A cohesive zone model (CZM) has been used in conjunction with both elastic and elastoplastic continuum behaviour to predict the response of a mixed mode flexure and three different lap shear joints, all manufactured with the same adhesive. It was found that, for a specific dissipated CZM energy (Gamma(0)) there was a range of CZM tripping tractions (sigma(u)) that gave a fairly constant failure load. A value of sigma(u) below this range gave rise to global damage throughout the bonded region before any crack propagation initiated. A value above this range gave rise to a discontinuous process zone, which resulted in failure loads that were strongly dependent on sigma(u). A discontinuous process zone gives rise to mesh dependent results. The CZM parameters used in the predictions were determined from the experimental fracture mechanics specimen test data. When damage initiated, a deviation from the linear load-displacement curve was observed. The value for sigma(u) was determined by identifying the magnitude that gave rise to the experimentally observed deviation. The CZM energy (Gamma(0)) was then obtained by correlating the simulated load-crack length response with corresponding experimental data. The R-curve behaviour seen with increasing crack length was successfully simulated when adhesive plasticity was included in the constitutive model of the adhesive layer. This was also seen to enhance the prediction of the lap shear specimens. Excellent correlation was found between the experimental and predicted joint strengths

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It is now well established that adhesives and fibre-reinforced polymers (FRPs) can fail at far lower loads under fatigue loading than under quasi-static loading. Most of the work on the fatigue of these materials has been under constant amplitude (CA); however, recent work has shown that fatigue crack growth acceleration and unstable crack growth can be seen under conditions of variableamplitude (VA) fatigue. In this paper, further evidence of this effect is given. This phenomenon is attributed to the influence of VA fatigue on the evolution of a damage zone ahead of the main crack front and results from the characterization of these damage zones are also presented. Finally, a simple fracture mechanics-based model for predicting crack growth under VA fatigue is proposed. This is shown to be entirely consistent with the experimental observations. NOTATIONa crack length da=dN fatigue crack growth rate C, m Paris law constants D Miner's damage sum FCG fatigue crack growth G strain energy release rate G arr critical value of G for crack arrest under quasistatic loading G C critical value of G for fast crack growth under quasi-static loading G max maximum value of G in a fatigue cycle G min minimum value of G in a fatigue cycle N number of fatigue cycles N C number of cycles to crack jump in variableamplitude fatigue N f number of cycles to failure N OL number of overload cycles N R ratio of overloads to constant-amplitude cycles R OL ratio of ÁG for constant-amplitude cycles to ÁG for overload cycles ¼ ÁG CA =ÁG OL R stress ratio ¼ min = max R displacement ratio ¼ min = max max maximum displacement in fatigue min minimum displacement in fatigue ÁG strain energy release rate range ¼ G max ÿ G min ÁG A applied value of ÁG ÁG AC critical value of ÁG A for unstable crack growth ÁG CA ÁG for constant-amplitude cycles in variableamplitude fatigue ÁG OL ÁG for overload cycles in variable-amplitude fatigue ÁG th threshold value of ÁG for negligible crack growth ÁK stress intensity factor range max maximum stress in fatigue loading min minimum stress in fatigue loading damage parameter quantifying a shift in the constant-amplitude fatigue crack growth curve to account for load history E