Uncoupled Fracture Indicator Dependent on the Third Invariant and the Level of Ductility: Applications in Predominant Shear Loading

This contribution presents an improved ductile fracture indicator that is dependent on the normalized third invariant and the plastic strain at fracture, which is in turn dependent on the stress state of the material. Lemaitre’s damage model is used to numerically evaluate the location and level of displacement required to initiate ductile fracture of the alloy. Along with this analysis, the constitutive model based on Gao’s criterion was implemented, to which the fracture indicator was coupled. The proposed fracture indicator is considered a damage denominator function dependent on the normalized third invariant and the plastic strain at the fracture. This damage denominator function seeks to regulate the rate of evolution of the degradation of the material according to its state of stress and level of ductility, which makes the indicator, coupled with the post-processing step of Gao’s model, satisfactorily predict the correct level of force, the appropriate location, and the displacement necessary for the fracture of the material. The predictions made with the help of the indicator were compared to the performance of Lemaitre’s model. In addition, specimens that describe the low triaxiality ratio were evaluated using AISI 4340 alloy at different ductility levels. The new indicator proved to be more advantageous in predicting the mechanical behavior in the fracture of the material for different types of loads, allowing a good estimate of the displacement in the fracture and the levels of force.