Fluorescent Penetrant Inspection (FPI) FAQs
Fluorescent penetrant inspection is one of the most sensitive surface-inspection methods used in industrial manufacturing. At AMT, we use FPI to test industrial components across a range of sectors and applications.
Use the FAQs below for answers to the questions we hear most often from manufacturers and industrial professionals across Indiana, Michigan, and Illinois. For a deeper walkthrough of the process, see our FPI 101 guide. To request testing services, request a quote, or contact our team to learn more.
FPI Basics
What is fluorescent penetrant inspection (FPI)?
FPI is a non-destructive testing (NDT) method that uses a fluorescent dye to reveal cracks and other flaws on a component's surface. The dye is drawn into any discontinuities, the excess is wiped away, and an inspector checks the surface under ultraviolet light. Wherever a flaw trapped some dye, it glows against the dark background, which reveals even very fine defects. Because nothing about the part itself is altered, it can move through repeated inspections at different points in production without causing any damage to the component.
What is the FPI process, step by step?
FPI runs through five general stages: pre-cleaning, penetrant application, excess removal, inspection, and post-cleaning. The part is cleaned, a fluorescent penetrant is applied and given time to dwell in any surface flaws, the excess is carefully removed, and the surface is examined under UV-A light for indications.
For a full breakdown of each stage and how AMT carries it out, visit our fluorescent penetrant inspection services page.
Choosing the Right Process: FPI Vs. Other NDT Methods
Is FPI the same as liquid penetrant inspection or dye penetrant inspection?
FPI is a type of liquid penetrant inspection. The distinction is in the dye: FPI uses a fluorescent penetrant viewed under UV light, while standard dye penetrant inspection (sometimes called visible dye penetrant testing) uses a colored dye viewed under normal light. Fluorescent penetrants are generally more sensitive to very small flaws, which is why FPI is common in aerospace and other high-reliability applications. For more on liquid penetrant testing methods, see our Liquid Penetrant Inspection resource.
FPI or MPI: which method do I need?
Material determines whether MPI is even an option. Ferromagnetic parts can use either MPI (magnetic particle inspection) or FPI. Non-ferrous metals, ceramics, and plastics can only use FPI, since MPI can't be used on them at all. For ferromagnetic parts where either method is technically viable, sensitivity is usually the deciding factor: FPI tends to detect finer surface flaws across a wider range of surface finishes, while MPI is generally faster and more cost-effective for high-volume ferromagnetic runs. In most cases, this decision has already been made before the part reaches us. If a drawing or specification names a method, that's what we test to.
Which testing standard or specification should I use for FPI?
That decision belongs to the design engineer, not the testing lab. The engineer knows the part's intended use, load conditions, and service environment, and NDT requirements are typically already documented on the drawing or purchase order by the time it reaches us. AMT doesn't have visibility into the full scope of how a part will be used, so we don't make that call. We test to whatever your documentation specifies, working within the frameworks outlined on our testing standards page.
FPI Testing Capabilities
What kinds of defects does FPI detect?
FPI is highly effective at finding surface-breaking discontinuities, including:
- Cracks from manufacturing, heat treatment, grinding, or fatigue
- Porosity and inclusions at the surface
- Seams and laps left over from forming
- Fractures too fine to see with the naked eye
Like MPI, FPI only catches flaws that break the surface. Radiographic inspection is the method used for internal defects.
What are the advantages and limitations of FPI?
FPI can test a wider range of materials than MPI, including non-ferrous metals, ceramics, and more. It's also cost-efficient, and can be used across large surface areas or high volumes without driving up the cost.
Like MPI, FPI only reaches defects that break the surface. Anything sitting deeper needs Radiographic Inspection.
Working with American Metal Testing
Which industries rely on fluorescent penetrant inspection?
FPI supports a wide range of sectors, from aerospace to wind energy to safety and rescue equipment. See our full list on the industries served page. Much of AMT's FPI work supports our sister company, Aero Metals, and we also test for outside manufacturers across these same sectors.
Does American Metal Testing serve my area?
AMT is based in La Porte, Indiana, and serves customers throughout Indiana, Michigan, and Illinois. Our lab is NADCAP-accredited and staffed by NAS-410-certified inspectors, so whether you're a large manufacturer or a first-time customer, your parts are evaluated to the same standard.
How do I get a part tested with FPI, or how can I request a quote?
Contact our team with your part specifications and any documentation outlining your NDT requirements, and we'll walk you through next steps. You can also request a quote directly.
