Grain size and surface roughness effect on the instability strains in sheet metal stretching

This paper examines the influence of surface roughness which is related directly to the grain size to the onset of plastic instability and necking conditions. A theoretical analysis is presented of thickness and grain size dependence on the limit strains in the process of stretch forming of anisotropic thin sheet metals. The theory assumes that the roughness can be represented by the modified sinusoidal nonlinear model thickness variation, and the limit strains are calculated based on the modified Al-Qureshi/Bressan theoretical model, taking into account the influence of surface asperities. Theoretical results show that the plastic instability strains decrease in all cases as thickness to grain size ratio decreases. Comparison between theoretical and experimental results for different classes of roughness demonstrated reasonable agreement. Other relevant parameters were also discussed.