Hysteretic behavior of post fire structural steels under cyclic loading

Abstract This paper presents a series of monotonic tensile tests and cyclic loading tests on structural steels after exposure to elevated temperatures. The generally used mild steel Q235, low alloy steel Q345 and high strength steel Q690 were included, and three cooling methods were considered. Strain ascending and strain alternate loading protocols were adopted to investigate the cyclic strength evolution patterns of postfire steels. The strength developments of post fire steels before and after cyclic loadings, and the effect of cyclic loading history and loading protocols on the cyclic strength evolutions were all compared and discussed. The strength evolution was cyclic hardening dominated in Q235 and Q345, and was cyclic softening dominated in Q690. When the experienced temperature exceeding 600 °C, the air-cooled specimens generally had yield strength reduction but also increased extent of cyclic hardening behaviors as the increase of temperature. The reduced tensile strength of postfire steels would be elevated and recovered after cyclic loading. The water-cooled specimens would induce both yield and ultimate strength increase after fire, and the strain hardening behaviors under cyclic loading were also increased. The water spray cooled fire heating specimens presented similar tensile strengths and the cyclic strength evolutions as the untreated ones.