Prediction of Sheet Metal Forming Limits in Multistage Forming Processes

Abstract

A three-dimensional stress state was implemented in a modified version of the Marciniak and Kuczynski model to predict Forming Limit Curves (FLC) with different through thickness stress values, and the sensitivity of the predicted FLC to the applied out-of-plane stress component was analyzed. Furthermore, the effect of normal stress on the formability of sheet metals under non-proportional loading was investigated. It was previously found that the influence of normal stress on the formability of sheet metals decreases with increased pre-strain, when the same normal stress is applied during both stages of deformation. For multistage metal forming processes, such as tube bending and hydroforming, the first forming stage can be performed without considering the normal stress, whereas the normal stress must be considered in the second forming stage. Alternately, different levels of normal stress may be applied in each forming stage. The present work aims to understand the influence of the normal stress applied in different stages of deformation on the sheet forming limits. Different normal stresses were applied during either the first or second forming stage, and the predicted final stress forming limits were compared in order to determine the influence of the normal stress for non-linear strain paths.