Development of an automatic crack detection method for cupping tests on sheets and foils with a wall thickness below 0.1 mm

Abstract

In deep drawing and stretch forming, in-situ crack detection is usually performed by detecting the drop in drawing force, by analyzing structure-borne sound or by reflected light methods. Friction, vibration, and machine noise are disturbance variables that prevent crack detection in thin sheets and foils using the first two methods and highly reflective as well as inhomogeneous surfaces prevent crack detection in the last method. In order to avoid the above-mentioned disadvantages of the existing crack detection methods, a method based on transmission-illumination was developed. Key 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. The detection process works as follows: the luminous punch penetrates the sheet and forms it accordingly. As soon as a crack is formed, light penetrates the crack and falls onto the image sensor of the camera. The image processing unit detects the jump in illumination and triggers the sudden stop of the forming process. Forming tests were performed that show the stable and operator independent performance of the newly developed detection process using sheet samples made from steel and aluminum.