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  • Journal article
    Fu H, Sharif Khodaei Z, Aliabadi MH, 2018,

    An event-triggered energy-efficient wireless structural health monitoring system for impact detection in composite airframes

    , IEEE Internet of Things Journal, ISSN: 2327-4662

    IEEE In this paper, a low-power high-response wireless structural health monitoring system (WSHMS) is designed, implemented and experimentally evaluated for impact detection in composite airframes. Due to the rare, random and transitory nature of impacts, an event-triggered mechanism is adopted for allowing the system to exhibit low power consumption when no impact occurs and high performance when triggered. System responsiveness, robustness and energy efficiency are considered and modelled. Based on system requirements and functions, several modules are designed, including filtering, impact detecting, local processing and wireless communicating modules. The system was implemented on a printed circuit board. The response time is about 12 us with an average current lower than 1 mA when the impact activity is lower than 0.1%. The system exhibits high robustness to ambient vibration noises and is also capable of accurately and responsively capturing multiple sensing input channels (up to 24 channels). This work presents a low-latency energy-aware WSHMS for impact detection of composite structures. It can be adapted to monitor of other rare, random and ephemeral events in many Internet of Things applications.

  • Journal article
    Li J, Khodaei ZS, Aliabadi MH, 2018,

    Dynamic dual boundary element analyses for cracked Mindlin plates

    , International Journal of Solids and Structures, ISSN: 0020-7683

    In this paper, a new dual boundary element formulation is presented for dynamic crack problems in Mindlin plates. The displacement and traction boundary integral equations are derived in the Laplace frequency domain to allow for a boundary-only formulation. The cracked plate is modelled with the dual boundary element method and dynamic plate bending stress intensity factors are evaluated. Four benchmark examples are presented including mode I and mixed mode deformation. Such stress intensity factors obtained are shown to be in excellent agreement with finite element results as well as published results.

  • Journal article
    Bekas D, Sharif Khodaei Z, Aliabadi MHF, 2018,

    An innovative diagnostic film for structural health monitoring of metallic and composite structures

    , Sensors, Vol: 18, ISSN: 1424-2818

    A novel lightweight diagnostic film with sensors/actuators and a multiple-path wiring option using inkjet printing was developed. The diagnostic film allows for systematic, accurate, and repeatable sensor placement. Furthermore, the film is highly flexible and adaptable for placement on complex configurations. The film can be attached to the surface of the structure through a uniform secondary boundary procedure or embedded within the composite layup during curing. The surface-mounted film can simply be peeled off for repair or replacement without scratching or damaging the part. The film offers significant weight reduction compared to other available technologies. A set of extreme temperature, altitude, and vibration environment test profiles were carried out following the Radio Technical Commission for Aeronautics (RTCA) DO-160 document to assess the durability and performance of the diagnostic film for onboard application. The diagnostic film was shown to be durable and reliable in withstanding the variable operational and harsh environmental conditions of tests representing the conditions of regional aircraft

  • Conference paper
    Goossens S, De Pauw B, Geernaert T, Salmanpour MS, Khodaei ZS, Thienpont H, Berghmans Fet al., 2018,

    Aerospace-grade compatible surface mounted optical fibre sensor for structural health monitoring of composite structures

    © 2018 The Author(s). We suggest a robust optical fibre sensor package and its installation method for structural health monitoring purposes on carbon fibre reinforced polymer structures with potential to meet aerospace standards.

  • Journal article
    Sharif Khodaei Z, Yue N, Aliabadi MH, 2017,

    Damage detectability model of pitch-catch configuration in composite plates.

    , Key Engineering Materials, Vol: 754, ISSN: 1013-9826

    Detectability of damage using Lamb waves depends on many factors such as size and severity of damage, attenuation of the wave and distance to the transducers. This paper presents a detectability model for pitch-catch sensors configuration for structural health monitoring (SHM) applications. The proposed model considers the physical properties of lamb wave propagation and is independent of damage detection algorithm, which provides a generic solution for probability of detection. The applicability of the model in different environmental and operational conditions is also discussed.

  • Journal article
    Dafydd IP, Khodaei ZS, 2017,

    Laser Vibrometer imaging of delamination interaction with Lamb waves using a chirp excitation method

    , Key Engineering Materials, Vol: 754, Pages: 375-378, ISSN: 1013-9826

    One method that has shown great potential in visualising and characterising the interaction of guided waves with damage in composites is Laser Vibrometry. A Laser Doppler Vibrometer (LDV) can be used to produce 2D wavefield images of guided Lamb waves but a single scan is very time consuming and normally multiple scans are required at various frequencies in order to determine the best input signal. This paper demonstrates the use of a chirp excitation method requiring only a single scan and a post-processing algorithm to obtain results corresponding to any narrowband signal within the frequency range of the chirp signal. The method was used on an artificially delaminated composite panel and showed that the S0 mode, dominant at higher frequencies, mainly caused mode conversions whilst the A0 mode, dominant at lower frequencies, mainly caused a change in phase and amplitude across the delamination.

  • Conference paper
    Yue N, Khodaei ZS, Aliabadi MH, 2017,

    Damage detectability model of pitch-catch configuration in composite plates

    , 16h International Conference on Fracture and Damage Mechanics, FDM2017, Publisher: Trans Tech Publications, Pages: 387-390, ISSN: 1013-9826

    Detectability of damage using Lamb waves depends on many factors such as size and severity of damage, attenuation of the wave and distance to the transducers. This paper presents a detectability model for pitch-catch sensors configuration for structural health monitoring (SHM) applications. The proposed model considers the physical properties of lamb wave propagation and is independent of damage detection algorithm, which provides a generic solution for probability of detection. The applicability of the model in different environmental and operational conditions is also discussed.

  • Conference paper
    Lambinet F, Sharif Khodaei Z, 2017,

    Damage detection in composite skin stiffener with hybrid PZT-FO SHM system

    , International Conference on Fracture and Damage Mechanics (FDM 2017), Publisher: Trans Tech Publications, Pages: 367-370, ISSN: 1013-9826

    A hybrid piezoelectric (PZT)/fibre optic diagnostic system has been developed for damage detection in built up composite structures. The hybrid system uses PZT transducers to actuate the structure and fibre optic (FO) sensors to capture the propagating wave. The diagnostic system will then have the advantages of both PZT and FO sensors. The applicability of the system is then tested for detecting an artificial damage at a skin/stiffener interface of a thick composite structure. The response of the FO sensors is then compared to PZT sensors and presented.

  • Conference paper
    De Toro Espejel JF, Khodaei ZS, 2017,

    Lightning strike simulation in composite structures

    , International Conference on Fracture and Damage Mechanics (FDM 2017), Publisher: Trans Tech Publications, Pages: 181-184, ISSN: 1013-9826

    Lighting strike is one of the critical threats to the safety of composite aircrafts during flight. This work reports on numerical simulation of lightning strike in composite structures. Different modelling techniques using the commercial software ABAQUS, together with damage models are studied to find the most appropriate one in comparison to experimental results. Once the numerical model is validated, the effect of insertion of carbon nanotubes (CNTs) and metallic mesh in the composite is investigated. It is concluded that inserting CNTs in the top layer of the composite can improve its lightning strike protection noticeably.

  • Conference paper
    Lambinet F, Khodaei ZS, 2017,

    Smart patch repair with low profile PVDF sensors

    , International Conference on Fracture and Damage Mechanics (FDM 2017), Publisher: Trans Tech Publications, Pages: 359-362, ISSN: 1013-9826

    Bonded repair of composite structures still remains a major concern for the airworthiness authorities because of the uncertainty about the repair quality. This work, investigates the applicability of conventional Structural Health Monitoring (SHM) techniques for monitoring of bonded repair with ring-shaped low profile sensors. A repaired composite panel has been sensorized with two Ring-Shaped Polyvinylidene fluoride piezopolymer Sensors (RSPS) and a piezoelectric (PZT) transducer. An electromechanical impedance (EMI) and Lamb wave analysis have been carried out to check the sensitivity of these sensors to detect an artificially introduced damage simulating a disbond of the repair. The state of the repair have been successfully monitored and reported by both methods.

  • Journal article
    Morse L, Sharif Khodaei Z, Aliabadi MH, 2017,

    Reliability based impact localization in composite panels using Bayesian updating and the Kalman filter

    , Mechanical Systems and Signal Processing, Vol: 99, Pages: 107-128, ISSN: 1096-1216

    In this work, a reliability based impact detection strategy for a sensorized composite structure is proposed. Impacts are localized using Artificial Neural Networks (ANNs) with recorded guided waves due to impacts used as inputs. To account for variability in the recorded data under operational conditions, Bayesian updating and Kalman filter techniques are applied to improve the reliability of the detection algorithm. The possibility of having one or more faulty sensors is considered, and a decision fusion algorithm based on sub-networks of sensors is proposed to improve the application of the methodology to real structures. A strategy for reliably categorizing impacts into high energy impacts, which are probable to cause damage in the structure (true impacts), and low energy non-damaging impacts (false impacts), has also been proposed to reduce the false alarm rate. The proposed strategy involves employing classification ANNs with different features extracted from captured signals used as inputs. The proposed methodologies are validated by experimental results on a quasi-isotropic composite coupon impacted with a range of impact energies.

  • Journal article
    Salmanpour MS, Khodaei ZS, Aliabadi MHF, 2017,

    Impact Damage Localisation with Piezoelectric Sensors under Operational and Environmental Conditions

    , SENSORS, Vol: 17, ISSN: 1424-8220

    Guided-wave structural health monitoring (SHM) systems with piezoelectric sensors are investigated for localisation of barely visible impact damage in CFRP plates under vibration and different thermal conditions. A single baseline set is used in a delay-and-sum algorithm with temperature correction for damage localisation in a large temperature range. Damage localisation is also demonstrated under transient thermal conditions, with signals recorded while the temperature is rapidly decreased. Damage severity due to successive impact events is studied under constant temperature. Damage is also localised when the plate is subjected to random vibration.

  • Journal article
    Geraci G, Aliabadi MH, 2017,

    Micromechanical boundary element modelling of transgranular and intergranular cohesive cracking in polycrystalline materials

    , ENGINEERING FRACTURE MECHANICS, Vol: 176, Pages: 351-374, ISSN: 0013-7944

    In this paper a cohesive formulation is proposed for modelling intergranular and transgranular damage and microcracking evolution in brittle polycrystalline materials. The model uses a multi-region boundary element approach combined with the dual boundary element formulation. Polycrystalline microstructures are created through a Voronoi tessellation algorithm. Each crystal has an elastic orthotropic behaviour and specific material orientation. Transgranular surfaces are inserted as the simulation evolves and only in those grains that experience stress levels high enough for the nucleation of a new potential crack. Damage evolution along (inter- or trans-granular) interfaces is then modelled using cohesive traction separation laws and, upon failure, frictional contact analysis is introduced to model separation, stick or slip. This is the first time inter- and trans-granular fracture are being modelled together by BEM, and DBEM is being extended to include cohesive approach for anisotropic materials. Finally numerical simulations are presented to demonstrate the validity of the proposed formulation in comparison with experimental observations and literature results.

  • Conference paper
    Mallardo V, Sharif Khodaei Z, Aliabadi MH, 2017,

    Sensor optimization for impact detection: A Bayesian approach

    , Pages: 1855-1865

    © 2018 International Center for Numerical Methods in Engineering. All rights reserved. A Bayesian optimization strategy resulting in optimal sensor locations for impact localization under operational conditions is developed and presented. The impact detection methodology is based on developing meta-models utilizing artificial neural network (ANN) through the recorded sensor signals generated by various impact events. The novelty of the proposed method is to include the probability of one or more sensors failing under operation as well as non-uniform probability of impact occurrence in the structure. Finally, the proposed optimization algorithm is applied to a composite stiffened panel.

  • Journal article
    Salmanpour MS, Sharif Khodaei Z, Aliabadi MH, 2016,

    Airborne transducer integrity under operational environment for structural health monitoring

    , Sensors, Vol: 16, ISSN: 1424-8220

    This paper investigates robustness of permanently mounted transducers used in airbornestructural health monitoring systems, when exposed to the operational environment. Typicalairliners operate in a range of conditions, hence SHM transducer robustness and integrity mustbe demonstrated for these environments. A set of extreme temperature, altitude and vibrationenvironment test profiles are developed using the existing RTCA/DO-160 test methods. Commerciallyavailable transducers and manufactured versions bonded to CFRP composite materials are tested. Itwas found that the DuraAct transducer is robust to environmental conditions tested, while the othertransducer types degrade under the same conditions.

  • Journal article
    Mallardo V, Sharif Khodaei Z, Aliabadi MH, 2016,

    A bayesian approach for sensor optimisation in impact identification

    , Materials, Vol: 9, ISSN: 1996-1944

    This paper presents a Bayesian approach for optimizing the position of sensors aimed at impact identification in composite structures under operational conditions. The uncertainty in the sensor data has been represented by statistical distributions of the recorded signals. An optimisation strategy based on the genetic algorithm is proposed to find the best sensor combination aimed at locating impacts on composite structures. A Bayesian-based objective function is adopted in the optimisation procedure as an indicator of the performance of meta-models developed for different sensor combinations to locate various impact events. To represent a real structure under operational load and to increase the reliability of the Structural Health Monitoring (SHM) system, the probability of malfunctioning sensors is included in the optimisation. The reliability and the robustness of the procedure is tested with experimental and numerical examples. Finally, the proposed optimisation algorithm is applied to a composite stiffened panel for both the uniform and non-uniform probability of impact occurrence.

  • Journal article
    Salmanpour MS, Sharif Khodaei, Aliabadi MH, 2016,

    Instantaneous baseline damage localisation using sensor mapping

    , IEEE Sensors Journal, Vol: 17, Pages: 295-301, ISSN: 1558-1748

    In this paper an instantaneously recorded baselinemethod is proposed using piezoelectric transducers for damagelocalisation under varying temperature. This method eliminatesneed for baselines required when operating at different temper-atures by mapping a baseline area onto the interrogation area.Instantaneously recorded baselines and current interrogationsignals are calibrated based on the sensor mapping. This allowsextraction of damage scatter signal which is used to localisedamage. The proposed method is used to localise actual impactdamage on a composite plate under varying temperatures. Themethod is also applied to a stiffened fuselage panel to accuratelylocalise impact damage.

  • Journal article
    Yue N, Sharif Khodaei Z, 2016,

    Assessment of impact detection tchniques for aeronautical application: ANN vs. LSSVM

    , Journal of Multiscale Modeling, Vol: 07, ISSN: 1756-9745

    The Impact localisation in composite panels is assessed using two machinelearning techniques: least square support vector machines (LSSVM) and artificialneural networks (ANN) with local strain signals from piezoelectric sensors. Sensorsignals from impact experiments on a composite plate as well as signals simulated by afinite element model are used to train and test models. A comparative study shows thatLSSVM achieves better accuracy than ANN on identifying location of impacts for acombination of large mass impact and small mass impact, in particular when less datais available for training which is more appropriate for real aeronautical application.Additionally, LSSVM is more capable of identifying new impact events which have notbeen considered in the training process.

  • Journal article
    Sharif Khodaei Z, Aliabadi MH, 2016,

    A Multi-Level Decision Fusion Strategy for Condition Based Maintenance of Composite Structures

    , Materials, Vol: 9, ISSN: 1996-1944

    In this work, a multi-level decision fusion strategy is proposed which weighs the Value of Information (VoI) against the intended functions of a Structural Health Monitoring (SHM) system. This paper presents a multi-level approach for three different maintenance strategies in which the performance of the SHM systems is evaluated against its intended functions. Level 1 diagnosis results in damage existence with minimum sensors covering a large area by finding the maximum energy difference for the guided waves propagating in pristine structure and the post-impact state; Level 2 diagnosis provides damage detection and approximate localization using an approach based on Electro-Mechanical Impedance (EMI) measures, while Level 3 characterizes damage (exact location and size) in addition to its detection by utilising a Weighted Energy Arrival Method (WEAM). The proposed multi-level strategy is verified and validated experimentally by detection of Barely Visible Impact Damage (BVID) on a curved composite fuselage panel.

  • Journal article
    Thiene M, Sharif Khodaei Z, Aliabadi MH, 2016,

    Optimal Sensor Placement for Maximum Area Coverage (MAC) for Damage Localization in Composite Structures

    , Smart Materials and Structures, Vol: 25, ISSN: 1361-665X

    In this paper an optimal sensor placement algorithm for attaining the Maximum Area Coverage (MAC)within a sensor network is presented. The proposed novel approach takes into account physical properties ofLamb wave propagation (attenuation profile, direction dependant group velocity due to material anisotropy)and geometrical complexities (boundary reflections, presence of openings) of the structure. A feature of theproposed optimization approach lies in the fact that it is independent of characteristics of the damagedetection algorithm (e.g. probability of detection) making it readily up-scalable to large complex compositestructures such as aircraft stiffened composite panel. The proposed fitness function (MAC) is independent ofdamage parameters (type, severity, location). Statistical analysis carried out shows that the proposedoptimum sensor network with MAC results in high probability of damage localization. Genetic algorithm iscoupled with the fitness function to provide an efficient optimization strategy.

  • Journal article
    Salmanpour MS, Sharif Khodaei Z, Aliabadi MH, 2016,

    Transducer placement optimisation scheme for a delay and sum damage detection algorithm

    , Structural Control and Health Monitoring, Vol: 24, ISSN: 1545-2255

    In this work, a transducer placement scheme based on wave propagation is proposed, which enhances damage localisation. The method was tailored to seek an optimal transducer network placement for a delay and sum damage detection algorithm. The proposed method determines a coverage index map and utilises a genetic algorithm to determine an optimal transducer network. It can also minimise the impact of faulty transducers, incorporate the effect of stiffeners and different damage types. The method is initially verified using numerically simulated signals. The optimal network outperformed the suboptimal for detection of holes and debonding in a stiffened panel. It is also shown that the coverage index reflected the localisation accuracy. The method is then validated with experimental results and the generated optimal transducer network compared with a suboptimal arrangement. The optimal network is shown to locate an actual crack with significantly higher accuracy than the suboptimal arrangement.

  • Journal article
    Salmanpour MS, Sharif khodaei, Aliabadi MH, 2016,

    Guided wave temperature correction methods in structural health monitoring

    , Journal of Intelligent Material Systems and Structures, Vol: 28, Pages: 604-618, ISSN: 1530-8138

    In this paper a method for addressing temperature effects using Lamb waves is developed withapplication to baseline comparison damage detection. The proposed method is based on baseline signal stretchwith an improved minimum residual allowing correction over a larger temperature range. The effectiveness ofthe proposed approach in detecting (artificial) damages is demonstrated experimentally over a large temperature.The method is also shown to accurately detect and localise a crack in an aluminium panel and actual impactdamage on a CFRP panel.

  • Journal article
    Sharif-Khodaei Z, Ghajari M, Aliabadi MH, 2016,

    Impact damage detection in composite plates using a self-diagnostic electro-mechanical impedance-based structural health monitoring system

    , Journal of Multiscale Modelling, Vol: 6, ISSN: 1756-9737

    In this work, application of the electro-mechanical impedance (EMI) method in structural health monitoring as a damage detection technique has been investigated. A damage metric based on the real and imaginary parts of the impedance measures is introduced. Numerical and experimental tests are carried out to investigate the applicability of the method for various types of damage, such as debonding between the transducers and the plate, faulty sensors and impact damage in composite plates. The effect of several parameters, such as environmental effects, frequency sweep, severity of damage, location of damage, etc., on the damage metric has been reported.

  • Conference paper
    Salmanpour MS, Sharif Khodaei Z, Aliabadi MH,

    TOWARDS REAL-TIME STRUCTURAL HEALTH MONITORING DAMAGE DETECTION WITHOUT USER INPUT

    , ECCOMAS Congress 2016

    In this work a real-time damage detection platform is presented, requiring little tono user input after initial installation and set-up. Diagnostic ultrasonic signals are generatedusing attached piezoelectric transducers, which also serve to capture the structural response.This paper shows real-time detection in a flat CFRP panel. The necessary data acquisitionand signal processing is carried out in an automated manner. A visualization of the damagemap is then given as the primary output highlighting the predicted damage location. The developedsystem is flexible in allowing scalable deployment to cater for an increased numberof transducers. The detection platform is experimentally demonstrated by real-time localizationof artificial damages at various locations on a CFRP panel. This was also done underoperational environment vibration loading, yielding accurate damage localization.

  • Conference paper
    Khodaei ZS, Aliabadi MH, 2016,

    An optimization strategy for best sensor placement for damage detection and localization in complex composite structures

    , Pages: 2999-3008

    In this paper an optimization strategy based on providing maximum area coverage (MAC) resulting in optimal sensor placement for damage localization in complex composite structures is proposed. The proposed optimization algorithm is uses genetic algorithm to minimize the defined fitness function based on geometrical and physical constraints of the structure, rather than probability of detection of the damage detection algorithm. The proposed fitness function is applicable to any damage detection technique based on ultrasonic guided wave in pitch catch configurations and is readily up-scalable to any structure. The optimization algorithm is then applied to a full wing demonstrator of an aircraft.

  • Conference paper
    Thiene M, Khodaei ZS, Aliabadi MH, 2016,

    Statistical analysis of SHM passive sensing systems

    , Pages: 241-244, ISSN: 1013-9826

    © (2016) Trans Tech Publications, Switzerland. Structural Health Monitoring (SHM) techniques have gained an increased interest to be utilised alongside NDI techniques for aircraft maintenance. However, to take the SHM methodologies from the laboratory conditions to actual structures under real load conditions requires them to be assessed in terms of reliability and robustness. In this work, a statistical analysis is carried out for a passive SHM system capable of impact detection and identification. The sensitivity of the platform to parameters such as noise, sensor failure and in-service load conditions has been investigated and reported.

  • Conference paper
    Thiene M, Sharif Khodaei Z, Aliabadi MH, 2016,

    Optimal sensor placement for damage detection based on ultrasonic guided wave

    , Pages: 269-272, ISSN: 1013-9826

    © (2016) Trans Tech Publications, Switzerland. In this work a methodology for effective positioning of sensors and actuators for damage detection and characterisation is described. The novelty of the proposed methodology is that the fitness function to be optimised does not contain probability of detection (POD) which needs to be obtained for every possible sensor combination. The proposed fitness function is to provide the maximum coverage of the structure via Lamb waves and reduce the negative effects of boundary reflections. Once the fitness function is defines, genetic algorithm (GA) is used as an optimisation strategy to result in optimal sensor positioning.

  • Conference paper
    De Luca A, Sharif-Khodaei Z, Caputo F, 2016,

    Determination of the impact location and damage characterization based on guided waves

    , Pages: 10-13, ISSN: 1013-9826

    © 2016 Trans Tech Publications, Switzerland. The aim of this paper is to understand the effects of the damage criteria modelling on the training phase (performed by means of Finite Element simulations) of an artificial neural network (ANN) enabled to locate impacts onto a CFRP laminate. The developed FE models have been also used to investigate the intra-laminar damage mode, which, among different ones, has the most effects on the residual strength of the panel.

  • Conference paper
    Thiene M, Khodaei S, Aliabadi MH, 2016,

    Uncertainty analysis of active SHM system

    , Pages: 249-252, ISSN: 1013-9826

    © (2016) Trans Tech Publications, Switzerland. Structural Health Monitoring (SHM) techniques have gained an increased interest to be utilised alongside NDI techniques for aircraft maintenance. However, to take the SHM methodologies from the laboratory conditions to actual structures under real load conditions requires them to be assessed in terms of reliability and robustness. In this work, a statistical analysis is carried out for an SHM system for damage detection and characterisation in composite structures. The sensitivity of the platform to parameters such as noise, sensor failure, placement tolerances and bonding has been investigated and reported.

  • Conference paper
    De Luca A, Sharif-Khodaei Z, Aliabadi MH, Caputo Fet al., 2016,

    Numerical simulation of the Lamb wave propagation in impacted CFRP laminate

    , 2nd International Symposium on Dynamic Response and Failure of Composite Materials, (Draf), Publisher: ELSEVIER SCIENCE BV, Pages: 109-115, ISSN: 1877-7058

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