T.O. 33B-1-1
3-23
3.3.4.1
Alternating Current (AC).
The use of alternating current in magnetic particle inspection is effective only for the detection of surface
discontinuities that comprise the majority of service-induced defects. Fatigue, overload and stress corrosion cracks are
examples of cracks usually open to the surface.
3.3.4.1.1
The shallow penetration of AC fields into the part at the usual power line frequencies of 50 and 60 Hertz precludes the
use of AC for the detection of subsurface discontinuities. The shallow penetration is due to a skin effect. The skin
effect is the crowding of magnetic flux or electric current outward and away from the part center. The crowding
phenomenon is caused by self-induced flux or currents that reduce the interior density of the flux or current. The skin
effect is the reason AC is recommended when inspecting for service-induced surface defects. However, the skin effect
of AC is less at lower frequencies, resulting in deeper penetration of the lines of force. At 25 Hertz, the penetration is
considerably deeper, and at frequencies of 10 Hz and less, the skin effect is almost nonexistent.
3.3.4.1.2
The alternating currents used in magnetic particle inspection have low excitation voltages. Currents, from stationary
equipment, range from about 100 amperes to 10,000 amperes depending upon the test part and the magnetization
technique. The high currents are obtained using step-down transformers that reduce line voltages to about 20 volts.
Lower amperages are available from hand-held devices that operate from standard 115-volt outlets. Alternating current
(AC) and half-wave direct current (HWDC) are obtained from single-phase systems or from one phase of three-phase
systems. Full-wave direct currents (DC) are usually obtained from three-phase systems using full-wave, three-phase
bridge rectifiers.
3.3.4.1.3
If the defects sought are at the surface, AC has several advantages. The rapid reversal of the field imparts mobility to
the particles, especially to the dry powders. The "dancing" of the powder helps it to move to the area of leakage fields
and to form stronger indications. This effect is less pronounced in the wet technique.
3.3.4.1.4
Alternating current has another advantage in that the magnetizing force is determined by the value of the peak current
(at the top of the sine wave of the cycle). The peak current is 1.41 times greater than the current value read on the
meter. Alternating current meters read more nearly the average current for the cycle rather than the peak value. To get
equivalent magnetizing effect from straight DC, more power and heavier equipment are required. Thus AC equipment
for a given output of magnetizing force can be lighter and less costly, and better adapted for portability.
3.3.4.2
Direct Current (DC).
Magnetic fields produced by direct current penetrate deeper into a part than fields produced by alternating current,
making possible the detection of subsurface discontinuities. For longitudinal magnetization DC magnetizes the entire
part's cross-section more or less uniformly. For direct contact (circular) magnetization a straight-line gradient of
strength (from a maximum at the surface to zero at the center) is experienced. Direct current generally is used with wet
magnetic particle techniques. In the presence of DC fields’ dry powder particles are relatively immobile and tend to
remain wherever they happen to land on the surface of a part. This is in contrast to what happens with dry powder
particles in the presence of AC or HWDC fields. In these fields the particles have mobility on a surface due to the
pulsating character of the fields. Particle mobility aids considerably in the formation of particle accumulations
(indications) at discontinuities.
3.3.4.2.1
Pure direct current can be obtained from automotive type storage batteries. Today this technique is seldom used except
occasionally in emergencies when a battery may be used to power a hand-held magnetizing device. The disadvantages
of using batteries are their weight, when a number of them must be used to obtain high currents, the frequent charging
and maintenance required, and their limited life and replacement cost.
