An experimental study of the effect of sheet thickness and grain size on limit-strains in biaxial stretching

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Abstract

Effects of grain size on the limiting strains attained in biaxial stretching have been investigated using 70% Cu-30% Zn brass sheets of 1.0 and 0.5 mm thickness with regular grain structures. The limit-strains were closely related to the ratio of sheet thickness, to, to average grain diameter, do, over a range of todo between 2 and 30. In the earlier stages of stretching, growth in the amplitude of surface roughening was independent of to and close to being proportional to doϵ3 where ϵ3 is thickness strain in equibiaxial stretching. The diffuse instability strain was rather insensitive to the effects of the short-range strain inhomogeneities developed during such stable extension. Surface roughening became sensitive to to at an applied strain ϵ3c which decreased with decreasing todo. In stretching beyond ϵ3c macroscopic strain localisation became established in the form of a network of surface grooves. It is concluded that the influence of grain anisotropy on limit-strains in biaxial stretching cannot be predicted in the terms of available continuum models of defect growth. An attempt is made to identify the dominant material and geometric factors which control the onset of localised necking when grain anisotropy is the principal source of inhomogeneous straining.

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