Acoustic Emission Barkhausen Techniques

Barkhausen Effect

If you zoom into the hysterysis loop on a magnetic flux density vs magnetizing field strength, instead of seeing a smooth curve, the jumps due to the barkhausen effect can be observed.The Barkhausen effect refers to the sudden change in size of ferromagnetic domains that occur during magnetization or demagnetization. During magnetization, favorably oriented domains develop at the cost of less favorably oriented domains. These two factors result in minute jumps of magnetization when a ferromagnetic sample (e.g. iron) is exposed to an increasing magnetic field (see figure). Domain wall motion itself is determined by many factors like microstructure, grain boundaries, inclusions, and stress and strain. By the same token, the Barkhausen effect is too a function of stress and strain.

Barkhausen Noise

Barkhausen noise can be heard if a coil of wire is wrapped around the sample undergoing magnetization. Abrupt movements in the magnetic field produce spiking current pulses in the coil. When amplified, the clicks can be compared to Rice Krispies or the crumbling a candy wrapper. The amount of Barkhausen noise is influenced by material imperfections and dislocations and is likewise dependent on the mechanical properties of a material. Currently, materials exposed to high energy particles (nuclear reactors) or cyclic mechanical stresses (pipelines) are available for nondestructive evaluation using Barkhausen noise, one of the many branches of AE testing.