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Introduction to Magnetic Particle Inspection

Introduction to
Magnetic Particle

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This method is suitable for the detection of surface and near surface discontinuities in magnetic material, mainly ferritic steel and iron.

The principle is to generate magnetic flux in the article to be examined, with the flux lines running along the surface at right angles to the suspected defect. Where the flux lines approach a discontinuity they will stray out into the air at the mouth of the crack. The crack edge becomes magnetic attractive poles North and South. These have the power to attract finely divided particles of magnetic material such as iron fillings. Usually these particles are of an oxide of iron in the size range 20 to 30 microns, and are suspended in a liquid which provides mobility for the particles on the surface of the test piece, assisting their migration to the crack edges. However, in some instances they can be applied in a dry powder form.

The particles can be red or black oxide, or they can be coated with a substance which fluoresces brilliantly under ultra-violet illumination (black light). The object is to present as great a contrast as possible between the crack indication and the material background.

The technique not only detects those defects which are not normally visible to the unaided eye, but also renders easily visible those defects which would otherwise require close scrutiny of the surface.

There are many methods of generating magnetic flux in the test piece, the most simple one being the application of a permanent magnet to the surface, but this method cannot be controlled accurately because of indifferent surface contact and deterioration in magnetic strength.

Modern equipments generate the magnetic field electrically either directly or indirectly.

In the direct method a high amperage current is passed through the subject and magnetic flux is generated at right angles to the current flow. Therefore the current flow should be in the same line as the suspected defect.

If it is not possible to carry out this method because of the orientation of the defect, then the indirect method must be used. This can be one of two forms:

  • Passing a high current through a coil which encircles the subject.
  • Making the test piece form part of a yoke which is wound with a current carrying coil. The effect is to pass magnetic flux along the part to reveal transverse and circumferential defects.

If a bar with a length much greater than its diameter is considered, then longitudinal defects would be detected by current flow and transverse and circumferential defects by the indirect method of an encircling coil or magnetic flux flow.

Subjects in which cracks radiating from a hole are suspected can be tested by means of the threading bar technique, whereby a current carrying conductor is passed through the hole and the field induced is cut by any defects. Detection of longitudinal defects in hollow shafts is a typical application of the threader bar technique.

The electricity used to generate the magnetic flux in any of these methods can be alternating current, half wave rectified direct current or full wave rectified direct current. A.C. generated magnetic flux, because of the skin effect, preferentially follows the contours of the surface and does not penetrate deeply into the material. H.W.D.C. penetrates more deeply but is inclined not to follow sharp changes in section. H.W.D.C. is useful for the detection of slightly subsurface defects. The pulsing effect of A.C. and H.W.D.C. gives additional mobility to the indicating particles. D.C. penetrates even more deeply but does not have this facility. Furthermore, demagnetising of the material after D.C. magnetising is far more difficult than after A.C. magnetising.

Normally, to ensure that a test piece has no cracks, it is necessary to magnetise it in at least two directions and after each magnetising - and ink application - visually examine the piece for crack indications.

Since this double process, which would include adjustment of the magnetising equipment controls in between each magnetising takes time it is obviously advantageous to have the facility to reduce the time required. The recent development of the 'Swinging Field' method of multi-directional magnetising will indicate all defects, regardless of their orientation on the surface, with one magnetising shot and therefore requires only one inspection.

Basically magnetic crack detection equipment takes two forms. Firstly, for test pieces which are part of a large structure, or pipes, heavy castings, etc. which cannot be moved easily, the equipment takes the form of just a power pack to generate a high current. This current is applied to the subject either by contact prods on flexible cables or by an encircling coil of cable. These power packs can have variable amperages up to a maximum of 2000 Amps for portable units, and up to 10,000 Amps for mobile equipments. Both A.C. and H.W.D.C. magnetising current is available. The indicating material is applied by means of a spray and generally the surplus runs to waste.

For factory applications on smaller more manageable test pieces the bench type of equipment, as represented by our 'Euromag' range, is normally preferred. This consists of a power pack similar to those described above, an indicating ink system which recirculates the fluid, and facilities to grip the work piece and apply the current flow or magnetic flux flow in a more methodical, controlled manner. The work pieces are brought to the equipment and can be individually tested in one operation. Subjects up to approximately 100" long can be accommodated is such equipments and can be loaded by crane if necessary. This type of universal equipment is ideally suited to either investigative work or routine quality control testing.

These bench type equipments often incorporate a canopy to prevent direct light falling on the subject so that ultra-violet fluorescent material can be used to the best effect. The indicating particles may be suspended in very thin oil (kerosene) or water. In some circumstances the indicating medium can be applied dry.

These equipments are suited to production work and in certain circumstances can be automated to the extent of loading, magnetising, inking and unloading. The work pieces still have to be viewed by eye for defect indications.

Specialised equipments are also frequently manufactured to test a particular size and type of test piece. Several examples of such equipments manufactured by Insight NDT have been included on this website.

Advantages of Magnetic Particle Crack Detection:

  • Simplicity of operation and application.
  • Quantitative.
  • Can be automated, apart from viewing.

Disadvantages of Magnetic Particle Crack Detection:

  • Restricted to ferromagnetic materials.
  • Restricted to surface or near surface flaws.
  • Not fail safe in that lack of indication could mean no defects or process not carried out properly.

This information is taken from the Insight NDT technical paper entitled 'A Brief Explanation of Non-Destructive Testing Methods'. A copy of the full paper in Adobe Acrobat format is available by clicking Here.

An additional paper entitled 'Faster Magnetic Crack Detection using the Swinging Field Multi-directional Magnetising Method' may also be of interest. A copy of this paper, also in Adobe Acrobat format, is available by clicking Here.

For details of other Insight NDT technical papers relating to MPI please refer to the 'Technical Library' section of this website, which is available from the main menu.

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