Micromechanical modeling of rate-dependent off-axis failure in thermoplastic composites

A micromechanical framework for modeling failure in unidirectional (UD) thermoplastic composites under rate-dependent off-axis loading is presented, with the aim to predict and analyze transverse matrix cracking under various load conditions. The onset of global softening in the micromodel corresponds to macroscopic matrix crack initiation. The problem addressed in this study is to include matrix plasticity and microcracking in the failure analysis of UD composites. A thin slice representative volume element (RVE) with periodic boundary conditions is used, which enables representation of 3D stress states. The testing conditions of a constant prescribed strain-rate and an off-axis uniaxial stress state are reproduced in the model with a dedicated arclength control method. The studied material system is carbon/PEEK composite material, where plasticity in the matrix is represented with the Eindhoven Glassy Polymer (EGP) constitutive law, while the fibers are modeled as transversely isotropic elastic material. In order to account for microcracking in the matrix, a cohesive surface methodology is applied. Cohesive elements are added on the fly with a stress-based initiation criterion. For this purpose, a power law microcrack initiation criterion is proposed. After initiation, the microcracking process is governed by a mixed-mode damage cohesive law. Geometric nonlinear effects are also included in the cohesive model, such that cohesive forces include material as well as geometric contributions. The model is validated with experimental data from tensile tests on UD material at different off-axis angles and strain-rates. The obtained maximum stress levels are used to generate Tsai-Hill failure envelopes for macroscopic transverse crack initiation. Additional capabilities of the model are demonstrated through examples with different fiber-volume ratios and temperature conditions