Experimental And Numerical Study On The Fracture Of Nodular Graphite Cast Iron QT600 Under Different Stress States

In this paper,we designed a series of round bars,'thin-wall circular tubes,plates and notched specimens of different sizes and shapes by nodular graphite cast iron QT600.Smooth solid round bar,notched solid round bar,notched plate,plate with center hole specimens were taken to uniaxial tension test;and thin-wall circular tube specimens were applied to torsion test.The load-displacement curves were obtained and the fracture strains of the material under different stress states were tested.During the test,the deformations at cracking point of the samples were traced by using optical microscope.The microstructure of the material was observed by metallographic method and Matlab program,and the effect of different factors on the mechanical properties of the nodular graphite cast iron was discussed.The fracture morphologies were observed by both optical microscope and scanning electron microscope system.The different fracture forms of different samples were discussed referring to the macro and micro fracture surfaces,which provide the basis for the study of the failure of the material under complex stress state.Furthermore,ABAQUS finite element analysis software was applied to analyze the elastic-plastic large deformation numerical simulation.Based on the numerical simulation associated with the microscopic observation,the stress state parameters of the cracking point were determined,and the relationship between fracture strain and stress triaxiality was further investigated.According to the researches,the main conclusions are obtained as follows:(1)Based on tensile and torsion tests and numerical simulation,the equivalent stress-strain curves of tension and torsion are basically consistent.(2)According to the experimental results and numerical simulation results,fracture strain of notched solid round bar specimen decreases with the increase of stress triaxiality,which is consistent with the failure law of ductile materials in accordance with the GTN model,but the failure law for notched plate specimen and plate with center hole specimen is not in conformity with the law of that fracture strain and stress triaxiality described by the GTN model.(3)For the case that fracture strain is same but stress triaxiality is different,the difference in Lode parameter is found great,so this parameter's value can be judged as an important factor to material fracture.