Fracture characterization of tailored Usibor® 1500-AS and damage modelling based on a coupled-micromechanical-phenomenological strategy

Abstract

Usibor® 1500-AS steel is commonly used in the automotive industry in the die-quenched form due to its very high level of strength after hot stamping. In order to increase the ductility of this steel, in-die-heated, tailored hot stamping can be utilized to promote the formation of softer phases instead of the hard martensitic phase normally formed during die quenching. In this study, the fracture behavior of the different microstructures of this steel, including fully martensitic, 60% martensitic plus 40% bainitic, and fully bainitic, were characterized through several mechanical testing. A coupled-micromechanical-phenomenological approach was developed to predict the fracture of Usibor® 1500-AS with mixed martensitic-bainitic microstructures under different loading conditions. In this strategy, the flow behavior of the steel as well as stress and strain partitioning between phases were predicted using mean-field homogenization (MFH) schemes, given the stress-strain curves of the constituent phases. At the same time, damage accumulation in the constituents was calculated using the phenomenological Generalized Incremental Stress-State-dependent damage MOdel (GISSMO) coupled with the fracture loci of the corresponding constituent phases based on the strain- and stress-state history within each phase. Finally, the onset of material fracture was predicted to correspond to the damage parameter for any phase reaching unity. In this framework, three MFH schemes, corresponding to the interpolative Mori-Tanaka, self-consistent, and Samadian-Butcher-Worswick-1 (SBW1) approaches, were evaluated using the first-order secant-based linearization approach. To investigate the accuracy of the numerical results, the predicted flow curves and fracture strains of a two-phase microstructure were compared with the corresponding measured data. The comparisons showed that the predictions based on all of the MFH models were in good accord with the measured data. However, the fracture locus predicted based on the SBW1 scheme, especially in the case of the plane-strain-tension stress state, provided the best agreement with the interpolated Bai-Wierzbicki fracture locus obtained using the experimental data.