Experimental And Theoretical Research On Ductile Fracture Of Structural Steels Based On Micromechanical Mechanisms

Earthquake-induced fractures of steel structures are usually characterized by high-strain low-cycle conditions and fracture under these conditions is called extremely low cycle fatigue fracture. Applications of traditional fracture mechanics, which are used to predict fracture of steels need work with the assumptions that the steel has little crack presented and the tip of the crack is subjected to high strain constraint. However, large amount of experimental and practical studies show that ductile fractures of structural steels can also take place without initial flaw under large scale strain cyclic loads condition. Consequently, traditional fracture mechanics is inaccurate to predict fractures under extremely low cycle fatigue. In recent years, theories and models of fracture based on micromechanical mechanisms showed great promise in predicting ductile fracture. But studies on the form of micromechanical models, the parameter calibrations of these models and related experiments are still limited. Particularly there exists a big controversy in the ductile fracture mechanism under cyclic loads and shear loads. In this case, theoretical models, fracture experiments and finite element analysis are combined to study the mechanisms of ductile fractures of structural steels in this paper. Overall, this research includes several aspects as follows:(1) Different kinds of specimens were made from Q235 steel and used to conduct tension coupon tests, cyclic tests and combined loading (shear and tensile) tests. From the tests, true stress strain curves of bars and boards were calibrated, as well, load-displacement curve, number of cycles, displacement and states at break for each specimen were all recorded to study the mechanisms of ductile fractures and shear fractures and to calibrate the parameters of fracture models;(2) Based on the tension coupon tests of smooth and notched bars and finite element analysis, the parameters of Void Growth Model (VGM), Stress Modified Critical Strain model (SMCS) and Johnson-Cook model were calibrated. The results showed that when stress triaxiality is within the range of smooth and notched bars test results in this paper, all the VGM, SMCS and Johnson-Cook model fit well with the test results. Furthermore, the conception of damage factor was introduced to develop VGM-DDF and JC-DDF models. The Ductile Damage Factor model is more applicable and it can capture the damage factors in real time Since Johnson-Cook model has more parameters than Void Growth Model, Johnson-Cook model and JC-DDF model are more accurate than Void Growth Model and VGM-DDF model, respectively.(3) Based on the cyclic tests of notched bars and finite element analysis, the parameters of Cyclic Void Growth Model (CVGM) and Degraded Significant Plastic Strain Model (DSPS) were calibrated. At the same time, the Jia-Kuwamura model was verified in prediction of the fracture of notched bars under large strain and high loading cycles in this paper. Moreover, Unified Ductile Damage Factor (UDDF) Model was proposed based on the combination of the mechanism of voids change under cyclic loading and the concept of a damage factor. Taking different kinds of response of voids under tensile and compressive stress into account, this model can predict fracture under both monotonic and cyclic loadings. The model can also capture damage in real time. The results showed that both Cyclic Void Growth Model and Degraded Significant Plastic Strain Model described well with the fracture law under cyclic loading. However, since Degraded Significant Plastic Strain Model ignored the variation of stress triaxiality during loading, it fit worse with test results than Cyclic Void Growth Model did. It also illustrated that Jia-Kuwamura model can not predict fracture under large strain and high loading cycles ideally. However, the prediction of fracture by VGM-UDDF model and JC-UDDF model proposed in this paper fit well with the cyclic tests results of notched bars.(4) Based on the combined loading (shear and tensile) tests and finite element analysis, the relationship between critical equivalent plastic strain and stress triaxiality as well as shear stress ratio were studied. Additionally, the parameters of Stress Triaxility Shear Damage (STSD) model and Shear Stress Ratio Shear Damage (SSRSD) model were calibrated. What’s more, the concept of damage factor was also introduced to the shear damage and formed SDF model. The results showed that Shear Stress Ratio Shear Damage model is more suitable for describing the shear fracture mechanism in the combined loading (shear and tensile) tests conducted in this paper. And the SDF model based on SSRSD factor fit well with the results of combined loading (shear and tensile) tests conducted in this paper. From the results of both tensile tests of smooth and notched bars and combined loading (shear and tensile) tests, it showed that the fracture mechanism was not very clear when stress triaxiality is within the range from 0.4 to 0.8. But, it can be approximately treated that when stress triaxiality is larger than 0.7, the critical equivalent plastic strain is a function of stress triaxiality and when stress triaxiality is between 0 and 0.7, the critical equivalent plastic strain is a function of shear stress ratio.