Penetrant testing (PT), also known as dye penetrant testing or liquid penetrant inspection, is a non-destructive testing method used to detect surface-breaking discontinuities in materials, such as cracks, pores, or seams.

It involves applying a liquid dye to the surface of a clean, dry part, allowing the dye to seep into flaws through capillary action. After the required dwell time, excess dye is removed, and a developer is applied to draw out the dye from defects, making them visible under appropriate lighting.