Void Nucleation Effects in Biaxially Stretched Sheets

TLDR

The effects of void nucleation during deformation on forming limit curves are examined for both in‑plane and punch stretching using a porous plastic solid constitutive model. The study simulates void nucleation with a two‑parameter criterion for plastic‑strain‑controlled and stress‑controlled processes, adopts a ductile‑rupture criterion limiting void volume fraction and defining a thickness trough in punch stretching, and considers only plastic‑strain‑controlled nucleation in out‑of‑plane stretching. In in‑plane stretching, plastic‑strain‑controlled nucleation can significantly destabilize the forming limit curve, whereas stress‑controlled nucleation shows no significant effect under plane‑stress theory, and in out‑of‑plane stretching the forming limit curves retain the shape previously obtained without void nucleation.

Abstract

The effects of void nucleation occurring during the deformation history on forming limit curves are considered for both in-plane and punch stretching employing a constitutive model of a porous plastic solid. Both plastic strain controlled and stress controlled nucleation processes are simulated by a two parameter void nucleation criterion. For in-plane stretching, plastic strain controlled nucleation can have, in certain circumstances, a significantly destabilizing effect on the forming limit curve. However, within the framework of plane stress theory which neglects the enhancement of the hydrostatic stress due to necking, a stress controlled nucleation process is not found to be significantly destabilizing. In punch stretching a ductile rupture criterion, which limits the maximum volume fraction of voids, as well as the appearance of a well defined thickness trough, is adopted as a localized necking criterion. Only plastic strain controlled void nucleation is considered here in out-of-plane stretching. The resulting forming limit curves have the same shape as those obtained previously with void nucleation neglected.