New ductile fracture criterion for prediction of fracture forming limit diagrams of sheet metals

A ductile fracture criterion is newly proposed to model fracture behavior of sheet metals for nucleation, growth and shear coalescence of voids during plastic deformation. In the new ductile fracture criterion, void nucleation is described as a function of the equivalent plastic strain, void growth is a function of the stress triaxiality and void coalescence is controlled by the normalized maximal shear stress. The new ductile fracture criterion is applied to construct a fracture forming limit diagram (FFLD) of a dual phase steel sheets of DP780 (1.0t). The FFLD is approximated using both the reverse engineering method and circle grid analysis (CGA) since DP780 fails with slight thickness reduction from the analysis of the fracture surface. Predicted FFLDs are compared to experimental results to validate the performance of the new criterion in the intermediate stress triaxiality between 1/3 and 2/3. The new criterion is also applied to construct the fracture locus of Al 2024-T351 (Bao and Wierzbicki, 2004) to validate the performance of the new criterion in the low and negative stress triaxiality. The fracture locus constructed by the new criterion are close to the experimental data points for all these two materials in a wide stress range from the uniaxial compression to the balanced biaxial tension. The new ductile fracture criterion is recommended to be utilized in finite element analysis to predict the onset of ductile fracture of sheet metals.