**1.2 Acoustic emission**

Non-destructive testing methods, especially acoustic emission methods, are used to condition monitor of engineering structures [3]. Acoustic emission as a phenomenon can be defined as transient elastic waves caused by internal microdisplacements in the materials of the tested structures. Acoustic emission, as defined by the American Society for Testing and Materials, refers to the class of phenomena whereby transient elastic waves are generated by the rapid release of energy from localized sources within material under stress [4]. Due to its high sensitivity, this method can detect processes such as micro-crack initiation and growth, displacement, failure, slip, leakage, or sediment separation [5]. **Figure 2** shows typical mechanisms that generate acoustic emission waves. The main sources of acoustic emission in metals are plastic deformation and crack growth processes, which are energy-release mechanisms at the scale of crystalline microstructure.

The acoustic emission method has advantages over other non-destructive testing methods, such as the dynamics of this inspection method and the ability to display crack growth online in the structure under load and during service. Other advantages of this method are high speed of testing, accurate location of defects, high efficiency, less sensitivity to the geometry of the part compared to other methods, and detection of very small and micro-scale defects [3].
