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Magnetic Particle Testing (MT):

 

MTis used to locate surface and slight subsurface discontinuities or defects in ferromagnetic materials. Such flaws present in a magnetized part will cause a magnetic field, i.e. flux, to leave the part. If magnetic particles are applied to this surface, they will be held in place by the flux leakage to give a visual indication.

 

While several different methods of magnetic particle tests can be used, they all rely on this same general principle. Therefore, any magnetic particle test will be conducted by creating a magnetic field in a part and applying the magnetic particles to the test surface.

 

To understand the principle of MT, it is necessary to have some basic knowledge of magnetism. Therefore, consider some of the important characteristics of magnetism in and around typical bar magnet.

 

First, there are magnetic lines of force, or magnetic flux lines, travelling from one end (or pole) of the magnet to the opposite end (pole). These poles are generally designated as the north and south. The magnetic flux lines form continuous loops that travel from one pole to the other. These lines generally remain parallel to one another and will not cross each other. Also, the force of these flux lines (and therefore the flux density of the resulting magnetic field) is greatest when they are fully contained within a ferromagnetic material. Although they will travel through air gaps, their intensity is reduced as the length of the air gap is increased.

 

If we now consider a horseshoe magnet, magnetic lines of force are travelling in continuous loops from one pole to the other. However, if a piece of steel or keeper is placed across the ends (poles) of the magnet, a continuous magnetic path for the lines of force is provided. While there is some flux leakage present at the slight air gaps between the ends of the magnet and the piece of steel, the magnetic field remains relatively strong because of the continuity of the path created by the keeper.

 

If a discontinuity or flaw is present in the steel bar (keeper) across the ends of the magnet in the vicinity of that flaw, a flux leakage field is created at the flaw surface because the magnetic field leaves the magnetic material and travels through air. If the steel bar is sprinkled with iron particles, such particles would be attracted and held in place by the flux leakage at the flaw. This will occur because the iron particles provide a continuous magnetic path for the lines of force just as the piece of steel across the ends of the magnet completed the magnetic circuit for the magnet.

 

Therefore, to perform MT, there must be a means of generating a magnetic field in the test specimen. Once the part has been magnetized, iron particles are applied to the surface. When discontinuities are present, these particles will be attracted and held in place to provide a visual indication of the flaw. In the examples discussed so far, permanent magnets.

 

However, use of a permanent magnet for MT is generally considered to be inferior to other methods of magnetic particle inspection.

 

Therefore, MT of pressure equipment is generally performed using a certain type of electromagnet. An electromagnet relies on the principle that there is a magnetic field associated with any electrical conductor.

 

Either alternating current (AC) or direct current (DC) can be used to induce a magnetic field. The magnetic field created by AC due to the “skin effect” is strongest at the surface of the test object. AC will also provide greater particle mobility on the surface of the part allowing them to move about freely to locate areas of flux leakage, even though the surface of the part may be irregular.

 

Direct current (DC) induces magnetic fields that have greater penetrating power and can be used to detect near surface discontinuities.

 

Limitations

 

• Materials or part being inspected must be ferromagnetic
• High currents can be used (c) Will only detect surface and slightly subsurface flaws
• Material or part may need to be demagnetized
• Material or part must be clean and relatively smooth
• Equipment can be bulky and heavy
• Power supply generally required
• Coating may mask indications
• Material or part permeability may affect results

 

Advantages

 

• Economical
• Aid to VT
• Can be fixed or portable equipment
• Instant repeatable results
• Effective inspection method
• Contrast or fluorescent consumables

 

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