Magnetic Particle Inspection: The "3D" Approach

The S-5000 "3D" Magnetic Particle System incorporates three modes for magnetizing a test part: current flow, coil induction, and field flow. Current flow magnetization involves sending current from the head stocks of the machine through the test piece (i.e., a "head shot"). This produces a circular field in the part. Coil induction passes a current through a large induction coil (two diameters are available) which can be used to produce a longitudinal field in the test part. The field flow mode provides a better way to produce a longitudinal field. The head stocks of the S-5000 act as large magnets so that when a test part is placed between them, it is magnetized. This results in a strong longitudinal magnetic field in the test part that is uniform over a length of up to 48".

Each magnetization mode can be controlled completely independently of the others. Thus, each mode can utilize AC, half wave rectified DC (HWDC), or full wave rectified DC (FWDC), with the magnitude of the current independently selectable. This can be done because, unlike other so-called multi-directional machines, the S-5000 incorporates three separate and discrete power trains.

Each magnetization mode can be used alone or they can be used simultaneously in "3D" mode. In reality, two or three magnetic fields cannot be imposed on a part simultaneously. The result would be a "washed out" vector average of the three fields. In "3D" mode, the fields are imposed 60° out of phase so that the magnetic domains inside the test part rapidly switch magnetization directions as the modes are switched. The magnetic particles forming the observable indications on the part's surface, however, do not have time to react to the rapid switching, so the "3D" mode gives the appearance of simultaneous magnetization in three directions.

Returning to the example of inspecting a 48" round shaft, the S-5000 could perform the inspection with a single "3D" shot. Current flow mode would be used to produce circular magnetization and, simultaneously, field flow magnetization would be used to generate the required longitudinal field. Thus, an inspection that would take four independent shots with a standard magnetic particle machine could be done in one shot with the S-5000. In addition, the "3D" approach has no difficulty in picking up the so-called oblique defects mentioned previously.

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