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186 Part III: Inspection and Test
Part III.D.1
As mentioned above, one of the main disadvantages of magnetic particle
inspection is that it works only on ferromagnetic material. On the other hand it
has many advantages such as:
• It is a simple concept compared to other concepts.
• It is a combination of two nondestructive testing methods, magnetic
flux leakage testing and visual testing.
• It can be used underwater.
• It is easy to apply, and objects’ surface preparation is not as critical
as with other NDT methods (Larson 2000) (Aircraft NDT)
(NDT-ED 2008c).
Liquid Penetrant. Liquid or dye penetrant inspection is considered an extension
of visual inspection and is used for detecting surface-breaking flaws on any nonabsorbent
material’s surface. For this type of test the object has to be cleaned thoroughly
to remove dirt and grease. Then the object is soaked in a fluorescent or
visible dye solution. After a specified dwell time, the excess penetrant is removed
from the surface and a developer applied. Often, a developer is applied to help
draw penetrant out of imperfections open to the surface, making them much more
visible. With fluorescent penetrants, the object must be viewed in a dark room
with an ultraviolet lamp, which sometimes is not practical. Some industries use
red dye in aerosol cans instead of the fluorescent material. This red dye doesn’t
need the ultraviolet light and can be viewed by the naked eye. Liquid penetrant
inspections are only able to detect surface-breaking flaws (Larson 2000) (Aircraft
NDT 2008) (Munns 2004). Figure 17.7 shows the following steps of liquid penetrant
inspection:
A. Sample before testing
B. Liquid penetrant applied
a. b.
UV lamp
c.
Eye
Visible
indications
d. e.
Figure 17.7 Liquid penetrant testing steps.