On the directionality of strain localization when stretching aluminum alloy sheets in biaxial tension
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geneities which become visible at the surface in biaxial bulge tests, and of preferred directions of failure in equibiaxial stretching, can provide useful indications of the structural features which exert a dominant influence in particular cases. The examples described in this communication relate to commercially processed sheets of aluminum and its alloys. Inhomogeneous deformation of grains situated close to a free surface is a general effect of incompletely constrained deformation which gives rise to a well-known form of surface roughening. Even in sheets having a regular distribution of grain sizes, a relatively coarse grain size (e.g., with average grain diameters only one order of magnitude smaller than the sheet thickness) can be a significant source of strain localization in biaxial stretching. 6 Less obviously, in fcc metals, grains having {110} planes parallel with a sheet surface can be a source of localized strain inhomogeneity even under conditions of deformation which might be expected to be fully constrained. Such grains have a strong tendency to deform in plane strain with extension along a (100) direction.9 In sheet textures having strong {110} components this form of inhomogeneous deformation leads to the development of an interlaced pattern of directional surface grooves which has been termed 'basket-weave roughening'. For example, in an AA 3003 alloy with a {110} (111) main component the grooves were developed at --+ 35 deg to the RD and, in equibiaxial stretching, failure occurred along these groove directions. ~0 In the majority of commercially processed aluminum alloy sheets ruptures in equibiaxial stretching occur either in the RD or in the transverse direction, TD. Failures in the RD are very often associated with some form of mechanical fibring generated in rolling which leads to alignment in the rolling direction of microstructural components having different plastic properties (Figure l(a)). The plastic inhomogeneities may be associated with second-phase particles, irregularities in grain distribution, or with chemical segregation but, in aluminum alloys, segregation of components of the crystallographic texture is a frequent cause. When the preferred direction of failure in equibiaxial stretching is not the RD, it is reasonable to suspect that planar anisotropy associated with the crystallographic texture is exerting a controlling influence. However, in certain aluminum alloys which develop negative strain-rate sensitivity at room temperatures (e.g., alloys in the 5XXX and 7XXX series), there is another form of directional inhomogeneity which can be inherited from the rolling process which may cause failure in the TD direction. If macroscopic shear banding is generated in cold roiling, upon recrystallization it can lead to the formation of narrow bands having a grain structure, and presumably texture, different from those in other parts of the sheet (Figure 2). Such defects lead to TD failures in biaxial stretching associated with transverse surface features which are characteristicall
