Micromechanical modeling of the nonlinear deformation of LFTS under consideration of the effects of interface damage

This work deals with the micromechanical finite element simulation of long fiber reinforced thermoplastics (LFT) under incorporation of the nonlinear deformation behavior of the matrix and the effects of fiber-matrix interface damage. The fiber-matrix debonding behavior is determined experimentally by single fiber push-out tests on thin slices, prepared from the cross section of the material. To extract the characteristics of the numerical interface model from the experimental curves, the push-out scenario is simulated and the properties of the interface are reversely determined in such a way that the experimental results of the push-out tests can be reproduced. The resulting values are then fed into a micromechanical finite element model of a LFT structure, described based on experimentally measured fiber orientation and length distributions. After implementation into the microstructural LFT model, the effects of interface damage can be captured in conjunction with the complex interactions on the material's microscale which arise from the locally varying fiber orientation, length and density. Thus, a mechanism-based interpretation of the stress-strain curve of macroscopic tensile tests on LFT specimens is enabled by comparison with the respective simulation results. The mechanisms considered are the plastic deformation of the matrix, the fiber-matrix interface debonding, the post-debonding friction and the brittle failure of the fibers. The potential increase in the material's strength and fracture strain by enhancement of its interface strength can finally be assessed by comparison of the simulations which account for a realistic and an imaginary, perfect interface.