MECHANICAL

The following characteristics affect key test parameters, including required magnetizing current, current on time, when to apply particles and demagnetization requirements.
1. Alloy, shape and magnetic condition of test object.
2. Type of magnetizing current.
3. Direction of magnetic field.
4. Sequence of operations.
5. Value of flux density.

TEST MATERIALS OF MPI

Wet Particles

Particle size distributions for both wet and dry tests are important considerations. For wet tests using true residual techniques, smaller particles may accumulate faster producing an observable indication. For wet tests, larger rounded particles may produce an observable indication faster under continuous magnetization techniques. Colors and backgrounds should be selected to provide optimum contrast when viewed by the technician. Wet particles, including oil based
visible/fluorescent and water based visible/fluorescent, fit various testing requirements and environments.

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Effects of Discontinuities on Materials

SURFACE DISCONTINUITIES

The magnetic field surrounding subsurface discontinuities drops off very rapidly with distance below the surface. Magnetic particle movement is critical to the collection of contrastingly colored or fluorescent particles at a discontinuity site. Thus, it is desirable to have fluctuating magnetic fields on the surface of the test object. 
Certain metallurgical discontinuities produce much higher collections of magnetic particles than other kinds of discontinuities of the same size.
The important condition of surface cracks includes metal grainto-grain tearing induced by surface tensile forces that produce the highest amount of magnetic field disruption for a given size of discontinuity. A leakage field at an air gap in a longitudinally magnetized test object is shown in Figure. This disruption is
produced by the alignment of magnetic domains in the stretched metal crack site. For this reason, cracks in the formation state are highly detectable and normally produce sharp, well defined indications. Open cracks that have been subject to large thermal, chemical or mechanical forces may have had their magnetic field
disruption characteristics greatly or entirely reduced and may not be detectable using magnetic particle testing. 
For high detectability of forming surface cracks, magnetic particle testing uses induced alternating field magnetization techniques as the primary method to test ferromagnetic objects inservice. Using wet alternating current fluorescent techniques will generally produce the highest sensitivity obtainable with this
technology.

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