Magnetic-particle inspection
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Magnetic particle inspection processes are non-destructive methods for the detection of defects in ferrous materials. They make use of an externally applied magnetic field and the principle that the magnetic susceptibility of a defect is markedly poorer (the magnetic resistance is greater) than that of the surrounding material.
The presence of a flaw (void) in the material causes distortion in the magnetic field through it, which in turn causes distortion of surrounding fields. This deformation of the magnetic field is not limited to the immediate locality of the defect but extends for a considerable distance; even through the surface and into the air if the magnetism is intense enough. Thus the size of the distortion is much larger than that of the defect and is visible at the surface of the sample.
The most common method of magnetic particle inspection uses finely divided iron or magnetic iron oxide particles, held in suspension in a suitable liquid. This fluid is referred to as ink and can include colored or fluorescent materials. The ink is sprayed or painted over the magnetized specimen to visualize the magnetic field. The magnetic particles are attracted by the surface field in the area of the defect and hold on to the edges of the defect to reveal it as a build up of particles.