Fatigue life prediction of z-fibre pinned composite laminate under mode I loading

A hybrid method is presented combining linear elastic fracture mechanics with nonlinear damage mechanics that can predict the fatigue crack growth rate in z-fibre pinned composites under mode I loading. The strain energy release rate is evaluated using the virtual crack closure technique via finite element analysis. Cohesive elements are used in the pinned region to represent the crack bridging force generated by the pins. The reduction of the pins' bridging force under the fatigue loading is accommodated by applying a degradation law, based on damage mechanics with empirical fitting parameters. A modified degradation law is proposed which is capable of accumulating fatigue damage under varying crack opening displacement ranges experienced by the pins during fatigue loading. Fatigue testing was performed with a z-pinned double cantilever beam at two different values of applied displacement amplitude. The predictions show reasonably good agreement with the test results in terms of the fatigue crack propagation rate and fatigue life.