Study of the relationship between J-integral and COD parameters under mixed mode I + II loading in aluminum alloy Ly 12

In this paper, the relationship between the J-integral and COD under mixed mode I+II loading was proposed and investigated. The J-integral was calculated by the Finite Element Method, and COD was defined by Rice's model and measured by a duplicating method in an aluminum alloy Ly12. The critical values of the J-integral and COD for a stable mixed crack initiation were also determined by a resistance curve. It shows that: (1) the mixed J-integral, J_M, and the mixed COD satisfy the relations of J_M = dn · σ₀ · CTOD + ds · τ₀ · CTSD and J_M = d · σ_yield · COD, where dn, ds and d are coefficients; CTOD and CTSD are the mode I and mode II components of COD, respectively; σ₀ and τ₀ are the tensile and shear stresses at the crack tip strip, respectively, and (2) the initiation values of the J-integral and COD of mixed stable crack growth increase with an increasing mode II component, the J_IIC value is 2 times greater than that of J_IC, and the COD_i for a pure mode II crack is 6 times greater than that of COD_i for a pure mode I crack.