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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.

A large number of steel moment frames fractured at the welds of beam-to-column connections in earthquake. As the weld damage behavior is a crucial factor for aseismic performance of steel frame connections, 20 local welded connections representing beam-to-column connections were tested under monotonic and cyclic loads to study the weld damage behavior in this paper. The failure modes and deformation curves were recorded; the effects of material strength, load types, and beam geometries on the connection damage behavior were analyzed. Three damage evolution models were calibrated, the relation between weld damage and macro connection performance was established. The results indicate weld cracking and plate buckling are the main causes of the connection damage; the range of cyclic loads is the key factor affecting the weld damage level; the plastic strain based damage model illustrate the damage process with good accuracy. This study provide technical basis for the damage evaluation and fracture prevention of the welded steel beam-to-column connections.

A simplified approach which utilizes an isotropic porous medium model has been widely adopted for modeling the flow through a compact heat exchanger. With respect to situations where the compact heat exchangers are partially installed inside a channel, such as the application of recuperators in an intercooled recuperative engine, the use of an isotropic porous medium model needs to be carefully assessed because the flow passing through the heat exchanger is very complicated. For this purpose, in this study the isotropic porous medium model is assessed together with specific pressure–velocity relationships for flow field modeling inside a rectangular channel with a built-in double-U-shaped tube bundle heat exchanger. Firstly, experiments were conducted using models to investigate the relationship between the pressure drop and the inlet velocity for a specific heat exchanger with different installation angles inside a rectangular channel. Secondly, a series of numerical computations were carried out using the isotropic porous medium model and the pressure–velocity relationship was then modified by introducing correction coefficients empirically. Finally, a three-dimensional (3-D) direct computation was made using a computational fluid dynamics (CFD) method for the comparison of detailed flow fields. The results suggest that the isotropic porous medium model is capable of making precise pressure drop predictions given the reasonable pressure–velocity relationship but is unable to precisely simulate the detailed flow features.

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.