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August 27 2013
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 referred 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
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.
Consider the inspection of a ring-shaped part. As shown in the figures below, such parts can have circumferential defects (on the left) or axial/radial defects (on the right).
Circumferential defects can be detected by setting up a toroidal field, a single line of which is shown as blue arrows in the picture on the left below. The toroidal field basically encircles the ring at every position along the ring's circumference. It allows for detection of circumferential defects to be detected on all faces of the ring. Using the central conductor approach, a circular field (below, right) can be established for the detection of any axial/radial flaws.
A toroidal field can be set up in a ring-shaped part by placing the ring in a coil with their axes parallel. Alternating current through the coil induces a circular electrical current in the part which, in turn, produces the toroidal magnetic field. An iron core can be placed along the common axis to enhance the field. The S-5000 "3D" approach is to use the field flow magnetization method to establish a strong axial magnetic field. As this field alternates its direction, a circular current is again produced in the ring resulting in a toroidal field. The ring is laying flat and is clamped between both head stocks.) The field produced in this case tends to be stronger and more effective for flaw detection than one produced with a coil. Thus, with the S-5000 in "3D" mode, a ring-shaped part could be inspected with a single shot, depending on the fill factor of the part
Another application of the toroidal field applies to the inspection of a landing gear trunnion fitting*. This part is basically ring-shaped but with many additional flanges, making it a very complex overall shape (see below). Five shims, numbered 1-5 in the picture below, were attached to the fitting. Each shim contained artificial defects oriented circumferentially and radially so that the appropriate inspection method could be developed and qualified.
The inspection was carried out using the "3D" mode of the S-5000. A central conductor was used to set up a circular field for detection of radial flaws and a toroidal field was used to detect circumferential flaws. The field flow magnetization method was utilized for producing the toroidal field by placing the part in contact with one of the head stocks.
In this case, with a 1" diameter central conductor rod, MIL-STD-1949A calls for eight shots with the part rotated between each one. Using the S-5000 and the five shims to prove the technique, it was found that this part could be completely inspected with only two "3D" shots, with the part rotated 180° on the central conductor.
* The description of the inspection of the landing gear trunnion fitting and the drawing of the part were taken from a detailed account of the issue, published in Quantitative Quality Indicators (QQI's) for Magnetic Particle Inspection by D.J. Hagemaier, presented to the 1990 ATA NDT FORUM in Montreal, Quebec, Canada on September 11-13, 1990. [Douglas Paper 8524]
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