Circular grid pattern based surface strain measurement system for sheet metal forming

Abstract

A surface strain measurement system (XJTUSM) for sheet metal forming is developed based on close range industry photogrammetry. A circular grid pattern is electrochemically etched onto the surface of flat sheet metal before forming. As the sheet metal is pressed into a desired shape, these circular grid nodes are deformed into approximate elliptical shapes. Original images are then recorded from different photo stations using a single camera. The three-dimensional (3D) coordinates of these ellipse centers are reconstructed according to image processing technologies, which include noise reduction, adaptive thresholding, sub-pixel edge detection, least-square ellipse fitting, camera calibration, fast matching and reconstruction of grid nodes. Subsequently, a 3D grid generation procedure is performed to build the topology of separate grid nodes and the deformation gradient tensor is calculated. Finally, the surface strains are derived from the deformation gradient tensor and the forming limit diagram (FLD) is thus obtained. To evaluate the accuracy of the novel system, a tensile experiment was carried out, and the high-accuracy surface strain measurement system XJTUDIC was employed for comparison. The surface strains measured by two systems agree well. The XJTUSM system can be used to diagnose production problems, identify critical areas and/or verify the accuracy of numerical modeling.