Magnetic Particle Inspection (MPI) FAQs
Magnetic particle inspection is one of the most widely used non-destructive testing methods in industrial manufacturing. Use the FAQ information below for answers to the questions we hear most often from engineers, quality teams, and manufacturers across Indiana, Michigan, and the greater Chicago area.
For a deeper walkthrough of the magnetic particle inspection process, see our MPI 101 guide. To request testing, contact our team or request a quote.
MPI Basics
What is magnetic particle inspection (MPI)?
Magnetic particle inspection is a non-destructive testing (NDT) method used to find surface and near-surface cracks and other discontinuities in ferromagnetic metal components. A technician magnetizes the part, applies fine magnetic particles to the surface, and looks for where those particles cluster. The part isn't damaged or altered during the process, so it can move forward in production as soon as it passes inspection.
What is the magnetic particle testing procedure, step by step?
At a high level, MPI runs through five stages: pre-cleaning, magnetization, particle application, inspection and interpretation, and post-cleaning. The part gets cleaned, a magnetic field is induced through it, magnetic particles are applied while that field is active, and a certified inspector evaluates any indications that form.
For a full breakdown of each stage and how AMT carries it out, visit our magnetic particle testing services page.
Materials and Method Selection for MPI
What materials can be tested with MPI?
MPI only works on ferromagnetic materials: metals that can be magnetized. That includes iron, nickel, cobalt, and most steel alloys. Non-ferrous metals such as aluminum, magnesium, titanium, and most austenitic stainless steels can't be magnetized, so this method won't work on them. Fluorescent penetrant inspection is usually the better fit for non-ferrous parts, since it works across a much wider range of metallic and non-metallic materials.
MPI or FPI: which method do I need?
The material being tested will determine whether MPI is an option at all. Ferromagnetic parts can use either method. Non-ferrous parts can only use FPI, since it also covers other materials.
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, though, 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 MPI?
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.
Testing Capabilities for MPI
What kinds of defects does MPI detect?
MPI is most effective at finding surface-breaking and near-surface discontinuities, including:
- Cracks from manufacturing, heat treatment, grinding, or fatigue
- Seams and laps left over from the forming process
- Porosity and inclusions near the surface
- Cold shuts in castings
What are the advantages and limitations of MPI?
MPI's biggest strength is speed and cost-effectiveness for high-volume testing. Indications appear as soon as the particles are applied, so parts typically move through inspection faster than they would with radiographic testing. MPI can also test components with rough surfaces and complex geometries.
MPI's tradeoff is testing depth. MPI identifies surface and near-surface flaws, so anything buried deeper can go undetected. Radiographic inspection is typically the better process for internal defects.
MPI is also limited to ferromagnetic parts (see the materials question above), and most parts need demagnetization once testing is concluded.
How long does an MPI inspection take?
MPI is one of the faster NDT methods available. Turnaround still depends on part volume, complexity, and any pre- or post-cleaning requirements, but MPI generally moves faster than radiographic inspection, which requires image processing and review.
Working with American Metal Testing for MPI Services
Which industries rely on magnetic particle inspection?
MPI serves any industry where ferromagnetic components need to be verified for structural integrity before they enter service, including aerospace, automotive and transportation, structural steel, wind turbine hardware, and safety and rescue equipment. See our full list on the industries served page. Much of AMT's MPI work supports our sister company, Aero Metals, and we also conduct testing for outside manufacturers across these sectors.
Does American Metal Testing serve my area?
AMT is based in La Porte, Indiana, and serves customers throughout Indiana, Michigan, and the greater Chicago area. Our lab is NADCAP-accredited and staffed by NAS-410-certified inspectors, so whether you're a manufacturer or a first-time customer, your parts are evaluated to the same standards.
How do I get a part tested with MPI, 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.
