EXPERIMENTAL STUDY ON FRACTURE BEHAVIOR OF Q355 STEEL IN THE WHOLE PROCESS OF FIRE UNDER HIGH STRESS TRIAXIALITY

The fracture of steel components may result in progressive collapse of a steel structure in fire events. Understanding the temperature-dependent fracture behavior of steel materials is the basis for investigating fire resistance of steel structures and for assessing their post-fire safety. Tensile fracture tests were thusly carried out on smooth and notched round specimens made of Q355 steel under different stages of fire including a heating stage, a cooling stage, and a post-fire stage. The influence of stress state (stress triaxiality) and temperature experience (peak experienced temperature and target temperature) on the engineering/true stress-strain behavior and fracture strain are studied. The micro fracture mechanism is investigated by scanning electron microscope. A fracture model is calibrated upon the test and numerical results. It is found that Q355 steel exhibit ductile fracture behavior in fire, which is greatly affected by the stress triaxiality and temperature experience. The higher the peak and target temperatures, the greater the fracture strain and the better the ductility. The post-fire fracture behavior is similar to that at ambient temperature. The stress triaxiality may affect the sensitivity of fracture behavior of materials to the temperature, while the temperature experience has an influence on the plastic stage of the true stress-strain curve. The SMCS fracture model can accurately predict the fracture behavior of Q355 steel during the whole process of a fire accident, where different parameters should be used to model the fracture behavior for the heating and cooling stages.