Magnetic Particle Testing

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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Current Level and Particle Application

Two methods of processing are used in magnetic particle testing. The method to use in a given case depends on the magnetic retentivity of the test object being tested and the desired sensitivity of the testing to be made. Highly retentive test objects may be tested using the residual method. The continuous method must be used on test objects having low retentivity. For a given magnetizing current or applied magnetizing field, the continuous method offers the greatest sensitivity for revealing discontinuities.

Wet method media may be applied to retentive test objects, which have been magnetized using the induced current method, while the test object is being magnetized or at any subsequent time. If applied to low retentivity test objects while being magnetized, careful control of the bath application is required to prevent washing away of indications after the magnetizing current has ceased.
On some test objects having very fine transverse discontinuities, it may be necessary to apply the liquid media (bath) while the test object is in a vertical or near vertical position. This allows the liquid to flow downward by gravity and across the discontinuities assisting in the formation of those indications that may be slow to develop. 

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Magnetic Particles and Methods of Application

The particles used in magnetic particle testing are made of ferromagnetic materials, usually combinations of iron and iron oxides, having a high permeability and low retentivity. Particles having high permeability are easily magnetized by and attracted to the low level leakage fields at discontinuities. Low retentivity is required to prevent the particles from being permanently
magnetized. Strongly retentive particles tend to cling together and to any magnetic surface, resulting in reduced particle mobility and increased background accumulation.
Particle sizes are very small, ranging from about 0.0005 to 0.015 cm (0.0002 to 0.006 in.) in commonly used formulations. Each magnetic particle formulation always contains a range of sizes and shapes to produce optimum results for the intended use. The smallest particles are more easily attracted to and held by the low level leakage fields at very fine discontinuities; larger particles can
more easily bridge across coarse discontinuities where the leakage
fields are usually stronger. Elongated particles are included, particularly in the case of dry powders, because these rod shaped particles easily align themselves with leakage fields not sharply defined, such as those that occur over subsurface discontinuities.

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Direct Current

Direct current, on the other hand, magnetizes the entire cross section more or less uniformly in the case of longitudinal magnetization, and with a straight line gradient of strength from a maximum at the surf ace to zero at the center of the bar in the case of direct contact (circular) magnetization. Magnetic fields produced by direct current penetrate deeper into a test object than fields produced by alternating current, making detection of subsurface discontinuities possible.
In the presence of direct current fields, dry powder particles behave as though they were immobile, tending to remain wherever they happen to land on the surface of the test object. This contrasts what happens with dry powder particles in the presence of alternating current and half wave fields. In these fields, the particles have mobility on a surface because of the pulsating character of the fields. Particle mobility aids considerably the formation of particle accumulations (indications) at discontinuities.

Pure direct current can be obtained from automotive type storage batteries, but today this method is seldom used, except occasionally in emergencies when a battery may be used to power a handheld magnetizing device. The disadvantages of using batteries are their weight when a number of them must be used to obtain high amperage currents, the frequent charging and maintenance required and their limited life and replacement cost.

Alternating currents used in magnetic particle testing are at low voltages. Current amperages range from about 100 A up to about 20 000 A, depending on the test object to be magnetized and the method of magnetizing. The higher amperages are obtained by using stepdown transformers that reduce power line voltages, while at the same time increasing current at about the same ratio. Exceptions are the much lower amperages drawn by handheld devices that operate from standard 120 V outlets. Alternating current and half wave direct current for magnetizing test objects are obtained from single phase systems or from one phase of three phase systems. Full wave direct currents are usually obtained from three phase systems using full three phase bridge rectifiers.

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