Uncoupled Ductile Fracture Models for Grade 8.8S Steel Bolts Considering Different Stress States and Elevated Temperatures

This paper presents the experimental results of Grade 8.8S high-strength steel coupons tensioned to fracture at room and elevated temperatures, followed by finite element analyses for investigating the ductile fracture behavior. Smooth round bars, notched round bars, flat shear sheets, and flat grooved sheets were tested at room temperature, which covered a wide range of stress states. The relationship between the ductility of the Grade 8.8S high-strength steel and the stress state was discussed. Extra smooth and notched round bars were also redesigned and tested at elevated temperatures. The nonlinear variations of material properties, including elastic modulus, yield strength, and tensile strength, were discussed. Different modeling strategies were used for the room and elevated temperatures. At room temperature, three uncoupled ductile fracture criteria were evaluated regarding their applications in the Grade 8.8S high-strength steel. Besides, in conjunction with a modified Johnson–Cook (JC) hardening model, the JC fracture criterion was improved and characterized by an exponential temperature function, as the original linear temperature function could not accurately describe the relationship between the fracture strain and temperatures. The developed finite element models closely traced most specimens’ load-displacement paths at the room and elevated temperatures.