MECHANICAL

Field Direction

The proper orientation of the magnetic field in the test object in relation to the direction of the discontinuity is a more important factor than the value or amount of the magnetizing current. Field direction for circular magnetization is shown in Figure. For best results, the magnetic lines of force should be at right angles to the discontinuity to be detected.

If the magnetic lines of force are parallel to the discontinuity, there will be little magnetic leakage at the discontinuity and therefore, if any indication is formed it is likely to be extremely small. Discontinuities located in the direction of current flow have the lowest probability of detection. Discontinuities perpendicular to current flow have the highest robability of detection.

Field direction for circular magnetization

Measurement of Magnetic Fields

The measurement of magnetic flux or field strength, either within a test object or at the test surface, is extremely complicated. There have been several attempts to develop practical methods or devices for this purpose, but these methods or devices have all been limited in success and contain serious limitations. They do serve a purpose in technique development if their limitations are understood. A procedure or technique typically will be developed for a particular test object using rules of thumb and past experience. The actual test object will then be subjected to the proposed procedure, and the
devices will be used to check the field strength at critical points as a comparative reference.
A hall effect gauss meter is a versatile magnetic field indicating instrument. It applies a current to a hall effect probe or sensor and amplifies the output voltage that is proportional to the measured magnetic flux density. There are several limitations that generally prevent its use as a shop instrument. First, it is a laboratory instrument and relatively fragile. It does not withstand normal use
required in a magnetic particle testing shop.

Another major limitation is that it measures only the flux passing through the probe or sensor at right angles. Placing the probe on the test object surface merely indicates the flux at that point and does not measure the field in the test object surface. Some attempt has been made to cut slots or drill holes in critical areas of sample test objects and then insert the probe.

This greatly improves the sensitivity and is perhaps the best approach at this time. Unfortunately, the availability of test objects in maintenance is extremely limited, and practical application of the hall effect gauss meter is confined to either the laboratory or new manufacturing operations.

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MAGNETIC PARTICLES TEST RESIDUAL METHOD

The residual method is a method of testing in which magnetic particles are applied to test objects after the test objects have been magnetized. The residual method is used only when test objects are magnetized with direct current. This method of testing is used when test objects have sufficient retentivity to form adequate magnetic particle indications at discontinuities. This method is used with both longitudinal and circular magnetization techniques, direct contact
and central conductor methods.

Residual testing requires the test objects to be retentive enough to hold magnetic particle indications at discontinuities. Usually, the use of the residual method is limited to the search for discontinuities that are open to the surface, such as cracks. Detection of subsurface discontinuities requires the stronger leakage fields at discontinuities that exist while the test object is being magnetized, as when the continuous method is used. Residual testing permits the magnetizing of test objects at one time and the application of magnetic particle media at a subsequent time.

When the central conductor method is used, testing of holes or bores is facilitated, since testing takes place after removal of the central conductor.
Care must be taken in the handling of test objects that have been magnetized, particularly test objects having smooth or machined surfaces, to avoid their being rubbed together or against other ferromagnetic test objects. Such rubbing may produce localized magnetized areas on surfaces that will attract and hold magnetic particles.

Magnetic particle indications produced on these areas are nonrelevant and are called magnetic writing. A technician maynotice that magnetic writing indications are not as sharp as those produced at surface cracks and is cautioned against misconstruing such indications as being caused by subsurface discontinuities. Whether an indication is caused by magnetic writing or by a
subsurface discontinuity can be determined by demagnetizing and
reprocessing the test object. Demagnetizing will remove the magnetic writing. If the indication returns after demagnetizing and reprocessing, it is an indication of a discontinuity at or near the surface.

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Magnetic Particles Wet Continuous Method

The continuous method implies that the magnetizing force is acting while the magnetic particles are applied. When the current is on, maximum flux density will be created in the test object for the magnetizing force being used. In some cases, usually when alternating current or half wave direct current is the magnetizing current being used, the current is actually left on, sometimes for
minutes at a time, while the magnetic particles are applied. This is more often needed in dry method applications than in the wet. To leave the current on for long durations of time is not practical in most instances, nor is it necessary. The heavy current required for proper magnetization can cause overheating of test objects andcontact burning or damage to the equipment if allowed to flow for
any appreciable length of time.

In practice, the magnetizing current is normally on for 0.5 s at a time. All that is required is that sufficient magnetic particles are in the zone and free to move while the magnetizing current flows. The bath ingredients are selected and formulated so that the particles can and do move through the film of liquid on the surf ace of the test object and form strong, readable indications. This is one reason why the viscosity of the bath and bath concentration are so important,
since anything that tends to reduce the number of available particles or to slow their movement tends to reduce the build up of indications.

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MAGNETIC PARTICLE TEST CONTINUOUS METHOD

The advantage of greater sensitivity for the continuous method is simple and basic. When the magnetizing force is applied to a ferromagnetic test object, the field within the test object rises to a maximum. This value or intensity is derived from the strength of the magnetizing force and the material permeability of the test object. 
When the magnetizing force is removed, the residual magnetism in the test object is always less than the field present while the magnetizing force was acting. The amount of difference depends on the retentivity of the material. The continuous method, for a given value of magnetizing current, is always more sensitive than the residual as determined by the strength of field in the test object.
Techniques have been developed for the continuous method that make it faster than the residual method. The indication is produced at the time of magnetization, whereas the residual method requires two steps: magnetization and application of particles, plus the added time for indications to build up if the immersion method is used. The continuous method is preferred unless special circumstances make the residual method more desirable.

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