From S-N to the Paris law with a new mixed-mode cohesive fatigue model for delamination in composites

Abstract

The relationship between fatigue life and fatigue crack propagation rate in composites is explored with a new cohesive damage model. The parameters of the model are obtained from idealizations of S-N diagrams used in engineering design. The model assumes that the quasi-static cohesive law that describes tearing is the envelope of the fatigue damage. Fatigue damage within the cohesive envelope accumulates at a rate that satisfies the S-N diagram and Miner’s cumulative fatigue damage rule. The fatigue model was implemented as a UMAT subroutine for Abaqus cohesive elements by adding fatigue damage accumulation within a cohesive model based on the Turon mixed-mode model. The analyses were conducted using a simplified cyclic loading procedure in which the maximum load is applied quasi-statically and load cycling is represented within the constitutive model. The predicted propagation rates of delamination in mode I and mixed mode were compared to experimental results for IM7/8552 graphite/epoxy tape. Several aspects of the results were investigated, including the effects of R-curves, the stress ratio R, and the difference between displacement and force control.