Acoustic Emission

What it is ...

The mechanical degradation of machinery is accompanied by various processes that are driven at a microscopic level by transients that release stored elastic energy in a localised region. Each of these events launches an elastic wave that propagates throughout the surrounding material. Acoustic Emission (AE) is a technology based on listening to the high frequency component of these structure borne sounds (typically between 25 kHz and 1 MHz). When machinery operates, the possible sources of AE include impacts, friction, particulate crushing and metal removal. The intensity and quantity of these sources generally increases as degradation progresses.

The importance of AE as a CM technique results from its extreme sensitivity and, in practice, it is largely unsurpassed in its ability to detect the earliest signs of machinery degradation. The high signal-to-noise ratio, which AE signals exhibit in the CM role, enables fault detection and trending to be conducted directly in the time domain with relatively simple signal processing.
Specific benefits of AE can be summarised as follows:

direct detection of fault signals without machinery specific data
wide applicability to rotating machinery including very slow rotation
applicable to reciprocating machinery and linear actions
inherently immune to audible noise and low frequency vibrations
front-line technique for use by maintenance and production personnel

Although AE signals contain much diagnostic information, identification of the nature of the fault usually requires additional analysis. In the case of rotating machinery this is based on frequency analysis of the enveloped AE signal and interpretation is done with reference to defect repetition frequencies (in a very similar way to Vibration analysis). For non-rotating machinery, additional diagnostic information can be extracted by analysing AE signals with respect to the phase of the machine cycle.

Commercially available AE equipment suitable for CM applications ranges from smart sensors through portable and on-line instruments to sophisticated multi-channel AE systems.