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In order to study the post-fire mechanical properties of Chinese LY225 low-yield strength steel, the tensile tests were conducted on LY225 steel specimens that had been exposed to temperatures between 500 °C and 800 °C, followed by air or water cooling. LY225 steel's mechanical properties after fire were acquired and compared to other steels commonly used in steel structures in China. The findings indicate that LY225 steel exhibits remarkable stability in its mechanical properties under air cooling conditions after being exposed to elevated temperatures, demonstrating significantly less variation compared to other steels. With the water cooling method, the increase in strength of LY225 steel is more pronounced than that of other steels, while the reduction in elongation is comparable to other steels. After being subjected to high temperature exposure, a mathematical model was established to calculate the residual mechanical properties of LY225 steel. A complete constitutive model considering the necking effect for LY225 steel was proposed, and the constitutive model parameters were calibrated through complementary finite element analysis. The constitutive model was validated by the tensile tests of perforated bolted plate specimens after elevated temperature exposure, which can facilitate the evaluation of the mechanical characteristics of LY225 steel connection components following a fire. • Monotonic tensile behavior of LY225 steel after high temperature exposure is presented. • Formulas are posed to calculate the residual strength and ductility of LY225 steel. • A post-fire complete constitutive model of LY225 steel is proposed in this study. • The constitutive model can simulate post-fire tensile test of a bolted plate with hole. [ABSTRACT FROM AUTHOR]

For the fracture problem of structural steel under strong earthquakes, a continuous damage model which comprehensively considers the influence of stress triaxiality and Lode parameter was proposed in this paper. According to the variation law of Mises stress under cyclic loading, this model decomposes the effective cumulative plastic strain into isotropic hardening and kinematic hardening components, assuming a linear proportional relationship between the fatigue damage induced by these two components. The basic mechanical properties and constitutive relationships of five common Chinese structural steels and four corresponding weld metals were investigated. The fracture failure tests for both notched round bar specimens and pure shear specimens subjected to monotonic tensile and ultra-low-cyclic loading were thoroughly examined on these materials. The continuous damage model parameters were calibrated by means of test results and complementary finite element simulations. Experimental and numerical simulation results indicate that Q690 high strength steel exhibits cyclic softening behavior, whereas the other steels show cyclic hardening characteristics. For pure shear specimens, plane strain specimens and shear energy dissipation cover plates, the continuous damage model was adopted to simulate and analyze the damage development and fracture failure process of the specimens. The predictions for crack initiation, crack propagation and fatigue life are consistent with experimental results, lending further credibility to the model. The calibrated continuous damage model of various steels provides an effective means for analyzing the ULCF fracture failure of structural steel under intense earthquakes. • A continuous damage model was presented to analyze ULCF failure of metallic materials. • The mechanical properties and constitutive models of steels and weld metals were obtained. • The CLCDM parameters of a series of structural steels and weld metals were calibrated. • The validity of the CLCDM was verified through experimental results of steel components. [ABSTRACT FROM AUTHOR]