SIMULATION ON FRACTURE PROPERTIES OF HIGH-STRENGTH BOLTS AND CONNECTIONS UNDER THE WHOLE PROCESS OF FIRE BASED ON SMCS MODEL

High-strength bolts have been widely used in steel connections. The fire-induced high temperature will affect its basic material properties and fracture behavior and may affect the fire resistance of bolted connections and even the collapse resistance of the whole structure. Based on the uniaxial tensile tests under the whole process of fire (heating stage, cooling stage and after fire) and finite element simulation on high-strength bolts, the SMCS fracture model of Grade 10.9 high-strength bolts with different temperature conditions and stress triaxialities is calibrated and compared with the results of bolt material properties tests and T-stub connection test. Parametric studies are conducted on the fracture behavior of T-stub connections in the whole process of fire, and the effect of damage criterion and heating history on the failure mode and deformation characteristics of T-stub connections is studied. The results show that the calibrated SMCS model can effectively and accurately predict the tensile fracture behavior of bolts and connections in the whole process of fire and high stress triaxialities (0.3-1.2), and the prediction errors are less than 12%. The tensile temperature and peak temperature are the main factors affecting the fracture capacity of high-strength bolts. The fracture capacity of bolts increases with the increase of temperatures. Under different heating histories, T-stub connections may have three failure modes: yielding fracture of flange plate, simultaneous yielding fracture of flange plate and bolt, and yielding fracture of bolt. The deformation capacity (ductility index) of connections is related to the failure mode. Determining the fracture model of base metal and bolts is the crux to accurately predict the failure mode of connections.