Modelling of Thermo-Viscoplastic Material Behavior Coupled with Nonlocal Ductile Damage

The postcritical behaviour due to mechanical loading of the high strength steel HX340LAD (ZStE340), typically used for cold forming of complex structures is modelled by means of a yield curve in the softening part of the material. Due to local heating, caused by viscoplastic deformations particularly for high strain rates, a thermo-mechanical coupled simulation is carried out by taking into account the conversion of plastic work into heat. Moreover, a temperature and rate dependent material model, coupled with ductile damage, is applied to allow the prediction of damage and failure of metal components caused by large plastic deformations during forging or sheet metal forming. The constitutive equations are implemented as a user defined material model into LS-DYNA and include the temperature dependency of the material parameters such as for the YOUNG's modulus, the initial yield stress, the nonlinear isotropic hardening parameter, the strain rate sensitivity as well as for the moduli of a continuum damage mechanics based approach. The nonlocal damage option *MAT_NONLOCAL in LS-DYNA is used to prevent localisation of the damaged zone for small elements. Test data of tensile specimens are considered under different strain rates from 0.006 1/s (quasistatic) up to 100 1/s for identifying the model parameters with the optimisation software LS-OPT. Finally, the numerically predicted stress-strain curves are compared to the according test data for the model verification. In addition, the computed heat evolution due to plastic flow is compared to the experimental measured data in terms of time-temperature courses. Finally, the plastic necking of the tensile specimen is investigated by means of the spatial strain distribution.