If the discontinuity, such as a crack, is parallel to the lines of magnetic flux, there will be no leakage and therefore no indication observed. To resolve this issue, each area needs to be examined twice. The second examination needs to be perpendicular to the first so discontinuities in any direction are detected.
The examiner must ensure that enough overlap of areas of magnetic flux is maintained throughout the examination process so discontinuities are not missed. History of Magnetic Particle Examination. Magnetism was first used as early as to check for cannon barrel defects.
Cannon barrels were first magnetized and then a magnetic compass was moved down the length of the barrel. If a discontinuity was present, the magnetic flux would leak out and cause the compass needle to move. Defects could be easily located with this technique. In the early s, William Hoke noticed metallic grindings from hard steel parts held by a magnetic chuck while being ground formed patterns that followed the cracks in the surface of parts he was machining.
He also found that by applying fine ferromagnetic powder to the parts, there was a build-up of powder at the discontinuities which formed a more visible indication. It was quicker and did not leave behind the white powder that required clean-up.
After an MT evaluation, only iron powder was left behind, which could easily fall off the part or be blown away. There are many different techniques and combinations of techniques of MT. There are two different ferromagnetic examination media: dry particles and wet particles.
Both forms can be either fluorescent or non-fluorescent visible, color contrast and come in a variety of colors to contrast with the tested material. Two of the most-used methods are the stationary horizontal system, using longitudinal and circular magnetization techniques, and the very portable yoke technique.
United States: N. Copy to clipboard. United States. Other availability. MPI will detect flaws perpendicular to the magnetic field, for this reason it is necessary to magnetise the material in at least two directions. During MPI the iron particles are in either dry or wet form.
In wet form they are either oil or water based. Electromagnet or Permanent Magnets are used for this inspection technique. Strength depends upon the number of interrupted lines of force.
Indications of maximum size are found when magnetic lines are at right angle to discontinuities. The Intensity of magnetization depends upon the susceptibility of the material to become magnetized.
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