Study on the behavior of elastic-plastic fracture under mixed I+II mode loading in aluminum alloy Ly12-J-integral analysis

The elastic-plastic fracture behavior of aluminum alloy Ly12 under mixed I+II mode loading was studied by finite element method and fracture test. A mixed mode elastic-plastic fracture criterion of J-integral was proposed by using the J-resistance curve, and the maximum fracture effective plastic strain ε_{p max} of different mixed ratios at crack tip were also calculated. The results show that, (1) the initiation J-integral values of different mixed ratios have the equation {JMC = J_Ii + J_IIi J_Ii/J_IC + J_IIi/J_IIC = 1, where J_Ii and J_IIi are the mode I and mode II components of the mixed initiation J-integral J_MC at a constant mixed ratio, respectively; (2) the relation between the J_MC and mixed ratio K_I/K_II is J_MC = K²_IJ_IC/(K²_I+αK ²_II) + αK²_II) + αK²_IIJ_IIC/(K² _I+αK²_II)+αK² _II), where α = J_IC/J_IIC; (3) J_MC increases with an increasing of mode II component, J_IIC is twice of J_IC for Ly12; and (4) the maximum fracture effective plastic strain ε_{p max} and stress triaxiality σ_m/σ of different mixed ratios at crack tip satisfy the formula of ε p max exp(3σ_m/2σ) = constant, where the constant is about 9.52 for Ly12. The relation with double parameters, ε _{p max} and σ_m/σ, can be used as the local fracture or damage mechanics parameter under mixed mode I+II loading.