Acoustic Emission Test (AET)
Acoustic Emission (AE) refers to the generation of transient elastic waves produced by a sudden redistribution of stress in a material. When a structure is subject to an external stimulus (change in pressure, load, or temperature), localized sources trigger the release of energy, in the form of stress waves, which propagate to the surface and are recorded by sensors. With the right equipment and setup, motions on the order of picometers (10 – 12 m) can be identified. Sources of AE vary fro natural events like earthquakes and rockbursts melting, twining, and phase transformations in metals. In composites, matrix cracking and fiber breakage and debonding contribute to acoustic emissions. AE’s have also been measured and recorded in polymers, wood, and concrete, among other materials.
Detection and analysis of AE signals can supply valuable information regarding the origin and importance of a discontinuity in a material. Because of the versaltility of Acoustic Emission Testing (AE), it has many industrial applications (e.g. assesing structural integrity, detecting flaws, testing for leaks, or monitoring weld quality) and is used extensively as a research tool.
Acoustic Emission is unlike most other nondestructive testing (NDT) technique in two regards. The first pertains to origin of the signal. Instead of supplying energy to the object under examination, AET siply listens for the energy released by the object. AE test are often performed onstructures while in operation, as this provides adequate loading for propagating defects and triggering acoustic emissions.
The scond difference is that AET deals with dynamic process, or changes, in a material. This is particulary meaningful because only active features (e.g. crack growth) are highlighted. The ability to discern between developing and stagnant defects is significant. Howevwe, it is possible for flaws to go in detected all together if loading is not high enough to cause an acoustic event. Furthermore, strength or risk of failure of a component. Other advantages of AET include fast and complete volumetric inspection using multiple sensors, permanent sensor mounting for process control, and no need to disassemble and clean a specimen.
Unfortunately, AE systems can only qualitatively gauge how much damage is contained in a structured. In order to obtain quantitative result about size, depth, and overall acceptability of a part, other NDT methods (often ultrasonic testing) are necessary. Another drawback of AE stems from load service environments which contribute extraneous noise to the signals. For successful applications, signal discrimination and noise reduction are crucial.
