LIQUID PENETRANT TESTING
NON DESTRUCTIVE TESTING
LIQUID PENETRANT TESTING
Liquid Penetrant Testing
Liquid penetrant testing is one of the oldest and simplists NDT methods where its earliest versions (using kerosene and oil mixture) dates back to the 19th century. This method is used to reveal surface discontinuities by bleedout of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a "clean" surface discontinuity by capillary action. After a period of time called the "dwell time", excess surface penetrant is removed and a developer applied. This acts as a blotter that draws the penetrant from the discontinuity to reveal its presence.
The advantage that a liquid penetrant inspection offers over an unaided visual inspection is that it makes defects easier to see for the inspector where that is done in two ways:
It produces a flaw indication that is much larger and easier for
the eye to detect than the flaw itself. Many flaws are so small
or narrow that they are undetectable by the unaided eye (a
person with a perfect vision can not resolve features smaller
than 0.08 mm).
It improves the detectability of a flaw due to the high level of
contrast between the indication and the background which helps
to make the indication more easily seen (such as a red indication
on a white background for visable penetrant or a penetrant that
glows under ultraviolate light for flourecent penetrant).
Liquid penetrant testing is one of the most widely used NDT methods. Its popularity can be attributed to two main factors: its relative ease of use and its flexibility. It can be used to inspect almost any material provided that its surface is not extremely rough or porous. Materials that are commonly inspected using this method include; metals, glass, many ceramic materials, rubber and plastics.
However, liquid penetrant testing can only be used to inspect for flaws that break the surface of the sample (such as surface cracks, porosity, laps, seams, lack of fusion, etc.).
Steps of Liquid Penetrant Testing
The exact procedure for liquid penetrant testing can vary from case to case depending on several factors such as the penetrant system being used, the size and material of the component being inspected, the type of discontinuities being expected in the component and the condition and environment under which the inspection is performed. However, the general steps can be summarized as follows:
1. Surface Preparation: One of the most critical steps of a liquid penetrant testing is the surface preparation. The surface must be free of oil, grease, water, or other contaminants that may prevent penetrant from entering flaws. The sample may also require etching if mechanical operations such as machining, sanding, or grit blasting have been performed. These and other mechanical operations can smear metal over the flaw opening and prevent the penetrant from entering.
2. Penetrant Application: Once the surface has been thoroughly cleaned and dried, the penetrant material is applied by spraying, brushing, or immersing the part in a penetrant bath.
3. Penetrant Dwell: The penetrant is left on the surface for a sufficient time to allow as much penetrant as possible to be drawn from or to seep into a defect. Penetrant dwell time is the total time that the penetrant is in contact with the part surface. Dwell times are usually recommended by the penetrant producers
or required by the specification being
followed. The times vary depending on
the application, penetrant materials
used, the material, the form of the
material being inspected, and the type
of discontinuity being inspected for.
Minimum dwell times typically range
from five to 60 minutes. Generally,
there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry. The ideal dwell time is often determined by experimentation and may be very specific to a particular application.
4. Excess Penetrant Removal: This is the most delicate part of the inspection procedure because the excess penetrant must be removed from the surface of the sample while removing as little penetrant as possible from defects.
6. Indication Development: The developer
is allowed to stand on the part surface
for a period of time sufficient to permit
the extraction of the trapped penetrant
out of any surface flaws. This
development time is usually a minimum
of 10 minutes. Significantly longer
times may be necessary for tight cracks.
7. Inspection: Inspection is then performed under appropriate lighting to detect indications from any flaws which may be present.
8. Clean Surface: The final step in the process is to thoroughly clean the part surface to remove the developer from the parts that were found to be acceptable.
Advantages and Disadvantages
The primary advantages and disadvantages when compared to other NDT methods are:
Advantages
High sensitivity (small discontinuities can be detected).
Few material limitations (metallic and nonmetallic, magnetic and nonmagnetic, and conductive and nonconductive materials may be inspected).
Rapid inspection of large areas and volumes.
Suitable for parts with complex shapes.
Indications are produced directly on the surface of the part and constitute a visual representation of the flaw.
Portable (materials are available in aerosol spray cans)
Low cost (materials and associated equipment are relatively inexpensive)
Disadvantages
Only surface breaking defects can be detected.
Only materials with a relatively nonporous surface can be inspected.
Pre-cleaning is critical since contaminants can mask defects.
Metal smearing from machining, grinding, and grit or vapor blasting must be removed.
The inspector must have direct access to the surface being inspected.
Surface finish and roughness can affect inspection sensitivity.
Multiple process operations must be performed and controlled.
Post cleaning of acceptable parts or materials is required.
Chemical handling and proper disposal is required.
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