A failure model for SFRP for FEM simulations is developed to describe the strain rate dependency, the influence of the local fiber orientation and of the stress state on the failure behavior. The material is considered as a continuum while internally calculating the micro-mechanics analytically. The described micro-mechanics are based on experimental observations and on analyzation with numerical studies. In particular the strain rate dependent delamination of fibers and matrix is incorporated in the model. The distortion energy density is defined as the driving value for failure and estimated by the model. This is achieved with the analytic solution by Eshelby for the stress field in the matrix and by introducing an additional phase for the plasticly deformed volume. The validation on characterization specimens as well as component test demonstrates that the influence of strain rate, fiber orientation, and stress state on the failure behavior can be described with only one material parameter, the critical distortion energy density.
For predicting the strain rate dependent failure of short fiber reinforced plastics (SFRP) a two-phase simulation model is developed using the finite element method and comparing the results to microscopic specimen tests for uniaxial tension under quasi-static (0.007/s) and dynamic loads (250/s). Experimentally the failure behavior of SFRP is observed to be strain rate dependent. The global strain at failure and the absorbed energy increase with strain rate. Moreover, locally an influence of the strain rate on the amount of material involved in the deformation can be observed. The suggested model can represent these effects accurately. Also, the present micro-mechanical effects and their influence on the strain distribution are investigated by unit cell simulations. Thereby the material model of the fibers, the matrix, and the boundary layer are varied respectively. These reveal the important role of strain rate dependent decohesion leading to a correct representation of the plastically deformed volume. Consequently, the distortion energy density is evaluated and is found to be constant at failure for all strain rates.
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