Formability and Strength of Sheet Metals Subjected to Complex Strain Paths

  • Trevor Davis

Student thesis: Doctoral ThesisDoctor of Philosophy

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.
Date of AwardAug 1985
Original languageEnglish
Awarding Institution
  • Aston University

Keywords

  • Formability
  • strength
  • sheet metals
  • complex strain paths

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