Unlocking the secrets of MPI testing: a comprehensive guide

The structural integrity of critical components in industry is hugely important; any hidden flaws could compromise safety and performance. 

Have you thought about how to ensure the structural integrity of the components you use? What methods would you use? 

Magnetic Particle Inspection (MPI) is a non-destructive testing technique widely used in various industries to detect surface and near-surface flaws in ferromagnetic materials. 

What makes MPI testing a go-to method for identifying concealed defects? How does it use the power of magnetism to expose imperfections hidden beneath the surface? 

In this comprehensive guide, we will explain more about MPI testing, exploring its techniques, benefits, and how it helps to uncover concealed defects.

Understanding MPI testing

Magnetic Particle Inspection (MPI)  relies on the principle of magnetism to identify defects in materials. 

Ferromagnetic materials, such as iron and steel, respond to magnetic fields by becoming magnetised. 

During the MPI process, the test material is magnetised, and ferrous particles—typically iron filings or magnetic powders—are applied to its surface. 

These particles are attracted to areas with magnetic flux leakage, highlighting the presence of defects like cracks, seams, or other imperfections.

Interested? NDT Group offers an MPI testing service and we have experienced technicians who aim for a fast turnaround on reporting. Contact us for more information. 

MPI testing techniques

There are two primary magnetic particle inspection techniques: wet visible and dry visible.

Wet visible MPI 

In this method, a wet fluorescent magnetic particle suspension is applied to the test surface. 

Ultraviolet (UV) light is then used to inspect the component, making indications visible under the fluorescent glow. 

This technique is highly sensitive, allowing for the detection of smaller flaws.

Dry visible MPI 

Dry visible MPI uses coloured magnetic particles that contrast with the surface of the material being tested. 

This technique is suitable for situations where wet inspections are impractical, and the components can be inspected in a dry state. 

Although less sensitive than the wet method, it is still effective for many applications.

MPI testing procedure

The MPI testing procedure involves several key steps:


The test surface must be thoroughly cleaned to ensure accurate results. Any contaminants can interfere with the inspection process.


The test component is magnetised using either a direct or indirect magnetisation method. 

Direct magnetisation involves applying the magnetic field directly to the component, while indirect magnetisation uses a separate device, such as a yoke or coil.

Application of magnetic particles 

The magnetic particles are applied to the magnetised surface. 

These particles may be suspended in a liquid, creating a slurry, or in a dry powder form, depending on the chosen MPI technique.


Our technicians observe the test surface for indications, which appear as visible lines or patterns of magnetic particles, indicating the presence of flaws.


After inspection, the test material is cleaned to remove the magnetic particles, allowing for further analysis or use of the component.

Benefits of MPI testing

MPI testing offers several advantages:


MPI is highly sensitive to surface and near-surface defects, making it an effective method for identifying hidden flaws that may compromise the integrity of materials.


MPI can be applied to a variety of ferromagnetic materials, making it suitable for a wide range of industries and applications.


Compared to other non-destructive testing methods, MPI is often more cost-effective, making it a preferred choice for routine inspections.

What role does MPI testing play in revealing defects? 

The ability of MPI testing to reveal hidden flaws lies in its sensitivity to magnetic flux leakage caused by defects in the material. 

As the magnetic field interacts with the material, any disruptions in the continuity of the material—such as cracks or voids—result in the leakage of magnetic flux. 

This leakage attracts the magnetic particles, creating visible indications that highlight the presence and location of flaws.

MPI testing: a powerful tool  

MPI testing is a powerful tool for identifying hidden flaws in ferromagnetic materials. 

Its sensitivity, versatility, and cost-effectiveness make it a valuable asset in ensuring the reliability and safety of critical components across various industries. 

Understanding the techniques and benefits of MPI testing helps you to make informed decisions about the inspection of materials, ultimately contributing to the longevity and efficiency of your systems and structures.

Responsive testing that delivers assurance that your equipment is safe.

Contact us for a quality testing service with a fast report turnaround. We can respond to any challenge.


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