Non-monotonic plasticity and fracture in DP1000: Stress-state, strain-rate and temperature influence

Abstract

Recent studies revealed a complex, non-monotonic relationship between flow stress, and temperature and strain rate for ultra-fine-grained DP1000 steel, including unexpected drops in ductility. Consequently, a comprehensive investigation into the interrelated effects of temperature, strain rate, and stress state on the plasticity and fracture behaviour of DP1000 steel is undertaken. To this purpose, eight purpose-designed specimen geometries, including a shear, a dogbone, a central hole, and various notched samples, are tested at strain rates covering 7 orders of magnitude from the quasi-static over the intermediate to the dynamic regime and in a temperature range from -40 to 400 °C. The experimental design thereby provides 336 unique combinations of stress state, strain rate and temperature. The study reveals phenomena typically associated with dynamic strain aging (DSA), such as flow stress serrations, negative strain rate sensitivity, thermal hardening, and embrittlement. As a result, the present study provides a comprehensive overview of the plastic, damage, and fracture behaviour of DP1000 steel. It also offers unique insights into the influence of external loading conditions on DSA. Increasing strain rates and decreasing temperatures suppressed DSA and its effects. However, a novel and important finding is that the impact of stress state on DSA and, consequently, on the behaviour of DP1000 steel cannot be solely explained by stress triaxiality and Lode angle. Lower stress triaxialities and Lode angle parameters amplify the occurrence and severity of DSA manifestations. However, non-proportional stress states, high-stress gradients and relatively low maximum in-plane shear stresses are also identified as contributing factors.