Dye Penetration Inspection (DPI)

This is a non-destructive testing (NDT) technique used to reveal surface-breaking defects that are invisible to the naked eye. A brightly colored liquid (the penetrant) is applied to the surface; it "wicks" into microscopic cracks via capillary action. After the excess is cleaned, a "developer" is applied to draw the dye back out, creating a vivid map of the cracks.