Mechanical properties of mild steel under combined effects of pre-damage and elevated temperatures in post-earthquake fire scenarios

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The earthquake-fire sequence is one of the most significant combinations of hazards, and exploring the cascading effects is essential for enhancing the multi-hazard resilience of structures. However, at the material level, the combined effects of the pre-damage and elevated temperatures on the mechanical properties remain unclear due to the lack of related research. Based on that, this study focuses on the material properties of mild steel with different pre-strain (including the extremely large strain of 0.15 mm/mm) at elevated temperatures (up to 750 °C). Twenty-eight Q235 steel specimens were subjected to two-stage test procedures, including the phase of inducing damage by monotonic loading at ambient temperature and then followed by the stage for tensile tests of damaged samples at elevated temperatures. According to the test results, stress-strain responses were derived and crucial mechanical characteristics, such as elastic modulus, yield and ultimate strengths, and fracture elongation, were analysed and compared between different combinations of pre-strain and temperature. Bivariate polynomial fitting equations were proposed to predict the mechanical properties of pre-damaged mild steel at elevated temperatures. Moreover, the whole stress-strain relationships with each combination of pre-strain and temperature were calibrated by a multi-coefficient explicit model. The findings highlight the necessity for considering both the impacts of pre-damage and elevated temperatures on the mechanical properties of mild steel, and in the meantime, provide practical methods to predict the material behaviour and facilitate further application in post-earthquake fire scenarios.