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Carbon nanotube-based composites have been deeply investigated in recent years. CNTs due to their unique physical properties have been employed for various applications in different disciplines of science and engineering. Due to the... more
Carbon nanotube-based composites have been deeply investigated in recent years. CNTs due to their unique physical properties have been employed for various applications in different disciplines of science and engineering. Due to the remarkable electrical conductivity of CNTs, one of practical applications is related to the development of strain sensing smart coatings. Up to now, high sensitivity strain sensors for micro- and macro-scale applications were proposed. However, controlling electro-mechanical properties of those devices is still a challenging issue. In order to facilitate the design process and to investigate multi-domain relationships between sensor's parameters and its properties, numerical models and simulations of CNT-based structures have been carried out with the primary focus on investigation of electrical conductivity for various concentrations of CNTs within the composite material. More accurate and detailed studies include analysis of the influence of deformation on changes in conductivity. However, due to significant microstructural complexity of the system (i.e. large number of CNTs within the structure) multiscale modeling and analysis approach must be employed. The main objective of this paper is to outline an relationships between micro- and macro-structural properties of CNT-based strain sensors and to discuss guidelines for a multiscale electro-mechanical model based on the Representative Volume Element (RVE) concept. The device employs the change in electrical conductivity of a CNT-based nanocomposite under applied deformation. The study investigates the impact of various micro-scale model parameters (e.g. size of an RVE, CNTs parameters etc.), on the macro scale model. Model parameters convergence studies are performed for different geometrical properties of CNTs and for various sizes of RVEs - revealing their critical mutual relationships. Also, the impact of boundary conditions at the micro-scale RVE structure is discussed.
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Nonlinear ultrasonic techniques provide improved damage sensitivity compared to linear approaches. The combination of attractive properties of guided waves, such as Lamb waves, with unique features of higher harmonic generation provides... more
Nonlinear ultrasonic techniques provide improved damage sensitivity compared to linear approaches. The combination of attractive properties of guided waves, such as Lamb waves, with unique features of higher harmonic generation provides great potential for characterization of incipient damage, particularly in plate-like structures. Nonlinear ultrasonic structural health monitoring techniques use interrogation signals at frequencies other than the excitation frequency to detect changes in structural integrity. Signal processing techniques used in non-destructive evaluation are frequently supported by modeling and numerical simulations in order to facilitate problem solution. This paper discusses known and newly-developed local computational strategies for simulating elastic waves, and attempts characterization of their numerical properties in the context of linear and nonlinear media. A hybrid numerical approach combining advantages of the Local Interaction Simulation Approach (LISA) and Cellular Automata for Elastodynamics (CAFE) is proposed for unique treatment of arbitrary strain-stress relations. The iteration equations of the method are derived directly from physical principles employing stress and displacement continuity, leading to an accurate description of the propagation in arbitrarily complex media. Numerical analysis of guided wave propagation, based on the newly developed hybrid approach, is presented and discussed in the paper for linear and nonlinear media. Comparisons to Finite Elements (FE) are also discussed.
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Nonlinear features of elastic wave propagation have attracted significant attention recently. The particular interest herein relates to complex wave-structure interactions, which provide potential new opportunities for feature discovery... more
Nonlinear features of elastic wave propagation have attracted significant attention recently. The particular interest herein relates to complex wave-structure interactions, which provide potential new opportunities for feature discovery and identification in a variety of applications. Due to significant complexity associated with wave propagation in nonlinear media, numerical modeling and simulations are employed to facilitate design and development of new measurement, monitoring and characterization systems. However, since very high spatio- temporal accuracy of numerical models is required, it is critical to evaluate their spectral properties and tune discretization parameters for compromise between accuracy and calculation time. Moreover, nonlinearities in structures give rise to various effects that are not present in linear systems, e.g. wave-wave interactions, higher harmonics generation, synchronism and | recently reported | shifts to dispersion characteristics. This paper discusses local computational model based on a new HYBRID approach for wave propagation in nonlinear media. The proposed approach combines advantages of the Local Interaction Simulation Approach (LISA) and Cellular Automata for Elastodynamics (CAFE). The methods are investigated in the context of their accuracy for predicting nonlinear wavefields, in particular shifts to dispersion characteristics for finite amplitude waves and secondary wavefields. The results are validated against Finite Element (FE) calculations for guided waves in copper plate. Critical modes i.e., modes determining accuracy of a model at given excitation frequency - are identified and guidelines for numerical model parameters are proposed.
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Acoustic Emission (AE) is a phenomenon of great importance for structural health monitoring (SHM). One of important applications of AE in SHM is the localization of a source of acoustic energy in the structure. For certain types of... more
Acoustic Emission (AE) is a phenomenon of great importance for structural health monitoring (SHM). One of important applications of AE in SHM is the localization of a source of acoustic energy in the structure. For certain types of structures and for large propagation distances where dispersion phenomenon is prominent, it is hard to estimate proper location of the damage. This paper discusses some aspects of AE source localization techniques. It is shown how an acoustic source in large plates can be localized using only six sensors and without apriori knowledge of any material pa- rameter, thus omitting the need of considering dispersion phenomena. The method is based on a predefined three-sensor setup, required to determine the Lamb wave propagation direction by detecting the angle at which the Lamb wave hits the array. As introduced in earlier works, the propagation direction of the Lamb wave is detected using arrival time differences between sensors, measured by correlating only the first dip and peak of the recorded signals. With another propagation direction detected by another sensor cluster the acoustic source can be localized as an intersection point of the two directions for planar structures. In this paper special orientation of the sensor cluster is suggested in order to apply the technique to a narrow plate. This modification enables to evaluate a wider area and avoid possible errors in detected propagation directions from some acoustic source locations. The method is compared with another source localization technique based on combined time-domain dispersion removal approach and localization. The dispersion removal technique reconstructs the source signal compensating distortions resulting from dispersion effects and transforms the time domain response directly into the distance one. Both methods are verified numerically and experimentally through artificial sources. doi: 10.12783/SHM2015/194
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In this work the authors propose a model-assisted POD application for SHM systems under uncertain crack configuration and system variability. The SHM system-related phenomena are studied via computer models. Other sources of variability... more
In this work the authors propose a model-assisted POD application for SHM systems under uncertain crack configuration and system variability. The SHM system-related phenomena are studied via computer models. Other sources of variability are accounted for by adding empirical noise to the numerical results. Computers models are solved making use of parallel processing technology which significantly speeds up the simulation time.
Research Interests: Computer Science and CUDA
This paper presents a cointegration-based method for condition monitoring and fault detection of wind turbines. The proposed method is based on the residual-based control chart approach. The main idea is that cointegration is a property... more
This paper presents a cointegration-based method for condition monitoring and fault detection of wind turbines. The proposed method is based on the residual-based control chart approach. The main idea is that cointegration is a property of some sets of nonstationary time series where a linear combination of the nonstationary series can produce a stationary residual. Then the stationarity of cointegration residuals can be used in a control chart as a potentially effective damage feature. The method is validated using the experimental data acquired from a wind turbine drivetrain with a nominal power of 2 MW under varying environmental and operational conditions. Two known abnormal problems of the wind turbine are used to illustrate the fault detection ability of the method. A cointegration-based procedure is performed on six process parameters of the wind turbine where data trends have nonlinear characteristics. Analysis of cointegration residuals—obtained from cointegration process of wind turbine data—is used for operational condition monitoring and fault/abnormal detection. The results show that the proposed method can effectively monitor the wind turbine and reliably detect abnormal problems.
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2013-2014 > Academic research: refereed > Refereed conference pape
Research Interests: Axle and Excitation
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Wavelet analysis is applied to identify the time-variant dynamics of adaptive structures. The wavelet-based power spectrum of the structural response, wavelet-based frequency response function (FRF) and wavelet-based coherence are used to... more
Wavelet analysis is applied to identify the time-variant dynamics of adaptive structures. The wavelet-based power spectrum of the structural response, wavelet-based frequency response function (FRF) and wavelet-based coherence are used to identify continuously and abruptly varying natural frequencies. A cantilever plate with surface-bonded macro fibre composite-which alters the structural stiffness-is used to demonstrate the application of the methods. The results show that the wavelet-based input-output characteristics-i.e. the FRF and coherence-can identify correctly the dynamics of the analysed time-variant system and reveal the varying natural frequency. The wavelet-based coherence can be used not only for the assessment of the quality of the wavelet-based FRF but also for the identification.This article is part of the theme issue 'Redundancy rules: the continuous wavelet transform comes of age'.
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Univelt, PO Box 28130, San Diego, CA 92198, USA. 2007. 453pp. Illustrated. $110. ISBN 9780-87703-538-1. The application of tethers in space has been considered since before the launch of the first artificial satellite, but only now it is... more
Univelt, PO Box 28130, San Diego, CA 92198, USA. 2007. 453pp. Illustrated. $110. ISBN 9780-87703-538-1. The application of tethers in space has been considered since before the launch of the first artificial satellite, but only now it is considered a viable solution in the field of space engineering as the technology matures and our understanding of the dynamics of such systems improves. This new book by Eugene Levin provides a very comprehensive treatise of the topic, providing not only theoretical development of the governing equations of motion of these complex systems, but also historical insight into the conceptual thinking, and practical issues regarding their application for addressing unique technical challenges. Eugene Levin is one of the leading specialists on space tether dynamics in the world whose research activity in the field spans almost three decades. Amongst the new missions considered by Levin, and receiving a very comprehensive dynamical analysis, are comet nucleus sample return, a reboost system for the Mir space station, electrodynamic autonomous transfer vehicles, momentum exchange tether systems, a deep space interferometer, and includes a lunar space elevator concept, and application of tethers to formation flying. Levin’s book brings great analytical insight into the often non-intuitive behaviour of tethered systems, and provides a perspective on survivability of tether missions, a notable omission being the inherent vulnerability of such systems to environmental factors such as meteoroid/debris impact. This minor shortcoming aside, the reader, from an undergraduate student seeking theoretical insight through to the established engineer wishing to consider such a system solution, will find a very comprehensive consideration of the topic. The text demands a significant understanding of vector dynamics of the reader but rewards them with a clear and logical development and interpretation of tether behaviour, and offers critical insights into the practical application of this maturing technology from Levin’s long involvement in tether projects most notably for the Jet Propulsion Laboratory, The Naval Research Laboratory and Marshall Space Flight Center. Professor Richard Crowther CEng, FRAeS
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Acoustic emission is a vital non-destructive testing technique and is widely used in industry for damage detection, localisation and characterization. The latter two aspects are particularly challenging, as AE data are typically noisy.... more
Acoustic emission is a vital non-destructive testing technique and is widely used in industry for damage detection, localisation and characterization. The latter two aspects are particularly challenging, as AE data are typically noisy. What is more, elastic waves generated by an AE event, propagate through a structural path and are significantly distorted. This effect is particularly prominent for thin elastic plates. In these media the dispersion phenomenon results in severe localisation and characterization issues. Traditional Time Difference of Arrival methods for localisation techniques typically fail when signals are highly dispersive. Hence, algorithms capable of dispersion compensation are sought. This paper presents a method based on the Time - Distance Domain Transform for an accurate AE event localisation. The source localisation is found through a minimization problem. The proposed technique focuses on transforming the time signal to the distance domain response, which would be recorded at the source. Only, basic elastic material properties and plate thickness are used in the approach, avoiding arbitrary parameters tuning.
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This paper deals with the problem of elastic constant identification in thin plates made of orthotropic composite materials. The approach is based on the analysis of Lamb wave propagation and the related dispersion curves to find the... more
This paper deals with the problem of elastic constant identification in thin plates made of orthotropic composite materials. The approach is based on the analysis of Lamb wave propagation and the related dispersion curves to find the underlying material elastic constants. In the proposed implementation a scanning laser Doppler vibrometer is used to measure Lamb wave dispersion curves. The Local Interaction Simulation Approach is used simultaneously to find a solution to a high-frequency wave propagation problem. The experimental and simulated data are combined in a Bayesian framework for parameter identification which is robust in condition of parameter, modeling and measurement uncertainty. The results are discussed and compared with the results from a deterministic optimization.
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Monitoring of trends and removal of undesired trends from operational/process parameters in wind turbines is important for their condition monitoring. This paper presents the homoscedastic nonlinear cointegration for the solution to this... more
Monitoring of trends and removal of undesired trends from operational/process parameters in wind turbines is important for their condition monitoring. This paper presents the homoscedastic nonlinear cointegration for the solution to this problem. The cointegration approach used leads to stable variances in cointegration residuals. The adapted Breusch-Pagan test procedure is developed to test for the presence of heteroscedasticity in cointegration residuals obtained from the nonlinear cointegration analysis. Examples using three different time series data sets—that is, one with a nonlinear quadratic deterministic trend, another with a nonlinear exponential deterministic trend, and experimental data from a wind turbine drivetrain—are used to illustrate the method and demonstrate possible practical applications. The results show that the proposed approach can be used for effective removal of nonlinear trends form various types of data, allowing for possible condition monitoring applica...
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The University of California at San Diego (UCSD), under a Federal Railroad Administration (FRA) Office of Research and Development (R&D) grant, is developing a system for high-speed and non-contact rail integrity evaluation. A prototype... more
The University of California at San Diego (UCSD), under a Federal Railroad Administration (FRA) Office of Research and Development (R&D) grant, is developing a system for high-speed and non-contact rail integrity evaluation. A prototype using an ultrasonic air-coupled guided wave signal generation and air-coupled signal detection, in pair with a real-time statistical analysis algorithm, is being developed. This solution presents an improvement over the previously considered laser/air-coupled hybrid system because it replaces the costly and hard-to-maintain laser with a much cheaper, faster, and easier-to-maintain air-coupled transmitter. This system requires a specialized filtering approach due to the inherently poor signal-to-noise ratio of the air-coupled ultrasonic measurements in rail steel. Various aspects of the prototype have been designed with the aid of numerical analyses. In particular, simulations of ultrasonic guided wave propagation in rails have been performed using a ...
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The paper presents a comparative study of damage detection in composite patch repairs. Three different nondestructive testing methods were used to characterize artificial damage in carbon fiber reinforced polymer (CFRP) samples with... more
The paper presents a comparative study of damage detection in composite patch repairs. Three different nondestructive testing methods were used to characterize artificial damage in carbon fiber reinforced polymer (CFRP) samples with external patch repairs. The damage was introduced in the form of Teflon inserts of different sizes and locations. The analyzed test methods include the ultrasonic c-scan, vibrothermography and shearography. Healthy specimens were used for verification of the applied test procedures. The paper gives details about the experimental procedures and equipment required to perform the tests.
The paper demonstrates how non-classical approaches can be used for structural health monitoring. Waveletbased modal analysis, various non-classical nonlinear acoustic techniques and cointegration are used for damage detection. These... more
The paper demonstrates how non-classical approaches can be used for structural health monitoring. Waveletbased modal analysis, various non-classical nonlinear acoustic techniques and cointegration are used for damage detection. These approaches are illustrated using various examples of damage detection in metallic and composites structures.
Fatigue crack detection is one of the major problems in maintenance of engineering structures. Ultrasonic guided waves are used for fatigue crack detection in metallic plate components. The application involves Lamb wave propagation and... more
Fatigue crack detection is one of the major problems in maintenance of engineering structures. Ultrasonic guided waves are used for fatigue crack detection in metallic plate components. The application involves Lamb wave propagation and the analysis of the diffused wave field. The cross-correlation function is used to obtain a damage index that reveals the initiation of fatigue crack. The major focus of the paper is on crack length estimation. The results demonstrate that crack length estimation based on Lamb wave propagation is challenging but not impossible.
Broadband environmental excitations from wind, ocean wave and earthquakes are especially dangerous for flexible tall structures such as wind turbines, towers or cable-stayed bridges. Liquid Dampers have been studied for almost thirty... more
Broadband environmental excitations from wind, ocean wave and earthquakes are especially dangerous for flexible tall structures such as wind turbines, towers or cable-stayed bridges. Liquid Dampers have been studied for almost thirty years in terms of their capability for suppression of vibration in such structures. The work presented in this paper focuses on the Tuned Liquid Column Damper, both open and sealed, and the identification of its time-varying properties of nonlinear damping, frequency and air pressure identification. Experimental tests are conducted on a full scale model of the damper which is subjected to both white noise and harmonic excitation by means of a hydraulic shaker. Exponential decay of the displacement of the liquid column was measured and analysed. The identification procedure was conducted step-wise, first, mode separation with the use of Continuous Wavelet Transform was carried out and then identification of the instantaneous damping ratio for the first m...
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The paper investigates the problem of structural health monitoring of composite patch repairs used in aerospace applications. Two diagnostic techniques are applied to evaluate the structural health of two carbon fiber reinforced composite... more
The paper investigates the problem of structural health monitoring of composite patch repairs used in aerospace applications. Two diagnostic techniques are applied to evaluate the structural health of two carbon fiber reinforced composite samples with bonded patch repairs; one test sample has a correctly manufactured patch repair without manufacturing defects while the second sample has a patch repair with multiple simulated debondings. Firstly, the classical ultrasonic c-scan is performed to confirm the assumed initial state of both samples i.e. exclude the presence of manufacturing defects in the first sample and to confirm the presence and location of the debondings in the second sample. Secondly, the non-classical nonlinear ultrasonic technique based on vibro-acoustic wave modulations is applied to verify its usefulness in testing of complex structural components. The paper gives technical details of the test samples and measurement techniques. Diagnostic procedures and results ...
It is well known that when cointegration is used for the analysis of data in structural health monitoring applications, the choice of lag length has strong influence on damage detection results. The paper demonstrates how this problem... more
It is well known that when cointegration is used for the analysis of data in structural health monitoring applications, the choice of lag length has strong influence on damage detection results. The paper demonstrates how this problem could be solved. The solution utilizes the inversely proportional relationship between damage severity and stationarity of cointegration residuals. The method is validated using Lamb wave data from a structure exposed to temperature variations. The experimental results show that the proper lag length selection is essential and this can be achieved with the appropriate statistical analysis.
The paper demonstrates a mobile and wireless smartphone platform that can be used for damage detection. This platform consists of sensors, electronics, Android-based software and a smartphone that is used for control, communication, data... more
The paper demonstrates a mobile and wireless smartphone platform that can be used for damage detection. This platform consists of sensors, electronics, Android-based software and a smartphone that is used for control, communication, data storage, damage detection analysis and the presentation of damage detection results. The method used for crack detection is based on nonlinear acoustics that utilizes vibro-acoustic wave modulations. The application of the smart damage detection platform is demonstrated in undamaged and cracked aluminium plates. The results demonstrate damage detection capability of the mobile smartphone-based system.
Lamb waves have been widely used for structural damage detection. However, practical applications of this technique are still limited. One of the main reasons is due to the complexity of Lamb wave propagation modes. Therefore, instead of... more
Lamb waves have been widely used for structural damage detection. However, practical applications of this technique are still limited. One of the main reasons is due to the complexity of Lamb wave propagation modes. Therefore, instead of directly analysing and interpreting Lamb wave propagation modes for information about health conditions of the structure, this study has proposed another approach that is based on statistical analyses of the stationarity of Lamb waves. The method is validated by using Lamb wave data from intact and damaged aluminium plates exposed to temperature variations. Four popular unit root testing methods, including Augmented Dickey–Fuller (ADF) test, Kwiatkowski–Phillips–Schmidt–Shin (KPSS) test, Phillips–Perron (PP) test, and Leybourne–McCabe (LM) test, have been investigated and compared in order to understand and make statistical inference about the stationarity of Lamb wave data before and after hole damages are introduced to the aluminium plate. The sep...
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Modelling and numerical simulation - based on the framework of the Local Interaction Simulation Approach - was developed to have more insight into nonlinear attributes of guided ultrasonic waves propagating in fatigued metallic materials.... more
Modelling and numerical simulation - based on the framework of the Local Interaction Simulation Approach - was developed to have more insight into nonlinear attributes of guided ultrasonic waves propagating in fatigued metallic materials. Various sources of nonlinearity were considered in this modelling work, with particular emphases on higher-order harmonic generation and accumulation of nonlinearity along wave propagation. The material hyper-elasticity was considered in the model using an energy density approach based on the Landau-Lifshitz formulation; and the "breathing" motion pattern of a fatigue crack in the material was interrogated using a spring model. Upon the successful validation with the model prepared in the commercial software based on the Finite Element Methods, two scenarios were comparatively investigated, i.e. the lower and higher frequency regime. In the first case propagation of a basic symmetric mode pair was simulated using the model to observe a cu...
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ABSTRACT Impact damage detection in composite structures has gained a considerable interest in many engineering areas. The capability to detect damage at the early stages reduces any risk of catastrophic failure. This paper compares two... more
ABSTRACT Impact damage detection in composite structures has gained a considerable interest in many engineering areas. The capability to detect damage at the early stages reduces any risk of catastrophic failure. This paper compares two advanced signal processing methods for impact location in composite aircraft structures. The first method is based on a modified triangulation procedure and Genetic Algorithms whereas the second technique applies Artificial Neural Networks. A series of impacts is performed experimentally on a composite aircraft wing-box structure instrumented with low-profile, bonded piezoceramic sensors. The strain data are used for learning in the Neural Network approach. The triangulation procedure utilises the same data to establish impact velocities for various angles of strain wave propagation. The study demonstrates that both approaches are capable of good impact location estimates in this complex structure.
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The paper presents a perturbation approach for calculating amplitude-dependent Lamb wave dispersion in nonlinear plates. Nonlinear dispersion relationships are derived in closed form using a hyperelastic stress-strain constitutive... more
The paper presents a perturbation approach for calculating amplitude-dependent Lamb wave dispersion in nonlinear plates. Nonlinear dispersion relationships are derived in closed form using a hyperelastic stress-strain constitutive relationship, the Green-Lagrange strain measure, and the partial wave technique integrated with a Lindstedt-Poincaré perturbation approach. Solvability conditions are derived using an operator formalism with inner product projections applied against solutions to the adjoint problem. When applied to the first- and second-order problems, these solvability conditions lead to amplitude-dependent, nonlinear dispersion corrections for frequency as a function of wavenumber. Numerical simulations verify the predicted dispersion shifts for an example nonlinear plate. The analysis and identification of amplitude-dependent, nonlinear Lamb wave dispersion complements recent research focusing on higher harmonic generation and internally resonant waves, which require precise dispersion relationships for frequency-wavenumber matching.
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We describe a novel fiber-optic system that is able to detect both ultrasonic Lamb waves and the location of their source. The aim of the system is to detect damage in structures such as those found in aerospace applications. Our system... more
We describe a novel fiber-optic system that is able to detect both ultrasonic Lamb waves and the location of their source. The aim of the system is to detect damage in structures such as those found in aerospace applications. Our system involves the use of fiber Bragg gratings, ...
Signal processing method based on wavelet transform used in non-linear acoustic test is presented in the paper. The method is applied for sidebands identification in response signal acquired during vibro-acoustic modulation test of... more
Signal processing method based on wavelet transform used in non-linear acoustic test is presented in the paper. The method is applied for sidebands identification in response signal acquired during vibro-acoustic modulation test of impacted carbon fiber reinforced plate (CFRP). The plate was impacted with known energy using drop-weight testing machine. The modulation effect in investigated specimen results from the interaction of low and high frequency excitation with damage. The paper investigates different than mono-harmonic low-frequency excitation usually used in non-linear acoustics tests. Application of aperiodic low-frequency excitation signal allows to omit the modal test, where natural frequency of the structure are estimated. However, this requires the use of dedicated signal processing methods.
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Composite materials play an increasing role in many engineering applications. High performance, strength, stiffness and low weight are the attractive factors which increase the use of these materials in aerospace, automobile, marine and... more
Composite materials play an increasing role in many engineering applications. High performance, strength, stiffness and low weight are the attractive factors which increase the use of these materials in aerospace, automobile, marine and rail industries. One of the major ...
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Abstract—A temperature-compensated strain-sensing scheme based on fiber Bragg gratings (FBGs) that is suitable for strain mapping applications is described. FBGs are bonded to a backing patch, together with an extra grating that is used... more
Abstract—A temperature-compensated strain-sensing scheme based on fiber Bragg gratings (FBGs) that is suitable for strain mapping applications is described. FBGs are bonded to a backing patch, together with an extra grating that is used for tempera- ture compensation/ ...
Research Interests: Optics, Medical Imaging, Fiber Optics, Photonics, Finite element method, and 15 moreFinite Element, Finite Element Analysis, Fiber Bragg grating, Society, Optical, Light, Fiber Bragg gratings, Optical physics, Optical fiber, Optical Fibres, Bragg Grating, Strain Gauges, Electrical And Electronic Engineering, Lightwave, and Strain Gauge
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ABSTRACT