Non-Destructive Testing Methods: Liquid penetrant testing (PT)

Like magnetic particle testing, this is a surface-only inspection method that can find imperfections such as cracks, surface porosity, seams, laps, etc. Unlike MT, it can be used on magnetic or nonmagnetic materials and welds, such as stainless steel and nickel alloys. In this method, a colored liquid dye known as a penetrant is applied to the surface that is being examined. The penetrant is left on the surface for a prerequisite amount of time (known as dwell time) to allow it to be absorbed into any imperfection by capillary action. After the dwell time is reached, the penetrant is removed with a solvent or water spray, and a developer is applied to the surface. The developer then draws any penetrant out of the flaws so that the indication is shown as a bright color on a contrasting background. PT is a relatively simple and low-cost testing method that can used on a wide variety of parts, welds, and surfaces. However, irregular or rough surfaces may cause false readings, and it cannot be used on high temperature parts.