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craft & technique
Dying to be Seen Dye penetrant is a relatively simple way to detect cracks— on the surface, at least Ron Sterkenberg
V
isual inspection is still the most common and economical way to inspect an aircraft or aircraft part for flaws and cracks. But the human eye is limited, and the use of more sophisticated inspection techniques like liquid penetrant, eddy current, ultrasound, and X-ray are needed to detect hidden flaws and cracks. Liquid penetrant testing is used to detect small cracks that are open to the surface and are not visi-
ble with the naked eye. You can’t detect cracks or flaws that are inside the material. There are two commonly used types of penetrant inspection methods: fluorescent liquid penetrant inspection, which uses a black light and fluorescent liquid, and the visible liquid penetrant inspection. The visible liquid penetrant inspection is the most suitable for the private-aircraft owner or small maintenance shop.
A crack in the landing gear trunnion is visible as a thin horizontal line toward the bottom of the smooth machined surface. 102
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For liquid penetrant to be effective, the material must be nonporous and the crack must be open to the surface. Liquid penetrant can be used for a variety of materials, including: metals, plastics, and glass.
Procedure For the visual liquid penetrant inspection method, the following materials are needed: a cleaner/remover spray container, visible liquid penetrant spray container or container and
Liquid dye penetrant is the most economical way to detect cracks, requiring cleaner, penetrant, developer, and a few rags.
Cracks show as a thin red line.
After cleaning, spray the surface to be tested and let it soak for five minutes.
brush, developer spray container, shop rag, and lint-free rag. Before beginning, always read the instructions on the spray container, because instructions of different manufacturers are not always the same. While the details of the spray itself may vary, the general method for using dye penetrants is the same. Step 1: Cleaning the part. One of the first and most important rules for successful liquid penetrant testing is that the cracks or flaws are open to the surface. If the cracks are filled with dirt or grease, the penetrant can’t enter and the crack won’t show up during the developing phase. If the surfaces are painted, it is best to remove the paint before testing. Spray on the area to be inspected, the cleaner/remover and wipe down thoroughly with a shop rag till all the dirt and grease are removed. Allow some time for the cleaner to evaporate before applying the penetrant. Step 2: Penetrant application. Spray a light coat of visible liquid penetrant on the surface, and wait 5 minutes before removal. The penetrant is drawn into the crack by capillary action. Use masking paper or plastic to prevent overspray on the workbench and floor, and tape off areas of the aircraft part you do not intend to inspect. This saves cleanup time later. EAA Sport Aviation
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craft & technique
In order to reduce cleanup, mask areas of the part you don’t care to inspect.
Too much developer will result in a blurred line. Clean and reapply the developer.
Step 3: Remove excess penetrant. After five minutes, wipe away the liquid penetrant with a clean, lintfree rag, and thoroughly clean the surface. Spray a little cleaner/remover on a clean rag and clean the surface. Repeat this process until all the penetrant is removed. The part is properly cleaned when no liquid penetrant is visible on the rag. Complete penetrant removal is very important, because if penetrant remains on the surface, it will show a false crack indication.
Proper developer application reveals a crack as a thin line.
Step 4: Apply developer. Spray a light, uniform coat of developer on the surface to be tested. The penetrant held in the crack will be drawn out by the blotting action of the developer and will show as a red line. If you use too much developer, the indication will be blurred. If this happens, clean off the developer and reapply a light coat. Step 5: Inspection. Carefully examine the tested surface. A crack in it will show up as a red line, but a red line may not be a crack. “False positive” indications can arise from improper penetrant removal or from penetrant being inadvertently transferred to the part by your hands. In the picture above you can see a faint red line, which indicates a crack in a landing gear trunnion. This crack is not visible with the naked eye. Step 6: Post-inspection cleaning. Remove all the developer as soon as possible with the cleaner/remover, and wipe down with a clean rag. The liquid penetrant process is relatively easy and inexpensive to perform. The results are good, but you can only find cracks that are open to the surface. It is very important to work in a clean environment and clean the parts thoroughly during the testing process. 104
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