MAGNETIC PARTICLE INSPECTION (MPI)
Also referred to as Magnetic Particle Inspection, this method is for the detection of surface and sub-surface defects in ferrous materials. They make use of an externally applied magnetic field or electric current through the material, and the principle that the magnetic flux will leave the part at the area of the flaw. As it does not require a high-degree of surface preparation, performing magnetic particle inspection is relatively fast and simple. This has resulted in MPI becoming one of the more commonly utilized NDT techniques.
The presence of a surface or near surface flaw (void) in the material causes distortion in the magnetic flux through it, which in turn causes leakage of the magnetic fields at the flaw. This deformation of the magnetic field is not limited to the immediate locality of the defect but extends for a considerable distance; even through the surface and into the air if the magnetism is intense enough. Thus, the size of the distortion is much larger than that of the defect and is made visible at the surface of the part by means of the tiny particles that are attracted to the leakage fields.
The method can be applied to any metals which can be strongly magnetized such as irons and ferritic steels - though typically not applicable to austenitic steels.
How MPI is Carried Out
There are two primary variations of the process - Dry Magnetic Particle Testing and Wet Magnetic Particle Testing. In either case, the process is initiated by running a magnetic current through the part. If there are any cracks or defects, this will interrupt the magnetic flow of current and will cause spreading. A resulting “flux leakage field” will form at the site of the damage.
The next step involves spreading metal particles over the part. If there are any flaws on or near the surface, the flux leakage field will attach particles to the area of damage. This step allows for a visible indication of the approximate dimension of the damage.
Advantages of MPI
There are a number of benefits of employing MPI compared to other non-destructive testing methods. For one, it is easily transportable, relatively inexpensive, and does not possess any intensive pre-cleaning requirements. Additionally, MPI is ideal for detecting minuscule, shallow surface cracks and it will quickly permeate through thin coatings. Lastly, it can be applied to wide range of test material of various shape and dimension.