Effects of exposure temperature on low-cycle fatigue properties of Q690 high-strength steel

Fire brings lasting effects on mechanical performance of structural steel. For the high strength steel (HSS) structures experiencing fire while do not collapse, it is necessary to evaluate the service performance after fire, including the ability to resist earthquake. During earthquake, the low-cycle fatigue load is applied on the structural steels. In this paper, the low-cycle fatigue properties of Q690 HSS exposed to different elevated temperatures are investigated experimentally. Totally, five exposure temperature levels (25, 600, 700, 800 and 900 degrees C) and four fatigue strain amplitudes (1%, 1.5%, 2% and 4%) are considered. After the heating test, the appearance characteristics and microstructures of Q690 HSS specimens are discussed. Later, the fatigue test is conducted and results show that the hysteretic curve tends to be fuller due to the increase in plastic strain when the exposure temperature ranges from 700 to 900 degrees C. Based on the test results, effects of exposure temperature and strain amplitude on cycles to failure and total dissipated energy density are analyzed. Besides, stress and strain coefficients are selected to analyze the stress and strain properties of post-fire Q690 HSS under low-cycle fatigue loading. Finally, a fatigue model is proposed to quantify the relation between total strain amplitude and cycles to failure, where the coefficient beta and exponent alpha of Q690 HSS exposed to different elevated temperatures are determined by test results, after which the predictive equations are suggested to the quantify effects of exposure temperature on beta and alpha.