Abstract: A diagnostic methodology of artificial defects in a single stage gearbox operating under various load levels and different defect states is proposed in the present work based on vibration recordings as well as advanced signal analysis techniques. Two different wavelet-based signal processing methodologies, using the discrete as well as the continuous wavelet transform, were utilised for the analysis of the recorded vibration signals and useful diagnostic information were extracted out of them. Both wavelet analysis techniques provided the ability of distinguishing between the healthy and the artificially defected gears. In this way, the health monitoring potential of vibration monitoring in the case of rotating machinery and gearboxes obtains a new dynamic under the prism of sophisticated time-frequency signal processing schemes, rather than conventional FFT-based approaches.
Abstract: This study focuses on the nondestructive monitoring of damage developed in carbon fiber reinforced polymers during fatigue loading via in situ acousto-ultrasonics (AU) measurements. Carbon nanotubes (CNTs) were used as modifiers of the epoxy matrix of quasi-isotropic carbon fiber reinforced laminates and consequently two composite material systems (one with matrix doped with CNTs and one with nondoped matrix) were manufactured and tested. The AU technique is utilized toward the monitoring of damage development, evolution, and accumulation during the fatigue tests. The AU waveforms acquired are processed using conventional (both in time and in frequency domain) as well as innovative wavelet-based signal processing techniques to extract parameters-indicators capable of monitoring and quantifying the damage accumulation into the test coupons. Based on the analysis of AU monitored signals, critical descriptors (parameters) were identified that show significant variation and monotonic behavior throughout the tests and thus can be proposed as potential candidate parameters for monitoring the damage development in composite materials. POLYM. COMPOS., 31:1409-1417,2010. (C) 2009 Society of Plastics Engineers
Abstract: A diagnostic methodology of artificial defects in a single stage gearbox operating under various load levels and different defect states is proposed in the present work based on vibration recordings as well as advanced signal analysis techniques. Two different wavelet-based signal processing methodologies, using the discrete as well as the continuous wavelet transform, were utilised for the analysis of the recorded vibration signals and useful diagnostic information were extracted out of them. Both wavelet analysis techniques provided the ability of distinguishing between the healthy and the artificially defected gears. In this way, the health monitoring potential of vibration monitoring in the case of rotating machinery and gearboxes obtains a new dynamic under the prism of sophisticated time-frequency signal processing schemes, rather than conventional FFT-based approaches.
Abstract: In the second part of this work, the acousto-ultrasonics waveforms acquired during the tests described in part I are processed using wavelet-based techniques in order to extract parameters capable of monitoring the damage accumulation in the C/C test coupons. A 10 level decomposition using the discrete wavelet transform is applied upon each one of the acquired acousto-ultrasonics waveforms. The continous wavelet transform was also used and the wavelet grey moment was calculated for each waveform. Some of the Parameters extracted from the wavelet-based analysis show all interesting monotonic variation and thus Call be Used as potential candidates for monitoring the damage development in the tested materials. (C) 2008 Elsevier Ltd. All rights reserved.
Abstract: The condition monitoring of a lab-scale. single stage, gearbox using different non-destructive inspection methodologies and the processing of the acquired waveforms with advanced signal processing techniques is the aim of the present work. Acoustic emission (AE) and vibration measurements were utilized for this purpose. The experimental setup and the instrumentation of each monitoring methodology are presented in detail. Emphasis is given on the signal processing of the acquired vibration and acoustic emission signals in order to extract conventional as well as novel parameters-features of potential diagnostic value from the monitored waveforms. Innovative wavelet-based parameters-features are proposed utilizing the discrete wavelet transform. The evolution of selected parameters/features versus test time is provided, evaluated and the parameters with the most interesting diagnostic behaviour are highlighted. The differences in the parameters evolution of each NDT technique are discussed and the superiority of AE over vibration recordings for the early diagnosis of natural wear in gear systems is concluded. (C) 2009 Elsevier Ltd. All rights reserved.
Abstract: 'Damage tolerance' is used to describe the attribute of a structure associated with the retention of the required residual strength throughout its service life, while irreversible damage mechanisms are active within the structure itself. 'Design for damage tolerance' is based on the identification and quantification of the various damage mechanisms that result in the alteration (mainly deterioration) of the material properties. These may alter the material response to thermomechanical loads. In the present paper, transparent glass fibre reinforced epoxy laminates were used to study the damage evolution sequence under tensile loading. Acoustic emission was employed as a non-destructive technique for the in situ monitoring of the active damage mechanisms until the final failure of the material. Pattern recognition algorithms were utilised to classify the acquired acoustic emission signals and associate them to active damage mechanisms. Experimental findings were compared to theoretical model predictions.
Abstract: The present work, first of two parts, deals with three types ofwoven carbon/carbon (C/C) composites having differentiations during the manufacturing procedure, which influences their fibre/matrix interface. All material types were tested under tensile loading in a load-unload-reload configuration, with online acoustic emission monitoring. Unsupervised pattern recognition algorithms were utilized to classify the acoustic emission (AE) data recorded during the tests. The resulted Clusters, Concluded by the analysis of AE hits, are associated with the damage mechanisms of the material, activated at the different load levels, and significant remarks were extracted regarding the damage evolution and its differentiation according to the different fibre/matrix interfaces. Emphasis is given on the impact of the different interface types upon the total mechanical behavior and damage accumulation at the test coupons. A qualitative evaluation of the interfaces using non-destructive testing data is also attempted. This first apart intends to Propose methodologies and procedures to analyze data front online acoustic emission monitoring in order to extract useful information regarding the damage evolution within C/C materials. (C) 2008 Elsevier Ltd. All rights reserved.
Abstract: In this study, CNTs were used as modifiers of the epoxy matrix of quasi-isotropic carbon fibre reinforced laminates. The prepared laminates were subjected to tensile loading and tension tension fatigue and, the changes in the longitudinal resistance were monitored via a digital multimeter. In addition, acoustic emission and acoustoultrasonic techniques were used for monitoring the fatigue process of the laminates. The nanoenhanced laminates, on the one hand, exhibited superior fatigue properties and on the other hand, they demonstrated the full potential of the material to be used as an integrated sensor to monitor the fatigue life.
Abstract: 'Damage tolerance' is used to describe the attribute of a structure associated with the retention of the required residual strength throughout its service life, while irreversible damage mechanisms are active within the structure itself. 'Design for damage tolerance' is based on the identification and quantification of the various damage mechanisms that result in the alteration (mainly deterioration) of the material properties. These may alter the material response to thermomechanical loads. In the present paper, transparent glass fibre reinforced epoxy laminates were used to study the damage evolution sequence under tensile loading. Acoustic emission was employed as a non-destructive technique for the in situ monitoring of the active damage mechanisms until the final failure of the material. Pattern recognition algorithms were utilised to classify the acquired acoustic emission signals and associate them to active damage mechanisms. Experimental findings were compared to theoretical model predictions.
Abstract: In this study, CNTs were used as modifiers of the epoxy matrix of quasi-isotropic carbon fibre reinforced laminates. The prepared laminates were subjected to tensile loading and tension tension fatigue and, the changes in the longitudinal resistance were monitored via a digital multimeter. In addition, acoustic emission and acoustoultrasonic techniques were used for monitoring the fatigue process of the laminates. The nanoenhanced laminates, on the one hand, exhibited superior fatigue properties and on the other hand, they demonstrated the full potential of the material to be used as an integrated sensor to monitor the fatigue life.
Abstract: The acoustic emission (AE) technique is a powerful nondestructive tool for health monitoring of structures and mechanical components, especially due to its sensitivity to capture high frequency signals, which are associated with the early stages of damage development and evolution. The aim of the present work is twofold. The first is the evaluation of a new concept of transducer mounting on rotating structures without the use of the expensive solution of the slip ring. The new concept is realized in a single stage in-house built gearbox setup. The second is the evaluation of the potential of the acquired with the new concept AE signals in distinguishing between different types of artificially induced damage on the gears. Run-in tests were carried out to study the effect of gear damage on the AE recordings. The acoustic emission signature of the healthy gears is first acquired. Then artificial defects are seeded and the acquisition is repeated. The AE signals are analyzed, and their root-mean-square values are calculated. The capability of the new approach of AE acquisition in discriminating between different loading and damage states is shown and discussed. Interesting findings on the effect of the oil temperature on AE recordings only speculated theoretically so far are also presented.
Abstract: The present work deals with the application of an innovative in-house developed wavelet-based methodology for the analysis of the acceleration responses of a human head complex model as a simulated diffused oedema progresses. The human head complex has been modeled as a structure consisting of three confocal prolate spheroids, whereas the three defined regions by the system of spheroids, from the outside to the inside, represent the scull, the region of cerebrospinal fluid, and the brain tissue. A Dirac-like pulse has been used to excite the human head complex model and the acceleration response of the system has been calculated and analyzed via the wavelet-based methodology. For the purpose of the present analysis, a wave propagation commercial finite element code, LS-DYNA 3D, has been used. The progressive diffused oedema was modeled via consecutive increases in brain volume accompanied by a decrease in brain density. It was shown that even a small increase in brain volume (at the level of 0.5%) can be identified by the effect it has on the vibration characteristics of the human head complex. More precisely, it was found that for some of the wavelet decomposition levels, the energy content changes monotonically as the brain volume increases, thus providing a useful. index of monitoring an oncoming brain oedema before any brain damage appears due to uncontrolled intracranial hypertension. For the purpose of the present work and for the levels of brain volume increase considered in the present analysis, no pressure increase was assumed into the cranial vault and, associatively, no brain compliance variation.
Abstract: Large Scale Bridging in SiC/MAS-L (ceramic glass matrix) composites was investigated by using DEN specimens under tensile loading conditions with in situ Acoustic Emission monitoring. The AE data were successfully classified using Unsupervised Pattern Recognition Algorithms and the resulted clusters were correlated to the dominant damage mechanisms of the material. The evolution in time of the different damage mechanisms is feasible after the pattern recognition classification. Microscopic examination was used to correlate the clusters to the damage mechanism they correspond and. thus to provide the failure mode identification based on AE data. (c) 2005 Elsevier Ltd. All rights reserved.
Abstract: The goal of the present study was to investigate the influence of carbon nanofibers (CNF) and/or piezoelectric (PZT) particles on the fracture behaviour of carbon fiber reinforced polymer laminates. For this purpose the fillers were added as dopants in the epoxy matrix of the laminates. An increase of 100% in fracture energy was observed after the addition of 1% CNF in the matrix of the laminates, while the introduction of PZT particles led to reduction in fracture energy, mainly due to the brittle character of the particle inclusions. In addition, the acoustic emission technique was used for monitoring the fracture process of the laminates. (c) 2006 Elsevier Ltd. All rights reserved.
Abstract: The Young's modulus determination of different types of ceramic fibres using both conventional quasi-static tensile tests and non-destructive elastic wave propagation techniques is the scope of the present work. A sufficient number of tensile tests were conducted on a number of different types of fibres such as carbon, Al<sub>2</sub>O<sub>3</sub> and SiC fibre bundles and the elastic modulus was obtained. In parallel a method to determine the elastic modulus via a non-destructive approach was of primary concern due to the uncertainties of the conventional approach. To this direction the acousto-ultrasonics method proved quite capable to provide time-of-flight measurements in a rather simple experimental setup. The results seem remarkably close to the manufacturer's data and verify the method's applicability. Both techniques are analytically described with the emphasis lying on the non-destructive one. Deviations obtained from the conventional approach are thoroughly discussed and the advantages of the non-destructive technique are highlighted.
Abstract: The aim of the present work is the classification of the characteristics of elastic waves, which are generated by fibre failures during quasistatic tensile fibre bundle tests and captured by the use of acoustic emission (AE) method. In addition, elastic waves generated out of the fibre bundle gauge length and propagated through the bundle at different stages of the loading process using an acousto-ultrasonic (AU) technique are also classified. To this target, a large number of tests were conducted on organic, ceramic and carbon fibre bundles according to DIN 53942. An in-house developed analysis and quantification methodology of the captured AE and AU waveforms is proposed in order to identify the frequency content of the fibre failure event, to characterize the medium of propagation and to investigate the effect of the acquisition system on the monitored signals. In fact, the application of the proposed analysis on the results of the conducted mechanical tests leads to the development of a useful database, concerning the ranges of AE features and the representative sets of frequency values that correspond to fibre fracture. Furthermore, the proposed database offers valuable knowledge for the role of material parameters, such as fibre structure and properties, on the characteristics of the recorded waveforms, constituting in this way a valuable tool that enables a better understanding of elastic waves initiation and propagation through continuous fibres. (C) 2003 Published by Elsevier Ltd.
Abstract: The Young's modulus determination of different types of ceramic fibres using both conventional quasi-static tensile tests and non-destructive elastic wave propagation techniques is the scope of the present work. A sufficient number of tensile tests were conducted on a number of different types of fibres such as carbon. Al2O3 and SiC fibre bundles and the elastic modulus was obtained. In parallel a method to determine the elastic modulus via a non-destructive approach was of primary concern due to the uncertainties of the conventional approach. To this direction the acousto-ultrasonics method proved quite capable to provide time-of-flight measurements in a rather simple experimental setup. The results seem remarkably close to the manufacturer's data and verify the method's applicability. Both techniques are analytically described with the emphasis lying on the non-destructive one. Deviations obtained from the conventional approach are thoroughly discussed and the advantages of the non-destructive technique are highlighted.
Abstract: In the present work, a new class of oxide/oxide composites made of Nextel((TM)) 720 fibre reinforcement and a mullite-based matrix, fabricated by using liquid polymer infiltration process, were studied. A fibre coating was applied via sol-gel in order to achieve improved damage tolerant behaviour. Mechanical properties were investigated at ambient temperature under quasi-static loading in the presence of continuous Acoustic Emission (AE) monitoring. Statistical pattern recognition analysis is the proposed tool for the classification of the monitored AE events. Lacking an a priori knowledge of different signal classes, unsupervised pattern recognition algorithms were used. A complete methodology including descriptor selection methods, procedures for numerical verification and cluster validity criteria is followed. Cluster analysis of AE data was achieved and the resulted clusters were correlated to the damage mechanisms of the material under investigation. This process was assisted by systematic microscopic examination. Furthermore, the initiation and evolution of each mechanism is described by plotting the cumulative hits of each class as a function of the applied load. (C) 2003 Elsevier Ltd. All rights reserved.
Abstract: Vapor growth carbon nanofibers (CNFs) were used as dopants for the epoxy resin (EP) matrix of unidirectional carbon fiber reinforced plastics (CFRPs). The doped laminates were compared to the reference one (with neat EP as matrix) with regard to their fracture toughness, and mechanical quasi-static and fatigue behaviour. Electrical resistance monitoring of the samples was used as a tool for sensing the damage propagation during the various loading configurations. Correlation between these results and acoustic emission (AE) data was also attempted.
