Magnetic particle test is old and one of the most applied NDT techniques in the world for detecting defects which open to surface or are in subsurface of ferromagnetic material. In traditional magnetic particle test, when test being carried out on a part with dark surface such as not ground weld, rolled steel plate, people generally apply a layer of white “paint”, named contrast agent on the surface where will be tested before test to improve the contrast between indications and background, then make it easy to find the indicators might occur during test. When test being performed on a large part, people must choose prod equipments or yoke equipments for they being convenient, portable and suitable for various part. Whether the contrast agent might mask the defects? What thickness of the contrast agent is suitable? Is there any difference of sensitivity of detecting defects, especially subsurface defects, between prod equipment and yoke equipment? How to choose suitable equipment for a specific test? However, just a few people know it. The paper reported details the results obtained from a number of trials that were carried out on test plates that are with various natural or artificial defects. The differences of results between tests that are with contrast agent and that are not with contrast agent, between tests that are with different thickness of layer of contrast agent, between tests that are performed with prod equipment and that are performed with yoke equipment, have been investigated. And then we come to an objective, scientific conclusion.
Keywords: yoke，prod，contrast agent， Magnetic particle test，defects，
