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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]

Beam-to-column welded joints in moment-resisting steel frames may undergo extremely low cycle fatigue in earthquake action. The aim of this paper is to investigate the fatigue failure behavior by means of experimental study and finite element analysis. Experiments were conducted on the six full scale beam-to-column welded joints. The seismic performance of welded joints with different weld access hole geometries was compared among them and the effect of crack initiation and growth on the load carrying capacity of welded joints was discussed. In addition, a continuum damage mechanics model used previously for monotonic loading was reformulated in order to account for the extremely low cycle loading condition. The model was implemented in a commercial nonlinear finite element software of ABAQUS, which was adopted to predict the fracture of beam-to-column welded joints subjected to monotonic loading and extremely low cycle loading. The predicted load displacement response agrees well with the test results. The distribution and evolution of damage in the welded joints were calculated during crack initiation and propagation. It is found that the methodology based on continuum damage mechanics proposed in the paper can successfully predict fracture behavior of beam-to-column welded joint with reasonable accuracy. [ABSTRACT FROM AUTHOR]

A micromechanics damage model which replaces a traditional method for fracture problem is used in this paper to investigate the crack initiation and propagation of welded beam-to-column joints subjected to monotonic loading. Firstly, parameters used in the micromechanics damage model for the area at steel base metal, heat affected zone and weld metal were calibrated respectively through uniaxial tension tests of notched specimens. Next, the position of a crack initiation and its growth in the welded joint were predicted by means of finite element numerical simulation. Compared with the available test results and numerical results based on a void growth model and a stress-modified critical strain model, the numerical results from the micromechanics damage model had better prediction accuracy. Finally, the effect of different shapes of welding access hole on fracture behavior of welded joints was studied numerically. It is concluded that the shape of a hole plays an important role in fracture mode. An oblong hole has better ductility and slower cracking than a round hole, but both shapes of holes show nothing different in load carring capacity. [ABSTRACT FROM AUTHOR]

Abstract: Moment-resisting steel frames often fail by fracture failure of beam-to-column welded joints during a strong earthquake. This paper provides a numerical methodology based on micromechanics damage model instead of traditional fracture analytical methods to investigate the crack initiation and propagation of welded beam-to-column joints subjected to monotonic loading and ultra low cycle loading conditions. Firstly, parameters used in the micromechanics damage model for steel base metal, heat affected zone and weld metal were calibrated, respectively, against uniaxial tension test results and cycle test results of notched specimens. The evolution of void growth in the notched specimens under different loading conditions was compared. Secondly, fracture process of the welded joints subjected to tensile loading was simulated based on the micromechanics damage model. The predicted load displacement response agrees well with the other researcher's test results. Finally, the micromechanics damage model was applied numerically to investigate the ultra low cycle fatigue fracture behavior of the welded joints under constant amplitude as well as variable amplitude inelastic cyclic loading. According to the distribution and evolution of void in the welded joints obtained from finite element analysis, crack initiation and propagation were presented and the number of cycles to fracture was predicted. It is shown that the fatigue life predicted from finite element analysis based on micromechanics damage model agrees well with the other's test results. [Copyright &y& Elsevier]

• A continuum damage model considering the effect of lode parameter was presented. • Monotonic loading tests were conducted on different state conditions of Q690D high strength steel. • Constitutive model of Q690D steel considering the influence of lode parameter were determined. • Damage parameters of Q690D steel were calibrated according to experimental and numerical results. • Fracture behavior of specimen with initial flaw was predicted. Chinese Q690D high strength steel has begun to be applied in building structures in recent years, but researches on ductile fracture behavior of Q690D steel are still insufficient, and most of the mechanical fracture models based on micro fracture mechanics mechanism ignore shear effect. In this paper, the yield surface size of materials considered the shear effect through Lode parameter, and a continuous damage model (LCDM) was also provided to consider the effect of Lode parameter. A batch of Chinese Q690D high strength steel round rods and plate shaped specimens were designed to reflect different stress states, which were submitted to monotonic tensile loading. The micro fracture mechanism under different stress states was revealed by scanning electron microscopy tests. According to experimental results and finite element simulation results, LCDM parameters of Q690D steel were calibrated and crack initiation positions of various specimens were analyzed. Finally, the LCDM was used to predict crack initiation and fracture failure process of the cruciform joint specimen with initial gap. It is found that the load displacement curve, crack initiation and evolution law obtained from finite element simulation are in good agreement with the test, and the prediction accuracy based on LCDM is higher than that based on the model without considering the effect of Lode parameter. [ABSTRACT FROM AUTHOR]