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Brittle cracks were observed in the welded beam-to-column connections of steel frames during an earthquake. The crack propagation and accumulated damage to the connections can lead to fractures at much lower ductility ratios. Understanding the connections’ damage behavior during an earthquake is crucial for the design of steel moment frames in seismic areas. Nine full scale beam-to-column connections were tested under constant amplitude and variable amplitude cyclic loading. The effects of loading amplitude, loading history, and peak load on the connection damage were analyzed. The damage characters were studied and three damage evolution models were calibrated and validated based on test results. The damage mechanism was investigated and an effective plastic strain index was developed to evaluate connection damage based on a ductile fracture mechanism. A fatigue fracture mechanics-based model, for evaluating the damage process of beam-to-column connections under cyclic loading, was proposed.

In order to investigate the ultra-low cycle fatigue (UCLF) fracture in welded connections of high-strength steel (HSS) under earthquakes, this paper studies micromechanical fracture models ...

An experimental study was conducted to investigate the mechanical behavior of four I-section steel beams, which were reinforced by welding a plate to the bottom and top flange, respectively. For all welding reinforced specimens, the ultimate capacity was obviously enhanced relative to that of the original beams. Preload while the beam is reinforced is proved to be an adverse factor for the capacity if the failure of the beam is controlled by lateral–torsional buckling. During the welding process, larger vertical deflection and web curvature were generated. The torsional section angle decreased if the appropriate welding procedure was adopted. In addition to the experimental study, a type of thermo-mechanical coupling analysis by ANSYS, considering the welding process, was conducted. A generally good agreement between FE analysis and the experiments could be observed from the comparison of the load versus deflection curves obtained from FE and the experiments. It was proved that the FE analysis is reasonable and can allow for the prediction of residual deflection after welding reinforcement. The FE analysis presented in this paper is a reasonable method for simulating the behavior of the steel beams reinforced while under load. With more accurate material parameters and heat source models, the simulation can be further enhanced.

Compared with the conventional steel structure, the high-strength steel structures are at more risk of brittle fracture, especially in cold regions. In the present study, a series of tests (such as uniaxial tensile test, Charpy impact test and three-point bending test) were carried out at low temperature to investigate the mechanical properties and toughness of Q460C steel and its butt welded joint, fracture micro-mechanisms were analyzed as well. The ductility indices and the toughness indices all decrease with temperature decreases, the heat affected zone (HAZ) in welded joint is more critical to fracture than the base material. The fracture toughness of high-strength steel Q460C is relatively lower than the other three conventional steels (i.e. Q235, Q345 and Q390). In this study, rich experimental data were collected so as to provide reference for the fracture resistant design of high-strength steel structures in cold regions.

The thick plate induces the variation of mechanical properties and fracture toughness, especially in cold regions. At the low temperature, the brittle behaviour of steel becomes worse. A series of tests (such as uniaxial tensile test and three-point bending test) were carried out at low temperature to investigate the mechanical properties and fracture toughness of structural steel plates of Q345B with thickness of 60 to 150 mm, as well as the fracture toughness of 150 mm thick butt welded plate. The test specimens are all manufactured from plates along thickness with small size, and the tensile test specimens included through-thickness specimens additionally. The ductility index (percentage reduction of area) and the fracture toughness index (critical CTOD values) all decrease with the temperature decreases and the distance from plate surface increases. The results obtained in this paper provide technical basis for preventing brittle fracture of thick plate steel structures in cold regions.