Magnetic Particle Inspection

Magnetic particle inspection is a non-destructive testing technique that pinpoints surface and near-surface discontinuities in ferromagnetic materials. It is the process of examining for features, such as cracks and welds, by using magnetic particles which are drawn to the surface defects by a magnetic field. The inspection process takes advantage of the fact that magnetically susceptible materials are attracted to magnetic fields.

Ferromagnetic materials can be magnetised, using permanent magnets, electromagnets or electrical currents, to a level where interruptions in the normal physical structure of the material, termed discontinuities, cause distortions to the magnetic field. These distortions cause localised fields, termed flux leakage fields, to emerge from the surface of the test specimen, adjacent to the discontinuity. Small, finely divided ferromagnetic particles are applied to the surface and are attracted to the leakage fields. These magnetic particles accumulate at the leakage to give a visible indication to the presence of the discontinuity.

Magnetic particle inspection can be used to detect a number of different types of surface and near surface discontinuities. It can be performed on raw materials, finished or semi-finished materials such as:

• Cast, rolled and forged products
• Billets
• Welds
• In-service assembled or disassembled products
  
  

Advantages of Magnetic Particle Inspection

The advantages to magnetic particle inspection include:

• It is a reliable and highly sensitive test method.
• It is relatively fast and simple.
• It has little limitation on size and shape of the asset being inspected.
• Set up costs are minimal.
• The asset does not require the stringent cleaning needed for Penetrant Testing, as the defect can still be detected if contaminated.

Limitations of Magnetic Particle Inspection

Magnetic particle inspection is not without limitation. These limitations can include:

• The component or area to be tested must be able to be magnetised (i.e. ferromagnetic).
• The detection of near surface discontinuities becomes increasingly difficult with relation to the depth from the examination surface.
• The magnetic field must be orientated at the correct angle to a discontinuity or the flux leakage may not be detected.
• Proper training and experience levels are required to ensure technicians obtain optimum results.
• Chemicals are often used which may present a fire or health hazard.

Where fluorescent magnetic particle techniques are employed the area must be darkened to below 20 lux to allow adequate viewing conditions for inspection under Ultraviolet light. This can sometimes be difficult to achieve.

The ALS Approach

ALS’s highly experienced technicians are across these limitations and are trained to identify, avoid and overcome these boundaries if they occur. When magnetic particle inspection is the best non-destructive testing option, our technicians are able to efficiently and accurately execute the testing.