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Procedia Engineering 10 (2011) 3252–3258

ICM11

Nano-engineered composites: a multiscale approach for adding toughness to fibre reinforced composites. Larissa Gorbatikh, Stepan V. Lomov, Ignaas Verpoest* Department of Metallurgy and materials Engineering, Katholieke Universiteit Leuven, Belgium

Abstract Fibre reinforced composites, and certainly carbon fibre composites (CFRP’s), are champions in combining stiffness, strength and low weight. This unique set of properties has resulted in a strong increase of the use of composites in airplanes (up to 50% of their structural weight), and supports the expectation that CFRP’s will soon make a breakthrough in automotive and other transport applications. One of the major limitations to the use of composites in primary structures is the rather low stress or strain threshold for damage initiation. Under static tensile loading, first matrix cracks in 0/90 or quasi-isotropic laminates appear at strains as low as 0.4%, whereas the final failure strain is as high as 1.5 to 2%. This early damage initiation can be further linked to the fatigue limit of UD-based laminates and, more generally, to the damage tolerance of textile based composites. In this research, different ways have been explored to increase the damage threshold of carbon and glass fibre reinforced composites by adding carbon nanotubes (CNT’s) to either the matrix or the fibre sizing, or by growing them on the fibre surface. The effect of CNT’s is not straightforward: some damage related properties are unaffected, but the interfacial shear strength, the interlaminar fracture toughness and the damage initiation threshold during tensile loading are strongly increased by adding CNT’s. © 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of ICM11

Keywords: composite materials, carbon nanotubes, carbon fibres, glass fibres, nano-engineering

1. Introduction Fibre reinforced composites are combining the excellent stiffness and strength of fibres with the lightness of polymers. For many applications, glass fibre composites (GFRP’s) offer the best cost/performance balance, and hence GFRP’s dominate the composites market, at least on a volume basis. Carbon fibres are more ‘sophisticated‘ than glass fibres: the oriented graphene layer structure gives them a three to five times higher stiffness, while maintaining a similar or even higher strength. This makes carbon fibre composites (CFRP’s) the champions in lightweight structural materials, because of their unparalleled combination of stiffness, strength and lightness. This unique set of properties has resulted in a strong increase of the use of carbon fibre composites in airplanes (up to 50% of their structural weight), and supports the expectation that CFRP’s will soon make a breakthrough in automotive and other transport applications.

* Corresponding author. Tel.: +32-16-321306; fax: +32-16-321990. E-mail address: [email protected]

1877-7058 © 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of ICM11 doi:10.1016/j.proeng.2011.04.537

Larissa Gorbatikh et al. / Procedia Engineering 10 (2011) 3252–3258

One of the major limitations to the use of composites in primary structures is the rather low stress or strain threshold for damage initiation. In typical carbon fibre reinforced epoxy composites, under static tensile loading, the first matrix cracks in cross-ply (0/90) or quasi-isotropic laminates appear at strains as low as 0.4%, and in woven textile based composites sometimes as low as 0.2%, whereas the final failure strain is as high as 1.5 to 2% [1]. This early damage initiation can be further linked to the fatigue limit of UD-based laminates and, more generally, to the damage tolerance of textile based composites. Carbon nanotubes (CNTs) have excited the materials science community over the past decade. Their excellent mechanical properties have created the expectation that they might replace carbon fibres as reinforcement in polymer composites. Their nanometer size, enabling a defect free tubular graphene structure, is at the same time a major drawback. When used as “fillers” for polymers, the maximum attainable volume fractions are limited to a few percent. Whereas this is largely sufficient for making polymers electrically conductive, the increase in mechanical properties is rather limited. On the contrary, reaching high strength and stiffness with micron-sized carbon fibres, made with the ‘traditional’ PAN-based manufacturing process, is much easier than with carbon nanotubes, grown in a CVD or arc discharge process. Hence, the idea has been raised to use carbon nanotubes as an additional reinforcement in carbon or glass fibre composites. In this way, a two-level reinforcement morphology is created; such composites will hereafter be named ‘nano-engineered composites’.

Property control

Meso Fabric architecture

stiffness Micro Fibers inside yarns Cracks in transverse ply

Debonding at matrix/fiber interface

Nano

failure resistance

Structure of the matrix Multiphase matrix

Carbon nanotubes in the matrix

Figure 1: schematic representation of the influence of the three morphological length scales on stiffness and failure resistance in fibre reinforced composites

The underlying hypotheses of the research reported in this overview paper, are illustrated in figure 1: the stiffness of a fibre reinforced composite is controlled on the meso- and micro-level. On the meso-level, the effect of the inplane and out-of-plane fibre orientation can be modeled using a combination of morphological descriptions (like WiseTex for textile based composites, [2]) and mesomechanical models (like TexComp, [2]). On the micro-level, the reinforcement can be described as a unidirectionally reinforced composite element, and it’s stiffness is controlled by the fibre volume fraction. In contrast, the initiation of damage during static, dynamic or cyclic loading is additionally, and mainly, influenced by the (ir)regularity of the spatial fibre distribution on one hand, and by the fibre, matrix and interphase properties on the other hand. It is on this level that the nano-scale starts playing a role. Existing examples are phase separated or rubber particle toughened epoxies and thermoplastic matrices with specific crystallite morphology (like transcrystallinity on the fibre surface). Also on this level, carbon nanotubes could play a role in controlling damage initiation and development in fibre reinforced composites. In this research, the carbon nanotubes will be located, in a controlled way, in three positions: x Included in the fibre sizing

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x Dispersed in the matrix x A combination of both. The effect of the addition of minor amounts of carbon nanotubes on these three positions will be evaluated for a number of mechanical properties that quantify the damage initiation and development in fibre reinforced composites, namely x The interlaminar fracture toughness, G1C, measured by DCB tests x Interfacial shear strength (IFSS), measured using a single fiber push-out test x Damage initiation and development under tensile loading of a cross-ply laminate, assisted with quantitative damage registration using acoustic emission, X-ray and SEM studies.

2. Influence of carbon nanotubes, dispersed in the matrix, on the interlaminar fracture toughness

2.1. Materials, manufacturing and testing procedures A drumwinder was used for the in-house production of the prepregs (Figure 2, left). The epoxy resin, based on diglycidylether of bisphenol A (DGEBA) formulated for hot-melt prepreg processing with hardener dicyandiamide (DICY, Aradur-5021) was impregnated into the carbon fibre tows, and wound on a rotating drum. From each wound drum, a prepreg sheet of 30 cm wide and 200 cm long can be cut. The glass transition temperature of the neat resin was ~90 °C and density ~1.18 g/cc. The carbon fibers (CFs) were supplied by Toho Tenax with 800 tex and tensile modulus of 235 GPa. Multiwalled carbon nanotube (MWCNT), thin-MWCNT (TMWCNT) and double-walled CNT (DWCNT) amine group functionalized were produced and supplied by Nanocyl (Belgium), already dispersed in an epoxy masterbatch, resulting in a relatively low CNT concentration (0.5 wt.%) in the epoxy matrix and about 0.2–0.25 wt.% in the final three phase composite. The unidirectional laminates were produced in an autoclave at vacuum í WR íEDUV DQG ZHUH FXUHG DW 120° for 60 min followed by a post curing step at 140 °C for 120 min. Interlaminar fracture toughness tests in order to determine G1C were carried out according to ASTM D5528. More details about the experimental procedures, and about other thermo-mechanical properties of this type of nanoengineered composites can be found in [3].

Drum winder

Prepregger

RTM

Figure 2: processing equipment for nano-engineered composites

2.2. Results and discussion Figure 3 shows the results of the mode I interlaminar fracture toughness tests. An important increase of the interlaminar fracture toughness, both for initiation and for propagation, was found for UD-carbon fibre composites containing ~ 0.25 w% multiwalled carbon nanotubes (MWCNT). The increase is even higher (+40% resp. +25%)


for the thinner MWCNT (TMWCNTs) based composites. This is possibly due to the greater effectiveness of thinner CNTs in penetrating the fibre yarns and avoiding the filtering effect, thereby providing a better microstructural homogeneity. Similar results of the increased toughness are obtained by the functionalized DWCNT, where covalent bonding helped in creating a much more stable and homogenous network of CNT’s in the matrix. However, formation of such network could also lead to stiffening of the matrix and to loss of the intrinsic toughness. The higher rigidity of the matrix could also explain the low values for DWCNT in comparison with TMWCNT and modified-MWCNT based composites. The combination of the compatibilizer with non-functionalized MWCNTs seems to be optimal for the dispersion and stability of the system throughout the process. The increased toughness values by +75% resp. 83% suggest an optimal dispersion and interfacial interaction and at the same time adequate intrinsic toughening properties of the used matrix.

Figure 3: effect on the initiation and propagation fracture toughness of the addition of ~0.25 wt% multiwall (MWCNT), thin multiwall (TMWCNT), amine functionalised double wall (DWCNT) and compatibiliser modified multiwall (MWCNT-modified) carbon nanotubes to the epoxy matrix of carbon fibre UD-composites

3. influence of the position of carbon nanotubes on the interfacial shear strength of nano-engineerd composites

3.1. Materials, manufacturing and testing procedures In a second series of tests, carbon nanotubes have been dispersed in the matrix, in the sizing, or in both. Multiwall, amine group functionalized carbon nanotubes (MWCNTs) were produced and supplied by Nanocyl (Belgium). The nanotubes have an average diameter of 9.5 nm, specific surface area of 250–300 m2/g and carbon purity >90%. Commercially available E-glass fibers have a diameter 15 μm, and a Young modulus of 72 GPa. These fibers have a commercial sizing and will be referred to further in the text as ‘‘virgin glass fibers” (VGF). An epoxy resin based on diglycidylether of bisphenol A (DGEBA) formulated for hotmelt prepreg processing with hardener dicyandiamide (Aradur-5021) was used as a matrix. The Young modulus is 3 GPa, and the tensile strength 82 MPa. The epoxy matrix without nanotubes is referred to below as EP. The glass fibers were directly coated with the MWCNT containing sizing without removal of the commercial sizing (Fig. 4, left). This was done by drawing the fibers through a water dissolved, epoxy-compatible phenoxybased formulation containing 0.5 wt.% MWCNTs and by subsequent drying them at 120 C. These fibers will be referred to further in the text as CNT-sized glass fibers or SGF. The obtained CNT-sizing was non-uniform and had

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a thickness of 1–2 μm. Alternatively, the MWCNTs were dispersed in the epoxy resin, at a concentration of 0.5 wt.%. A homogeneous dispersion was achieved using calendaring equipment consisting of three rolls which generate high shearing forces by controlling the spacing between the rolls and their speed. 1 450 150

1 2

360 120

2

270 90

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180 60

IFSS increase

CNTs are inside the sizing

90

30

Virgin Glass Fibers

Sized Glass Fibers (CNTs)

0

(a)

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0

VGF/EP

SGF/EP VGF/EPNT SGF/EPNT

Figure 4: bobbins of virgin glass fibres (left) and glass fibres covered with a carbon nanotube containing sizing (middle); interfacial shear strength of (1) CNT-sized glass fibres in epoxy (SGF/EP), (2) virgin glass fibres in CNT-epoxy (VGF/EPNT), (3) CNT-sized glass fibres in CNT-epoxy (SGF/EP), and (4) virgin glass fibres in epoxy (VGF/EP)

The interfacial shear strength was measured using the microindentation test method [4], and were carried out by dr. Kalinka at the BAM laboratories in Berlin. More detailed information about the test method and about the materials used can be found in [5,6], where also a more extended discussion of the experimental results is presented.

3.2. Results and discussion Figure 4 (right) shows the measured interfacial shear strength values (IFSS), obtained with the micro-indentation test on the four different nano-engineered glass fibre composites. The reference value for the virgin glass fibres in epoxy, namely 57 MPa, is in line with literature data on glass-epoxy composites. Adding carbon nanotubes only to the sizing seems to be the most efficient, as the IFSS increases to almost double, namely 109 MPa, which is a very high value for glass-epoxy systems. Adding carbon nanotubes only to the matrix results in a moderate increase (84 MPa). If however carbon nanotubes are added both to the sizing and to the matrix, the IFSS drops again to 75 MPa. It is obvious that carbon nanotubes in the sizing have a strong positive influence on the interfacial shear strength. More in-depth experimental and modeling research will be needed to explicitely correlate this increase in IFSS with either an intrinsic increase of the fibre-matrix adhesion (interface effect), or to the creation of a gradient interphase. Indeed, the applied sizing is rather tick (1-2 μm). Even when some interdiffusion of the matrix into this thick sizing layer would have occurred, a non-uniform distribution of the carbon nanotubes has to be expected. This might result in a gradual transition from the stiff, isotropic glass fibres to the much weaker epoxy matrix, and hence to a decrease in the stress concentrations around the glass fibres (typically a stress concentration factor 2 to 3 under shear loading), because this stress concentration factor depends on the stiffness mismatch between fibre and matrix. The lower IFSS-values for both systems that contain carbon nanotubes in the matrix have not been fully explained yet. It is assumed that some agglomeration might have occurred, reducing the shear strength of the matrix, or at least it’s shear plasticity.


4. Influence of carbon nanotubes on the damage development during tensile loading of nano-engineered woven carbon fabric composites

4.1. Materials, manufacturing and testing procedures A master batch based on liquid Bisphenol-A (Bis-A) epoxy resin containing high concentration of CNTs from Nanocyl (Epocyl NC R128-02) was used to prepare the matrix. The CNTs used in the master batch are multi-wall CNTs (MWCNTs) that have average diameter of 9.5 nm, specific surface of 250–300 m2/g and carbon purity >90%. Epoxy resin Epicote 828VEL was used to dilute the master batch ( dilution factor 12) to obtain 0.25wt% of CNTs. The hardener Dytek® DCH-99 and diluted CNT modified epoxy were then mixed in the proportion 15/100. A carbon twill 2/2 fabric (Hexcel G986 'Injectex') was used as reinforcement. The fabric has an areal density of 300 g/m2, and is made of 6K Carbon AS4C GP carbon tows, with 3.5 yarns/cm in warp and weft. Composite plates comprising 7 plies were fabricated with the RTM process (fig. 2, right) using a spacer of 2.1 mm thickness. The materials with neat resin and the resin containing 0.25 wt% of MWCNT were produced. Samples were tested on Instron 4505 with a loadcel of 100kN. The samples were loaded at a constan (2mm/min). During these tensile tests, acoustic emissions signals, originating from microcracks, were captured and analysed. More details about the experimental procedure and an extensive discussion of the results can be found in [7.8]. 4.2. Results and discussion In figure 5, the cumulative energy curves of the acoustic emission signals, captured during tensile loading of the nano-engineered carbon fabric composites are presented as a function of the applied tensile strain. It can be clearly observed that the onset of microdamage (indicated by sharp increase of the cumulative AE events curve) is shifted towards higher strains by about 0.2% strain, which constitutes almost a doubling of the damage thresholds.

virgin

CNT modified

Figure 5: cumulative AE-energy as a function of tensile strain Figure 6: microcracking in transverse fibre bundles: (a) in nano-engineered composites, (b) in virgin composite

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The increase of the damage threshold can be correlated with a change in microcrack pattern. In the virgin composites (not containing CNT’s), the microcracks are sharp, whereas in the CNT-containing composites, a diffuse crack pattern can be observed; at some fibre distances away from the main crack, additional microcracks can be found, creating a ‘damage zone’ around the main crack. Such damage zone could relax the intensity of the stress field around the main crack, and hence reduce the damage development rate, as suggested by the acoustic emission measurements.

5. Conclusion In this overview paper, the potential of using carbon nanotubes as a second, nano-scale reinforcement for (micron-scale) fibre reinforced composites has been explored. Three examples have been presented, suggesting that the beneficial effects of adding carbon nanotubes on damage related properties are larger than the effect found on neat epoxy resins reinforced with carbon nanotubes. In the latter, a very limited increase in stiffness, strength or toughness is usually observed. In this paper, an up to 80% increase in interlaminar fracture toughness of UD-carbon fibre composites, a doubling of the interfacial shear strength in glass fibre composites and a doubling of the damage threshold during tensile loading of carbon fabric composites have been observed. Hence, there seems to exist a synergetic effect between the micro-reinforcement of the (glass or carbon) fibres and the nano-reinforcement of the carbon nanotubes. Ongoing research at the Composite Materials Group of K.U.Leuven explores the mechanisms that might be responsible for this synergy.

ACKNOWLEDGEMENTS The authors would like to acknowledge the company Nanocyl SA (Belgium) for providing the carbon nanotubes and dr. Kalinka (BAM, Germany) for carrying out the micro-indentation experiments.

REFERENCES [1] S. V. Lomov, D. S. Ivanov, T. C. Truong, I. Verpoest, F. Baudry, K. Vanden Bosche, H. Xie, Experimental methodology of study of damage initiation and development in textile composites in unaxial tensile testComposites Science and Technology, Vol. 68, Issue 12, 2008, pp. 23402349 [2] I. Verpoest, S.V. Lomov, Virtual textile composites software Wisetex: integration with micro-mechanical, permeability and structural analysis, Composites Science and Technology, 2005; 65(15-16): 2563-2574. [3] A. Godara, L.Mezzo, F.Luizi, A. Warrier, S.V. Lomov, A.W. VanVuure, L. Gorbatikh, P.Moldenaers, I. Verpoest, Influence of carbon nanotubes reinforcement on the processing and the mechanical behaviour of carbon fiber/epoxy composites, Carbon, (2009) 47(12), 2914-2923. [4] Desaeger M, Verpoest I. On the use of the micro-indentation test technique to measure the interfacial shear strength of fiber-reinforced polymer composites. Compos Sci Technol 1993;48:215. [5 A.Warrier, O.Rochez, A.Godara, L.Mezzo, F.Luizi, L.Gorbatikh, S.Lomov, A. VanVuure, I.Verpoest,, The effect of adding carbon nanotubes to glass/epoxy composites in the fibre sizing and/or the matrix, Composites Part A 41/4 (2010) 532–538 [6] A. Godara, L. Gorbatikh, G. Kalinka, A. Warrier, O. Rochez, L. Mezzo, F. Luizi , A.W. van Vuure, S.V. Lomov, Verpoest: Interfacial shear strenght of a glass fiber/epoxy bo,ding in composites modified with carbon nanotubes, Composites Science and Technology, Vol.70, Issue 9, September 2010, pp. 1346-1352 [7] De Greef N, Gorbatikh L, Godara A, Mezzo L, Lomov SV, Verpoest I. Potential of carbon nanotubes to hinder damage development in carbon fiber/epoxy composites. Carbon (under review). [8] N. De Greef, L. Gorbatikh*, S. V.Lomov, I.Verpoest, Damage development in woven carbon fiber/epoxy composites modified with carbon nanotubes under tension in the bias direction. Submitted to Composites A