The liquid penetrant test is one of the
oldest methods of nondestructive testing.
The method relies on a penetrant seeping into a discontinuity. It is
applicable only to surface defects or to subsurface defects with surface
openings. It is applicable to all
homogeneous materials except those of a generally porous nature where penetrant
would seep into and leak from the entire surface. Penetrant testing is therefore best adapted
to inspection of all types of surface cracks, porosity, laminations, and lack
of bond at exposed edges of joined materials, and of leaks in tubing, tanks,
welds and the like. It has been used
with excellent success on ferrous and nonferrous metals, ceramics, powdered metal
products, ceramics and glass, as well as on some plastic and synthetic
materials.
Penetrant inspection is basically a very
simple process. First a liquid penetrant
is applied to the surface of a part. It
is permitted to remain on the surface for a period of time, during which it
penetrates into any defects open at the surface. After the penetrating period, the excess
penetrant that remains on the surface is removed. Then an absorbent, light colored, powdered material
called a developer is applied to the surface.
This developer acts as a blotter and draws out a portion of the
penetrant which has previously seeped into the surface openings. As the penetrant is drawn out, into diffuse
into the coating of the developer, forming indications that are much wider than
the surface openings with which they are associated. An inspector then views the part and looks
for these colored indications against the background of the developing powder.
source : PCA Engineering
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