Magnetic Particle Inspection is a non-destructive testing method used for detecting surface and near surface flaws and defects. MPI is fast and relatively easy to apply, and surface preparation is not as critical as it is for other NDT methods. These characteristics make MPI one of the most widely utilized non-destructive testing methods. MPI uses magnetic fields and small magnetic particles similar to iron filings to detect flaws in components. The only real limitation is that the component being inspected must be made of a ferromagnetic material such as iron, nickel, cobalt, or one of heir alloys. The method is used to inspect a variety of product forms including castings, forgings, and welds. The structural steel, automotive, petrochemical, power generation, and aerospace industries are a few examples that utilize magnetic particle inspection.
This method of non-destructive testing tends to supplement rather than displace radiography. For example, radiography ordinarily cannot detects small cracks, especially when they are too small to be seen with human eye.
This method of inspection is used on magnetic ferrous weldments for detecting invisible surface of slightly subsurface defects*. Deeper subsurface defects are not satisfactorily detected because the influence of the distorted line of magnetic flux (owing to a discontinuity) on the magnetic particle spread over the job surface becomes weaker with the distance, so that sensitivity falls away rapidly with the depth.
The defects commonly revealed by magnetic particle inspection are quenching cracks, thermal cracks, seams. lap, grinding cracks, overlaps, non-metallic inclusions, fatigue cracks, hot tears, etc.
magnetic particle inspection is a relatively simple and easy technique. It is almost free from any restriction as to size, shape, composition and heat-treatment of a ferromagnetic specimen.