Effect of Void Shape and Its Growth on Forming Limits for Anisotropic Sheets Containing Non-Spherical Voids

Most failures of ductile materials in metal forming processes occurred due to material damage evolution- void nucleation, growth and coalescence of neighboring voids. Recently, Gologanu-Leblond-Devaux (J. Mech. Phys. Solids, Vol.41 (1993), pp.1723-1754) extended the classical Gurson model (ASME J. Engng. Mater. Technology, Vol.99 (1997), pp.2-15) to a ductile material containing an oblate ellipsoidal cavity. And, they proposed a new approximate yield function incorporating the initial void shape effects, which is significant especially at low stress triaxiality. In the present work, the Gologanu-Leblond-Devaux's yield function for anisotropic sheet materials containing axisymmetric prolate ellipsoidal cavities is adopted in evaluating analytically forming limits of sheet metals under biaxial stretching by Marciniak and Kuczynski (M-K) model. The effect of a void shape and growth on the forming limits of sheet metals under biaxial tensile loading is introduced and examined within the framework of the M-K model, along with the effect of including a first-order strain gradient term in the flow stress. To confirm the validity of the proposed model, the predicted FLDs were compared with experimental results for steel sheets. The predicted forming limits for the voided sheets were found to agree well with the experimental data.