Why can liquid-penetrant testing detect surface flaws but not internal flaws?

Liquid-penetrant testing is a nondestructive testing method that relies on the capillary action of a dye or fluorescent liquid to fill surface-breaking discontinuities. Because the process requires the liquid to be applied to the exterior of the material and subsequently wiped away from the surface, it can only reveal flaws that have an opening to the outside.

Internal flaws remain undetected because they are sealed within the material's structure, preventing the penetrant from entering the defect or being drawn out for visual inspection. Consequently, this technique is strictly limited to identifying surface-level cracks and flaws, as it lacks the mechanism to penetrate or interact with subsurface features.1