How can you determine that a surface scratch in an aluminum alloy propeller blade is not actually a crack?

• A. Clean the damage out to a saucer-shaped depression and spray the area with a dye-penetrant liquid; allow it to seep into a crack if present, wipe all of the liquid off the surface, spray with a developer; if the damage is actually a crack, the developer will pull the penetrant out and it will form a visible line
• B. Visually inspect and rely on appearance alone
• C. Use radiography to confirm crack presence
• D. Use ultrasound to confirm crack presence

Answer: (A)

Dye-penetrant testing is used to reveal any surface-breaking defects by letting a colored or fluorescent dye seep into openings and then showing where it leaks out with a developer. For a surface scratch that isn’t a crack, there isn’t an open path for the dye to reside or be drawn out, so no distinct indication appears. If a crack is present, the penetrant gets into the crack and, after applying the developer, is drawn back out along the crack, creating a visible line or indication on the surface. This makes dye-penetrant inspection a direct and dependable way to tell whether a surface flaw is just a scratch or a real crack on aluminum alloy propeller blades. Visual inspection alone can miss tiny cracks; radiography and ultrasonic testing can detect cracks but are not as suited to quickly distinguishing surface cracks at the blade surface, especially for shallow, surface-breaking flaws.