Postfire Constitutive Model and Fracture Performance of Grade 14.9 Superhigh Tension Bolts

In recent years, superhigh tension bolts (SHTB) with ultimate strengths exceeding 1,400 MPa have been increasingly utilized in steel structures due to their significant economic and environmental advantages. However, the performance of these high-strength bolts under extreme conditions, such as fire, remains a critical concern. Fire is one of the most severe hazards to steel structures because high temperatures can significantly degrade their mechanical properties. Even after the fire has been extinguished, the residual effects of heat exposure may lead to a gradual loss of structural integrity, and in some cases, structures may not collapse immediately. Given that bolts are essential components of joints, their postfire performance is crucial for ensuring structural safety. Despite the importance of this issue, current research primarily focused on Grades 10.9 and 12.9 bolts, with very limited studies on SHTB. Therefore, this study aims to investigate the postfire performance of Grade 14.9 SHTB. In this study, hardness tests and coupon tests were conducted to investigate the postfire performance of Grade 14.9 SHTB. Performance indicators of the bolts after being exposed to fire and subsequently cooled were obtained. Metallographic analysis revealed that the phase transition temperature of Grade 14.9 SHTB is above 750°C, which differs from the phase transition temperatures of bolts of other grades, which are below 750°C. An assessment procedure was proposed for the rapid determination of exposure temperature and the residual performance of bolts after a fire, by combining appearance characteristics and hardness test data. Using hardening law, parameterized constitutive models considering necking after cooling were established. Based on the independent model, a unified postfire constitutive model was proposed, with relative errors for all temperature groups being less than 1.5%. Subsequently, the postfire SMCS fracture model at representative temperature points was calibrated, demonstrating high predictive accuracy through comparisons with experimental results, with fracture prediction errors for all temperature groups being less than 6%. A convenient evaluation method and comprehensive theoretical data for the postfire performance of Grade 14.9 SHTB have been provided in this study.