Research Of Material Failure Basic On Stress Triaxiality

The dynamic fracture is very important for the survivability of the critical structure.Choosing the most suitable fracture criterion is the key steps for the safety assessmentwhen using FEM method. At present the constant strain criterion and Johnson-cookfailure model are the most common failure models in engineering application. Theconstant strain criterion is too simple and independent of stress triaxiality. Johnson-cookfailure model is a criterion in the stress triaxiality space, but we can see that it is amonotonic function which is in the wide range stress triaxiality. The accuracy needsfurther study. In this paper, a series tests including upsetting tests and tensile tests on14MnNbq providing clues to fracture characteristics for a wide range of stress triaxialitywas carried out. Several aspects outlined below summarized the research：(1) Based on the rectangular plate collision test, A series of low velocity impact testsand numerical simulation results are presented in this paper, which examine the fracturecharacteristics of plates struck by a sharp nosed project. The Johnson-Cook constitutiveand failure model were used in numerical simulation and the mesh size effect wasconsidered. The fracture characteristics of different thickness plate were discussed. Theresults showed that the parameters of the failure model and mesh size in the paper couldpredict the fracture process exactly. The way of increasing the thickness of plate couldimprove the crashworthiness obviously.(2) Quasi-static tensile tests on smooth and notched specimens were carried out,meanwhile numerical simulations of the tests were performed. It was found that the stresstriaxility during the entire deformation is not constant and equivalent strain to fracturedecreases with the stress triaxiality.(3) A pure shear test and combined loading (shear and tension) were developed andcarried out to get the information of fracture characteristics under shear stress state. The results showed that the fracture strain increases with the stress triaxiality.(4) A series of uniaxial compression tests of short cylindrical specimens which had asame diameter and different heights were carried out to get the information of fracturecharacteristics in negative stress triaxiality. The analysis results illustrated that that thereis a cut-off value of stress triaxiality equal to-1/3, below which fracture never occurs.The work of present thesis showed that the fracture locus can not be described byone smooth monotonic function as suggested by Johnson and Cook, but consists threebranches. It is should be pointed that there is a cut-off value of stress triaxiality equal to-1/3, below which fracture never occurs.