A device of this kind is described in German patent publication No. 34 40 471. In this device a robot brings the article to be examined from a conveyor into a strong magnetic field. A crack in the surface gives rise to a particularly strong magnetic field on the surface in the region of the crack. If a solution which contains a magnetic powder is applied to the magnetized article to be examined the magnetic particles concentrate on the crack and are held there while the magnetic powder particles are washed away from the undamaged parts of the surface by the solution. On irradiation by means of a UV Lamp the crack-free surface only glows diffusely while the magnetic powder, which has a fluorescing agent added to it, gives a sharp crack indication in the region of the crack. This crack indication is registered by means of an image pick-up camera and is input into the digital image processing and control unit, where it triggers a sorting report.
To ensure reliable automatic testing a device such as this must be checked periodically. For this purpose it is known to provide a control body with defect patterns which can only just be detected reliably. This control body serves to demonstrate the sensitivity and to test the performance of the entire system. It is also possible to use test workpieces having natural defects.
For continuous monitoring of an automatic device for detecting and evaluating surface cracks a test body would have to be repeatedly passed through the device and then again demagnetised and cleaned. Practical experience however has shown that cracks, like artificial defects, grow with time and that the intensity of the crack indication becomes weaker after even a few test cycles, so that as time goes on monitoring with repeatedly used test bodies becomes ineffective.
Furthermore introducing test bodies or defective parts into an automatic system causes problems as there is always the danger that the test body or the defective part may not be removed again and therefore cause damage. Moreover test bodies having artificial defect patterns are expensive, which is of particular significance if they can only be used a few times.