T.O. 33B-1-1
3-31
b. Quantitative to some extent.
c. Ultra-high permeability. Virtually no rententivity.
d. Can bend in one direction to conform to very tightly curved surfaces. The 0.002” thick QQIs can
conform to radii down to about 1/8”.
e. Can be re-used with careful application and removal practice.
3.3.9.3.2.3 Disadvantages of the QQI.
a. Parts must be clean and dry for application.
b. Usefulness readily destroyed with careless handling.
c. Not well adapted to dry powder applications.
d. Physical size limits application to some areas.
3.3.9.3.2.4 Application of the QQI.
To be effective, the QQI must be placed flaw side down and in intimate contact with the part surface. Also, it must be
emphasized that since the QQI responds to the field in its immediate vicinity, indications can be produced in the QQI
when no other ferromagnetic material is present. Obviously, the primary rule of assuring that the part is ferromagnetic
before attempting an inspection applies with the use of QQIs.
3.3.10
Sensitivity Level.
Any factor that affects the formation of magnetic indications at a discontinuity affects the sensitivity of that magnetic
particle inspection. Three of the most important factors are the field direction, current level and control of the
magnetic particle inspection media.
3.3.10.1
Current Level.
The formation of magnetic particle indications at discontinuities depends upon the strength of the corresponding
leakage fields. Since the leakage fields result from the field generated by the magnetizing current, the greater the
magnetizing current, the greater will be the strength of the leakage fields. Thus, the sensitivity of a magnetic particle
inspection is directly related to the exciting current. A current level that is too low produces leakage fields too weak to
form readily discernible indications; and a current level that is too high creates a heavy background accumulation of
particles which masks an indication. In circular magnetization, a high current level may also burn the contact points of
a part.
3.3.10.2
Inspection Media.
Sensitivity level is affected not only by the current amperage, but also by the kind of magnetic particle inspection
media, its control and its applications.
3.3.10.2.1
Liquid suspensions, because of the smaller particle sizes, are the most sensitive for the detection of surface
discontinuities. Dry powders can be better for detecting subsurface defects. Fluorescent materials have a higher
apparent sensitivity than do those used with visible light, such as the black and red particles.
3.3.10.2.2
Inspection of parts made of materials only moderately retentive requires careful control of the way the inspection media
is applied. Usually, maximum sensitivity is obtained by applying the media while a part is being magnetized and
ending it before the magnetizing field is removed. This is also true in the case of automatic wet-method inspection in
which the main bath stream is shut off shortly before the magnetizing current is ended; to avoid washing off indications
already formed.
