The forming limit curve for bending processes

Abstract The maximum strain that metal can suffer in some bulk forming processes is limited by the phenomenon of shear band formation, which precedes ductile fracture. The process of strain localization in the shear band has been analysed theoretically on the basis of the theory of plastic flow. It is assumed that the material does not only undergo strain-hardening, but also strain-softening, due to the growth and coalescence of voids and microcracks. Taking into account the post-stability stage of the strain localization process the limit strain, i.e. the maximum strain outside the shear band has been determined as a function of stress ratio. The theoretically predicted forming limit curve for bending processes shows satisfactory correlation with the curve obtained experimentally for 0.39% C carbon steel. In both cases the limit strain reaches its minimum for plane states of strain. The analysis does not include plastic anisotropy, strain rate sensitivity and other metallurgical features of the material.