Influence of in-plane fibre orientation on mode I interlaminar fracture toughness of stitched glass/polyester composites

The influence of in-plane fibre orientation on the mode I interlaminar fracture toughness, G_Ic of unstitched and stitched glass/polyester composites is investigated in this paper. The G_Ic of planar specimens depends on the fibre orientation, θ in the layers adjacent to the fracture plane, in addition to the property of matrix material. The mode I fracture toughness and fracture behavior of unstitched and stitched 0/0, 30/−30, 45/−45, 60/−60, 90/90 and 0/90 interfaces of unidirectional fibre mats (UD) and 30/−30, 45/−45 and 90/90 interfaces of woven roving mats (WRM) are studied. WRM layer orientation is represented by the direction of warp fibres. Stitching is done by untwisted Kevlar fibre roving of Tex 175 g/km at the stitch densities (number of stitches per unit area) of 10.24 and 20.48 stitches/inch². The specimens having same stitch density, but different stitch distributions are prepared, and the influence of stitch distribution on G_Ic is studied. Double cantilever beam (DCB) tests are carried out and the G_Ic is determined using modified beam theory. The G_Ic of both unstitched and stitched specimens increases with increase in orientation angle, θ upto 45° above which it decreases. The G_Ic values of unstitched 45/−45 delamination interface is around 2.4 times that of the unstitched 0/0 interfaces. The influence of fibre orientation on G_Ic is clearly observed in unstitched specimens, whereas in the stitched specimens, stitching plays an important role in improving the G_Ic and suppresses the influence of fibre orientation; degree of suppression increases with increasing stitch density. When the value of θ is above 45°, transverse cracks are observed in the delamination interface surrounded by UD layers; while in the delamination interface surrounded by WRM layers, transverse cracks are not initiated irrespective of the fibre orientation angle.     

An application of the single specimen technique to a double cantilever beam specimen to determine GIC from a single test record

The single specimen technique is effectively used in classical fracture mechanics for evaluating the J-integral from a single test record. It was previously shown using a cracked lap shear specimen that the technique can be applied to composite structures and the energy release rate of composite laminates can be determined from a single test specimen. The objective of this investigation is to determine the η_el form of a double cantilever beam (DCB) specimen, and to compare it with that of a compact tension (CT) specimen. The result showed that while η_el is linearly related to crack length for the CT specimen, it is proportional to the reciprocal of the crack length for the DCB composite specimen. Also, G_Ic determined from the compliance method was compared with that determined from the η_el approach. The result showed that the η_el form determined from a single beam analysis can be used to determine G_Ic from a single test record.     

Numerical investigation to prevent crack jumping in Double Cantilever Beam tests of multidirectional composite laminates

During the experimental characterization of the mode I interlaminar fracture toughness of multidirectional composite laminates, the crack tends to migrate from the propagation plane (crack jumping) or to grow asymmetrically, invalidating the tests.The aim of this study is to check the feasibility of defining the stacking sequence of Double Cantilever Beam (DCB) specimens so that these undesired effects do not occur, leading to meaningful onset and propagation data from the tests. Accordingly, a finite element model using cohesive elements for interlaminar delamination and an intralaminar ply failure criterion are exploited here to thoroughly investigate the effect of specimen stiffness and thermal residual stresses on crack jumping and asymmetric crack growth occurring in multidirectional DCB specimens.The results show that the higher the arm bending stiffness, the lower the tendency to crack jumping and the better the crack front symmetry. This analysis raises the prospect of defining a test campaign leading to meaningful fracture toughness results (onset and propagation data) in multidirectional laminates.     

Intralaminar fatigue crack growth properties of conventional and interlayer toughened CFRP laminate under mode I loading

Intralaminar and interlaminar fatigue crack growth behaviours under mode I loading were investigated with conventional and interlayer toughened unidirectional CFRP laminates. For intralaminar crack growth tests, initial defects were introduced using “intralaminar film insertion method”, in which a release film is inserted inside a single lamina prepreg. A fatigue test under a constant maximum energy release rate, G_max, was carried out using DCB specimens. It was found that the intralaminar fatigue crack growth property of the interlayer toughened CFRP laminates was the same as that of the conventional CFRP laminates. For the interlayer toughened CFRP laminates, the G_max with a given crack growth rate, da/dN, was much lower for intralaminar crack growth than for interlaminar crack growth. The da/dN-G_max curve at zero crack extension, Δa = 0, which was estimated by extrapolating the da/dN-Δa relationship, was not affected by bridging fibres, and most conservative for the interlayer toughened CFRP laminates.     

Mixed Bending-Tension (MBT) test for mode I interlaminar and intralaminar fracture

A new Mixed Bending-Tension (MBT) test is proposed for mode I fracture of laminated composites. The MBT specimen consists of a relatively small pre-cracked laminate adhesively bonded to pin-loaded steel beams. This design reduces significantly the bending stresses that prevent successful application of DCB tests to certain laminates. The MBT was here applied to carbon/epoxy unidirectional [0°]₂₆ and [90°]₂₆ laminates with starter delaminations. Interlaminar initiation G_IC values of [0°]₂₆ laminates agreed well with previous DCB test results, while [90°]₂₆ laminates exhibited 50% higher values. Significant lengths of fairly planar intralaminar crack propagation were seen in the latter laminates. The results showed a fibre bridging related R-curve, which was more pronounced in [0°]₂₆ laminates. The consistency of the present results indicates that the MBT opens new possibilities for the interlaminar and intralaminar mode I fracture.     

Bridging effect on mode I fatigue delamination behavior in composite laminates

This paper discusses the bridging effect of fibres on mode I fatigue delamination growth in unidirectional and multidirectional polymer composite laminates based on a series of double cantilever beam (DCB) tests. From the results, there is sufficient evidence that fibre bridging can decrease the crack growth rate da/dN significantly, and using only one fatigue resistance curve to determine the delamination behavior in composite materials with large-scale fibre bridging may be inadequate. The bridging created in fatigue delamination is different from that of quasi-static delamination at the same crack length. So it is incorrect to use the resistance curve (R-curve) from quasi-static delamination tests to normalize fatigue delamination results.     

Fracture toughness of weft-knitted fabric composites

The mode I inter-laminar fracture toughness of advanced knitted textile composites was investigated. Two complex weft-knitted glass fabrics were selected for the study: a triple rib knit and a Milano knit were impregnated with a tough epoxy resin and tested using a double cantilever beam geometry. For both knitted composites, the influence of the growth direction was studied by investigating crack propagation in both the wale and course directions. The fracture toughness was quantified by determining the critical strain energy release rate (G_IC) using the modified beam theory. The specimens had to be stiffened with layers of glass woven composites added on top and bottom of the beams. This was necessary in order to avoid plastic deformation of the beams and crack deviation out of the inter-laminar plane. The results clearly showed that knitted fabric composites have exceptional inter-laminar fracture toughness properties, namely, more than 7000 J/m². The origin of the high G_IC values, which are superior to woven or UD laminates, lies in the very complex fabric architecture. The three-dimensional loop structure induces various energy consuming mechanisms, which do not occur in other composites. Toughening mechanisms such as crack branching, friction, yarn bridging and breakage were identified using scanning electron microscopy.     

Effects of the marine environment on the interfacial fracture toughness of PVC core sandwich composites

An experimental study was undertaken to investigate the facesheet/core interfacial fracture toughness of E-Glass/Vinylester facesheet, closed-cell polyvinyl chloride (PVC) core, sandwich composites. To determine the effects of a marine environment (temperature and sea-water) on conditioned specimens with a crack present, an interfacial crack was induced prior, as well as subsequent to, 5000 h of elevated temperature (80 °C), elevated temperature and moisture (80 °C, 90%+ relative humidity), and sea-water (submersed) conditioning. The interfacial fracture toughness from room temperature double cantilever beam tests for each environmental condition was then compared using the critical strain energy release rate, G_C. The G_C was reduced considerably (greater than 50%) in specimens submerged in sea-water, and significantly (approximately 90%) due to 5000 h of the ‘hot/wet’ and hot/dry exposure. Results showed that elevated temperature exposure contributes greatest to the PVC core degradation, whereas sea-water exposure mostly degrades the facesheet/core interface. Exposure to elevated temperatures, along with inducing cracks between the facesheet and a PVC core degraded by elevated temperature exposure, appear to be the most detrimental to interfacial fracture toughness.     

On compressive failure of multidirectional fibre-reinforced composites: A fractographic study

The compressive failure of multidirectional carbon fibre-reinforced composites is investigated in this paper. Cross-ply and multidirectional compact compression IM7/8552 specimens were tested to deduce the failure mechanisms that occurred during compressive loading. The experimental results and the subsequent fractographic analysis revealed that the stacking sequence had a significant effect on the performance of multidirectional composites under compression. Delamination and in-plane shear fracture were the dominant failure mechanisms both in cross-ply and multidirectional configurations. While multidirectional configurations exhibited a stiffer response and higher failure load compared to cross-ply configurations, they were also more prone to delaminations and post-failure damage. Multidirectional laminates also exhibited significantly more complex fracture morphologies, which made the failure process interpretation more difficult. The sequence of events that lead to global fracture in multidirectional fibre-reinforced composites is presented.     

Strength of carbon/epoxy laminates with countersunk hole

The aim of this study is to predict the static strength of carbon/epoxy laminates with countersunk hole. Also, three-point bend (TPB) specimens with the same lay-up were analysed. For this purpose, the notched strength of the laminates was analysed by a damage zone model (DZM), where damage around the notch is represented by an ‘equivalent crack’ with cohesive forces acting between the crack surfaces. The DZM requiring only basic properties of the laminate such as unnotched tensile strength, δ₀, fracture energy, G_c^*, and stiffnesses of the laminate. However, the complex geometry around the countersunk hole implies that both δ₀ and G_c^* will vary in this area, and in order to avoid this problem an approximate geometry of the countersunk hole is used in the DZM-calculations. With this approximation, good agreement between experimental and calculated strength was observed for the laminates with countersunk hole. This was also the case for the TPB specimens.     

Monitoring of delamination onset and growth during Mode I and Mode II interlaminar fracture tests using guided waves

A delamination monitoring method was proposed to characterize Mode I and Mode II delamination onset in carbon fiber/epoxy (CF/EP) composite laminates through interrogation of guided waves activated and captured using piezoelectric actuators and sensors in a pitch–catch configuration. Mode I and Mode II interlaminar fracture tests were conducted using double cantilever beam (DCB) and end notch flexure (ENF) specimens to evaluate the proposed method. The changes in wave propagation velocity and wave magnitude (or attenuation), and the degree of waveform similarity between excitation and response signals, were calculated as delamination-sensitive wave parameters and plotted versus displacement recorded using a materials testing system. The kink points determined from wave parameter–displacement curves agreed well with the deviation from linearity (NL), visual observation (VIS) and maximum load (Max) points, which are often used in conventional methods for determining interlaminar fracture toughness. The propagation characteristics of the A₀ wave mode in a low frequency range were demonstrated to have high sensitivity to Mode I and in particular Mode II delamination onset in CF/EP composite laminates. It was concluded that the guided waves propagating in the DCB and ENF specimens were capable of determining Mode I and Mode II interlaminar fracture toughness, complementing current practices based on visual inspection or trivial interrogation using load–displacement curve alone.     

Characterization of delamination behavior of unidirectional graphite/PEEK laminates using cracked lap shear (CLS) specimens

Delamination failure criterion is an important tool for characterizing the fracture behavior of laminated composites under mixed loading. In this paper, a fracture envelope was built based on the energy release rate as a fracture criterion of graphite/PEEK laminates. Unidirectional cracked lap shear (CLS) specimens were employed to calculate mode I and mode II energy release rates (G_I, G_II. Static fracture tests were conducted using the specimens with two different lap to strap thickness ratios in order to obtain a wide range of G_I/G_II values. The G_I/G_II values for each thickness ratio were calculated numerically using finite element analysis. The results showed that as a delamination length changes, the G_I/G_II varies from 0.13 to 0.48 depending on the lap to strap thickness ratio. It was also found that a linear fracture envelope may be appropriate for a CLS composite specimen.     

DCB test simulation of stitched CFRP laminates using interlaminar tension test results

A simulation model for the delamination extension of stitched CFRP laminates and 3-D orthogonal interlocked fabric composites (3-D OIFC) has been developed using a 2-D finite element method incorporating interlaminar tension test results to simulate the experimental results of their DCB tests. The mechanical properties of through-the-thickness fiber were determined from the results of interlaminar tension tests in which the specimen included only one through-the-thickness yarn. The fracture phenomena around the through-the-thickness thread, such as debonding from the in-plane layer, slack absorption, fiber bridging, and the pull-out of broken threads from the in-plane layers, are also introduced into the FEM model. The present FEM simulation results were compared to DCB test results for certain stitched laminates and a 3-D OIFC, and the simulation results showed good agreement with the experimental results of DCB tests, including the load–displacement curve and Mode I strain energy release rate (GI). While it was difficult to estimate GI accurately when the DCB test specimen included different types of z-fiber fracture modes, the present model of FEM analysis can simulate the experimental results of DCB tests of stitched laminates and 3-D OIFC. It is suggested that the GI of CFRP with arbitrary z-fiber densities can be predicted by using this FEM analysis model together with interlaminar tension test results.     

An investigation of non-linear elastic behavior of CFRP laminates and strain measurement using Lamb waves

This paper investigates the non-linear elastic behavior of unidirectional and cross-ply CFRP laminates and proposes a new method to measure tensile strain using Lamb waves. Young’s modulus was measured as a function of strain in situ using Lamb wave velocity during a tensile test. The stiffening effect of the carbon fibers on [0]₈ specimens and the softening effect of the epoxy matrix on [90]₈ specimens were accurately evaluated. Young’s modulus of the 0° ply was obtained as a quadratic function of strain. Using the function and the rule of mixture, the dependence of Young’s modulus on strain was accurately predicted for cross-ply laminates. Based on the results, the tensile strain was quantitatively correlated with the corresponding arrival time of the Lamb waves. The strains obtained from the proposed method agreed well with those from the strain gauge. Finally, the effect of transverse cracks on the in situ Young’s modulus of the cross-ply laminate under a tensile load was investigated. This method clearly detected even a small decrease in the Young’s modulus due to the transverse cracks in stiffening cross-ply laminate.     

Mechanics and mechanisms of delamination in a poly(ether sulphone)—Fibre composite

The interlaminar fracture behaviour of AS4/PES (poly(ether sulphone)) composite has been investigated in Mode I, Mode II and for fixed Mode I to Mode II ratios of 0·84, 1·33 and 2·13. The data obtained from these tests have been analysed using several different analytical approaches. The results obtained show that in Mode I the interlaminar crack growth in double cantilever beam (DCB) specimens is accompanied by fibre bridging behind the crack tip and by splitting at the crack tip, and in Mode II by the formation of a damage zone at the crack tip. These failure mechanisms are shown to increase the value of the interlaminar fracture energy considerably as the crack propagates through the composite, i.e. a rising ‘R-curve’ is measured. It is shown also that the value of the interlaminar fracture energy at crack initiation in Mode I, G_CI (init), increases as the length of the initial precrack is increased. The lowest G_IC (init) value obtained for the poly(ether sulphone) (PES) composite in this study is 0·8 kJm⁻², and this value was ascertained from a specimen with the precrack being grown by about 2 mm ahead of the initial crack (a₀ = 23 mm, a_p = 25 mm). The typical Mode II steady-state propagation energy, G_IIC (s/s-prop), value obtained for the specimens was about 2·0 kJm⁻². The length of the initial precrack had no significant effect on the G_IIC (init) and G_I/IIC (init) values. The Mode II tests gave values of G_IIC (init) = 1·25 kJm⁻² and of G_IIC (s/s-prop) = 1·85 kJm⁻². Finally, the failure loci for the PES composite have been constructed and theoretical expressions to describe these data considered.     

Mechanical behavior and optimization of multidirectional laminate specimens under delamination by bending

Delamination process in the multidirectional laminates under bending loading conditions is often accompanied by excessive deformations ahead of the crack tip and non-linearities due to large displacements and transverse microcracks, which appear in the most strained plies especially for thin materials. Since linear elastic fracture mechanics cannot be directly used in order to characterize delamination, our objective is to highlight, experimentally and analytically, the delamination peculiarities on DCB specimens, then to optimize specimen thickness to the value, which makes it possible to avoid the appearance of transverse cracking before starting of delamination. The material used is E-glass/epoxy multidirectional laminates. The results of optimization obtained are in good agreement with the experimental data. __________ Translated from Problemy Prochnosti, No. 6, pp. 66–78, November–December, 2006.     

XFEM simulation of delamination in composite laminates

In this paper, the extended finite element method (XFEM) is extended to simulate delamination problems in composite laminates. A crack-leading model is proposed and implemented in the ABAQUS® to discriminate different delamination morphologies, i.e., the 0°/0° interface in unidirectional laminates and the 0°/90° interface in multidirectional laminates, which accounts for both interlaminar and intralaminar crack propagation. Three typical delamination problems were simulated and verified. The results of single delamination in unidirectional laminates under pure mode I, mode II, and mixed mode I/II correspond well with the analytical solutions. The results of multiple delaminations in unidirectional laminates are in good agreement with experimental data. Finally, using a recently proposed test that characterizes the interaction of delamination and matrix cracks in cross-ply laminates, the present numerical results of the delamination migration caused by the coupled failure mechanisms are consistent with experimental observations.     

Relationship between microstructure and fracture behavior of bioabsorbable HA/PLLA composites

Hydroxyapatite particles of four different shapes, that is, micro, nano, spherical and plate, were used to fabricate hydroxyapatite filled poly(l-lactic acid) (HA/PLLA) composites. Effects of HA particle shape on the fracture behavior of HA/PLLA were investigated by mode I fracture testing, fracture surface measurement and scanning electron microscopy. It was found that the micro-HA/PLLA has the highest critical energy release rate, G_IC, with the largest surface roughness, while G_IC of the nano-HA/PLLA was lowest corresponding to the smallest surface roughness. The micro-HA/PLLA composites exhibited interfacial debonding and local ductile deformation of the PLLA matrix, indicating higher fracture energy and therefore, the highest G_IC. On the other hand, the nano-HA/PLLA composites showed brittle fracture surface due to nano-scale interaction between PLLA fibrils and primary HA particles, corresponding to lower fracture energy and hence the lowest G_IC.     

Plane strain fracture toughness of MP35N in aged and unaged conditions measured using modified CT specimens

The multiphase alloy MP35N (35% Ni, 35% Co, 10% Mo, 20% Cr) is a high strength, high toughness alloy of choice for several safety-critical applications in aerospace, oil drilling, and biomedical industries. Several previous attempts in literature to measure the plane strain fracture toughness of commercially drawn MP35N did not produce reliable values since they violated one or more of the criteria stipulated by ASTM standards for a valid measurement of K_Ic. In most cases, the requirements for plane strain and small scale yielding conditions were not met, since the commercially drawn material was available only with limited cross-sectional dimensions. In this investigation specially designed specimens (modified compact tension (CT) specimens) have been used to measure the plane strain fracture toughness of MP35N in both the unaged and the aged conditions. The K_Ic of the commercially drawn (53% reduction level) MP35N was measured to be 126 MPa√m, while that of the commercially drawn and aged MP35N was measured to be 98 MPa√m. Both these measurements satisfied all the required criteria stipulated by ASTM standards for valid measurements of K_Ic. The new procedure used in this study has been verified by the measuring of fracture toughness of Al alloy, using both the modified specimen, as designed here, and the standard one. The results for plane strain fracture toughness of MP35N alloy have been verified by the standard measurement of J_Ic values for both the aged and the unaged alloys. Finally, on a suitably normalized plot, introduced in this paper, the toughness-strength envelop for MP35 is higher than most of structural alloys, but significantly lower than that of the TRIP steels.     

Adhesive repair of bismaleimide/carbon fiber composites with bisphenol E cyanate ester

The adhesive strength and repair efficiency of bisphenol E cyanate ester (BECy) is investigated for the injection repair of high temperature polymer-matrix composites (PMCs) by lap shear (LS), short beam shear bending (SBSB), and double-cantilever beam (DCB) tests. Bismaleimide/carbon fiber (BMI-cf) composites were chosen as a model substrate. The BECy resin showed similar strength at room temperature to a benchmark epoxy adhesive and outperformed the epoxy at high temperature (200 °C) in all mechanical tests performed. The influence of moisture content of the PMC substrate on the adhesive strength of BECy was systematically investigated. Drying of PMC before repair was necessary for excellent repair performance. Both the flexural strength of repaired SBSB specimens and the inter-laminar fracture toughness of repaired DCB specimens were significantly higher than that of the control composites and stable over a broad temperature range.