Acoustic Emission as a Powerful Non- Destructive Technique - Article Example

Acoustic Emission as a powerful Non- Destructive Technique (NDT) for applying diagnosis evaluation of rotating machines Introduction: Acoustic emissions (AE) which is taken to mean transient elastic waves produced from a fast discharge of strain energy caused by a deformation or damage within or on the surface of a material. Acoustic Emission Technology (AET) has offered great scope for application in diverse industrial fields including railways, petrochemical, power generation together with air transport and oil industries. “In the application to rotating machinery monitoring,” D. Mba & Raj B.K.N. Rao remark, “AE are defined as transient elastic waves generated by the interaction of two media in relative motion. Sources of AE in rotating machinery include impacting, cyclic fatigue, friction, turbulence, material loss, cavitation, leakage, etc.” (Mba & Rao 2006, P.3-16). Initially built up for the non-destructive testing of static structures, acoustic emission (AE) is now applied in numerous areas including the diagnosis evaluation of rotating machines, such as bearings, gearboxes, pumps, etc One of the most characteristic features of AET in comparison with other NDT methods is that it assists in the detection of the incipient failure in machines and this quality makes AET the favourable monitoring tool in aforementioned industries. It is primary to acknowledge that acoustic emission, through its increased sensitivity, contributes to the improvement of earlier defect/failure detection. Therefore, there is significant research material available on the advantages of the uses and application of AE technology. In this paper, the focus has been a review of the physical disciplines of the AET along with the discussion of the different case study in which the AE technology has been applied effectively. Acoustic Emission Technology (AET) in the diagnosis evaluation of rotating machines: In order to understand the Acoustic Emission (AE), it may be defined as a unsurprisingly taking place fact through which peripheral incentives including mechanical loading produce sources of elastic waves. Acoustic Emission can be understood as the result of the production of a small exterior dislocation of a material. It is due to the stress waves produced during a quick release of energy in a material that the AE occurs. The main use of the waves produced by the source of the AE is that it can be effectively implemented in the methods which arouse and arrest Acoustic Emission in an unnatural way. The importance of the AET has been that it is one of the most effective Non-Destructive Techniques. “Acoustic emission technique is a very powerful non-destructive evaluation method. It has been widely used for detection and characterization of acoustic events, such as crack opening, crack propagation, fibre breaking, de-lamination, impact damage, and so forth. It has also been used for manufacturing process monitoring such as cutting tool wear monitoring. Traditionally the acoustic emission measurements have been used in a more statistical way… However, acoustic emission can be also quantitative and analytic, which is known as quantitative acoustic emission. The basic approach of quantitative acoustic emission is to first obtain a number of transient wave signals, mostly on the surface of the sample under testing.” (Lee., Lin & Kuo 2004, P. 471-478). It is relative that the acoustic emission technique (AE) makes use of one or several sensors to ‘listen’ to a wide range of events that may take place inside a solid material. There are broadly three application are on the basis of the source of this high frequency sound. They are: 1. The structural testing and surveillance. 2. The process monitoring and control. 3. The materials characterisation. These three applications effectively confirm the usefulness of AE technique in the diagnosis of the rotating machinery. “In the first case the source is probably a defect which radiates elastic waves as it grows. Provided these waves are detectable, AE can be used in conjunction with other NDT techniques to assess structural integrity. Advances in deterministic and statistical analysis methods now enable data to be interpreted in greater detail and with more confidence than before. In the second area the acoustic signature of processes is monitored. In the third area, AE is used as an additional diagnostic technique for the study of fracture, because it gives unique dynamic information on defect growth.” (An Introduction to Acoustic Emission 2008, P.946-953). The usefulness of the acoustic emission technology in the various areas such as the railways, bridge maintenance, the pipeline, aircraft, defence and nuclear reactor industries etc has been beyond question. In the case of the railway bridge maintenance, for example, it offers great effectiveness. The non-destructive evaluation technique of acoustic emission monitoring is used in the case of a bridge issue. It listens for the noises from a growing crack and these cracks usually grow only when a train passes on a bridge. “The AE equipment has to sort out the noises of a growing crack from all the other noises in a bridge when a train passes. Once a crack is detected, usually by visual inspection, other non-destructive evaluation techniques, such as dye penetrant, magnetic particles and ultrasonic testing, can help determine the size and depth of a crack. AE monitoring can then be used to give an indication of how fast the crack size is changing with the passage of trains.” (Wanek 2003). Acoustic Emission Technology (AET), it is proved, is an effective technique among the various Non-Destructive Technology and it is particularly found to be useful in the rotating machinery monitoring and diagnosis. The earliest documented evidences of the applications and the effectiveness of the technology date back to the late 1960s. However, in the recent years, some pertinent studies provided comprehensive and critical review on the application and the usefulness of Acoustic Emission Technology to condition monitoring and diagnostics of rotating machinery. According to one recent study, “Acoustic emission (AE) was originally developed for non-destructive testing of static structures, however, over the last 35 years its application has been extended to health monitoring of rotating machines, including bearings, gearboxes, pumps, etc. It offers the advantage of earlier defect/failure detection in comparison to vibration analysis due to the increased sensitivity offered by AE.” (Mba & Rao 2006, P.3-16). It is proved that the AE includes a wide frequency range, i.e. 100 kHz to 1MHz. it is also mentioned that there are two types of the time domain waveforms connected with AE and they are burst and continuous. The former type of Acoustic Emission is not perceptible whereas the latter type refers to a waveform. In the rotating machinery, one finds the application of both waveform types, i.e. a continuous type emission as well as burst type emission. Significantly, a burst type could be associated with transient rolling action of meshing bears and the continuous type emission is the result of tumultuous fluid flow within a peep. It is particularly remarkable that continuous type emission is the operational noise used in rotating machinery. The different parameters of the most frequently measured Acoustic Emission for diagnosis traditionally include amplitude, r.m.s, energy, kurtosis, crest factor, counts and events. According to D. Mba & Raj B.K.N. Rao, Acoustic Emission Technology has become one of the continuously evolving multi-disciplines. It has given rise to a number of concentrated researches and application based studies. They provide the vast amount of discovered, generated and disseminated knowledge in this evolving discipline as the exact confirmation of its varied applicability. According to them, “the interests to develop new technologies to overcome the many hitherto unsolved problems in condition monitoring and diagnostics of complex industrial machinery applications offers immense opportunities for the AE Technology to grow unabated. This is also reflected by the significant growth in the global demands for the AE sensors. With the accelerating speed in the growth of intelligent information, sensor and data acquisition technologies, combined with the rapid advances in intelligent signal processing techniques, a healthy growth in the application of AE in many engineering, manufacturing, processing and medical sectors is to be expected.” (Mba & Rao 2006, P.3-16). However, there is yet much to be explored in the area of Acoustic Emission, especially the application of Acoustic Emission in the diagnosis of rotating machines. It is of significant implication that the acoustic emission (AE) technology offers great advantage in the diagnosis of the rotating machines. Among the various factors, the capacity of the Acoustic Emission technology in the detection of the arrival times of the signals at three or more sensors to calculate the location of the source in the test object proves to be one important advantage. “One of the important aspects of acoustic emission (AE) technology is the ability to use the arrival times of the signals (from a single event) at three or more sensors to calculate the location of the source in the test object. Often the two-dimensional location in a plate is determined. The standard technique combines the measured arrival times with an appropriate single propagation velocity to calculate the source location. In most AE applications, the arrival times of the signals are obtained from the time that the signals first penetrate a fixed voltage threshold.” (Hamstad., O’Gallagher & Gary 2002). In an analysis of the physical disciplines behind the Acoustic Emission technique, one comes to realise that it essentially makes use of the elastic waves which is the result of the quick discharge of energy from localized sources within a multi-layered structure of the blade. In acoustic emission technique, therefore, the use of the elastic waves has a great significance. In the detection of the probable failure of blades in the real time and during their testing in laboratory environment, the acoustic emission technique has been found greatly useful. Though, there are some evidences that suggest the defects of the technique, some important studies unmistakably point to the advantages of the technique. It is also noticeable that the acoustic monitoring has a strong relationship with vibration monitoring, though there is also a major difference. Thus, the acoustic sensors execute registration of the constituent response, whereas the vibration sensors are tightly mounted on the constituent involved and register the local motion. “There are two types of acoustic condition monitoring: passive type - when the excitation is performed by the component itself and active type - when the excitation is externally applied. In the case of acoustic emission application for the testing of a fibreglass structure, the source of emission is cracking of matrix and fibres. The internal crack inside the fibreglass structure will involve all these mechanisms. Which of the emission generation mechanisms is dominating will depend upon the material characteristics and the locally applied stress field in every region of the material. The presence of acoustic emission in a fibreglass structure subjected to an external load indicates a local failure in some part of the structure. However, during the initial loading or at loads low compared with the normal load, fibreglass structure generates some acoustic emission by itself.” (Raisutis 2008). The effectiveness of Acoustic Emission Technology: Case Studies: The application of the Acoustic Emission technology as a diagnosis method in the rotating machines has been a thrust area of several researches and the results of these studies confirm the effectiveness of AET as one of the foremost Non-Destructive Techniques. It has been found in various studies that an acoustic emission technique for a rotating machine is an effective diagnosis method. In one of the studies which relates to a diagnosis method and apparatus using an acoustic emission technique for a rotating machine, it was concluded that AE technique was greatly useful. In fact, this has been concluded by several such studies. Some of the serious affairs with the accidents happening during the operation of rotating machines include steam turbines, electric generators, water turbines, rolling mills, and so on and the mechanical accidents of rotating parts are especially found hazardous. Perceiving abnormality in its early stages has pertinent value in the prevention of such accidents from occurring. It is at this point that the Acoustic Emission technology proves to be useful. “AE techniques have been proposed as one of means for detecting abnormality in its early stages. According to these AE techniques, an ultrasonic signal is detected by an AE sensor mounted on a part of a rotary machine and subject to signal processing… The prior art abnormality detection systems are useful in detecting abnormal conditions one by one. However, the prior type systems are not useful in detecting a plurality of kinds of abnormality.” (Sato & Yoneyama 1987). Thus, in this case study, it has been proved that a rotating machinery diagnosis method and machinery with an AE technique is effective in distinguishing a plurality of kinds of abnormality that crop up at the time of the operation of a rotating machine. It is of especial note at this point that there are several studies that focus on the effectiveness of the Non-Destructive Technique (NDT) and the various techniques used in the mentoring and diagnostics of rotating machinery. A profound analysis of the Acoustic Emission Technique (AET) in comparison with the other techniques proves that the AET has been found a greater technique. In the different industrial sectors the techniques for machine condition monitoring and diagnostics have been proved as gaining acceptance. According to one of the case studies in the area, these techniques have been proved to be effective in predictive or proactive maintenance and quality control. “In this study, multisensors are used to collect signals of rotating imbalance vibration of a test rig. The characteristic features of each vibration signal are extracted with an auto-regressive (AR) model. Data fusion is then implemented with a Cascade-Correlation (CC) neural network. The results clearly show that multisensor data-fusion-based diagnostics outperforms the single sensor diagnostics with statistical significance.” (Liu, & Wang 2001, P.203-210). Conclusion: Acoustic Emission Technology is one important contributor to the modern world as it offers different industrial application in the important areas including railways, petrochemical, power generation together with air transport and oil industries. Among the various advantages of the Acoustic Emission Technology (AET) is the fact that it can be used in detecting the embryonic failure in machines. The various physical disciplines behind the AET contribute to the usefulness of the technique in several key industrial applications. The available case study in which the AE technology has been well applied also suggests the success of the technology as one of the important methods of Non-Destructive Technique. There is room for further researches in the area which will definitely help in the clear understanding of the effect of the AET. Bibliography An introduction to Acoustic Emission: Abstract. (2008). P.946-953. [Online]. IOP. Last accessed 27 July 2008 at: http://www.iop.org/EJ/abstract/0022-3735/20/8/001 HAMSTAD, M A., O’Gallagher, A., & Gary, J (2002). A Wavelet Transform Applied To Acoustic Emission Signals: Part 2: Source Location. Vol.20. [Online]. Acoustic Emission Group. Last accessed 27 July 2008 at: http://www.engr.du.edu/profile/pdf/20-062.pdf LEE, Yung-Chun., Lin, Zibin., & Kuo, Shi Hoa (2004). An Advanced Acoustic Emission Sensor for Transient Elastic Wave Measurements. Vol.270-273. P. 471-478. [Online]. Scientific.Net. Last accessed 27 July 2008 at: http://www.scientific.net/0-87849-948-2/471/ LIU, Qing (Charlie)., & Wang, Hsu-Pin (Ben) (2001). A Case Study on Multisensor Data Fusion for Imbalance Diagnosis of Rotating Machinery. Vol.15. P.203-210. [Online]. Cambridge Journals. Last accessed 27 July 2008 at: http://journals.cambridge.org/action/displayAbstract;jsessionid=01A449407E76D69A256F10719419EB84.tomcat1?fromPage=online&aid=80117 MBA, D., & Rao, Raj B K N (2006). Development of Acoustic Emission Technology for Condition Monitoring and Diagnosis of Rotating Machines; Bearings, Pumps, Gearboxes, Engines and Rotating Structures. Vol.38. No.1. P. 3-16. [Online]. The Shock and Vibration Digest. 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