24 Liquid Penetrant Examination (PT)
Liquid penetrant examination (PT), also known as dye penetrant testing, is an nondestructive examination technique that uses dyes suspended in high-fluidity liquids to penetrate solid materials and indicate the presence of discontinuities. It is low cost and requires little training to use.
Liquid penetrant examination is used to detect defects open to the surface, particularly in nonferrous metals such as aluminum, which cannot be examined by magnetic particle testing. (However, it cannot be used on materials with excessively porous surfaces, such as sintered metals.) The surface of a part must be completely clean and dry before administering liquid penetrant examination. The surface of the weldment is coated with a thin film of the penetrant, which is left on the surface for a predetermined amount of time known as dwell time. After the penetrant has been wiped clean, developer is applied to the surface to accelerate bleedout and enhance indication contrast.
Why It Works
Liquid penetrant examination uses the force of capillary action, which draws the liquids into all surface defects. This can be observed if two pieces of glass slides are held tightly together in a container of water. Capillary action causes the liquid between the slides to rise to a level above that of the water in the container.
Procedure
Liquid penetrant examination consists of six steps, followed in a set sequence to ensure accuracy and reproducibility. The PT procedure requires a cleaner, a penetrant (a solution or suspension of dye), and a developer (a material that is applied to the test surface to accelerate bleed-out and enhance the contrast of indications).
- Apply a cleaner to the surface to ensure that the surface is clean and free from dirt, oil, grease, or other materials that may adversely affect the test.
- Dry the surface.
- Coat the surface of the weldment with a thin film of the penetrant, which is allowed to remain on the surface for a predetermined amount of time, known as the dwell time.
- After allowing time for the penetrant to flow into the defects, remove excess penetrant from the surface. Only the penetrant in the defects remains.
- Apply the developer to the weldment.
- Visually examine the part to locate indications and defects.
Notes on the use of penetrants:
- Dwell time is the total time penetrant is in contact with the component surface, including application and drain times.
- Dwell time is directly related to the size and shape of anticipated discontinuities since discontinuity size determines the rate of penetration. For example, tight cracks require more than 30 min for penetration if an adequate indication is to be achieved, while gross discontinuities may be suitably penetrated in 3 min to 5 min.
- Penetrant application is done by immersion, spraying, or swabbing (brushing) on dry parts over the areas to be examined.
- The application is extremely messy. Care must be taken to minimize penetrant overspray onto other components and to control the runoff of excessive penetrant from the part being examined.
Notes on the use of developers:
- The specific developer used is selected according to the manufacturer’s recommendation for the type of penetrant used.
- The developer acts as a blotting agent, accentuating the presence of penetrant in a discontinuity by causing the penetrant within a discontinuity to seep over a greater area so that the size of the indication in the developer is larger than the actual size of the discontinuity.
- The developer also serves as a color-contrast background for the dye.
- Application consists of coating the test surface with a material to accelerate bleed-out and enhance indication contrast.
- Developer should be applied in several very light layers rather than in one thick coating, as too thick a coating of developer could prevent the penetrant from being drawn to the surface.