A distinctive approach to testing and modeling thermal creep in ultra-high strength steel

The literature on the thermal creep of steels with nominal yield strengths greater than 700MPa is scarce. Conventionally, mathematical models simulating the thermal creep of steel are developed based on steady-state tests. Transient-state testing, however, replicates structural fire conditions more realistically, and consequently, yields more reliable results for the fire design of structures. Hence, in response to the existing knowledge gap, the thermal creep of Grade 1200 ultra-high strength steel (UHSS) is examined and modeled in this study using the transient-state testing approach. First, by performing transient-state tests at a 10°C/min heating rate, the stress-strain diagrams of the UHSS at elevated temperatures are obtained, in which thermal creep has been implicitly included. Thermal expansion, elastic modulus, and yield strength of the material are derived at temperatures up to 750°C and compared with the predictions given by three world-leading design codes of practice. Due to the distinct microstructure and chemical composition of the UHSS, a significant disparity is observed between the results obtained for the UHSS and specifications provided by the design codes for mild steel. Then, based on transient-state tests at the heating rates of 5 and 20°C/min, an analytical thermal creep model is developed for the UHSS. Exclusively tailored to the transient-state conditions such as real fires, the proposed creep model is capable of simulating tertiary creep, and hence, ideal for structural fire design applications. The model is validated against the experimental data obtained for the heating rate of 10°C/min, where the model predictions are found to be in very good agreement with experimental results. By the explicit inclusion of thermal creep using the creep model, it is then shown that under 5 and 20°C/min heating rates, the yield strength of the UHSS can experience a difference as great as 30% at fire temperatures; hence, the necessity of the explicit modeling of thermal creep for the safe and accurate fire design of UHSS components is concluded.