Strain hardening at high strain in aluminum alloys and its effect on strain localization

The modes of strain localization in the tensile testing of a sheet sample are diffuse necking, localized necking and, in some materials, localization in an unstable shear band. In a tensile test of a rate insensitive material, the normalized strain hardening parameter,H = (1/σ)(dσ/dε) has the values ofH = 1 for diffuse necking andH = 0.5 for localized necking. Curves ofH vs strain were obtained up to large values of plastic strain using the hydraulic bulge test. The materials selected were commercially important sheet alloys in the condition normally used for forming. It is shown that the materials have similarH vs strain curves in the range of uniform tensile straining, but the curves diverge widely at higher strains whereH falls below 1. This has important consequences on strain localization behavior. The limit strains of the alloys in simple tension and punch stretching show reasonable correlation with their values ofH and those alloys which are susceptible to catastrophic shear failure have low values ofH at high strains. Strain rate sensitivity adds to or subtracts from theH values obtained in this study and has an additional influence on strain localization. Formerly with Alcoa Laboratories, Alcoa Center, PA 15069, U.S.A