Testing of a metallic insert for subpreform assembly

Bachelorarbeit cand. mach. Ivan Guerrero Matrikelnr.:

1625998

Testing of a metallic insert for subpreform assembly and reinforcement of the joint wbk Institute of Production Science Karlsruhe Institute of Technology (KIT) Kaiserstraße 12 76131 Karlsruhe Prof. Dr.-Ing. Jürgen Fleischer Prof. Dr.-Ing. Gisela Lanza Prof. Dr.-Ing. habil. Volker Schulze

Statement of Originality I sincerely affirm to have composed this thesis work autonomously, to have indicated completely and accurately all aids and sources used and to have marked anything taken from other works, with or without changes. Furthermore, I affirm to have observed the constitution of the KIT for the safeguarding of good scientific practice, as amended. Karlsruhe, October 17th 2017

_______________ Ivan Guerrero

Acknowledgement I would like to thank my supervisor, Fabian Ballier, for his expert advice and encouragement throughout this project. I also would like to extend my thanks to the “wbk Institute of Production Science” for giving me the tools to carry this investigation out. Finally, I would like to thank my family and friends for their support and encouragement throughout my study.

Abstract In this work an experimental study of the effect of a novel reinforcement technique on co-cured CFRP-joints is presented. The toughening mechanism consists of a metallic plate with arrowpins, which act as trough-thickness reinforcement between carbon fiber preforms. Five different designs of joints where manufactured and then tested with help of a single lap shear test. The specimens reinforced with the metallic insert showed a worse performance compared with a conventional co-cured joint without trough-thickness reinforcement. The negative effect on the joint is attributed to the bad adherence between the resin and the metal plate. A statistical analysis of the data obtained during the test campaign suggests that the metallic arrow pins and the surface treatment of the metal do not have a significant influence on the failure resistance of the joint when tested under shear conditions.

wbk Institute of Production Science

Table of Contents

Table of Contents Table of Abbreviations

III

1

1

2

Introduction 1.1 Motivation

1

1.2 Objective

1

1.3 Structure of the Thesis

2

Fundaments 2.1 Fiber-reinforced plastics

3

2.1.1 Fibers

3

2.1.2 Matrix materials

5

2.1.3 Dry textile fabrics

5

2.1.4 Preforming

6

2.1.5 Manufacturing Processes

8

2.2 Joining technologies

11

2.2.1 Bolted joints

11

2.2.2 Bonded joints

12

2.2.3 Co-cured joints

13

2.3 Testing and characterization of bonded joints

14

2.3.1 Single lap shear test

14

2.3.2 Typical failure modes of FRP bonded joints

14

2.4 Statistics and data analysis

3

3

15

2.4.1 Descriptive statistics

15

2.4.2 Inferential statistics

18

State of the art

22

3.1 Z-pinning

22

3.2 Z-anchoring

23

3.3 Metallic Arrow-Pin Reinforcement

24

4

Own approach

25

5

Results

27

5.1 Experiment planning

27

5.2 Lap shear test

30

5.2.1 Type 0

31

5.2.2 Type 1

32

cand. mach. Ivan Guerrero

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6

5.2.3 Type 2

33

5.2.4 Type 3

35

5.2.5 Type 4

35

5.2.6 Descriptive analysis

37

Assessment 6.1 Analysis of variance (ANOVA-test)

42 42

6.1.1 Tensile strength

42

6.1.2 Tensile shear strength

44

6.2 Breaking area analysis

7

Table of Contents

45

6.2.1 Type 0

46

6.2.2 Type 1

47

6.2.3 Type 2

48

6.2.4 Type 4

49

Summary and Outlook

50

7.1 Summary

50

7.2 Outlook

50

List of Figures

I

List of Tables

III

8

IV

Publication bibliography


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Table of Abbreviations

Table of Abbreviations Symbol

Measurement

ANOVA

Analysis of variance

-

CAD

Computer-aided design

-

CFRP

Carbon fiber reinforced plastic

-

FRP

Fiber reinforced plastic

-

RMT

Resin transfer molding

-

UTM

Universal testing machine

-

VARIM

Vacuum assisted resin infusion molding

-

VARTM

Vacuum assisted resin transfer molding

-


unity

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1 1.1

1 Introduction

Introduction Motivation

The application of fiber reinforced plastics (FRP) is constantly increasing in the industry, due to their superior properties (Kotresh M.Gaddikeri, M.Subba Rao 2002, p. 1). The process chain of resin transfer molding (RTM) offers one approach for producing structural components made of fiber reinforced plastic in large quantities (Eickenbuschn, H.: Krauss, O, p. 25). This method makes use of preassembled stacks of textile fabric sheets also called preforms. When working with a large or complex preform, it can be composed of an assembly of single sub-preforms. These sub-preforms are easier to drape and can be produced within an automated line. Once the assembly is ready, the sub-preforms can be co-cured in order to manufacture a FRPcomponent. (Ballier et al. 2015, p. 312) Although integrally co-cured structures have the advantage that the assembly costs are reduced and the weight of mechanical fasteners is eliminated, they have a recurring problem. A joint, in this case the joint between two sub-preforms, always represents a weak spot which can lead to the collapse of a structure. For this reason, different toughening mechanisms have been developed in the last decades and have shown notable results. However these reinforcement techniques are far from being ideal and we still face major challenges in the pursue of better cocured structures with superior failure resistance and damage tolerance. (Heimbs et al. 2014, p. 16)

1.2

Objective

The aim of this work was to investigate the influence of a novel reinforcement technique in the failure behavior of different designs of carbon fiber-reinforced plastic (CFRP) joints. The toughening mechanism consists of a thin metallic sheet with arrow-shaped pins which, once they are bent, will act as through-thickness reinforcement. This metallic insert is intended to be placed between two sub-preforms prior to the infiltration process. The expected result is a greater resistance to failure due to the mechanical interlocking effect and plastic deformation of the arrow-shaped pins of the insert.

Figure 1.1: CAD-model of the metallic insert.


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1.3

1 Introduction

Structure of the Thesis

This work consists of seven chapters; the first one is an introduction which describes the motivation of this work and the objective. The second chapter explains the necessary fundamentals required to understand this document in depth. First of all, the components and manufacturing processes of fiber reinforced plastics will be presented. Then, an explanation of the most used joining technologies will be provided. Afterwards, a test for composite bonded joints will explained and guidelines for the characterization of the failure modes will be provided. Finally some basics concepts of statistics and data analysis will be reviewed. Chapter three gives an overall picture of the toughening mechanisms that are currently in-service or in development. Chapter four presents the procedure model for this work and a brief explanation of the main steps is offered. In chapter five the manufacturing process of the test-specimens is described and the results of the single lap shear test are presented. In chapter six a statistical analysis is performed with the data from chapter five and a characterization of the failure modes of the specimens is done. Finally, in the seventh chapter, the conclusions and outlook for further projects will be analyzed.


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2 2.1

2 Fundaments

Fundaments Fiber-reinforced plastics

Composites can be defined as a blend of two or more materials, whose combination achieves better qualities than its single components. (Schürmann 2008, p. 13) Fiber-reinforced plastics are composite materials consisting of reinforcement fibers embedded in a polymer matrix. The function of the matrix is to protect the fibers against external factors and to fix them in position. The fibers constitute a reinforcing structure which withstands the loads. The boundary surface between matrix and fibers is called interface and its function is to transfer the loads to the fibers. (Neitzel, Mitschang 2004, p. 25)

Figure 2.1: CFRP body of a BMW i3 in a production plant located in Leipzig, Germany. (Jacob 2014, p. 18) These types of composites stand out among traditional materials because of their exceptional qualities such as low weight, corrosion resistance and high fatigue strength. Although the concept of matrix materials reinforced by flexible filaments has always been present in the nature, for example in the structure of plants, it was not until the second half of the 20 th century that FRP began to be really developed. Currently, FRP have uses in all areas. Some examples are automotive, marine and ground transport industries, and particularly aerospace industry, in which FRP has become the dominant form of structural material. (Bunsell, Renard 2005, p. 1)

2.1.1

Fibers

The outstanding characteristics of FRP are due to the fibers used to reinforce the matrix. Because their function is to bear the loads, mechanical strength and stiffness are required. (Schürmann 2008, p. 21) Reinforcement fibers can be classified according to their composition (organic - inorganic), production/extraction (artificial - natural) or qualities (high-strength – high-stiffness). However,


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there is no category large enough that can encapsulate the large amount of reinforcement fibers currently available. (Neitzel, Mitschang 2004, p. 25) The most relevant type of fibers for this thesis is carbon fiber. The next section offers a brief overview of the properties of this material. 2.1.1.1 Carbon Fiber Carbon fibers are the reinforcements par excellence for high performance composites because of its extraordinary properties. Similarly, CFRP have become a standard for military and civil aircraft and is slowly emerging into the sports goods and competition cars sector.(Bunsell, Renard 2005, p. 5–6) Some of the advantages that this type of fibers offers are: 

Carbon fibers are light with a density of only 1.8 g/cm 3. Compared to glass fiber (ρ ≈ 2.54 g/cm3), carbon fiber is usually better suited for constructions where the limiting factor is the weight.  Carbon fibers combine high elastic modulus and extreme high tensile strength. These qualities of the material are directly linked to a very strong bond between the atoms of the fibers. During the manufacturing process, these two properties can be adjusted to the requirements of the costumer.  Carbon fiber has great resistance against fatigue. This quality has spread the use of this kind of fibers through the aviation field.  Carbon fibers are very resistant to corrosion and are compatible with every synthetic polymer. This material is even compatible with human tissues and bones (Biocompatibility), making the use of these fibers very appealing for the medical field. (Schürmann 2008, pp. 39–40) On the other hand, there are some negative aspects of carbon fibers which have to be considered during the material selection process:  The elongation at break of carbon fibers is for some applications too small.  Fractures and delamination between the plies of carbon fiber are very difficult to notice with the naked eye. A nondestructive analysis, for example ultrasound, might be needed in order to verify the state of the material.  Compared with its tensile strength, the compression resistance of carbon fibers is very small. This can be a constraining aspect in the design of some structures.  The most limiting factor and the reason why most structural components are still being manufactured with traditional metals, such as steel or aluminum, is the high price of carbon fiber. (Schürmann 2008, pp. 40–41) There are different ways to manufacture carbon fiber. From a technical point of view, the difference lies on the type of precursors used during the manufacturing process. One of the most used production methods consists on the conversion of a modified form of polyacrylonitrile (PAN) into carbon fibers. (AVK – Industrievereinigung Verstärkte Kunststoffe e. V 2010, p. 140) Many fibers, for example cellulose filaments, can be converted into carbon fibers. For this purpose the precursor fibers should be able to carbonize rather than melt when heated. (Bunsell, Renard 2005, p. 46)


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2.1.2

2 Fundaments

Matrix materials

Polymers have established themselves as matrix materials in most applications. They have attracted attention because they are light in weight, highly resistant to corrosion and can impregnate the fibers at low pressures and temperatures when used as a solution or molten. Load transfer between fibers, and therefore an effective use of the fiber properties, is achieved by elastic deformation of the matrix material. (Bunsell, Renard 2005, p. 82) The polymers used as matrix materials can be classified by their molecular structures in two main groups: 2.1.2.1 Thermosetting resins Thermosetting resins are currently the most used polymers in the manufacturing of FRP. These materials undergo an irreversible chemical reaction where a hardener forms strong covalent links between the molecules of the resin, which then becomes solid. Some of the positive aspects of these polymers are the high resistance against corrosion, low creep tendency and high thermal and chemical stability. (Neitzel, Mitschang 2004, p. 34) Some problems linked to the use of this kind of resins are the impossibility of modifying the final form after manufacture and the difficulties of recycling thermosetting materials (Bunsell, Renard 2005, p. 83). The most important thermosetting resins used in the production of FRP are epoxy (EP) and phenolic resins, as well as unsaturated polyester (UP) and vinylester (VE) resins (Neitzel, Mitschang 2004, p. 34). 2.1.2.2 Thermoplastics Thermoplastics are polymers that undergo dramatic changes in their mechanical properties when they are heated above a certain temperature, but, unlike thermosetting polymers, these changes are reversible. This is an advantage at the moment of manufacturing FRP. The forming can be very rapid because the material only requires to be heated to a high enough temperature. (Bunsell, Renard 2005, p. 84) Some positive aspects of thermoplastics are the resistance to delamination and chemical stability. In principle they can be recycled and the final form of a structure can be modified just by reheating it (Neitzel, Mitschang 2004, p. 43). The disadvantages of using thermoplastics in the production of FRP are the creep tendency and the high melt viscosity during the forming process (Neitzel, Mitschang 2004, p. 43). Although processing can be extremely rapid, the fiber impregnation is not as easy as with thermosets since the thermoplastic has to be molten and fiber-matrix adhesion controlled (Bunsell, Renard 2005, p. 84). The most widely used thermoplastics are polypropylene (PP), polyamides (PA) (Nylons), saturated polyester polyethylene terphtalate (PET), polphenylene sulphide (PPS) (Bunsell, Renard 2005, p. 87).

2.1.3

Dry textile fabrics

There are a multitude of different products made out of fibers which are used in composite structures. They can be composed of continuous or discontinuous fibers, they can be supplied as dry fabrics or pre-impregnated with resin and the fibers can be oriented or disoriented (Campbell 2003, pp. 12–14). Although the range of textile products is very wide, the most relevant for this thesis are the dry textile fabrics. By definition, a textile fabric is a manufactured assembly of fibers, which has a considerable surface area in relation to its thickness and enough mechanical strength to give the assembly an


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inherent cohesion. The two dominant textile fabrics in the FRP field are woven and knitted structures. The fundamental difference between woven fabrics and knitted fabrics lies in the way the fibers are interconnected. (Rozant et al. 2000, p. 1168) Woven fabric (Figure 2.2 a) is the most common continuous dry textile fabric. They are produced by the weaving of straight fibers. A very important aspect that defines the behavior of the fabric is the weave pattern, because it affects the drapeablility, structural properties and the handleability. Many weave patterns are available in the market, each one with its advantages and disadvantages. Some general positive aspects include drapeablility, structural efficiency, market availability and the capacity to achieve high fiber volume contents. (Campbell 2003, p. 15) Knitted fabrics have a structure formed by the intermeshing of yarn loops. There are two types of knitted structures in the market: warp-knitted (Figure 2.2 b) and weft-knitted (Figure 2.2 c) fabrics. Warp-knitted fabrics are obtained by intermeshing loops along the length of the fabric while weft-knitted fabrics are produced by intermeshing loops across its width. Advantages of knitted fabrics include stiffness, dimensional stability and good drapeablility because of their high extensibility due to stretching of the loops. (Rozant et al. 2000, p. 1168)

(a)

(b)

(c)

Figure 2.2: Different textile architectures (Rozant et al. 2000) Other important dry textile fabrics which deserve to be mentioned are stitched fabrics. They consist of unidirectional fibers oriented in specified directions that are then stitched together to hold them in place, forming a fabric. A positive aspect is the reduction of total costs due to labor decrease. When using multi-ply stitched materials, less plies are required to cut and handle during the fabrication of a part. Another advantage is that the ply orientation remains intact during handling due to the stitches. Disadvantages include the reduced drapeablility and the availability of some specific stitched ply set designs. (Campbell 2003, p. 15)

2.1.4

Preforming

The term preform refers to a textile structure of fibers which has been formed into a shape close to the final part, but hasn’t been impregnated yet. This structure can consist of one or more preshaped elements called sub-preforms. This reinforcing structure will be impregnated with a matrix system in a subsequent process in order to form a FRP.(Campbell 2003, p. 16) Due to the constant increase of requirements on the part of the customers, like higher fiber volume content and more complex geometries of the manufactured components, preforms have


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been widely used in combination with liquid molding processes like RTM since 1940. (Neitzel, Mitschang 2004, p. 73) Preform technologies offer a wide range of advantages to engineers and designers like an increment of the fiber volume content, reduction of manufacturing cycle times and the potential to reduce composite part costs. Furthermore, the use of preforms allows a tailored orientation of the fibers, optimizing the mechanic properties of the structure. (Campbell 2003, pp. 323–324) Some negative features are the fiber wettability for complex shapes, compatibility with the matrix if a binder is used in the preform and the limited flexibility if design changes are required. (Campbell 2003, p. 16) In general, two different forms of preforming can be distinguished: directed and sequential fiber preforming. Direct fiber preforming denotes methods for manufacturing 3D-structures directly with fibers and additives, for example binders. In this category, some processes are worth mentioning like fiber spraying, water-slurry-method and the P4-process (Programmable Powdered Preform Process). In addition, 3D-textile processes based on knitting and braiding technologies are also applied in the manufacturing of preforms. (Neitzel, Mitschang 2004, pp. 77–80) Figure 2.3 shows a preform of a helmet manufactured entirely with a 3-D knitting process.

Figure 2.3: Preform manufactured with a 3D-knitting process (Neitzel, Mitschang 2004, p. 80) Unlike directed fiber preforming, Sequential preforming methods require a number of steps to create a three dimensional structure. All this methods rely on a textile fabric which must be prepared in advance for the preforming process. Usually there are two ways to proceed, in the first, the sub-preforms are joined with a binder and then the structure is deformed to its final form. In the second, the sub preforms are sewed together creating an assembly with the desired contour. Figure 2.4 shows a T-profile preform manufactured with help of a sequential preforming method


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Figure 2.4: Sewed T-Profile preform 2010, p. 231)

2.1.5

2 Fundaments

(AVK – Industrievereinigung Verstärkte Kunststoffe e. V

Manufacturing Processes

There are different types of manufacturing processes for fiber-reinforced plastics. Many aspects need to be considered in order to choose one that meets the requirements of the customer. As a general rule, the size and the complexity of the parts to be made are the most essential aspects in the selection of a manufacturing process. The loads to which the component is subjected are the dominant factor at the time of selecting the material for the product. Also, the number of pieces, the area of application and the customer requirements should not be forgotten during the decision making process. (Neitzel, Mitschang 2004, pp. 156–157) In general, the production of FRP is based on manufacturing processes, whereby a polymer is bonded to a reinforcing fiber structure. From a technical point of view, these manufacturing processes can be decomposed in three main steps: impregnation, consolidation and solidification. Impregnation is understood as the moistening of the single filaments of the reinforcing structure and the filling of the spaces between them with a matrix. (Neitzel, Mitschang 2004, pp. 160–161) Consolidation is the process step in which complete bonding of the fiber layers is achieved; in consequence a void-free FRP is obtained. During this step, the air is squeezed out from the space between the fibers, increasing the fiber volume fraction and enhancing the uniformity of the fiber distribution in the composite. (Colton et al. 1992) At last, the mixture of reinforcing fibers and liquid matrix solidifies due to heat dissipation or chemical cross-linking, forming a solid body (Neitzel, Mitschang 2004, p. 161). Although there are different types of manufacturing processes, the most relevant for this thesis are resin transfer molding (RTM), Vacuum assisted resin infusion molding (VARIM) and Vacuum Assisted Resin Transfer Molding (VARTM). 2.1.5.1 Resin Transfer Molding (RTM) RTM is one of the most widely used manufacturing techniques in the industry. The process starts with the placement of a preform inside a mold, which is then closed. Mold closing pressures are in the range of 100-600 kPa. After that, a mixture of resin and curing agents is injected into the mold under low pressure in order to avoid alterations in the fiber placement. Figure 2.5 offers a schematic overview of the manufacturing process. Generally the resin´s viscosity is low, in the range of 0.1 to 1 Pa/s. During the injection, the resin spreads through the preform, impregnating the fibers and displacing the air out of the mold trough escape holes. The permeability of the preform is important for the correct impregnation of cand. mach. Ivan Guerrero

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the fibers. The number of pieces to be produced will determine the material used for the mold. For example, molds made of epoxy compound can be used to produce up to 4000 pieces. Metallic molds made of stamped steel o aluminum will allow the production of tens of thousands of pieces. (Bunsell, Renard 2005, p. 143)

Figure 2.5: Resin Transfer Molding (RTM) (Neitzel, Mitschang 2004, p. 279) One of the many advantages of this manufacturing technique is the short cycle time of production. In general, an RTM cycle takes around 5 to 25 min., being the injection and the solidification of the matrix the most time consuming steps. Another positive aspect, is the capacity of making pieces with complex geometries with high dimensional accuracy and exceptional surface quality. This reduces the post-production processes, lowering the total cost. (Neitzel, Mitschang 2004, pp. 278–279) 2.1.5.2 Vacuum assisted resin infusion molding (VARIM) VARIM (Goren, Atas 2008, p. 118), also called vacuum assisted resin infusion process (VAP) (Heimbs et al. 2014, p. 18), is a simpler method for manufacturing composite parts. A small number of elements are needed to set this process up like a mold, a vacuum-pump and a container for the resin. Usually a one sided mold is used and the other part is replaced by a foil or a cheap plastic cover to reduce costs even more. Like all resin injection processes, VARIM starts with the placement of a preform inside a mold, which is then closed. Then the pump generates a vacuum inside the mold, which forces the resin inside it. The pressure difference between the sprue and the outlet of the mold drags the resin through the fibers, displacing simultaneously the air out of the cavity (Figure 2.6). Pieces with good quality and reduced cost can be manufactured with this process and it is especially suitable for parts with ripping. The disadvantage lies in the size of the components that are going to be manufactured. When a one sided mold is used, the pressure difference shouldn’t exceed 1 bar, this restricts the size of the parts that can be produced and therefore the areas of usage of VARIM. (Neitzel 2014, p. 365)


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Figure 2.6: Vacuum assisted resin infusion molding (VARIM) (Neitzel 2014, p. 365)

2.1.5.3 Vacuum Assisted Resin Transfer Molding (VARTM) VARTM uses the principle of pressure difference to achieve better results during the manufacturing process. Just like RTM, VARTM starts with the placement of a preform inside a mold, which is then closed. The difference lies in the injection process. While the resin is injected into the mold, a pump displaces the air out of the cavity creating a bigger pressure difference, with the benefit that the absolute pressure of the injection is not increased (Figure 2.7). This helps the resin to impregnate the fibers more efficiently. (AVK – Industrievereinigung Verstärkte Kunststoffe e. V 2010, p. 380) Although the cost of this manufacturing process is normally higher than conventional RTM, the products have very low porosity and the surface finish is very good. (Bunsell, Renard 2005, p. 144)

Figure 2.7: Vacuum Assisted Resin Transfer Molding (VARTM) (Neitzel, Mitschang 2004, p. 280)


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2.2

2 Fundaments

Joining technologies

New materials and innovative constructive principles are only applicable in the industry when appropriate joining technologies are available. In the case of FRP, techniques developed at first for the joining of metals and plastics have been customized to the requirements of this type of materials (Dipl.-Ing. N. Burkardt, pp. 2–3). A general classification of these techniques is presented in Figure 2.8. Hereafter, a brief explanation of the most relevant technologies will be offered, being the co-curing process the most significant for this document.

Figure 2.8: Classification of joining technologies for FRP (Neitzel 2014, p. 469)

2.2.1

Bolted joints

Bolted joints are the most ancient technique and one of the most investigated methods so far. This kind of joining technology is used to a larger extent when structural components with a flat surface need to be joined. In the majority of cases an overlapping of the components is required. Some of the positive aspects of bolted joints are:    

The facility to join different types of materials to each other. The joints are separable and the bolts can be reused. The price of bolts in the market is inexpensive. Surface treatment of the components to be joined is not required.

(Schürmann 2008, pp. 513–515) Despite the widespread use of bolted joints, they present some problems when used for joining components made of FRP. The nature of this technic requires drilling holes, which causes fiber cut, delamination and stress concentration around the holes. (Shin et al. 2000, pp. 123–124)


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Figure 2.9: Common bolted joint and force flow within the structural components. (Schürmann 2008, p. 530)

2.2.2

Bonded joints

Adhesive bonding is a widely used industrial joining method in which a polymeric material called adhesive is used to join two or more pieces called adherends. An schematic overview of different types of bonded joints is displayed in figure 2.10. Between the most prevalent adhesives in the industry we can find epoxies, nitrile phenolics and bismaleimides. Bonded joints may be preferred for thin composite sections with well-defined load paths. The advantages of bonded joints include:   

More uniform stress distribution than mechanical fasteners by eliminating stress concentration peaks, leading to a better fatigue life in comparison to mechanical joints. Bonded joints are usually lighter than mechanically fastened joints and are cheaper in most cases, due to elimination of mechanical fasteners. Bonded joints are electrically insulated, which prevents galvanic corrosion of metal adherends.

Figure 2.10: Different types of bonded joints (Neitzel 2014, p. 490)


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Bonded joints also have some disadvantages, including:   

2.2.3

Bonded joints should be considered inseparable joints. Disassembly is difficult and often results in damage of the adherends. Adhesive bonding requires in most cases a surface preparation of the adherends. At present there is no reliable nondestructive test method for determining the strength of bonded joints. Therefore, test specimens must be fabricated and destructively tested. (Campbell 2003, pp. 242–244)

Co-cured joints

The assembly of sub-preforms into a final complex structure enables the impregnation of the whole structure in only one step. This process is known as co-curing. (Kotresh M.Gaddikeri, M.Subba Rao 2002, p. 1) A co-cured joint is an adhesively bonded joint in which uncured composite parts are cured and bonded simultaneously during the same curing process to other cured or uncured composite fragments, metallic elements or core materials. (Campbell 2003, p. 242) When working with FRP, the co-curing process has several positive aspects. In addition to the advantages of classic bonded joints, co-cured joints are simpler to design and easier to manufacture since no further joining processes are required, reducing the assembly cycles and providing the parts with better structural integrity. (Shin et al. 2000, pp. 124–125).

Figure 2.11: Experimental set-up for the tensile test of a co-cured single lap joint specimen. In this case the adherends are FRP and steel. (Shin et al. 2000, p. 129)


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2.3 2.3.1

2 Fundaments

Testing and characterization of bonded joints Single lap shear test

The single lap shear test is one of the most common used methods for testing adhesive bond strength. In this destructive technique, a test specimen is placed in the universal testing machine (UTM) and slowly extended until it fractures. During this process, the elongation of the specimen and the applied force are measured. The lap shear strength is given by the failure stress in the adhesive, which is calculated by dividing the breaking force by the bond area. (Campbell 2003, p. 250) While this test is relatively easy to perform, it does not give a true measure of the shear strength due to andherend bending and induced peel loads. However, the single lap shear test is an effective method for comparison and evaluation of adhesives or surface preparation and for inprocess control. (Campbell 2003, p. 251)

Figure 2.12: Principle of the single lap shear test (Fraunhofer-Institut für Fertigungstechnik und Angewandte Materialforschung IFAM)

2.3.2

Typical failure modes of FRP bonded joints

When testing or characterizing adhesive materials, the failure mode for all the specimens should be examined in order to gain a full understanding of the properties of the adhesive and the joint. According to the standard ASTM D5573, there are seven different failure modes and they are: adhesive failure, cohesive failure, thin-layer cohesive failure, fibre-tear failure, light-fibre-tear failure, stock-break failure and mixed failure, being the last one a combination of the other six modes. (Banea, da Silva 2016, pp. 8–9)


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Figure 2.13: Possible failure modes of bonded joints. (Banea, da Silva 2016, p. 9) Extensive investigation with structural adhesive bonds has repeatedly demonstrated that adhesive durability and longevity depends on the surface preparation of a material prior to the bonding process. With this in mind, some failure modes may be acceptable and others may suggest a preparation problem of the specimen. This is the case of specimens which exhibit an adhesive failure at the adherend-adhesive interface rather than a cohesive failure within the adhesive. This type of failure is undesirable and may be an indication of an inadequate surface treatment that generally results in decreased joint durability. (Campbell 2003, pp. 250–252) When cohesive failure within the thin adhesive layer or within the first ply of the composite laminate occurs, the maximum strength of the material in a co-cured single lap joint is reached. Normally, this failure mode is desired and is a signal of a proper bonding process. (Shin et al. 2000, p. 132)

2.4

Statistics and data analysis

Statistics is a collection of methods which help us to describe, summarize, interpret, and analyze data. In this part of the document a brief explanation of descriptive and inferential statistics will be offered.

2.4.1

Descriptive statistics

Descriptive statistics are a group of methods which help us describing data and provide estimates of population parameters. First, some terminology will be introduced. The units on which data is measured – such as persons, cars, animals or plants – are called observations. The collection of all units is called population and if a selection of observations is considered, then this subset is called sample. (Heumann et al. 2016, p. 3) Observations are usually evaluated for three characteristics: central tendency (or location), variation (or dispersion), and shape. Location and variation are evaluated quantitatively, that is with numeric measures, and the distribution shape is usually evaluated qualitatively such as by cand. mach. Ivan Guerrero

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interpreting a graph. Measures of location and variation determined from a population are called parameters of the population. (Mathews 2005, p. 19) 2.4.1.1 Measures of central tendency A data set may contain many observations. However, we are not always interested in each of the measured values but rather in a summary which interprets the data. The measures of central tendency fulfil the purpose of summarizing the data in a meaningful yet concise way. The most commonly used are the sample mean ͞x and the sample median ͠x. 

The mean is the sum of the individual values of each observation divided by the number of observations. The sample mean is determined from:

𝑥̅ =

1 𝑛

∑𝑛𝑖=1 𝑥𝑖

Formula 2-1

Where xi are the individual values in the sample and the summation is performed over all n of the values in the sample. The mean of a population is denoted with the letter μ and its value is calculated in an analogous way. (Mathews 2005, p. 20) 

The median is the value which divides the observations into two equal groups such that at least 50% of the values are greater than or equal to the median and at least 50% of the values are less than or equal to the median. The position of the median in the data set, when the data are organized by size from the smallest to the largest value, is given by: 𝑛+1 2

Formula 2-2

Where n is the size of the sample. For a data set containing an odd number of values, the median will be equal to the middle value in the data set. For a set containing an even number of data points, the median position falls between two values in the data set. (Heumann et al. 2016, p. 40) 2.4.1.2 Measures of dispersion The variation or dispersion of observations around any particular value, such as the mean, is another property which characterizes the data and its distribution. The most common statistics used to measure variation of sample data are the range R, the variance and the standard deviation s. 

The range is equal to the difference between the largest and smallest values of a sample:

𝑅 = 𝑥𝑚𝑎𝑥 − 𝑥𝑚𝑖𝑛


Formula 2-3

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

2 Fundaments

The standard deviation is determined by considering the dispersion of the data points of a data set from the sample mean ͞x. The deviation of the i-th data point from x͞ is:

ε𝑖 = 𝑥𝑖 − 𝑥̅

Formula 2-4

The sample standard deviation is given by:

𝑠=√

2 ∑𝑛 𝑖=1 𝜀𝑖

𝑛−1

Formula 2-5

The standard deviation measures how much the observations vary or how they are dispersed around the arithmetic mean. A low value of the standard deviation indicates that the values are highly concentrated around the mean. A high value of the standard deviation indicates lower concentration of the observations around the mean, and some of the observed values may even be far away from the mean. The population standard deviation σ is calculated in almost the same way as the sample standard deviation:

𝜎=√

2 ∑𝑛 𝑖=1 𝜀𝑖

𝑁

Formula 2-6

Where εi =xi – μ and N is the population size. (Heumann et al. 2016, pp. 50–52)

Figure 2.14: The figure shows the normal distribution of two samples. Although the both groups of observations have the same mean, the data represented by the taller graph suggests a higher concentration of values around the mean. (Amity Global Business) 2.4.1.3 Distribution shape: The normal distribution Although there are many probability distributions that play important roles in statistics, one of the most important distributions is the normal probability distribution. The equation for this bellshaped curve is:


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𝜑(𝑥; 𝜇, 𝜎) =

2 Fundaments

1 √2𝜋𝜎

1 𝑥−𝜇 2 ( ) 𝜎

𝑒2

Formula 2-7

This function is called the probability density function, where μ is the population mean and σ is the standard deviation of the normal distribution. It helps us calculate the probability that the value x of an observation falls within a specified range of values. This probability is given by: 𝑏

𝛷 (𝑎 < 𝑥 < 𝑏; 𝜇, 𝜎) = ∫𝑎 𝜑(𝑥; 𝜇, 𝜎)𝑑𝑥

Formula 2-8

Where Φ stands for the cumulative normal probability distribution. (Mathews 2005, pp. 26–28) In Figure 2.15 the bell-shaped curve of a normal distribution is presented. The gray area under the curve gives the cumulative normal probability that x will have a value in the interval a < x < b. The z-value is an auxiliary variable which helps in the calculation of the cumulative normal probability Φ and can be expressed like x = μ + zσ. The magnitude of z indicates how many standard deviations x falls away from μ.

Figure 2.15: Normal distribution and the cumulative normal probability distribution (Mathews 2005, p. 27)

2.4.2

Inferential statistics

Unlike descriptive statistics, inferential statistics are methods used to draw conclusions about a population of interest using the obtained data from the samples. These kinds of statements are called inferences. The most important tools for statistical inference are hypothesis tests and confidence intervals. (Mathews 2005, p. 52) 2.4.2.1 Confidence intervals: When sample data is collected, we expect this information to be representative of its population. For example, we expect the sample mean ͞x to be a reasonable estimate for the population mean


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2 Fundaments

μ. However, the statistic calculated from the sample, also called point estimate, does not take into account the precision of the estimate. The deviation between the point estimate and the true parameter, in the example | ͞x – μ|, can be substantial, especially when the sample size is small. To incorporate the information about the precision of an estimate, a confidence interval can be created. It is a random interval with lower and upper bounds, Il(X) and Iu(X), such that the unknown parameter θ is covered by a prespecified probability of at least 1 − α:

𝑃𝜃 (𝐼𝑙 (𝑋) ≤ 𝜃 ≤ 𝐼𝑢 (𝑋)) ≥ 1 − 𝛼

Formula 2-9

The probability 1 – α is called the confidence level, Il(X) is called the lower confidence limit and Iu(X) is called the upper confidence limit. Frequent used confidence levels are 0.8, 0.9 and 0.95. It is important to note that the bounds are random and the parameter is a fixed value. This is the reason why we say that the true parameter is covered by the interval with probability 1 – α and not that the probability that the interval contains the parameter is 1 − α. (Heumann et al. 2016, pp. 195–196)

Figure 2.16: Sample mean and 95% confidence intervals for 6 random samples. (Heumann et al. 2016, p. 197)

2.4.2.2 Hypothesis testing Hypothesis testing is a very important statistical inference technique. Occasionally, a precise estimate of a parameter is not significant for the statistician, but rather only an analysis is needed of whether a statement about a parameter of interest is true or not. Suppose that the unknown mean of a population has a certain value or if it differs from this value. These statements are written: 𝐻0 : 𝜇 = 𝜇0 𝐻𝐴 : 𝜇 ≠ 𝜇0


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2 Fundaments

Where μ is the true but unknown mean of the population, μ0 is the value we think μ might have, and H0 and HA are complementary statements about the relationship between μ and μ0. H0 and HA are called the null and alternative hypotheses, respectively. If appropriate and sufficient data are collected from a population, then a statistical estimate of its parameter will be consistent with one or the other of the two statements. Then, one hypothesis can be accepted and the other rejected. (Mathews 2005, pp. 42–43) Although there are different methods for decision making, the most important for this document is the method based on the p-value. It can be calculated from the following equation:

1 − 𝑝 = 𝛷(−𝑧𝑝/2 < 𝑧 < +𝑧𝑝/2 )

Formula 2-10

Where z is the z-value mentioned in 2.4.1.3 and:

𝑧𝑝/2 =

𝑥̅ −𝜇0 𝜎𝑥

Formula 2-11

Once the p-value is identified, the decision to accept or reject the null hypothesis is made comparing the p-value with the value of α from the confidence level:  

If p < α then reject H0. If p > α then accept H0 or reserve judgment.

(Mathews 2005, pp. 46)

Figure 2.17: Relationship between x, its corresponding z-value and the p-value. (Mathews 2005, p. 47) A helpful way to think about the p-value is to interpret it as the probability of obtaining the observed test statistic if the null hypothesis was true. This means that when p is small, the


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2 Fundaments

observed statistic would be a rare event if H0 was true, so H0 is more likely to be false. If p is large, a value like the observed statistic is an expected result if H0 was true, so we should accept H0 or reserve judgment. (Mathews 2005, pp. 47–48) Although interpreting the p-value as a probability can be useful for the understanding of this concept, this interpretation should not be taken literally and some aspects must be clarified. Both a test and the p-value can only provide a yes/no decision: either H0 is rejected or not. In this context, hypotheses are true or false and no probability is assigned to them. In other words, a decision based on this method has equal credibility in every case that p is smaller than α, either if the p-value is very small or big enough to be close to the value of α. (Heumann et al. 2016, p. 215) 2.4.2.3 One-way analysis of variance (ANOVA) An Analysis of variance (ANOVA) is used to test the hypothesis that the means of two or more populations are equal. The null hypothesis assures that all population means are equal while the alternative hypothesis assures that at least one is different. This method is really helpful to summarize the effect of categorical variables (Heumann et al. 2016, p. 274) To ensure valid results, the following guidelines should be considered:  The data should include only one categorical factor.  The response variable should be continuous.  Each observation should be independent from all other observations.  Sample data should be from a normal population or each sample should be greater than 15. Otherwise the results can be misleading. (Minitab Inc. 2016) Most statistics software packages are able to perform an analysis of variance. A table is frequently shown which should contain the following key results: 

P-value: The p-value is used to define whether a statistically significant difference between group means exists. If p is smaller than or equal to α, the null hypothesis can be rejected and the conclusion that not all population means are equal can be drawn. If p is greater than α, there is not enough evidence to reject the null hypothesis and judgment should be reserved.  Interval plot: The interval plot displays the mean and confidence interval for each group. The results shown in this plot are important for multiple comparison tests.  Multiple comparison tests: If the p-value is smaller than α, that means that some of the group means are different. The problem is that it is unknown which pairs of groups have different means. For this purpose, multiple comparison tests help us to determine whether the mean difference between specific pairs of groups are statistically significant and to estimate by how much they are different.  Goodness-of-fit values: These values are used to determine how well the model fits the data and to assess how well the model predicts the response for new observations. The most common results are S, R-sq and R-sq (pred). (Heumann et al. 2016, pp. 274–275)


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3

3 State of the art

State of the art

One of the fields that present the highest number of challenges in the industry is the field of joining technologies. Despite the many improvements made in this field, a joint will always represent a weak spot in a structure. For this reason, different toughening mechanisms for FRP joints have been developed. A brief explanation of the most trending methods and recent progress in this field is presented below.

3.1

Z-pinning

Z-pinning is a through-thickness localized reinforcement technique. The process consists in the insertion of thin reinforcing rods into a stack of ply layers with the help of an ultrasonic device. These thin rods are called Z-pins and they are commonly made from extruded metal wire or pultruded fibrous composite. This toughening mechanism is the only available through-thickness reinforcement technique for prepreg composites so far, although it can be also used with preforms based on dry textile fabrics. (Boisse 2011, p. 163) The main steps in the ultrasonically assisted Z-pinning process are shown schematically in Figure 3.1.

Figure 3.1: Ultrasonically assisted Z-pinning insertion method. (Qin, Ye 2015, p. 163) These kind of reinforcements have successfully been tested for the improvement of the ultimate failure strength and elongation limit of assemblies by the action of z-pins bridging the bond-line (Chang et al. 2004, pp. 615–620). The bridging effect has been shown to be more effective in delamination cracks where stress is applied normal to the plane of the crack growth (mode I), than in delamination cracks where a shear stress acts parallel to the plane of the crack growth and perpendicular to the crack front (mode II). Figure 3.2 offers a schematic overview of the three different types of crack modes. Inclining the z-pins such that the angle between the longitudinal axis of the pin and the shear force load vector is reduced, has recently been proved to be an efficient way to improve the Mode II delamination resistance of Z-pins. (M'membe et al. 2016, pp. 565–572)


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3 State of the art

Figure 3.2: Fracture crack modes (Philipp et al. 2013, p. 2)

3.2

Z-anchoring

Z-anchoring is a process which involves punching thin rods through a fabric preform to create zbinders. In contrast to other through-thickness reinforcement techniques which use separate yarns to create the z-binders, z-anchoring uses the in-plane yarns of the fabric preform to create the z-binders. After the rods have crimped the in-plane fibers they are removed, leaving behind through-thickness z-binders called z-anchors. When the process is complete, the fabric is then infused with liquid resin using conventional liquid molding processes. (Boisse 2011, p. 181)

Figure 3.3: Z-anchoring process (Boisse 2011, p. 181) Different studies have demonstrated that the mode I interlaminar fracture toughness was improved effectively by the z-anchor reinforcement. Experimental results assure that the fracture toughness of the composite increases almost linearly with the z-anchor density (T. Kusaka, M. Hojo, T. Fukuoka, M. Ishibashi 2006, pp. 1105–1111).Other examinations report that the fatigue resistance and static mode I interlaminar fracture toughness values for z-anchor reinforced laminates were 3.4 to 5 times higher than those without z-anchor reinforcement (Hojo et al. 2010, pp. 37–45). cand. mach. Ivan Guerrero

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3 State of the art

Z-pinning and z-anchoring act in a similar fashion; none of them prevent actual crack initiation, but both are effective, to different extents, in reducing or even preventing delamination crack growth. These toughening mechanisms need to be appreciated in the context of damage tolerance of structures rather than the prevention of the first failure. (Qin, Ye 2015, p. 161)

3.3

Metallic Arrow-Pin Reinforcement

This toughening mechanism is a contribution from a German research team. Aiming at an increase in failure resistance of composite T-joints, a metallic plate with arrow-pins has been proposed as reinforcement. This creates a hybrid joint which benefits from the plasticity of the reinforcing pins and the mechanical interlocking effect of the arrow heads under pull-out loads. (Heimbs et al. 2014, pp. 16–18)

Figure 3.4: CAD-model of the metallic arrow-pin reinforcement (Heimbs et al. 2014, p. 18) Pull-out tests were performed with T-joints reinforced with the metallic reinforcement under quasi-static and high-rate dynamic loading conditions. The results of a comparison between a conventional monolithic composite T-joint without trough-thickness reinforcement and the reinforced joint showed an impressive improvement in energy absorption up to 720%. (Cloud et al. 2017, pp. 36–38)

Figure 3.5: Composite T-joint with metallic reinforcement (Heimbs et al. 2014, p. 17)


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4

4 Own approach

Own approach

As already mentioned, researchers find a lot of challenges in the field of joining technologies. In this work a novel reinforcement technique for composite joints in through-thickness direction has been proposed. The formulated hypothesis affirms that the metallic insert with arrow pins will prevent delamination crack growth and will have a positive effect on the mechanical properties of bonded joints. In order to prove the efficiency of this toughening element, the scientific method will be used. In this thesis different types of joint designs will be examined. The procedure model of this work will be explained below. Question

Formulate hypothesis: The metallic insert will have a positive effect on the mechanic properties of the joint.

Background research

Experiment planning: -Manufacturing process selection -Designs for the joints -Ply-book generation -Testing method selection -Analysis methods selection

Manufacture specimens: -Textile fabric cutting -Preform assembly -VARIM-process -Specimen cutting

Single lap shear test

Analyze results and draw conclusions: -Statistical analysis -Breaking area analysis

Report results

Figure 4.1: Procedure model for this thesis. cand. mach. Ivan Guerrero

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4 Own approach

After the hypothesis was proposed, a background research phase was executed to make decisions about the tools that where going to be used in further stages of the work. At the beginning of the research phase, a suitable manufacturing process for the specimens was selected. Then, from a selection of different designs, five types of joints were chosen and an appropriate lay-up for the plies of the specimens was calculated. After that, the single lap shear test was elected as testing method. Finally, a statistical method for data analysis, in this case the ANOVA-test, was chosen. During the manufacturing stage, the plies for the preforms where modeled with a CAD-software called “NX” and a ply-book was generated. Then, the textile fabric was cut with help of a cuttingtable and the sub-preforms were assembled by hand. Finally, through a VARIM-process, FRPplates were manufactured and the specimens were cut with the final dimensions of 30 X120 X 2 millimeters. In the next stage, a single lap shear test was performed and the resulting data was collected for further analysis. After that, during the analysis phase, a statistical analysis of the data with the software-package “Minitab 18” was carried out and an examination of the breaking area was performed in order to characterize the different types of joints and therefore discover the effects of the insert in the mechanic properties of the bond. The different stages of this work will be discussed in further depth in chapter 5.


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5 5.1

5 Results

Results Experiment planning

The specimens were manufactured in the facilities of the “wbk Institute of Production Science” according to the norm DIN EN 1465. For this purpose, different composite plates where made using a VARIM-process. As matrix material the epoxy resin “NEUKADUR EP 986” in combination with the hardener “NEUKADUR Härter 244” was chosen. As reinforcing fibers, a dry textile woven fabric with plain weave called “SIGRATEX CW305-PL1/1” was selected. It had been agreed that the plates would have the dimensions of 150 x 150 x 2 millimeters, a bonding area of 25 mm and a fiber volume ratio between 45% and 55%. The calculations for the number of n plies needed for the production of the plates where made according to the following formula found in the book “Konstruieren mit Faser-kunststoffverbunden” from Helmut Schürmann:

𝑛 = 𝑡𝑠𝑜𝑙𝑙 ∗ 𝜑 ∗

𝜌𝑓 𝑚𝑓 ( ) 𝐿∗𝑏

Formula 5-1

With: 𝑡𝑠𝑜𝑙𝑙 = 𝑊𝑎𝑙𝑙 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝜑 = 𝐹𝑖𝑏𝑒𝑟 𝑣𝑜𝑙𝑢𝑚𝑒𝑛 𝑟𝑎𝑡𝑖𝑜 𝜌𝑓=𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑏𝑒𝑟𝑠 𝑚𝑓 ( ) = 𝐴𝑟𝑒𝑎𝑙 𝑤𝑒𝑖𝑔ℎ𝑡 𝐿∗𝑏 (Schürmann 2008, pp. 161–164) In this case the given thickness of the walls is 2 mm. The density of the fibers is equal to 1.8 g/cm3 and the areal weight is 305 g/m2. According to this data, the number of plies n to be stacked is equal to 6 with a fiber volume ratio of 49.8%. The stacking sequence (Figure 5.1) for all preforms and sub-preforms was: (0/90; ±45; 0/90; 0/90; ±45; 0/90). This sequence was taken from the design guidelines explained in Figure 5.2, where the laminate configuration should be as close as possible to a quasi-isotropic pattern.

0/90

±45

Figure 5.1: Stacking sequence of the preforms.


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5 Results

Figure 5.2: Desirable lay-ups for bolted composite joints (Military Aircraft Systems Division 1997, 12-2) As indicated, five different types of plates where manufactured: 

Design 0 is a conventional co-cured composite joint without through-thickness reinforcement. This type of joint acts as a reference of an in-service technology.



Design 1 has the same features as design 0, with the difference that this joint has one ply less in the bonding area, with a total of 5, and is reinforced with the metallic insert and the support arrow-pins.



Design 2 is built in a similar way to design 1, except that in this design the arrow-pins are not bent out.



Design 3 is a monolithic plate. This type specimen acts as a reference of the best-casescenario.



Design 4 is reinforced just like design 1, with the difference that this joint has total of 6 plies in the bonding area and the surface of the insert was treated according to the norm EN 13887.

In the following table the types of joints investigated in this work are presented schematically: Table 1: Overview of composite joint designs considered in this work. Design 0


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5 Results

Design 1

Design 2

Design 3

Design 4

After all the plates were manufactured, they had to be cut in single test specimens with the dimensions of 30 X 150 X 2 millimeters according to the norm DIN EN 1465 as shown in Figure 5.3. The specimens had the following identification numbers:

20170719 - 3 - 1 - 0 - 3 Date of production

Batch number

Type of joint

Specimen number

Place in the mold Although the standard recommends an amount of 5 test specimens, a total of 16 specimens for each joint design were manufactured in order to reduce the dispersion of the obtained data. Figure 5.4 shows a schematic diagram of the process for the manufacturing of the final test specimens.


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wbk 5 Results

2 mm

Institute of Production Science

30 mm 150 mm Figure 5.3: Schematic illustration of a test specimen with its final dimensions.

Figure 5.4: Brief outline of the specimen manufacturing.

5.2

Lap shear test

The lap shear test of the specimens was performed in the facilities of the “IAM Institute for Applied Materials”, for this purpose the UTM “Zwicker 200” was used. The guidelines of the normative DIN EN 1465 have been taken into account for the execution of the test and the report of the results. According to this standard, three different parameters for the analysis of the joints should be considered. The first one is the peak force, which is the highest force that the joint can withstand before break. The second is called tensile strength and it is the result of dividing the cand. mach. Ivan Guerrero

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5 Results

peak force by the transversal area of the specimen. Finally, the tensile shear strength is equal to the peak force divided by the bonding area of the specimen. It’s important to remember that type 3 joints are composed of a monolithic body and a calculation of the tensile shear strength is not possible. In the following sections a summary the of the lap shear test results for every type of joint will be presented in form of a chart. In addition, the particular load-strain curve of different test specimens will be displayed. For aesthetic purposes, only a few curves where plotted, however each one acts as a good representative of all the specimens manufactured from the same test-plate.

5.2.1

Type 0

In Table 2 a summary of the results of the lap shear test for joints of the type 0 is displayed. Figure 5.5 shows the particular load-strain curve of four different test specimens, each one as a representative of each one of the four plates which were used for the production of the specimens.

Figure 5.5: Load-strain curves for different type 0 joints. Table 2: Summary of the lap shear test results for type 0 joints. Probe Code

Type of joint

Peak force

Tensile Strength

Tensile shear strength

[kN]

[MPa]

[MPa]

20170713-1-1-0-1

0

16,964

169,645

19,123

20170713-1-1-0-2

0

14,142

188,329

15,819

20170713-1-1-0-3

0

13,771

185,687

16,135

20170713-1-1-0-4

0

12,692

166,791

14,862


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5 Results

20170719-3-1-0-1

0

14,354

226,674

16,940

20170719-3-1-0-2

0

14,866

234,164

17,295

20170719-3-1-0-3

0

14,455

228,746

17,085

20170719-3-1-0-4

0

13,876

159,174

16,514

20170719-3-4-0-1

0

12,264

176,098

18,490

20170719-3-4-0-2

0

12,850

184,020

18,892

20170719-3-4-0-3

0

13,510

192,586

20,029

20170719-3-4-0-4

0

11,524

163,363

16,691

20170721-4-3-0-1

0

14,505

179,827

17,890

20170721-4-3-0-2

0

14,952

232,232

18,751

20170721-4-3-0-3

0

15,625

245,995

19,881

20170721-4-3-0-4

0

15,700

252,729

19,675

5.2.2

Type 1


Figure 5.6: Load-strain curves for different type 1 joints.


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5 Results

Table 3: Summary of the lap shear test results for type 1 joints. Probe Code

Type of joint

Peak force

Tensile strength


[kN]

[MPa]

[MPa]

20170721-4-1-1-1

1

10,513

168,34

12,595

20170721-4-1-1-2

1

10,622

173,063

13,011

20170721-4-1-1-3

1

10,039

160,36

11,912

20170721-4-1-1-4

1

11,041

128,122

12,689

20170721-4-4-1-1

1

10,774

249,549

13,206

20170721-4-4-1-2

1

12,324

159,629

14,943

20170721-4-4-1-3

1

10,411

179,848

12,636

20170721-4-4-1-4

1

8,018

151,745

9,924

20170731-5-2-1-1

1

7,249

108,868

9,337

20170731-5-2-1-2

1

7,208

106,451

9,171

20170731-5-2-1-3

1

7,779

113,925

10,253

20170731-5-2-1-4

1

8,295

123,536

11,093

20170731-5-3-1-1

1

6,381

103,496

8,548

20170731-5-3-1-2

1

5,703

93,266

7,722

20170731-5-3-1-3

1

7,587

122,421

10,136

20170731-5-3-1-4

1

7,032

113,491

9,351

5.2.3

Type 2



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5 Results


Type of joint

Peak force

Tensile Strength


[kN]

[MPa]

[MPa]

20170807-6-2-2-1

2

5,877

87,952

8,546648645

20170807-6-2-2-2

2

6,116

90,625

8,806411035

20170807-6-2-2-3

2

4,685

70,328

6,549349357

20170807-6-2-2-4

2

4,366

64,253

6,014546208

20170807-6-3-2-1

2

8,232

134,732

11,39609775

20170807-6-3-2-2

2

7,803

127,000

10,79502182

20170807-6-3-2-3

2

7,992

130,987

11,13390424

20170807-6-3-2-4

2

7,652

123,991

10,56508915

20170808-7-1-2-1

2

8,277

135,867

10,92372438

20170808-7-1-2-2

2

8,133

134,475

10,81183338

20170808-7-1-2-3

2

7,229

119,588

9,638854817

20170808-7-1-2-4

2

7,223

119,045

9,571222751

20170808-7-4-2-1

2

6,409

98,3296

8,535011261

20170808-7-4-2-2

2

7,188

110,131

9,603432124

20170808-7-4-2-3

2

7,471

114,058

9,991538417

20170808-7-4-2-4

2

8,226

124,594

10,91446802


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5.2.4

5 Results

Type 3

In Table 5 a summary of the results of the lap shear test for joints of the type 3 is displayed. Figure 5.8 shows the particular load-strain curve of a test specimen.

Figure 5.8: Load-strain curve for a type 3 joint.

Table 5: Summary of the lap shear test results for type 3 joints. Probe Code

Type of joint

Peak force

Tensile Strength


[kN]

[MPa]

[MPa]

20170807-6-1-3-1

3

40,413871

531,7615

20170808-7-2-3-3

3

46,526359

696,736

5.2.5

Type 4



Page 35


5 Results


Type of joint

Peak force

Tensile Strength


[kN]

[MPa]

[MPa]

20170823-8-1-4-1

4

8,645

122,727

11,481

20170823-8-1-4-2

4

9,782

140,342

13,051

20170823-8-1-4-3

4

8,602

134,998

11,771

20170823-8-1-4-4

4

7,864

105,603

10,475

20170823-8-2-4-1

4

6,995

94,020

9,402

20170823-8-2-4-2

4

6,951

89,716

9,258

20170823-8-2-4-3

4

7,402

94,762

9,893

20170823-8-2-4-4

4

7,107

90,6836

9,539

20170823-8-3-4-1

4

7,693

103,651

9,886

20170823-8-3-4-2

4

8,536

121,

11,010

20170823-8-3-4-3

4

7,558

107,408

9,626

20170823-8-3-4-4

4

8,548

130,042

11,003

20170823-8-4-4-1

4

7,061

87,9491

9,428

20170823-8-4-4-2

4

8,909

116,819

12,289

20170823-8-4-4-3

4

9,053

118,934

12,131

20170823-8-4-4-4

4

8,901

121,181

11,924


Page 36


5 Results

0

1

3

2

4

Figure 5.10: Breaking area of the bonded joints after the lap shear test.

5.2.6

Descriptive analysis

In order to offer a better overall assessment of the test results, descriptive statistics have been used to analyze each category. The individual value plot and the interval plot have been created with the statistical software “Minitab 18”. The mean of the resulting values, the standard deviation, as well as the 95% confidence interval for each category have been calculated with the abovementioned formulas in chapter 2.4.1 and they are presented in the following section.


Page 37


5 Results

5.2.6.1 Peak force Table 7: Descriptive analysis of the peak force for each type of joint. Type of Joint

N

Mean

StDev

0

95% CI

[kN]

[kN]

16

14,129

1,386

(13,369. 14,888)

1

16

8,811

1,977

(8,052. 9,571)

2

16

7,055

1,238

(6,296. 7,815)

3

2

43,47

4,32

(41,32. 45,62)

4

16

8,101

0,879

(7,342. 8,860)

Individual Value Plot of Peak force [kN] vs Type of Joint 50

(a)

Peak force [kN]

40

30

20

10

0 0

1

2

3

4

Type of Joint

Interval Plot of Peak force [kN] vs Type of Joint 95% CI for the Mean

50

(b)

Peak force [kN]

40

30

20

10

0

1

2

3

4

Type of Joint The pooled standard deviation is used to calculate the intervals.

Figure 5.11: Individual value plot (a) and interval plot (b) for the peak force.


Page 38


5 Results

5.2.6.2 Tensile strength Table 8: Descriptive analysis of the tensile strength for each type of joint. Type of Joint

N

Mean

StDev

0

95% CI

[MPa]

[MPa]

16

203,49

33,82

(188,28. 218,69)

1

16

136,65

30,05

(121,45. 151,85)

2

16

111,62

22,85

(96,42. 126,83)

3

2

614,2

116,7

(571,2. 657,3)

4

16

111,26

16,91

(96,05. 126,46)

Individual Value Plot of Tensile Strength [MPa] vs Type of Joint 700

(a)

Tensile Strength [MPa]

600 500 400 300 200 100 0 0

1

2

3

4

Type of Joint

Interval Plot of Tensile Strength [MPa] vs Type of Joint 95% CI for the Mean

700

(b)

Tensile Strength [MPa]

600 500 400 300 200 100 0

1

2

3

4


Figure 5.12: Individual value plot (a) and interval plot (b) for the tensile strength.


Page 39


5 Results

5.2.6.3 Tensile shear strength Table 9: Descriptive analysis of the tensile shear strength for each type of joint. Type of Joint

N

Mean

StDev

0

95% CI

[MPa]

[MPa]

16

17,755

1,567

(16,941. 18,569)

1

16

11,033

2,023

(10,219. 11,848)

2

16

9,612

1,593

(8,798. 10,426)

4

16

10,761

1,231

(9,947. 11,575)

Individual Value Plot of Tensile Shear Strength[MPa] vs Type of Joint

(a)

Tensile Shear Strength[MPa]

20,0

17,5

15,0

12,5

10,0

7,5

5,0 0

1

2

4

Type of Joint

Interval Plot of Tensile Shear Strength[MPa] vs Type of Joint 95% CI for the Mean

(b)

Tensile Shear Strength[MPa]

20 18

16 14

12

10

0

1

2

4


Figure 5.13: Individual value plot (a) and interval plot (b) for the tensile shear strength.


Page 40


5 Results

5.2.6.4 Maximum elongation Table 10: Descriptive analysis of the maximum elongation for each type of joint. Type of Joint

N

Mean [%]

StDev [%]

95% CI

0

16

3,6939

0,3339

(3,3926. 3,9951)

1

16

2,270

0,563

(1,969. 2,571)

2

16

2,0400

0,3857

(1,7388. 2,3413)

3

2

9,55

3,35

(8,69. 10,40)

4

16

2,1903

0,387

(1,8891. 2,4916)

Individual Value Plot of Strain [%] vs Type of Joint 12

10

(a)

Strain [%]

8

6

4

2

0 0

1

2

3

4

Type of Joint

Interval Plot of Strain [%] vs Type of Joint 95% CI for the Mean

11 10 9

(b)

Strain [%]

8 7 6 5 4 3 2 0

1

2

3

4


Figure 5.14: Individual value plot (a) and interval plot (b) for the maximum elongation.


Page 41


6

6 Assessment

Assessment

Although the data displayed in chapter 5 can throw some suggestions about the effect of the metallic insert on the mechanical properties of the joint with just a quick check, it is necessary to proof that the differences between the means of the results really exist and they are not just a product of chance. For this purpose, the acquired data has to be tested for statistical significance with help of inferential statistics. In the following section an ANOVA-test will be performed with the data from chapter 5 to proof that an effect of the insert on the joint exists and an analysis of the failure mode will be executed in order to find some hints about the bad performance of the reinforced joints.

6.1

Analysis of variance (ANOVA-test)

The following ANOVA-tests were performed with the software “Minitab 18”.

6.1.1

Tensile strength

Table 11: Results of the analysis of variance for the tensile strength. Null hypothesis

All means are equal

Alternative hypothesis

Not all means are equal

Significance level

α = 0,05

Factor Information Factor

Levels

Values

Type of Joint

5

0. 1. 2. 3. 4

Analysis of Variance Source

DF

Adj SS

Adj MS

F-Value

P-Value

Type of Joint

4

525540

131385

142,03

0,000

Error

61

56426

925

Total

65

581966 Model Summary

S

R-sq

R-sq(adj)

R-sq(pred)

30,4142

90,30%

89,67%

82,28%

Tukey Pairwise Comparisons


Type of Joint

N

Mean

Grouping

3

2

614,2

A

0

16

203,49

1

16

136,65

B C

Page 42


6 Assessment

2

16

111,62

C

4 16 111,26 C Means that do not share a letter are significantly different.

Figure 6.1: Tuckey Pairwise Comparison plot.

According to the results of the one-way ANOVA-test, the p-value is less than 0.05. This result indicates that the mean differences are statistically significant. To find which pair of groups has different means a Tuckey Pairwise Comparison was performed. The result of this comparison is a 95% confidence interval which covers the real value of the difference between two means. The graph and the table show that the confidence intervals for the difference between the means of 1 - 0, 2 – 0, 3 – 0, and 4 – 0 do not contain zero. Therefore, the difference between these means is significant. The difference between the type 3 joint and the other joints was expected and has been confirmed with help of this method. The confidence intervals for the remaining pairs of means all include zero, which indicates that the differences are not significant and a statement can’t be drown from the data. With this information we can assure that joints of the type 0 have better mechanical properties and they are able to withstand a bigger amount of stress, in terms of tensile strength, than joints from the type 1, 2 and 4. This means that the hypothesis proposed at the beginning of this work is not true and the metallic insert has a negative impact on the tensile strength of the bonded joint. A significant difference between the joints of the type 1, 2 and 4 can’t be confirmed. However, the difference between the means of the results is so small that a further analysis has no practical value. Hence, the results might suggest that the arrow-pins or the surface treatment of the insert has no substantial effect in the properties of the joint.


Page 43


6.1.2

6 Assessment


Table 12: Results of the analysis of variance for the tensile shear strength. Null hypothesis

All means are equal

Alternative hypothesis

Not all means are equal

Significance level

α = 0,05

Factor Information Factor

Levels

Values

Type of Joint

4

0. 1. 2. 4

Analysis of Variance Source

DF

Adj SS

Adj MS

F-Value

P-Value

Type of Joint

3

655,3

218,417

82,40

0,000

Error

60

159,0

2,651

Total

63

814,3 Model Summary

S

R-sq

R-sq(adj)

R-sq(pred)

1,62808

80,47%

79,49%

77,78%

Tukey Pairwise Comparisons Type of Joint

N

Mean

Grouping

0

16

17,755

1

16

11,033

B

4

16

10,761

B

A

2 16 9,612 B Means that do not share a letter are significantly different.

According to the results of the one-way ANOVA-test, the p-value is less than 0.05. This result indicates that the mean differences are statistically significant. Like before, to find which pair of groups has different means a Tuckey Pairwise Comparison was performed. The graph and the table show that the confidence intervals for the difference between the means of 1 - 0, 2 – 0, 3 – 0, and 4 – 0 do not contain zero. Therefore, the difference between these means is significant. The confidence intervals for the remaining pairs of means all include zero, which indicates that the differences are not significant and a statement can’t be drown from the data. With this information we can assure that joints of the type 0 have better mechanical properties and can withstand a bigger amount of stress, in terms of tensile shear strength, than joints from the type 1, 2 and 4. This means that the hypothesis proposed at the beginning of this work is not true and the metallic insert has a negative impact on the tensile shear strength of the bonded joint.


Page 44


6 Assessment

Just like in section 6.1.1, a significant difference between the joints of the type 1,2 and 4 can’t be confirmed. However, the difference between the means of the results is so small that a further analysis has no practical value. With this in mind, the results might suggest that the arrow-pins or the surface treatment of the insert has no substantial effect in the properties of the joint.


6.2

Breaking area analysis

This analysis was performed with a light microscope in the facilities of the “wbk Institute of Production Science”. The damaged area of the bonded joints was observed and some conclusions were drawn from the images displayed in the following sections. It is important to mention that the specimens of the type 3 where tested only as an example of the “best-casescenario” and since it has been analyzed in-depth in other works, a detailed analysis of the breaking area for type 3 joints was considered not necessary.


Page 45


6.2.1

6 Assessment

Type 0

Figure 6.3: Close-up of the breaking area of a type 0 joint. In Figure 6.3, images of the damaged area between the two adherends of a type 0 joint are displayed. The images show an irregular surface with rests of cracked resin. This suggests a failure within the thin resin layer that acts as adhesive, meaning a cohesive failure. As already discussed in the section 2.3.2, a cohesive failure is always desired over other failure modes because it is a sign of a proper bonding process and the joint exhibits the best mechanical properties. Therefore, it is not surprising to see that this type of joint exhibits the greatest results on the lap shear test compared to specimens of the type 1, 2 and 4.


Page 46


6.2.2

6 Assessment

Type 1

A

B

C

Figure 6.4: Close-up of the breaking area of a type 1 joint. In Figure 6.4, images of the damaged area of a type 1 joint are displayed. It is important to clarify that this type of joints did not brake completely after the lap shear test and its adherends had to be separated manually for an analysis under the microscope. Some of the marks seen in the surface of the damaged area are a result of this process. With a quick check of the top left image, it is evident that there are two different regions on the surface of bonding area. The first one is the interface between the fibers and the metallic plate; the other one is the interface between the fibers of the two preforms, located in the gaps around the arrow pins. A close-up of these two regions is shown in image “A” and “B”. Image “B” shows an irregular surface in the area where the fibers of the two preforms have contact with clear signs of resin cracking. This suggests a failure within the thin resin layer and is characteristic of cohesive failure. Picture “A” shows a regular surface with no visible marks of cracking or rests of the metallic adherend on the interface between the sub-preform and the metallic plate. After an analysis of the surface of the metallic plate, displayed in picture “C”, it is clear that the surface is clean of resin remnants. The lack of cracking of the resin, the absence of rests of adhesive on the surface of the metal and the lack of fibers bonded to the insert are clear signs of adhesive failure on the interface between fibers and metal.


Page 47


6.2.3

6 Assessment

Type 2

D

E

F

Figure 6.5: Close-up of the breaking area of a type 2 joint. In Figure 6.5, images of the damaged area between one adherend and the metallic insert are displayed. Two different regions can be noticed on the top left image. The first one is the interface between the fibers and the metallic plate; the other one is the region where the gaps of the insert were located and allowed the contact of the fibers of the two preforms. A close-up of these two regions is shown in image “D” and “E”. Picture “D” presents a regular surface with no traces of cracked resin. Besides that, an analysis of picture “F” reveals a complete separation of the resin layer from the steel. This suggests an adhesive failure in the region where the metal insert and the fibers where bonded. On the other hand, image “E” shows an irregular surface which is a product of the cracking of the resin. This suggests a failure within the thin resin layer. This is a sign of cohesive failure in the gaps of the insert where the fibers of the two preforms have contact. As already discussed in the section 2.3.2, an adhesive failure is undesired because it is a sign of improper bonding process or improper surface treatment. The bad adherence of the adhesive to the steel and the lack of a surface treatment of the insert can be the reason of the bad performance of these types of specimens in the lap shear test.


Page 48


6.2.4

6 Assessment

Type 4

G

H

I

Figure 6.6: Close-up of the breaking area of a type 4 joint. In Figure 6.6, images of the damaged area of a type 4 joint are displayed. It is important to clarify that this type of joints did not brake completely after the lap shear test and its adherends had to be separated manually for further analysis. Some of the marks seen in the surface of the damaged area are a result of this process. Like in the other images of joints with this toughening mechanism, two different regions can be noticed on the surface of the sub-preforms. The first one is the interface between the fibers and the metallic plate; the other one is the interface between the fibers of the two preforms, located in the gaps around the arrow pins. A close-up of these two regions is shown in image “G” and “H”. Image “H” shows an irregular surface in the area where the fibers of the two preforms have contact. This suggests a failure within the thin resin layer and is a sign of cohesive failure. Although picture “G” doesn’t shows a regular surface, there are no signs of resin cracking whatsoever. After an analysis of the surface of the metallic plate, displayed in picture “I”, it is clear that the lines on the surface of the resin are a reproduction of the patterns on the surface of the insert. These lines are a product of the surface treatment of the metallic sheet prior to the bonding process. The lack of cracking of the resin, the absence of rests of adhesive on the surface of the metal and the lack of fibers bonded to the insert suggests an adhesive failure on the interface between fibers and metal.


Page 49


7 Summary and Outlook

Summary and Outlook

7 7.1

Summary

In this work, the efficiency of a novel toughening mechanism for joints of sub-preform assemblies was tested. For this purpose, different types of joints were examined with help of a lap shear test. From the experimental investigations, different conclusions were derived, and they are summarized in the following points: 1. The failure mechanism of the specimens that didn’t have an insert was entirely cohesive. 2. The failure mechanism of the specimens with insert was cohesive where the gaps of the metallic sheet allowed a contact of the fibers of the two preforms and adhesive in the interface between fibers and metal. 3. From all the investigated specimens excluding specimens of the type 3, the samples from the type 0 had the best performance in terms of failure resistance and maximum elongation. 4. The metallic insert had a negative impact on the tensile strength, tensile shear strength and the maximum elongation of the specimens. This is attributed to the bad adherence of the resin to the metallic surface. 5. Neither the failure resistance nor the maximum elongation was significantly affected by the action of the arrow-pins or the surface treatment of the insert.

7.2

Outlook

For a further work, it would be interesting to analyze the configuration displayed in Figure 7.1. In this design the insert is placed between the plies of one of the sub-preforms in the expectation to reduce de adhesion problem in the bond line. Also, a finite element analysis of the joints could be helpful in the understanding of the effect of the interlock between the arrow-pins and the fibers and could provide answers to the uncertainty of why the pins didn’t have a substantial influence in the test results.

Figure 7.1: Proposed design for further examinations.


Page 50


List of Figures Figure 1.1: CAD-model of the metallic insert.

1

Figure 2.1: CFRP body of a BMW i3 in a production plant located in Leipzig, Germany. (Jacob 2014, p. 18) 3 Figure 2.2: Different textile architectures (Rozant et al. 2000)

6

Figure 2.3: Preform manufactured with a 3D-knitting process (Neitzel, Mitschang 2004, p. 80)

7

Figure 2.4: Sewed T-Profile preform 2010, p. 231)

(AVK – Industrievereinigung Verstärkte Kunststoffe e. V 8

Figure 2.5: Resin Transfer Molding (RTM) (Neitzel, Mitschang 2004, p. 279) Figure 2.6: Vacuum assisted resin infusion molding (VARIM) (Neitzel 2014, p. 365)

9 10

Figure 2.7: Vacuum Assisted Resin Transfer Molding (VARTM) (Neitzel, Mitschang 2004, p. 280) 10 Figure 2.8: Classification of joining technologies for FRP (Neitzel 2014, p. 469)

11

Figure 2.9: Common bolted joint and force flow within the structural components. (Schürmann 2008, p. 530) 12 Figure 2.10: Different types of bonded joints (Neitzel 2014, p. 490)

12

Figure 2.11: Experimental set-up for the tensile test of a co-cured single lap joint specimen. In this case the adherends are FRP and steel. (Shin et al. 2000, p. 129) 13 Figure 2.12: Principle of the single lap shear test (Fraunhofer-Institut für Fertigungstechnik und Angewandte Materialforschung IFAM) 14 Figure 2.13: Possible failure modes of bonded joints. (Banea, da Silva 2016, p. 9)

15

Figure 2.14: The figure shows the normal distribution of two samples. Although the both groups of observations have the same mean, the data represented by the taller graph suggests a higher concentration of values around the mean. (Amity Global Business) 17 Figure 2.15: Normal distribution and the cumulative normal probability distribution (Mathews 2005, p. 27) 18 Figure 2.16: Sample mean and 95% confidence intervals for 6 random samples. (Heumann et al. 2016, p. 197) 19 Figure 2.17: Relationship between x, its corresponding z-value and the p-value. (Mathews 2005, p. 47) 20 Figure 3.1: Ultrasonically assisted Z-pinning insertion method. (Qin, Ye 2015, p. 163)

22

Figure 3.2: Fracture crack modes (Philipp et al. 2013, p. 2)

23

Figure 3.3: Z-anchoring process (Boisse 2011, p. 181)

23

Figure 3.4: CAD-model of the metallic arrow-pin reinforcement (Heimbs et al. 2014, p. 18)

24

Figure 3.5: Composite T-joint with metallic reinforcement (Heimbs et al. 2014, p. 17)

24

Figure 4.1: Procedure model for this thesis.

25

cand. Mach. Ivan Guerrero

Page I


Figure 5.1: Stacking sequence of the preforms.

27

Figure 5.2: Desirable lay-ups for bolted composite joints (Military Aircraft Systems Division 1997, 12-2) 28 Figure 5.3: Schematic illustration of a test specimen with its final dimensions.

30

Figure 5.4: Brief outline of the specimen manufacturing.

30


31


32


34

Figure 5.8: Load-strain curve for a type 3 joint.

35


36

Figure 5.10: Breaking area of the bonded joints after the lap shear test.

37

Figure 5.11: Individual value plot (a) and interval plot (b) for the peak force.

38

Figure 5.12: Individual value plot (a) and interval plot (b) for the tensile strength.

39

Figure 5.13: Individual value plot (a) and interval plot (b) for the tensile shear strength.

40

Figure 5.14: Individual value plot (a) and interval plot (b) for the maximum elongation.

41


43


45

Figure 6.3: Close-up of the breaking area of a type 0 joint.

46


47


48


49

Figure 7.1: Proposed design for further examinations.

50


Page II


List of Tables Table 1: Overview of composite joint designs considered in this work.

28

Table 2: Summary of the lap shear test results for type 0 joints.

31


33


34


35


36

Table 7: Descriptive analysis of the peak force for each type of joint.

38

Table 8: Descriptive analysis of the tensile strength for each type of joint.

39

Table 9: Descriptive analysis of the tensile shear strength for each type of joint.

40

Table 10: Descriptive analysis of the maximum elongation for each type of joint.

41

Table 11: Results of the analysis of variance for the tensile strength.

42

Table 12: Results of the analysis of variance for the tensile shear strength.

44


Page III


8

Publication bibliography

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