Liquid Penetrant Testing (PT) is one of the oldest, widely used and important methods of nondestructive testing (NDT) available today. Used since the 1890s, the PT method applies oil or another liquid to the surface of an object and, in a variety of steps, enables the tester to isolate discontinuities. While modern materials have improved the basic penetrant process, despite the passage of time, the fundamental method remains unchanged. The apparent simplicity of PT inspection, however, can be deceptive. It is crucial that for all NDT technicians to be adept in PT, they meticulously employ the precise techniques needed to easily and quickly detect objects’ discontinuities.
The first known use of PT dates back to the development of the railroad in the U.S. during the late 1800s and early 1900s as a method for detecting cracks. Its primary use was to ensure the integrity of tracks, wheels, and other car components. The first method was the so-called “oil and whiting” process where an oil solvent was applied to the surface of an object, wiped clean using kerosene-soaked cloths and examined following the application of a chalk coating. Any surface discontinuities would then be visible as a result of staining by the chalk coating. The oil and whiting test was used less frequently after the introduction of the fluorescent dye materials process patented in 1941. The dye, typically red, was added to the oil that penetrates the objects. It would then be illuminated under fluorescent light to reveal the discontinuities. Since then, penetrant systems have continued to evolve from the oil and whiting through developments with dye materials, penetrant oils, various materials and developing agents. Fundamentally, the precise procedure of PT has changed very little.
The PT method can be used to detect surface breaking discontinuities on a variety of non-porous magnetic and non-magnetic materials, including metals, ceramics, glass and many plastics. PT is used on the final finished surface of an object to detect discontinuities. It is also used to detect fatigue damage or production problems. PT inspection will reveal discontinuities that break the surface of the object, including cracks, seams, laps, porosity and other surface features.
The process has three basic steps that must be performed properly and precisely. The basic component of PT testing is the application of penetrant, a low viscosity, highly fluid liquid to the surface of the object being inspected. Prior to application, the testing surface must be thoroughly cleaned to be free of any foreign materials or liquids that could block the penetrant from entering voids. If a meticulous cleaning is not administered, the effectiveness of the PT inspection is greatly reduced. Once the liquid has been applied to the clean surface, it is allowed to remain on the object for a sufficient time to allow the liquid to enter and fill any surface openings or discontinuities. Referred to as “penetrant dwell time,” this ensures the liquid can fully do its job before the appropriate moment when any excess is carefully removed. The final step of the PT process is the application of a “developer” which extracts the penetrant to reveal the discontinuities that previously were invisible to the naked eye.
There are several advantages to the PT method, particularly that it is a quick and relatively cost-effective way to inspect a large surface area or a large volume of parts or materials. The inspection process can also be portable with the dye penetrant contained in easy to carry aerosol spray cans, making the process convenient and inexpensive. Because of these and other characteristics, PT today is widely used in the oil and gas, power and aerospace industries. The greatest advantage for these industries is that PT can examine the entire surface of an object with a unique or complex shape such as engine parts, turbines or blades.
Penetrant Testing is typically one of the first methods learned by most new technicians and a critically important inspection method for NDT technicians to have in their skillset. While the PT method is seemingly straightforward, as it does not require highly technical equipment, the process can be deceptive. The smallest variation during the application and removal of the penetrant and application of the developer can result in vast discrepancies in inspection results. It is critically important, therefore, that both the operator and inspector possess the knowledge, skills and experience for reliable PT inspections.
Given PT’s feasibility, portability and relatively low cost, it is a valuable and widely used NDT method. While the process of PT inspection is easy to learn, it’s critical for inspectors and operators to take care to perform the test properly to avoid potential false results. This makes the results of the inspection completely dependent on the operator and inspector. NDT professionals learn new “tricks” as this dynamic field continues to evolve, and new technologies become available. But the proverbial old dog can confidently apply this time-tested approach with training and experience. Work smarter, not harder!
