MAGNETIC PARTICLE 

The test material is electrically magnetized. Any cracks (discontinuities) present will cause flux leakage. These defects  are detected by the application of magnetic particles, which are attracted to the flux leakage. The magnetic particles come in various colors for contrast on different materials, including fluorescent. This method is effective for locating cracks in all ferromagnetic materials and is often used on welded joints, structural steel, bolts and castings.