Stress-state-dependency of post-necking hardening rule and its influence on ductile fracture prediction

Abstract

This paper presents the investigation of the stress-state-dependency of the post-necking hardening rule of the Q355B structural steel and its influence on fracture prediction. The comparisons of the testing result and the simulation based on the Mises model with the stress-state-independent hardening rule indicate that triaxiality dependency under axisymmetric tension is negligible, while the reduced yield stress increases with the rising triaxiality under plane strain. In addition, the overestimated predicted force based on the Mises model is continuously ramping up along with the plastic deformation under the shear-dominant condition. It can be considered by the reduced post-necking hardening rate under the requirement of convexity of the yield surface. The stress-state-dependent hardening model (SDH model) is herein proposed including the modified triaxiality-dependent Hosford yield criterion and state-dependent post-necking hardening rule. The comparative studies of the proposed and conventional models show similar calculated fracture strains under high triaxiality, but considerably larger fracture strain under low triaxiality from the proposed SDH model, since the reduced post-necking hardening rate intensifies the localization of plastic deformation. Using the proposed pressure-dependent maximum shear stress (PMSS) fracture model, the calibrated fracture parameters based on the data from SDH the model produce better fracture simulations under various stress states, especially the complex condition in multi-hole plate (MHP) specimen. It emphasizes the necessity of the consideration of stress-state-dependency in the post-necking hardening rule for accurate fracture prediction.