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What is Magnetic Particle Testing? - A Definitive Guide

What is Magnetic Particle Testing?

Magnetic Particle Testing, also known as Magnetic Particle Inspection (MPI) is the process whereby a component is magnetised to produce lines of magnetic flux within the component. When the lines of flux meet a discontinuity that lies in a generally perpendicular orientation to the direction of flux flow, a flux leakage field is formed at and just above the component surface. The flux leakage is revealed by the use of finely divided ferromagnetic particles that are caused to flow over the surface, some of which are gathered and held by the leakage field. These magnetic particles, which may be either in the form of a powder (dry) or in a liquid suspension (wet), are used to form an outline of the discontinuity and indicate its location, size, shape and extent.

What are the Methods of Magnetisation?

There is essentially two methods of producing a magnetic flux within the component under investigation. A flow of magnetic flux can be generated by introducing an external magnetic field into the component from a source such as a permanent magnet, an electro-magnet or a current carrying conductor adjacent to the component or formed into a coil. Alternatively, an electric current can be passed directly through the component that will induce a magnetic flux into the component, which is perpendicular to the direction of the current flow. The current waveform used can range from DC to full AC. As a general rule, AC is very sensitive to surface breaking discontinuities, while DC has the potential to detect slightly subsurface discontinuities.

Dry and Wet Magnetic Particles 

The particles used in magnetic particle testing are important in indicating discontinuities within a test part. The properties of the magnetic particles used in magnetic particle testing need to include high magnetic permeability (the ability to be magnetised) to allow the particles to be attracted to the magnetic flux leakage fields and low retentivity (ability of particles to retain magnetisation) to prevent the particles from attaching to each other or the surface of the component. There are two types of magnetic particles used:

Dry Magnetic Particles this method involves using a variation of particle sizes to indicate different sizes of flux leakage sites. Smaller particles are more sensitive to flux leakage sites/fields and are therefore able to identify smaller defects/flaws and larger particles are able to identify larger flux leakage sites.

Wet Magnetic Particles – this technique includes magnetic particles being suspended in a substance such as water or oil. This method is more sensitive to flux leakage sites caused by defects and flaws than dry magnetic particles due to the particles being more mobile whilst in a suspended state. This method also allows a larger surface area to be covered, therefore indicating more flux leakage sites.

TWI Services

Magnetic particle testing is a non-destructive test method, which can be used to inspect a variety of ferromagnetic components and products. These include castings, forgings and weldments. The method is used across different industry sectors, especially when determining if a component is fit-for-service. Industries that use magnetic particle testing include, aerospaceautomotivemarinepoweroil and gas, components and structures, engineering and fabrication

TWI have used their 40 years’ of experience to develop a wide variety of non-destructive testing (NDT) techniques and have provided services within the industry related to magnetic particle testing. Examples of this work include:

Components and Structures

  • TWI inspected a component within a mechanical system using magnetic particle inspection to detect surface or near surface flaws or defects.
  • TWI investigated the technique of warm prestressing. TWI looked at the effects of these pressure tests and how they impact the tests that are carried out throughout a component/structure’s life. This article also looks at the uses of the magnetic particle testing performed within this research.
  • The Magnetic particle inspection method was carried out by TWI to investigate a welded manifold valve (a component/piece of equipment that connects valves of a hydraulic system).

Oil and Gas




  • TWI inspected a repaired reactor at a nuclear power station using magnetic particle inspection. TWI’s main responsibility was to ensure that the repaired component was free from defects including surface and sub-surface cracks.

The Welding Institute

As a Member of The Welding Institute you have access to our membership benefits. As an engineer, technician or personnel engaged in magnetic particle testing The Welding Institute has benefits that can support you, these include:

Click here to view all of our Membership Benefits!

Magnetic Particle Testing Training Courses

Check out our Magnetic Particle Testing – TWI Training and Examinations Insight to find out more about the courses that TWI Training and Examinations offer!

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