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An electromechanical impedance (EMI) based structural health monitoring (SHM) approach is proposed for the localisation of skin-core debonds in composite sandwich structure (CSS). Towards this, laboratory experiments and numerical simulations of EMI in a CSS with core to bottom face-sheet debond have been carried out using a network of piezoelectric transducers (PZTs). The frequency-domain analysis of the registered EMI signals shows that the presence of inter-facial debonds in the CSS significantly influences the conductance magnitudes of the registered EMI data. It was also noticed that the conductance magnitudes of the signals are dependent on the debond-to-PZT distances. In all the study cases, an agreement between the simulation and experimental results is observed. Eventually, a simulated SHM approach is proposed that uses a debond detection algorithm to calculate the changes in conductance magnitudes to effectively locate such debonds in CSS. The study is further extended for the detection of debonds at different locations in the CSS, including a debond located at the edge to assess the potential of the proposed SHM approach.

Compliant mechanism finds its application in high precision devices for precise movements attaining positioning accuracy, micro actuation, aerospace, biomedical engineering, etc. A detailed study of compliant mechanisms capable of transferring coplanar motion/ force along with geometric amplification was undertaken and is presented. An iterative approach was undertaken successively to design and develop a displacement amplifying compliant mechanism (DaCM) within a specified design domain with the objective to obtain geometric amplification (GA) of 3. Also, an application of object holding mechanism using topology optimization has been presented. The optimized topology was further iterated for attaining desired objective function and modifications in the geometry are undertaken considering design for manufacturing and design for assembly principles. With the modifications undertaken, a reduction in (GA)T is observed and the volume fraction of the mechanism is increased than the base model resulting into more strength required for object holding. However, the designer needs to balance between stiffness and compliance based on the objective function of specific application.

Guided waves (GW) allow fast inspection of a large area and hence have received great interest from the structural health monitoring (SHM) community. Fiber Bragg grating (FBG) sensors offer several advantages but their use has been limited for the GW sensing due to its limited sensitivity. FBG sensors in the edge-filtering configuration have overcome this issue with sensitivity and there is a renewed interest in their use. Unfortunately, the FBG sensors and the equipment needed for interrogation is quite expensive and their number is restricted. In the previous work by the authors the number and location of the actuators was optimized for developing a SHM system with single sensor and multiple actuators. But through the use of the phenomenon of acoustic coupling, multiple locations on the structure may be interrogated with a single FBG sensors. As a result, a sensor network with multiple sensing locations and few actuators is feasible and cost effective. Hence this paper develops the optimization problem for designing an SHM network for use with FBG sensors making use of acoustic coupling. The optimization problem is implemented on a simple aluminum plate. The directionality, bond efficiency and the factors influencing the acoustic coupling are taken into consideration for optimizing the sensor network. A multi-objective optimization problem is defined and solved using non-sorting genetic algorithm (NSGA). The results indicate that indeed a multi-objective optimization is necessary and has potential to improve the SHM system performance.

Recent advancements in fiber optic methods have enabled their use for guided wave sensing. It opens up new possibilities for Structural Health Monitoring. The aim of this paper is to provide insight for the physics related to guided wave propagation and coupling between the optical fiber and solid structure. For this purpose, a new approach for non-matching interface based on Lagrange multipliers and the time domain spectral element method was developed. A parallelized code has been implemented in order to simulate the guided wave propagation in the structure, its coupling into the optical fiber and the propagation in the fiber in a computationally efficient way. The paper presents four studies showing the efficacy of the modeling approach. The paper first shows the improvement in the computation speed through the use of parallelization and a more efficient implementation. Then the results of the simulation of wave propagation in the fiber are compared with results from previous simulation studies using commercially available software. The third study shows that the spectral element method is able to capture the directional sensitivity of optical fiber based sensors. Lastly, the simulation is used for detection of simulated damage using the spectral element method based simulation. The results indicate that indeed the spectral element implementation is able to recreate the wave coupling phenomena, capture the physics of the system including directional sensitivity and reflections from damage.

Concentrated damage such as cracks is one of the most common types of damage in beams. It is essential to detect such damage early to avoid structural failure. Vibration-based damage detection methods that employ changes in structural dynamic parameters such as natural frequencies have been extensively studied, and it is commonly acknowledged that natural frequencies depicting structural global dynamic properties are incompetent to portray local damage. Differing from extant work, this study presents a concept of relative natural frequency change (RNFC) curves for local damage characterization and especially ascertains the relationship between RNFC curves and mode shapes, leading to an explicit equation of RNFC. With RNFC curves and measured values of RNFCs, a two-step method for localizing and quantifying damage is created: a novel probabilistic damage indicator is developed using Bayesian data fusion for localizing single and multiple damage; moreover, a damage severity factor defined as the stiffness reduction ratio of the damaged element is formulated to quantify damage. The proposed method features localization, quantification, and evolution monitoring of damage, relying solely on natural frequencies. The efficacy of the method is verified numerically and then validated experimentally on cracked beams. The numerical and experimental results demonstrate the capability of the method to localize single and multiple damage and to estimate damage severity. This mechanism of characterizing damage relying solely on natural frequencies provides the foundation for developing practical local damage detection and monitoring technologies for beam-type structures.

Non-uniform crack identification in plate-like structures is of significance in mechanical, aerospace, and civil engineering. The 2D modal curvature is often utilized to identify damage in plates, but its capability to precisely characterize non-uniform cracks has not been resolved well, largely due to two noticeable deficiencies, lack of sensitivity to cracks and susceptibility to noise interference. With this concern, this study formulates the wavelet 2D modal curvatures by ameliorating the 2D modal curvature using the real and complex wavelet transform, respectively. Crack-caused singular peaks appear in crack locations can designate the presence and locations of cracks. Superior to the 2D modal curvature, the real-wavelet 2D modal curvature and complex-wavelet 2D modal curvature enjoy the multiscale analysis capability introduced by the wavelet transform, whereby noise interference can be eliminated. Moreover, the complex-wavelet 2D modal curvature is capable of synchronously eliminating noise and characterizing cracks, which endows it with the capability to identify and characterize non-uniform cracks under noisy conditions. The capability of the complex-wavelet 2D modal curvature is analytically and experimentally validated on plates with non-uniform cracks; its applicability is further explored in a numerical scenario of a plate-like sluice gate with non-uniform cracks. The results show the complex-wavelet 2D modal curvature can characterize the length, extent, and non-uniformity of non-uniform cracks, providing an appropriate tool for identifying and characterizing such cracks in plate-like structures under noisy conditions.

In this paper results of numerical analysis of S0/A0 elastic wave mode conversion at discontinuities in glass fibre reinforced polymer (GFRP) plates are presented. Analysis of S0/A0 mode conversion effect at Teflon inserts simulating delamination (circular Teflon inserts located at a different depth, Teflon inserts with different shapes at the same depth) and impact damage with energies 5 J, 10 J and 15 J is conducted. Numerical analysis is based on spectral element method (SEM) in the time domain. Numerical model includes a composite plate, a piezoelectric transducer and bonding layer for transducer and damage. Numerical model takes also into account the influence of material damping. Numerical results are compared with experimental results based on full wavefield measurements using scanning laser Doppler vibrometry (SLDV). It is shown that based on effects of S0/A0 mode conversion it is possible to detect the damage and determine its shape and size.

Identification of delaminations in the early stage is crucial to ensure the safety of CFRP laminates. In the modal domain, high spatial sampling resolution of a mode shape is required for precise location of incipient delamination. Non-contact measurement relying on laser scanning technology allows measured mode shapes to be densely sampled. Unfortunately, noise components usually contaminate such densely-sampled mode shapes, easily causing intense noise interference that likely masks actual delamination features. To address this issue, a noise-robust damage indicator is formulated for designating the presence and location of incipient delamination in CFRP laminates under noisy environments. This damage indicator is created by introducing a 2D multi-resolution modal Teager-Kaiser energy for characterizing delamination-caused singularity of mode shapes. Proof of concept of the damage indicator is numerically undertaken in finite-element CFRP laminates with incipient delaminations, with emphasis on sensitivity to damage as well as robustness against noise. The applicability of the damage indicator is experimentally validated on a CFRP laminate with an incipient delamination, mode shapes of which are precisely acquired via the non-contact measurement using a scanning laser vibrometer. The numerical and experimental results show that the damage indicator can effectively identify the presence and location of incipient delamination in CFRP laminates under noisy environments.

Recently, in many industrial branches (e.g. aviation, marine or civil engineering) metal elements are replacing by composite ones. To ensure their reliability structural health monitoring systems also based on fibre optics sensors (embedded into or mounted on composite structure) are widely applied. In the paper THz spectroscopy technique was proposed for detection and localisation of glass fibre optics embedded into a glass fibre reinforced polymer . The analysis was performed on a four-layer sample manufactured by infusion method. During this investigation both the sample surfaces roughness as well as fibre optics arrangements (according to the THz spectrometer axes) were examined. For this purpose THz images of internal structure of the sample were presented. Additionally two methods (in time and frequency domains) for quick determining of fibre optics location are presented. The first one is based on local changes in signal amplitudes while the second – on differences in power spectrum.

A coordinated semi-analytical, finite element, and experimental analysis of damage induced acoustic emission (AE) wave propagation and source monitoring in a honeycomb sandwich composite structure (HSCS) have been carried out. Towards this, a global matrix method based two-dimensional (2D) semi-analytical formulation is first applied to obtain the AE response and dispersion characteristics in the HSCS. The finite element simulation of AE-wave propagation in the HSCS is then carried out using different AE-sensor network configurations. Eventually, the damage source locations are efficiently identified by applying a source localization algorithm, which uses the registered AE signals from the sensor networks. In order to verify the robustness of the proposed damage source monitoring technique, the localization is further broadened for randomly selected multiple damage source locations for each sensor network configuration. The obtained results distinctly represent the efficiency of the proposed online monitoring strategy for localizing the damage-source in such complex and advanced structures. Moreover, it is shown that the proposed damage source monitoring technique is reliable and independent of sensor positions.

Guided waves (GW) allow fast inspection of a large area and hence have received great interest from the structural health monitoring (SHM) community. Fiber Bragg grating (FBG) sensors offer several advantages but their use for GW sensing has been limited due to their limited sensitivity. FBG sensors in the edge-filtering configuration have overcome this issue with sensitivity and there is a renewed interest in their use. It has been seen that depending on the ratio of the wavelength of the propagating wave to the gauge length of the FBG, the mechanism of the transduction of the wave measurement is different. A large ratio leads to a more uniform strain over the FBG leading to a shift in the frequency, while a non-uniform strain due to a short wavelength, leads to the peak widening. The present paper studies this phenomena and develops a signal processing technique for the filtering of the modes.

Guided waves (GW) are one of the most promising tools for structural health monitoring (SHM). They allow fast inspection of a large area. Thus GW based SHM is finding applications in several fields like aerospace, automotive, wind energy, etc. Recently, the use of optical fiber sensors and more specifically fiber Bragg grating (FBG) sensors are getting increasing attention from researchers for GW sensing. The FBG sensors are deployed in the edge-filtering configuration for the sensing. The FBG sensors offer several advantages such as light weight and small size. They also offer the chance to be embedded, and multiplexed. Unfortunately, they are passive sensors and show directional sensitivity. Also they are relatively expensive. Thus there is a need to optimize the usage of the sensors to obtain maximum information with minimum cost. Several different optimization strategies have been proposed in the literature including gradient based as well as evolutionary algorithms. For a non-linear, non-differentiable cost function evolutionary algorithms (EA) such as genetic algorithm, particle swarm optimization, etc. have shown a lot of promise. In this paper the Covariance-Matrix Adaptation Evolution Strategy (CMAES) has been employed for the sensor network optimization for GW based SHM. First, the ability of the algorithm to obtain the global optima is investigated and then the results are compared with the genetic algorithm (GA) which is the most commonly used EA. 3 different criteria for the optimization are used namely, the coverage with 3 actuator-sensor (AS) pairs (cov3), coverage with 1 AS pair (cov1), and the scalarized combination of cov3 and cov1. The results indicate that indeed CMAES is a powerful optimization tool for the sensor placement optimization problem.

Fibre reinforced polymers are commonly used in many industrial branches. The continuous technical progress in the applied science and technology requires more and more advanced materials. The desire to reduce the weight of structure and reduce their manufacturing costs may affect the safety and reliability of designed structure. The structural damage can occur due to many factors which are difficult to predict in advance. The possible hidden damage can be the source of a further mechanical deterioration of the composite structural element. Damage detection in structures is an active research area since decades. Number of studies is carried out in order to meet this objective, particularly, the identification of the damage size and location. Safety and reliability requirements results in development of a variety of non-destructive testing (NDT) techniques. One of the non-destructive testing techniques that can be applied for evaluation of internal structure of non-conductive materials is THz spectroscopy. The method can be used for identification of material structural disintegrations that results in changes of absorption coefficient, refractive index or scattering of THz waves propagating throughout analysed material. The goal of the paper is to study the sensitivity and applicability limitations of THz spectroscopy method used for evaluation and observation of internal structures of composite element with reference to simultaneous influence of temperature and moisture.

Guided waves (GW) allow fast inspection of a large area and hence have received great interest from the structural health monitoring (SHM) community. Fiber Bragg grating (FBG) sensors offer several advantages but their use has been limited for the GW sensing due to their limited sensitivity. FBG sensors in the edge-filtering configuration have overcome the issue with sensitivity and there is a renewed interest in their use. The FBG sensors have directional sensitivity and are passive in nature. This makes it difficult to apply the existing signal processing and damage detection techniques such as tomography, phased array approach etc. As a result, there is a need to develop novel damage detection techniques which take into consideration the specific limitations of the use of FBG sensors. This paper applies the ellipse and hyperbola based techniques for a FBG sensor based network. The paper identifies the specific challenges and applies solutions to overcome those challenges. A comparative assessment of the two methods is presented with the help of experimental data.

Fiber Bragg grating (FBG) sensors have long been thought of as the ideal sensors for structural health monitoring (SHM) due to their small size, light weight, ability to be embedded and ability to be multiplexed. So, FBG sensors have been commonly used for strain based SHM. Their use for ultrasonic guided wave (GW) measurements in the traditional wavelength division multiplexing approach was tried as well, but did not get accepted widely due to the low sensitivity. In the recent times, a renewed interest is seen in the use of FBG sensors for GW measurements using the edge reflection approach which increases the sensitivity several folds. This paper reports a very interesting phenomena, shown by the FBG sensors. The mechanism of the measurement of the incident GW is different based on the relative ratio of the wavelength of the incident GW ( λ G W ) and the grating length (L) of the FBG sensor. For λ G W / L ≫ 1 the propagating wave leads to uniform strain over the FBG resulting in the shift of the Bragg wavelength. For the λ G W / L ≈ 1 the FBG experiences non-uniform strain over the FBG which results in the distortion of the spectrum (widening or narrowing) of the peak. By separating these effects on the FBG, mode filtering may be achieved. The mode-filtered data is then used for damage detection and damage localization. Modifications have been proposed to the elliptical approach which makes use of the filtered modes and removes the unwanted peaks in the signal stemming from the other mode. The methodology is validated on a simple aluminum plate with simulated damage scenarios (added mass). The results indicate that indeed the mode-filtered signals improve the damage localization performance of the actuator-sensor network.

The performance of Lamb wave based monitoring systems, both in terms of diagnosis time and data complexity, can be enhanced by increasing the number of transducers used to actuate simultaneously the guided waves in the inspected medium. However, in case of multiple simultaneously-operated actuators the interference among the excited wave modes within the acquired signals has to be considered for the further processing. To this aim, in this work a code division strategy based on the Warped Frequency Transform is presented. At first, the proposed procedure encodes actuation pulses using Gold sequences. Next, for each considered actuator the acquired signals are compensated from dispersion by cross correlating the warped version of the actuated and received signals. Compensated signals form the base for a final wavenumber imaging meant at emphasizing defects and or anomalies by removing incident wavefield and edge reflections. The proposed strategy is tested numerically, and validated through an experiment in which guided waves are actuated in a plate by four piezoelectric transducers operating simultaneously.

Fibre reinforced polymers are commonly used in many industrial branches. The continuous technical progress in the applied science and technology requires more and more advanced materials. The structural damage can occur due to many factors which are difficult to predict in advance.

We present the results of extended non-destructive testing (ENDT) methods for bond line quality assessment in adhesive joints. The results presented were derived for important application scenarios with regards to aircraft manufacturing and the in-service repair of composite structures. The electromechanical impedance (EMI), laser shock adhesion testing (LASAT), and nonlinear ultrasound scanning (NUS) were used on flat coupon samples, scarfed samples, and curved samples. The EMI method applied to the flat coupons showed some relation of the frequency shift to the level of contamination. For the curved samples, there was insufficient sensitivity to differentiate distinct levels of contamination, while for scarfed samples in most cases both detection and distinction were possible. The LASAT method gave good results for the coupon samples, which were also in accordance with the results of the $${\text{G}}_{\text{IC}}$$ G IC and $${\text{G}}_{\text{IIC}}$$ G IIC tests. For coupon samples with multiple contaminations, we obtained results with varying significance. In the case of NUS, the measurements revealed an increase in nonlinearity affected by contamination at the interphase between the CFRP adherend and the adhesive layer for the majority of scenarios comprising single contamination of flat coupons and scarfed samples. The effect of multiple contaminations was a decrease in nonlinearity for the curved samples.

The first chapter highlights the relevance of both adhesive bonding technology and in-process quality assessment for mastering twenty-first-century challenges in joining functional and lightweight materials like carbon fibre reinforced polymers. The ongoing developments of the relevant technological and regulatory procedures and frameworks are hereby outlined, following trends for data-driven innovation and standardisation. Advances from monitoring process variables towards the in-depth and objective Extended Non-destructive Testing (ENDT) of material-related features are presented, based on methodological and technological innovation and insights from recent European joint research projects like Horizon 2020s ComBoNDT—“Quality assurance concepts for adhesive bonding of aircraft composite structures by advanced NDT”. Introducing ten heuristic principles for quality assessment in bonding processes, a concept is demonstrated for establishing empirically consolidated sets of quantitative material and process-specific correlations between design-relevant joint features and quality data measured during the manufacture or repair of adhesive joints using ENDT. Each correlation is obtained by systematically introducing disturbances of relevant process features identified by experts and is levelled once by linking findings from standardised mechanical tests with ENDT results obtained for joints that have intentionally been manufactured or repaired in an off-specification way. Subsequent chapters will demonstrate the suitability of the broadly applicable process.

Information about exact material properties may be of great importance in many areas where CAD/CAE software is used. It is also a key component of properly operating model-based SHM systems. Unfortunately, composite laminates producers are not providing sufficient and/or precise enough materials data sheets to meet such requirements. This is the reason why material properties identification techniques are attracting considerable interest.

As guided ultrasonic waves are sensitive to changes in the acoustic impedance of the structure, knowledge of the waves behaviour with respect to different materials and varying temperatures is crucial for reliable damage detection. Hence, we present how specific wave characteristics, such as velocity and amplitude, behave for typical materials at various frequencies and temperatures, and how this knowledge can be integrated into a damage detection technique. We also provide an analysis of the thickness of the adhesive layer since we have experimentally observed a strong temperature-related effect on guided wave signals.

This chapter introduces various extended non-destructive testing (ENDT) techniques for surface quality assessment, which are first characterized, then enhanced, and finally applied to assess the level of pre-bond contaminations intentionally applied to carbon fiber reinforced plastic (CFRP) adherends following the procedures described in the previous chapter. Based on two user cases comprising different scenarios that are characteristic of either aeronautical production or repair, the detailed tests conducted on two types of sample geometry, namely flat coupons and scarfed pilot samples with a more complex shape, form the basis for applying the advanced ENDT procedures for the monitoring of realistic and real aircraft parts, as will be described in Chap. 10.1007/978-3-319-92810-4_5. Specifically, the reported investigations were performed to assess the surface quality of first ground and then intentionally contaminated CFRP surfaces using the following ENDT tools: the aerosol wetting test (AWT), optically stimulated electron emission (OSEE), two differently implemented approaches based on electronic noses, laser-induced breakdown spectroscopy (LIBS), Fourier-transform infrared (FTIR) spectroscopy, laser-induced fluorescence (LIF), and laser vibrometry.

Electromechanical Impedance (EMI) method employs high frequencies range in assessing the local structural damage, using various damage metrics. These dam-age metrics are used as a tool to separate quantitatively EMI spectra into classes of the damage presence level and location in the structure. This paper describes the EMI based damage quantification using two piezoelectric transducers on the steel beam structure. The series and parallel combinations were studied and compared with the output of single sensors. The theoretical approach for simulating the serial and parallel connections was proposed and tested. The performance comparison was done in the selected frequency range: 600–800 kHz. The advantage of using the parallel connection was shown on the considered example of the steel beam with and additional mass. The mass was successfully detected and the time needed for measurements was reduced. It was also shown that the simplified model of parallel connection gives the comparison result to the real connection of the two transducers.

In this chapter, we outline some perspectives on embracing the datasets gathered using Extended Non-destructive Testing (ENDT) during manufacturing or repair process steps within the life cycle of bonded products. Ensuring that the ENDT data and metadata are FAIR, i.e. findable, accessible, interoperable and re-usable, will support the relevant stakeholders in exploiting the contained material-related information far beyond a stop/go decision, while a shorter time-to-information will facilitate a prompter time-to-decision in process and product management. Exploiting the value of ENDT (meta)data will contribute to increased performance by integrating all defined, measured, analyzed and controlled aspects of material transformation across process and company boundaries. This will facilitate the optimization of manufacturing and repair operations, boosting their energy efficiency and productivity. In this regard, some aspects that are currently driving activities in the field of pre-process, in-process and post-process quality assessment will be addressed in the following. Furthermore, some requirements will be contemplated for harmonized and conjoint data transfer ranging from a bonded product’s beginning-of-life through its end-of-life, the customization of stand-alone or linked ENDT tools, and the implementation of sensor arrays and networks in joints, devices and structural parts to gather material-related data during a product’s middle-of-life application phase, thereby fostering structural health monitoring (SHM).

This chapter highlights two advances towards a higher maturity of versatile extended non-destructive testing (ENDT) procedures. Full-scale demonstration tests are presented in realistic user application cases that involve typical production or repair scenarios. Subsequently, the investigations used to assess the probability of detection (POD) are detailed for the respective ENDT processes and application-relevant scenarios in a realistic environment. Although some results indicated that some additional in-depth investigations would be even more enlightening, these demonstrations still clearly showed that developments and progress described in the previous chapters have enabled some of the technologies to achieve a maturity that is sufficient to proceed towards industrial implementation. Some ENDT techniques revealed the presence of contaminants on real structural parts with unknown contaminant amounts. For the first time, POD results obtained for ENDT investigations are presented. Some ENDT procedures permitted POD results to be obtained for several scenarios, while others showed technologically relevant POD only for certain scenarios. For two ENDT techniques, determining the POD helped to enhance the respective testing and evaluation procedures. In most of the cases, it was possible to estimate a preliminary quantification of POD by giving the POD90/95. For some techniques, this value was below the lowest contamination degree.

International audience; Research on phononic crystal architectures has produced many interesting designs in the past years, with useful wave manipulation properties. However, not all of the proposed designs can lead to convenient realizations for practical applications, and only a limited number of them have actually been tested experimentally to verify numerical estimations and demonstrate their feasibility. In this work, we propose a combined numerical-experimental procedure to characterize the dynamic behavior of metamaterials, starting from a simplified 2D design to a real 3D manufactured structure. To do this, we consider a new design of a resonator-type geometry for a phononic crystal, and verify its wave filtering properties in wave propagation experiments. The proposed geometry exploits a circular distribution of cavities in a homogeneous material, leading to a central resonator surrounded by thin ligaments and an external matrix. Parametric simulations are performed to determine the optimal thickness of this design, leading to a large full band gap in the kHz range. Full-field experimental characterization of the resulting phononic crystal using a scanning laser Doppler vibrometer is then performed, showing excellent agreement with numerically predicted band gap properties and with their resulting effects on propagating waves. The outlined procedure can serve as a useful step towards a standardization of metamaterial development and validation procedures.

This paper presents a natural frequency-based method for concentrated mass localization in beam-like structures. The localization of a concentrated mass is achieved by developing a novel index using relative natural frequency change (RNFC) curves and measured RNFCs. The index shows the exact location of the concentrated mass for asymmetrical beams and shows general locations of the concentrated mass including the actual location and its fake counterpart at the symmetrical position for symmetrical beams. With the general locations known, some local inspection methods can be used to identify the specific location. The effectiveness of the proposed method is confirmed by experimental investigation of a simply-supported beam and a clamped-free beam carrying a concentrated mass. In the experiment, the natural frequencies of the beam before and after adding a concentrated mass are measured by non-contact measurement using a scanning laser Doppler vibrometer.

The deployment cost of the structural health monitoring (SHM) system is the major argument against the more widespread use of the structural health monitoring techniques. Optimization of sensor placement offers an opportunity to reduce the cost of the SHM system without compromising on the quality of the monitoring approach. Several studies in the area of optimization of sensor placement for SHM applications have been undertaken but the approach has been rather application specific. This article is an attempt to present an unbiased state of the art of the work carried out in the area. The article is targeted towards researchers working in the field of structural health monitoring and optimization of sensor placement as well as practising engineers. This article reviews the work in the area of optimization of sensor placement. It first presents the definition of the optimization problem and then describes each step of the optimization. The current state of the art is then classified based on the techniques for which the optimization of sensor placement has been optimized. The article covers vibration-based monitoring, strain monitoring and elastic wave-based monitoring, as in the eyes of the authors these three techniques are most commonly used and accepted in the SHM community. The article later discusses the different optimization algorithms that have been applied in the literature. The article highlights the different pitfalls of the optimization algorithms and the countermeasures different researchers have proposed to overcome the known shortcomings. In the later section, the multi-objective optimization or the problem definition, keeping in mind the structural as well as executional demands, is discussed. A section has also been developed to showcase the use of optimization of sensor placement techniques’ data fusion–based systems.

Guided waves (GW) allow fast inspection of a large area and hence have received great interest from the structural health monitoring (SHM) community. Fiber Bragg grating (FBG) sensors offer several advantage but their use has been limited for the GW sensing due to their limited sensitivity. But the use of the edge-filtering approach in the remote bonded configuration has enhanced their sensitivity and allows use of these sensors for GW measurement. Although these sensors are light in weight and need no additional wiring there is still significant motivation to reduce the number of sensors while maintaining the quality and reliability of the assessment due to their high cost. In addition, for the safety and reliability of the structures it is of utmost importance that the entire structure can be investigated. Hence it is necessary to optimize the locations of the sensors in order to maximize the coverage while limiting the number of sensors used. The problem posed for the optimization of the FBG sensors for GW is different from any other work in this area due to the directional sensitivity shown by these sensors. Hence a novel optimization problem is developed for the application of FBG sensors for the GW based SHM. This paper develops a genetic algorithm based optimization methodology to incorporate the directional sensitivity. The methodology is shown analytically, using inputs from experimental investigations.

Vibration-based data-driven structural damage identification methods have gained large popularity because of their independence of high-fidelity models of target systems. However, the effectiveness of existing methods is constrained by critical shortcomings. For example, the measured vibration responses may contain insufficient damage-sensitive features and suffer from high instability under the interference of random excitations. Moreover, the capability of conventional intelligent algorithms in damage feature extraction and noise influence suppression is limited. To address the above issues, a novel damage identification framework was established in this study by integrating massive datasets constructed by structural transmissibility functions (TFs) and a deep learning strategy based on one-dimensional convolutional neural networks (1D CNNs). The effectiveness and efficiency of the TF-1D CNN framework were verified using an American Society of Civil Engineers (ASCE) structural health monitoring benchmark structure, from which dynamic responses were captured, subject to white noise random excitations and a number of different damage scenarios. The damage identification accuracy of the framework was examined and compared with others by using different dataset types and intelligent algorithms. Specifically, compared with time series (TS) and fast Fourier transform (FFT)-based frequency-domain signals, the TF signals exhibited more significant damage-sensitive features and stronger stability under excitation interference. The utilization of 1D CNN, on the other hand, exhibited some unique advantages over other machine learning algorithms (e.g., traditional artificial neural networks (ANNs)), particularly in aspects of computation efficiency, generalization ability, and noise immunity when treating massive, high-dimensional datasets. The developed TF-1D CNN damage identification framework was demonstrated to have practical value in future applications.

Line-type damage is abundant in engineering structures that have been in service for a long time. Vibration-based nondestructive testing as a global method has been extensively used in structural damage identification. In this study, a two-dimensional directional continuous wavelet transform (2D-DCWT)-based damage identification algorithm using “Morlet2d” wavelet for line-type damage detection in plate structures is presented. An analytical long narrow notch-type damage model of simply-supported plates is proposed to evaluate line-type damage effect on plate vibration performance. Based on the proposed model, the biggest challenge in parameter selection of wavelet analysis is solved. The fundamental aspects of 2D-DCWT algorithm in term of notch location and orientation are assessed. A multi-mode shape fusion approach is presented to make the 2D-DCWT algorithm apply to the high-order mode shapes. A comparative study with isotropic “Mexcan-hat2d” wavelet-based damage identification is performed, and it demonstrates that the proposed 2D-DCWT algorithm is superior in noise immunity. The efficiency and practicability of the 2D-DCWT algorithm are certified by an experimental modal test on damage detection of an artificially-induced notched aluminum alloy clamped plate using excitation-response system of the Piezoelectric Leadzirconate-Titanate (PZT) and Scanning laser Doppler Vibrometers (SLDV). As demonstrated in both the analytical and experimental investigation, the suggested approach may successfully identify line-type damage in plate-type structures.