FRACTURE PREDICTING IN BULK METAL-FORMING

An important concern in forming is whether the desired deformation can be accomplished without failure of the work material. This paper desc ribes the utilization of ductile fracture criteria in conjunction with the finite element method for predicting failures in cold bulk metal forming. Four previously published ductile fracture criteria are selec ted, and their relative accuracy for predicting and quantifying fractu re initiation sites is investigated. Experiments with ring, cylindrica l, tapered and Banged upset samples are performed to investigate the v alidity of the workability criteria under conditions of stress and str ain similar to those usually found in bulk metal forming processes. Th e implementation of ductile fracture criteria into a rigid-plastic fin ite element computer program is presented. Local stress and strain dis tributions throughout the deformation are computed and compared with e xperimental measurements. A general good agreement is found. However, only two of these workability criteria have successfully predicted the location at which fracture initiates for all the upset tests performe d in this work. The paper concludes with a discussion of the importanc e of the critical damage at fracture to remain independent from the te chnological processes.