Plastic instability and Lüders bands in the tensile test: the role of crystal orientation

Considèreʹs plastic stability criterion in tensile testing is rewritten by separating the effects of intrinsic and geometrical hardening for single and polycrystals deforming by slip. The Taylor factor is employed to describe the effect of orientation change in the stability of plastic deformation for which a new instability parameter has been defined. Analytical results are presented for single slip in f.c.c. and h.c.p. crystals, while numerical simulations were done for the polycrystal case using the Taylor model. Localization may lead to the formation of Lüders bands if the strains are limited either by intrinsic hardening or by geometric hardening (discussed on the example of a zinc single crystal).