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Liquid Penetrant Testing: Fundamentals and NDT Personnel Training

Liquid Penetrant Testing (LPT) is a non-destructive testing (NDT) technique used to detect surface flaws that may not be detectable to the naked eye under standard lighting conditions. Liquid penetrant testing is typically used for non-magnetic metals to reveal surface flaws and in some cases plastics. Penetrant testing is one of the simplest and most economical non-destructive testing techniques. Additionally, it requires the least investment in personnel in terms of formal technical training and on-the-job hours. This article discusses some of the basic principles of penetrant testing and how to qualify NDT personnel to perform NDT in this method. TKS training courses are eligible for Workforce and Economic Development Network of Pennsylvania (WEDnetPA) funding for Pennsylvania based students and employers.

Figure 1: Penetrant Testing – Affordable Non-destructive Testing.

There are couple technical principles associated with liquid penetrant testing that drive the quality control process: Surface Wetting and Capillary Action. PT is most effective when the penetrant uniformly wets the surface of the part tested. Surface tension exists between the liquid penetrant molecules and the inspection surface due to cohesive forces. When a liquid penetrant interacts with a specimen’s surface, a cohesive force responsible for surface tension competes with an adhesive force between the molecules of the liquid and the solid surface. This causes the surface to be wet, or to have a good wetting ability. This results in a contact angle between the penetrant and the surface of the test surface. If the angle is equal to or greater than a 90-degree angle, the wetting is considered to be poor.

Figure 1 Penetrant testing and surface wetting based off contact angle.

As the penetrant dwells on the surface of the specimen, it slowly migrates into cavities of interest that are open at the surface of the test piece. This method depends on a few critical factors that allow it to function correctly and yield results. The ability of the liquid to wet the entire surface, cleanliness of the flaw, and other variables contribute to success or failure of the NDT method.

Capillary action is the second physical mechanism that drives excellent testing results. Capillary action drives dye penetration into the depth of a surface crack and draws the penetrant from the crack depth to be contrasted against the developed.

Figure 2: Penetrant testing – capillary action.