Abstract
First introduced in 1963, forming limit diagrams (FLDs) have been empirically constructed to describe the strain states, or combinations of major (e1) and minor (e2) principal strains, at which a highly localized zone of thinning or necking becomes visible in the surface of sheet metal. Laboratory rigid-punch and pressshop experiments have shown that a single, gage-corrected FLD — known as the Keller-Goodwin Curve — is applicable to most grades of low-carbon, low-strength, 1008-type steels and is unaltered by normal production variations in material properties and cleanliness. Different FLDs are obtained, however, for steels whose properties differ widely from 1008 steels — such as high-strength or stainless steels — and for nonferrous metals. For some metals, ductile fracture terminates usable deformation and becomes the fracture-forming limit. Distinction is made between FLDs obtained from rigid punch tests and those derived from in-plane stretching of flat sheets in free space. Recently, theoretical calculations of FLDs have achieved some measure of success. Unfortunately, knowledge of the peak strain limit described by the FLD is insufficient to completely characterize the formability of a sheet metal. Equally important is the ability of the sheet metal to distribute the strain as uniformly as possible. Since the level of the FLD is relatively fixed for a given material, control of the strain distribution is the primary practical means available to sheet-metal producers and users to improve formability limits. The goal of current research is to apply forming limit criteria during the design or blueprint stage.
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© 1978 Plenum Press, New York
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Keeler, S.P. (1978). Forming Limit Criteria — Sheets. In: Burke, J.J., Weiss, V. (eds) Advances in Deformation Processing. Springer, Boston, MA. https://doi.org/10.1007/978-1-4613-4024-9_4
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