AN EXPERIMENTAL-STUDY OF LARGE FINITE PLASTIC-DEFORMATION IN ANNEALED 1100 ALUMINUM DURING PROPORTIONAL AND NONPROPORTIONAL BIAXIAL COMPRESSION

A new nonproportional biaxial compression experiment has been developed where a rectangular block first undergoes uniaxial compression and then, after finite deformation, it is subjected to large deformations (up to 45% strain) under biaxial compression. Experimental results on rectangular specimens of annealed 1100 aluminum subjected to uniaxial and biaxial compressive loadings are presented. The experiments include proportional and nonproportional loading paths. Stress and strain measures, directional changes of plastic strain increment, and existence of the von Mises equivalent stress-strain relation at large finite strains are discussed. Small volume changes were computed from the measured strains; however, density measurements after unloading indicate negligible change in density or volume. The classical Prandtl-Reuss equations are shown to predict acceptable results when the true or logarithmic strain loading paths are approximately proportional; however, many constitutive models can predict the experimental results during proportional loading paths. On the other hand, during sharp directional changes in loading paths, the predictions using Prandtl-Reuss equations in terms of deviatoric stress ratios and incremental strain ratios do not agree with experimental data for nonproportional loading paths, especially in the neighborhood of loading path change. © 1990.