Liquid penetrant testing, sometimes referred to as penetrant testing, dye penetrant testing or LT, is a method used to test for cracks and other surface defects in non-porous materials like ceramics, metals or plastics, which does not result in destruction of the material being tested. While LT has a wide variety of applications, it is commonly used in industrial settings like the petrochemical industry, where the technique is used to test for flaws in pipelines and other machinery parts to help prevent equipment failure and accidents. LT is designed to be especially effective at testing for the presence of hairline and fatigue cracks and seepage or porosity. In addition to the gas and oil industry, LT is commonly used in the aerospace, engineering, automotive and precious metals testing industries as well as a host of other industries and applications.
Using liquid penetrant testing
After carefully cleaning the material to be tested, removing any dirt, grime or residue, the special liquid penetrant solution is applied to the surface, usually as a spray. Once applied, the solution remains on the surface for a predetermined period of time, allowing it to penetrate any tiny cracks, crevices and fissures on the surface of the test material. After the predetermined time has elapsed, the liquid penetrant solution is wiped off using a clean cloth and a chemical-based developer is applied directly over the test area. The developer interacts with the liquid penetrant solution, drawing it out of the cracks and fissures while coloring the areas where the surface defects occur to make them highly visible to the technician.
The effective use of liquid penetrant testing relies on the ability of the technician to clearly see the colors left behind by the developer. As a result, many products use fluorescent dyes to provide color that is easily visible, even in environments that are relatively dimly lit or when the test surface has colors or other features that would make standard dyes difficult to see. In fluorescent testing, the process is essentially the same except that a penetrant using a fluorescent dye is applied to the surface. After the developer is used, an ultraviolet light is used to help enhance areas where the fluorescent dye has penetrated. Under ultraviolet light, areas that retain the fluorescent dye will glow, making them much easier to visualize during follow-up inspection.
Advantages of liquid penetrant testing
Unlike other non-destructive testing methods, liquid penetrant testing is low cost and easy to perform, making it accessible to companies of all sizes and employees of all levels of expertise, requiring considerably less training to apply than other non-destructive testing methods like radiography, ultrasonic testing and magnetic particle testing. In most testing situations, all that is required to successfully perform liquid penetrant testing are the penetrant and developer solutions, a cleansing solution when needed, cleaning rags and, in the case of fluorescent testing, an ultraviolet light source.
Since so few materials and equipment are needed, liquid penetrant testing is also extremely versatile, enabling it to be used in tight spaces where testing equipment used by other non-destructive testing methods would be too large to be used. Due to these three factors – low-cost, ease of use and versatility – liquid penetrant testing remains popular among many companies and industries that rely on non-destructive testing methods.