The verification of strains obtained by grid measurements using digital image processing for sheet metal formability

Grid marking, which is applied before mechanical tests, is a crucial step in formability tests. In this study, four different grid marking methods, serigraphy, electro-chemical etching, photo-chemical etching, and laser marking techniques, were investigated for the stability of grids and their measurement accuracy. After examining the performance of the grid marking methods, laser marking has been selected to obtain grids on the surface of aluminum alloy parts. To evaluate the efficiency of the strains calculation using grids, tensile tests were conducted that considered different levels of elongation. A forming limit diagram of the 6061-T6 Al alloy was constructed to demonstrate the influence of possible measurement errors. Verification of grid strain evaluation revealed that the error in size measurements is dependent on overall strain values and reduced with increasing overall strain as expected. The maximum error of the implemented method does not exceed 1% when 9% of overall uniform elongation is reached. The approach used in the study provides sufficient accuracy for evaluating strains in principal axis of deformed circular grids with minimalized costs.