Liquid Penetrant Testing

It is a widely applied and low-cost inspection method used to locate surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). PT is used to detect casting, forging and welding surface defects such as hairline cracks, surface porosity, leaks in new products, and fatigue cracks on in-service components.

Liquid penetrants are generally classified by type according to the dye contained in the liquid penetrant. The liquid penetrant testing process relies on liquid penetrant entering a discontinuity and subsequently being drawn back out and made easily visible on the surface of the part. The amount of liquid penetrant material entrapped in discontinuities is usually very small. If the discontinuity is to be detected, the very small amount of liquid penetrant must be highly visible. Colored dyes are added to the penetrants to increase visibility. Based on the dye, liquid penetrants are classified as one of three types: fluorescent, visible and dual mode. A further classification is by the removal method of the penetrant. This can be water washable, solvent removable or postemulsifiable.

A developer is applied after removal of the penetrant. The developer improves the intensity of fluorescent penetrants and provides contrast of visible dye penetrants. Developer also draws the penetrant from the discontinuity to the surface as well as spread the liquid to the surface enlarging the appearance.

Typical examples of components inspected using liquid penetrant testing are welds, valve parts, generator retaining rings, generator wedges, fan blades, non-ferrous parts of rotors and stationary blading and bearings.