Acoustic Emission

Acoustic Emission (AE) is a naturally occurring phenomenon whereby external stimuli such as mechanical loading generate sources of elastic waves. AE occurs when a small surface displacement of a material is produced. This occurs due to stress waves generated when there is a rapid release of energy in a material, or on its surface. The wave generated by the source of the AE, or, of practical interest, in methods used to stimulate and capture AE in a controlled fashion for study and/or use in inspection, quality control, system feedback, process monitoring and others.

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Organizations which support the study of Acoustic Emission

Several organizations worldwide support the study and use of AE through various activities including technical meetings on Acoustic Emission.

• AEWG The Acoustic Emission Working Group [www.aewg.org].
• EWGAE The European Working Group
• JCEA The Japanese Working Group on Acoustic Emission
• GLEA The Latin American Working Group on Acoustic Emission [1].

The 6th international meeting on Acoustic Emission (IAE6), a collaboration of the AEWG, the European Acoustic Emission Working Group *(EWGAE) and the Japanese Working Group on Acoustic Emission is being held as this revision to entry is being revised (Oct 29-Nov 2, 2007). A request to the participants has been made to expand this article during the meeting.

The websites for these organization contain information on their histories.

Acoustic Emission Publications

The number of technical papers on Acoustic Emission exceed 10,000. Because of the diverse range of Acoustic Emission, papers on Acoustic Emission appear in Journals across the scientific spectrum. A notable source of AE information is especially noteworthy:

The e-Journal & Database of Nondestructive Testing NDT.net [2]

The Acoustic Emission Phenomenon

Acoustic emission testing is used as a type of nondestructive testing technology. It is in the ultrasonic regime, typically within the range between 100 kHz and 1 MHz (although this range is not absolute). Acoustic emissions can be monitored and detected in frequency ranges under 1 kHz and have been reported at frequencies up to 100 MHz. Rapid stress-releasing events generate a spectrum of stress waves starting at 0 Hz and typically falling off at several MHz, but one strength of the technique is that background noise, particularly airborne, falls off more quickly, so the signal-to-noise ratio reaches an optimum value around the conventional frequency range.

A commonly accepted definition for AE is a transient elastic waves within a material due to localized stress release. Hence, a source which generates one AE event is the phenomenon which releases elastic energy into the material, which then propagates as an elastic wave. AE events can also come quite rapidly when materials begin to fail, in which case AE activity rates are studied as opposed to individual events. AE events that are commonly studied include the extension of a fatigue crack, or fiber breakage in a composite material among material failure processes. AE is related to an irreversible release of energy, and can be generated from sources not involving material failure including friction, cavitation and impact.

Transducers are attached to the material to detect these waves. Most of these sensors are in the frequency range of 20 kHz to 650 kHz. Some geophysical studies with AE use much lower frequency sensors, while sensors in the MHz range are also available commercially.

AE tools do not actively produce waves (or "insonify") as in conventional ultrasonics. Rather, they passively detect emissions from acoustic sources.

AEs from within a material are monitored to locate and/or define their source event. AE is even more commonly used to correlate when activity occurred with the level of stimuli or length of time before something occurred, such as determining the onset of cracking, documenting the failure of a part during unattended monitoring or the level of reoccurrence of AE during multiple load cycles. The last method listed is the basis for many safety inspection methods utilizing AE. Parts inspected with AE can remain in service.

In addition to non-destructive testing acoustic emission monitoring has application in process monitoring. Applications where AE monitoring has successfully been used include detecting upsets in fluidized beds and detecting end points in batch granulation.