Effects of Geometric Array and Size of Internal Voids on Tensile Ductility of AZ31 Magnesium Alloy Sheets

This study aims to investigate the contributions of the geometric array and size distribution of perforated cylindrical voids to the tensile elongation of a metallic sheet through the numerical correction of the plastic constraint factor with regard to the geometric array of internal voids. In AZ31 alloy sheets fabricated via strip casting, internal voids were formed via mechanical drilling in the form of perforated cylindrical holes. The tensile strain decreased significantly (from 0.18 to 0.01) as the void area fraction increased to 40%, and its nominal value decreased with an increase in the void size, even when the geometric array (ratio of void size to ligament distance between voids, a/l) was maintained. A plastic constraint factor describing the geometric array and size distribution of perforated cylindrical voids was proposed through the modification of an original form for isolated spherical voids, and the theoretical prediction using a modified constitutive model agreed well with the experimental results. The tensile ductility of a metallic sheet with perforated cylindrical voids depends more sensitively than isolated spherical voids on the variation in the void area fraction, because the effective void area fraction is overestimated by the exclusion of the strain-hardening exponent term in the modified plastic constraint factor. Additionally, increases in the void size and void area fraction at a fixed a/l ratio lead to a reduction in the tensile strain, by the increase of the plastic zone and the intensification of stress concentration around a void.