Formability and Strength of Sheet Metals Subjected to Complex Strain Paths

Abstract

The effects of changes in strain path on the plastic flow behaviour of steel sheet have been investigated for plain carbon, re-phosphorised, and dual-phase steels. Prestraining has been carried out in simple tension, plane-strain tension, and equi-biaxial stretching. A wide range of prestrain levels has been used in each case. Final testing is done in simple tension with continuous monitoring of flow stress, work-hardening behaviour, and plastic anisotropy throughout the test. The tensile tests have been carried out in several directions relative to the prestrain direction. The results show that the work-hardening characteristics and instability conditions can be greatly modified by the changes in strain path investigated. The major conclusions are: A) All of the steels behaved similarly. B) At low prestrains, a Bauschinger Effect is observed (a 'Bauschinger Hump'). C) Increasing prestrain leads to ‘latent hardening’ and a drastic loss of ductility at a critical prestrain. This is due to changes in the dislocation substructure restricting the number of active slip systems. D) A near-constant strain to instability is observed at high prestrains. This is attributed to activation of previously unfavourable dislocation sources. E) There is a transient increase in r-value after uniaxial and plane-strain prestraining. This is most likely due to exhaustion of mobile dislocations left over from the prestraining operation. F) The dual-phase steel does not show any increase in necking strains after biaxial stretching. This is attributed to failure of the martensite/ferrite interfaces.