FPI research at CNDE has been funded primarily by the Federal Aviation Administration in support of ensuring the continued airworthiness of the commercial fleet. Much of the work has been in cooperation with industry partners and is recorded in an archival website with details at http://www.cnde.iastate.edu/faa-casr/fpi/index.html. The basic approach has focused on the generation of quantitative data using a combination of luminance (brightness) measurement as a function of processing parameters, recording of UVa indications and more traditional probability of detection studies. The major conclusions of the prior work are as follows:
Flourescent Penetrant Inspection
- Detects surface breaking defects
- Requires clean, dry surface
- Requires proper processing
- Both fluorescent and nonfluorescent techniques are available, most aviation applications involve the use of fluorescent penetrants in a bulk process
- Liquid penetrant is applied to the precleaned surface of a part to be inspected. The liquid penetrant is drawn into defects by capillary action.
- Excess penetrant is gently removed from the surface, taking care not to remove penetrant from any defects. The crack remains full of penetrant.
- A thin layer of developer is applied to the part surface.
- The developer acts as a blotter to draw penetrant out of the flaw. The developer also helps to provide contrast so the colored penetrant can be viewed more easily. The part is then inspected for signs of penetrant, indicating the presence of a defect.
- Selection of cleaning method should be stringent enough to remove soil of concern without detrimental effect on the component or on FPI performance
- Four-step process shown to be most effective for Ni
- Hot water rinse (>150F, 15 minutes) recommended for Ti
- Mechanical cleaning can lead to metal smear and entrapment of cleaning media if recommended practices aren’t adhered to
- Use of wet-glass bead is not recommended prior to FPI
- Water is a contaminant which degrades penetrant process, i.e., drying is an important step in preparation for FPI
- No statistically significant differences found between flash dry and oven dry methods when applied correctly
- Increased penetrant dwell time can improve brightness performance, particularly for “tight cracks”
- Use of red dye prior to FPI led to detrimental effect on luminance of subsequent FPI processing
- Differences were found between penetrant method with Level 4 found to be more sensitive than Levels 3 or 2. Differences between levels 2 and 3 were not significant for the rinse times used in this study.
Control of contact time was shown to be most critical factor.