Home - Education Resources - NDT Course Material - Acoustic Emission
 

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Introduction to Acoustic Emission Testing

Introduction

History

Theory - AE Sources

Theory - AE Waves

Equipment

Signal Features

Data Display

Source Location

Barkhausen Noise

Applications

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AE Barkhausen Techniques

Barkhausen Effect
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.