Study on ultra low cycle fatigue fracture behavior of Q690D high-strength steel after fire exposure

Abstract

In order to investigate the fracture behavior of Q690D steel high-strength steel after exposure to elevated temperature due to the fire, standard coupon specimen, notched coupon specimen and pure shear specimen representing various stress states were tested under monotonic loading and ultra low cycle fatigue loading. For air cooling condition, the tensile test results of Q690D steel show that when exposed to the temperature above 500 °C (up to 800 °C), the strength decreases while the elongation increases. These variation trends are basically the same under different stress states, and the predictive equations are proposed to determine the residual strength and ductility after exposure to high temperature. The effect of exposed temperature on the ductility under monotonic loading is significantly greater than that on the ultra low cycle fatigue life. In addition, as compared with air cooling, water cooling can significantly reduce the deformation capacity and fatigue life of the specimens, and even lead to a failure mode close to brittle fracture. The Lode parameter enhanced cyclic void growth model (LCVGM) parameters of Q690D steel after being exposed to various temperatures and air cooling were calibrated based on the fracture failure test results, and the post-fire LCVGM of Q690D high-strength steel was established to facilitate the fracture evaluation. Finally, the LCVGM was employed to predict the post-fire ultra low cycle fatigue life of Q690D steel notched plane strain specimen, and its estimation accuracy was verified by the comparison between tests and simulations. This fracture prediction model LCVGM can be recommended to evaluate the post-fire seismic performance of Q690 high-strength steel members.