# Machine-Learning-Based Methods for Acoustic Emission Testing: A Review
## 2 Acoustic Emission Testing Methods
### Acoustic Emission Sources  
In heterogeneous materials, fracture occurs at the maximum of progressive damage due to the applied loads or the severity of the environmental conditions. The microfracturing process is accompanied by a fast dislocation motion, which is associated with a rapid spontaneous release of energy in the form of transient elastic waves or acoustic emissions (AEs). The AE event manifests itself as an elastic wave that propagates through the material toward the surface of the element and can be detected by appropriate sensors that transform it into an electrical signal. The transient stress wave ends when a new equilibrium configuration is reached, in which the resulting forces acting on each volume element vanish. The AE signal carries with it a certain amount of information that characterizes it, identifying its origin. It is generated only when the crack grows or when its edges touch each other. AE can therefore provide information on the origin of the discontinuity in a component subjected to loads and on its subsequent development when the component is subjected to continuous and repetitive stresses [24]. As already said, when a solid is subjected to a mechanical stress of a certain intensity, it releases energy, which travels in the form of elastic high-frequency waves. These waves are captured by a sensor that converts their energy into an electrical signal. This signal is then electronically amplified and, using special circuits, processed as an AE signal. The data analysis includes the characterization of the signals according to the location of the source, intensity of the voltage, and frequency content. The onset of this phenomenon is found in situations of various kinds such as mechanical deformations and fractures, phase transformations, corrosion, friction, and processes of magnetic nature [25]. In addition to the signals deriving from the AE, there may be signals deriving from other causes such as noise or from sources of AE not relevant to the objective of the test. The main causes of noise are friction and impacts. The sources of friction are stimulated by structural loads that cause movements in the joints. The sources of impact, on the other hand, can include rain, windblown dust, or flying objects. A relevant part in tests using AE is, therefore, the ability to eliminate all these noise sources and focus on the one relevant to the test. This is achieved by selecting an appropriate setup, taking practical precautions to prevent noise sources as much as possible, and recognizing and removing noise from recorded data. All this depends on the experience of those who carry out the measurements, and it is necessary to have a wide range of data on which to rely to make a correct interpretation of the results [26]. AE, appearing in the form of an elastic wave, is always accompanied by vibrations. Vibrations can be studied from the point of view of wave motion. Each continuous system has masses and elastic forces continuously distributed. These systems consist of an infinite number of particles and, therefore, require an infinite number of coordinates to describe motion. The system must be modeled, so that the motion of each point in the system can be specified as a function of time. The resulting differential equations that describe the motion of particles are the wave equations and describe the propagation of the same in a solid. Wave propagation in solids is very complex, and it is, therefore, necessary to consider only waves directly relevant for the study of vibrations. To remove parasitic oscillatory components, due to structure vibrations and noise, AE detection instruments consider only part of the frequency spectrum detected: other components are removed through the application of band-pass filters. AE has a frequency between 20 kHz and 1 MHz, but the vibrations at lower frequencies (20-100 kHz) can be masked by external noises, and those at higher frequencies tend to very quickly dampen, so the AE's survey range is reduced to the range 100-700 kHz [27].