New material modeling approaches for thermoplastics, composites and organic sheet

In this paper, new anisotropic elastic-viscoplastic constitutive models for simulating thermoplastic materials, endless fiber reinforced composites and organic sheets are presented. The anisotropic material models address the same main features as the isotropic SAMP model (MAT_187 in LS-DYNA). These are in particular pressure dependent yielding allowing for different yielding in tension, compression, shear and biaxial loadings, tabulated input of hardening data for each stress state and a non-associated flow rule for a correct prediction of the volumetric plastic strains. Hence, the anisotropic material models represent a consistent further development of the isotropic SAMP material model (SAMP-1 or MAT_187 in LS-DYNA 971). ) The anisotropy is incorporated by an invariant formulation using structural tensors. This provides interesting modeling techniques for short fiber reinforced thermoplastics and for organic sheets. When modeling short fiber reinforced thermoplastics, the fiber orientation tensor is directly integrated into the constitutive equations and an automated homogenization is performed. That is, the fiber orientation tensor “weights” the structural tensors representing the preferred directions and in the limiting case “all fiber directions equally distributed in all directions”, the isotropic SAMP model is recovered as a special case. When modeling organic sheets, the finite fiber rotations observed under certain loading conditions can be simulated. That is, an initial misalignment of the yarns due to the draping process and also a loading induced misalignment of the yarns due to the forming process can be incorporated easily, letting the structural tensors rotate against each other. The applicability of the anisotropic material models will be shown with three examples. First, simulation results of a short fiber reinforced polymer PA6GF60 are presented. These are in particular the material characterization tests (tensile, compression and shear tests) and quasi-static and dynamic 4a-Impetus bending tests. Secondly, simulation results of quasi-static and dynamic off-axis compression tests of a carbon epoxy IM7-8552 are presented, predicting the experimentally observed pre-failure nonlinearities. Finally, the applicability of the anisotropic model to organic sheets is discussed. The experimentally observed highly non-linear behavior under shear dominated loadings due to the finite fiber rotations and the quasi brittle behavior in uniaxial tension and compression in the main directions can be predicted. In future developments, the whole process chain drape simulation, forming simulation and crash simulation will be addressed.