A physically based micromechanical approach to model damage initiation and evolution of fiber reinforced polymers under fatigue loading conditions

The hypothesis of this work is that the fatigue behavior of a composite material is governed by its matrix. By characterizing and modeling the quasi-static and cyclic behavior of the pure polymer matrix, the transverse crack initiation and evolution of a composite under fatigue loading can be studied on a micromechanical level. Extensive characterization of the epoxy resin system Araldite LY564/Aradur22962 is conducted with special emphasis on the hystersis energy. A novel physically based fatigue failure criterion for polymers under multiaxial loading conditions is derived from these experimental results. To overcome the limitations of experimental accuracy and scatter, a compensation procedure is presented.For the incorporation in a micromechanical analysis, a viscoplastic material model from the literature is modified and utilized. A linear viscous network of Maxwell elements is compared with a nonlinear approach. It is found that even though the results show an indication of viscous nonlinearity, the linear network is capable of capturing the cyclic response with sufficient accuracy. For both models, a multiaxial generalization and a calibration procedure is presented in order to incorporate the material model in the commercial finite element software Abaqus.With the implementation of the material model and the developed failure criterion, a micromechanical model of a fiber reinforced polymer is set up. With the developed fatigue modeling framework, the damage initiation and evolution are evaluated using data available in the literature. The damage behavior is in good qualitative agreement with the reported mechanisms proving the general suitability of the failure criterion.