Acoustic emission (AE) is a relatively new non-destructive Testing (NDT) technique. Structural testing and assessment, material characterization, and process monitoring are three important application areas of AE. A comprehensive introduction to the AE technique can be found in .
The AE technique is one of the most reliable NDT techniques for detecting and monitoring damages and defects in different structures. AE has been effectively employed for fracture behavior monitoring and fatigue detection in various materials including composites, metals, concrete, fiberglass, ceramics, plastics, and wood. It has also been used for fault and pressure leak detection in pipes, tanks, and vessels.
There are several electronic instruments that can be used to digitize and store large numbers of high-speed digital waveform signals of AE. Common instruments used in AE include preamplifiers, amplifiers, filters, sensors and other data collection, analysis, and storage equipment such as computers, oscilloscopes, and voltmeters. Preamplifiers are used to amplify a weak signal and to reduce the interference from noise, while the piezoelectric sensors are used for the conversion of mechanical AE waves into electrical voltages. The overall objective of the measurement is to determine the various AE parameters such as the frequency range (controlled by filters) that exist in the system by observing and measuring the performance of AE amplifiers and sensors. These are very useful mechanisms for measuring the essential AE parameters such as event, count, energy moment, maximum amplitude, hit, energy, arrival-time difference, RMS (root mean square) voltage, rise time, spectrum, frequency, and duration .
2. Applications of AE techniques
Application areas of AE span numerous fields, including aerospace, automotive, biomedical, manufacturing, civil, and materials engineering fields.
Monitoring the condition and predicting the life of the main structures of an aircraft play significant role in guaranteeing the flight safety. AE techniques are successfully employed in damage and crack identification and monitoring in aircraft composite and steel structures [3, 4].
AE is extensively used in the automotive industry in fault diagnosis of internal combustion engines (ICEs). Reference  deals with advanced techniques based on vibro-acoustic signals that can diagnose and monitor ICE malfunctions under vehicle operating conditions. Reference  develops new AE models and effective wavelet-based AE signal processing techniques for monitoring lubrication conditions.
The primary use of AE technique in biomedical field is bone condition assessment under different loading conditions, in osteoporosis and in fracture healing process monitoring . AE can be used for detecting defects in tissues and materials, predicting failure, and monitoring damage progression in real time .
AE has been used as a widely applied technique in manufacturing process monitoring due to its sensitivity to process parameters. The use of AE as a monitoring technique for machining operations comes with more advantages, one of which is its ability to detect machine vibrations from those of AE signals due to high-frequency range and sensitivity of AE signals, thus preventing it from interfering with the cutting operation [9, 10].
Structural health monitoring (SHM) in civil engineering involves AE technique for detecting cracks in structures. This technique relies on the high-frequency ultrasonic waves generating energy that is rapidly emitted from a material throughout from the initiation to growth progression of cracks. The wide applicability of AE technique is evident in several metal piping system evaluations and fiberglass-reinforced plastics (FRP) and concrete bridges [11, 12].
AE originates from stress waves generated as a result of the growth or movement that takes place in solid defects. When a composite material is subject to a mechanical load, it can experience matrix cracking, debonding, and delamination. AE is a powerful technique capable of detecting these damage types in composites [13, 14].
Scruby CB. An introduction to acoustic emission. Journal of Physics E: Scientific Instruments. 1987; 20(8):946
Grosse CU, Ohtsu ME. Acoustic Emission Testing. Berlin, Heidelberg: Springer-Verlag; 2008
Diamanti K, Soutis C. Structural health monitoring techniques for aircraft composite structures. Progress in Aerospace Sciences. 2010; 46(8):342-352
Pullin R, Eaton MJ, Hensman JJ, Holford KM, Worden K, Evans S. Validation of acoustic emission (AE) crack detection in aerospace grade steel using digital image correlation. Applied Mechanics and Materials. 2010; 24:221-226
Delvecchio S, Bonfiglio P, Pompoli F. Vibro-acoustic condition monitoring of internal combustion engines: A critical review of existing techniques. Mechanical Systems and Signal Processing. 2018; 99:661-683
Wei N, Gu JX, Gu F, Chen Z, Li G, Wang T, et al. An investigation into the acoustic emissions of internal combustion engines with modelling and wavelet package analysis for monitoring lubrication conditions. Energies. 2019; 12:640-659
Shrivastava S, Prakash R. Assessment of bone condition by acoustic emission technique: A review. Journal of Biomedical Science and Engineering. 2009; 2(03):144
Kohn DH. Acoustic emission and nondestructive evaluation of biomaterials and tissues. Critical Reviews in Biomedical Engineering. 1995; 23(3-4):221-306
Govekar E, Gradisek J, Grabec I. Analysis of acoustic emission signals and monitoring of machining processes. Ultrasonics. 2000; 38(1-8):598-603
Jemielniak K, Arrazola PJ. Application of AE and cutting force signals in tool condition monitoring in micro-milling. CIRP Journal of Manufacturing Science and Technology. 2008; 1(2):97-102
Grosse CU, Reinhardt HW, Finck F. Signal-based acoustic emission techniques in civil engineering. Journal of Materials in Civil Engineering. 2003; 15(3):274-279
Ai Q, Liu CX, Chen XR, He P, Wang Y. Acoustic emission of fatigue crack in pressure pipe under cyclic pressure. Nuclear Engineering and Design. 2010; 240(10):3616-3620
Ohtsu M. The history and development of acoustic emission in concrete engineering. Magazine of Concrete Research. 1996; 48(177):321-330
Wevers M. Listening to the sound of materials: Acoustic emission for the analysis of material behavior. NDT and E International. 1997; 30(2):99-106