Magnetic Particle Inspection
Magnetic particle inspection is a nondestructive testing method used for surface and near surface defect detection. MPI is a fast and relatively easy to apply and surface preparation is not as critical as it is for some other NDT methods. These characteristics make MPI one of the most widely utilized nondestructive testing methods.
MPI uses magnetic fields and small magnetic particles, such as iron filings to detect flaws in components. The only requirement is that the component being inspected must be made of a ferromagnetic material such iron, nickel, cobalt, or some of their alloys. Ferromagnetic materials are materials that can be magnetized to a level that will allow the inspection to be effective.
The method is used to inspect a variety of product forms such as castings, forgings, and weldments. Many different industries use magnetic particle inspection for determining a component’s fitness-for-use. Some examples of industries that use magnetic particle inspection include
- Structural steel
- Automotive
- Petrochemical
- Oil & Gas
- Power generation, and
- Aerospace industries.
- It is quick and relatively uncomplicated
- It gives immediate indications of defects
- It shows surface and near surface defects, and these are the most serious ones as they concentrate stresses
- The method can be adapted for site or workshop use
- It is inexpensive compared to radiography
- Large or small objects can be examined
- Elaborate pre-cleaning is not necessary
- It is restricted to ferromagnetic materials - usually iron and steel, and cannot be used on austenitic stainless steel
- Most methods need a supply of electricity
- It is sometimes unclear whether the magnetic field is sufficiently strong to give good indications
- The method cannot be used if a thick paint coating is present
- Spurious, or non-relevant indications, are probable, and thus interpretation is a skilled task
- Some of the paints and particle suspension fluids can give a fume or fire problem, particularly in a confined space