Micromechanical investigation of the effect of the crystal orientation on the local deformation path and ductile void nucleation in dual-phase steels

The effect of the crystal orientation on the local stress-strain path and ductile damage initiation in dual-phase steels was characterized through an integrated experimental-numerical study. Interrupted tensile tests were carried out on two different steels with varying microstructure to clarify the damage initiation mechanisms, their locations, sequences and proportions. It was found that damage initiated early and mainly from fer-rite/martensite decohesion and martensite/martensite cracking with different ratio in the two materials. Finite element simulations were carried out on semi-synthetic microstructures generated from optical micrographs of specimen surfaces, using either an isotropic elasto-plastic model or a phenomenological crystal plasticity model calibrated from macroscopic stress-strain curves. The accuracy of the simulations was assessed through a comparison of the equivalent plastic strain fields estimated experimentally by digital image correlation and predicted numerically. It was found that both models predicted similar plastic strain distribution but slightly different stress triaxiality and Lode angle fields which highlighted the effect of the crystal orientation. However, in the vicinity of strain concentration regions where voids nucleated, the two fields were relatively close suggesting that the phase distribution and strength contrast had a predominant effect on these fields. The local stress-strain paths at void sites were extracted and were observed to significantly differ from that predicted by macroscopic simulations, especially in the case of ferrite damage which highlighted the importance of considering microstructural effects. A general ductile damage model dependent on the equivalent plastic strain, the stress triaxiality and the Lode angle was calibrated based on the stress-strain path history of void sites, taking into account experimental uncertainties. Similar fracture loci were predicted by the isotropic and the crystal plasticity model. Estimated damage initiation parameters suggested that martensite fracture was insensitive to the stress triaxiality and the Lode angle, while a certain sensitivity against the stress triaxiality was observed for ferrite/martensite decohesion.