE model of an existing Flecksteel© toecap was carried out to simulate the compression test and compared with the experimental results to observe and verify the performance of the model. The model was then enhanced with greater capabilities in material behaviour and failure modes to simulate the sisal-PP toecap samples. The verified model was then further developed and incorporated with an orthotropic composite material model to simulate the structural strength of flax-PLA toecaps made from both hot-pressed and solution impregnated prepregs. This chapter details the construction procedure and modelling output of the FE models for each of the three development stages. The simulation results generated will later be used for optimisation and comparison with actual experiments in Chapter 6 and 7. 5.1 Toecap Geometry and Mesh Generation The structure of Flecksteel© and composite toecaps commercially available incorporates a 3D concave shape formed by thin sheets of metal or composite laminates. The geometry was deliberately made with rounded corners and equal thickness throughout the part to avoid …. -43-

1. Finite Element Modelling and Analysis stress concentrations. To ensure identical geometry for all FE models and custom-made toecap samples, a reference toecap shape was obtained by digitizing the outer surface geometry of the available Fleck steel toecap. The scanned surface was then processed in CAD packages to design the aluminium moulds for sample manufacturing as explained in section 1.3.2. FE modelling on the other hand requires detail inspection and mesh generation for the digitized surface before setting up the model. In this research, the toecap surface, associate structural mesh and model setups were processed across four different software packages: PTC Pro/E Wildfire, ANSYS Advance Meshing, Altair Hypermesh and LSTC LS-DYNA Prepost. Geometrical files were converted into different forms such as .iges, .prt, .vda, .hm and .k in order to transfer files between different software packages: Firstly, the digitized geometry was trimmed in PTC Pro/E Wildfire to eliminate unwanted surfaces and preserve only the essential geometry, the geometry file was then converted from .prt into .iges format and loaded in ANSYS® Advanced Meshing Tool packages to check for geometry defects. Defects such as broken edges, overlapping/penetrating surfaces and unnecessary geometry points were removed or repaired to create a “smooth surface” before transferring into Altair® HyperMesh for mesh generation. Due to the irregular concave shape of the toecap, a radiating-mapped mesh was generated to provide smooth mesh coverage for all irregular surfaces with relatively similar mesh size as shown in Figure 5-1. The constructed mesh with 2mm in size was then saved from .hm into a LS-DYNA R971 keyword file to be operated on in LS-DYNA Prepost. Considering the toecaps were constructed from thin sheets of equal thickness, shell elements were used to define the model structure using LS-DYNA Prepost. With the coordinate systems, material data and testing conditions available, modelling setup for simulating the compression and impact tests could proceed.

b

a

c

d

Figure 5-1: Evolution of digitized toecap geometry a) scanned surface b) fixed c) repaired d) mesh

5.2 Modelling of Manufacturing Process and Fibre Orientation -44-

Finite Element Modelling & Analysis

The fibre direction plays a very important role in determining the structural strength of the composite parts. For composite parts with irregular 3D geometry, accurate designation of fibre orientations in the FE model has long been a difficult task as described in Chapter 2. Fibre orientation of each laminate has a critical effect on the final structural strength of the product, and usually requires careful measurements to accurately assign the fibre directions at each location. In this research, two different approaches have been carried out in order to seek a simple and time-saving solution, which is capable of providing reasonable accuracy to the fibre direction issue: The production modelling method, and the manual assigning method.

5.2.1 Production Modelling Method The proposed production modelling method considers defining the fibre orientation from the early stage of manufacturing, before the pressing of the prepreg. This method recognises that although it is difficult to obtain a numerical prediction of the fibre direction for the 3D toecap geometry, fibres in the prepreg remains unidirectional before compression moulding is executed. The method therefore aims at simulating the compression moulding process, where unidirectional fibres can be easily assigned at the beginning. After the solver launches its computation, it will calculate the deformation of the prepreg fibres at each time step, and provide the final fibre orientation at all locations automatically by the end of the simulation. Locations where wrinkles and folds are likely to take place can also be determined. This can be useful for proper actions to be taken before commencing the actual manufacturing process. Trial modelling of the compression moulding process was constructed by editing the CAD files of the moulds into male and female surface files. The two surfaces were then aligned into their compression moulding positions combined into one file, and imported into LS-DYNA Prepost [49]. A flat surface 1.5mm above the female mould representing the blank material was constructed as shown in Figure 5-2. The *SECTION_SHELL card was used to define all surfaces with shell elements, while the *MAT_RIGID material model was used to assign a rigid material to both halves of the mould, *MAT_PIECEWISE_LINER was used to assign isotropic elastic-plastic material behaviour to the flat surface for simulating the composite prepreg. *BOUNDARY cards were used to fix the female mould in place, and also to define the downward movement of the male mould to simulate the compression moulding procedure. The *CONTACT_AUTOMATIC_SURFACE_TO_ SURFACE card was used to define contact nodes between the blank material surface and the moulds, which allowed the

-45-

1. Finite Element Modelling and Analysis modelling of contact and therefore the deformation of the prepreg surface upon contact with the tooling.

Figure 5-2: Production modelling setup: male mould (blue), female mould (orange), prepreg (green)

Figure 5-3: Simulation of deformed prepreg after compression moulding.

-46-

Finite Element Modelling & Analysis The modelling results of the compression moulding process for 2mm thick rectangular PLA sheets reveals the simulated deformed shape of the PLA sheets at each time step. This trial model provides an approximate prediction of the evolution of the compression process, and also reveals the region where wrinkles and folds are likely to occur as shown in Figure 5-3. From the result shown, intensive folds and wrinkles are more likely to arise at the corners of the toecap cavities, where the materials nearby is pulled into the mould during compression moulding. Aiming at minimizing toecap surface defects and usage of materials, the area with wrinkles and folds indicated were trimmed and reshaped manually to improve the surface finish for the simulated toecap geometry. By applying a trial and error approach, regions of possible wrinkles and folds on the rectangular surface were deleted and further trimmed to save material, while preserving sufficient coverage for the desired toecap geometry to be formed. This was done by eliminating excessive material outside the toecap cavities, and careful trimming around the cavity edges were conducted to form the desired toecap shape. Finally, the right toecap elements were also deleted to form the left toecap surface model as shown in Figure 5-4

Figure 5-4: Trimming of prepreg surface to eliminate surface defects

The left toecap surface was then expanded in LS-DYNA Prepost, and further adjusted to match the compression of the mould. The procedure led to the final shape of a mushroom-like surface that minimises folds and wrinkles on the toecap surfaces while possessing sufficient material to form the required toecap geometry as shown in Figure 5-5. -47-

1. Finite Element Modelling and Analysis

Figure 5-5: Final mushroom-shape prepreg

Once the mushroom-shaped prepreg geometry was developed, the composite fibre orientation can be assigned onto the shell element prepreg using orthotropic composite material models such as *MAT_ COMPOSITE_DAMAGE or *054-055_MAT_ENHANCED_COMOPSITE_ DAMAGE. The material card allows the fibre orientation to be defined for all elements using the AOPT material axis option. For this research, AOPT=3 was used to assign the fibre direction using x, y, and z vector components. A single layer composite prepreg with fibres orientated in the X direction was assigned for observation. As the mushroom-shaped surface had been positioned in the global X-Y plane, all elements shared an identical normal vector and therefore had identical X fibre directions, indicated by white lines in Figure 5-6 and 5-7.

Figure 5-6: Compression moulds with mushroom-shape prepreg

-48-

Finite Element Modelling & Analysis

Figure 5-7: Assigned fibre direction for mushroom-shape prepreg

The compression model of the mushroom-shaped prepreg and fibre direction settings were then re-computed in LS-DYNA to obtain the final fibre layups for the toecap elements. The computed result illustrated in Figure 5-8 shows the simulated compressed toecap product computed from the production model, with original prepreg fibres orientated in the Xdirection. From the developed compression moulding simulation, a wrap-folding behaviour of the mushroom-like prepreg was also observed during the compression of the male mould. This motion poses a significant effect to the final fibre layups in the toecap samples, as shown in Figure 5-9. Upon contacting the male mould, the prepreg was pressed into the female mould cavity. The two side wings wrap join up with the tail fin forming the toecap shape, while the unidirectional fibres embedded in the prepreg were also wrapped and eventually formed the final toecap layup.

Figure 5-8: Compressed toecap model with simulated fibre layups.

-49-

1. Finite Element Modelling and Analysis

T=6.327

T=11.750

T=17.174

T=22.597

T=28.020

T=33.444

T=38.867

T=44.291

T=49.714

T=53.700

Figure 5-9: Compression moulding simulation: evolution of the prepreg wrap-folding motion

-50-

Finite Element Modelling & Analysis From the results illustrated above, the production modelling approach has successfully provided a forecast of fibre orientations for toecap samples, and also improved the manufacturing quality using the developed mushroom-shaped cut-out. The mushroom shaped cut-out allowed material to be conserved while simplifying the production process, thus being more economical. The simulated geometry with the fibre direction data can then be saved as LS-DYNA .k file and used to conduct further setup for the compression test simulation. The objectives defined for this approach can therefore be fully achieved. 5.2.2 Manual Assigning Method The second method used to define fibre orientations was done by directly assigning the local fibre directions onto the toecap geometry. Using the combination of the LS-DYNA material model and the unique wrap-folding layup characteristic of the prepreg, the continuous fibres wrapping of the toecap geometry can be assigned using the AOPT function available in LSDYNA Prepost that defines the material axis direction by assigning a normal vector. The AOPT = 3: locally orthotropic material axis option was used for this section. The user assigns an AOPT vector v using V1, V2 and V3, which represents the product of X, Y and Z vector components originating from the space origin O. The element material orientation is then computed by cross multiplying the element normal n to v to yield the primary material direction as shown below:

i  N X  V1   N   V   N X  Y   2  N Z  V3  V1

j NY V2

k NZ V3

 i ( N yV3  N zV2 )  j ( N xV3  N zV1 )  k ( N xV2  N yV1 )

Eq [5-1]

Eq [5-2]

The result can also be visualized in a 3D environment. Figure 5-10 illustrates the assigned AOPT vector v in a defined space with shell elements and the material normal vector n located nearby. From equation 5-5 and 5-6, material direction of the elements v  n is hence perpendicular to vector v and n at all times regardless of their position and shape.

Figure 5-10: Assignment of material primary direction using AOPT=3

-51-

1. Finite Element Modelling and Analysis The AOPT=3 material assignment can also be better visualized by considering Figure 5-11, which illustrates an AOPT vector v[1,0,0] defined from the origin O with two shell elements located nearby. The primary material direction for the elements can be seen as “radiating” from the vector v and projected onto the shell elements.

Figure 5-11: Illustration of assigning primary material direction using AOPT=3

Due to the special concave shape of the toecap, and also the wrap-folding behaviour of the mushroom-shaped prepreg simulated in the production modelling method, toecap fibre orientations appear to possess a radiatinglike layup similar to the system used in AOPT. This can potentially offer the possibility for manual assignment of fibre directions. Aiming at establishing a quick and simple method of assigning the toecap fibre layup, efforts were therefore made to observe the wrap-folding behaviour.

A paper cut out of the mushroom-shape with marked x-direction lines was made as shown in Figure 5-12. The paper cut-out -52-

Figure 5-12: Mushroom shape paper cut-out with X direction mark lines.

Finite Element Modelling & Analysis was wrapped around the male mould profile and fully compressed in order to form the final toecap shape. As the result, the marked lines originally plotted in X direction have formed into 3D geometry that represents the final fibre layup directions. In Figure 5-13 a) and b), the marked X-direction lines are shown in green and have evolved into series of circular curves passing horizontally through the toecap shell, which can also be seen as radiated from a virtual axis v’ illustrated by the red axis. The direction of the virtual axis was approximated using the fact that all existing fibres shall have their orientation perpendicular with respect to the v’ axis at all times. Its location was determined by passing through the concave wrapping centre point P, which possesses a circular fibre orientation surrounding the point. With the near-symmetric shape of the toecap, the v’ axis was assumed to be aligned on the mid-plane, and therefore has a Vz vector component of zero.

Figure 5-13: Toecap geometry formed from mushroom paper cut-out with virtual axis a) front view b) side view

From the observation, the toecap fibre layup can be approximated by assigning the proper virtual axis v’ to both compression simulated and scanned toecap geometry as shown in Figure 5-14. Development of the method commenced by locating the virtual axis v’ for the compression formed toecap simulated in the production model previously

Figure 5-14: Mushroom shaped paper cut-out with virtual axis.

-53-

1. Finite Element Modelling and Analysis Illustrated in Figure 5-15, the simulated fibre angles were used as indicators to the direction of v’, which has been approximated manually as 36.87 ゚ from the bottom toecap surface. The angle was then defined using AOPT function in the material model to assign the primary material axis for all elements:

tan 36.86 

Vx

Vy

 0.75

Eq [5-3]

 V x : V y  V1 : V2  0.75 : 1

Eq [5-4] Figure 5-15: Vector component of v’

To position the v’ axis, the concave wrapping centre P was approximated at the front lower edge of the toecap geometry for the axis to pass through. As the v’ axis originates from the space origin, positioning was done by adjusting the location of the toecap geometry with respect to the space origin. These steps allowed the virtual axis v’ to be fully defined. The material angle can also be varied globally by using the MANGLE function to simulate different prepreg angles before compression. Information of AOPT setup is therefore listed in Table 3-1 as below. Table 5-1: Manually defined vector axis: AOPT and position data in LS-DYNA AOPT

V1

V2

V3

MANGLE

3

0.75

1

0

0 for x dir

Finally, the vector axis setup was applied onto the scanned toecap geometry for defining the fibre direction as shown in Figure 5-16 and 5-17.

Figure 5-16: Layup of toecap fibre direction for scanned geometry using manual assignment method.

-54-

Finite Element Modelling & Analysis

Figure 5-17: Toecap fibre direction from X-direction prepreg Layup

This approach has provided a simple and quick adjustment mechanism for global fibre direction that applies to all the toecap elements. The digitized toecap surface incorporated also eliminates geometry issues such as wrinkles and folds experienced by the compression simulated model, and thus provide the result with higher accuracy. The objectives defined for this approach have therefore been achieved successfully.

5.3 Compression Test Modelling The construction of compression test models in this research aims to provide reliable simulation outputs of the structural strength and failure mode of the toecap samples. Simulation results can then provide a basis of comparison with the actual testing results. The computed stress distribution over the toecap geometry can also pinpoint critical stress concentration areas, and eventually allow structural optimisation by re-orientating the fiber layup angles. The modelling was carried out following a step by step verification process, from simple isotropic piecewise plastic-elastic material models to orthotropic enhanced composite material models with breakage simulation capability. FE modelling of the toecap compression test was conducted for the toecap samples made from four different materials: Fleck Steel, sisal-PP prepreg, flax-PLA hot-pressed prepreg and solution impregnated prepreg. All modelling work was performed using LS-DYNA© Prepost and solved by the LSDYNA R971 solver. -55-

1. Finite Element Modelling and Analysis 5.3.1 Model Setup for Compression Test According to the EN12568:1998 compression test standard mentioned in Chapter 3, toecap specimens shall rest upon a flat surface and be subjected to compression between two flat plates. A CAD assembly comprised of the toecap as well as other associated parts for testing was therefore created.

Setup Geometry: Two flat plates 120 x 80 x 5mm were created inside the toecap geometric models for both the production modelling file and scanned-geometry file. One plate is placed 0.5mm below the toecap flange surface as testing bed, the other is placed 4.5mm above the apex of the toecap surface to simulate the compressing apparatus. The toecap surface was located in the middle between the two plates as shown in Figures 5-18 and 5-19.

Figure 5-18: Compression test model of the scanned-geometry toecap [47]

Figure 5-19: Compression test model of the production model toecap [47]

-56-

Finite Element Modelling & Analysis Sections:

The toecap specimen was represented using section *SHELL elements. The

associated number of integration points (NIP) was defined for composite toecaps according to the setup (number of prepreg layers) from the different materials used. Shell thickness (T1~T4) settings were also dependant on the material prepregs properties and the setup of toecap samples. On the other hand, the two flat plates were assigned using *SHELL with *MATERIAL _RIGID function.

Boundary Conditions: To imitate the Degree of Freedom (DOF) restriction of the two compressing plates, all nodes located at the bottom plate were defined as fully constrained with no translation and rotation respect to the X, Y, and Z axes. All nodes at the top compression plate were defined to allow translation in the X direction only, all other DOF were constrained. The toecap samples in actual testing were placed on the flat surface with no other contacts, therefore there were no restrictions defined for the toecap elements.

Compressing Velocity: With the defined timestep of 0.0001 seconds for obtaining sufficient data for observation, applying specified overhead speed of 5mm/min ±2mm/min as listed in the testing standard will take unreasonable amount of time to compute. Moreover, with no visco-elastic material model used for this research, the modelling results are not sensitive to the compression speed within reasonable limits. Therefore the initial velocity of the top compressing plate was defined to move at 2500 (mm/s) to shorten the computation time, while having enough time steps to acquire sufficient modelling data. Compressing velocity of the top plate was applied by defining a LCID velocity curve of 2500 (mm/s) across the duration of the simulation, and later linked using *Prescribed _Motion_Rigid card in the boundary condition setup.

Contacts: Two sets of contacts were defined using the same contact model *Automatic_ Single_Surface : 1) top plate - toecap elements, 2) toecap elements - base plate. The two contact sets were setup using *SetD card, and then linked in the contact model with node set type (SSTYP) = 2.

Termination Time:

According to regulations mentioned in Chapter three, a minimum

clearance of 21.5mm needs to be maintained for the current toecap specimen after the test to satisfy the standard. With compression speed of 2500 mm/s and clearance of 45.5mm between the plates, the time to reach termination when clearance falls below 21.5mm is therefore:

-57-

1. Finite Element Modelling and Analysis

(45.5-21.5) / 2500 ≈ 0.01 second.

Result Output:

The essential outputs from the compression model include force-

displacement curves, failure mode prediction and stress/strain distribution contours. Force values were obtained by summing all forces experienced by the nodes of the bottom plate, while the deformation values were taken from the compression distance of the plate. Contours of stresses and strains can be viewed using the *FCOMP card in LS-DYNA Prepost across different time steps with predicted failure modes. These graphical results can also be exported into images and animations for further analysis. 5.3.2 Fleck Steel Toecap Model To begin the construction of the FE model, commercially available metallic toecaps made from FleckSteel© was first simulated. As this model requires no assignment of fibre direction, only the laser-scanned toecap geometry was used. From investigation, FleckSteel© used in the commercial toecaps possess a thickness of 1.8mm with similar material properties to Carbon Steel AISI 1030. The material model *024-Piecewise_Linear_Plasticity was therefore used with the estimated properties defined to simulate the isotropic metallic behavior of the thin metallic shell as shown in Table 5-2. RO (Density)

7.85E-9

ton/mm

E (Young’s Modulus)

2.1E+5

Mpa

PR (Poisson Ratio)

0.33

SIGY (Yield Stress)

400

Mpa

ETAN (Tangent Modulus)

2000

Mpa

Table 5-2: FleckSteel© properties defined

The result from the compression test model shows strong agreement with the experimental results, where the toecap was compressed and experienced plastic deformation following the trend of the force-displacement curve obtained from actual experiment. As shown in Figure 520, the top compression plate begins by moving downwards and contacts the apex of the top surface. The top of the toecap bulges and remains in contact with the plate as it deflects downwards. As the plate begins to compress the toecap wings, the compressed top surface bulge begins to separate away from the plate. The wings eventually compressed and deform outwards until termination. Comparison between the force-displacement curves from -58-

Finite Element Modelling & Analysis simulation and experiment shows good correlation of the results, with the data existing in the same domain reaching similar force values across the deformation scale as shown in Figure 521. The different curves correspond to different mesh sizes and show the improvements in accuracy for a more finely meshed model.

Figure 5-20: Force and deformation of simulated FleckSteel© toecap [47]

Figure 5-21: Comparison of Force-displacement curve for FleckSteel© toecap [47]

-59-

1. Finite Element Modelling and Analysis The model also reveals information on stress distribution and associate failure modes of the toecap geometry during compression. Shown in Figure 5-22, stress concentrations indicated by the models match up with the locations of failure on the compressed FleckSteel© samples. This provides a reliable indication of stress distribution of the target toecap product. The model was therefore accepted for the next stage of modelling orthotropic composite materials.

Figure 5-22: Modelling result of stress distribution and failure modes for FleckSteel© toecap [47]

5.3.3 Sisal-PP Toecap Model Considering sisal-PP prepreg being a short fibre random orientated composite as explained in Chapter 4, the prepreg possesses a 2D isotopic behaviour with liner elastic modulus up to the failure stress, which appears to be similar to the metallic materials. Shell elements of thickness 1.5mm were thus defined using the *SECTION card. Material model *024Piecewise_Linear_ Plasticity applied in previous metallic toecap modelling was also used for the sisal-PP toecaps simulation. From the properties listed in section 4.2, the material data defined into LS-DYNA is listed in Table 5-3 as follows.

Table 5-3: Sisal-PP toecap properties defined

RO (Density)

1.25E-9

ton/mm

E (Young’s Modulus)

1980

MPa

PR (Poisson Ratio)

0.33

SIGY (Yield Stress)

33

MPa

ETAN (Tangent Modulus)

100

MPa

As the compression testing condition remained unchanged, so did simulation setups from the previous model. Only the laser-scanned toecap geometry was used for this model due to its isotopic nature which does not require the definition of fibre orientations.

-60-

Finite Element Modelling & Analysis 5.3.4 Flax-PLA (Hot-Pressed) Model flax-PLA toecaps made from several layers of long-fibre uni-directional prepreg possess orthotropic material properties. The structural strength of the toecap is therefore strongly dependant on the number of prepreg layers and their associate fibre directions. From the literature survey conducted in Chapter 2, *054-055_ENHANCED_COMPOSITE_DAMAGE material model was selected for simulating the behavior of flax-PLA toecaps. The composite material model defines the material strength in three principle directions: a represents the longitudinal direction, b the transverse direction, and c the thickness direction which was not taken into account by the material model. From the properties listed in section 4.3.3, the data listed in Table 5-4 was inserted into the composite material model: Table 5-4: Flax-PLA (hot-pressed) toecap properties defined

RO (Density)

1.33E-9

ton/mm

EA (Y modulus - a dir)

1.8E+4

MPa

EB (Y modulus - b dir)

3500

MPa

EC (Y modulus - c dir)

3500

MPa

PRBA (poisson’s ratio)

0.2

GAB (shear modulus)

1750

AOPT

3

DFAILM (max matrix ε)

8%

DFAILT (max fibre ε tension)

8%

MPa

DFAILC (max fibre ε compress) 8% XC (max compress strength - a)

250

MPa

XT (max tensile strength - a )

250

MPa

YC (max compress strength - b)

60

MPa

YT (max tensile strength - b)

60

MPa

For flax-PLA composite toecaps, stacking of several prepreg layers with different fibre orientations was proposed to offer a stronger structural support. Modelling of the laminate structure was therefore required for the section elements to simulate the behaviour of the combined composite layers. To assign composite layers with specific fibre directions, number of integration points (NIP) was defined depending on the number of layers demanded required in *SECTION_SHELL card. The assigned NIP value will therefore divide each shell elements with numbers of integration points depending on the input. Principle material directions at -61-

1. Finite Element Modelling and Analysis each integration point can then be defined by material angle function (Bi) in the *SECTION_ SHELL card. This function also divides the elements into small layers with equal thickness as discussed in Chapter 2. In order to establish a guideline for defining fibre directions for the toecap layers, and also to establish an analysis system for future structural optimisation, a layup structure using even number, symmetric laminates with four fibre orientations of 0, 90, +45, -45 degrees was decided upon. 0 degree angle was defined as the orientation formed by the x-direction prepreg, other angles can therefore be defined accordingly by offsetting with individually specified angles using MANGLE function discussed in section 5.2.2. This system provides a reasonable number of possible layers-direction arrangements, which helps narrow down the option during the selection of arrangements, and also helps to simplify the optimisation process which will be covered in the next chapter. Total number of available arrangements versus number of layers used can therefore be calculated as below:

NS

t 2

Eq [5-5]

Where N = total number of arrangements S = total number of fibre direction options ( 0, 90, +45, -45) t = total number of composite layers used (in even numbers) The results from the calculation are shown in Table 5-5 below: Table 5-5: Flax-PLA total number of layer-direction arrangements

t

S

N

2

4

4

4

4

16

6

4

64

8

4

256

10

4

1024

12

4

4096

14

4

16384

16

4

65536

18

4

262144

20

4

1048576

For the hot-pressed flax-PLA toecap modelling, a trial arrangement of 4 layers with a fibre direction of [90/0/0/90] and thickness of 5.25mm has been proposed for examination. Modelling for the compression test settings was carried out for the both laser-scanned and production modelling generated geometry in order to observe the output from both models, -62-

Finite Element Modelling & Analysis and also to evaluate their accuracy by comparing them to manufactured samples which will be discuss later in Chapter 7. 5.3.5 Flax-PLA (Impregnated) Model The compression test model of impregnated flax-PLA toecap shares a similar setup with the hot-pressed model described previously. The main difference lies in the defined material directions and the material properties. As indicated in section 4.4.3, stronger material properties has been entered into the material model to simulate the material behaviour as shown in Table 5-6 below:

Table 5-6: Flax-PLA (impregnated) toecap properties defined

RO (Density)

1.27E-9

ton/mm

EA (Y modulus - a dir)

3.99E+4

MPa

EB (Y modulus - b dir)

3500

MPa

EC (Y modulus - c dir)

3500

MPa

PRBA (poisson ratio)

0.2

GAB (shear modulus)

1750

AOPT

3

DFAILM (max matrix ε)

8%

DFAILT (max fibre ε tension)

8%

MPa

DFAILC (max fibre ε compress) 8% XC (max compress strength - a)

302

MPa

XT (max tensile strength - a )

302

MPa

YC (max compress strength - b)

65

MPa

YT (max tensile strength - b)

65

MPa

Due to the structural strengthening process described in section 4.4.3, each material prepreg consists of two fibre layers with an orientation offset of 90 degrees. Each prepreg was therefore modelled as a 2 layers laminate. An arrangement for the impregnated flax-PLA toecap trial model was proposed as having 4 prepreg sheets, 8 layer symmetric laminate composites with a fibre direction of [90/0/90/0/0/90/0/90] and thickness of 2.8mm. The setup for impregnated flax-PLA toecap will be modelled in laser scanned geometry in order to evaluate the toecap performance and failure mode..

-63-

1. Finite Element Modelling and Analysis 5.4 Modelling Results Modelling results for composite toecaps made from sisal-PP and flax-PLA prepregs were obtained in order to evaluate their structural strength, stress distribution and failure modes. After the verification of the FleckSteel© toecap model carried out in the previous chapter, material data from sisal-PP and flax-PLA prepregs were applied to construct their associate toecap compression test models. Both the hot-pressed and solution impregnated flax-PLA toecap model with simulated high structural strength were then investigated further using the impact testing model. Toecap designs with potential of reaching 15kN benchmark strength were then proposed for optimisation analysis. 5.4.1 Sisal-PP Toecap Results Simulation of toecap compression tests were performed using the sisal-PP material model with the mesh size of both 2mm and 4mm. Apart from the standard 2mm single layer sisal-PP prepreg, simulation of a 4mm double layer prepreg was also conducted to observe the effects of shell thickness versus the structural strength. Figure 5-23 illustrates the force-displacement curve up to its maximum force values. The modelling results of the toecap using a 4 mm and 2 mm mesh shown very similar behavior before reaching a compression distance of 10mm. The finer 2mm mesh however possess slightly higher strength beyond 10mm distance, and eventually reached the maximum compressive load of 660.65N as oppose to the 614.23N from 4mm mesh size. The double layer model on the other hand demonstrates significantly higher rigidity and strength, which is capable of reaching 2.65 times higher strength of 1752.90N compares to the single layer model.

Figure 5-23: Results of sisal-PP compression testing simulation [47]

-64-

Finite Element Modelling & Analysis Results of simulated deformation and stress distribution were also evaluated. Figure 5-24 presents the distribution of stress and deformation for both single and double layer toecaps at 15 mm compression distance, where both models possess similar failure modes. Higher stress level for double-layer model can also be seen as expected due to its larger thickness of 3mm.

Maximum stress value occurred along the upper frontal rim as the material provided vital structural support for resisting the compression from the upper plate. Stress concentration areas at the frontal region to the far edge of the left and right wings were also observed, which indicates intensive stress suffered by the material and eventually bent inwards that led to structural failure. In regards to deformation, the top surface apex was first compressed by the top plate, the upper surface was then deflected into a flat shape. As the toecap been compressed further, the top surface bulged away from the top plate while the left and right wings were squashed outwards. The wings together with frontal area were then compressed until failure.

Figure 5-24: Result of 2mm mesh sisal-PP toecap models showing (a) single-layer von-mises stress (top left) (b) single layer deformation (bottom left) (c) double-layer von-mises stress (top right) (d) double-layer deformation (bottom right) [47]

The compression test modelling results of sisal-PP composite toecap have shown good agreement with the literature survey as discussed in Chapter 2. The simulated strength difference between 2mm and 4mm thickness will later be used to compare with the experimental results from the manufactured samples, which can provide better understanding of the relationship between layer thicknesses and toecap strength. -65-

1. Finite Element Modelling and Analysis 5.4.2 Flax-PLA (Hot-Pressed) Toecap Results Results of hot-pressed flax-PLA compression test simulation were obtained from both laserscanned and production-simulated geometry models, with the defined 4 layer laminates with material direction of [90/0]s and thickness of 4mm. Shown in Figure 5-25, the two results shared similar trend in force-displacement curve up to 12mm displacement. The curve from laser-scanned model possesses a more stable gradient with its smooth surface that better distributed the stresses across the surface, and achieved a maximum compressive load of 6251.28N. The compression-simulated model on the other hand demonstrated a zigzag shape in its force-displacement curve due to several stress concentration locations resulted from the uneven surfaces and broken gaps. The structure was therefore weakened and reached only 4499.17N in terms of maximum compressive load.

Figure 5-25: Comparison between toecap compression test results with laser-scanned & compressionsimulated geometry.

Comparison between stress distribution and failure modes was also carried out to understand the effect of uneven surfaces and associated fibre directions towards the computed compressive load. Shown in Figure 5-26, the uneven surfaces of compression-simulated toecap possesses stress concentrations around the area of apex and the side wings, which weakened the structure and led to early failures at t=0.005. With the uneven surface and lower compressive load, it was therefore believed that the compression-simulated toecap was not suitable for further application due to geometric modelling inaccuracy. The laser-scanned toecap geometric model was therefore chosen for carrying out all further simulation work.

-66-

Finite Element Modelling & Analysis

Figure 5-26:

Comparison of deformation and failure modes between laser-scanned (left) and

compression-simulated (right) geometric model.

-67-

1. Finite Element Modelling and Analysis 5.4.3 Flax-PLA (Impregnated) Toecap Results Modelling results from the impregnated flax-PLA toecap have shown similar forcedisplacement behaviour compared to the previous hot-pressed model. The stronger material properties with 8 layer structure, thinner 2.8mm thickness and fibre direction of [0/90/0/90]s offered a higher gradient in the force-displacement curve and achieved greater values of 7237.41N for maximum compressive load as shown in Figure 5-27.

Figure 5-27: Comparison between flax-PLA hot-pressed and impregnated toecap simulation results

5.5 Evaluation of Results The three results obtained from the modelling of flax-PLA and sisal-PP toecap compression tests show strong agreements in the force-displacement curve in terms of graphical trend. With the aim of developing high-strength composite toecaps, investigation with different materials and thickness has been carried out during these simulation studies, comparison between different toecap material models was also performed in order to search for the features which can possibly be optimised in later studies for achieving higher compressive load.

As flax-PLA toecaps modelled showed stronger strength properties compared to sisal-PP toecap, difference in deformation and failure modes between the hot-pressed and impregnated -68-

Finite Element Modelling & Analysis toecap model were investigated. Shown in Figure 5-28, both impregnated and hot-pressed toecap models possesses great similarity in compressive resistance value and form of deformation. As the displacement reached 12.5mm, stress concentration located at the right wing of the impregnated model increased beyond the maximum tensile stress defined for the material, and therefore led to the deletion of elements that eventually causes breakages and leads to structural failure. The hot-pressed model on the other hand found its stress concentration located at the upper front rim that resulted in the breakage of the upper dorm, which soon propagates to the side wings that leads to total failure. The difference in failure is therefore observed to be critical to the maximum compressive load reached.

Figure 5-28:

Comparison between flax-PLA impregnated (left) and hot-pressed (right) toecap

deformation results

-69-

1. Finite Element Modelling and Analysis Also shown in Figure 5-29, simulated strength of flax-PLA toecaps have achieved more than 3 times higher the compressive load than sisal-PP prepreg. However when compared to the 15kN strength listed in the EN12568:1998 standard, all composite toecaps models have suggested insufficient strength for the current model settings. Nevertheless, the modelling results have revealed several possible approaches for achieving higher strength, which includes: stronger materials, number of prepreg laminates and optimised fibre orientations. Comparing among the materials modelled, the stronger hot-pressed and impregnated flaxPLA toecaps have so far managed to achieve close to 50% of the aimed 15kN strength, and was therefore been considered as the possible design for the targeted high-strength toecap materials. Increasing structural strength of flax-PLA toecaps by controlling the number of layers used and the associate fibre orientation was thus decided as the two main objectives for the optimisation studies conducted in the next chapter.

Figure 5-29: Comparison of toecap simulation results obtained

-70-

6 6. Design Optimisation Optimisation analyses for the toecap models were carried out by varying two critical control factors: number of prepreg layers, and fibre orientation. These two factors were controlled by changing the material definition from the existing FE models for both flax-PLA hot-pressed and impregnated toecaps. Models with different setting were then computed to investigate the relationships between factors and their resulted strength. Prior to the analysis, a modelling result matrix which included the strength results from all possible arrangements under the defined condition was obtained in advance in order to evaluate the accuracy of the optimisation approaches. Optimisation analysis employed in this research includes the LayerTournament (L-T) method and the Taguchi method. The two methods perform their analyses based on two separate methodologies: the Layer-Tournament method uses a layer-by-layer approach with a defined direction to achieve the best result, while the Taguchi method focuses mainly on exploring all critical experiment outputs for seeking the relationship between best results and its associate factors.

6.1 Modelling of Prepreg Layers & Fibre Orientation To initiate the analysis, models with different factor settings were firstly constructed to obtain sufficient data for the proposed optimisation studies. Compression test models were duplicated and then modified to accommodate different setups for the two targeted factors: fibre directions and number of prepreg layers. Modelling of fibre orientation in the toecap geometry can be altered using the *MANGLE function in the *MATERIAL card as discussed …. -71-

1. Design Optimisation in Section 5.2.2. With the four fibre orientations: 0, 90 +45, -45 been defined as the available modelling conditions, fibre orientation was therefore modified as shown in Figure 6-1 to represent the four conditions of the factor. The assigned fibre directions can be displayed in LS-DYNA PrePost with the *IDENT card. This function however is limited in showing only the direction settings of the first layer. 0゚

90 ゚

゚゚

゚゚

45 ゚

-45 ゚

゚゚

゚゚

Figure 6-1: Assignment of fibre layout for 0, 90, 45, -45 degrees

Number of prepreg layers on the other hand were modified using the *SECTION_SHELL card. Prepreg layers and the associated total thickness were re-defined using the NIP and Tx function to assign the desired number of layers. Finally, material orientation of each layer was inserted using the [Bi] function to complete the property setup as shown in Figure 6-2.

Figure 6-2: Assignment of fibre orientations for 0, 90, 45, -45 degrees layup sequences

-72-

Design Optimisation In order to conduct optimisation analysis for the toecap models incorporating various number of layers, laminates with different material properties were required for the FE model to perform simulations following the procedures of the applied optimisation approaches. To model laminates with different materials, the *INTERGRATION_SHELL card was used to define the thickness and the material of each layer.

This function allows the user to model the desired layer properties through controlling the number of integration points (NIP), weight fraction (WF), position (S) and the material (PID). An example of four-layer laminate is illustrated in Figure 6-3. NIP defines the number of the divided layers through the laminate thickness, therefore in this example NIP=5 was used to divide the laminate into 4 portions. The weight fraction (WF) tab defines the thickness proportion ratio of the specified layer versus the entire laminate, where the total laminate is equal to 1. For 4 layers with equal thickness, a WF value of 1/4 = 0.25 was thus defined for all layers. The position (S) tab defines the centre position of each layer with respect to the centre axis. The user enters the distance between the centre of the defined layers and the laminates to assign the position of the layers, while the top and bottom surface were defined as -1 and 1 respectively by the system. The positions of the four layers were therefore inserted as -0.75, -0.25, 0.25, 0.75 from top to bottom. At last, the PID defines the material of each layers according to the properties entered in the *MATERIAL card.

Figure 6-3: *INTEGRATION_SHELL card in LS-DYNA Prepost

Definition of the layers with different material properties using *INTEGRATION_SHELL was conducted for the Layer-Tournament optimisation approach, which will be introduced in later sections. This function can also be useful for other modelling purposes such as defining sandwich structures and boundaries between fibres and composite matrix. An excel program for calculation of WF and S is also included as Appendix D to aid the calculation.

-73-

1. Design Optimisation 6.2 Optimisation and Evaluation Approaches Optimisation and evaluation approaches employed for this research comprises the complete modelling result matrix, the Layer-Tournament (L-T) method, and the Taguchi method.

6.2.1 Complete Modelling Matrix Matrices enlisting the modelling results of maximum compressive load from all possible arrangements were created from the output of each toecap design. The total number of possible arrangements for each toecap design can be calculated using the equation below: A f

l 2

Eq [5-6]

Where A = total number of possible arrangements l = number of laminate layers (even numbers) f = number of options for fibre orientation From the complete modelling matrix, information regarding the optimum modelling designs and their ranking among all simulations was obtained to offer an indication on accuracy to aid the optimisation analysis performed later. The matrices also revealed the relationship between fibre directions and possible failure modes, which provided a good reference for understanding the behaviour of composite toecaps. An example of the matrix shown in Table 6-1 details the arrangement of fibre directions and their associated models. The four fibre orientations defined were used as the top row and the left column which represent the orientation sequence of the piles on one side of the symmetrical composite stack respectively. Multiplication of the four orientations thus formed 4 x 4 = 16 different arrangement models. Letters of the alphabets were assigned to each of the model for identification purposes. For instance, the assigned G matrix possesses the fibre orientation of [45/90/90/45] as indicated from the matrix. For the toecap design with more than 16 arrangements, multiple matrices were used in order to accommodate all possible arrangements. Table 6-1: Layout of the complete modelling matrix

-74-

symm

[0]

[90]

[45]

[-45]

[0]

A

B

C

D

[90]

E

F

G

H

[45]

I

J

K

L

[-45]

M

N

O

P

Design Optimisation 6.2.2 Layer-Tournament Approach A custom-designed approach was developed from the standard tournament optimisation method as discussed in Chapter 2. This approach utilizes the characteristics of the composite laminate structure, where the outer-most layers experience the highest stress among the laminates when experiencing bending moment based on the beam theory. The method therefore analyzes the structure from the outer-most layer through to the centre layer. Strength of the laminates were then evaluated progressively by defining different fibre orientations layer after layer following the priority order established, and eventually provided the best arrangement from the final comparison. The following describes the optimisation analysis process carried out for the hot-pressed flax-PLA toecaps made from a 4 layers symmetrical laminate, which has 16 possible arrangements with the four fibre orientations defined.

Being the most critical factor for this analysis, the number of toecap layers was first confirmed for initiating the analysis. Figure 6-4 shows the cross-sectional view of the 4 layer structure. Layers 1 and 4 were laminated as the outer layers, while layers 2 and 3 were sandwiched as the inner layers. The symmetrical layup with respect to the centre line offered the same fibre direction for layers 1 and 4, as well as for layer 2 and 3.

Figure 6-4: Cross-section view of 4 layer composite laminates

The Layer-Tournament (L-T) optimisation analysis was then performed by calculating the number of analyzing levels, which equals to half the number of layers used in the laminate. In this example, two levels were used in total. Beginning from level one, models with the two outer-most layers orientated into the four available directions were constructed. Four separate models with flax-PLA fibre direction in 0, 90, 45, -45 degrees for layer 1 and 4 were thus created, while layer 2 and 3 were temporarily modelled with PLA properties into the “dummy model” as illustrated in Figure 6-5. The first four dummy models were then solved using LSDYNA to calculate their maximum structural strength. The arrangement with highest strength values among the four was then promoted into the next level. For the 4 layer flax-PLA toecaps, analysis of the first level indicated layer 1 and 4 with fibre direction of 0 degrees -75-

1. Design Optimisation returned the highest strength values. The 0 degree setup was therefore promoted to level two and remained for further analysis. 0゚

90 ゚

45 ゚

-45 ゚

゚゚

゚゚

゚゚

゚゚

0゚

90 ゚

45 ゚

-45 ゚

゚゚

Figure 6-5: Four ゚゚ dummy models created゚゚for level one analysis

゚゚

In the second level, layer 2 and 3 of the promoted model was further modified to create another four separate dummy models with the flax-PLA properties and the available fibre orientations as shown in Figure 6-6. The optimised design was obtained by comparing the maximum strength values among the final four models to identify the strongest one. The results from second level analysis indicated that the orientation layup of [0/90/90/0] possesses the highest maximum strength among the four models, and was therefore identified by the Layer-Tournament approach as the best design among all arrangements 0゚

0゚

0゚

0゚

0゚゚゚

90 ゚゚゚

45 ゚゚゚

-45 ゚゚゚

0゚

90 ゚

゚゚ 0゚ ゚゚ ゚゚

Figure 6-6:

45 ゚ ゚゚ ゚゚ 0゚ 0゚゚゚ ゚゚ ゚゚ ゚゚ Four models created for level two analysis

-45 ゚

゚゚ 0゚ ゚゚ ゚゚

Shown in Table 6-2, the L-T method has proposed an optimised design for the 4-layer laminate by using only 8 analysis models, 4 at each level, as opposed to the full modelling matrix of 16 arrangements. An increase in difference between the numbers of analysed models versus all possible arrangements can also be seen as the number of layers increases. The method is thus capable of providing great savings in both computation power and time, which can be very useful for aiding the development of composite products.

Table 6-2: Comparison of required number of model from L-T approach vs complete arrangements Number of layers (l)

4

6

8

10

12

14

16

18

20

L-T Approach

(L)

8

12

16

20

24

28

32

36

40

All Arrangements (A)

16

64

256

1024

-76-

4096 16384 65536 262144 1E+06

Design Optimisation Analysis index E is the ratio of the possible arrangements over the arrangements used in the analysis. With the analysis index, the scale of reduction in computation power and solving time compare to the construction of all possible arrangement models can be better analyzed. From the assigned number of layers and fibre orientation options, E can therefore be calculated as below in Equation 5-7 and 5-8: A f

For

E

E  Where

l 2

,

L f 

l 2

,

x

l 2

A fx  L fx

Eq [5-7]

f ( x 1) x

Eq [5-8]

l = number of laminate layers (even numbers) f = number of options for fibre orientation L = number of models required from Layer-Tournament Analysis A = total number of possible arrangements

For a fixed fibre orientation options of four ( l = 4 ), relationship between the analysis index versus the number of layers and can also be visualized using Figure 6-7:

Figure 6-7: Efficiency vs number of layers curve for the Layer-Tournament approach

-77-

1. Design Optimisation 6.2.3 Taguchi Approach Taguchi approach was chosen as the second optimisation analysis method for this research due to its emphasis on statistical results, as opposed to the L-T approach which relies on the FE simulation of the dummy models as described previously. With the defined four fibre orientations, the array of the four-level factors with minimum number of experiments was required in order to satisfy the conditions of analyzing the toecap modelling results with the least number of models. From the literature survey performed in Chapter 2, a modified L-16’ orthogonal array was chosen for performing the optimisation analysis in four levels. Aiming at obtaining the optimised toecap with the highest compressive load, the criteria of “the larger the better” was therefore selected for calculating the signal noise ratio.

To initiate the analysis, the required information on factors and levels was first defined. Table 6-3 shows the four fibre orientation options were defined as the four levels, while the symmetric layer pairs that were defined as factors A, B, C and so on for the analysis. However due to its required input of minimum 16 experiments, which is identical to the total number of possible arrangements for 4-layer toecaps, performing optimisation analysis using modified Taguchi L-16’ array to 4-layer toecap design was considered meaningless since all modelling data would be included. The approach was, therefore, applied to only the toecap design with the number of possible fibre orientation arrangements above 16. Table 6-3: Assignment of factors and levels for Taguchi Analysis

Factors and Level Description Layers

Factors

Level 1

Level 2

Level 3

Level 4

1 (skin)

A

0

90

45

-45

2……..

B

0

90

45

-45

3 (core)

C

0

90

45

-45

With the factors and levels defined, the modelling results were inserted by referencing the modified L-16’ array shown in Table 6-4. As the toecap layers have been considered as independent factors, interaction relationships should therefore be prevented between the chosen columns to ensure the reliability of the optimisation analysis. After the modelling results have been entered, the associate signal to noise ratio (S/N ratio) was then calculated using “the larger the better” criteria defined as:

-78-

Design Optimisation

S N bigger

 1 2   yi   10 log   n   

Eq [5-

9] Table 6-4: Taguchi modified L-16’array 1 1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

2 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

3 1 2 3 4 2 1 4 3 3 4 1 2 4 3 2 1

4 1 2 3 4 3 4 1 2 4 3 2 1 2 1 4 3

5 1 2 3 4 4 3 2 1 2 1 4 3 3 4 1 2

With the factors and levels defined, modelling setups and their results were entered into the Taguchi matrix layout for the calculation of the S/N ratio. Referencing the L-16’ array shown in Table 6-4, results associated with fibre orientations defined by the array was arranged and inserted into Table 6-5. For instance, the array defines Model 1 with fibre orientations of [0/0/0]s for layer A, B and C respectively. The associate modelled maximum compressive load was then entered in to the far right column. To further simplify the data analysis, the large strength value data was reduced by 1000 times and used as the result data for the analysis. Finally, S/N ratio for each model was calculated using Equation 6-3. Table 6-5: Taguchi S/N ratio calculation from modelling results S/N Ratio Calculation Models Layer A 1 2 3

1 1 1

S/N Ratio Results Larger the better Max F

Layer B

Layer C

1

1

6.51

16.28

2

9.23

19.30

6513.8 9227.17

3

8.43

18.51

8427.4

14.57 19.58

5352.4 9525.03

2 3

4

1

4

4

5.35

5

2

1

3

9.53

-79-

1. Design Optimisation To analyse the response of S/N ratio of each level (fibre orientation) to each factor (symmetric layer pair), the S/N ratios of the models with the same level at a particular factor was averaged and entered into Table 6-6 to evaluate the their response. For example, response A1 was obtained by averaging S/N ratio of the Model 1~4 for possessing the same level at layer A. The complete response can therefore be obtained referencing the full L-16’ array. Table 6-6: Taguchi S/N ratio calculation from modelling results S/N Response Table Level 1 Layer A Layer B Layer C

Mean S/N ratio (dB) Level 2 Level 3

A1 B1 C1

A2 B2 C2

A3 B3 C3

Level 4 A4 B4 C4

By completing the response table, the optimised arrangement of fibre orientation (level) from each symmetric layer pair (factor) was finally obtained by selecting the three highest S/N ratio values from each layer common. The suggested optimised arrangement therefore possess the format of [Ax/By/Cz]s.

For the Taguchi method described above, the number of required models for conducting the analysis was solely depended on the assigned array. With the defined four fibre orientations, the analysis is thus constrained to four levels, the available arrays suggested by the method for this research are therefore the applied modified L-16’ array and the L-32 array. According to literature survey in Chapter 2, the L-16’ array is suitable only for levels 2~5, while the L-32 is able to perform levels 6~9. The design with maximum number of layers suitable for this Number of layers

(l)

4

6

8

10

12

14

16

18

Taguchi Approach (T)

16

16

16

16

32

32

32

32

All Arrangements (A)

16

64

256

1024

4096 16384 65536 262144

analysis is therefore 9 x 2 = 18 layers.

Table 6-7: Comparison of required number of model from Taguchi approach vs complete arrangements

Shown in Table 6-7, using the L-16 and L-32 arrays the Taguchi method demonstrates the capability of offering significant savings in computation power and time for the toecap design

-80-

Design Optimisation with less or equals to 18 layers. The analysis index E calculated below also shows great reductions in the required number of models:

E

T fx  A 16

or l 10

fx 32

Eq [5l 10

10]

Where

x = l/2 = symmetric layer pair f

= number of options for fibre orientation

A = total number of possible arrangements T = number of models required from Taguchi Analysis

Figure 6-8 illustrates the relationship between analysis efficiency versus number of layers for the fixed fibre orientation options of four ( l = 4 ).

Figure 6-8: Efficiency vs number of layer curve for Taguchi approach

6.3 Analysis Procedures and Results In order to obtain the strongest optimum design capable of fulfilling the 15kN strength requirement stated in the EN 12568:1998 standard with the least computing power and time, the three optimisation approaches described in the previous section were conducted for both long fibre flax-PLA hot-pressed and impregnated toecap designs. The results obtained were then validated by cross referencing the L-T and Taguchi results with the data collected from the Complete Analysis Matrix.

-81-

1. Design Optimisation

6.3.1 Flax-PLA (hot-pressed 4 layers) Toecaps Two optimisation approaches were applied for the 4 layers hot-pressed flax-PLA toecaps: Complete Analysis Matrix and the L-T approach. Due to the 4 layer prepreg structure which possess only 16 possible arrangements, the Taguchi approach using L-16’ array was considered unnecessary as it would provide identical results as the Complete Modelling Matrix. 6.3.1.1 Complete Modelling Matrix From the constructed compression test model, the complete modelling matrix for hot-pressed flax-PLA toecaps made from symmetrical 4 layers composite prepregs of thickness 3.5mm were computed after performing the full 16 simulations that covered all possible fibre orientations. The modelled results are shown in Table 6-8 below:

Table 6-8: Complete modelling matrix for 4 layer hot-pressed flax-PLA toecap symm

[0]

[90]

[45]

[-45] symm

[0]

[90]

[45]

[-45]

[0]

A

B

C

D

[0]

4453.72

6251.28

3909.95

2996.95

[90]

E

F

G

H

[90]

6191.46

3426.37

4061.38

2763.21

[45]

I

J

K

L

[45]

5774.2

5589.24

2812.21

3682.45

[-45]

M

N

O

P

[-45]

4894.64

3734.13

5407.6

2162.99

As shown in the matrix, the results from the toecaps with different fibre layup models ranged between 2000N ~ 6500N. This wide range of simulated values indicates the significant influence the fibre orientations have towards the resulting toecap strength. Among all 16 results, arrangement B [90/0/0/90], highlighted in red, possesses the highest compressive load of up to 2.89 times higher than the weakest 2162.99N from arrangement P [-45/-45/-45/-45] as shown in green. Table 6-9: Ranking matrix for 4 layer hot-pressed flax-PLA toecap modelling results

-82-

symm

[0]

[90]

[45]

[-45]

[0]

7

1

9

13

Design Optimisation [90]

2

12

8

15

[45]

3

4

14

11

[-45]

6

10

5

16

A ranking matrix of the 16 modelling results was also created as Table 6-9, with the highest top five values among all the modelling data highlighted with green background. The matrix indicates that four of the top five strongest toecaps have their outer-most layers orientated in 0 or 90 degrees, while the toecaps with -45 degree outer-most layers possess the weakest performance among all data. With the top five arrangements comprised with orientations from all four options, the ranking matrix therefore shows no preference on any particular orientation for achieving high compressive load. Proper combination of prepreg orientations was therefore the vital factor of developing high-strength toecaps. Fibre orientations of the simulated models also pose significant influences to their failure models under compression. Among all modelling results, three main failure models were observed from the FE models: top-dome failure, side-wing failure, twist collapse. The top-dome failure mode is mostly observed from the models with high compressive load. The process begin by having stress concentrations appear at the top dome apex and the upper front region, the high stresses soon caused the material to yield and resulted in cracks which propagate from the dome to the front upper region. This resulted in the failure of the the top dome, and eventually led to the collapse of the structure as shown in Figure 6-9.

Figure 6-9: Top-dome failure mode of the compression test model (chronologically left to right)

-83-

1. Design Optimisation The toecap models with side-wing failure mode often possess the maximum compressive load in the mid to low range. The typical phenomenon of the failure mode includes stress concentrations appearing at the upper rim of the side wings during the early compression stage. The stress later causes the failure of the materials at the side wings, where the structure looses its side support, and finally leads to the collapse of the top dome as shown in Figure 610.

Figure 6-10: Side-wing failure mode of the compression test model (chronologically left to right)

Twist collapse mode is the result of the large deformation of the toecap side wings, and often causes low structural strength for the toecap under compression. The failure mode was usually caused by the diagonal ±45 degrees orientation of the fibres, which causes the side wings to twist to the side and generates large bending moments in the materials. The structure then collapses from the absence of vertical support as shown in Figure 6-11.

Figure 6-11: Twist collapse mode of the compression test model (chronologically left to right)

The relationship between the toecap structural strength and their associate failure models can also be seen using the force-displacement curve Figure 6-12. Each curve shows the forcedisplacement output of the associate models from the initial state to 0.65mm displacement after reaching their maximum force values. Among the data from the 16 models, results from arrangement B, L and P each possessing one of the three different failure modes were highlighted using solid lines for better visualization.

-84-

Design Optimisation

Figure 6-12: Force-displacement curve of 4 layers flax-PLA toecap models

The presented results provided a clear indication of the failure force range for each of the failure models described. Twist collapse of arrangements P, K and H can be clearly seen failing between 2000~2500N, while arrangements C, L and N fails rapidly by side-wing failure between 3500~4000N. The stronger toecap arrangements on the other hand were structurally sound and withstood high compressive loads up to 5500N before the yielding and deletion of the top dome elements, which eventually causes the failure of the top dome.

This analysis has explored the distribution of maximum compressive load, their associate behaviour and failure models among the possible fibre arrangements. The data was thus used as benchmark reference data for evaluating the accuracy of the optimisation analysis performed in the following section.

6.3.1.2 Layer-Tournament Analysis Approach L-T analysis approach for the 4 layer hot-pressed flax-PLA toecaps was performed to generate the suggested optimised design and to evaluate its ranking among all simulated results from the 16 arrangements. By constructing four models for each of the analysis level, the suggested optimised arrangement from the two level analyses has been obtained as shown in Table 6-10 below:

Table 6-10: L-T approach optimisation results for 4-layer hot pressed flax-PLA toecap Level1

0/x/x/0

45/x/x/45

-45/x/x/-45

90/x/x/90

5692.83

3595.92

2631.62

5121.2

-85-

1. Design Optimisation

Level 2 Arrg’mt

0/-45/-45/0 0/90/90/0

0/45/45/0

0/0/0/0

4894.64

6191.46

5774.2

4453.72

M

E

I

A

The analysis result suggested an optimised arrangement of [0/90/90/0] for the 4 layer flaxPLA toecap. Referring back to Table 6-9, the maximum compressive load of the suggested E[0/90/90/0] arrangement was ranked 2nd among all 16 simulation results. The method therefore reached the top 12.5% of the entire data pool with the analysis index of 2 as calculated previously using Equation 5-8.

6.3.1.3 Evaluation Comparing the proposed arrangement E from the L-T analysis to the data in the complete model matrix, the L-T approach has demonstrated high accuracy in the optimisation of toecap fibre layups by achieving a similar result. The required computation power and time as opposed to the complete model matrix were also greatly reduced. The approach was therefore validated as an effective method for conducting optimisation analysis.

However, the modelling results from all 16 possible arrangements reached only 41% of the 15kN strength targeted. With an effective optimisation approach available, a thicker composite laminate comprises of 6 flax-PLA layers was therefore proposed for further developments in the next section in order to design the demanded high strength composite toecaps. Moreover, from this analysis the maximum compressive load and their associate behaviour have been studied. With the occurrence of the three failure modes depending on their fibre orientation layout, the strongest arrangements B, along with the arrangement L and P each with different simulated failure models were also proposed for sample manufacturing in the next chapter, in order to evaluate the accuracy of the models in both compressive load and structural behaviour.

6.3.2 Flax-PLA (hot-pressed 6 layers) Toecaps

-86-

Design Optimisation Optimisation analysis for 6 layers hot-pressed flax-PLA toecap was conducted aiming at providing the design that is capable of satisfying the 15kN compressive load requirements as stated in the EN 12568:1998 standard. With the six 6 layer symmetrical layup structure, there are thus 64 possible fibre layup arrangements as calculated using Equation 5-6. Three optimisation approaches have been utilized: complete modelling matrix, L-T approach, and the Taguchi approach. 6.3.2.1 Complete Modelling Matrix Due to the large number of simulated models, the created 64 models were first assigned with a specific code combining of 3 call-signs depending on their fibre orientation layup. They were then separated into four main groups W, X, Y and Z depending on the orientation of their outer-most layers as shown in Table 6-11 and 6-12.

Table 6-11: Fibre orientation call-signs for 6 layer flax-PLA toecaps

call-sign

orientation

0

0 degrees

4 5

45 degrees -45 degrees

9

90 degrees

Table 6-12: Complete model matrix with 64 arrangements W - series

X - series

00x

04x

05x

09x

40x

44x

45x

49x

0

000

040

050

090

400

440

450

490

4

004

044

054

094

404

444

454

494

5

005

045

055

095

405

445

455

495

9

009

049

059

099

409

449

459

499

Y - series

Z - series

50x

54x

55x

59x

90x

94x

95x

99x

0

500

540

550

590

900

940

950

990

4

504

544

554

594

904

944

954

994

-87-

1. Design Optimisation 5

505

545

555

595

905

945

955

995

9

509

549

559

599

909

949

959

999

As shown above, each toecap arrangement was first defined using three call-sign numbers representing the layup of the symmetric laminate. They were then allocated into one of the four series according to their outermost layer: W for 0 degrees; X for 45 degrees; Y for -45 degrees; Z for 90 degrees. For example, arrangement 094 has the fibre layups of [0/90/45/45/90/0] with the outer-most layer orientated in 0 degrees, and was therefore allocated in the W series. Models of 16 in each series were then computed and analysed following identical procedures as the previous 4-layer design, the maximum compression stresses reached were then inserted into the complete model matrix for comparison. The obtained results and the associated ranking matrix are listed in Table 6-13 and 6-14 as below.

W

X

00x

04x

05x

09x

40x

44x

45x

49x

0

6513.82

8116.68

8438.26

10583.2

7562.07

5842.27

8939.34

9077.09

4

7030.57

8427.41

7106.13

12609.3

7194.46

5067.22

7028.52

8249.54

5

7643.63

8210.45

5352.38

12183.4

7570.27

7140.14

7075.65

7796.31

9

10153.9

11978.3

7399.56

9227.17

7758.11

5841.83

5893.11

6814.99

95x

99x

Y 50x

-88-

54x

Z 55x

59x

90x

94x

Design Optimisation 0

6171.32

6308.63

3662.1

6011.46

14927.3

12633.4

7395.12

8007.43

4

5020.62

7186.25

3987.52

5075.68

9525.03

8781.69

7355.37

7701.66

5

4865.39

6005.93

3305.07

3989.13

10377.6

10610.1

5288.77

6213.57

9

4847.93

6334.53

3412.86

4323.72

6575.34

7552.8

5580.88

6901.58

Table 6-13: Comple te

modelling matrix results for 6 layer hot-pressed flax-PLA toecap

Table 6-14: Complete modelling matrix results for 6 layer hot-pressed flax-PLA toecap W

X

00x

04x

05x

09x

40x

44x

45x

49x

0

41

19

15

7

26

49

13

12

4

36

16

34

3

31

55

37

17

5

24

18

52

4

25

33

35

21

9

9

5

28

11

22

50

48

39

Y

Z

50x

54x

55x

59x

90x

94x

95x

99x

0

45

43

62

46

1

2

29

20

4

56

32

61

54

10

14

30

23

5

57

47

64

60

8

6

53

44

9

58

42

63

59

40

27

51

38

From the matrices shown above, the results varied greatly between arrangements and ranged from 3305N to nearly 15000N, which is over 4.5 times difference in strength. The fibre orientations thus showed an even greater significance to the strength of the toecap compare to the previous 4-layer design. Among the 64 models, the strongest arrangement 900 [90/0/0/0/0/90] shown in red demonstrated the maximum compressive load of 14927.3N, which nearly reached the 15kN targeted strength stated in the EN standard. The lowest result came from the arrangement 555 [-45/-45/-45/-45/-45/-45] as shown in green, which only achieved 3305.07N.

The ranking matrix of the 64 models shown in Table 6-14 indicates that the top nine strongest toecaps with their background highlighted in green were all from either W or Z series, which points out the orientation of either 0 or 90 degrees in the outer-most layers for all arrangements reaching above 10000N. Nevertheless, the results do not show special favour from the W and Z series in particular. Models with poor results reaching less than 5300N can

-89-

1. Design Optimisation also be found from the two series, and hence further confirms the importance of proper combination of layer orientations.

In regards to failure modes, top-dome failure mode has been experienced by the strongest 900 arrangement as shown in Figure 6-13. Stress concentrations led to the failure of the top dome, which actually cause the top dome to break off and collapsed the structure. This result has shown strong agreement with the 4-layer toecap discussed previously.

Figure 6-13: Top-dome failure mode experienced by 900 arrangement (left to right)

For the weakest 555 arrangement on the other hand, twist collapse failure mode initiated from the high stress located on the upper rim of the side-wings as shown in Figure 6-14. The wings were soon twisted with the failed upper rim and cause the structure to fall to one side and eventually collapsed. The low compressive load from the failure mode also matched up with previous observations.

Figure 6-14: Top-dome failure mode experienced by 555 arrangement (left to right)

6.3.2.2 Layer-Tournament Approach L-T analysis approach for the 6 layer hot-pressed flax-PLA toecaps was carried out to suggest an optimum fibre layups of the toecap among all 64 possible arrangements. A three level analysis was performed for the 6 layer laminate with the final results shown in Table 6-15 below: Table 6-15: L-T approach optimisation results for 6-layer hot-pressed flax-PLA toecap

-90-

Design Optimisation

Level 1

Level 2

Level 3

0xx

4xx

5xx

9xx

13237.9

8888.11

6909.13

12913.3

00x

04x

05x

09x

9596.92

8887.63

7421.59

11299.2

090

094

095

099

10583.2

12609.3

12183.4

9227.17

From the results listed above, the optimised arrangement of 094 [0/90/45/45/90/0] was suggested. The arrangement is ranked 3rd among all 64 models with its maximum compressive load of 12094N, the analysis therefore achieved the top 4.6% among the data pool with the analysis index of 5.34 as calculated using Equation 5-8.

6.3.2.3 Taguchi Approach Taguchi optimisation analysis was applied to the 6 layer hot-pressed flax-PLA toecap models as a second approach to serve as a comparison against the results provided by the L-T approach. Taguchi approach suggests the optimised arrangements solely via the inserted experimental data, and thus does not require the “dummy model” as used in the L-T approach. For this analysis Layer A, B and C represents the outer-most, middle and inner-most layer in the laminate respectively. The three layers were regarded as independent factors, and therefore column 1, 2 and 4 in the L-16’ used to avoid interactions. After inserting the data of the defined models according to the L-16’ array shown in Table 6-4 previously, the S/N response table were then constructed as shown in Table 6-16 below. The complete data for S/N ratio calculation can be found in Appendix E.

Table 6-16: S/N response table for 6-layer hot-pressed flax-PLA toecap S/N Response Table Mean S/N ratio (dB) Factors Level 1 Level 2 Level 3 Level 4 A B C

17.17 16.79 18.23

18.10 17.27 15.82

17.57 17.86 17.11

14.22 15.14 15.90

-91-

1. Design Optimisation The S/N response data can also be better visualized by plotting the response graph as illustrated in Figure 6-15. Parameters shown on the x-axis represents the factors with their associate levels. From the response graph, the three largest ratios from each of the factor group A2, B3 and C1 were chosen and highlighted using the green circles. These three S/N ratios represent the largest S/N responses of the fibre orientations to the specific layer. By referencing the assigned factors and levels listed in 6-3, A2/B3/C1 can therefore be interpreted into arrangement 940 [90/45/0/0/45/90].

Figure 6-15: Force-displacement curve of 4 layers flax-PLA toecap models

According to the ranking of the complete model matrix from Table 6-13, 940 arrangement is ranked 2nd among all 64 models with the strength of 12633N. The analysis therefore achieved the top 3.1% among the data pool with the analysis index of 4 as calculated using Equation 510. To investigate the influence of each layer (factor) towards the resulted strength, the analysis of variance (ANOVA) was also carried out as shown in Table 6-17.

Table 6-17: ANOVA for 6-layer hot-pressed flax-PLA toecap ANOVA Table for S/N ratio CF 825.5204 Layer Sum of Sq DF Mean Sq V F % Contribution

-92-

A B C Error

23.36 10.55 13.57 25.34

3 3 3 6

TOTAL

72.82

15

7.79 3.52 4.52 4.22

1.84 0.83 1.07 -

32.09 14.49 18.63 34.80

Design Optimisation The analysis show a stronger contribution of 32.09% from the outer-most layer A, which indicates a greater influences towards the final structural strength from the fibre orientation of the outer-most layer. The contributions from middle and inner layers are 14.49% and 18.63% respectively, which were nearly half of the percentages of the outer-most layer, and therefore were regarded as less significant. 6.3.2.4 Evaluation From the L-T and Taguchi approaches performed, the arrangements suggested from the two approaches have shown high accuracy among the 64 models analysed in the complete model matrix. As shown in Figure 6-16, the high rankings of 2nd and 3rd achieved by Taguchi and LT approach for arrangements 940 and 094 respectively have shown great potential from both methods of providing optimised high strength toecap fibre layups, while providing great savings for computation power and time. Together with the strongest arrangement 900, topdome failure mode experienced by the two toecaps also show agreements with the observations from the previous 4-layer toecap design.

Figure 6-16: Force-displacement curve of the 6-layer hot-pressed flax-PLA toecaps

Finally, in regards to compressive load the strongest arrangement 900 has shown great potential of fulfilling the targeted 15kN strength with its maximum compression force of 14927N. Decision was therefore made to manufacture the toecap samples with this particular arrangement in order to produce the high-strength composite toecap desired.

6.3.3 Flax-PLA (impregnated 4 prepregs) Toecaps

-93-

1. Design Optimisation Impregnated 4 prepreg flax-PLA toecap possesses a stronger material strength compare to the previous hot-press toecap. However, each material prepreg was made from 2 layers of unidirectional fibres laminate in 90 degree offset angle as discussed in Section 4.4.3. This structure inevitably added complexity to the Taguchi analysis, where the defined settings in the Taguchi arrays can no longer be satisfied. To obtain valid optimisation analysis results for the impregnated flax-PLA toecap, the L-T approach was conducted for the 4-layer impregnated flax-PLA toecap design, while the Taguchi approach was discarded for the study of impregnated flax-PLA toecap. The complete model matrix was also performed for the 4 prepreg impregnated toecap to observe the resulting strength and failure modes.

6.3.3.1 Complete Modelling Matrix Complete modelling matrix for impregnated flax-PLA toecaps made from symmetrical 4 prepregs laminate of thickness 2.8mm were computed after performing the full 16 simulations. The simulated results are shown in Table 6-18 below. To avoid confusion with the hot-pressed toecap results, English characters in small case were assigned to the models as oppose to the large case used previously. Table 6-18: Complete modelling matrix for 4 prepreg impregnated flax-PLA toecap symm

[0/90] [90/0] [45/-45] [-45/45] symm

[0/90]

[90/0]

[45/-45]

[-45/45]

[0/90]

a

b

c

d

[0/90]

6819.71

4446.81

4490.58

4920.04

[90/0]

e

f

g

h

[90/0]

7237.41

6604.53

5690.36

4846.71

[45/-45]

i

j

k

l

[45/-45]

6115.93

5870.62

4642.69

4748.87

[-45/45]

m

n

o

p

[-45/45]

6441.55

6657.99

4594.42

4161.31

The matrix above shows the results ranged between 4000N ~ 7300N, where the strength of the models can vary up to 1.7 times. The strongest arrangement e [0/90/90/0/0/90/90/0] shown in red possess the maximum compressive load of 7237.41N, while the weakest arrangement p [-45/45/-45/45/45/-45/45/-45] reached the low strength of only 4161.31N shown in green. A ranking matrix of the 16 modelling results shown in Table 6-19 indicates the highest top six models achieved over 6000N with their background highlighted in green. The ranking matrix has demonstrated clear advantages in terms of stronger strength from the models with the outer-most prepreg orientated in the [90/0] and [0/90] direction, while seven of the eight models with the outer-most prepreg orientation from the remaining two directions were ranked at the bottom 50% of the data pool. -94-

Design Optimisation

Table 6-19: Raking matrix for 4 prepreg impregnated flax-PLA toecap

Force-displacement results of all 16 models shown in Figure 6-17 also indicates a clear separation in force values achieved between the models with the outer-most layer in 0/90 degrees and ±45 [0/90]

[90/0]

[45/-45]

[-45/45]

[0/90]

2

15

14

9

[90/0]

1

4

8

10

tends

results

[45/-45]

6

7

12

11

above

latter

[-45/45]

5

3

13

16

seldom

orientations. degrees outer layer higher while

the

degrees symm

Models with 0/90 to

achieve 5500N, reach

beyond 5000N. The fibre direction of the outer-most layer for 4-prepreg impregnated toecaps is believed to be very critical to the structural strength of the final product.

Figure 6-17: Force-displacement curve of the 4-prepreg impregnated flax-PLA toecaps

Failure modes simulated for the 4-prepreg impregnated toecaps demonstrated different behaviours from the previous hot-pressed analysis, where most of the models performed either side-wing failures or twist collapse before structural collapse. Figure 6-18 and 6-19 illustrates the progression of the side-wing failure mode of arrangement e, and twist collapse of arrangement p. The distribution of stresses demonstrated very similar patterns from the -95-

1. Design Optimisation previous observation before the failure modes occur, and thus lays the potential of predicting the final failure modes by observing the stress distribution during the early compression stage.

Figure 6-18: Top-dome failure mode experienced by e arrangement (left to right)

Figure 6-19: Top-dome failure mode experienced by p arrangement (left to right)

6.3.3.2 Layer-Tournament Approach Results of the L-T analysis approach for the 4 prepreg impregnated flax-PLA toecaps have been obtained. A two level analysis was performed with the final results shown in Table 6-20 below: Table 6-20: L-T approach optimisation results for 4 prepreg impregnated flax-PLA toecap

Level1

Level 2

From

the

[0/90/x/x]s [90/0/x/x]s [45/-45/x/x]s 6863.14

6807.97

3846.28

[-45/45/x/x]s 4284.52

[0/90/0/90]s [0/90/90/0]s [0/90/45/-45]s [0/90/-45/45]s 6819.71

7237.41

6115.93

6441.55

a

e

i

m

Arrg’mt

results listed

above, the optimised arrangement of e [0/90/90/0]s suggested is ranked first among all 16 possible arrangements with its maximum compressive load reaching 7237.41N, the analysis therefore achieved the top value among the data pool with the analysis index of 2 as calculated using Equation 5-8.

-96-

Design Optimisation 6.3.3.3 Evaluation From the L-T approaches performed, the arrangements suggested have once again achieved a high ranking among the 16 toecap arrangements. The approach has demonstrates good reliability over the analysis performed above, and was therefore been regarded as an effective analysis for the development of toecap arrangement for this research. 6.3.4 Comparison of Approaches From the analysis of the four flax-PLA toecap designs, the L-T approach has demonstrated simplicity in the analysis process and very good reliability with the suggested arrangements achieving high rankings among their associated data pool. The Taguchi approach on the other hand reveals several problems during the optimisation studies. The restrictions from L-16’ and L-32 arrays has limited their use for analyzing various toecap designs, this is especially true for the impregnated prepregs where the material comprises of two layers with 90 degree offset angle. Also, the approach was conducted only for one toecap design in this research. The data is therefore insufficient for judging the accuracy of the approach. It was therefore concluded that the L-T approach was by far the best optimisation approach for this study, and has been recommended for future optimisation analysis of other toecap designs.

-97-

7 7. Manufacture and Evaluation of Composite Toecap Samples Manufacturing of the composite toecaps was carried out by incorporating the knowledge gained from the literature survey and the results from FE models. The objectives for producing the toecap samples were to verify the modelling results regarding compressive resistance, deformation and failure modes resulting from different materials and fibre layup orientations as simulated in LS-DYNA. Samples from three different materials were manufactured, one of them is the sisal-PP toecap made for the purpose of becoming familiar with the production and verification process. The remaining two are the flax-PLA hot-pressed and impregnated toecaps aiming to comply with the industrial standard. In order to obtain valid statistical data, a minimum of three samples were produced for each of the specified toecap designs. 7.1 Sisal-PP Samples sisal-PP prepreg was chosen for its high-resin composition and low cost that suits the need of researching the proposed manufacturing procedures for 3D toecap geometry. The randomlyorientated short fibres also allow simplification of the material models during the early stage of FE modelling. The experimental results from the produced samples were thus used mainly for verifying the modelling setup conditions. Samples of the two designs having different thicknesses were made to observe the accuracy of the material models. The results from the …. -98-

Manufacture and Evaluation of Composite Toecap Samples experiment were also used to observe the influence of thickness towards the final compressive load.

7.1.1 Production Process Manufacturing of the sisal-PP toecaps was conducted using the hot-pressing method with the available prepregs using the custom-made pneumatic hot-press and the toecap mould developed at CACM as introduced in Chapter 2. The production procedures are explained as follows: sisal-PP prepreg with a thickness of 2mm was first trimmed into a rectangular shape measuring 110mm x 160 mm. Three folding cuts were made at one of the longer edges for allowing the prepreg to fold and drape into the mould cavity when softened by heat. One of the three cuts was positioned from the mid-point of the longer edge to the prepreg centreline, while the other two begin from 10mm away for the corners and possess a 75 degree angle towards the first cut until reaching centre line as shown in Figure 7-1.

160mm

゚゚

110mm Figure 7-1: Sisal-PP rectangular prepreg with folding cuts

Figure 7-2: Trimmed sisal-PP prepregs soften using baking oven

-99-

1. Manufacture and Evaluation of Composite Toecap Samples The trimmed prepregs were dried in the vacuum oven for 12 hours at 50 degrees after being trimmed to remove excess moisture. From the vacuum oven, the prepregs were moved into the baking oven and heated up to 120°C and softened as shown in Figure 7-2, while the aluminum mould attached to the pneumatic press was also heated up to the same temperature. The prepregs were then pre-stamped using the aluminum mould at 120°C for 10 seconds, and were later returned back to the oven with the pre-stamped cup-like shape. The aluminum mould was heated up to 185°C and remained for 10 minutes for the temperature to stabilize. The prepreg were then placed into the mould and pressed at 185°C for 30 minutes under 50kPa as shown in Figure 7-3, before being fan-cooled to 70°C for demoulding and later trimmed into the final shape. The 4mm samples were produced by following the identical procedures with two prepregs used at once.

Figure 7-3: Sisal-PP prepreg pressed in the aluminium mould

To verify the modelling results for toecap thicknesses of 2mm and 4mm, a total of 12 toecaps were manufactured using 18 prepregs. This comprise of six samples from each design as shown in Figure 7-4. Among them, four samples possessed serious porosity, breakages and uneven thickness due to the poor control of moulding time and demoulding temperature during the beginning stage as shown in Figure 7-5. These samples were therefore considered not of sufficient quality for conducting compression testing and were later disposed of. The remaining eight samples suitable for conducting compression test were made of four 2mm thick toecaps and four 4mm thick toecaps.

-100-

Manufacture and Evaluation of Composite Toecap Samples

Figure 7-4: Manufacture 18 sisal-PP toecap samples

Figure 7-5: Faulty sisal-PP toecap samples with excessive porosities and uneven thickness

7.1.2 Testing Results and Evaluation of Samples The compression tests were performed using the Instron 5500R universal tensile testing machine following the testing conditions and procedures defined in European Standard EN12568:1998 as previously explained in Chapter 3. The obtained force-displacement data was then compared to the FE simulation results as shown in Figure 7-6 to verify the models, and also to study the behavior of the toecap structure. -101-

1. Manufacture and Evaluation of Composite Toecap Samples

Figure 7-6: Comparison of sisal-PP toecap testing data vs modelling results

Data from the manufactured sisal-PP toecap samples demonstrated good consistency among the same sample groups. The compression test data from the 2mm thickness toecap samples show strong agreement with the modelling results from models with 2mm and 4mm mesh size. The curves from the experiments started with a low gradient with the top plate compressing downwards with a small contact area to the toecap dome. As the dome was fully compressed by the top plate, the gradient slowly increases and eventually reached over 600N before structural failure occurred as predicted by the FE models. The models were thus verified as having a very high accuracy. The toecap samples with 4mm thickness however performed with weaker strength and stiffness than predicted. The samples initiated with a similar gradient to the modelling result. However, the testing data failed to maintain the rate of climb for the gradient upon reaching 400N, and were soon separated from the modeled path and eventually failed between 1300 ~ 1400N. After the experiments, the compressed samples were examined in order to find the possible reasons that caused the difference discussed above. From visual observations, the bottom flanges of the toecap samples were manufactured with varying quality, where cracks existed -102-

Manufacture and Evaluation of Composite Toecap Samples prior to the compression experiment of several samples. The bottom flanges were therefore unable to provide structural support when the top dome was under compression. The cracks also attracted stress concentrations which soon led to the breakage of the toecap frontal regions as shown in Figure 7-7 marked by red arrows, and thus weakened the structure. This problem can be solved by accurately aligning the male and female mould, as well as extending the prepreg sheets to improve the coverage at bottom flange region.

Figure 7-7: Pre-existed cracks in sisal-PP toecap before (left) and after (right) compression test

From this study, a comparison between the FE model and experimental data has shown good agreement in the strength and stiffness. The force-displacement curves have also indicated a very close match in structural behavior with the experimental results. The FE model, therefore, demonstrated a high accuracy in offering an accurate forecast for the behavior of the composite toecap. In regards to production, the toecap samples have offered testing results that agrees with the FE models with very high consistency.

Improvements were also

proposed to solve the current manufacturing defects in the previous chapter. The production process was therefore considered reliable for further manufacturing of the toecap designs.

7.2 Flax-PLA(Impregnated) Samples

Samples of 4-prepreg impregnated flax-PLA toecaps were produced and compression tested to observe their structural behaviour, compressive load, and failure modes. From the optimisation analysis conducted in Chapter 6, fibre layup arrangements with the potential of providing a better understanding of the structural behaviour of the toecaps were introduced to this Section for sample manufacturing. The toecap samples manufactured included: three specimens of each of the arrangements e [0/90/90/0]s and p [-45/45/-45/45]s.

-103-

1. Manufacture and Evaluation of Composite Toecap Samples 7.2.1 Production Process

The productions of impregnated flax-PLA toecaps were conducted using a similar process to the previous sisal-PP samples. The main difference lies in the use of the long fibre prepregs which were trimmed into mushroom-shape cut-outs. The cut-outs were pre-shaped and then stacked together before being compressed fully in the mould to produce the toecap product. Detail procedures are explained as below:

In order to manufacture 6 toecap samples, 3 for each of the two arrangements, a total of 24 flax-PLA impregnated prepregs were first mass produced as shown in Figure 7-8. The prepregs were then divided into two halves and hot-pressed with a 90 degree offset angle as described in Section 4.4.3. The hot-pressed sheets were then trimmed into a mushroom-like shape as shown in Figure 7-9 with the assigned fibre orientations according to the setup of the toecap designs.

Figure 7-8: Mass produced 24 impregnated flax-PLA prepregs

Figure 7-9: Impregnated flax-PLA prepreg trimmed into mushroom-like shape

-104-

Manufacture and Evaluation of Composite Toecap Samples The mushroom shaped prepregs were then dried in the vacuum oven for 12 hours at 50 degrees to remove excess moisture. The dried cut-outs were then moved to the baking oven and softened at the temperature of 120°C. With the low matrix volume fraction of 35%, stronger compression pressure is required to fully compact the toecap laminate and form the solid structure. The aluminum mould was therefore removed from the original pneumatic press with maximum pressing output of 1 ton, and placed in a new hydraulic press with the maximum pressure output of 10 tons. The aluminum mould was then heated up to the temperature of 120°C in the hydraulic press as shown in Figure 7-10.

Figure 7-10: Toecap mould inside the hydraulic press

Figure 7-11: Spacer strips used

Each prepreg cut-out was pre-stamped inside the hydraulic press under a pressure of 50 kPa at 120°C for 10 seconds, and was then removed from the mould in the preshaped form and returned back to the oven. As each prepreg has a thickness of 0.7mm, the total thickness for a 4 layer laminates was therefore 2.8mm. Two spacer strips of 2.8mm thick were therefore placed between the male and female mould to maintain a 2.8mm internal gap as shown in Figure 7-11. The pre-shaped piles with different fibre orientations were then stacked together according to the assigned toecap design layups, and molded in the hydraulic press under 306 kPa at 185°C for 20 minutes. The mould was then fan-cooled to 60°C before demoulding the flax-PLA product as shown in Figure 7-12. Finally, the products were trimmed and shaped into the toecap samples of the e and p arrangements for compressive testing as shown in Figure 7-13. Figure 7-14 shows the 6 toecap models manufactured for compression testing. -105-

1. Manufacture and Evaluation of Composite Toecap Samples

Figure 7-12: The demoulded impregnated flax-PLA product

Figure 7-13: The completed impregnated flax-PLA composite toecap sample

Figure 7-14: Total of 6 impregnated flax-PLA samples produced

-106-

Manufacture and Evaluation of Composite Toecap Samples 7.2.2 Testing Results and Evaluation of Samples Compression testing results from the impregnated flax-PLA toecap samples have demonstrated the structural strength being significantly weaker than the prediction offered by the FE models. Figure 7-15 presents the force-displacement data from both arrangements e and p of toecap samples, with arrangement e having an offset of 10mm in displacement for better visualization purposes. The results show that the manufactured samples shown in dotted lines have achieved less than 50% of the modelled compression forces shown in solid lines, while the forces from both results failed to reach above 2500N. The difference in strength between arrangement e and p was also less significant. This outcome indicates the existence of a special failure mode which is dominated mainly by the geometry, and is less sensitive to the arrangement of fibre orientations.

Figure 7-15: Comparison of impregnated flax-PLA toecap testing data vs modelling results

The produced samples show high consistency in laminate thickness with an average value of 2.86 mm, the problem with thickness variation has therefore been eliminated with the use of spacer strips. To investigate the reason of under-performing from the toecap samples, careful observations were carried out to examine the failure modes of the samples. Shown in Figure 7-16, the top domes of the samples were damaged by serious de-lamination from the outer-107-

1. Manufacture and Evaluation of Composite Toecap Samples most layers. Early de-lamination of the dome layers resulted in the cracks propagating inside the laminates, which quickly spread into the structure of the side-wings as the toecap is further compressed. With the de-lamination of the outer-layer, the flexible core layers can no longer withstand the stress and were easily deformed which led to the failure of the sidewings, and eventually caused the collapse of the structure.

Figure 7-16: De-lamination of the top dome from impregnated flax-PLA toecaps

As the current FE models do not have the capability to simulate de-lamination effect between layers, the behaviour of the impregnated flax-PLA prepregs therefore cannot be modelled with acceptable accuracy and reliability. The material was therefore not considered for further developments.

7.3 Flax-PLA(Hot-Pressed) Samples

Toecap samples made from hot-pressed flax-PLA prepregs were simulated to achieve the highest compressive load among the three materials researched. The arrangements applied for sample production include: arrangements B, L and P from the 4 layer toecap design, and arrangement 900 from 6 layer toecap design. In particular, arrangement 900 possesses the highest prediction in compressive load reaching above 14000kN, and was therefore chosen in the hope of producing the toecap samples with high enough strength to fulfil the 15kN requirements listed in the EN 12568:1998 standard. The 4-layer and 6-layer samples were produced using identical material prepregs and manufacturing procedures, while the 6 layer design required an additional compression step due to its thicker laminate. Compression tests were conducted for all produced samples to obtain the information regarding their stiffness and maximum structural strength, failure modes of each design were also investigated and later compared with the modelling results for verification.

-108-

Manufacture and Evaluation of Composite Toecap Samples 7.3.1 Production Process

Due to the lower fibre volume fraction of 35% from the hot-pressed flax-PLA prepreg, higher temperature settings were used to soften and melt the PLA matrix during moulding. A production process similar to the impregnated samples explained previously was also applied. To manufacture three samples for each of the 4 arrangements: P, L and B of 4-layers and the 900 of 6 layers, a total number of 54 hot-pressed flax-PLA prepregs were manufactured in order to carry out the production process. The prepreg sheets were later trimmed into the mushroom-shape cut-outs as shown in Figure 7-17 and left in the vacuum oven to remove excess moisture.

Figure 7-17: Hot-pressed flax-PLA prepregs trimmed into mushroom-shape cut-outs

The dried flax-PLA cut-outs were then heated up to 140°C for the PLA matrix to soften, while the aluminum mould inside the 10 ton hydraulic press was heated to 150°C and remained for 10 minutes for the temperature to stabilize. The prepregs were then pre-shaped inside the mould and then stacked up according to the assigned arrangements to form the precast as shown in Figure 7-18.

Figure 7-18: Hot-pressed flax-PLA precast

-109-

1. Manufacture and Evaluation of Composite Toecap Samples For the 4 layer toecap model, the precast was then heated up to 150°C in the baking oven, while the temperature of the aluminum mould was raised to 185°C in the hydraulic press. With the 0.875mm thickness for each prepreg, a laminate of 4 layers thus possesses a thickness of 3.5mm. Stepping strips with 3.5mm thickness was therefore used for the mouldpressing stage. Finally, the precasts were hand-laid into the aluminum mould and pressed under a pressure of 350kPa at 185°C for 30 minutes, before fan-cooled for demolding as shown in Figure 7-19 and 7-20. In total 9 toecap samples from the 4 layer arrangements P, L and B were produced. Figure 7-21 shows the manufactured toecap samples with a good surface finish and consistency in thickness of the laminates. The manufactured 8 of the 9 toecaps are presented in Figure 7-22.

Figure 7-19: Precast been hand-laid into the mould

Figure 7-20: Product demoulding

Figure 7-21: Hot-pressed 4 layer flax-PLA sample

-110-

Manufacture and Evaluation of Composite Toecap Samples

Figure 7-22: Eight samples of the 4 layer hot-pressed flax-PLA toecap

In regards to the 6 layer toecaps, samples were made following the same procedure with an additional compression step involved. The calculated thickness for the 6 layer laminate was 5.25mm. Due to this greater thickness, the aluminium mould was not able to be fully closed and positioned into the hydraulic press. An extra compression step was therefore applied: After heating the 6 layer stacked precast to 150°C, it was then manually laid into the 185°C aluminum mould and compressed inside a motor powered press down to the height of 90mm, before relocated back into the hydraulic press for molding. Stepping strips of 5.25mm were used during press-molding to maintain the required clearance between the moulds. Figure 723 shows the outer and inner surface finish of the produced samples, all three samples of arrangement 900 produced are also presented in Figure 7-24.

Figure 7-23: Hot-pressed 6 layers flax-PLA sample

-111-

1. Manufacture and Evaluation of Composite Toecap Samples

Figure 7-24: Three samples of the 6 layer hot-pressed flax-PLA toecap

The three 6-layer flax-PLA samples manufactured have shown consistent geometry and good surface finish. However, variation in laminate thickness was observed at the bottom toecap flanges and the side-wings. The bottom flanges shown in Figure 7-23 possess an average thickness of 2mm as opposed to the 5.25mm defined thickness, which the thickness of the side wings were also observed to vary between 3~4 mm. 7.3.2 Testing Results and Evaluation of Samples

In general, samples made from the hot-pressed flax-PLA prepreg possessed better surface finishes and higher structural strength compared to the other toecap samples manufactured previously. The results from compression testing also demonstrated good agreement with the simulated data. Observations of the toecap during the compression test can be explained as below:

From the constructed FE models, the top plate was defined to travel downwards by a distance of 21.5mm which compressed the toecap beyond its structural failure point in order to investigate the structural behaviour and the failure modes. To correlate the experimental data with the modelling results, compression tests were conducted by compressing up to 21.5mm -112-

Manufacture and Evaluation of Composite Toecap Samples to provide observations of the actual toecap performance beyond the structural failure point. Shown in Figure 7-25, the behaviour of the compressed toecaps including the modelling data and the experimental results of arrangement L are presented in the form of force-displacement curves and can be separated into three stages. In stage I, the total compression force increases with a gradual gradient. The bulged dome of the toecaps contacts the top compressing plate as shown in Figure 7-26a, and is forced downwards until the dome come into full contact with the top plate. In stage II, the top plate was further lowered and begins to compress the sidewings of the toecap as shown in Figure 7-26b, the compression force rises rapidly until reaching the structural failure point, which is represented by the red and black dots for modelling and experimental results respectively in Figure 7-25. The toecaps with a failed structure were further compressed in stage III until the termination of the experiments as shown in Figure 7-26c.

Figure 7-25: Force-displacement curves of arrangement L showing the three compression stages

a)

b)

c)

゚゚

゚゚

゚゚

Figure 7-26: The three compression stages: a) Stage I b) Stage II c) Stage III

-113-

1. Manufacture and Evaluation of Composite Toecap Samples The point of structural failure was decided by observing the toecap structure during the compression tests to locate the breakage of the top dome or the side wings depending on their failure modes. These structural breakages were also reflected on the force-displacement curves in the form of “notches” and can therefore be easily identified by synchronizing the observation with the force-displacement curve plotted in real-time.

From Figure 7-25, the difference in toecap behaviour beyond the failure load was also observed between the FE model and manufactured samples. The force-displacement curve of the simulated model shows a steep and permanent decline in structural strength after reaching the failure load, while the results from the samples experienced a short and steep decline, but soon regained strength and resumed climbing until termination. This difference is due to the failure mechanism of the material model used in the FE simulation. In LS-DYNA, the value of maximum tensile stress and maximum failure strain were defined in the *MATERIAL card and applied onto the elements as described in Chapter 5. Therefore, when the computed stress and strain exceeds the defined values in the elements, the elements were deleted from the domain to simulate the “cracks” shown in Figure 7-27a.

The actual toecap samples on the other hand failed from the breakages inside the laminates, the fibres inside the failed laminates however remained in place and were still capable of transferring loads as shown in Figure-27b. The prediction of the simulation model was therefore unreliable beyond the structure failure load. Furthermore, to fulfil the European Standard EN12568:1998, an internal clearance of 21.5mm is required from the toecap samples at all times. Compressed toecap samples beyond their structural load were observed to be unlikely to maintain the required clearance. The comparison between the experiment and the modelling results was therefore conducted only for the data within Stages I and II up to the structural failure point, and data beyond the structural failure point was not considered.

Figure 7-27: Difference in laminate failure mechanism a) FE model (L) b) actual toecap sample (R)

-114-

Manufacture and Evaluation of Composite Toecap Samples Figure 7-28 illustrates the force-displacement curves comparing experimental data with simulation results conducted for samples of arrangements P, L, B and 900, with their displacement values offset by 0, 11, 22, and 37mm respectively. This allows better visualisation of the difference in structural strength and stiffness between the four arrangements. With such an offset, the results were separated into four series each comprising of data from the FE model plotted as the thicker lines, along with the other three experimental data curves from the manufactured samples plotted as thinner lines. The maximum compressive forces simulated by the FE models are labelled by the red dots along with their corresponding values, while the failure strength of the toecap samples are labelled by the black dots.

The results from arrangement P, L and B samples matched up very well with the modelling results, where the maximum failure loads occurred within the predicted regions. The close match between the force-displacement curves also indicates the accurate predictions in stiffness among the toecap samples. The FE model was therefore believed to be reliable for modelling designs of 4-layer hot-pressed flax-PLA toecap. For the 6-layer 900 arrangement, the results have shown a significant difference in the prediction of both maximum structural load and stiffness. The samples reached the maximum load of only 10421N, or 70% of the predicted load. Data from the three tested samples show good consistency in both stiffness and failure load reached, where the structure demonstrated high properties in strength during compression with no major breakages until reaching the failure load.

Toecap samples of arrangement 900 on the other hand have shown a significant difference in both the failure load and structural stiffness. While the experimental results indicated good consistency between the three samples, high structural strength with no major breakage until structural failure was also observed during the compression test. Investigation focusing on the variation in the manufactured geometry was therefore conducted to understand the disagreement between modelling and experimental results. Shown in Figure 7-29, the compressed toecap sample possesses cracks propagating through the laminates with thinner thickness. The thickness at the bottom flanges and side-wings positions highlighted by the red and green arrows respectively have an average value of 1 ~ 2mm, which is far from the 5.25mm defined. The thinner laminates at the side-wings have possibly resulted in the lower structural load achieved by the samples.

-115-

1. Manufacture and Evaluation of Composite Toecap Samples

Figure 7-28: Compression results of the hot-pressed flax-PLA composite toecap samples.

Figure 7-29: Cracks at the bottom flanges and the side wings where thickness was less than expected

To investigate the issue of uneven thickness, CAD models of the toecap moulds were observed in order to identify the possible cause. Shown in Figure 7-30a, seven probe points were created along the centre plane of the female toecap mould. By defining a thickness clearance between the male and female mould, the shortest distance from each probe point to the surface of the male mould can then be measured. According to the CAD model, the uneven thickness was caused due to the thicker 6-layer flax-PLA prepreg, which requires greater clearance between the male and female mould. As shown in Figure 7-30b and 7-30c, the use of stepping strips with 3.5mm and 5.25mm thickness respectively causes variation to the clearance between the seven positions. This is due to the irregular curved shape of the two moulds, which cannot offer a uniform clearance by simply changing the stepping distance. The measurements of the clearances are listed in Table 7-1. -116-

Manufacture and Evaluation of Composite Toecap Samples

a)

゚ ゚

b)

step = 3.5mm

゚゚

゚゚

c)

゚゚

step = 5.25mm

゚゚

Figure 7-30: CAD models of the toecap mould: a) seven probe points b) 3.5mm clearance c) 5.25mm clearance

-117-

1. Manufacture and Evaluation of Composite Toecap Samples Table 7-1: Clearance measurements from the toecap mould models with 3.5mm and 5.25mm clearance

clearance 3.5mm 5.25mm t1 3.01 3.02 t2 3.06 3.31 t3 3.13 3.61 t4 3.46 5.09 t5 3.49 5.24 t6 3.27 4.14 t7 3.49 5.24

Table 7-1 shows that with the current aluminium mould, the greater stepping clearance demanded by 6-layer laminates have resulted in greater variance in the laminate thickness from ranging from 3.02mm to 5.24mm, while the 4 layer 3.5mm toecaps possess lower variance in the thickness, and are thus better in quality. Also, with the use of the 10 tonne hydraulic press, higher stepping clearance can easily lead to the occurrence of misalignment between the moulds as shown in Figure 7-31. Therefore, the combined effect of the two issues described above eventually caused variations in laminate thickness of the toecap samples.

Figure 7-31: Misalignment of the toecap mould due to shifting.

In order to produce the high-strength flax-PLA toecaps capable of achieving the 15kN targeted strength, the proposed solution is to first finalize the layer thickness using the optimisation approach explained in Chapter 6 previously. The finalized thickness can then be incorporated into the CAD files of the toecap mould design, and finally exported to the CNC machine for producing new toecap moulds with the defined uniform clearance. Positioning pins can also be used to prevent misalignment between the new male and female mould.

-118-

Manufacture and Evaluation of Composite Toecap Samples 7.4 Result Evaluation

Verification of the FE models was conducted by using a step by step comparison method as discussed in the sections of this chapter. Results from sisal-PP toecaps presented good correlations with the simulated output; the experimental data from flax-PLA toecaps further proves the accuracy and reliability of the composite toecap models created. Apart the indication of maximum failure loads; the accuracy of the FE models becomes even more apparent when the prediction of failure modes for the hot-pressed flax-PLA toecapsare examined. Shown in Figure 7-32, the failure modes possessed by samples of arrangement P, L, B and 900 have also demonstrated good agreements with the behaviour modelled previously.

900

900

゚゚

゚゚

L

L

゚゚

゚゚

B

B

゚゚

゚゚

P

P

゚゚

゚゚

-119-

1. Manufacture and Evaluation of Composite Toecap Samples Figure 7-32: Comparison of failure modes ( top to bottom ): arrangement 900 , L, B, P

A top-dome failure model has been exhibited by specimens manufactured according to arrangements 900 and B with the typical split-breakage behaviour occurring at the top dome, accompanied bulging side-wings that remained structurally sound until the collapse of the structure. Side-wing failure is shown by arrangement L which matches up with the FE model. Breakages occurred at the top rim of the side wings that eventually led to the collapse of the top dome, while the dome remains unharmed.

Twist collapse of arrangement P demonstrated a complex failure process with a twisting motion from the side wings in the early compression stage. This soon led to breakages at various places including at the top-dome, side wings, bottom flanges and also the frontal area, which soon collapse the structure. Modelling results of the above four models have shown strong agreement with the experimental outcome in Figure 7-32. The composite material model of the FE simulation has hence successfully simulated the variation in strength and behaviour caused by the different application of fibre orientations.

-120-

Manufacture and Evaluation of Composite Toecap Samples

-121-

8 8. Conclusion From the studies performed in this thesis, the research objectives defined were fulfilled. The selected flax-PLA material possess the properties of high-strength, light weight and biodegradability as desired. Composite toecaps of different setups have been successfully analysed by constructing FE models in LS-DYNA, which simulates the structural failure process and the associated force-displacement relationship with reasonable accuracy. The designs were further optimised with Taguchi and Layer-Tournament approaches and led to the final design and manufacture of a 6 layer hot-pressed flax-PLA composite toecap capable of reaching loads beyond 10kN. The following are main conclusions drawn from each objective.

8.1 Material Selection In relation to the selection of composite prepreg for composite toecaps, it was observed that tensile strength results obtained from initial material testing related closely to the final structural strength of the toecap specimens, however increase in tensile strength by raising the volume fraction can result in severe de-lamination problems between the laminates due to insufficient matrix material between the layers. From this research, the toecap samples made from flax-PLA hot-pressed prepreg have provided the maximum tensile stress of 250Mpa, which is capable of reaching the load of 10.2kN under compression.

…. -122-

Conclusion 8.2 FE Structural Modelling With respect to numerical modelling, LS-DYNA has proven to be a suitable software package for simulating composite shell parts with large deformation and breakages. Numerical models simulating the compression test of composite toecaps have been successfully created. The simulation of the failure modes also matched up closely with the actual failure progress and provided a good indication of the stress distribution inside the structure geometry during compression testing. The feature of radiating fibre orientation assignment from AOPT also demonstrates good correlation with the actual fibre layup.

Comparing to test results, the simulations data follows the actual force-displacement curve accurately until reaching beyond the structural failure load. The modelling result appeared to be less accurate afterwards due to the current use of shell element and the material model which does not include the capability of simulating de-lamination and post-breakage behaviour beyond the collapse of the structure. The final optimised design achieved the maximum compressive load of 10421kN, and a 30% error for the maximum strength predicted by the FE model.

8.3 Structural Optimisation The two optimisation methods applied have provided very similar results. In the final 6 layers flax-PLA design, the suggested design from both approaches reached the top 5% quartile among all 64 combinations for maximum compression resistance. The Taguchi approach however possesses limitations in the experimental data imported; minimum of 16 or 32 sets of data are required in order to perform the optimisation analysis. The custom designed LayerTournament approach on the other hand requires four experimental runs for every symmetric layer pairs. The approach was therefore been recommended as the better approach which can analyse the product laminates with indefinite number of layers.

8.4 Product Manufacturing Manufacturing of the product was initially performed using a custom-made pneumatic mould press and a CNC-made aluminium mould produced previously, the methods and parameters were later transferred to a larger manual hydraulic press for better manufacturing quality. The control of the temperature has been observed to be an important factor for the final strength of the matrix material. The layup method of the prepreg, designed to match the FE model, was -123-

1. Conclusion also proven to be a simple and effective solution that yielded satisfactory results. However the robust design in the mould also resulted in uneven thickness and wrinkles in the products when increasing the blank thickness beyond 3.5mm. The combination of these defects weakened the structure and resulted in weaker structural strength of the toecap products comparing to numerical modelling results.

-124-

…. -125-

9 9. Future Work It is recommended that future research be carried out in three main areas. These are: detail modelling of the compression moulding process; FE modelling of composite toecaps using solid elements; manufacture of new toecap mould to ensure even thickness of the laminates. 

An accurate hot-press moulding model at the micro-level needs to be developed to better predict the deformation and thickness variation of the targeted products. The model can potentially offer benefits in predicting the position of surface defects including wrinkles and folds, and thus eliminating the cause by modifying the geometry of the laminates during early stage of development. The simulation in press-draping can also help predicting the fibre orientation for products with complex geometry.



FE modelling using solid elements offer extra capability for the current simulation including de-lamination effect and post-breakage material support, to better simulate the actual failure mechanism in the toecap product. Furthermore, with the use of solid elements, the stress distribution in the thickness direction can also be modelled with higher accuracy.



The flax-PLA composite prepregs used in this research possesses lower to medium strength when comparing to non-bio-degradable composites such as Twintex®, searching for bio-degradable composites with higher strength can effectively increase the strength and stiffness properties of the laminates, and eventually leads to better products.

…. -126-

Future Work

-127-

References 1. Yang, C.C., FEA Modelling and Analysis of Steel Toecaps Using LS-DYNA: Project Report. Department of Mechanical Engineering ,School of Engineering, The University of Auckland, 2008. 1. 2. Kuhn, M., Nowacki, J., Himmel, N., Development of an innovative high performance FRP protective toe cap. In Proceedings of the PAM Users Conference in Europe (EuroPAM), Nantes, France, 2000. 3. FLIEGER, et al., Biodegradable plastics from renewable sources. Vol. 48. 2003, Praha, TCHEQUE, REPUBLIQUE: Academia. 18. 4. SHIMAO, M., Biodegradation of plastics. Curr.Opin.Biotechnol. , 2001(12): p. 242–247. 5. SCHLECHTER, M., Biodegradable Polymer (Report P-175) Business Communications Company, Norwalk (USA) 2001. 6. CAHILL E., S.F., DUCATEL K., MÜNKER T., AGUADO M., EDER P., LEONE F., HERNANDEZ H. , The Futures Project Technology Map. EC TECS (Report EUR 19031EN), Brussels 1999. 7. KIMURA T., I.N., ISHIDA Y., SAITO Y., SHIMIZU N, Hydrolysis characteristics of biodegradable plastic (poly-lactic acid). . J.Japan. Soc.Food Sci.Technol. , 2002. 49, : p. 598–604 8. Hoshino, A. and Y. Isono, Degradation of aliphatic polyester films by commercially available lipases with special reference to rapid and complete degradation of poly(Llactide) film by lipase PL derived from Alcaligenes sp. Biodegradation, 2002. 13(2): p. 141-147. 9. Shirai, Y., Biodegradable plastics-present and future. . Kagaku Kogaku 63, 1999 p. 438– 441.

…. -128-

References 10. Perego, G., G.D. Cella, and C. Bastioli, Effect of molecular weight and crystallinity on poly(lactic acid) mechanical properties. Journal of Applied Polymer Science, 1996. 59(1): p. 37-43. 11. Tokiwa, Y.a.T.S., Hydrolysis of copolyesters containing aromatic and aliphatic ester blocks by lipase, . J. Appl. Polym. Sci. , 1981, . 26: p. 441-448. 12. Lee, S. and J.W. Lee, Characterization and processing of Biodegradable polymer blends of poly(lactic acid) with poly(butylene succinate adipate). Korea-Australia Rheology Journal, 2005. 17(2): p. 71-77. 13. Liu, X., et al., Thermal and mechanical properties of poly(lactic Acid) and poly(ethylene/butylene Succinate) blends. Journal of Polymers and the Environment, 1997. 5(4): p. 225-235. 14. [email protected], Natural fibres: Ancient fabrics, high-tech geotextile. 2009. 15. Joseph, P.V., K. Joseph, and S. Thomas, Effect of processing variables on the mechanical properties of sisal-fiber-reinforced polypropylene composites. Composites Science and Technology, 1999. 59(11): p. 1625-1640. 16. Rong, M.Z., et al., The effect of fiber treatment on the mechanical properties of unidirectional sisal-reinforced epoxy composites. Composites Science and Technology, 2001. 61(10): p. 1437-1447. 17. Li, Y., Y.-W. Mai, and L. Ye, Sisal fibre and its composites: a review of recent developments. Composites Science and Technology, 2000. 60(11): p. 2037-2055. 18. Andersons, J., et al., Strength distribution of elementary flax fibres. Composites Science and Technology, 2005. 65(3-4): p. 693-702. 19. Baiardo, M., E. Zini, and M. Scandola, Flax fibre-polyester composites. Composites Part A: Applied Science and Manufacturing, 2004. 35(6): p. 703-710.

-129-

1. References 20. Stamboulis, A., C.A. Baillie, and T. Peijs, Effects of environmental conditions on mechanical and physical properties of flax fibers. Composites Part A: Applied Science and Manufacturing, 2001. 32(8): p. 1105-1115. 21. Arbelaiz, A., et al., Mechanical properties of flax fibre/polypropylene composites. Influence of fibre/matrix modification and glass fibre hybridization. Composites Part A: Applied Science and Manufacturing, 2005. 36(12): p. 1637-1644. 22. Baley, C., Analysis of the flax fibres tensile behaviour and analysis of the tensile stiffness increase. Composites Part A: Applied Science and Manufacturing, 2002. 33(7): p. 939948. 23. Maker, B. X.Z., Input Parameters for Metal Forming Simulation using LS-DYNA. 2000. 24. Maker, B, X.Z., Input Parameters for Springback Simulation using LS-DYNA. 2001. 25. Shipway, P., Hutchings, M, Fracture of brittle spheres under compression and impact loading. I. Elastic stress distributions. Philosophical Magazine A, , 1993. 67(6 ): p. 1389 — 1404. 26. Filiz, Ç., ANALYSIS OF COMPOSITE BARS IN TORSION. Dokuz Eylul University Graduate School Of Natural And Applied Sciences. 2005. 27. Anderson, T.A., A 3-D elasticity solution for a sandwich composite with functionally graded core subjected to transverse loading by a rigid sphere. Composite Structures. 60(3): p. 265-274. 28. KOGUT, et al., Elastic-plastic contact analysis of a sphere and a rigid flat. Vol. 69. 2002, New York, NY, ETATS-UNIS: American Society of Mechanical Engineers. 29. Prasad, G.L.E. and N.K. Gupta, An experimental study of deformation modes of domes and large-angled frusta at different rates of compression. International Journal of Impact Engineering, 2005. 32(1-4): p. 400-415.

-130-

References 30. Lamers, E.A.D., S. Wijskamp, and R. Akkerman, Modelling shape distortions in composite products, in Seventh ESAFORM Conference on Material Forming. 2004: Trondheim, Norway. 31. Sidhu, R.M.J.S., et al., Finite element analysis of textile composite preform stamping. Composite Structures. 52(3-4): p. 483-497. 32. Cui, Z., D. Bhattacharyya, and G. Moltschaniwskyj, Experimental and numerical study of buckling response of composite shells compressed transversely between rigid platens. Composites Part B: Engineering, 2005. 36(5): p. 478-486. 33. Cui, Z., G. Moltschaniwskyj, and D. Bhattacharyya, Buckling and large deformation behaviour of composite domes compressed between rigid platens. Composite Structures. 66(1-4): p. 591-599. 34. Mills, N.J. and A. Gilchrist, Finite-element analysis of bicycle helmet oblique impacts. International Journal of Impact Engineering, 2008. 35(9): p. 1087-1101. 35. Tiernan, S. and M. Fahy, Dynamic FEA modelling of ISO tank containers. Journal of Materials Processing Technology, 2002. 124(1-2): p. 126-132. 36. LSTC, Getting Started with LS-DYNA. 2002. 37. LSTC, LS-DYNA Keyword Manual 2007. I. 38. Lee, S.M., T.S. Lim, and D.G. Lee, Damage tolerance of composite toecap. Composite Structures, 2005. 67(2): p. 167-174. 39. Chartiri, M., An Assessment of the new LS-DYNA layer solid element. 7th European LSDYNA Conference, 2009. 40. Tsau, L.-R., Y.-H. Chang, and F.-L. Tsao, The design of optimal stacking sequence for laminated FRP plates with inplane loading. Computers & Structures, 1995. 55(4): p. 565580. 41. Park, J.H., et al., Stacking sequence design of composite laminates for maximum strength using genetic algorithms. Composite Structures, 2001. 52(2): p. 217-231.

-131-

1. References 42. Kere, P. and J. Koski, Multicriterion stacking sequence optimisation scheme for composite laminates subjected to multiple loading conditions. Composite Structures, 2001. 54(2-3): p. 225-229. 43. Kim, C.W., et al., Stacking sequence optimisation of laminated plates. Composite Structures, 1997. 39(3-4): p. 283-288. 44. Miravete, A., Optimisation of Design of Composite Structures. 1996. 45. Roy, R.K., Design of Experiments Using The Taguchi Approach: 16 steps to product and process improvement. 2001. 46. Roden, M., Feasibility study for composite toecaps: Project Report. Department of mechanical Engineering, The University of Auckland. 2008. 47. Yang, C.C., Duhovic M , Lin R J T, Bhattacharyya D. finite element Modelling and Analysis of Composites Toecaps. IOP Conf. Series: Materials Science and Engineering, 2009. 48. Duhovic, M., S. Horbach, and D. Bhattacharyya, Improving the Interface Strength in Flax Fibre Poly(lactic) Acid Composites. Journal of Biobased Materials and Bioenergy, 2009. 3: p. 188-198. 49. Yang, C.C., Duhovic, M., Lin, R.J.T., BHATTACHARYYA, D., finite element Modelling for Manufacturing and Structural Analysis of Composite Toecaps. Eighteenth International Symposium Processing And Fabrication of Advanced Materials (PFAM XVIII), 2009. 4: p. 2057-2066. 50. Mcgregor,

O.,

Duhovic,

M.,

BHATTACHARYYA,

D.,

Manufacturing

and

Characterization of High Strength Biodegradable Composites. Eighteenth International Symposium Processing And Fabrication of Advanced Materials (PFAM XVIII), 2009. 4.

-132-

References

-133-

Appendix A.

Toecap Compressive Resistance Test Results: 6 Layers Hot-Pressed flax-PLA

…. -134-

Appendix B.

Toecap Compressive Resistance Test Results: 4 Prepregs Impregnated flax-PLA

…. -135-

1. Appendix

C.

Toecap Compressive Resistance Test Results: 4 Layer Hot-Pressed flax-PLA

-136-

Appendix

D.

IRID Calculation Program

IRID Calculators

layer (mm)

0.4375

sheet (mm)

0.875

Layers Equal thickness laminate 0.4375 mm

No. layers Thickness

8 3.5

Ip 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

S

WF -0.875 -0.625 -0.375 -0.125 0.125 0.375 0.625 0.875 0 0 0 0 0 0 0 0

0.125 0.125 0.125 0.125 0.125 0.125 0.125 0.125 0 0 0 0 0 0 0 0

-137-

1. Appendix

E.

Taguchi Optimisation Analysis Data Matrices: 6 Layer Hot-Pressed flax-PLA

S/N Ratio Calculation Arrgm't No.

Layer A

1

1

2

Layer B 1

1

3

S/N Ratio

2

1

3

Layer C

Sq Results Larger the better Error

Dir.

1

16.28

0.24

2

9.23

19.30

6.44

6513.8 000 9227.17 099

3

8.43

18.51

3.06

8427.4 044

4

1

4

4

5.35

14.57

4.81

5

2

1

3

9.53

19.58

7.92

5352.4 055 9525.03 904

6

2

2

4

6.21

15.87

0.80

6213.57 995

7

2

3

1

12.63

22.03

27.74

12633 940

8

2

4

2

5.58

14.93

3.35

4

7.57

17.58

0.67

5580.88 959 7570.27 405

18.33

2.45

9

3

1

10

3

2

3

8.25

11

3

3

2

5.84

15.33

2.05

19.03

5.12

8249.54 494 5841.83 449

12

3

4

1

8.94

13

4

1

2

4.85

13.71

9.32

14

4

2

1

6.01

15.58

1.40

15

4

3

4

6.01

15.57

1.42

6011.46 590 6005.93 545

16

4

4

3

3.99

12.01

22.56

3987.52 554

114.93

16.76

99.35

Mean of Strength Level 2 Level 3

Level 4

Total

40

40

40

Mean Response Table Level 1 Layer A Layer B Layer C

7.38 7.11 8.52

8.49 7.43 6.37

7.65 8.23 7.55

5.21 5.97 6.29

S/N Response Table Level 1 Layer A Layer B Layer C

-138-

Act F

6.51

17.17 16.79 18.23

Mean S/N ratio (dB) Level 2 Level 3 18.10 17.27 15.82

17.57 17.86 17.11

Level 4 14.22 15.14 15.90

8939.34 450 4847.93 509