A NEW CRITERION TO PREDICT NECKING FAILURE UNDER BIAXIAL STRETCHING

Localized necking limits useful forming of sheet materials. Several theoretical models predict limit strains under biaxial stretching conditions. Models that assume infinite-length defects (M-K approach) or finite compact defects (FEM approach) have been successful in predicting limit strains. In both these approaches, a failure criteria is required to predict the limit strain during simulation. Failure criteria based on comparing the principal strains, strain rates in the defect and the bulk region of the deforming sheet are reviewed. A new failure criterion is developed, which is defined in terms of thickness gradients that develop during biaxial stretching. This new criterion can be used under a wide range of forming conditions to predict limit strains. FLDs predicted using the new criterion are compared to: (a) FLDs predicted by other existing criteria and (b) Experimental FLDs. The effect of material parameters on the shape of the FLDs is predicted using the new criterion.