Forming limit diagrams (FLDs) are a convenient and often used tool for the classification of the formability and for the evaluation of the forming process of sheet materials. One possibility for the determination of a FLD is the Nakazima test method using a grid measurement method. In general, a circle grid that is electrochemically etched on the sheet surface is measured after the forming. This measurement method causes, in addition to the high effort of the evaluation, a low local resolution. Therefore an optical strain measurement system was developed that allows to meet the requirements of both short determination times and significantly higher accuracies at the determination of FLDs. Instead of a costly grid pattern, a stochastic spray structure is used that is applied by spraying. Based on the assignment of every pixel in two images under different strain levels, this method uses the correlation technique to determine the displacement fields of a sheet surface. In addition to the theoretical derivation of the optical strain measurement, experimental results of the determination of FLDs are presented. | 
