An Experimental Investigation of Necking in Thin Sheets

Abstract

Abstract Due to its simplicity in instrumentation and data analysis, uniaxial tensile tests are commonly used for determining the plastic stress-strain relation of a sheet metal. However, the tensile stress-strain data obtained using the standard procedure is valid only up to the moderate strain level just before the onset of diffuse necking in the specimen gage section. Once the diffuse necking starts, the plastic deformation of the specimen becomes increasingly non-homogenous within the necked region. Questions arise regarding the validity of extrapolating the uniaxial stress-strain data to very large strains when one analyzes the forming failure of the sheet metal. Here an experimental technique and the related application procedure for obtaining rate-dependent, anisotropic, tensile stress-strain data of sheet metals at large strains up to fracture are described. Detailed plastic strain mapping results of a tapered commercially pure titanium thin sheet specimen are presented to characterize necking of the thin sheet. A brief discussion of a hybrid experimental and numerical methodology for estimating the strain hardening, strain rate sensitivity, and plastic anisotropy of sheet metals at large tensile strains is given. Preliminary results on necking in aluminum alloys, copper alloys, and low-carbon steel thin sheets are also presented.