Liquid Penetrant Inspection
Liquid penetrant inspection is a process that is both easy to use and inexpensive. As such, it is one of the most widely used inspection processes used in aviation.
Because of its ease of use and wide availability, many in the industry take dye penetrant for granted, allowing anyone, under any conditions to inspect critical aviation parts without the benefit of proper training and controlled conditions.
Like any other inspection process, liquid penetrant, whether visible or fluorescent, must be applied through a controlled process in order for it to be accurate. These controls include such things as dwell time, rinsing practices, wiping practices, penetrant selection for the conditions and parts being inspected, black light intensity, etc.
The following is a basic review of good practices for using liquid penetrants.
According to Magnaflux, one of the manufacturers of liquid penetrants and related equipment, the liquid penetrant inspection process uses the natural seepage of a liquid around a suspected flaw to create a recognizable indication of all types of cracks or surface opening defects.
Penetrants are useful only for surface breaking cracks in a solid and non-porous material. Its sensitivity is greater than that of magnetic particle inspection if it is applied properly.
General instructions for use
According to Magnaflux, penetrant inspection is accomplished in five simple but critical steps. One of the most important steps in the process is the initial cleaning of the workpiece in order to "open" potential cracks to the surface. Oil, water, dirt, grease and other contaminants that are not thoroughly cleaned will tend to accumulate penetrant that can either mask real indications or create false indications of defects. A cleaner/remover and rag should be utilized, while rags and paper towels that leave residue on the inspection surface should be avoided. It is critical that the cleaned surface be completely dry before the penetrant is applied. This is particularly important when using water-based cleaners.
The desired degree of sensitivity and cost are usually the most important factors in selecting the proper penetrant method for a given application. The "visible" liquid penetrant method is utilized primarily for localized applications. The fluorescent method is used for larger pieces and for volume applications and is actually more accurate. When using either method, the next step in the inspection process is the application of the penetrant fluid (usually red in color for visible or a transparent green cast for fluorescent) to the cleaned surface, forming a film over the area. The aerosol spray or liquid should spread freely and uniformly over the surface and migrate into any openings. With the visible method, the vivid red indications contrast the light background of developer under visible light. Application time varies depending on crack size, shape, characteristics of the defect, physical characteristics of the fluids and environmental conditions. Generally, you should allow at least ten minutes for complete penetration. If the temperature falls below 40F, increase the dwell time to compensate. When using immersion, the dwell time can vary from three to five minutes. Check with the manufacturer of the penetrant, however, to ensure proper dwell times. Also with immersion, parts can be immersed one at a time, or small parts can be batch processed. When batch processing a number of parts at once, they must be separated from each other during the immersion and dwell period. Contact between parts interferes with the formation of a smooth, even coating of penetrant.
The visible liquid penetrant inspection method employs color contrasting water wash, post emulsifiable, and solvent removable penetrants to satisfy all but the highest range of sensitivities. The fluorescent method can be used at all ranges of sensitivity.
Post-emulsifiable penetrants are penetrants that cannot be completely removed from the part surface with water. They require a second processing step to convert the surface penetrant layer into a mixture that can be removed with water.
Removing the excess penetrant may be the most difficult step in the entire process. When using the visible method, use a dry cloth, then a damp cloth to remove excess penetrant. Keep in mind that insufficient cleaning will leave a background of penetrant on the surface, creating only a slight visible difference between the defect and the background, a contrast that may not be sufficient for detection. Cleaning too aggressively, however, may remove the penetrant from the upper area of the defect, not allowing the developer to reach the penetrant and, therefore, offering no possibility of indicating a flaw. Due care must be exercised to avoid over-removal.
For fluorescent dip tank applications, the wash procedure will vary depending on which type of penetrant used — water wash, lipophilic (PE), hydrophilic (PR) or solvent-removable.
The water wash method involves simple equipment and is ideal for large parts. It requires washing the penetrant off with 65F to 75F water (30 to 120 seconds according to manufacturer's specs.
The lipophilic method offers greater control than any other penetrant method, but it is more difficult to handle large parts effectively and generates more waste watter. This method requires removal of penetrant by dipping in emulsifier first, then washing with water as with the water wash method.
The hydrophilic method is a bit more complex, yet it is the most common in the industry. The process involves a dip, pre-rinse, application of a penetrant remover, and a water wash.
The last step in the process involves developing the remaining penetrant as it seeps from the cracks, and a visual examination for indication of penetrant bleedback from surface openings.
If inspecting visibly, be sure you are using suitable lighting conditions.
Fluorescent inspection requires the use of a suitable black light.
Remember that although many manufacturers don't specify a shelve life for their products, old penetrant that has been sitting in a tank for a year may not be as sensitive as new penetrant. So check it regularly and change it when needed.