Forming limit diagrams with the existence of through-thickness normal stress

Since necking in some processes such as hydroforming can occur at locations where in addition to the in-plane stresses a through thickness compressive stress acts, the plane stress assumption is not proper for these processes. Therefore, in this study, the effect of normal stress has been examined on the prediction of the forming limit diagrams (FLDs). Determination of the FLD is based on the Marciniak and Kuczynski (M–K) model with some modifications on the stress states for consideration of the normal stress effects. The yield function has been used in three dimensional form and a modified energy equation has been introduced in the groove zone. To verify the results, the published experimental data for AA6011 and STKM-11A have been used. It has been shown that the FLD shifts up when the normal compressive stress increases.