Laser digitizer-based sheet metal strain and surface analysis

This paper presents a new approach to sheet metal strain and surface analysis using data collected with a high-accuracy coordinate measuring machine mounted laser digitizer. First, the flat sheet metal blank is electrochemically etched with a hollow centre circular grid pattern. As the blank is pressed into the desired shape, the resulting strain deforms the circles into approximately elliptical shapes. The sheet metal is then laser digitized every 0.1 mm and the (X,Y,Z)(X,Y,Z) surface coordinates plus reflected light intensity (grey level) are simultaneously recorded. Parallel computer-based image processing separates the darker ellipse grid points from the lighter background, and orthogonal least-squares data fitting estimates the major and minor surfaces strains. Results can be presented as a forming limit diagram, or as a derived thickness strain plot. Surface analysis is achieved by computing and plotting a maximum curvature map and then quantifying feature depth as the normal direction amplitude between the locally highest and lowest points. Implementation of the research for a representative industrial part is included.