During cold forming of components and semifinished products damages may occur, like cracks, thinning, double draws or wrinkles and so on. Most production processes do not allow to analyze the product during the process, due to reasons regarding the safety and production. Only the finished product will reveal the damage done in the process.
Extensive post-process testing is to prevent further use of damaged components. A reasonable in-process detection of defects or monitoring the production parameters is seldom possible because of space problems or high mechanical stress on the sensors. High-Frequency-Impulse-Measurement (HFIM) is a reliable alternative.
The piezoelectric sensors of the HFIM process need force-fit contact with the forming tool, which has direct contact with the component. Structure borne noise is emitted by component and machine, travels through the whole installation and is detected by the sensors.
Both images show HFIM signals of a cold forming process. During this process, steel sheets were formed to produce a car body component. The left image shows a flawless process, the section shown is limited to emission that in friction between the steel sheet and the forming tool. The right image shows signals of the same forming process; the difference between the two images is that the amplitudes are risen in every frequency range. The reason for the high amplitudes are cracks inside the component which developed because of modified parameters of the cold forming process.