Acoustic Emission is a phenomenon whereby transient elastic waves are generated by, e.g., plastic deformation, crack propagation, erosion, corrosion, impact, leakage [EN 1330-9].
The mechanism giving rise to emission of transient elastic waves is a sudden release of elastic energy in case of plastic deformation, crack propagation, erosion, corrosion and impact. In case of leakage, transient elastic waves are generated by friction of flowing medium.
Once a transient elastic wave is generated, it will propagate in its medium and eventually reach a surface. The interaction of the transient elastic wave with a surface will cause surface motion. Acoustic emission sensors mounted on the surface are able to pick up the faintest surface motion and convert it into an electrical signal. This electrical signal can be processed and features extracted. Feature data and transient signal data (waveforms) can be stored and analyzed in real-time.
An AE measurement obtains information about active damage processes in a material. AE data analysis is capable of monitoring defects at the time of their occurrence. The advantage is that only a limited number of sensors are needed to monitor large structures. Additionally, the spatial location of the signal origin can be calculated by using the signal arrival times at a number of sensors.
Application in Materials Research
The acoustic emission technique finds one of its largest application fields in materials research. Examples are the detection of the point of damage initiation and the rate of damage evolution under mechanical loading (tensile, bending, fatigue, creep), the discrimination between different damage phenomena occurring simultaneously in composite materials (matrix cracking, delamination, fiber fracture), the study of phase transformations, the detection of coating wear, etc. Generally, the acoustic emission technique can be used to obtain information about the microstructural changes that are occurring in any loaded material. It thus contributes to a better and more profound understanding of the behavior of these materials.
Application in Structural Integrity Monitoring
The main area where the acoustic emission technique has found practical and industrial applications is in the field of structural integrity monitoring. By equipping a loaded, safety critical structure with a number of Acoustic Emission sensors, information can be gathered about the evolution of damage in the structure during its service life. Alternatively, structural integrity can be monitored on a periodic basis by applying proof loads and correlating it with detected AE. It is important to realize that the AE method requires the monitored structure to be under load and that only active damage processes are detected. Good examples of this kind of application are the monitoring of pressure vessels, chemical reactors or storage tanks.
Because the physical process of acoustic emission occurs in a wide variety of materials and under a large range of loading conditions, the method offers great potential for use as an online monitoring tool. Due to its inherent advantages as compared to other techniques, it should always be considered when real-time detection and location of defects is required.