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

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