Sound Emission Testing

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Signal sources and signal processing

Sound emission testing is a quasi-non-destructive testing method that is linked to damage-inducing processes. The sound emissions released in the process can be caused by mechanical, biological or chemical stresses in both the microscopic and macroscopic range. The definition of sound emissions according to Bardenheier, who states in [1]:

‘Sound emissions (SE) occur in every solid whenever elastic energy is released in the form of mechanical stress waves when certain material stresses are exceeded.’

The mechanical stress waves propagate spherically from the source and can be converted into analogue electrical signals using piezoelectric transducers (SE sensors). However, the original signal, which can be defined as a square wave, undergoes significant changes due to dispersion and reflections as it propagates through the material. The square wave can thus become a long, slowly rising and falling signal, which also exhibits an exponential decrease due to inherent loss mechanisms. Figure 1 schematically shows the signal processing in sound emission analysis (SEA).

Fig. 1:

Signal processing in sound emission analysis based on [1]

Continuous emissions and burst signals

The recorded signals can basically be classified into two types. DIN EN 1330-9 [2] defines a signal that cannot be separated despite high time resolution as a continuous emission and events that can be separated from each other as burst emissions or transient signals. Characteristic examples of continuous emissions are plastic, homogeneous deformations of metals, leakage flows or flow processes. Discontinuous events such as crack formation and crack propagation processes, as well as fibre pull-out and fibre breakage in fibre-reinforced plastics, lead to burst emissions. (see: fracture behaviour, short fibre composites).

Principle of sound emission testing

Figure 2 on the left shows a schematic representation of the principle of acoustic emission measurement using a measuring chain consisting of an ultrasonic sensor, a preamplifier and an analyser. A typical transient signal with the derivable measured variables is shown in the right-hand Fig. 2b.

Fig. 2:

Principle of sound emission measurement (a) and example of a transient signal with the characteristic measured variables that can be derived (b)

A signal is referred to in the literature as an event or hit, whereby the term hit as defined in DIN EN 1330-9 [2] is used in this procedure. Another variable not shown in the Figure is the signal energy E_AE, which is obtained by integrating the acoustic signal using the equation below. Due to the amplitude-time representation, the unit of energy is abbreviated as eu for energy unit, which physically corresponds to 1 nVs.

E_{AE}\,=\,\int _{0}^{t_{ED}}U\left(t\right)\,dt

Signal analysis

The recorded signals can be analysed in a variety of ways using sound emission analysis. For example, impulse, energy and event measurements can be selected for display in both sum (cumulative) and rate (distributive) modes. In addition, amplitude and frequency analysis can be performed. The possibilities for evaluating sound emissions are shown schematically in Fig. 3 below. The representation of the results in sum and rate form serves to characterise the signal dynamics and thus the load- and deformation-related damage development and accumulation. In contrast, the representation of the amplitude values and the frequency analysis can be used to draw conclusions about the damage mechanisms and their temporal assignment. However, [3] points out that the representation via amplitude analysis only allows limited conclusions to be drawn about the damage due to the weakening of the mechanical stress waves by the distance travelled in the material and thus by the distance between the sound emission source and the sensor. In these cases, the damage mechanisms can only be assigned by means of frequency analysis [4, 5].

Fig. 3:

Possible representations of acoustic emission analysis based on [1]

Sound emission testing is applied in two main areas. On the one hand, applications in the technical sense are possible, e.g. in the area of component monitoring of containers or in the monitoring of crack growth (see: crack propagation) on bridges and dams. On the other hand, the area of application in materials science lies in the investigation of damage mechanisms and the correlations to material properties that can be derived from them [2, 5].

[1]	Bardenheier, R.: Schallemissionsuntersuchungen an polymeren Verbundwerkstoffen – Part I: Das Schallemissionsmeßverfahren als quasi-zerstörungsfreie Werkstoffprüfung. Zeitschrift für Werkstofftechnik, 11 (1980) 41–46
[2]	DIN EN 1330-9 (2009-09): Non-destructive Testing – Terminology– Part 9: Terms used in Acoustic Emission Testing (withdrawn)
[3]	Ramirez-Jimenez, C. R., Papadakis, N., Reynolds, N., Gan, T. H., Purnell, P., Pharaoh, M.: Identification of Failure Modes in Glass/Polypropylene Composites by Means of the Primary Frequency Content of the Acoustic Emission Event. Compos. Sci. Technol. 64 (2004) 1819–1827 ; https://doi.org/10.1016/j.compscitech.2004.01.008
[4]	Bohse, J.: Acoustic Emission Characteristics of Micro-Failure Processes in Polymer Blends and Composites. Compos. Sci. Technol. 60 (2000) 1213–1226; https://doi.org/10.1016/S0266-3538(00)00060-9
[5]	N. N., Kompendium Schallemissionsprüfung – Acoustic Emission Testing (AT) – Grundlagen, Verfahren und praktische Anwendung. DGZfP-Fachausschuss Schallemissionsprüfverfahren

Signal sources and signal processing

Continuous emissions and burst signals

Principle of sound emission testing

Signal analysis

See also