How Magnetic Particle Inspection works 

In the world of non-destructive testing (NDT), various techniques detect flaws and defects in materials without causing any damage. One of the most widely used techniques is Magnetic Particle Inspection (MPI). At NDT Group, we regularly use this technique when carrying out NDT inspections. 

This method uses magnetic fields and fine magnetic particles to detect surface and near-surface defects. In this blog, we’ll explore how MPI works, its applications, and the advantages and disadvantages of this technique.

Understanding the basics of Magnetic Particle Inspection (MPI)

MPI primarily detects flaws in metals that can be magnetised (called “ferromagnetic” materials), such as iron and steel. It works on the principle that when a magnetic field is applied to a material, it creates magnetic poles, creating flux lines that flow through the material. In the presence of a surface or near-surface defect, the magnetic field is distorted, leading to magnetic flux leakage. If you can identify the leakage, you can identify the flaw. 

Our process of MPI

1. Cleaning

Step one is to pre-clean the test area using a solvent cleaner and lint-free wipe.

2. Paint application

Then we need to apply a white contrast paint to the test area.

3. Magnetisation

The next step in MPI is to create a magnetic field within the test piece. Our technicians use various methods to do this, such as applying an electric current through a coil or using a permanent magnet. 

The magnetic field should be strong enough to completely cover or “saturate” the material, meaning that the material is magnetised to its maximum potential. Saturation ensures that any defects present in the material will disrupt the flow of magnetic flux, making them easier to detect.

4. Particle Application 

Once the material is magnetised, our technicians apply a fine iron-based powder or liquid containing magnetic particles to the surface of the test piece. These magnetic particles are usually coated with a black colour to enhance visibility. 

As the particles are applied, they stick to the surface due to the presence of magnetic field lines. These particles are attracted to areas with magnetic flux leakage, forming visible indications that can be easily detected.

After the application of magnetic particles, our technicians examine the test piece to identify indications of defects. This step requires a keen eye and experience. 

Any surface or near-surface defects, such as cracks or discontinuities, will be indicated by the buildup of particles, creating a visible indication.

Our technicians may use various lighting techniques, such as ultraviolet (UV) or white light, to enhance the visibility of the indications.

5. Evaluation 

Our technicians analyse the indications to determine their size, shape, and significance. This evaluation helps classify defects as acceptable or unacceptable. This judgement is based on established acceptance criteria.

6. Demagnetisation 

Once the inspection is complete and the indications have been recorded, it is essential to remove any remaining magnetic fields from the material. Demagnetisation can be achieved through various methods, including passing the test piece through a demagnetising coil or applying an alternating current. 

Demagnetisation is crucial to prevent any residual magnetism that may interfere with the material’s future use or cause inaccuracies in future inspections.

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Applications of MPI 

MPI testing can be used for all types of components – from small items such as castings, weldings, and forgings to inspection on in-service components. 

In aerospace, critical components such as engine parts, landing gear, and turbine blades undergo MPI to detect cracks, fatigue, or other flaws that could compromise performance. 

Our technicians also use MPI tests for welding components, such as pressure vessels, pipelines and bridges. It helps identify potential defects like cracks, porosity, and lack of fusion (where two components are not properly welded together). 

Advantages of MPI

1. Sensitivity 

MPI can detect both surface and near-surface defects, making it highly effective in identifying potential flaws that might compromise the structural integrity of welds.

2. Versatility 

MPI can be used on various shapes and sizes of ferromagnetic materials, making it suitable for a wide range of applications in different industries.

3. Real-time Inspection 

MPI provides immediate results, allowing us to work with our clients to make prompt decisions regarding the acceptability of welds and take necessary actions for repair or rework. 

4. Cost-Effectiveness 

Ultimately, saving time, saves money, as components can be repaired and brought back into operation quickly. This makes the test highly cost-effective too. Compared to other NDT methods, MPI is relatively affordable and widely accessible.

Disadvantages of MPI

There are some limitations to Magnetic Particle Inspection.  

It is only applicable to ferromagnetic materials such as iron, nickel, and cobalt and their alloys. Non-ferromagnetic materials like aluminium or copper cannot be inspected using this method. 

Furthermore, the size and depth of defects that can be detected are limited by the magnetic field strength and the size of the magnetic particles used.

MPI: Ensuring integrity

Magnetic Particle Inspection is a valuable non-destructive testing technique that plays a significant role in ensuring the integrity of ferromagnetic materials. 

MPI’s sensitivity, ease of use, and cost-effectiveness make it a popular method in various industries, including aerospace, energy, and manufacturing. While it does have limitations, the benefits it offers make it an essential tool in maintaining quality and safety standards.

This non-destructive testing method plays an essential role in industries worldwide, ensuring that welded components meet stringent quality standards. 

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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