T.O. 33B-1-1
3-22
3.2.6.2
Considerations.
If the critical area of an assembly is completely accessible for inspection without any disassembly, and if the inspection
medium (magnetic powder or paste) can be removed after inspection, then it is acceptable to inspect those area or parts
in place without disassembly. For example, steel propeller blades may be inspected in the blade area when they are in
place on the aircraft, but to inspect the shank area, which is concealed by the hub, it is necessary to disassemble.
SECTION III
MAGNETIC PARTICLE INSPECTION TECHNIQUES
3.3
MAGNETIC PARTICLE INSPECTION TECHNIQUES.
3.3.1
General.
a. Magnetic particle inspection is a method of nondestructive testing that uses very small ferromagnetic
particles to reveal discontinuities in parts capable of being magnetized. Magnetizable parts are those
parts made of metals classed as ferromagnetic. To find flaws in metals that are not ferromagnetic, such
as aluminum, titanium, brass, etc., other inspection methods must be used.
b. This section includes a description of the types of electric currents used; the various techniques,
procedures and equipment used to magnetize parts; and the kinds of magnetic particles available for
either wet or dry application. Special magnetization techniques are also included.
3.3.2
Factors determining the choice Of Technique.
The choice of technique for a particular magnetic particle inspection depends upon:
a. The type of discontinuity or defect being sought.
b. The part's material, shape, and size.
c. The magnetic particle inspection equipment available.
3.3.3
Technique Variations.
The following points must be considered and the appropriate alternatives selected in order to achieve a particular
inspection result. A discussion of each alternative and the effect that it has upon the results of the inspection will
follow.
a. Type and amount of electric current to be used for magnetization.
b. The direction of the resultant magnetic fields obtained using these currents.
c. The kind of magnetic particles to be used, i.e., wet or dry.
d. How the inspection media is to be applied.
3.3.4
Types Of Electric Current.
Commonly, three types of electric current are used in magnetic particle inspection: alternating current (AC), direct
current (DC), and half-wave direct current (HWDC). Alternating current is used for the detection of surface
discontinuities only, whereas direct or half-wave direct currents are used for both surface and subsurface
discontinuities.
