An aspect of plasticity with compressibility

Ductile metals containing initially negligible micro-void density are treated. Such materials are popularly used in metal forming processes. Variation of material density due to plastic deformation is investigated experimentally using very severe plane strain compression, and uni- and equi-biaxial tension. No volume or density change is measured even after very severe plane-strain compression, in contrast with its change after tensile deformation. With reference to the experimental results, the existing plasticity theory with compressibility is examined thoroughly, and a new proposition is presented. The decomposition of the void evolution rate into growth and nucleation terms is absolutely improved, in addition to other faults in the existing theory such as an unrealistically sharp rise in density decrease at early deformation. Several examples of the relationship between density change and the given equivalent strain are illustrated regarding tensile loadings to confirm the significance of the new proposition. Apart from density change, it is found in plane strain compression that, surprisingly enough, pure copper can deform stably up to about 34 in the logarithmic strain measure, which is equivalent to an elongation of about 4.3×1014 times its initial length in uniaxial tension.