Scaling of fracture response in Over-height Compact Tension tests

An experimental investigation into in-plane scaled Over-height Compact Tension (OCT) [45/90/−45/0]_4s carbon/epoxy laminates was carried out to study the scaling of fracture response. The dimensions of the baseline specimens were scaled up and down by a factor of 2. Interrupted tests were carried out for specimens of each size in which the tests were stopped after certain load drops in order to study the failure mechanisms. X-ray Computed Tomography (CT) scanning was applied after the interrupted tests to examine the damage development and its effect on the fracture response. The test results showed that the scaling of the initial propagation of fracture follows Linear Elastic Fracture Mechanics (LEFM), but the development of the damage process zone differs with specimen sizes. The OCT specimens were found to be not large enough to generate a self-similar damage zone during propagation, and so no conclusions could be drawn regarding the R-curve effect.     

A comparative study of fatigue behaviour of flax/epoxy and glass/epoxy composites

Experimental investigations on flax and glass fabrics reinforced epoxy specimens, i.e. FFRE and GFRE, submitted to fatigue tests are presented in this paper. Samples having [0/90]_3S and [±45]_3S stacking sequences, with similar fibre volume fractions have been tested under tension–tension fatigue loading. The specific stress-number of cycles to failure (S–N) curves, show that for the [0/90]_3S specimens, FFRE have lower fatigue endurance than GFRE, but the [±45]_3S FFRE specimens offer better specific fatigue endurance than similar GFRE, in the studied life range (<2 × 10⁶). Overall, the three-stage stiffness degradation is observed in all cases except for [0/90]_3S FFRE specimens, which present a stiffening phenomenon of around 2–3% which could be related to the straightening of the microfibrils.     

Compression creep rupture behavior of a glass/vinyl ester composite laminate subject to fire loading conditions

The growing use of polymer matrix composites in civil infrastructure, marine and military applications provides the impetus for developing mechanical models to describe their response under combined mechanical and fire loading. A viscoelastic stress analysis using classical lamination theory is conducted on an E-glass/vinyl ester composite. The model includes a characterization of the non-linear thermo-viscoelasticity and its inclusion into a compression strength failure criterion for the prediction of laminate failure under combined compressive load and temperature profile simulating fire exposure. By accounting for the viscoelastic non-linearity at T_g, the proposed model yields good predictions for lifetimes of the studied composite ([0/+45/90/−45/0]_S).     

Monotonic properties of unidirectional glass fiber (U)/random glass fiber (R)/epoxy hybrid composites

The main objective of the present paper is to study the tensile and bending behaviors of unidirectional glass fiber (U)/random glass fiber (R)/epoxy hybrid composites with total fiber volume fraction (V_fT) = 37%. Six kinds of laminated composites of average thickness 5.5 mm were manufactured using hand lay-up technique; i.e. [R]₅, [U/R/U/R/U], [U/0.5R/U]_S, [0.5R/U/U]_S, [U/U/0.5R]_S, and [U]₅. In bending test, notched and unnotched specimens were tested. For this purpose different circular notch sizes (D = 3, 6, 9 mm) were drilled at the specimen center. Tensile strength, tensile modulus, Poisson’s ratio, bending strength and bending modulus were determined experimentally. The effect of stacking sequences, random fiber relative volume fraction (V_fR/V_fT), and notch diameter on the mechanical properties of the mentioned composite types were studied. Failure modes of all specimens were investigated.     

Fracture criterion for kinking cracks in a tri-material adhesively bonded joint under mixed mode loading

The fracture behavior of a composite/adhesive/steel bonded joint was investigated by using double cantilever beam specimens. A starter crack is embedded at the steel/adhesive interface by inserting Teflon tape. The composite adherend is a random carbon fiber reinforced vinyl ester resin composite while the other adherend is cold rolled steel. The adhesive is a one-part epoxy that is heat cured. The Fernlund-Spelt mixed mode loading fixture was employed to generate five different mode mixities. Due to the dissimilar adherends, crack turning into the adhesive (or crack kinking) associated with joint failure, was observed. The bulk fracture toughness of the adhesive was measured separately by using standard compact tension specimens. The strain energy release rates for kinking cracks at the critical loads were calculated by a commercial finite element analysis software ABAQUS in conjunction with the virtual crack closure technique. Two fracture criteria related to strain energy release rates were examined. These are (1) maximum energy release rate criterion (G_max) and, (2) mode I facture criterion (G_II = 0). They are shown to be equivalent in this study. That is, crack kinking takes place at the angle close to maximum G or G_I (also minimum G_II, with a value that is approximately zero). The average value of G_IC obtained from bulk adhesive tests using compact tension specimens is shown to be an accurate indicator of the mode I fracture toughness of the kinking cracks within the adhesive layer. It is concluded that the crack in tri-material adhesively bonded joint tends to initiate into the adhesive along a path that promotes failure in pure mode I, locally.     

Retention of mechanical performance of polymer matrix composites above the glass transition temperature by vascular cooling

An actively cooled vascular polymer matrix composite containing 3.0% channel volume fraction retains greater than 90% flexural stiffness when exposed continuously to 325 °C environmental temperature. Non-cooled controls suffered complete structural failure through thermal degradation under the same conditions. Glass–epoxy composites (T_g = 152 °C) manufactured by vacuum assisted resin transfer molding contain microchannel networks of two different architectures optimized for thermal and mechanical performance. Microchannels are fabricated by vaporization of poly(lactide) fibers treated with tin(II) oxalate catalyst that are incorporated into the fiber preform prior to resin infiltration. Flexural modulus, material temperature, and heat removal rates are measured during four-point bending testing as a function of environmental temperature and coolant flow rate. Simulations validate experimental measurements and provide insight into the thermal behavior. Vascular specimens with only 1.5% channel volume fraction centered at the neutral bending axis also retained over 80% flexural stiffness at 325 °C environmental temperature.     

Analysis of impact properties of 3D-carbon-carbon composite material by the instrumented charpy test

Impact properties of 3D-carbon-carbon composite materials have been studied through fractographic analysis and failure parameters determination using instrumented Charpy testing. Different methods have been used to assess dynamic toughness. Making use of static formulae for homogeneous materials, critical stress intensity factors have been calculated at the maximum of the load-time response of the 3D-C-C notched specimens. Applying a method which takes into account the inertia effect, dynamic stress intensity factors have been determined. Comparison between the related failure parameters K_IDy, K_IDp and K_ISm and their correlation with the corresponding values of the J-integral show the reliability of the results. Gathering fractographic analysis, qualitative examination of load/time curves and dynamic toughness assessment contribute to a better understanding of rupture mechanism in 3D-C-C composite materials.     

An investigation into the damage development and residual strengths of open-hole specimens in fatigue

An extensive experimental program was carried out to investigate and understand the sequence of damage development throughout the life of open-hole composite laminates loaded in tension–tension fatigue. Quasi-isotropic carbon/epoxy laminates, with stacking sequence [45₂/90₂/−45₂/0₂]_S, [45/90/−45/0]_2S and [45/90/−45/0]_4S were examined. These were selected on the basis that under quasi-static loading the [45₂/90₂/−45₂/0₂]_S configuration exhibited a delamination dominated mode of failure whilst the [45/90/−45/0]_2S and [45/90/−45/0]_4S configurations showed a fibre dominated failure mode, previously described as “pull-out” and “brittle” respectively. Specimens were fatigue loaded to 1 × 10⁶ cycles or catastrophic failure, which ever occurred first. A number of tests were interrupted at various points as the stiffness dropped with increasing cycles, which were inspected using X-ray computed tomography (CT) scanning. A static residual strength program was carried out for run-out specimens of each configuration.     

Measurement of the fracture toughness associated with the longitudinal fibre compressive failure mode of laminated composites

The fracture toughness associated with the fibre compressive failure was obtained from testing notched unidirectional carbon/epoxy four-point-bend specimens. Microscopy of failed specimens revealed that onset of damage was characterised by the formation of a single line of fibre breaks at approximately 45° to the plane of the initial notch. A micromechanical finite element model was used to investigate this failure scenario and it was concluded that the most probable cause of the damage morphology was compression-induced shear failure of the composite. An intrinsic material property in this case was deemed to be the mode II critical strain energy release rate associated with the initiation of the 45° crack. For IM7/8552, this was measured to be G_IIc = 4.5 ± 0.8 kJ/m².     

Postbuckling strengths of composite laminate with various shaped cutouts under in-plane shear

The aim of present investigation is to study the buckling and postbuckling response and strengths under positive and negative in-plane shear loads of simply-supported composite laminate with various shaped cutouts (i.e., circular, square, diamond, elliptical-vertical and elliptical-horizontal) of various sizes using finite-element method. The FEM formulation is based on the first order shear deformation theory which incorporates geometric nonlinearity using von Karman’s assumptions. The 3-D Tsai-Hill criterion is used to predict the failure of a lamina while the onset of delamination is predicted by the interlaminar failure criterion. The effect of cutout shape, size and direction of shear load on buckling and postbuckling responses, failure loads and failure characteristics of quasi-isotropic [i.e., (+45/−45/0/90)_2s] laminate has been discussed. In addition, the effect of composite lay-up [i.e., (+45/−45/0/90)_2s, (45/−45)_4s and (0/90)_4s] has also been reported. It is observed that the cutout shape has considerable effect on the buckling and postbucking behaviour of the quasi-isotropic laminate with large size cutout. It is also observed that the direction of shear load and composite lay-up have substantial influence on strength and failure characteristics of the laminate.     

Predicting evolution of ply cracks in composite laminates subjected to biaxial loading

An energy-based model is developed to predict the evolution of sub-critical matrix crack density in symmetric multidirectional composite laminates for the case of multiaxial loading. A finite element-based numerical scheme is also developed to evaluate the critical strain energy release rate, G_Ic, associated with matrix micro-cracking, a parameter that previously required fitting with experimental data. Furthermore, the prediction scheme is improved to account for the statistical variation of G_Ic within the material volume by using a two-parameter Weibull distribution. The variation of G_Ic with increasing crack density is also accounted for based on reported experimental evidence. The simulated results for carbon/epoxy and glass/epoxy cross-ply laminates demonstrate the ability of the improved model to predict the evolution of multidirectional ply cracking. By integrating this damage evolution model with the synergistic damage mechanics approach for stiffness degradation, the stress-strain response of the studied laminates is predicted. Finally, biaxial stress envelopes for ply crack initiation and pre-determined stiffness degradation levels are predicted to serve as representative examples of stiffness-based design and failure criterion.     

Crashworthiness characteristics investigation of silk/epoxy composite square tubes

This research concentrates on the evaluation of crashworthiness characteristics of natural silk/epoxy composite square tubes energy-absorbers. Composite laminate specimens were subjected to static axial compression load and experimental evaluation of the energy absorption capability of silk/epoxy composite. Specimens were in the form of square cross-sections with the dimension of 80 mm × 80 mm and a radius curvature of 5 mm. The variables in the experiment were the length of the tubes built 50 mm, 80 mm and 120 mm. Meanwhile, the thickness of the walls, consisting of laminates of silk/epoxy of 12, 24 and 30 plies, correspond to equivalent wall thickness of 1.7 mm, 3.4 mm and 4.2 mm, respectively. The parameters measured were the total absorbed energy (E_total), and the crash force efficiency (CFE). E_total is the measure of the amount of energy that the structure can withstand without failure and thus is a measure of its strength, while CFE gives a quantitative indication of the mode of failure of the composites. The mode of failure was observed using photography.     

Compressive response and failure of fiber reinforced unidirectional composites

The compressive response of polymer matrix fiber reinforced unidirectional composites (PMC's) is investigated via a combination of experiment and analysis. The study accounts for the nonlinear constitutive response of the polymer matrix material and examines the effect of fiber geometric imperfections, fiber mechanical properties and fiber volume fraction on the measured compressive strength and compressive failure mechanism.Glass and carbon fiber reinforced unidirectional composite specimens are manufactured in-house with fiber volume fractions ranging over 1060 percent. Compression test results with these specimens show that carbon fiber composites have lower compressive strengths than glass fiber composites. Glass fiber composites demonstrate a splitting failure mode for a range of low fiber volume fractions and a simultaneous splitting/kink banding failure mode for high fiber volume fractions. Carbon fiber composites show kink banding throughout the range of fiber volume fractions examined. Nonlinear material properties of the matrix, orthotropic material properties of the carbon fiber, initial geometric fiber imperfections and nonuniform fiber volume fraction are all included in an appropriate finite element analysis to explain some of the observed experimental results. A new analytical model predictionof the splitting failure mode shows that this failure mode is favorable for glass fiber composites, which is in agreement with test results. Furthermore, this modelis able to show the influence of fiber mechanical properties, fiber volume fraction and fiber geometry on the splitting failure mode.     

Fiber direction and stacking sequence design for bicycle frame made of carbon/epoxy composite laminate

According to the maximum stress theory and the results of strength-to-stress ratios, the fiber direction and stacking sequence design for the bicycle frame made of the carbon/epoxy composite laminates have been discussed in this paper. Three testing methods for the bicycle frame, i.e. torsional, frontal, and vertical loadings, are adopted in the analysis. From the finite element results, the stacking sequences [0/90/90/0]_s and [0/90/45/−45]_s are the good designs for the composite bicycle frames. On the contrary, the uni-directional laminates, i.e. [0/0/0/0]_s, [90/90/90/90]_s, [45/45/45/45]_s and [−45/−45/−45/−45]_s, are the bad designs. In addition, weak regions of failure occur at the fillets and connections of the frame, i.e. the stress concentration regions. All weak points occur at the inner or outer layer of the laminated composite tube. The 0°-ply and 90°-ply located on the inner and outer layer of the tube can effectively resist the higher stress at its location.     

Effects of laminate carbon/epoxy composite patches on the strength of double-strap adhesive joints: Experimental and numerical analysis

In this study, mechanical properties of double-strap joints with aluminum or composite patches of different orientation angles at their overlap area were investigated under tensile loading. For this purpose, AA2024-T3 aluminum was used as adherend, while patches were either AA2024-T3 aluminum or 16-ply laminate of carbon/epoxy composite with five different orientation angles ([0]₁₆, [90]₁₆, [0/90]₈, [45/−45]₈, [0/45/−45/90]₄). A two-part paste adhesive (DP 460) was used to bond adherend and patches. Six different types of joint samples were subjected to tensile loading. The effect of patch material on failure load and stress distribution was examined experimentally and numerically. As a result, it was concluded that the data obtained from 3-D finite element analysis were coherent with experimental results and additional to that fiber orientation angles of the patches markedly affected the failure load of joints, failure mode and stress distributions appeared in adhesive and composite.     

Characterization of the in-situ non-linear shear response of laminated fiber-reinforced composites

A simple procedure to determine the non-linear in-plane lamina shear response of laminated composites is presented. Using the ±45° symmetric laminate tensile test results, in conjunction with computational micromechanics, a method was developed and validated to characterize the lamina shear response and the in-situ matrix shear response. Load, and axial and transverse strains measured in the tests were used to calculate the non-linear shear stress–shear strain response of the composite. From this result, the in-situ matrix equivalent stress–strain response was obtained, with some simplifying assumptions, and subsequently used in a micromechanics-based representative finite element (FE) model of the ±45° symmetric laminate tensile test to determine the accuracy of the non-linear response of the in-situ matrix. Results from the FE model of a representative cell (RC) that depicts fiber diameter, fiber volume fraction (V_f) and angled fiber packing of the ±45° symmetric laminate were found to match the tests result well. Thus, the procedure to extract the non-linear lamina shear response and the non-linear in-situ matrix response from the ±45° symmetric laminate tensile test was validated.     

Effect of off-axis ply orientation on 0°-fibre microbuckling

The aim of the present work is to study both experimentally and theoretically the compression failure mechanisms in multi-directional composite laminates, and especially the effect of the off-axis ply orientation on fibre microbuckling in the 0°-plies. The critical mechanism in the compressive fracture of unidirectional polymer matrix composites is plastic microbuckling/kinking. In multi-directional composites with internal 0°-plies, catastrophic failure also initiates by kinking of 0°-plies at the free-edges or manufacturing defects, followed by delamination. When 0°-plies are located at the outside, or in the case of cross-ply laminates, failure rather tends to occur by out-of-plane buckling of the 0°-plies. T800/924C carbon-fibre–epoxy laminates with a [(±θ/0₂)₂]_s lay-up are used here to study the effect of the supporting ply angle θ on the stress initiation of 0°-fibre microbuckling. Experimental data on the compressive strength of laminates with θ equal to 30, 45, 60 or 75° are compared to theoretical predictions obtained from a fibre kinking model that incorporates interlaminar shear stresses developed at the free edges at (0/θ) interfaces. Initial misalignment of the fibres and non-linear shear behaviour of the matrix are also included in the analysis.     

Mechanical properties of cross-ply and quasi-isotropic composite laminates processed using aligned multi-walled carbon nanotube/epoxy prepreg

This study examined the processing and mechanical properties of cross-ply and quasi-isotropic composite laminates processed using aligned multi-walled carbon nanotube/epoxy prepreg sheets. Three kinds of CNT/epoxy laminates, ([0°/90°]s, [60°/0°/?60°]s, [0°/45°/90°/?45°]s) were successfully fabricated using aligned CNT/epoxy prepreg sheets. The CNT volume fraction was approximately 10%. No visible void or delamination was observed in composite laminates, and the thickness of each layer was almost equal to that of the prepreg. To evaluate the elastic moduli, E₁₁, E₂₂, and G₁₂, of each ply in the laminates, on-axis and off-axis tensile tests (0°, 45°, 90°) were conducted of aligned CNT/epoxy lamina specimens. The Young’s modulus of CNT/epoxy cross-ply and quasi-isotropic laminates agreed with the theoretical values, which were calculated using classical laminate theory and elastic moduli of CNT/epoxy lamina. The respective failure strains of [0°/90°]s, [60°/0°/?60°]s, and [0°/45°/90°/?45°]s laminates are 0.65, 0.92, 0.63%, which are higher than that of 0° composite lamina (0.5%). Results suggest that the failure strain of 0° layer in composite laminates is improved because of the other layers.     

Predicting the thermal conductivity of composite materials with imperfect interfaces

This paper compares the predicted values of the thermal conductivity of a composite made using the equivalent inclusion method (EIM) and the finite element method (FEM) using representative volume elements. The effects of inclusion anisotropy, inclusion orientation distribution, thermal interface conductance, h, and inclusion dimensions have been considered. Both methods predict similar overall behaviour, whereby at high h values, the effective thermal conductivity of the composite is limited by the inclusion anisotropy, while at lower h values, the effect of anisotropy is greatly diminished due to the more dominant effect of limited heat flow across the inclusion/matrix interface. The simulation results are then used to understand why in those cases where it has been possible to produce CNF reinforced Cu matrix composites with a large volume fraction of well dispersed CNFs, the measured thermal properties of the composite have failed to meet the expectations in terms of thermal conductivity, with measured conductivities in the range 200–300 W/m K. The simulation results show that, although degradation of the thermal properties of the CNFs and a poor interfacial thermal conductance are very likely the reasons behind the low conductivities reported, great care should be taken when measuring the thermal conductivity of this new class of materials, to avoid misleading results due to anisotropic effects.     

Failure behavior of quasi-isotropic laminates with three-pin loaded holes

This paper presents the effect of hole positions on the failure behavior of glass–epoxy laminated composite plates fabricated from stacking sequence [0/90/±45]_S subjected to a traction force by three-pins. Three different hole distance parameters, namely the ratio of free edge distance to the outer holes/pin diameter (E/D = 1, 2, 3, 4, 5), the ratio of longitudinal distance between the holes/pin diameter (F/D = 2, 4, 6) and the ratio of transverse distance between the parallel holes/pin diameter (G/D = 3, 4, 5) were used to investigate the effects of hole positions on failure load and failure mode. Shear out failure mode which is not the desired mode for a structure occurs around the first and second pin holes very close to the free edge of the specimen (E/D = 1). The other specimens were damaged in bearing mode which is the most desired mode.