Study On Micromechanical Fracture Criteria Of G20Mn5QT And Its Applications To Fracture Prediction Of Connections

After the earthquake in 1994 in the United States and the Kobe earthquake in Japan in 1995,it is found that the fracture of the joint is usually accompanied by large plastic deformation,which can be attributed to the ultra-low cycle fatigue?ULCF?failure of the stress concentration area.As cast steel joint has become more and more popular with the application of spatial structure and new-style structure where connection node becomes more and more complex,prediction for the ULCF of cast steel joint has great research value.This paper focuses on the microscopic mechanics model for both monotonic loading case and cyclic loading case for the G20Mn5QT material.The accuracy of the calibration model was verified by the test of cast steel joints carried out by other researchers.The main research is as follows:?1?The microscopic mechanism model,SMCS model and VGM model,under monotonic load,and the micro mechanism model of the super low cycle reciprocating load,namely the CVGM model,the DSPS model and the ICVGM model,were introduced.?2?Based on the G20Mn5QT material standard specimen subjected to monotonic tensile test and the corresponding finite element analysis,the parameters?and?of microscopic mechanism model were determined,and its accuracy was verified through the bolt hole design test.?3?The calibrated DSPS and CVGM models were used to predict the fracture position and fracture load for 4 cast steel joints tensile test carried out by Liu Mingjie of Tianjin University.The prediction has high accuracy compared with the experimental results which verifies the applicability of the DSPS and CVGM model to predict the actual cast steel joints under monotonic loading of ductile fracture.?4?Based on the G20Mn5QT material standard sample for reciprocating loading test and the corresponding finite element analysis,the parameters?_DSPS,?_CVGMand?_ICVGMof microscopic mechanism model were determined,and its accuracy was verified through the bolt hole reciprocating loading test.?5?The ULCF service life prediction of the cast steel hollow spherical node was carried out by using the calibrated CVGM,DSPS and ICVGM models.The prediction ability of these models was investigated.The results show that the three models have close ability to predict the ULCF life for nodes under symmetrical cyclic loading,while ICVGM model is more effective for nodes under asymmetrical cyclic loading with small non-elastic compression.Based on the experimental study and the finite element analysis method,two parameters of the micro mechanism model under monotonic loading and three parameters of the micro mechanism model under cyclic loading were determined.The calibrated fracture prediction models were validated by examples,which mean they can be applied to the fracture prediction analysis of cast steel joints.This is the first time to study the micro mechanism model of cast steel material G20Mn5QT,and it has great application value to effectively judge the ULCF property of the cast steel joint.