Research On Failure Criteria And Damage Evolution Of Ⅰ/Ⅱ Mixed Mode Lip-shaped Crack

Lip-shaped crack is a widespread class of cracks for engineering materials.In engineering problems,the defects in material are not just some linear cracks or regular curves,but usually consist of the curves with polygon.This kind of crack has the characteristics of linear crack and oval crack at the same time.Therefore,it is more practical to describe by using lip-shaped cracks in engineering applications.Firstly,the off-axis load model of lip-shaped crack was established,and the theoretical solutions of the stress intensity factors of type I and type II were obtained by using conformal transformation and complex function theory.The expression of the energy release rate at the crack tip was deduced,and then the failure criterion of the maximum energy release rate of the I/II mixed mode lip-shaped crack was established.And combined with the Irwin small-range yield assumption,the lip-shaped crack failure model was plastically modified.Secondly,based on aluminum alloy AL7050,lip-shaped crack specimens with width-to-length ratios of 0,0.15 and 0.6 and with different crack deviation angles were prepared.And tensile experiments were conducted with the aid of DIC equipment in order to verify the established failure criterion of the I/II mixed-mode lip-shaped crack.The fracture load,fracture initiation angle of the crack tip and the DIC strain full-field was obtained.In addition,the mechanism of the abdomen fracture of the lip-shaped crack was discovered through experiments.The DIC strain full-field image was compared with the results of finite element analysis to complete the failure model of lip-shaped crack.Finally,based on titanium alloy TC4,high temperature tensile experiments of lip-shaped crack were carried out,and the tensile displacement-load curves,fracture loads and fracture initiation angles were obtained for each specimen.The finite element analysis of lip-shaped crack at high temperature was performed to explore the effect of temperature on the failure mechanism of the lip-shaped crack.