Comparison of Different Surface Strain Measurement Methods Used for Deformed Sheet Metal Parts

A recently developed automated method for measuring surface strains on a deformed three dimensional part is compared with a careful manual measurement method and the differences are discussed. The overall experimental procedure consisted of marking the sheet steel surface with square grids, forming them into different shapes, and measuring surface strains by the two different methods. The manual method involved measuring the distances in each square using a traveling microscope. The automated method is based on digital image processing techniques and is capable of measuring strain distributions over an area of interest. The measured results from both methods agree reasonably well, but some discrepancies are observed at locations where strains are localized.