Magnetic Particle Testing Principle MPI Method Techniques of Magnetic Particle Testing
Magnetic particle testing is a nondestructive method of revealing surface and slightly subsurface discontinuities in magnetizable materials. It may be applied to raw materials such as billets, bars and shapes; during processes such as forming, machining, welding, heat treating and electroplating; and in testing for service related discontinuities. Magnetic particle procedures cannot be used with
nonmagnetizable materials such as aluminum or copper.
Magnetic particle testing is based on the principle that magnetic flux in a magnetized test object is locally distorted by the presence of a discontinuity. This distortion causes some of the magnetic field to exit and reenter the test object at the discontinuity. This phenomenon is called magnetic flux leakage. Flux leakage is capable of attracting finely divided particles of magnetic materials that in
turn form an outline or indication of the discontinuity.
One of the objectives of magnetic particle testing is to detect discontinuities as early as possible in the processing sequence, thus avoiding the expenditure of effort on materials that will later be rejected. Practically every process, from the original production of metal from its ore to the last finishing operation, may introduce discontinuities. Magnetic particle testing can reveal many of these,
preventing components with discontinuities from entering service.
Even though magnetic particle testing may be applied during and between processing operations, a final test is usually performed to ensure that all detrimental discontinuities have been detected. The test itself consists of six basic operations.
1. Clean the test object.
2. Establish a suitable magnetic flux in the test object.
3. Apply magnetic particles in a dry powder or a liquid suspension.
4. Examine the test object under suitable lighting conditions.
5. Interpret and evaluate the test indications.
6. Demagnetize, if necessary.
Techniques of Magnetic Particle Testing
Magnetic particle testing is a method of nondestructive testing that uses very small magnetic particles to reveal discontinuities in test objects capable of being magnetized. Magnetizable objects include those test objects made of metals classed as ferromagnetic.
To find discontinuities in metals that are not ferromagnetic, such as aluminum, copper and brass, other testing methods must be used.
This chapter includes a description of the various methods, procedures and equipment used to magnetize test objects, the electric currents and amperages used and the kinds of magnetic particles available for either wet or dry application. Special magnetization methods are also included.
The technique to use for a particular magnetic particle test depends on the following factors.
1. The type of discontinuity being looked for in a test object.
2. The material, shape and size of the test object.
3. The magnetic particle testing equipment available.
This chapter also describes technique variations along with the effect each has on the results that will be attained. The following points must be considered and selected to achieve a particular test result.
1. The type and amount of electric current to be used for magnetization. If the discontinuities sought are at the surface, alternating current has several advantages. The rapid reversal of the field imparts mobility to the particles, especially to the dry powders. The dancing of the powder helps it move to the
area of leakage fields and to form stronger indications. This effect is less pronounced in the wet method.
2. The direction of the resulting magnetic fields obtained using these currents.
3. The kind of magnetic particles to be used.
4. How the testing media is to be applied.