Plasticity and fracture modeling of the heat-affected zone in resistance spot welded tailor hardened boron steel

Five hardness grades of 22MnB5 are considered, covering the full strength-range from 600 MPa in the ferritic/pearlitic range to 1500 MPa in the fully hardened, martensitic state. These five grades form the basis for a hardness-based material model for the heat-affected zone found around resistance spot welds in tailor hardened boron steel. Microhardness measurements of resistance spot welds in all five grades are used to determine the location and shape of the heat-affected zone and for mapping of the hardness distributions into FE-models of the specimens used for model calibration. For calibration of the strain hardening of the heat-affected zone, a specially designed asymmetric uni-axial tensile specimen is used that features a well-defined strain field up to fracture initiation. Both the measured force–displacement curves and the strain fields are used as input for an inverse FEM optimization algorithm that identifies suitable strain hardening model parameters by minimizing the differences between experimental and simulated results. A strain-based fracture model is calibrated using a hybrid experimental/numerical approach, featuring two additional specimens in which fracture initiates in the HAZ under different stress states. Strain hardening and fracture strains are assumed to be linearly related to the as-welded material hardness. The calibration and modeling approach are validated by comparing measured and predicted force–displacement curves and strain fields of welded coupon tensile tests.