Modern acoustic emission technique and its application in aviation industry
Introduction
Acoustic emission (AE) testing is an important branch of non-destructive testing (NDT). The main advantage of AE is twofold: the first one is that AE is a dynamic method capable of continuously testing and in situ monitoring structures under supervision without the need of their stoppage or shutdown. The second one is that AE is a passive means without the need of injecting signals from outside to the object under investigation, thus causing little disturbance to the object. The main disadvantage of AE, on the other hand, is the noise, or background noise interferences, and the difficulty in realizing the quantitative evaluation of this defect because of the lack of understanding of the AE source mechanism, the distortion of sound wave during propagation.
The condition monitoring and life prediction of aircraft main structures have received great concern because they have close relations with flight safety. The early detection of fatigue crack initiation and growth is in general beyond the capability of various conventional NDT means, AE however is more suitable to undertake the task. The other important issue is the environment-related aircraft life, which is also called as the calendar life of an aircraft or a group of aircrafts. It is noticed very often that a military aircraft uses up its calendar life much earlier than its fatigue life. A method for determining fatigue life is available and has been developed mature to some extent, on the other hand, however, the means for determining and evaluating the calendar life of a military aircraft is still mostly based on experience and lacks systematic and scientific foundation. Therefore, it is of a great concern to determine the calendar life of military aircrafts based on scientific research results. The calendar damage, which is mainly caused by corrosion, and is sometimes called corrosion damage, must be first investigated in order to determine the calendar life. Hence, monitoring and investigating corrosion damages of aircraft structures, such as bulkhead, fastener holes, wing skin and fuel-tank baffle, are of significance. The work can supply scientific basis for determining calendar life.
Section snippets
Basic element of modern AE technique
Modern AE covers a much wider aspect than the commonly used modal AE [1], [2], although both have the same abbreviation, MAE. The term MAE is uniquely given to modern acoustic emission in this paper. In essence, MAE is a method based on the combination of AE parameter and waveform analysis. The first one, the parameter analysis, covers a much wider aspect than before, and it includes data trend analysis, correlation and modern data processing means. The waveform analysis is based on the sound
Role of AE in evaluating corrosion damages
AE sources of corrosion process are diverse. The exfoliation and rubbing of corrosion products, and the breakage of hydrogen and air bubbles produced by corrosion, the breakage of passivation film are all possible AE sources and can produce detectable AE signals [3]. In order to study the calendar life, the calendar damage, which is mainly caused by corrosion, must be first investigated. The combination of AE waveform and parameter analysis can give a clear picture about the features of AE
Aircraft fatigue crack prediction and fatigue life evaluation
The prediction and evaluation of fatigue life of an aircraft are normally fulfilled by a fatigue test, especially the fatigue test of a full-size aircraft body. Acoustic emission technique is a good choice, but is facing problems caused by background noise. Genuine AE signals can be well identified by using AE waveform analysis instead of conventional parameter analysis. Due to the time span and large data involved during a fatigue test, it is very difficult, if not impossible, to perform
Conclusions
AE technique is of significance in safeguarding aircraft safety. Due to the lack of quantitative appraisal of defect and of the standard and code, its application in aviation is limited to very narrow areas. Traditional AE parameter analysis supplies a rapid, simple and cost-effective means of inspecting and monitoring structure defect and failure for the aviation industry, it however, due to the lack of understanding of AE source mechanism or the failure to grasp the physical essence of AE
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