In sheet metal testing, in-situ crack detection is either performed manually by purely visual inspection by the machine operator or automatically by a crack detection system. The automatic crack detection method, commonly integrated in sheet metal testing machines, evaluates the drawing force during forming. However, friction, vibration, and machine noise prevent reliable crack detection in thin sheets and foils. The same disturbance variables also prevent robust crack identification in thin sheets and foils by systems that analyze structure-borne sound. Crack detection systems that use reflected light methods, on the other hand, necessitate homogeneous illumination and are interfered by highly reflective as well as inhomogeneous sheet surfaces. In order to avoid the above-mentioned disadvantages of the currently existing crack detection methods, a procedure based on transmission-illumination was developed. Fundamental to this new development is the use of a translucent punch, which incorporates a light source and a camera connected to an image-processing unit. Building up on previous research performed with a 3D printed punch made from PVB, a punch made from alumina-based ceramics is investigated. The detection process works as follows: the translucent punch draws the sheet. As soon as a crack is formed, light penetrates the fracture and falls onto the image sensor of the camera. The image-processing unit detects the jump in illumination and triggers the immediate stop of the forming process. Forming tests were conducted to confirm the stable and operator independent performance of the newly developed detection process and the toughness of the translucent alumina-based ceramics punch.
