Failure by fracture in bulk metal forming

Abstract This paper revisits formability in bulk metal forming in the light of fundamental concepts of plasticity, ductile damage and crack opening modes. It proposes a new test to appraise the accuracy, reliability and validity of fracture loci associated with crack opening by tension and out-of-plane shear under loading conditions different from those found in conventional tests for bulk formability based on cylindrical, tapered and flanged specimens. The new formability test consists of expanding rings of various wall thicknesses with a stepped conical punch and allows investigating the onset of failure by cracking under three-dimensional states of stress subjected to various magnitudes of stress triaxiality. The presentation is supported by finite element analysis and experimentation in aluminium AA2030-T4 and results show that failure by fracture under three-dimensional loading conditions can be easily and effectively characterized in the space of equivalent strain to fracture and stress triaxiality.