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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 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]