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The past two decades have marked significant research interest in nonlinearities produced in micro-cracked and cracked solids. These investigations have been based on the analysis of nonlinear static stress-strain characteristics and various dynamic elastic wave interactions. Crack-induced elastic, thermo-elastic and dissipative-type nonlinearities have been observed within various material scales and for various strain levels. This talk reports recent developments in the field, covering various aspects related to classical and non-classical nonlinear mechanisms and their applications for structural damage detection. The focus will be on modelling, numerical simulations, implementation and monitoring strategies.

Materials such as aluminum alloys are widely used in aircraft structures. In the case of the use of Al-alloys in aircraft structures, fatigue cracks occur because of excessive and repeated loading and vibrations experienced during frequent flights. Ultrasonic propagation imaging (UPI) is a damage visualization technique that is used in structural inspections employing a laser scanning system and ultrasonic sensors. However, conventional UPI or other scanning systems, such as a scanning laser Doppler vibrometer, only permit a single area to be inspected at one time. It is also difficult to inspect inaccessible areas, such as the upper skins of the aircraft wings. In this work, we describe a multi-area scanning UPI system built in a hangar and a mobile UPI system with a serially connected piezoelectric sensor array that is able to rapidly scan at a maximum pulse repetition rate of 20 kHz. After acquiring the generated ultrasonic wave signal induced by laser excitation, UPI videos for the in-plate guided wave are displayed. Finally, internal damage can be identified in a damage visualization platform. The developed hanger-based multi-area scanning UPI system is demonstrated on wings of an actual aircraft containing back surface cracks. The multi-area scanning technique is enabled to simultaneous scanning by two synchronized laser mirror scanners. The multi-area scanning UPI system with tilting mirror systems installed in the hangar ceiling permitted a clear visualization of the damage. Meanwhile, the mobile UPI system also was demonstrated with a serially connected piezoelectric sensor array connected with conductive fabric tape to cover large scan area by using multiple PZT sensors. The serial-connected piezoelectric sensor network is installed on the skin of metallic aircraft fuselage side. The mobile UPI system with a serial-connected piezoelectric sensor network was allowed to confirm aircraft fuselage containing back surface cracks. The damage visualization results confirm that the proposed multi-area scanning UPI system, a mobile UPI system with a serially connected piezoelectric sensor array and their approaches have excellent applicability as a built-in UPI system and mobile UPI system for a Smart Hangar, which is a future SHM solution that will be used to realize a full-scale structural inspection of an actual aircraft.

Acousto Ultrasonics (AU) is a Structural Health Monitoring (SHM) technique based on a permanently installed piezoelectric transducer network, which actuates and receives ultrasonic guided waves to provide information concerning the structure integrity. A reliable damage detection procedure needs to be developed so that AU represents a feasible alternative to the currently used Non-Destructive Testing (NDT) in aircraft structures. This work focuses on the damage detection and assessment of barely visible impact damages that occur after impacting a composite aircraft door surrounding structure. A generic door surrounding structure with an integrated SHM-network is manufactured within the project. The features of the structure include omega stringers, frames, skin thickness variations, clips and intercoastals. The door surrounding structure is afterwards impacted with a mobile gas gun and interrogated with the SHM-network, generating the experimental data required for the damage assessment. A methodology for the identification of damages based on probability-based diagnostic imaging has been developed in order to perform the damage assessment. The performance of the methodology is evaluated through comparison with the position and size of the damages observed with traditional NDT methods. The study presents the performance of the damage assessment methodology when it is applied to monitor the door surrounding structure. The benefits and limitations of the developed methodology are depicted through multiple representative cases involving real damages. The key aspects of the analysis are the accuracy in damage location and sizing as well as the sensitivity to damages of different nature induced via impacting, such skin delamination or disbond of structural parts. Finally, an approach towards the improvement of the methodology is outlined.

A major safety and maintenance concern in aerospace structures are fatigue induced cracks for which quantification is a very critical component for achieving design performance. Such incipient damages, once initiated, can grow extremely fast and can lead to catastrophic failure of the structure. Traditional nondestructive inspection (NDI) techniques offer solutions which are offline, time consuming and quite expensive. Structural Health Monitoring (SHM) techniques are known to overcome some of these drawbacks. However, in SHM the challenges associated with quantification of fatigue cracks come mainly from the differential structural acoustic-ultrasonic response in similar material due to system installation errors or are caused by environmental usage conditions. Studies have shown that these variations in acoustic response come primarily from sensor positioning and the characteristics of crack e.g. orientation, crack length etc. This study presents a robust, multipath and scalable unit-cell approach for quantification of fatigue cracks. Multiple paths from a sensor configuration with a four sensor unit called a unit-cell is used along with an adaptive weighted averaging method to mitigate the effects of sensor positioning errors and/or uncertainties associated with crack orientation for quantification in SHM systems. Coupon tests that have been conducted to verify and validate the performance of this unit-cell approach for quantification purposes.

In scope of the SYMOST project a turbo-prop military aircraft, monitored by a net of piezoelectric transducers, was subjected to a long-term full-scale fatigue experiment. Lamb waves excited by the sensor net were used for the detection and localization of fatigue damage. Several algorithms for Lamb wave signal processing and interpretation were implemented and tested in the project. This paper presents selected results of the full-scale fatigue experiment of the PZL-130 Orlik TC II Aircraft, including: four methods of temperature compensation for Lamb waves and diagnose obtained by ensembles of neural networks operating on groups of damage indices. Results of aircraft monitoring by means of different SHM methods over a period of one year are presented and commented. Different approaches to damage detection are compared in terms of their reliability and their limitations for practical applications.

The interest in Structural Health Monitoring (SHM) Systems that are able to detect and characterize events such as impacts for in-service aircraft is increasing. One of the main challenges for these systems is the detection of low energy accidental BVID on thin-walled structures such as fuselages or wing’s skin. These structures may be metallic or composite but the emphasis of this research has been placed on composite structures. The Airbus DS stress methods team has designed a set of Numerical Simulations that has been validated with physical tests, to increase the knowledge of the structural response in terms of elastic waves to support the development of SHM Systems using these phenomena. The target of this study is to develop a simulation methodology that enables research on structural trend behaviour in support of the development of a reliable SHM System for real aircraft structures while only carrying out a limited number of physical tests.

Research and development of active monitoring systems for reinforced concrete structures should lead to improved structural safety and reliability. Implementation of active structural health monitoring systems with capability of damage detection and structural diagnosis represent one of the main challenges in this field of research. In this paper we propose a numerical modeling of damage detection process in a concrete beam with piezoelectric smart aggregates, which may be used both as actuators and sensors. The modeling procedure involves modeling of piezoelectric smart aggregates – developed using implicit finite element method and modeling of the wave propagation – developed using explicit finite element method. Based on the numerically generated actuation waves and sensors signals, a one-dimensional damage index based on energy variations of the output sensor signals is formed using the wavelet signal decomposition and the principle of root-mean-square deviation. The paper presents the original numerical models with parametric analysis of the damage index variation problem depending on the size, position and orientation of the cracks.

It is a big challenge to relate acoustic emission (AE) signal events to specific damage modes developed in composites under hygro-thermo-mechanical loading. This study provides further insight into the AE monitoring of a 3D angle interlock (AI) glass fibre composite and has revealed the complex nature of the relationship between the principal characteristics of recorded AE events on the one hand and the mechanical behaviour of the material on the other. This paper presents experimental and simulation results on the use of AE on 3D AI glass fibre composites for structural health monitoring (SHM) of matrix cracks, during quasi-static tension of flat plates. Fibre-reinforced composite materials are used extensively in the aerospace industry because of their high specific strength and stiffness, superior corrosion resistance and improved fatigue properties. In addition to the manufacturing costs and production rates, damage tolerance has become a major issue for the composite industry. Three-dimensional (3D) woven composites have superior through-thickness properties compared to two-dimensional (2D) laminate, for example, improved impact damage tolerance, high interlaminar fracture toughness and reduced notch sensitivity. The performance of 3D woven preforms is dependent on the fabric architecture which is determined by the binding pattern. Different combinations of 3D woven preforms can be produced given the variation of the binding pattern. They can be classified into angle interlock and orthogonal interlock according to the binder orientation, or through-the-thickness and layer-to-layer if the penetration depth of binders is involved. For this study, AI structures with through-thickness and layer-to-layer binding were manufactured. Monitoring of AE during mechanical loading is an effective and widely used tool in the study of damage processes in glass fiber-reinforced composites. Tests were performed with piezoelectric sensors bonded on a tensile specimen acting as passive receivers of AE signals. A new set of experimental data has been generated which will be useful for validating numerical models, providing insight into the damage behaviour of novel 3D AI glass fibre composites, and may ultimately lead to more effective material selection and determination of design limits. The paper finishes with conclusions and suggestions for further work.

The performance of Acoustic Emission (AE) systems is modelled using the linear time-invariant systems approach. This is done to aid the understanding of how AE systems perform on real aircraft structure. The motivation for this is to improve the understanding of AE results from current structural tests and to inform future AE system design. The focus of the work is to model the propagation of ultrasonic guided waves across features that are present in typical aircraft structure. Wave propagation has a large influence on the performance of an AE system yet is either not considered or only considered simply in many industrial tests. An empirical model for propagation across an L-section bolted to a plate has been created from experimental results for a range of incident angles. The bolted L-section is a simplification of stringers which are a common feature on aircraft structure. The accuracy of the feature model is assessed; first with experimental results from a plate with 2 bolted L-sections and then with experimental results collected from a section of A320 wing skin with real stringers. Regions where the model predicts sensitivity to AE events above that of the experimental results are found and the implications of these regions for system design are discussed. The feature model is included in an overall model of an AE system which includes realistic models of AE events. Example results from the overall model are used to show the affects of system parameters and processing algorithms from a real AE system on system performance. These examples show the benefits and limitations of this modelling approach.

Our purpose addresses accurate fatigue damage prognostic in composite aircraft structure based on an SHM approach. A methodology based on the combination of an active sensing SHM technique and a state-parameter estimator to predict the fatigue damage and compute the remaining useful life of a structure is proposed. Active sensing SHM utilizes active sensors, such as piezoelectric sensors (PZTs) for monitoring local defects and can potentially interrogate large structural areas. With known inputs, the difference in local sensor measurements based on the same input is strongly related to a physical change in the structural condition, such as the introduction of damage. The major challenge of the current sensor-based health monitoring systems is to quantify the damage based on sensor measurements. In this study an ultrasonic guided wave active sensing SHM method will be used to detect and quantify fatigue damage in composite structures using appropriate simulation scenarios, as well as experimental data obtained from laboratory composite coupons. The ultrasonic wave propagation signals will be used for the extraction of appropriate damage indices based on which the quantification of fatigue damage will be achieved. Next, these features will be used as inputs in an appropriate fatigue model to enable the damage prognostics of the composite structure. The phenomenological fatigue model derived from stiffness degradation rule proposed by Wu and Yao, verified with experimental data is considered in this study. The Extended Kalman Filter (EKF) based on a recursive digital processing, appropriate to real-time applications, is used to estimate the Wu and Yao model parameters. The behavior of the EKF is first studied by using simulated data. The impact on the estimation of parameter and variance initial values has been studied. Process and measurement noise influence have also been studied. Glass and carbon fiber composite materials under various fatigue loading have been tested. A similar general trend but different estimation error magnitudes are observed. A better error estimation is always obtained with a process noise smaller than the measurement noise. Then real data are considered to process the state-parameter estimation. Based on sensitivity analysis, as previously, it is shown that some materials under estimation initial conditions provide better results.

In this paper, a recently proposed refined time-reversed Lamb wave method for damage detection is tested experimentally for detecting block mass type damage in isotropic plates. The best reconstruction frequency has been found out for the actuator-plate-sensor system by performing the time reversal process for a certain range of frequency, which is found to be very different from the sweet spot frequency, hitherto recommended for improving the performance of the time-reversal process (TRP) based techniques. Damage indices (DIs) computed by using the conventional main wave packet of the reconstructed signal are less sensitive to an increase in damage size, which is consistent with some recently reported experimental results by other groups. The present method with extended signal length shows excellent sensitivity to damage, and also ensures a low threshold for the undamaged case when used at the best reconstruction frequency. The DIs based on present method reflect the true severity of damage. It was observed that a putty on the plate has no significant change in the DIs, whereas a baseline method would identify it as a damage due to very significant scattering by the putty .

Optical fiber sensors (OFSs) have been recently used for impact monitoring, impact force reconstruction, Lamb wave monitoring and acoustic emission monitoring. In particular, most of the studies available in the structural health monitoring (SHM) literature perform local measurements by means of fiber Bragg gratings, which provide high signal to noise ratios but are associated to expensive acquisition systems, making the scalability of this diagnostic solution not sustainable for realistic structure monitoring. Optical fiber interferometers have been also exploited for acoustic and ultrasonic sensing, although one main drawback of interferometric OFS is that the entire length of the leading optical fiber is sensitive, while punctual measurements are desirable in many SHM applications. A cost-effective solution for ultrasonic wave monitoring is proposed in this paper for high frequency SHM applications. The sensor relies on a fiber optic Michelson interferometric architecture associated to an innovative coherent detection scheme, which retrieves in a completely passive way the phase information of the received optical signal. The optical fiber is organized into multiple loop structures glued on an aluminum thin panel in order to increase the phase signal relative only to the sensing points of interest. The ultrasonic wave emitted by a piezoelectric transducer placed on the aluminium panel is then acquired by the coherent FOS with different sensor layouts as a function of the number of sensor fiber loops and the fiber gage length. Measurements carried out with the coherent fiber optic sensor are then compared with traditional piezoelectric sensors. The proposed fiber-optic solution represents a promising, cost-effective and highly sensitive measurement technique for impact monitoring, including both passive impact identification (detection, localization and energy estimation) and active damage monitoring, e.g. by means of Lamb wave scatter analysis.

Structural health monitoring based on guided wave plays an important role in the damage evaluation of practical application. However, the damage evaluation under time-varying environments which introduces undesired uncertainties to guided wave features is difficult to achieve reliably. In this paper, an approach of guided wave based Hidden Markov Model (HMM) method is proposed to improve the reliability of damage evaluation under time-varying environment. With this method, a left-right continuous HMM which is composed of several hidden states is trained firstly based on the time-varying affected guided wave features of different damage states. Each hidden state of HMM represents a damage state of a monitored structure under time-varying environment. Then a maximum average posterior probability based on the HMM can be obtained to evaluate the damage when a new guided wave feature is obtained during an on-line damage process. Finally, the method performance is validated by monitoring the hole-edge crack of an aluminum tensile specimen under fatigue load condition and result shows that the reliability of damage state evaluation is improved.

This paper presents a study on the scattering of the fundamental flexural Lamb wave mode (A0) by an edge delamination around the circumference of a circular through hole in a plate. Although edge delaminations are an important form of structural damage in fibre-composite laminates, there is little previous work on Lamb wave scattering by them. A 3D laser vibrometer is used to obtain experimental characterisation of the angular dependence of the scattered field from an edge delamination, for an incident probing wave at various angles of incidence. These experimental observations are correlated with computational results obtained by using the commercial Finite Element package ABAQUS, showing excellent correlation. Aluminium is used instead of a composite laminate, as this removes further complications that arise due to the anisotropic and multilayer nature of the material in composites. A semi-circular delamination is introduced at the boundary of a circular hole of 20mm diameter, in the mid plane location of the plate using Electrical Discharge Machining. The probing signal is a narrowband 5.5-cycle toneburst with a centre frequency of 200kHz, for which the corresponding A0 wavelength is λ = 10 mm. Relationship between the scattered wave amplitude and damage size is studied for a fixed oblique angle of incidence, and for damage radius a varying from 1- 4mm in 0.5mm increments. The relatively small damage size corresponds to early damage detection which is the main area of interest for SHM applications The Scattered Amplitude Pattern is used to study the relationship between the scattered field and damage orientation relative to the incident wave. Results obtained indicate a strong dependence between the scattering amplitude pattern and angle of incidence, and between the scattering amplitude and damage size. The results show that the scatter field amplitude and directivity information can be used to detect and quantify the damage in any orientation around the through hole, which confirms and extends the correlations obtained in our previous work for delaminations at a straight edge. This forward scattering problem provides a necessary pre-requisite for addressing the more challenging inverse problem of characterising the damage from observations of the scattered field, particularly for complex geometries involving detection on blind side of through holes.

A long-term target for SHM systems based on guided waves is the coverage of large areas with sparsely distributed sensors. Future systems will need detailed information about the monitored structure and flexible but fast algorithms to reconstruct wave propagation. Multiple stiffeners, thickness changes and different materials cause complex signals in aircraft parts trough reflection and mode conversion of the initial waves. Computation of wave propagation will require a certain amount of approximation to enable calculation within reasonable times. Sensor network optimization and damage localization are fields which can benefit from such fast but simplified algorithms. A minimal model to approximate wave propagation in complex structures has been proposed in previous works of the authors. Core principle of the model is a reduction of inhomogeneities, like stiffeners, to interaction parameters and homogeneous areas to phase velocities. Combined with the plate like shape of aircraft parts relevant for SHM, a two-dimensional model is sufficient. Paths taken by the waves from actuator to sensor are estimated with ray tracing. Finally, signals at selected sensor positions can be calculated with a signal synthesis algorithm. Several characteristic values of the structure significant to guided waves have to be calculated beforehand. Preparation of such a database can be costly to a certain extend, but is only necessary once per part. With such a database in place, the proposed technique offers a very fast way to approximate wave propagation in large and complex structures. The minimal model allows individual activation of many effects that influence wave propagation. This includes among others refraction, attenuation and diverse types interaction. This paper discusses some of these effects and their influence on wave propagation, sensor signals and simulation accuracy. To support this, measured signals are compared to results of the minimal model.

Quantitative non-destructive evaluation for small fatigue cracks still remains a significant challenge for structural integrity management, especially for cracks in hidden or hard-to-inspect locations. Recently, Lamb waves have attracted considerable research interest in structural health monitoring (SHM) due to their desirable properties, which includes slow geometrical decay of amplitude with propagation and hence a potential for rapid wide-area inspection. This paper presents a 3D laser vibrometry experimental study of the interaction between edge-guided waves and a small through-thickness edge crack in a 3mm thick aluminium (isotropic) plate. A piezoelectric transducer is bonded on the edge of the plate to generate the incident wave. The excitation signal consisted of a 5.5 cycle Hann-windowed toneburst of centre frequency 200 kHz, which is well below the cut-off for the first order Lamb wave modes (A1 and SH1). A 2D FFT procedure is applied to the incident and scattered field along radial lines emanating from the crack mouth so as to obtain both the amplitude of the incident field and the amplitude of the scattered field as a function of scattering angle. In principle, edge waves are one dimensional and therefore their amplitude should not decay with propagation distance, which makes them well suited for SHM. In practice, a small decay with propagation distance was observed, especially for the antisymmetric edge wave, which may be attributed to difficulties in distinguishing between the edge wave and the bulk A0 mode in the 2D FFT plots. Nevertheless, it is shown that the scattered wave field due to a small crack length, a, (compared to the wavelength of incident wave, λ) can be considered to be equivalent to a point source consisting of particular combinations of body-force doublets. It is found that the amplitude of the scattered field increases as a power function of a/λ, whereas the scattered wave pattern is independent of crack length for small cracks a << λ. This forward problem of determining the scattered wave field from a known crack size is an essential study to guide a judicious approach to the inverse problem of crack detection and sizing.

Guided waves have attracted a considerable interest as a mean to perform inspections in plate-like structures. Generally, dense networks of transducers are employed to actuate and receive the elastic waves. Besides being used for the inspections the same transducers can be adopted to communicate the results of the inspection, for example a damage indicator, to a central processing unit [1], thus avoiding the need of communication cables or radio modules. To this aim phase-modulated excitation signals, known from CDMA-communication systems, can be employed. This class of signals enables a simultaneous transmission from all piezoelectric transducers leading to a significant reduction in overall system complexity, because channel switching is not required anymore. In addition, phase modulated signals are useful to improve the signal-to-noise ratio. However, phase modulated signals loss their effectiveness for guided wave communications since classical decoding procedures based on matched filters fail because of the detrimental effect of dispersion. To tackle this problem, a new decoding procedure capable to compensate for dispersion guided waves signals propagating in regular and irregular waveguides is proposed in this work. Dispersion compensation is particularly challenging in irregular waveguides composed by segments with (i) different material properties, (ii) different cross-section, (iii) tapered geometry, as well as (iv) different radii of curvature (for instance a bent pipe in a pipeline), and thus characterized by a varying dispersive behavior. The method is based on an accurate computation of the group delay associated to the transmitter-receiver propagating path [2]. The presented procedure is suitable for guided waves communications in irregular waveguides and can be applied in many structural health monitoring (SHM) applications. A proof-of-concept related to a tapered Aluminum plate will be demonstrated. [1] Y. Jin, Y. Ying, and D. Zhao, “Data Communications Using Guided Elastic Waves by Time Reversal Pulse Position Modulation: Experimental Study,” Sensors, 13(7): 8352–8376, 2013. [2] De Marchi L, Marzani A, Speciale N, Viola E. Prediction of pulse dispersion in tapered waveguides. NDT & E Int 2010;43(3):265–71.

The increasing usage of Carbon Fiber Reinforced Plastics (CFRP) for primary aerospace structures involves dealing with the principal susceptibility of composite laminates to impact loads as well as the occurrence of barely visible impact damages. Therefore the assessment of impact damages in CFRP structures poses an attractive application case for Structural Health Monitoring by Guided Ultrasonic Waves. Wave propagation phenomena at impact damages as well as the utilized signal processing to extract a damage related feature (i.e. damage index) contribute to the sensitivity of SHM system and thus to the reliability of such SHM systems. This work bases on data from the EU-funded project SARISTU, where a generic CFRP door surrounding fuselage panel with an integrated sensor network has been build and tested by introducing plenty of impact damages. Wave interaction at delamination and debonding of different size and morphology in Omega-stringer stiffened structures are examined in detail, in order to highlight the factors contributing to the sensitivity. Subsequently criteria for a reliable design are extracted. Common damage indicator formulations for use with imaging algorithms are applied on data from various damage cases and compared to these criteria.

Multi-wire cables are commonly used in many civil engineer structures. Particularly in the case of power lines, Aluminium Conductor Steel Reinforced (ACSR) cables are frequently used. The ACSR cables can be exposed to sea side or industrial atmospheres and these conditions could lead to the deterioration or the corrosion of the cable. Ultrasonic Guided Waves (UGW) testing has been proved as a potential technique for its implementation as a SHM system of the cables. The present paper investigates the performance of dry-couple piezoelectric transducers with two configurations for the inspection of the cables using L(0,1) and T(0,1). The ability of these two vibrational modes for the detection of notches has been investigated through a scattering analysis in the peripheral wires of a 4 meter “Dog” ACSR cable specimen. This analysis has been carried out in a wide range of frequencies to obtain the optimum frequency range for these two modes, as well as an attenuation investigation in this frequency range. A 3D laser vibrometer has been used for the reception of the signals as well as for area displacement examination. The results show the superior capabilities of L(0,1) for the inspection of the cables with respect to T(0,1) as well as the effect of the excitation frequency in the wave propagation behaviour of the wave in the cables.

Classic guided wave based defect-detection strategies employ the dispersive behaviour of waves propagating in solids to determine the reflected and transmitted wavemodes from a time of flight measurement. One of the main difficulties with these techniques is the big effort that has to be undertaken to ensure the excitation of only desired wavemodes. The purpose of this paper is to present a novel guided wave technique for the detection of inhomogeneities in pipes, which is not relying on the excitation of one specific wavemode. The method works in the frequency domain and can be divided into three steps: (i) measurement of the surface-displacement of a pipe that is excited by an unknown combination of wavemodes at one discrete frequency, (ii) localisation of the defect based on the surface-displacement, (iii) fitting the measured surface-displacement in the pipe-section before and behind the defect with a set of possible wavemodes. After this step, the contribution of every wavemode to the displacement-field and thus the wave-scattering of the incident modes at the defect is known. More details about the defect can be obtained by means of the known wave-scattering. The measurements (step i) are simulated using a hybrid Wave and Finite Element / Finite Element method. Thereby the complete cross-section of the pipe in the vicinity of the defect is modelled with Finite Elements while the rest of the pipe is modelled with wave-vectors found in the WFE analysis. Applying the above described technique to those simulated measurements, the wave-scattering of all flexural and longitudinal incident wave-modes could be determined without any up-front knowledge about the excited wavemodes. This property can contribute to overcome the difficulties of classic guided wave based defect-detection strategies to excite only specific wavemodes.