T.O. 33B-1-1
3-52
3.4.4.4
Pulsed Direct Current.
Pulsed direct current combines the strong magnetic field of direct current with the particle mobility of alternating
current. Pulsed direct current is produced by rectifying single-phase alternating current. This pulsating direct current
pulses at a rate and level to produce a noticeable surge peak in addition to providing the necessary vibratory action for
magnetic particle mobility. Even though pulsed, the direct current aspect permits the residual method of inspection to
be used.
3.4.4.5
Permanent Magnet.
When permanent magnets are placed on a ferromagnetic surface, the magnetic field travels through the surface from
one pole to the other. The flux field will be relatively straight along a line between the poles and strongest near the
poles. Field strength will vary and be weakest at a point midway between the poles. The actual field strength at any
point will depend upon the strength of the magnet and the distance between the poles.
3.4.4.6
Field Direction.
Regardless of the current selected, AC or DC, or the position of the legs, the magnetic flux field induced in a test
surface always traverses a path in the same direction, from one pole or leg to the other. The yoke is therefore oriented
in a transverse direction to the discontinuities being sought to obtain optimum results.
3.4.4.7
Current / Particle Application.
Magnetic particles may be applied either dry or in suspension in a liquid carrier. The part may be magnetized first and
the particles applied after the magnetizing force is removed (residual method of inspection applicable to DC or
specially designed AC units only), or the particles may be applied while the magnetizing force is still acting
(continuous method of inspection). In order to select the proper variations to obtain optimum results, the inspector
must understand the variations and how each affects the desired end result.
3.4.4.8
Dry powder / Wet Suspension.
The type of magnetic particles to be used is a choice primarily between the dry and the wet method, and secondarily
among the various colors that are available, including fluorescent colors. The decision is influenced principally by the
following considerations:
a. Size of the discontinuity. Dry powder is excellent for surface defects of moderate size. The wet method
is usually best for very fine and shallow defects.
b. Convenience. The wet method offers the advantage of easy, complete coverage of the surface of parts of
all sizes and shapes. Dry powder is more often used for very local inspections.
3.4.4.8.1
Selection of the color of particles to use is essentially a matter of securing the best possible contrast with the
background of the surface of the part being inspected. The differences in visibility among the black, gray, red, and
yellow particles are considerable on backgrounds which may be dark or bright, and when viewed in various kinds of
light, may be difficult to see. If some difficulty is experienced in seeing indications, the inspector should try a different
color of powder. In the case of the wet method, the ultimate in visibility and contrast is obtained by the use of
fluorescent particles. The fluorescent wet method has been used in constantly increasing numbers of inspection
applications for many years, principally because of the ease of seeing even the faintest indications.
3.4.5
Applications.
Hand-held yokes, because of their compact size, low voltage requirements, and minimal weight, are versatile, general-
purpose magnetic particle test equipment. They may be used at an inspection facility where parts are brought for
inspection, or they may be taken to the inspection site. They are used to test large castings and weldments, assembled
and welded structures, or component parts of assemblies without the necessity of disassembly. Yokes are used on parts
subject to arc burns, in the detection of surface cracks in welds and castings, and to locate fatigue cracks of large
assemblies that may not be conveniently inspected with either mobile or stationary equipment. Where no source of
electric current is available or where because of fire or explosive hazard the use of electric current is not permitted, a
