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This method of NDT may detect surface discotinuitys. MPI can only be used on ferromagnetic materials. A magnetic field is introduced into the specimen to be tested. Fine particles of ferromagnetic powder or particles in a liquid suspension are then applied to the test area. |
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Any discontinuity which interrupts the magnetic lines of force will create a leakage field which has a north and south pole either side of it. This attracts the ferromagnetic particles in great numbers. The discontinuity may show as a black indication against a contrasting background (usually white paint). A fluorescent indicator, which is more sensitive, may also be used and the results are viewed under a ultraviolet light. Also,by using DC or half wave rectified current, sub-surface discontinuities up to 2mm below the surface may be found. |
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