Fracture behavior of Grade 10.9 high-strength bolts and T-stub connections in fire: Fid-Bau Portal

Fracture behavior of Grade 10.9 high-strength bolts and T-stub connections in fire

Lu, Yaoliang / Jiang, Jian / Chen, Qiao / Cai, Wenyu / Chen, Wei / Ye, Jihong

Abstract High-strength bolts have been widely used in steel bolted connections, and fire-induced high temperatures will affect its basic mechanical properties and fracture behavior, leading to failure of the connection and even collapse of the overall structure. Tensile experiments are conducted on Grade 10.9 high-strength bolts to investigate the effect of stress triaxiality and heating history on their mechanical and fracture properties in the whole process of fire. The true stress-strain curves are obtained, and microscopic fracture mechanism is analyzed by SEM. The SMCS fracture model is calibrated based on test results, and parametric studies are conducted on fracture performance of T-stub connections in fire. The experimental results show that the material properties in the heating-cooling stage depend on the maximum temperature, which is inaccurate and unsafe if being replaced by those in the heating stage. The fracture performance of high-strength bolts is greatly affected by maximum and target temperatures, and the bolt ductility increases with the increase of temperatures, up to 3.5 times the ambient-temperature value. High-strength bolts exhibit ductile fracture characteristics in the whole process of fire, and the SMCS model can effectively predict the fracture behavior of Grade 10.9 high-strength bolts for a stress triaxiality range of 0.3 to 1.2. The ductility factor of T-stub connections is related to its failure mode with a variation range of 0.6 to 1.6. It is necessary to consider the fracture performance of high-strength bolts for determining the bearing capacity of bolted connections in the whole process of fire.

Highlights • Tests are conducted on fracture behavior of high-strength bolts under different stress triaxialities and temperatures. • The stress triaxiality, maximum and target temperatures are key factors affecting behavior of high-strength bolts. • SMCS model is calibrated and validated for fracture behavior of high-strength bolts. • The behavior of T-stub connections significantly depends on failure modes of high-strength bolts. • It is unsafe to directly use EC3 for material properties of high-strength bolts in fire.