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 cannot 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 of a white background for visible penetrant or a penetrant that glows under ultraviolet light for fluorescent 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.).
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• The method has few material limitations, i.e. metallic and nonmetallic, magnetic and nonmagnetic, and conductive and nonconductive materials may be inspected.
• Large areas and large volumes of parts/materials can be inspected rapidly and at low cost.
• Parts with complex geometric shapes are routinely inspected.
• Indications are produced directly on the surface of the part and constitute a visual representation of the flaw.
• Aerosol spray cans make penetrant materials very portable.
• Penetrant materials and associated equipment are relatively inexpensive.
• Only materials with a relatively nonporous surface can be inspected.
• Precleaning is critical since contaminants can mask defects.
• Metal smearing from machining, grinding, and grit or vapor blasting must be removed prior to LPI.
• 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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