Determination of Forming Limit Strains in Sheet Metals using an Automated Optical Procedure

Industry is progressing very rapidly as a consequence of continuous technological advances. Rigorous quality control has become an unavoidable step in the chain of production. The forming limit diagram (FLD), a plot of the maximum major principal strains that can be sustained by sheet materials prior to the onset of localized necking, is a useful concept for characterizing the formability of sheet metal. Both experimental and numerical results in the literature have shown that the level of the FLD depends strongly on the strain path. In this work, we purpose a system based on a fast, optical, coordinate acquisition device that calculates all the strain parameters of an industrial piece, to improve the accuracy of FLD determination. Those results are obtained with high accuracy and in real time. An application of this improved system is described: the strain paths on a sheet metal stamped part, during the different stages of the metal forming process, are obtained. These measurements are of great importance, even when the final results are placed below the forming limit curve (FLC). The deformation undergone by the material in some of the forming steps might cause later necking, tearing or localized thinning of the piece.