Fracture characteristics of a cold-rolled dual-phase steel

In this study, the fracture characteristics of a cold-rolled, low-strength, high-hardening steel sheet (Docol 600DL) under quasi-static loading conditions are established using five different test set-ups. In all the tests, the sheet material is initially in plane-stress states. Optical field measurements with digital image correlation were used to determine the strain fields to fracture, to calibrate the material model for the sheet material and to validate the finite element models of the tests. Based on the field measurements, a novel method for experimental determination of the stress triaxiality and the Lode parameter is presented for isotropic materials and plane-stress states. These parameters were also obtained from finite element simulations. Comparisons show that the two methods give approximately the same average values of the stress triaxiality and the Lode parameter up to fracture. The sheet material displays only moderate variation in ductility as a function of the stress triaxiality and the Lode parameter within the investigated range of these parameters. The most critical through-thickness position in the specimens was found to be in the centre where the strains and the stress triaxiality are highest.