Acoustic Emission (AE)

Acoustic emission is the technical term for the noise emitted by materials and structures when they are subjected to stress. Types of stresses can be mechanical, thermal or chemical. This emission is caused by the rapid release of energy within a material due to events such as crack initiation and growth, crack opening and closure, dislocation movement, twinning, and phase transformation in monolithic materials and fiber breakage and fiber-matrix debonding in composites.
The subsequent extension occurring under an applied stress generates transient elastic waves which propagate through the solid to the surface where they can be detected by one or more sensors. The sensor is a transducer that converts the mechanical wave into an electrical signal. In this way information about the existence and location of possible sources is obtained. Acoustic emission may be described as the "sound" emanating from regions of localized deformation within a material.
Until about 1973, acoustic emission technology was primarily employed in the non-destructive testing of such structures as pipelines, heat exchangers, storage tanks, pressure vessels, and coolant circuits of nuclear reactor plants. However, this technique was soon applied to the detection of defects in rotating equipment bearings.