When a non-porous surface needs to be inspected for defects without being damaged, a lab technician will turn to one of several types of NDT. A very common form of this testing is dye penetrant examination. This can be applied to all types of surfaces including ceramic, metal and plastic. Surface defects can be found quickly, with low expense and little training using this method.
To begin the Penetrant Inspection process, an operator cleans the object to be tested, using either a solvent or other common degreasing method. This ensures no grease or coating will prevent the penetrant fluid from being absorbed into a defect. After the part is cleaned, it is uniformly coated with the penetrant fluid and allowed to soak. After a period of time that corresponds to the type of penetrant being used, usually ranging from five to 30 minutes, excess penetrant fluid is removed from the object being tested. The removal process can be as simple as wiping off the fluid, to as complex as using a special emulsifier to bond with the fluid on the surface, allowing it to be washed away, while leaving the penetrant that has been drawn into the defects behind.
During the soaking period, penetrant fluid is drawn into cracks and other defects through capillary action. After the excess penetrant is cleaned through the Penetrant Inspection process, the inspector needs a method to view the defects through the penetrant left behind. To accomplish this, a substance called a developer is applied to the part. There are many types of developer with some being liquid and some being a powder. They are all white, however, and their primary function is to draw the penetrant out of the defects to make them visible to the naked eye.
Penetrant may simply be a bright color, usually pink or green, and when soaked up into the developer, the high contrast makes defects obvious to the inspector. The inspector can then mark the defects for either repair or further analysis. For very small defects, where the unaided eyed will have difficulty detecting any fluid pulled from the defect, there are penetrant fluids available that have fluorescent properties. The operator can then view the part under a ultra-violet light source, which will cause the fluorescent penetrant fluid to glow, making the defect detectable to the naked eye. Finally, after the defects have been marked or the part declared undamaged, the surface is cleaned to remove all developer and any remaining detection fluid. In addition to being very cost effective, this inspection is portable, requiring at a minimum, just a few spray cans containing a cleaner, penetrant fluid and developer. This makes dye penetrant Inspection a highly desirable testing solution for structures and other objects that are not practically moved to a lab.
