Acoustic emission testing
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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.
Acoustic emission testing is used as a type of nondestructive testing technology is in the ultrasonic regime, typically within the range between 100 kHz and 1 MHz. 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 0Hz 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 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, it can be generated from sources not involving material failure including friction, cavitation and impact.
Transducers are attached to the material in order 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 in order 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.
