Transient stress-strain curves of ultra-high strength steel tubes at high temperatures incorporating thermal creep effects

In contrast to room temperature, at high temperatures above 400°C, significant time-dependent deformations occur in steel materials. Known as “thermal creep”, this material behavior should be considered in the fire design of steel structures. According to EUROCODE 3-1-2, the creep strains occurring in the event of a fire are generally not explicitly taken into account in the design procedure, but they are implicitly included in the transiently-determined material laws. Accordingly, by conducting transient tensile tests at temperatures of up to around 800°C on standard coupons taken from a 3.2 mm thick Grade 1200 ultra-high strength steel tube, this paper investigates the fire-temperature mechanical properties of these tubular elements. To better simulate structural fire conditions, in a transient heating regime, the temperature is steadily increased when the specimen is subjected to a constant tensile load. From the coupon tests, temperature-strain curves are obtained, wherein the strain contains all mechanical-, thermal- and time-dependent strain fractions. The temperature-strain curves are then converted to elevated-temperature stress-strain diagrams suitable for the fire design applications. Taking the thermal creep of UHSS into account, the results clearly differ from those of the steady-state tests previously conducted on the same UHSS tubes.