Stress-state dependent ductile fracture initiation characterization of Q355 structural steel after exposure to elevated temperatures

Elevated-temperature exposure significantly alters the mechanical performance of structural steels. Previous studies have mainly focused on conventional mechanical properties, such as yield strength, ultimate strength, and elastic modulus, whereas ductile fracture behavior under different stress states remains insufficiently understood. This gap limits accurate prediction of the large-deformation response of steel structures after fire exposure. In this study, a hybrid experimental-numerical investigation is conducted to characterize the stress-state dependent ductile fracture initiation behavior of Q355 structural steel after elevated-temperature exposure, considering combined effects of exposure temperature, stress triaxiality, and the Lode angle parameter. A total of 72 coupons were tested after exposure to six temperature levels ranging from 20 °C to 1000 °C. The results show that the strength of Q355 structural steel decreases with increasing exposure temperature, whereas its ductility, characterized by plastic deformation energy, shows no monotonic trend. Furthermore, the effects of elevated-temperature exposure on ductile fracture initiation under various stress states are quantified. Based on the experimental and numerical results, the Hosford–Coulomb (H–C) ductile fracture model is extended to post-elevated-temperature conditions, and a temperature-continuous H–C model incorporating a damage accumulation law is phenomenologically calibrated.