Parameter Calibrations And Application Of Micromechanics Fracture Models Of Q235B Steel

It is more realistic to predict ductile fracture in steel structures based on micromechanics facture models. Micromechanics damage model(GTN) has several parameters. Stress modified critical strain model(SMCS) and void growth model(VGM) include parameters and respectively, which indicate the material toughness. They aresuitable to predict metal materials fracture initiation under monotonic loading. The corresponding degraded significant plastic strain model(DSPS) and the cyclic void growth model(CVGM) model include not only parametersand, but also degraded parameters and that reflect the damage effect of cyclic loading. Both models are fit for fracture prediction under ultra low cycle fatigue loading. In addition, characteristic length is involved in the above models. This paper used the results of uniaxial tensile test, cyclic loading test and scanning electron microscope tests conducted by Zhang Pei. In combination with the finite element analysis, this paper calibrated parameters of the above five micromechanics fracture models of Q235 B steel. Specimens were made by four kinds of materials: parent metal of Q235 B hot rolled seamless pipe, cold bending steel pipe(base metal, weld seam and heat affected zone). This paper carried out the finite element analysis of each specimen. In comparison with the results of tests, VGM can predict the fracture displacement and fracture load more accurately. Meanwhile, the GTN model performs obviously better than VGM and SMCS in predicting the development of bearing capability after fracture point. And the result of calibrated parameters can be applied to the prediction of material fracture damage.This paper used the micromechanics damage model(GTN) to investigate the crack initiation and propagation of welded beam-to-column joints under monotonic loading. Being compared with the results based on a void growth model(VGM) and a stress-modified critical strain model(SMCS), the curves of Δ-P based on GTN are more similar to the previous results of tests in the aspect of the development after fracture. Also it was concluded that the shape of a welding access hole plays a significant role in fracture behavior. Though the larger hole can reduce the load capacity lightly, the joints with larger hole perform better in the ductility and the cracking point became slower than before obviously. This paper analyzed the triaxial stress and equivalent plastic strain of element that failure firstly. It was founded that the equivalent plastic strain reached its maximum when fracture occurred while the triaxial stress reach its peak before the fracture. This paper also did some weaken design on TS-1, by comparing the three weaken parameters’ effect on the carrying capacity, it was found that influence of the weaken depth is most obvious and the weaken length almost has no influence on the bearing capability of joints. By analyzing the mises stress distribution along the length of the beam direction, this paper discovered that though the weakening design lead to lower bearing capacity, reasonable weakening parameters can make the plastic hinge be transferred to weaken areas and reduce the stress of the beam root.