Magnetic Particle NDT (MT)

Magnetic particle testing is also a fairly popular NDT technique because of its fast execution where no surface preparation is needed.

In magnetic particle testing, the part is placed between permanent magnets or electromagnets. The strength of the field is an important factor since a stronger field gives better results.

When the part under inspection is placed into the field, a magnetic current starts flowing through the specimen. If there’s no defect, an uninterrupted magnetic flux field is obtained.

But if it comes across a defect, the magnetic field bends and a part of it leaks out. This leakage is also known as the flux leakage field.

In order to identify the defects via these leakage points, magnetic particles are used. These particles are applied to the test specimen and they are pulled into these leakage points because of the uneven magnetic flux density.

We may either use magnetic particles that can’t be seen with the naked eye or fluorescent ones for better visibility. 

The width of the magnetic particle strips is wider than the defect’s width. As a result, it can reveal minute defects with an opening width of up to 0.001 mm and depth of 0.01 mm.

With this technique, we can detect defects such as cracks, pores, laps, inclusions, seams, laminations, shrinks, flakes, welding defects, machining tears and also service-related or fatigue cracks.

Advantages of magnetic particle testing:

  • Easy to use
  • Portable setup
  • High sensitivity
  • Immediate results
  • Usually inexpensive
  • Can work through thin surface coatings
  • Parts with complex geometries are also suitable
  • Visual indication of the shape and size of the defect
  • Can detect surface defects well. Also works for subsurface defects to an extent

Disadvantages of magnetic particle testing:

  • Can only test small areas at a time
  • Does not work with non-magnetic materials
  • Testing may burn the particle if the field is too strong
  • Coatings thicker than 0.1 mm need removal for testing
  • Demagnetisation of test specimens is necessary but may be tricky
  • Can only work for subsurface defects that have a depth of up to 3 mm