FPI

Fluorescent penetrant inspection (FPI) is a heavily utilized NDE method when detection of surface breaking cracks is needed. The method includes multiple steps which should be optimized for the material, surface condition, anticipated crack morphology/location/size, and inspection environment. While FPI is used by many industries, it is a major contributor to the inspection requirements of the aerospace industry. Over 90% of aviation components are inspected with FPI at least once during their lifetime. For this reason, the Federal Aviation Administration invested in a significant research effort to understand what steps in the FPI process were more critical to a successful inspection. Full details of that research effort are maintained for the benefit of the NDE community at a separate website. CASR Website

 

Technique Basics:

There are a number of penetrant materials available for use in component inspection with the choice of method and level often dictated by the customer and dependent on the crack size of interest. For aviation inspections, the most common methods are post-emulsifiable, hydrophilic (Method D) and water washable (Method A). Because detection of tight fatigue cracks is often the requirement, Level 3 and Level 4 are the most common sensitivity levels. While CNDE research has focused primarily on Level 4, Method D penetrants other capabilities are available for use in research and evaluation efforts.

white_light_UVA_Ti_0-600x334

White light micrograph and UVA indication for a tight fatigue crack in Titanium sample

 

After selection of the penetrant material for a given application, it is important that the part be clean and dry. Any surface residue can prevent penetrant from entering the flaws, rendering the inspection useless. Once a clean part is available, the following general steps are used:

  1. Apply penetrant to the part surface for the recommended dwell time. For most components, a dwell time of at least 20 minutes is recommended.
    fpi_step1-600x450
  2. Excess penetrant is then removed from the surface by water rinse. It is important that all surfaces be rinsed. However, it is also imperative that any given region not be “over-rinsed” such that penetrant is removed from the defect.
    FPI_step2-600x450
  3. In the case of post-emulsifiable pentrants, an emulsification step is required. It is important that the emulsification time is not exceeded. For typical applications, emulsification dwell should not exceed 2 minutes. It is also important that recommended concentrations are adhered to.
    fpi_step3-600x450
  4. To stop the emulsification process, a post rinse is used. For both the pre- and post-rinse, it is important that recommended water pressures not be exceeded.
  5. The final step prior to inspection is the application of the developer. In the research efforts completed by CNDE to date, developer application was found to be the most critical step. In most cases, a dust cloud chamber or spray wand system is used to apply a dry powder developer. It is important that adequate developer coverage occur and that all surfaces of interest can be reached by the developer materials. Significant differences were found between part surfaces depending on the location, i.e., better detectability was found for indications on the top of a sample compared to the bottoms or sides of a part. For this reason, the application of developer to critical areas using either a bulb or the use of non-aqueous wet developer (NAWD) is recommended. For complete details of the research on each of the steps in the FPI process, please go to the Center for Aviation Systems Reliability (CASR) website. CASR is an FAA-funded program managed by the Center for NDE.