Magnetic Particle Inspection (MPI)
Magnetic Particle Inspection (MPI testing) detects all surface- and near surface- crack type defects which, corresponding to their position and size, proportionally influence the magnet field.
MPI, commonly referred to as Magnetic Testing (MT), is utilized in industry as a quality assurance method to test all ferromagnetic materials including all kinds of steel and its alloys with the exception of austenitic steel and cast iron. As a 'rule-of-thumb', reliable surface crack detection requires that the width-depth-length dimensions correspond to the ratio: 1: 10: 50.
Typically the lowest detection limits are a 1-µm crack width with a 10-µm crack depth.
During the magnetization of a ferromagnetic material, magnetic field lines of flux flow through the magnetically conducting medium.
If the magnetic flux lines hit an area of low magnetic conductivity (crack filled with air), a portion of the flux lines leak out of the material and are diverted out and above the surface of the inspected part. A magnetic stray flux field emerges from the part.
To show this external stray flux field, iron powder particles are applied as dry powder or with a special liquid applicator onto the part undergoing inspection.
The iron powder particles are attracted through the magnetic effect of the magnetic stray leakage flux field and essentially create a powder outline - a clear visual indication for the human eye to see.
For easier recognition of the iron powder crack patterns, the basic iron powder can be coloured with a fluorescent dye. Under UV light the powder pattern indications will be enhanced through black light illumination. Optimum crack detection occurs when the magnetic field lines are at right angles to the defect. The angle between the field direction and the expected defect position should not be greater than 30°.
Magnetic powder techniques are usually considered to be surface crack detection techniques; there is the possibility, however, that near-surface defects of favourable position and adequate size can be indicated. The surface structure of a test piece has a significant influence on the detectability of defects.
The depth of a defect should be at least twice the associated surface roughness. Furthermore, defect detect-ability can be reduced by false indications arising from magnetic stray fields, associated with surface condition due to scoring, scratches, scale, slots etc..
Magnetic powder techniques cannot be recommended for the detection of internal defects because the possibility of a defect indication rapidly decreases when the defect is more than 0.2 mm below the surface. In spite of optimum magnetization, cases can occur where it is difficult to generate the force required for a positive defect indication.
Typical unsuitable conditions are relatively wide defects, which are recognizable with the naked eye and defects with rounded sides, shallow surface scabs or laps.