Calibration And Application Of Micromechanical Fracture Models For G20Mn5QT Cast Steel

Cast steel joints and connections,with the advantages of flexibility and adaptability,have been widely used in complicated building structures in recent years,such as in super high-rise buildings and large-span spatial structures.Since cast steels are prone to defects during the casting process,which may leads to fracture when subjected to loadings afterwards,it is of vital importance to study the fracture behavior of cast steel joints.It is known that steel structures are more likely to fracture under low temperatures,so studying the negative effects of low temperature on fracture behavior of cast steel joints and connections is also of great value.This thesis studies the fracture behavior of G20Mn5 QT cast steel using void growth model from the perspective of microscopic fracture mechanism.Both experimental and numerical investigations were carried out to calibrate the fracture parameters of micromechanical fracture models under different temperatures.In order to verify the applicability,these calibrated models were used to predict fracture performance of components and connections made of G20Mn5 QT.The specific research contents of the thesis are as follows:(1)The basic principle of micromechanical fracture models for monotonic loadings,i.e.the void growth model(VGM)and the stress modified critical strain(SMCS)model,were introduced.(2)Mechanical properties of G20Mn5 QT cast steel under the temperatures of20?,-20?,-40? and-60? were obtained by tension coupon tests,respectively.Ductile-brittle transition temperature of G20Mn5 QT cast steel was-12.31?,which was obtained by Charpy impact toughness tests.(3)Tests and finite element analyses on smooth notched tensile specimens were conducted to calibrate the fracture parameters of VGM and SMCS model for G20Mn5 QT cast steel.The fracture parameters of VGM under 20?,-20?,-40? and-60? are 1.32,1.14,1.00 and 0.88 respectively;while the fracture parameters of SMCS model are 1.26,1.06,0.94,0.86 respectively.Characteristic lengths of G20Mn5 QT cast steel under these same temperatures were obtained through scanning electron microscopy tests and set to be 0.214 mm,0.197 mm,0.167 mm and 0.139 mm.(4)The calibrated models were verified by conducting uniaxial tensile tests on double-hole plate specimens of G20Mn5 QT cast steel.Based on VGM fracture propagation method,the whole fracture process of double-hole specimen under monotonic loading was simulated.On the basis of finite element analysis and the calibrated micromechanical fracture models under temperatures,the effects of low temperature on the fracture initiation and crack propagation of double-hole specimens were analyzed.(5)The calibrated VGM and SMCS model were applied to beam-column cast steel connections to study the fracture behavior.The effect of low temperature on cast steel connections was also studied.The micromechanical fracture models of G20Mn5 QT cast steel calibrated in this thesis will have great application value for the analysis on fracture prediction and the study on fracture behavior under low temperatures of cast steel joints and connections.