Aspects of automated measurement of proportional and non-proportional deformation in sheet metal forming

This paper evaluates techniques of surface strain analysis in sheet metal forming based on a two-dimensional grid. Deficiencies of circle grid analysis, which in general cannot be used to measure non-proportional deformation, are discussed. An improved technique utilising a grid of squares or quadrilaterals for the automatic measurement of strains and stresses is presented. The advantage of applying a stress limit curve (SLC), instead of the forming limit curve (FLC), in the experimental analysis of deformation and in the evaluation of forming severity is discussed.