Magnetic Particle Inspection

NDT of a weld using MPI

Image: MPI of Welds 

Magnetic Particle Inspection (MPI) is a non-destructive testing (NDT) process for detecting surface and near-surface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. The method uses magnetic fields and small magnetic particles (like fine iron filings) to detect flaws in components.

Here’s how MPI works:

  1. The object being inspected is magnetized either locally or overall. When the material is magnetized, magnetic lines of force are established within the material.
  2. If there are any discontinuities (such as cracks, seams, or inclusions) present, they will interrupt the flow of magnetic lines of force, causing leakage at the site.
  3. The applied magnetic particles, which can be dry or in a liquid suspension, are attracted to this area of flux leakage, clustering to form a visible indication that mirrors the shape of the discontinuity.
  4. The indications are then evaluated to determine if they are relevant defects that require action.


Busy workshop with MPI of HME Boom

Image: MPI of an excavator boom during refurbishment

MPI is valuable because it is a quick and relatively simple means of testing for surface cracks and other defects in magnetic materials. 

Advantages of MPI:

  • Sensitivity to Small Defects: MPI can detect very fine cracks and other surface discontinuities that might be invisible to the naked eye.
  • Immediate Results: The results of a magnetic particle inspection are visible as the test is taking place. There is no need for complex processing or extended waiting periods.
  • Minimal Preparation: The surface needs only to be free from oil, dirt, scale, or other coatings that might keep the particles from moving freely.
  • Versatility: It can be used to inspect various product forms including castings, forgings, and welds.
  • Cost-Effectiveness: MPI is generally less expensive and more available than other forms of non-destructive examination.

Disadvantages of MPI

  • Material Limitations: MPI is only applicable to ferromagnetic materials. Non-ferromagnetic materials, like aluminium, plastic, or certain stainless steels, cannot be tested using this method.

  • Surface Accessibility: MPI can only detect discontinuities that are open to the surface or just below it. It cannot detect flaws that are deeper inside the material.

  • Surface Preparation: The surface must be relatively clean and free of oil, grease, scale, or other coatings that could potentially prevent the magnetic particles from freely moving to the site of the flaw.

  • Cleanup Required: After testing, the parts need to be demagnetized and cleaned to remove the magnetic particles, which can be an additional process step.

  • Limited to Certain Shapes: Complex geometries can be challenging to inspect because uniform magnetization may be difficult to achieve, leading to misleading indications or missed defects.

MPI is valuable in various industries, including mining, power generation, oil, gas chemical, aerospace, automotive, and manufacturing, where it is essential to ensure the integrity of critical components without causing damage to the materials. It plays a crucial role in the quality assurance process, helping to prevent failures, accidents, and costly downtime.

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