Strain rate dependent ductile fracture behavior of Q960 ultra-high-strength steel

Predicting the ductile fracture behavior of ultra-high-strength structural steels under high-speed loading remains a significant challenge in impact engineering within civil engineering. In this study, the strain rate effect on the ductile fracture behavior of Q960 ultra-high-strength structural steel, which is stress-state dependent, is reported for the first time. In the testing programme, four types of stress-state-dependent specimens were designed and tested, including uniaxial tension, notched tension, plane strain tension and pure shear specimens. These specimens were subjected to five nominal strain rates: 10⁻³ s⁻¹, 10⁻¹ s⁻¹, 10⁰ s⁻¹, 10¹ s⁻¹, 10² s⁻¹. The shear-dominated fracture strain decreased sharply with increasing strain rate, even exhibiting a brittle trend at intermediate strain rates. To accurately describe the plasticity behavior undergoing large deformation at intermediate strain rates, a stress-state-dependent plasticity model was implemented in conjunction with a deformation resistance model that accounts for strain hardening, strain rate effect, and thermal softening under adiabatic conditions. Accordingly, the loading paths to fracture for all specimens at different strain rates, i.e., the evolution of the equivalent plastic strain in terms of the stress triaxiality, the Lode angle parameter and the equivalent plastic strain rate, were extracted based on a hybrid experimental-numerical approach. To characterize the stress-state-dependent effect of strain rate on fracture strain, a new rate-dependent Hosford-Coulomb ductile fracture initiation model was proposed to capture this specific strain-rate-induced mechanism. With calibrated parameters in a user-defined material subroutine, the proposed rate-dependent Hosford-Coulomb fracture model was successfully validated based on the experimental results.