Dependence of Localized Necking Behavior on the Specimen Shapes in Tensile Testing of Ultra-Thin Copper Alloy Sheet

Abstract The effect of specimen shape on the fracture angle in tensile testing of copper alloy sheets was investigated. Specimens with a width-to-thickness ratio of more than 30 were investigated, and it was found that relatively thick and narrow copper alloy sheets exhibited a fracture angle oblique to the loading direction. In contrast, the fracture angle was perpendicular to the loading direction in the thinner and wider specimens. To understand the mechanism of the change in fracture direction, digital image correlation (DIC) analysis was performed. The DIC analysis showed that the oblique localizations of the strain rate appeared in two directions in any thickness and width after diffuse necking. In thick specimens, one of the localizations was selected by the oblique fracture. In contrast, two localizations remained until the perpendicular fracture in thin and wide specimens. The analysis of the ratio of local strain rate revealed that the length strain over width strain was basically unchanged until fracture. However, in a few cases, the length strain over width strain drastically changed just before the fracture. These results suggest that the remaining two localized strain rates and the change in the strain state lead to the perpendicular fracture in thin and wide specimen. Regarding the edge of the specimen, the fracture angle tends to be oblique. This is presumably due to the less constrained condition of the edge, which is the possible reason for the oblique fracture of the narrow specimen.