LIMITING FORMABILITY OF ORTHOTROPIC SHEET METAL

摘要

The bulge test assesses formability of sheet metal under frictionless, biaxial tension. Two criteria of instability are required to predict forming limit strains when bulging sheet by lateral pressure through a die. A pressure maximum is observed for circular dies and this marks the onset of necking that diffuses thereafter around the pole under falling pressure. Dies with sharp elliptical apertures produce plane strain failures under a rising pressure, lying along the rim at one end of the minor axis. Predictions to the critical pressure are derived for each failure and applied to rolled sheet metals. The theory conforms to experiments which show (ⅰ) a pressure maximum initiates thinning around the pole before a split appears and (ⅱ) edge failures occur suddenly with uniform thinning from pole to rim.