Magnetic Particle Inspection
Magnetic particle inspection (MPI) is a nondestructive testing (NDT) method used for detection of surface defects. MPI is fast and relatively easy to apply, and component surface preparation is not as critical as for some other NDT methods. These characteristics make MPI one of the most widely used NDT testing methods in the world.
MPI uses magnetic fields, induced by a electric magnet and small magnetic particles (i.e.iron filings) to detect flaws in components. The only requirement is that the part being tested is a ferromagnetic material such as iron, nickel, cobalt, etc. Ferromagnetic materials means that the material can be magnetized enough to allow the inspection to take place and be effective.
MPI is used to inspect a large variety of products including castings, forgings and welding. Many different industries use MPI to determine the fitness-for-use of components, among those industries are automotive, petrochemical, power generation, marine engineering and even the aerospace industry.
In order to do MPI inspection, we first have to magnetize the component that is to be inspected. If any defects are on or near the surface, the defects will create a leakage field. After magnetizing the component, we apply a iron particle, either in a dry or wet suspended form, onto the surface of the magnetized component. These particles will be attracted and will cluster at the flux leakage fields area, forming a visible indication.
The advantages of MPI are:
- MPI inspection is generally simple to operate and apply.
- Can inspect parts with irregular shapes easily
Precleaning of components is not as critical as it is for some other inspection methods. Most contaminants within a flaw will not hinder flaw detectability.
- The test is visual, so any indications can be seen and recorded (i.e. Photos).
- Very fast and effective to test large areas for minor defects
Considered low cost compared to many other NDT methods.
- Is a very portable inspection method especially when used with battery powered equipment
Some of the disadvantages are:
Cannot inspect non-ferrous materials such as aluminum, magnesium or most stainless steels.
Inspection of large parts may require use of equipment with special power requirements.
Some parts may require removal of coating or plating to achieve desired inspection sensitivity.
Limited subsurface discontinuity detection capabilities. Maximum depth sensitivity is approximately 1.5cm (under ideal conditions).
Post cleaning, and post demagnetization is most often necessary.
Alignment between magnetic flux and defect is important