Using image processing on Erichsen cup test machine to calculate anisotropic property of sheet metal

Abstract

Abstract Erichsen cup test is widely used to test sheet metal formability and ductility according to standard testing methods such as: ISO 8490, EN ISO 20482 and JIS Z-2247. During the Erichsen cup test, the sheet metal is pushed by a ball-shape punch in multi-axis forming until the specimen cracks. In this study, a CCD camera was mounted on top of the testing machine and a LED ring light was used to provide the light source. The forming videos were captured in real-time and sent to an image processing computer program. The OpenCV library was used to develop the sheet-metal-forming image analysis program. The image was first converted into grayscale. The threshold methods were used to separate bright and dark areas. Then, the fitellipse function was used to identify the ellipse shape and position. The lengths of major and minor axes of ellipse were measured and the ratio between minor and major axis was calculated. The tests were then repeated for 0, 45 clockwise, 45 anti-clockwise and 90 degree orientations of the metal sheet. The anisotropic values of the specimens were calculated from the ration between major-minor axis. The calculated anisotropic results from the image analysis of the known material were compared with the conventional tensile tests results. It was found that the ratio between major-minor axes at different orientations showed a few variations. However, the values of planar anisotropic and average anisotropic were in good agreement.