Combined effects of triaxiality, Lode parameter and shear stress on void growth and coalescence

Shear stress has been recognized as an important parameter, in addition to stress triaxiality and Lode parameter, that characterizes the stress state and influences the ductile fracture process. Micromechanical analyses are performed on a cubic unit cell with a centrally embedded spherical void in this paper. An effective method for controlling triaxiality, Lode parameter and shear stress component throughout the loading history is provided. The combined normal and shear stress is applied to investigate the effects of shear stress as well as the triaxiality and Lode parameter on the void growth and coalescence as well as material response. The results show that increasing triaxiality, shear stress and Lode parameter decrease the loading carrying capacity of the ductile solid and the rate of void growth at high and intermediate triaxialities. The unit cell model is then used to analyze two different void coalescence mechanisms, including internal necking at high and intermediate triaxialities and shearing at low triaxialities. The increasing weight of shear stress component facilitates the internal necking at high and intermediate triaxialities. At low triaxialities, the shear stress benefits to the void rotation, elongation, coalescence and reducing the distance of ligament between adjacent voids. Comparison of the state variables at the onset of void coalescence between the numerical modeling and the analytical criteria is performed. The results indicate that improvement to the current criteria is needed to obtain more accurate predictions of the onset of void coalescence under combined normal and shear stress over a wide range of triaxialities.