Determination of strain hardening parameters of tailor hardened boron steel up to high strains using inverse FEM optimization and strain field matching

In this article, an inverse FEM optimization strategy is proposed for identification of the strain hardening parameters of boron alloyed steel 22MnB5 in five different hardness grades. In the proposed elasto-plastic constitutive model, the strain hardening is represented by a nonlinear combination of the Swift hardening law and a modified version of the Voce law. Initial fits of these two classical strain hardening equations are constructed based on experimental data from uni-axial tensile tests up to the point of diffuse necking. The strain hardening response beyond the point of diffuse necking is determined from a 3D strain field analysis of notched tensile and equibiaxial tension tests. Both the measured force-displacement curves and the strain fields are used as input for the optimization algorithm that identifies suitable material model parameters by minimizing the differences between experimental and simulated results. In order to show the contribution of the different parts of the elasto-plastic model for representing the real material response, three simplified versions of the proposed model and the parameter identification procedure are applied on two selected hardness grades, confirming the importance of a flexible strain hardening law, suitable yield criteria and accurate experimental data up to high plastic strains. The calibrated model was shown to accurately capture the elasto-plastic response of 22MnB5 in different hardness grades, with an excellent representation of the strain fields up to the point of fracture.