LAW OF MAGNETISM
When an external magnetic field is applied to ferromagnetic materials, the magnetic domains align parallel to the applied fields. These domains are very small, but larger than the atoms of the material. It is the interchange action between atoms and the alignment of the magnetic domains that are the cause of increased flux density. As the magnetizing force increases, the aligned
domains increase in volumetric steps. This incremental increase is detectable, and is often referred to as the barkhausen effect. When all the domains in a material are aligned, the material is said to be magnetically saturated.
Materials Influenced by Magnetic Fields
Some materials are attracted by magnets, whereas others are
repelled.
1. Paramagnetic: weakly attracted magnetic materials not normally tested by magnetic particle testing.
2. Ferromagnetic: materials having high magnetic permeability, such as nickel, iron, cobalt and many transuranic isotopes like gadolinium.
3. Diamagnetic: materials that are weakly repelled by a magnetic field. This kind of magnetism is often used to locate buried and hidden conductors.
4. Nonferromagnetic: nonmagnetic materials. This classification of materials includes materials that are not strongly attracted or repelled to a magnetic field. This encompasses the majority of metals and other nonmetallic materials, and is caused by their low degree of magnetic permeability. Because of the inability to react to a magnetic field, inspection using the magnetic particle test method cannot be performed on these materials and is such limited only to ferromagnetic materials. Nonferromagnetic materials, if nonporous, may be inspected using alternative test methods such as liquid penetrant testing.