A review of certain aspects of the Bauschinger effect in metals

Certain aspects of the Bauschinger effect (B.E.) in metals (in particular two-phase systems) are reviewed. The B.E. is usually determined from simple forward and reverse loading tests, and is manifested by the reverse flow curve exhibiting a reduced elastic limit, a well-rounded appearance of the initial plastic portion and permanent softening vis à vis the forward hardening curve. Even under such simple loading systems a reliable a priori estimate of the reverse flow behaviour is still lacking. A number of macroscopic and microscopic models which have been proposed for reproducing the forward and reverse flow behaviour are discussed. The macroscopic models are phenomenological in nature and base on the continuum theory of plasticity. The microscopic approach aims to identify the hardening mechanisms and the magnitude of the internal stresses created within the deforming material; the B.E. is a natural consequence of the unrelaxed internal stresses that are developed. Some of the strengths and weaknesses of the models are discussed in the light of experimental data available from quasi-static forward and reverse loading tests on a variety of materials. Recourse is also made to data from some dynamic cyclic loading tests. In two-phase systems the volume fraction of the second phases appears to be one parameter that can be correlated (at least qualitatively) with certain reverse flow features. A definitive account of the role of second-phase constituents is still lacking. In conclusion the implications of the B.E. in plate fabrication processes which involve reverse strain cycles, such as some pipemaking processes, are discussed.