The acoustic emission technique (AET) is considered quite unique among the non-destructive testing (NDT) methods. The question of whether this method should be classified as completely non-destructive is often posed, since fracture of the material is necessary for testing. In contrast to other NDT methods, however, AET is usually applied during loading, while most others are applied before or after loading of a structure. For example the ultrasound method, which is a typical NDT technique, is able to detect the geometric shape of a defect in a specimen using an artificially generated source signal and a receiver, whereas the AET detects the elastic waves radiated by a growing fracture. Therefore, the acoustic emission (AE) method should be considered to be a "passive" non-destructive technique, because it is only able to identify defects developing during the test. AE is often used to detect a failure at a very early stage of damage long before a structure completely fails. Nevertheless, it is useful to compare the results of acoustic emission tests with other NDE test methods, such as a visual inspection of the surface, ultrasound, X-Ray or RADAR.

There are numerous ways to apply AE techniques. Many reports of its successful application come from laboratory investigations of the behavior of various kinds of materials (e.g. steel, polymers, wood, concrete, fiber reinforced materials, etc.) or of real structures in the field, such as pressure vessels. After some distrust of this method in the early 1980's due to doubtful measurements of vessels, AE is nowadays becoming more and more "popular" and is successfully applied to numerous measurement problems.

Some reasons for its increasing acceptance and use are the advancements in microelectronics and in computer-based analysis techniques. AE is usually dealing with high signal rates and events at relatively high frequencies (from 20 kHz up to several megahertz). Recording and analysis devices need powerful techniques to handle these data.

Another point motivating the development of AE techniques was the interconnection between AE and seismology. The close relationship of these two disciplines inspired scientists to transfer measurement and analysis techniques from earthquake engineering to acoustic emission applications in the 1980's. It is remarkable that even sophisticated techniques such as the three dimensional localization of events, the moment tensor inversion or wavelet techniques are nowadays routinely applied in the AE environment and it is expected that other methods will stimulate further developments.

New developments raise new problems, however, therefore the demands on the equipment are still very high. This is particularly true concerning the sensor technology. Resonant transducers are increasingly replaced by sensors with broader frequency characteristics. Issues of flat response, sensitivity and calibration have to be addressed more carefully in the future. Other sensor techniques that are currently discussed in the field of AE applications (e.g. in structural health monitoring) use network techniques, wireless communication and Micro-Electro-Mechanical Systems (MEMS). These promising ideas ensure that the acoustic emission technology will be a field of interesting future developments and applications.

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