Combined instability and crack failure criterions for prediction of multilayer aluminum material behavior

Beside of the classical failure prediction in sheet metal forming based on the forming limit curve, also other failure limits for bending cracks and shear cracks are needed. The experimental evaluation of those additional limits is still not sufficiently investigated. In the present paper two different methods are investi-gated. The first one bases on the direct evaluation of the optical strain field measurements methods. The second method uses thinning measurements on micrographs in the cracked zone of the Nakajima speci-mens. The so specified fracture limits will be used and compared with the prediction of critical angles in hemming experiments. The results shows that especially the second method delivers reasonable predictions of the critical strains under bending conditions. The investigation was done for special Al-alloy materials with monolithical as well as three layer composite structure.