Guided Wave Structural Health Monitoring Research Articles

Towards spacecraft applications of structural health monitoring

The first part of the paper presents recent developments in the field of structural health monitoring (SHM) with special attention on the piezoelectric wafer active sensors (PWAS) technologies utilizing guided waves (GW) as propagating waves (pitch-catch, pulse-echo), standing wave (electromechanical impedance), and phased arrays. The second part of the paper describes the challenges of extending the PWAS GW SHM approach to in-space applications. Three major issues are identified, (a) cryogenic temperatures; (b) high temperatures; and (c) space radiation exposure. Preliminary results in which these three issues were address in a series of carefully conducted experiments are presented and discussed. The third part of the paper discusses a new project that is about to start in collaboration between three Romanian institutes to address the issues and challenging of developing space SHM technologies based on PWAS concepts. The paper finishes with conclusions and suggestions for further work.

Modeling and signal processing for guided wave structural health monitoring

Guided wave approaches are a key technique for the implementation of structural health monitoring with a sparse sensor array, offering good sensitivity over a reasonable range. There are however several difficulties, particularly how the system can be modelled to predict performance and how acquired data can be used most effectively post capture. This paper looks at both these issues, starting with a study of the modelling of the scattering of guided waves from features. Models of both the scattering from structural features and predicted damage types can then be used to determine the sensitivity of any network and the process of their derivation and use is described here. In order to maximise this sensitivity it is important to use the data in the most efficient way possible. This is accomplished through the use of statistical models of the changes in the signal and fitting maximum likelihood estimators to these. In choosing a good model of the signal a statistical detector can be designed to optimise the performance for a given use case, be that detection or localisation.

Enhanced detection through low-order stochastic modeling for guided-wave structural health monitoring

Guided wave structural health monitoring offers the potential for efficient defect detection and localization on large plate-like structures. The aim of this article is to demonstrate that the performance of guided wave structural health monitoring on complex structures can be quantified, predicted, and enhanced through basic stochastic modeling and application of a likelihood-based detector. These are necessary steps in enabling such structural heath monitoring techniques to be used more robustly on safety-critical structures. Extensive ensembles of measurements were taken using a sparse array of transducers on an aluminum plate with geometric complexities (stiffeners), before and after introducing several sizes of damage at several locations, and in the presence of large noise processes. This enabled a full statistical treatment of performance evaluation using a modification of the standard receiver operating characteristic curve. The incorporation of stochastic modeling produced on average a 44% reduction in missed detections among the damage modes tested, with up to a 95% reduction in some cases.

Cointegration and Nonstationarity in the Context of Multiresolution Analysis

Cointegration has established itself as a powerful means of projecting out long-term trends from time-series data in the context of econometrics. Recent work by the current authors has further established that cointegration can be applied profitably in the context of structural health monitoring (SHM), where it is desirable to project out the effects of environmental and operational variations from data in order that they do not generate false positives in diagnostic tests. The concept of cointegration is partly built on a clear understanding of the ideas of stationarity and nonstationarity for time-series. Nonstationarity in this context is 'traditionally' established through the use of statistical tests, e.g. the hypothesis test based on the augmented Dickey-Fuller statistic. However, it is important to understand the distinction in this case between 'trend' stationarity and stationarity of the AR models typically fitted as part of the analysis process. The current paper will discuss this distinction in the context of SHM data and will extend the discussion by the introduction of multi-resolution (discrete wavelet) analysis as a means of characterising the time-scales on which nonstationarity manifests itself. The discussion will be based on synthetic data and also on experimental data for the guided-wave SHM of a composite plate.

Maximum-likelihood estimation of damage location in guided-wave structural health monitoring

This paper describes the formulation of a maximum-likelihood estimate of damage location for guided-wave structural health monitoring (GWSHM) using a minimally informed, Rayleigh-based statistical model of scattered wave measurements. Also introduced are two statistics-based methods for evaluating localization performance: the localization probability density function estimate and the localizer operating characteristic curve. Using an ensemble of measurements from an instrumented plate with stiffening stringers, the statistical performance of the so-called Rayleigh maximum-likelihood estimate (RMLE) is compared with that of seven previously reported localization methods. The RMLE proves superior in all test cases, and is particularly effective in localizing damage using very sparse arrays consisting of as few as three transducers. The probabilistic basis used for modelling the complicated wave scattering behaviour makes the algorithm especially suited for localizing damage in complicated structures, with the potential for improved performance with increasing structure complexity.

Piezoelectric Transducer Diagnostics via Linear Reciprocity for Guided Wave Structural Health Monitoring

Guided waves using spatially distributed piezoelectric transducers are being applied to many structural health monitoring applications. These surface-mounted transducers, which are typically lead zirconate titanate, are generally assumed to be both undamaged and properly bonded to the host structure during usage. However, this assumption may not be valid, particularly after long-term operation under realistic environmental conditions. The transducer integrity becomes even more critical for aerospace applications because of the inevitably harsh operational conditions. In this study, a methodology for piezoelectric transducer diagnosis is developed to identify both damaged and poorly bonded transducers by quantifying the degree of signal reciprocity for guided waves propagating between pairs of surface-mounted transducers. The proposed method does not require direct comparison of signals to baselines, and it is independent of structural complexities such as geometrical features and boundaries. The method is also insensitive to environmental and operational conditions, such as temperature and applied loads, and is therefore not susceptible to false alarms caused by such variations. The efficacy of the proposed diagnostic technique is evaluated via two-dimensional simulations followed by experiments under varying environmental and operational conditions for piezoelectric transducers mounted on an aluminum plate.

A Wavelet-Based Damage Detection Approach for Acousto-Ultrasonic <i>In Situ</i> Monitoring Systems

In this research, an advanced signal processing technique using wavelet analysis has been developed for a guided wave structural health monitoring system. The approach was applied for the detection of delamination in carbon fibre reinforced composites. A monolithic piezoceramic actuator was attached to a laminate plate for wave generation while laser vibrometry was used to facilitate the measurements of the wave response in a sensor network. This database of wave response was then processed using the continuous wavelet transform to obtain the positional frequency content. Transforms between damaged and undamaged states were compared to ascertain the presence of defects by evaluating the total energy of the time-frequency density function. Results show high damage detection indices depending on the location of the sensor and normalisation factor applied while there are positive indications that this methodology can be extended for damage characterisation.

Guided Wave Beamsteering using MFC Phased Arrays for Structural Health Monitoring: Analysis and Experiment

Phased array beamsteering is an effective tool for damage detection and assessment method in a guided wave structural health monitoring system. Monolithic piezoceramic (PZT) actuators have been widely utilized for beamsteering by assuming omnidirectional point sources for each actuator. However, this assumption can lead to erroneous results for a phased array of actuators with anisotropic actuation, such as macro-fiber composites (MFC). The MFC actuators are investigated in this research for beamsteering considering the main lobe width, main lobe magnitude, and side lobe levels and compared to equivalently sized PZT actuators. Analytical models of the MFC and PZT actuators attached to an isotropic plate structure are presented and the genetic algorithm is used to determine the optimal timing sequences of the phased array elements for beamforming enhancement. The analysis results show that the MFC phased arrays have reduced the main lobe width and side lobe levels compared to the PZT phased arrays for a range of beamsteering angles. The area ratios of the MFC arrays are also found to be greater than the PZT arrays for these beamsteering angles. Experiments using the PZT and MFC phased arrays on an aluminum plate are performed and compared to the analysis results. The experiment results agree with the analysis results and demonstrate improved beamsteering of the MFC phased arrays for specific applications.

Multi-site damage localization in anisotropic plate-like structures using an active guidedwave structural health monitoring system

A new approach for structural health monitoring using guided waves in plate-like structureshas been developed. In contrast to previous approaches, which mainly focused on isotropicor quasi-isotropic plates, the proposed algorithm does not assume any simplificationsregarding anisotropic wave propagation. Thus, it can be used to improve the probability ofdetection.In this paper the mathematical background for damage localization in anisotropic plateswill be introduced. This is an extension of the widely known ellipse method. The formalismis based on a distributed sensor network, where each piezoelectric sensor acts in turn as anactuator. The automatic extraction of the onset time of the first waveform in thedifferential signal in combination with a statistical post-processing via a two-dimensionalprobability density function and the application of the expectation-maximization algorithmallows a completely automatic localization procedure. Thus, multiple damages can beidentified at the same time.The present study uses ultrasonic signals provided by the spectral element method. Thissimulation approach shows good agreement with experimental measurements. A local linearneural network is used to model the nonlinear dispersion curves. The benefit of using aneural network approach is to increase the angular resolution that results from the sparsesensor network. Furthermore, it can be used to shorten the computational time for thedamage localization procedure.

Efficient temperature compensation strategies for guided wave structural health monitoring

The application of temperature compensation strategies is important when using a guided wave structural health monitoring system. It has been shown by different authors that the influence of changing environmental and operational conditions, especially temperature, limits performance. This paper quantitatively describes two different methods to compensate for the temperature effect, namely optimal baseline selection (OBS) and baseline signal stretch (BSS). The effect of temperature separation between baseline time-traces in OBS and the parameters used in the BSS method are investigated. A combined strategy that uses both OBS and BSS is considered. Theoretical results are compared, using data from two independent long-term experiments, which use predominantly A 0 mode and S 0 mode data respectively. These confirm that the performance of OBS and BSS quantitatively agrees with predictions and also demonstrate that the combination of OBS and BSS is a robust practical solution to temperature compensation.

Guided wave structural health monitoring using CLoVER transducers in compositematerials

The guided wave (GW) field excited by piezoelectric wafers and piezocomposite transducersin carbon-fiber composite materials is experimentally investigated with applications tostructural health monitoring. This investigation supports the characterization of thecomposite long-range variable-length emitting radar (CLoVER) transducer introduced bythe authors. A systematic approach is followed where composite configurations withdifferent levels of anisotropy are analyzed. In particular, unidirectional, cross-ply[0/90]3S andquasi-isotropic [0/45/–45/90]2S IM7-based composite plates are employed. A combination of laser vibrometry and finiteelement analysis is used to determine the in-plane wave speed and peak-to-peak amplitudedistribution in each substrate considered. The results illustrate the effect of the materialanisotropy on GW propagation through the steering effect where the wavepackets do notgenerally travel along the direction in which they are launched. After characterizing theeffect of substrate anisotropy on the GW field, the performance of the CLoVER transducerto detect damage in various composite configurations is explored. It is found that thedirectionality and geometry of the device is effective in detecting the presenceand identifying the location of simulated defects in different composite layups.

Potential and Limitations of a Deconvolution Approach for Guided Wave Structural Health Monitoring

Ultrasonic guided waves offer the possibility of inspecting large areas of structures from a small number of sensor positions. However, inspection of complex structures is difficult as the reflections from different features overlap. Estimating the number and amplitude of the wave packets in ultrasonic time traces is crucial for the development of a guided wave inspection system, in order to detect and locate damage. Deconvolution has been extensively used in geophysical applications to resolve overlapping echoes in the recorded signals. The main objective of this work was to evaluate the applicability of the deconvolution approach for enhancing the resolution of ultrasonic time traces in structural health monitoring (SHM). Numerical simulations on strongly overlapping signals were carried out to evaluate the performance of the two techniques that have been considered: (i) Wiener filter, (ii) single most likely replacement. It was shown that the relatively narrow bandwidth of the input signals and phase shifts between different reflections limit the benefits obtained from deconvolution and it was concluded that deconvolution is unlikely to be useful for guided wave SHM applications.

Low-frequency vibration modulation of guided waves to image nonlinear scatterers forstructural health monitoring

Guided wave structural health monitoring offers the prospect of continuous interrogation oflarge plate-like structures with a sparse network of permanently attached sensors.Currently, the most common approach is to monitor changes in the received signalsby subtraction from a reference signal obtained when the structure was knownto be defect-free. In this paper a comparison is made between this defect-freesubtraction approach and a technique in which low-frequency vibration modulationof guided wave signals is used to detect nonlinear scatterers. The modulationtechnique potentially overcomes the need for the defect-free reference measurement asthe subtraction is now made between different parts of an externally appliedlow-frequency vibration. Linear defects were simulated by masses bonded onto a plate andnonlinear scatterers were simulated by loading a similar mass against the plate. Theexperimental results show that the defect-free subtraction technique performs well indetecting the bonded mass whereas the modulation technique is able to discriminatebetween the bonded and loaded masses. Furthermore, because the modulationtechnique does not require a defect-free reference, it is shown to be relativelyindependent of temperature effects, a significant problem for reference based subtractiontechniques.

Potential and Limitations of a Deconvolution Approach for Guided Wave Structural Health Monitoring

Potential and Limitations of a Deconvolution Approach for Guided Wave Structural Health Monitoring

Effects of Elevated Temperature on Guided-wave Structural Health Monitoring

Elevated temperatures can cause significant changes in guided-wave (GW) propagation and transduction for structural health monitoring (SHM). This work focuses on GW SHM using surface-bonded piezoelectric wafer transducers in metallic plates for the temperature range encountered in internal spacecraft structures (20—150°C). First, studies done to determine a suitable bonding agent are documented. This is then used in controlled experiments to examine changes in GW propagation and transduction using PZT-5A piezoelectric wafers under quasi-statically varying temperature (also from 20 to 150°C). Modeling efforts to explain the experimentally observed increase in time-of-flight and change in sensor response peak-to-peak magnitude with increasing temperature are detailed. Finally, these results are used in detection and location of mild and moderate damage using the pulse-echo GW testing approach within the temperature range.

Strategies for guided-wave structural health monitoring

Structural health monitoring (SHM) using guided waves is one of the only ways in which damage anywhere in a structure can be detected using a sparse array of permanently attached sensors. To distinguish damage from structural features, some form of comparison with damage-free reference data is essential, and here subtraction is considered. The detectability of damage is determined by the amplitude of residual signals from structural features remaining after the subtraction of reference data. These are non-zero due to changing environmental conditions such as temperature. In this paper, the amplitude of the residual signals is quantified for different guided-wave SHM strategies. Comparisons are made between two methods of reference signal subtraction and between two candidate sensor configurations. These studies allow estimates to be made of the number of sensors required per unit area to reliably detect a prescribed type of damage. It is shown that the number required is prohibitively high, even in the presence of modest temperature fluctuations, hence some form of temperature compensation is absolutely essential for guided-wave SHM systems to be viable. A potential solution is examined and shown to provide an improvement in signal suppression of approximately 30 dB, which corresponds to two orders of magnitude reduction in the number of sensors required.

Active health monitoring of an aircraft wing with embedded piezoelectric sensor/actuatornetwork: I. Defect detection, localization and growth monitoring

This work focuses on an ultrasonic guided wave structural health monitoring (SHM) systemdevelopment for aircraft wing inspection. In part I of the study, a detailed description of areal aluminum wing specimen and some preliminary wave propagation tests on the wingpanel are presented. Unfortunately, strong attenuation and scattering impede guidedwaves for large-area inspection. Nevertheless, small, low-cost and light-weightpiezoelectric (PZT) discs were bonded to various parts of the aircraft wing, in a formof relatively sparse arrays, for simulated cracks and corrosion monitoring. ThePZT discs take turns generating and receiving ultrasonic guided waves. Pair-wisethrough-transmission waveforms collected at normal conditions served as baselines,and subsequent signals collected at defected conditions such as rivet cracks orcorrosion detected the presence of a defect and its location with a novel correlationanalysis based technique called RAPID (reconstruction algorithm for probabilisticinspection of defects). The effectiveness of the algorithm was tested with severalcase studies in a laboratory environment. It showed good performance for defectdetection, size estimation and localization in complex aircraft wing structures.

An Investigation Into the Temperature Stability of a Guided Wave Structural Health Monitoring System Using Permanently Attached Sensors

When testing complex structures using a deployable guided wave system, the measured signals are often far too complex to directly interpret without a priori knowledge of the geometry of the structure. However, in a structural health monitoring (SHM) scenario, the sensors are permanently attached to the structure and so changes in the system can be monitored by comparison to an earlier baseline measurement, taken when the structure was undamaged. This paper investigates such a system for the SHM of plate-like structures using guided waves. The basis of the system is a distributed array of permanently attached piezoelectric guided wave transducers where pairs of transducers are used in pitch-catch configuration allowing the detection and localization of damage in the plate. It has been observed that the use of a single baseline measurement suffers in the medium to long-term due to the variation of environmental conditions. A key parameter in the instability is the change in the temperature of the test structure. A method of using a database of baselines, termed optimal baseline subtraction (OBS), is described and applied to an experimental SHM system. The objective of this paper is to investigate the suitability and robustness of OBS under varying rates of temperature change and its use in the localization of damage. Results have shown that through the OBS an improvement of up to 20 dB in signal-to-coherent noise ratio may be achieved compared with single baseline subtraction

Review of Guided-wave Structural Health Monitoring

In this paper we present the state of the art in the field of guided-wave structural health monitoring (SHM). We begin with an overview of damage prognosis, and a description of the basic methodology of guided-wave SHM. We then review developments from the open literature in various aspects of this truly multidisciplinary field. First, we discuss different transducer technologies, including both piezoelectric and non-conventional popular and non-conventional piezoelectric transducers. Next, we examine guided-wave theory, tracing its early history down to modern developments. Following this, we detail the efforts into models for guided-wave excitation by SHM transducers. Then, we review several signal processing related works. The next topic in Section 6 is guided-wave SHM system development, and we explore various packaging ideas, integrated solutions and efforts to examine robustness to different service conditions. We also highlight the broad spectrum of applications in which this technology has been tested. We then present some investigations that have attempted to combine guided-wave approaches with other complementary SHM technologies for better system performance. Finally, we propose desirable developments for further advancement of this field.

3-D Elasticity-Based Modeling of Anisotropic Piezocomposite Transducers for Guided Wave Structural Health Monitoring

Anisotropic piezocomposite transducers (APTs), such as macro fiber composites and active fiber composites, have great potential to be used as structurally integrated transducers for guided-wave (GW) structural health monitoring. Their main advantages over conventional monolithic piezoceramic wafer transducers are mechanical flexibility, curved surface conformability, power efficiency, their ability to excite focused GW fields, and their unidirectional sensing capability as a GW sensor. In this paper, models are developed to describe excitation of GW fields by APTs in isotropic structures. The configurations explored are plane Lamb-wave fields in beams with rectangular cross-section, axisymmetric GW fields in cylinders, and 3-D GW fields in plates. The dynamics of the substrate and transducer are assumed uncoupled. The actuator is modeled as causing shear traction at the edges of the actuator’s active area along the fiber direction. The sensor is modeled as sensing the average extensional strain over the active area along the fiber direction. The work is unique in that the formulation is based on 3-D elasticity, and no reduced-order structural assumptions are used. This is crucial to model multimodal GW propagation, especially at high frequencies. A formulation is also proposed to model the behavior of APTs as GW sensors. Finally, results from experimental tests to examine the validity of the models are discussed and the possible sources of error are examined in detail.