Ductile fracture under in-plane biaxial tension and out-of-plane compression

Abstract

A new experiment for determining the equivalent plastic strain at fracture of a ductile material subjected to loading under a state of stress of in-plane biaxial tension and out-of-plane compression is introduced. In this experiment a small diameter spherical punch is pushed into a thin specimen plate that is backed up by another plate made of a different material. Upon loading, the material element at the back surface of the specimen along the center line of the punch that is in contact with the backing plate deforms under a state of stress of in-plane biaxial tension and out-of-plane compression and eventually fractures. A combined experimental–numerical approach is used to determine the state of stress throughout the test and the final strain at the fracture point. The experiment is simulated using an assumed plasticity model, and the results are validated by comparing the simulated and measured quantities (force and displacement of the punch and strain at the back surface of the specimen). The results from the experiment provide data needed for the construction of a surface of plastic strain to fracture as a function of triaxiality and Lode parameter. In testing specimens made of 2024 aluminum and a backing plate made of annealed copper, the equivalent plastic strain to fracture is determined for Lode parameter of about −1 and several values of triaxiality.