Influence of superimposed hydrostatic tension on void growth in the neck of a metal sheet in biaxial stress-fields .2. plastic instability

The growth of a void in the neck of a metal sheet under superimposed hydrostatic tension as modelled in part-I is used in this paper to study its influence on the sheet metal forming limits using the M-K analysis incorporating the correction for triaxial stress state in the neck, and void growth equivalent of Marciniak inhomogeneity index as proposed by Rao and Chaturvedi. It has been observed that consideration of the triaxial stress state enhances the forming limits as compared to the M-K model but simultaneous incorporation of the void growth and coalescence criteria in the neck reduces them in the equibiaxial strain region. This analysis is also used to study the effect of sheet thickness on forming limits. The predictions based on this model show relatively faster increase in forming limits with higher sheet thickness as compared to the predictions based on the void growth and coalescence model assuming biaxial stress state in the neck. This is in accordance with the experimental observations of Haberfield and Boyles