Magnetic Barkhausen Noise 

MBNMagnetic Barkhausen Noise is a physical phenomenon that can be used as a non-destructive technique for measuring changes in mechanical properties in magnetic materials or for detecting grinding damage in high-strength steels. When a changing magnetic field is applied to a ferromagnetic material, the magnetic domains in the material randomly align with the field in fits and starts, which in turn causes jumps in the magnetic flux in the material. These jumps can be detected by corresponding transitions in the voltage induced in a coil placed near to the material.
The MBN phenomenon was discovered in 1919 by Heinrich Barkhausen, hence the name, who detected the induced voltage jumps as noise in an earphone. MBN can give information on stresses within a material or compositional microstructure, such as the amount of impurities or crystal dislocations. It is essentially a surface technique, with a depth of penetration up to about 1 mm.

For more information on MBN see:
www.nde-ed.org/EducationResources/CommunityCollege/Other Methods/AE/AE_BarkhausenTechniques.php

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