Ductile fracture under proportional and non-proportional multiaxial loading

Abstract

This paper revisits proportional and non-proportional experiments performed earlier by the authors, with the aim to establish the fracture envelope of the material. The experiments involved inflation of thin-walled AA6260-T4 tubes under axial force and internal pressure. By controlling the force/pressure ratio during the experiments, proportional (i.e., radial) and non-proportional (i.e., corner) loading paths were generated. These experiments are reviewed here. Since fracture is of interest, a computationally-efficient finite element model with solid element discretization around the anticipated fracture location and shell elements in the rest of the tube is described. The Yld2000-2D and Yld2004-3D yield criteria are calibrated and implemented in the simulations. In concert with earlier findings, these criteria are found to offer significantly better agreement with the experiments than von Mises. In particular, the local normals to the yield locus show discrepancies under 3°, while 8°–12° are typical for von Mises. This has significant impact in the prediction of the induced strain paths. The fidelity of the modeling framework is established by comparing the predictions of average stress-strain responses, as well as post-mortem local surface strains, to experiments. Then, the finite element models are used to establish the fracture locus of the material, under proportional loading. The fracture strains are found to increase monotonically as the triaxiality approaches zero from above, i.e., no spike is found at triaxiality of 1/3. The fracture strains also increase at higher triaxialities. The presence of the radial stress shifts that spike from triaxiality of 2/3 (equibiaxial tension) to lower values. By superposing the non-proportional results, it is found that the fracture locus is path-dependent.