Effect of Material Damage on Thermal Forming Limit Prediction for Hot Stamping

In the hot stamping process, the formability of high strength steel is limited by the evolution of void damage inside materials. Thus, to evaluate the formability of hot metal blank, the damage evolution must be fully considered. In the present work, the extended Gurson-Tvergaard-Needleman (GTN) damage model for normal anisotropic materials is used to characterize material damage behavior. The prediction method of forming limits considering void damage is developed with the framework of Marciniak and Kuczynski (M-K) theory. To confirm the validity of the proposed prediction method, the forming limit strains contained by Nakazima experiments are compared with the prediction results and good accordance is acquired. Then, the thermal forming limit diagrams (TFLDs) at different temperatures are established. Simultaneously, the effect of void damage on forming limits and the void growth process under typical deformation modes are investigated.