Magnetic Particle Inspection Flashcards
Presentation for thursday (Currently Tuesday)
What is the principle of MPI?
The principle of magnetic particle inspection (MPI) is that when a ferromagnetic material is magnetized, any surface or near-surface discontinuities, like cracks, disrupt the magnetic field, creating “flux leakage” which attracts magnetic particles applied to the surface, thus visually indicating the presence of a flaw.
What is the Purpose of MPI?
It detects discontinuities at or near the surface in Ferro-magnetic materials.
Describe the method of Yoke-type electro-magnets.
When poles of a horseshoe magnet or magnetic yoke are placed on a steel surface, a magnetic flux is set up in the steel from one pole of the magnet to the other. Cracks between 90°- 45° to flux path can be detected if the flux strength is sufficient.
How is the magnetizing current affected?
Permeability of the metal, the shape and thickness of the test piece, and the type of discontinuity sought.
What are Wet Particles in Magnetic Particles?
- Suspension of magnetic particles in a light petroleum distillate or light oil.
- coloured or have fluorescent coating viewing with black light.
- applied by dipping, immersing or spraying the part.
What are Dry Particles in Magnetic Particles?
- powder form, in a variety of colours.
- applied by using hand shakers, spray bulbs, shaking screens or an air stream.
What is the purpose of performing demagnetization?
To remove residual magnetization after the magnetic force has been removed.
Why is it critical to remove residual magnetism?
Residual magnetism may interfere or cause adverse effects on the parts or components.
How is demagnetization performed?
Subject the part to an alternating diminishing magnetic field.
Advantages of MPI
(i) Can detect both surface and near sub-surface defects.
(ii) Can inspect parts with irregular shapes easily.
(iii) 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.
(iv) Fast method of inspection and indications are visible directly on the specimen surface.
(v) Considered low cost compared to many other NDT methods.
(vi) A very portable inspection method especially when used with battery powered
equipment.
Limitations of MPI
(i) Cannot inspect non-ferrous materials such as aluminum, magnesium or most stainless
steels.
(ii) Inspection of large parts may require use of equipment with special power requirements.
(iii) Some parts may require removal of coating or plating to achieve desired inspection
sensitivity.
(iv) Limited subsurface discontinuity detection capabilities. Maximum depth sensitivity is
approximately 0.6” (under ideal conditions).
(v) Post cleaning, and post demagnetization is often necessary.
(vi) Alignment between magnetic flux and defect is important.
