Modelling of yield point phenomenon in bake-hardening grade steel

In this study the yield point phenomenon in Bake-Hardening grade steel is predicted using a physically based thermo-mechanical model. A modified Taylor equation is proposed with a physically based dislocation density evolution approach. The softening that follows the higher yield point is incorporated with a Voce type decaying exponential function. The strain rate dependency of the plastic hardening is also incorporated in the model. The yield point in the decay function is also strain rate dependent but does not follow the same dependency of plastic hardening. This was solved by making the decay function strain rate dependent by adding a modified strain rate stress term to the exponential function. This parameter is calculated based on tensile experiments. Due to the softening behavior of the material, the numerical model is mesh size sensitive. Hence, a lower order strain gradient enhanced approach is implemented. The gradient is in a form of an additional hardening term assigned in the locally strained bands based on the plastic strain gradient. Hill48 yield criterion is used to assimilate the anisotropy in the steel grade. The numerical results show good correspondence with experimental tensile tests. The regularization significantly reduced the mesh size dependency of the numerical results.