The Anisotropic Yield, Flow and Creep Behaviour of Pre-Strained En24 Steel

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Abstract
Tests under combinations of tension, torsion and internal pressure have been performed at constant stress ratio on En24 steel, previously annealed, and then subjected to a pre-stress in either axial or circumferential tension or torsion. Post-yield behaviour showed marked room-temperature creep by all strain components in the logarithmic form ε = a In t + c. The initial direction of the incremental plastic strain-ratio vector was markedly different from isotropic behaviour and remained constant in direction with time. Increased loading resulted in a progressive rotation towards the isotropic direction. Anisotropic yield loci were established from the normality rule and from the backward extrapolation of curves of creep coefficient versus stress and stress versus ‘long-time’ strain. The yield locus was translated to the pre-stress point and this local work-hardening was accompanied by softening in both the transverse and reverse directions, i.e. the cross-effect and Bauschinger effect respectively. Yield loci in planes not containing the pre-stress axis showed softening in all directions and under axial tension-internal pressure the cross-effect caused a rotation of the locus. All yield loci were smooth and continuous. Yield criteria derived from the theories of Edelman and Drucker and also Williams and Svensson were in good agreement with experiment over the whole locus. Hill's theory was thought to be more appropriate to material behaviour following large deformations.

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