- Transducers – They come in two types single or double. Singles serve as receiver and transmitter. They are small and subject to wear and tear of frequent use. Transducers must be used with a couplant that allows a high frequency signal to travel to the test piece. Couplants as a rule are generic but certain applications may call for a specific formulation.
- Double type transducers are a separate receiver and transmitter. The generating probes are placed on one side of the test piece with the receiver probe on the other side. The signal passes through the piece and shows any flaws. UT systems using double probes are often used by manufacturers for large regular shaped pieces that can be submerged in a liquid medium that acts as a couplant.
- Cables – Check for condition (fraying, broken or loose connections).
- Test box – The test box generates and displays the signal from the test piece. Check the unit for condition; if not out of line have a technician demonstrate use on a test standard.
Magnetic particle (MP)
Inspection of ferrous materials is often accomplished using magnetic particle inspection methods. Methods of inspection include the use of fluorescent and nonfluorescent types of fluid. Dry methods of flaw indication using fine powder are also used and are effective. Parts are placed in the magnetic particle bench unit where a current creates an indication of any flaws as metal particulate aligns with the flaw in the surface of the test piece. Mag particle inspection can be accomplished on the work site using a portable yoke. It can inspect small local areas.
Key areas to check:
- Fluid must be checked for particulate concentration. The manufacturer sets the minimum particulate levels required. A small glass container called a centrifuge tube is used and the tube graduations identify the level particle concentration.
- Tooling and procedures for de-magnetizing parts.
- Fluorescent fluid requires the use of a black light. A test for light intensity must be periodically accomplished and recorded.
- An area for cleaning or removal of paint and other coatings.
- If dry powder is used, it must be periodically cleaned of any accumulated contamination, such as oils, paint chips, and dirt.
- Verify that the testing bench is functional and that no parts are missing.
- If possible have a test standard placed on the bench so that the test flaw can be observed. Observe the use of the preparation procedures, cleaning, and unit operation.
Liquid penetrant (LP)
Liquid dye penetrant is probably the most common NDI method used after visual inspection. Penetrant methods are an easy and cheap method for inspection of nonporous type metals. Personnel can be easily trained to accomplish this type of inspection and its use is very common in the field.
Penetrant inspection involves a series of steps beginning with cleaning the part or inspection area, application of the penetrant liquid with a waiting period that allows the penetrant to soak in (dwell time). The part is then cleaned and a developer powder is applied to draw any remaining penetrant from a flaw that can be seen under a bright light (dye penetrant) or black light (fluorescent penetrant). The remaining penetrant reveals the crack or flaw as it is drawn out by the developer.
- The auditor should check that penetrant and developers are controlled by a shelf life and they are properly stored.
- If fluorescent penetrant is used, check that the black light is periodically checked for intensity. It should be on a schedule. There are meters that can be used to check intensity and they should be on the calibrated tooling system as well.
- Large penetrant tanks for large articles will be clean and protected from local contaminants such as dust, paint, etc. There should be a location for preparation, application of penetrant, dwell time, and final inspection. Parts segregation should be maintained with unserviceable parts separated from serviceable or parts awaiting inspection.
- Compare the repair station internal processes and controls for accomplishing penetrant inspections with observed activity. Identify and observed deviations.
Radiological inspections (RI)
Check X-ray equipment for condition and type. X-ray methods may be electronically generated by a cathode ray tube – others use an isotope source. Darkroom facilities are required to develop the film although some NDI services are moving to digital imaging. X-ray exposures and methods are described in the inspection information for each aircraft. The NDI provider should be able to demonstrate their capabilities to meet those requirements. Safety is paramount as well. Check exposure meter records and tracking. Film storage procedures are necessary to preserve film quality. Developer and fixing solutions should be properly stored and, if manufacturer requires, under a shelf life control.
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