Magnetic Particle Inspection: The "3D" Approach

Successful Magnetic Particle Inspection (MPI) requires that the magnetic field set up in and on the test piece is at least roughly perpendicular to the defect or discontinuity to be detected. In practice, this means that several "shots" of the test part will be required to thoroughly inspect it for defects.

Consider the case of a circular rod or shaft, such as the one shown below. A circular magnetic field, indicated by the blue arrows, is useful for detecting defects (shown in red) whose axes are along the length of the shaft but not for ones with axes along the circumference of the shaft. In the picture below, the defect marked by the green arrow would be detected using the field shown, but the other defect would not.

In order to detect the other flaw shown in the picture, a longitudinal magnetic field must be set up in the test part. Such a field is shown in the picture below. The flaw marked by the green arrow would be detected with this field, but the other flaw would not. Thus, to detect both of these defects would require two independent "shots".

It is important to note that all flaw orientations are possible and may be encountered during testing. Thus, discontinuities with orientations intermediate between the ones pictured here (sometimes refered to as oblique defects) could be encountered. Neither the circular nor the longitudinal fields would necessarily detect these.

Circular fields are set up in a test part by one of two primary methods: (1) the so-called "head shot", in which electrical current is passed through the length of the test part and (2) the central conductor or "rod shot", in which a conducting rod is used to carry a current and the test piece, which is open in the center, is hung on the rod. Longitudinal fields are generally set up by placing the test piece inside of a coil. This method is sufficient to produce a reasonably uniform longitudinal field along about 18" of the test piece.

Thus, to inspect a 48" long shaft for defects of arbitrary orientation would require at least the following shots:

1. Head Shot - detects axial defects along length of shaft
2. Coil Shot 1 - detects circumferential defects in first third of shaft
3. Coil Shot 2 - detects circumferential defects in middle third of shaft
4. Coil Shot 3 - detects circumferential defects in last third of shaft

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