Abstract It is proposed that the increase in surface roughness which occurs with increasing strain reduces the effective thickness of a plastically stretched sheet metal so that a tensile instability condition occurs at surface strains smaller than those predicted by the classical analysis. It is thought that the empirical relationship for growth of surface roughness is introduced in a realistic manner although it is recognized that there is a need for more precise information of the effect of the surface grains on the load carrying capacity of the sheet metal. Limit strains are predicted on the assumption that an incipient neck at the instability condition develops into a groove. The growth of surface roughness is taken into account throughout the deformation and note is taken of the fact that the surface roughness effect is superimposed on the developing groove.
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