Dye Penetrant Inspection
Dye penetrant examination is an important method of
non-destructive testing of materials.
This is the basic principle: often the surface of a
workpiece is covered with rust or paint which masks the material defects as
shown in the sectional view.
Therefore, the first step is always the pre-cleaning of the
test piece surface.
This must be done very thoroughly so that any defects
present here it is a crack are open to the surface.
In the second step that penetrant is applied to the test
piece usually a red colored low viscosity oil.
Due to the penetrance high surface wetting capability it
penetrates into the defects and cracks.
After several minutes of dwell time the third step is to
rinse the path surface cautiously with water.
The water removes the penetrance completely from the surface
but leaves it in the crack.
Now the fourth step after drying the test piece a developer
supplied on to the surface.
The developer is a fine grained white powder suspended in a
liquid it forms an even coating on the surface after drying it draws penetrant
from the crack out onto the surface.
The location of the crack is indicated clearly on the
surface and this is how it is carried out in practice.
We wish to inspect two samples a segment of a worm gear
shaft and a broken bicycle crank arm. Both test pieces have already been
cleaned.
First in line is the worm gear shaft.
The material tester applies the penetrant.
After a dwell time of about 10 minutes she removes the
excess penetrant from the surface by rinsing the test piece gently with water.
She takes care not to rinse for an unnecessary long time.
Because the penetrant is supposed to stay in the defects.
Now she dries the surface of the sample with a cloth to
absorb the excess water.
As the final step she applies the developer and lets it dry.
That's it. Several cracks are indicated on the surface in
deep red color.
Next the broken bicycle crank arm has to undergo the die
penetration test.
Spraying with penetrant, allowing to soak, rinsing, drying
and applying developer.
Clearly additional cracks are indicated on the fragments.
Finally the pros and cons at a glance:
Advantages of this test method are
- almost all materials can be tested and the method is
simple and low-cost for a single inspection.
Disadvantages are:
- only suitable for surface defects
- no information about the depth of flaws can be gained and
rough surfaces are difficult to test.
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