This PDF file contains the front matter associated with SPIE Proceedings Volume10972, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Nowadays there is pressure for commercialization of research from founding agencies, but the process of commercialization of research needs time and support to be successful because there are several stages and limits which should be crossed to achieve a market product. Some of them are related to technical issues but some are related to business problems. One possible path of commercialization is creating new start-up companies. During this presentation, problems of research commercialization will be listed and discussed. Some case studies related to SHM technology will be presented. Particularly, the talk will focus on active thermography and applications, predictive maintenance based on AI algorithms and its application, on sensors design and application, and some UAV-based solutions.

Composite materials make up an increasing portion of today’s aerospace structures (see, e.g. Boeing 787 and Airbus 380). These aircrafts’ fuselage, for example, is composed of a laminated composite skin connected to composite stringers and C-frames. Of primary importance is the detection of damage in these built-up structures, whether caused by the manufacturing process or in service (e.g. impacts). A related issue is the characterization of the composite elastic mechanical properties, that can also be related to the quantification of potential damage. Guided elastic waves propagating in the ~100s kHz regime lend themselves to provide the necessary sensitivity to these two conditions (damage and mechanical properties). This presentation will discuss the use of these waves to provide information on both damage and mechanical properties of composite structures that are typically used in modern commercial aircraft fuselages. In particular, a scanning system using air-coupled ultrasonic transducers and transfer function reconstruction will be presented for the detection and the quantification of impact-induced damage in laboratory test panels representative of fuselage construction. An optimization scheme that uses Simulated Annealing and the Semi-Analytical Finite Element (SAFE) technique as the forward model will be used to identify the layer-by-layer elastic properties of the composite skin laminate by observation of the guided wave phase velocity dispersive behavior.

Condition assessment of pipeline networks is critical to ensure operation safety considering that the network around the US is aging. This work aims at combining guided ultrasonic waves and advanced tomographic algorithms to locate corrosion-induced defects in both inner and outer surfaces of steel pipes. Particularly, it employs guided ultrasonic waves that propagate on helical paths around pipes. The novelty of this work is in using high orders of the so-called helical guided ultrasonic waves (HGUW), which can significantly increase the inspection area with a minimum number of sensors. Algebraic reconstruction technique (ART) is then implemented in order to gather information from the wave propagation through the pipe and asses possible locations where defects might exist. To validate the proposed imaging algorithm, numerical simulation and experiments were carried out. The final results suggest that the proposed imaging algorithm can be effectively used for continuous monitoring of corrosion damage in pipelines.

Micro-cracks can be induced in thin monocrystalline silicon wafers during the manufacture of solar panels. High frequency guided waves allow for the monitoring of wafers and characterization of defects. Selective excitation of the first anti-symmetric A₀ guided wave mode was achieved experimentally using a custom-made wedge transducer. The Lamb wave scattered field in the vicinity of artificial defects was measured using a noncontact laser interferometer. The surface extent of the shallow defects varying in size from 30 μm to 100 μm was characterized using an optical microscope. The characteristics of the scattered wave field were correlated to the defect size and the detection sensitivity was discussed.

Stiffened carbon-fibre-reinforced composite structures are extensively used in the aerospace industry for constructing aircraft wings, fuselage, and several other structural components. These structures are often prone to damage due to ageing, cyclic loading and impact. The wave propagation based structural health monitoring technique is widely used for identifying such damage in these structures. This paper presents the analysis of guided wave propagation in a repaired stiffened composite aircraft-wing panel, in order to understand the wave propagation phenomenon in such complex multi-layered structure. Towards this, a coordinated theoretical, numerical and experimental investigation has been carried out. The dispersion curves for the structure are theoretically obtained by using a fast and efficient semi-analytical model to study the dispersion characteristics of the propagating guided waves at the high-frequency range. An extensive finite element based numerical simulation of guided wave propagation in the sample structure is carried out in ABAQUS. Based on the theoretically obtained dispersion curves, different wave modes in the signals are effectively identified. It is observed that the presence of a localized patch repair region in the structure significantly influences the wave mode amplitudes and propagation velocities. Laboratory experiments are then conducted, in order to verify the numerical simulation results. A good agreement is noticed between the simulation and experimental results, in all the cases studied. A series of parametric study is also numerically carried out, in order to check the influence of repaired region size on the propagating guided wave modes in the structure.

In a nonhomogeneous specimen, if the acoustic source and receiving sensors are located in different media then the acoustic source localization becomes very difficult. In this paper, a recently developed source localization technique is extended to non-homogeneous plates by appropriately considering and modeling the refraction phenomenon. The modified technique is applied to two-layered structure. The proposed new technique gives a relatively simple way to localize the acoustic source without the need to solve a system of nonlinear equations, and thus it avoids the problem of multiplicity, converging to local minima instead of global minimum and giving wrong solution. The proposed technique works for both isotropic and anisotropic structures. The finite element simulation shows that this modified technique considering refraction at material interfaces can localize the acoustic source better than when this modification is not considered.

This study focuses on localizing and characterizing acoustic emission (AE) sources in metallic panels with rivetconnected doublers. In particular, a deep learning-based framework is proposed that first performs zonal localization with only one sensor and then depending on the zone in which the source occurs, either finds the coordinates of the source or characterize it based on its source-to-rivet distance. The performance of the framework is assessed in typical scenarios in which the training and testing conditions of the deep networks are not identical, and Hsu-Nielsen sources were carried out for validation.

This paper describes the electromechanical impedance (EMI) method based structural health monitoring using sensor data fusion approach. The data fusion of different attributes is more effective over a single data in achieving reasonable accuracy and precision. The paper investigates an electromechanical impedance (EMI) method applied to the structural health monitoring of sensor network of thin composite plates using distributed sensor data fusion techniques and a single sensor for different levels of delamination. The information from multiple sensors and single sensor is studied in the frequency domain and a new optimized fused criteria of variable admittance (Y) and conductance (G) is explained by damage metric root mean square deviation (RMSD). The experiments are performed on a thin composite plate with attached piezoelectric transducers at different locations and a plate with single transducer with different delamination levels.

Structural health monitoring (SHM) is a growing field with many applications in the aerospace, civil, and mining industries. There has been a desire to develop SHM systems to operate in the microsecond timescale during highly dynamic events. Current efforts have focused on creating an impedance measurement system using the electromechanical impedance (EMI) method technique. In order to consider ways to decrease the time required to measure the impedance of a system, researchers have considered taking measurements at higher frequencies. As part of this research, it is important to consider the sensitivities and capabilities of the sensors to detect changes in the structure at higher frequencies (up to MHz). The goal for this study is to evaluate the sensitivity of the EMI method to damage using a PZT disk bonded to a cantilevered aluminum beam using a finite element (FE) model as well as experimental data. Damage was created by adding holes along the length of the beam, incrementally moving closer to the PZT disk. As a result of this study, an FE model has been developed using previously introduced methods to characterize the material properties of a PZT disk with an optimization algorithm. While initial coefficients resulted in a significant deviation of FE resonance peaks from experimental results, when using the optimized parameters the FE model accurately matches the experimental data. Modeling of the PZT when bonded to the aluminum beam showed a similar trend, there is not an exact match between the model and experimental data. This can be attributed to the material properties of the aluminum beam, which are from a general data sheet for the 6061-T6 and not data from the actual beam. In addition, the bonding layer is not modeled in the FE simulation, which can be a cause of the error in the modeling results. In both the model and experimental data, indications of damage from the impedance curves occurred below 600 kHz.

In this paper idea of load and structural health monitoring (LSHM) system for assessment of composite powerboat has been presented. One of the structural health assessment method utilised in this system is electromechanical impedance (EMI). Authors have presented results related to influence of load and temperature on EMI method. EMI method is based on electromechanical coupling of piezoelectric transducer (PZT) with host structure. Due to this coupling, mechanical resonances of structure can be seen in electrical impedance characteristic of PZT. According to the literature EMI method is sensitive to change of loads in the structure and to changing temperature. Influence of changing operation and environmental conditions should be eliminated by compensation algorithms in order to reduce risk of false alarms in damage assessment system. Results of research related to simple specimen experienced the influence of changing temperature and transverse load and EMI method have been presented. Strains at bending condition for beam have been measured using fibre Bragg grating strain sensors. Some preliminary results of measurements related to powerboat have been also presented and discussed. Measurements have been related to self-diagnosis of piezoelectric transducers using EMI method and to measurements of accelerations using MEMS accelerometer.

Topology has recently emerged as a principle governing unique wave transport phenomena through interface or edge modes that are impurity-immune and potentially unidirectional. In mechanics, these phenomena arise by marrying the notion of material and structure, and are expected to lead to functionalities at the mesoscale that are unattainable solely based on the properties of constituents. Beyond the mere notion of a material, these meta-structures draw their unique characteristics from their finite size and the existence of interfaces. The resulting structural assemblies are expected to feature unprecedented performance in terms of stress wave mitigation, wave guiding, acoustic absorption, and vibration isolation. The seminar illustrates investigations on the effects of nonlinearities on topological properties, and the study of quasi-periodic assemblies. Investigation of the effects of nonlinearities on topological properties allows the exploration of the appearance/robustness of edge and localized modes in the presence of nonlinearities. In addition, the study of lattices with quasi-periodic configurations uncover additional unique properties related to vibration localization in one-dimensional and two-dimensional systems. These can be as interesting if not more useful than the interface modes that are found in periodic structures, as the quasi-periodicity framework provides a consistent methodology that leads to vibration confinement in systems that are not ordered, but are described by deterministic property distributions. Beam and plate structures with quasiperiodic arrangements of grounding springs and lumped masses are presented as structural components which support a variety of localized modes and that are suitable for the experimental characterization of the dynamic behavior of these configurations.

Reciprocity is a fundamental principle in acoustics, posing constraints on the way we process acoustic signals. Breaking reciprocity with spatiotemporal modulations provides an opportunity to design compact, low-energy, integrated non-reciprocal acoustic devices. Here, we design and experimentally demonstrate a space-time modulated programmable metamaterial beam with electromagnet resonators controlled by currents. A numerical approach based on the finite element method is developed for wave dispersion calculations of space-time modulated programmable metamaterials with complex geometries. Unidirectional band gaps are demonstrated experimentally and numerically in a good agreement. We quantify effects of the modulation amplitude and material damping in terms of band gap width and attenuation factor of the unidirectional band gaps in the space-time modulated metamaterial beam. Lastly, the unidirectional band gaps due to the second-order mode coupling caused by strong modulations are identified and examined numerically. Our design as well as the numerical approach provide a practical solution for the applications of non-reciprocal acoustic devices with spatiotemporal modulations.

Non-reciprocal wave propagation in elastic structures has received considerable attention lately. A common mechanism to break elastic wave reciprocity is the use of phononic materials with traveling-wave-like properties. Among the popular methods to study wave dispersion in periodic media are the plane wave expansion and transfer matrix method (TMM). However, owing to the time-variant nature of such non-reciprocal systems, the implementation of both methods requires the truncation of harmonic terms. In this work, we adopt the TMM to extract the dispersion patterns of a moving phononic material with a prescribed velocity. In the presence of a temporal modulation of material properties in one direction accompanied by physical motion in an opposing direction, both effects cancel out and the problem becomes effectively time-invariant. This facilitates the analysis and yields interesting results. Subsequently, we exploit the well-established relationship between the momentumenergy spaces of moving and stationary elastic media to reconstruct exact dispersion diagrams of a stationary space-time-periodic system. The proposed approach provides a platform to incorporate the TMM in the analysis of non-reciprocal time-variant materials. Finally, given the lack of harmonic truncation, the accuracy of the new method does not diminish as the modulating speed increases.

Metastructures exhibit vast potential for novel control of elastic wave propagation through careful engineering of their geometry. Recent research has studied such engineered materials and structures that utilize the coupled magneto-mechanical response of magnetoactive elastomers (MAE) to enable adaptive control of dynamic properties. However, MAEs exhibit viscoelastic behavior that strongly influences their frequency-dependent vibration transmission. Here, we computationally study the influence of viscoelasticity on the vibration transmission of an example metastructure using finite element method (FEM) simulations. Frequency-sweep simulations of the metastructure show strong dips in the transmission spectra that are associated with band gaps. A viscoelastic material model is employed, and the loss factor is parametrically varied to study the influence of different damping intensities. Furthermore, the effect of spatially-varying damping on the transmission spectrum is studied through partitioning of the material models used in the FEM simulations. The results show that increased damping causes a smoothing of structural peaks and widening of the transmission trough, with the maximum attenuation unaffected and even enhanced.

Lightweight engineering structures often suffer from environmental vibration that is difficult to suppress due to its low frequency and multiple polarizations. The emerging field of metastructure offers a practical solution for the lowfrequency vibration reduction without introducing extra isolators that have gigantic size and heavy weight. In this research, 3D printed subwavelength-scale microstructures are embedded into a honeycomb structure to form a lightweight metastructure which can suppress vibrations with different polarizations at targeted frequencies. Moreover, by simply rotating the fabricated resonators from horizontal embedment into vertical embedment, the bandgaps as well as the vibration isolations can be easily switched for different vibration sources. The multi-polarization vibration suspensions have also been demonstrated with strategically positioned resonators following interval and segment arrangements. Finally, metastructures with quasi-zero dynamic stiffness are designed to achieve the ultra-low frequency vibration isolation while maintaining their lightweight.

Confidence in the ability to find defects in NDE and SHM using ultrasound depends on knowlege of the strength of the reflection of the ultrasound from the defect. Roughness of a defect, such as on the surface of a crack, has a strong effect on the reflection, but every rough defect has a different surface, so the usual methods of assessing the sensitivity of inspection cannot be used. Research at Imperial College has pursued a stochastic approach to predict the statistical expected scattering. The talk will provide a summary of the method, its results, and its validation using numerical modelling.

In this research, at first, several general analytical methods such as global matrix method (GMM), transfer matrix method (TMM) and stiffness matrix method (SMM) were employed to predict the guided waves in composites. It was found that GMM is more stable over other methods. However, GMM has missing roots at high frequency. Other methods such TMM provides spurious roots at high frequency and SMM has some missing roots at low frequency despite having computationally efficient. Therefore, an improved analytical method was implemented to calculate wavenumbers corresponding to propagating, evanescent and complex guided wave mode of composite materials. The evanescent and complex wavenumber guided wave modes are very important in studying the interaction between the guided waves and composite damage. The generic analytical method may not work efficiently for finding threedimensional complex wavenumber, frequency roots in composite materials. Christoffel’s equation for composite lamina was used to obtain the eigenvalues and eigenvectors. The eigenvalues and eigenvectors were used to calculate state vectors and field matrix. In this improved analytical method, the exponential part containing wavenumber of the field matrix is expanded as Taylor series expansion with respect to initial wavenumber guess. Then the problem becomes a polynomial eigenvalue problem. Upon solving the eigenvalue problem, it provides wavenumber, frequency solutions for propagating, evanescent and complex guided wave modes. The advantage of the current method is that it is computationally efficient and can provide exact stress mode shapes. As a proof case, the solution was developed first for isotropic (aluminum) materials, and the results were compared with the available analytical solution of the Rayleigh- Lamb equation. Then the solution was extended for unidirectional CFRP composites.

Ultrasonic guided-wave testing can greatly benefit from (1) an ability to provide quantitative information on the damage that is being detected, and (2) an ability to select the best mode-frequency combination for maximum sensitivity to a given type of damage. Achieving these capabilities in complex structures (e.g. nonprismatic structures such as a stiffened panel in aerospace fuselages) is a nontrivial task. This paper will discuss an improved Global-Local (GL) method where the geometrical “local” discontinuity (e.g. the stiffener) is modelled by traditional FE discretization and the rest of the structure (“global” part) is modelled by Semi-Analytical Finite Element (SAFE) cross-sectional discretization. The boundaries of the “local” domain and the “global” domain are then matched in terms of wave displacement and stresses. GL models have been proposed in the past using theoretical (Lamb) wave solutions that only apply to isotropic plates. The authors have also previously studied GL methods using the SAFE approach for application to multi-layered anisotropic plates for which theoretical solutions are either not existent or hard to obtain. This work will extend recent research on these methods by optimizing the Matlab routine that is used to run the GL code, correcting some formulation errors that were present in the previous edition, and studying the specific case of a composite panel stiffened with cocured stringers that is representative of modern commercial aircraft construction (e.g. Boeing 787). The newlyformulated GL method will be shown to provide excellent results that can help designing a guided-wave test on these aircraft components for optimum detection of relevant damage that can be induced by impacts (including skin delaminations, stringer heel cracks, and stringer to skin disbonds). Other applications of the GL methods beyond stiffened aircraft panels will be discussed.

In this paper results of numerical and experimental analysis of S0/A0 elastic wave mode conversion phenomenon at various discontinuities in glass fibre reinforced polymer (GFRP) panels are presented. Analysis of S0/A0 mode conversion effect at Teflon inserts simulating delamination and impact damage with energies 5 J, 10 J and 15 J is conducted. In the case of Teflon inserts circular inserts with the same diameters located at different depth and inserts with various shapes located at the same depth were investigated. Numerical analysis is based on the time domain spectral element method (SEM). SEM computational algorithm is adapted for parallel calculation using graphic processor units (GPUs). 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 the effects of S0/A0 mode conversion it is possible to detect the damage and determine its shape and size. In the research, auxiliary Non-Destructive Testing (NDT) method is also utilised. The aim of its application is to indicate the depth of discontinuity and to prove that delamination was created in the case of impact damage. The auxiliary method is based on terahertz spectroscopy (THz) where the analysis of propagation of electromagnetic waves in the terahertz band is conducted. The THz spectroscopy is a useful technique for damage assessment in the dielectric materials.

In this paper, numerical and experimental investigations of wave damage interaction in metals and composites were conducted. The frequency and direction dependent complex-valued wave damage interaction coefficients (WDIC) were used to model the scattering and mode conversion phenomena of guided wave interaction with damage. These coefficients were extracted from the harmonic analysis of small-size finite element (FE) model with non-reflective boundaries (NRB) and they are capable of describing the amplitude and phase of the scattered waves as a function of frequency and direction. Commercial finite element package ANSYS 17.0 was used to implement and realize the small-size FE model. First, the proposed method was used to extract the WDIC of a through hole in an aluminum plate. Then, the FEM WDIC was compared with the analytical model of A0 wave scattering at through hole, and the experimental validation was performed using scanning laser Doppler vibrometer (SLDV) measurements on an aluminum plate. It is shown that a good agreement between FEM WDIC and experimental WDIC is achieved. Finally, the harmonic analysis of small-size FE models was extended to extract the WDIC of composites. Through hole and delamination in a unidirectional composite plate were investigated. It can be observed that the scattering energy is mainly concentrated in the fiber directions when the A0 waves interact with the through hole.

We have successfully designed a linear two-way one-dimensional acoustic diode by a combined use of the heterogeneity of functionally graded material and the finite size of the practical phononic device. In this study, we further explore the large deformation characteristics of hyperelastic soft material. We first showed through numerical simulation that, the pre-deformation can be well used to tune the working performance of the soft acoustic diode by changing the direction of the unidirectional wave propagation. Experiments are also conducted to validate the theoretical predictions.

The overall mechanical properties of an origami can be programed by its pattern of crease, which introduces various interesting mechanical properties, such as tunable stiffness, multistability and coupled deformations. Once obtaining the knowledge about the properties of the side plates, the creases and the folding procedure, the mechanical response of origami can be completely determined. Therefore, origami with highly designable and tunable abilities offers new possibilities for the metamaterial design. In this research, we aim to combine origami with elastic metamaterials. By introducing the tunable twisting origami structure into the subwavelength-scale resonator design, a three-dimensional elastic metamaterial with low-frequency dynamic performance has been proposed, which, at the same time, has the advantages of lightweight and controllablility. The geometrical nonlinearity of the origami building block is first studied, which indicates that the large structural deformation can be harnessed to tune the effective stiffness of the origami. Further research discovers the quantitative relationship between the overall stiffness and each geometric parameter through the potential energy analysis. Then, the designed origami cell is used as an attachable resonator to control the flexural wave propagation in a metamaterial beam. Finally, both static and dynamic experiments are conducted on the origami cell and the metamaterial beam to verify the tunable stiffness and the on-demand bandgaps, respectively.

Real-time manipulating elastic waves in solid materials is crucial for several applications ranging from active noise and vibration cancellation to inverse methods aiming to either reveal or dissimulate the presence of foreign bodies. Here, we introduce a programmable elastic metasurface with sensing-and-actuating units following feedback control loops. The active units governed by local transfer functions encoded into a digital controller and offering highly flexible and independent phase and amplitude engineering of both transmitted and reflected waves. Through numerical and experimental demonstrations, the programmable metasurface can be a perfect absorber with flexural waves incident from left to right and a perfect transparent mirror with flexural waves incident from right to left. Various other significant demonstrations include steering transmitted (reflected) waves without reflection (transmission) and simultaneously control in both transmitted and reflected domains. Finally, we unveil the relations of the programmable elastic metasurface with nonreciprocity to an effective Willis medium. The design will pave a new efficient way in the field of manipulating of elastic waves.

In recent years, Non-Hermitian physics has drawn considerable attention. Bender and Boettcher suggested that the parity(P) and time(T) symmetries can be responsible for purely real spectra of non-Hermitian operators[1]. Then, it was further developed and extended to optics and acoustics[2, 3]. The PT symmetric system experience PT phase transition at critical point. This kind of PT symmetric system can be used to design single mode lasers, coherent perfect absorb, isolators and invisible sensor. However, it is difficult to apply the PT symmetric system in elastic waves because of its multi-modes and the balance of the loss and gain. In addition, the gain materials are hard to be found in nature. In this work, we first use the mass-spring model to exhibit the PT symmetric system’s characteristics. Then we expand PT symmetric system to continuous model, and analyze a PT symmetric beam structure. We use piezoelectric patches based on active shunted circuit to control the critical point of PT symmetric beam successfully. Results show that this work is useful and has potential to be developed as non-disturbance sensors for nondestructive examination method based on guided waves.

Secondary bonded or embedded sensors are usually adopted in Structural Health Monitoring of composite structures. Each type of sensor has advantages and drawbacks when used separately although their proper integrated combination may improve the overall performance of a SHM system. The aim of the present work is to evaluate the feasibility of an efficient hybrid SHM system able to sense and locate low velocity impacts and to localize eventual impact damages at their early stage with few sensors. A CFRP panel demonstrator with embedded FBGs has been fabricated by NOVOTECH srl through an advanced out-of-autoclave manufacturing process based on the following main phases: production of dry preforms laid down with a laser assisted automated fiber placement robot (AFP); insertion of the FBGs network within the stack of the preform according to the design requirements; high temperature liquid resin infusion process. Disk shaped piezo patches (PZT) are then secondary bonded to the panel. First, an impact sensing mode is enabled resorting to both PZTs and FBGs as receivers, gathering strain waves from the impact location. A material independent technique, that needs neither a priori knowledge of the material properties even for anisotropic plates nor a dense array of sensors, will be used to locate the impact. To assess the integrity of the panel after impact localization, the damage location mode is then performed using the most effective sensors, where the selection of these sensors is possible thanks to the knowledge of the impact location carried out in the previous mentioned impact sensing mode. The damage localization technique is based on guided waves generated by PZTs and sensed by both PZTs and FBGs. Again, thanks to the knowledge of the impact location, a reduced number of PZTs is selected to perform the damage localization. The main advantage of the proposed technique is the capability to sense impact events and to activate the damage localization mode only when required. Moreover, by the knowledge of the impact location, only a few sensors (sources and receivers) need to be used in favor a reduced number of acquisition channels required and acquisition data to be managed.

Composite structures need structural monitoring systems to improve maintenance and design processes. Maintenance may be supported by prompt detection of damage insurgence, moving towards condition-based rather than scheduled approaches. Design can attain adequate safety levels with lighter structures, in force of a continuous knowledge of their status. Required practices and systems are dependent on damage type, each with its own particularity; therefore, complex systems are necessary to respond to such a necessity. Among the many, bonding defects are particularly important. They can be classified as bonding deficiency, as adhesive misses in some parts, or de-bonding, as attachment collapses. Moving from activities performed within OPTICOMS, a project funded within the European flagship Clean Sky 2 JTI, the present work focuses on the preliminary characterization of bonding imperfections effect on selected composite aircraft components. In detail, how local adhesive absence influences static structural response and how this flaw type can be detected through a proprietary algorithm is investigated. A multi-element beam is referred, representing a main spar of the primary structure. A large numerical campaign is conducted on a tuned FE model, implementing different defect layouts, for size and location. Numerical structural response is computed through a representation of a distributed strain sensing system. Supported by a basic theoretical discussion, results are processed and commented, to individuate specific parameters that can describe applied failures. Finally, an in-house code verifies preliminarily its capabilities in exposing presence and size of the applied imperfections, correlating numerical outcomes with performed estimations.

This paper describes the results of an experimental study of guided ultrasonic wave interaction with damage in an alpine ski. The ski is chosen to act as a surrogate for built-up composite structures commonly used in aerospace and many other engineering applications. The study begins by characterizing the optimal actuation signal to generate observable guided wave signals in sensors located along the ski. Using this frequency, the experiment continues by determining the effect of through-thickness hole damage. The damage is located between the sensors to allow for both pitch-catch and pulse-echo approaches to be considered. Data were collected for various hole diameters to determine the sensors’ ability to detect worsening damage in the ski. Interestingly, results showed the damage difference signals collected by comparing damaged and pristine cases actually decreased in value as the diameter of the hole increased. These results indicate further study is required to fully understand how the lateral boundaries of the structure affect the signals obtained from damaged configurations.

Guided acoustic wave techniques have been found to be very effective for damage detection. In this investigation Lead Zirconate Titanate (PZT) transducers are used to generate guided acoustic waves for structural health monitoring of a variety of composite specimens. Multiple sets of composite plate specimens are inspected for impact induced damage detection using PZT transducers. Composite samples are divided into two groups for comparative studies i.e. glass fiber composites and basalt fiber composites. They are damaged by impactors having different levels of impact energy. A chirp signal is excited and propagated through the specimens in a single sided excitation/detection setup to investigate the damages induced by impacts of varying intensity. Signal processing of the recorded signals for damage analysis involved both linear and nonlinear analyses. Linear ultrasonic analysis such as change in the time-of-flight of the propagating waves, Fast Fourier Transform and S-Transform of the recorded signals were tried out while the nonlinear ultrasonic analysis involved the Sideband Peak Count or the SPC technique

Sensorised structures aimed at Structural Health Monitoring implementation in aircraft components are among the most promising approaches for a next future evolution of the current maintenance procedures (and consequently for operative costs reduction) but also for relevant modifications of design rules and manufacturing processes. Adhesive bonds represent an efficient manufacturing process for composite structural parts due to the possibility to co-bond or co-cure together thin plates and stiffeners leading to weight saving, although they have not been massively employed yet. One of the showstoppers for the full implementation of adhesive bonds in composites are the airworthiness certification requirements for composite aircraft structures as presented within the FAA Advisory Circular 20-107B. In that document the general methods for substantiating the limit load capacity of any bonded stiffener, the failure of which would result in catastrophic loss of the airplane, are prescribed. Among the suggested methods, the only one really permitting to achieve the optimal bonding efficiency without the addiction of disbond stoppers (i.e. rivets), is a “repeatable and reliable non-destructive inspection techniques ensuring the strength of each joint”. This paper presents experimental activities that the authors have carried out to characterize two stiffened plates that are “nominally” equal, but obtained by different manufacturing processes; the two plates have been statically characterized performing strength tests, inspected by traditional NDI (ultrasonic C-Scan) and implementing a guided waves based SHM system designed for stringer debonding detection and characterization. A critical analysis of the experimental results as well as a comparison with the expected nominal structural performances will be presented. Afterward a comparison of the three approaches adopted for structural health status characterization before and after stiffener disbonding will be presented looking at the possible implementation of a SHM system committed at satisfying the certification requirements of the AC 20-107B.

Glass composite structures are recently very popular in many branches of industry, such as marine (e.g. ship hull), civil engineering (e.g. composite bridge deck) or energy (e.g. wind turbine blades). Due to high safety requirements related to the objects structural health monitoring systems based on fiber optics techniques are recently widely applied. One of problems that can influence on material durability is moisture introduced into element structure during its manufacturing or exploitation processes. Moisture changes material characteristics, affect element durability and can be a damage origin especially during exposure on negative temperature influence. One of the non-destructive techniques that can be applied for evaluation of internal structure of non- conductive materials (like glass fiber reinforced polymers) is THz spectroscopy. This 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 analyzed material. The paper presents an application of THz spectroscopy for inspection of glass composite samples internal structure. The method was used for evaluation of internal material structure as well as detection, localization and determination of size of internal damage due to influence of moisture and exposure on negative temperature. During analysis the limitations of proposed method will be determined.

Advanced composite materials have gained popularity in high-performance structural designs applications (like aircraft, yachts, wind turbine blades) that require lightweight components with superior mechanical properties in order to perform in demanding service conditions as well as provide energy efficiency and safety to mankind and nature. Due to high safety requirements related to the objects structural health monitoring systems based on fiber optics techniques are recently widely applied. One of the problems that can influence on material durability is moisture introduced into element structure during its manufacturing or exploitation processes. Moisture changes material characteristics, affect element durability and can be a damage origin especially during exposure on negative temperature influence (resulting in burst or ripping of an element). One of the non-destructive techniques that can be applied for evaluation of the internal structure of composite materials (like glass and/ or carbon fiber reinforced polymers) is infrared thermography (pulse and/ or vibrothermography). The methods can be used for identification of material structural disintegration that results in changes in the behavior of temperature field changes. The paper presents an application of infrared thermography (vibrothermography) for inspection of composite (glass fiber reinforced polymers) sample internal structure. The method will be used for detection, localization, and determination of the size of internal damage due to the influence of moisture introduced into material structure and exposure on negative temperature influence. During analysis, the limitations of the proposed method will be determined.

In the last decade, Lamb wave based structural health monitoring (SHM) has been gaining attention for real-time monitoring of plate-like structures. Although the capabilities of Lamb wave in detecting cracks in metallic plates have been studied and proven, there is not a systematic method explaining how to predict network coverage of a sparse array of sensors to send and receive Lamb waves. In this study, initial steps are taken to simulate and predict coverage area of simple sensor network configurations for detecting a specific type of under-surface crack. Computer models are developed for Lamb wave propagation simulation using Finite Element Method (FEM). Using the models, an initial framework is developed to estimate coverage area of a sensor network to detect one type of under-surface crack. Using the framework, coverage area of three different simple sensor arrangements to detect horizontal cracks in an aluminum plate is estimated. The first arrangement is a single transducer, whereas the two other arrangements consist of two and three transducers/sensors respectively. Coverage area is increased as number of transducers in the sensor network is increased.

Beamforming with an array of sensors is an advanced signal processing technique for directional signal transmission or reception. This directionality is achieved by phase shifts of received signals of each sensor for the constructive interference of wavefronts, resulting in the amplification of the signal from a particular direction. In this study, the use of an asymmetric sensor array is proposed to reduce the effects of ‘spatial aliasing’, which is typically encountered in the structural health monitoring (SHM) practice when employing beamforming. In this technique, a sensor array is asymmetrically and closely deployed for robust beamforming based source localization. This sensor deployment has a great effect on suppressing the phenomenon that wave signals are constructively shifted even on other directions. The proposed asymmetric sensor array is to reduce the spatial aliasing error without using any advanced signal processing techniques. In order to demonstrate the proposed sensor array technique, several simulation and experimental investigations are carried out with various symmetric and asymmetric sensor arrays. For the experiments, a complicated composite structure including honeycomb core, spars, and holes is used. The performance comparison is then made to demonstrate the performance of the proposed sensor array technique. The superior robustness of the asymmetric sensor array is confirmed by both simulation and experiment results in complicated structures.

Structural Health Monitoring (SHM) and Non Destructive Evaluation (NDE) is considered a necessity for optimized lightweight structures. Thus several SHM strategies have been suggested for assessing the condition of such structures. The key aspect of any SHM method is the choice of the damage sensing feature. Ideally the damage sensing feature should be sensitive to small levels of damage, insensitive to measurements noise and ambient condition changes, like loading conditions, temperature, moisture etc. For beam structures the Neutral Axis (NA) is known as a very informative damage sensing feature. The NA position depends on the condition of the structure alone and it is insensitive to the load level. Through the use of advanced data processing tools like the Kalman Filter (KF), the NA estimation can be made robust in the presence of measurement noise and changing ambient conditions. In this paper, the use of a KF based NA estimation technique is investigated for determining the size of a damage. The methodology is employed in experiments on a thin-walled beam with rectangular cross-section under 4-point bending. Cracks of different lengths are introduced in the beam and based on the strain measurements the NA is estimated. The estimated position of the NA is then correlated with the size of the damage.

Smart sensors based on optical fibers are popular in recent years, and one of them is optical fiber-based acoustic/ultrasonic sensors, which plays a vital role in areas from scientific research to nondestructive testing. However, as the sensitivity of traditional fiber optic sensors is limited, it is necessary to develop highly sensitive optical fiber-based sensors for ultrasound detection. Here, we present a 3D printed polymer-based Fabry-Perot interferometer (FPI) directly on a single mode fiber tip. The fabrication is based on femtosecond laser writing through two-photon polymerization. The resolution can reach up to ~100 nm, which is less than 1/10 wavelength within the C-band. The spectral characteristics of the sensors are presented. Due to the properties of polymer materials, the devices have a higher sensitivity to acoustic waves that can modify the length of the cavity, which can be utilized for designing ultrasonic sensors. However, the optical quality of the fabricated FP sensor is lower, which is not suitable for high-frequency ultrasound detection. In this research, we propose a tunable erbium-doped fiber ring laser with the 3D printed FPI, which acts a wavelength filter and a reflector of the fiber ring laser. The stability and thermal variations around the modal interferometers are investigated. The spectra are symmetric with a maximal power difference about 35 dB between the lasing modes and the average of the side mode suppression ratio, which is tuned into the C-band with a resolution of 0.02 nm. An unbalanced interferometer-based demodulator using a PID controller is presented to demodulate the ultrasonic signal, which is applied directly on the fabricated FPI. The results show that this sensing scheme offers low wavelength drift, good signal to noise ratio and high-power stability, and can therefore be used for acoustic sensing applications.

We experimentally demonstrate the existence and the characteristics of acoustic quantum valley Hall edge states in a continuous topological elastic waveguide. The waveguide is obtained by subtractive manufacturing, hence cutting a non-resonant truss-like lattice from an initially uniform aluminum thin plate. The fabricated lattice includes two different domains characterized by broken space inversion symmetry and contrasted with each other in order to create a physical interface (i.e. a domain wall) capable of inducing a topological transition. Guided modes in the waveguide are generated using piezoelectric actuators and are measured using laser vibrometry. Data show the existence of well-confined edge states with negligible backscattering at the sharp corners along the domain wall. A methodology to precisely excite a uni-directional edge state is also demonstrated. In addition to the experiment results, the coupling between valley modes is also further investigated and linked to a chiral flux of the mechanical energy.

Zero-energy topological oppy edge modes have been demonstrated in families of kagome lattices with geometries that differ from the regular case composed of equilateral triangles. In this work, we explore the behavior of these systems in the limit of continuum elasticity, which is established when the ideal hinges that appear in the idealized models are replaced by ligaments capable of supporting bending deformation, as observed in realistic physical lattices. Under these assumptions, the oppy edge modes are preserved but shifted to finite frequencies, where they spectrally overlap with the acoustic bulk modes. The net result is the establishment of a relatively broad low-frequency regime over which the lattices display strong asymmetric wave transport capabilities. By simply varying the thickness of the ligament of the unit cell, we can obtain a variety of lattices with different localization capabilities. Through theoretical analysis and finite element simulations, we parametrically explore the localization capabilities of different configurations, thus establishing a qualitative relation between the topological descriptors of the unit cell and the effective global transmission properties of the lattice. Using simple elasticity arguments, we provide a mechanistic rationale for the observed range of behaviors. Our study has implications for the design of mechanical filters, structural logic components, and acoustic metamaterials for wave manipulation at large.

While most of the conventional acoustic liner which comprised of a micro-perforated panel (MPP) backed by a honeycomb structure shows good acoustic performance at high or medium frequency range, there is still very challenging for them at low-frequency range. Difference from MPP where resonance frequency of a MPP based liner is much depended on the backing space and shifting the absorption peak of the liner to low-frequency range generally makes the structure bulky and down-grades the absorbing quality, Helmholtz resonators shows easy frequency-tuning ability and outstanding absorption at low-frequency. In this research, by carefully design, the proposed designs of acoustic liner based on multiple Helmholtz resonators (MHR) show very good absorption in a broadband at low-frequency. First, a super unit-cell which is an annulus disk comprised of three segments or Helmholtz resonators is numerically studied to tune the working frequency band into the desired region, to form a broadband working region and to have the supercells absorbing potential. Afterward, the proposed liner constructed by patterning the supercells along the central axis of the liner is studied on absorption by using both commercial code (COMSOL) and theoretical analysis. The results show high absorption (>80%) at low-frequency broadband (400Hz to 650Hz) and good agreement between numerical and theoretical analysis. The proposed design and analysis can be served as effective model and tool for design and constructing advanced acoustic liners.

Floquet Topological Insulators (FTIs) have inspired analogues in photonics, optics, and acoustics, in which non-reciprocal wave propagation in topologically protected materials, with topological immunity against structural defects and disorder, is achieved due to the breaking of time-reversal symmetry induced by time-modulation. This study aims at investigating a possibility for the existence of a mechanical analogue to the Quantum Hall Effect (QHE) in one and two-dimensional periodically modulated lattices. In 1D, the system shows topologically protected one-way edge modes as the time-modulation is turned on, which is demonstrated by the principle of bulk-edge correspondence and transient numerical simulations. The study is then extended to 2D hexagonal lattices to demonstrate the existence of Floquet topological edge modes. The band diagram and helical edge states characteristic of QHE are obtained by using the Plane Wave Expansion (PWE) method. Given the breakage of the time-reversal symmetry, the system switches from the trivial state to the nontrivial topological one that is quantified in terms of topological invariant Chern numbers. Last, the robustness of one-way edge modes and their immunity to backscattering by sharp corners, defects and modulation disorders are analyzed and assessed numerically.

The manipulation of acoustic wave has broad applications in ultrasonic medical equipment,acoustic detection,noise isolation and many other fields. As a way to control acoustic waves, metasurface is not only small in size, simple to manufacture, but also can control acoustic waves in various patterns with high precision. Up till now, most metasurfaces are fixed structures that can only realize one changeless acoustic wave manipulation purpose. This presentation intends to study a new type of tunable ultrasonic metasurface to control different refraction angles or focus plane waves. According to Snell's formula, the refraction angle of a wave can be controlled by the phase gradient along the interface. Here we design a metasurface, which can manipulate wave in different patterns by simple mechanical movement without external circuits and electromagnets. This presentation demonstrates the feasibility of the design based on theory and finite element simulation, and will provide a novel perspective in the field of acoustic wave control research.

Asphalt concrete is one of the most widely used materials in transportation infrastructure, covering the surface of approximately 94% of more than 4 million miles of highways in United States. Self-healing is an intrinsic property of asphalt material, which can reverse the cracking process in asphalt pavements and therefore extend their pavement service life. The present study utilizes an Acoustic Emission (AE) approach to provide quantitative assessment of self-healing of thermally induced cracks in asphalt concrete materials. Asphalt concrete specimens were subjected to eight cooling cycles and effects of resting time between cooling cycles on self-healing were investigated. The AE test results showed gradual degradation in self-healing capability of asphalt mixtures as the material was exposed to increasing numbers of cooling cycles. However, it was also observed that the rate of self-healing degradation was not constant as it was higher at beginning and then gradually reduced until it reached almost zero after the fourth cooling cycle. Moreover, AE results also indicated that the 12 hours of resting time between cooling cycles significantly increased the self-healing by more than 30% and allowed the material to regain most of its self-healing capabilities.

In this paper we present principles of NDE and structural health monitoring (SHM) of rockbolts using ultrasonic guided waves. Specialized techniques and instruments are presented that apply low frequency ultrasound to investigate bolts status, especially its load imposed by movements of the surrounding rock. Guided waves (GW) that propagate in real rockbolts are dispersive and multimodal. The technique based on the use of modal resonances in bolts is presented. In this technique an application tailored ultrasound probe is employed that transmits the high-energy, low frequency (below 100kHz) guided waves and is capable of receiving weak echoes reflected from the bolt-end. The received end-echoes are amplified and sent back in the sing-around mode. The echo, related to the selected guided wave mode, will generate resonant oscillations with frequency that depends on the velocity of that wave mode, bolt's length and also, in result of the acoustoelastic effect, its load. The latter effect can be used for the evaluation of the load imposed on the bolt by means of the sing-around setup that converts wave velocity to the frequency of self-oscillations. In practical applications the frequency band has to be limited by a suitable band-pass filter to avoid frequency jumps due to the presence of resonances generated by other wave modes. Simulation results are presented that facilitate the choice of the frequency band where the acoustoelastic effect is significant and the frequency jumps can be minimized.

This paper proposes a guided wave-based approach for monitoring stress redistribution in prestressing strands under corrosion. The stress dependence of wave velocity is leveraged for stress measurement, while targeting advantageous frequencies of higher-order wave modes to eliminate the geometric effects of corrosion. For practical longterm monitoring scenarios, where sensor reattachment/replacement may introduce artificial noise, a technique for eliminating such effects (called modal modulation) is also proposed. To demonstrate the approach, accelerated corrosion testing was carried out on a strand while actively generating and receiving higher-order modes. The strand was subjected to 29 cycles of accelerated corrosion (reaching 45% mass loss), with the 29th cycle terminated at the simultaneous fracture of three peripheral wires. The measurements from several higherorder modes were processed into a single estimate using a data fusion approach. The data fusion estimate showed good agreement with the measured stress values even under significant surface roughness (up to 20% mass loss), and especially under a large stress increase due to fracture. To evaluate the modal modulation technique, stress estimations obtained without applying the technique were also shown, which yielded incoherent results.

This article presents a nondestructive evaluation (NDE) method to infer the axial stress in thick beams with the aim to extent the methodology to continuous welded rails. The method relies on the propagation of highly nonlinear solitary waves generated at one end of a chain of spherical particles in contact with the beam to be evaluated. The opposite end of the chain is in contact with the beam to be evaluated. Here the waves are reflected back to chain and the hypothesis is that the axial stress influences the amplitude and speed of the reflected waves. To verify this hypothesis a general finite element model of thermally stressed beams was developed and coupled to a discrete particle model able to predict the propagation of the waves along a L-shaped granular medium. The models were validated experimentally. The hypothesis was proven valid by both the numerical and the experimental results. In the future, these findings may be used to refine a NDE method to assess stress in columns, to infer the neutral temperature of continuous welded rails, and to prevent thermal buckling of critical structures.

Harvesting trees that contain internal defects such as knots and cracks are neither financially nor environmentally sustainable. In hardwood plantations, it is impossible to produce sawlogs from knotty or cracked timber. The challenge is to identify internal defects in a timely and cost-effective manner prior to harvesting. The aim of this paper is to investigate non-destructive testing (NDT) methods to rapidly detect the presence of internal defects in standing live trees in plantation plots. The study highlights that whilst several methods exist, few have been actively applied in-field harvesting operations to optimise log handling and to increase transportation efficiencies. Key constraints are portability of the NDT equipment for use in-field, speed versus accuracy of measurements undertaken and the usability of different evaluation approaches for decision-support. In this paper, the field assessment involved using two non-destructive techniques, ground penetrating radar (GPR) and ultrasonics that use electromagnetic and ultrasonic sound waves respectively to penetrate the internal structure of standing trees. These assessment techniques can assist forest growers to more accurately evaluate the quality of growing stems in the field. They also open the opportunity to investigate differences across a wide selection of growing conditions and forest types to generate data that may support the generation of a software algorithm for predictive imputation of likely internal defect rates within particular forests and under particular growing conditions. The plan being to integrate this predictive imputation software into existing geographical information systems owned by industry partners to enable accurate mapping of land areas where high ratios of defects are likely to be detected to further optimise infield harvesting.

Coda wave has been demonstrated to be a powerful tool for non-destructive evaluation and test (NDT) since it is very sensitivity to changes in media. This sensitivity is attributed to the fact that its trailing parts have traveled a large volume and may have traversed the defect region repeatedly. The diffusion equation, describing the propagation of the average energy, is one of the basic theories in current coda wave-based NDT techniques. Diffusion coefficient is usually assumed to be independent of stress changes and defect positions in concrete structures; however, the heterogeneity and inhomogeneity inherent of concrete materials may cause this assumption problematic, especially for large-size concrete structures. Here, a typical four-bending test with varied loads is performed on a real-size reinforced concrete beam. A couple of transducers are installed to cover its top and side surfaces to collect coda waveforms at each loading step. Then diffusion coefficient values are calculated under varied external loads and at multiple receiver locations by applying the diffusion equation to the associated coda wave measurements. The results show a trend that diffusion coefficient values in general increase with loads, but minute cracks break this trend and lead decreases in its values by introducing more tortuous propagation paths. These results are also consistent with the trend in our direct wave velocity measurements. Diffusion coefficient complementing other wave attributes such as direct wave velocity may offer a novel potential approach for concrete structural NDT applications.

This research presents an evaluation of the damage detection capabilities of displacements calculated from strain measurements in beam-like structure. Displacements can provide useful information for the monitoring and assessment of a bridges health and safety. The displacement of a structure is correlated with the curvature of a structure, so any unusual behavior of the structure that alters the curvature will also affect the displacement of the structure. Additionally, monitoring the displacement of a structure is useful for evaluating service limits as excessive displacement are uncomfortable to users and can cause damage to surrounding structures. Direct displacement monitoring of a real-life structure can be challenging, especially for long-term measurements. Because of this, the focus of this research is on in-direct displacement monitoring using strain-based displacement estimation methods with FBG strain sensors. A numerical analysis was initially performed to evaluate the potential of the methods, which illustrated the damage detection capabilities for a strain-based displacement method. Additionally, small-scale laboratory tests were performed using an aluminum beam instrumented with FBG strain sensors. The beam was tested under static and dynamic loading conditions as well as a cantilever and simply supported boundary conditions. Increasing levels of damage were applied to the beam by reducing the cross-section of the beam in one location to represent a crack. Both an evaluation of the method for damage sensitivity and a comparison with additional strain-based damage detection methods are performed.

This paper presents the Scanning Laser Vibrometry (SLV) imaging of fatigue cracks by taking advantage of the nonlinear ultrasonic guided wave scattering and mode conversion phenomena. The investigation starts with the numerical modeling using the Local Interaction Simulation Approach (LISA) to demonstrate the distinctive scattering and mode conversion features at rough fatigue cracks. During the wave crack interactions, nonlinear higher harmonics are generated from Contact Acoustic Nonlinearity (CAN). In addition, the microscale rough crack surface condition may introduce mode conversion between the symmetric and antisymmetric Lamb modes. After the theoretical analysis, SLV experiments are conducted on an aluminum plate, where fatigue cracks are nucleated from a rivet hole. The damage imaging scheme utilizes the post-processing techniques via Fast Fourier Transform (FFT), frequency domain filtering, and Inverse Fast Fourier Transform (IFFT) to eliminate the linear wave field, leaving only the scattered higher harmonics in the images. In this way, the fatigue cracks can be distinguished from structural features such as rivet holes and stiffeners. This paper finishes with summary, concluding remarks, and suggestions for future work.

Previous work has reported how a phased array can be used in two operating modes to detect nonlinear features. Specifically firing each element individually, as in a classic full matrix capture, and firing all elements at once with an appropriate delay to form a physical beamform, as in classic beamforming. If the difference in the energy of these two approaches is compared then nonlinearity in the material can be measured. This approach has proven effective, but greater sensitivity is desired. This paper studies how the phase of the received signal can be used to characterise nonlinearity. The underlying approach is first investigated and it is shown that phase information is preserved in the diffuse ultrasonic field. Then analytical and numerical models are presented to show that a tightly closed crack in a part should lead to an offset in the measured phase. Experimental results are then presented to validate these conclusions and the benefits of such an approach discussed.

The present work aims to address open issues hindering the development of nonlinear wave modulation SHM/NDE methods for the detection of delaminations in composite laminates. Specifically, the mechanisms generating the nonlinear indices are clarified and the relationship of the latter with damage and wave parameters is investigated. A robust and computationally fast, time domain spectral finite element, containing high-order layerwise laminate mechanics is further extended to model delaminated composite strips. Contact mechanisms enabling impacts between the delaminated interfaces are included in the formulation. Simulations of high-frequency antisymmetric and symmetric ultrasonic wave propagation in Carbon/Epoxy strips with various delaminations sizes are presented. The simulations reveal complex nonlinear phenomena involving interactions between wave conversion and contacts in the delaminated region, which subsequently result in frequency harmonics in the dynamic response, manifesting the presence of delamination. The dependence of the generated harmonics and their modulation factor, to the type of assumed contact, the size of damage and the frequency/wavelength of the excited wave are further studied. Finally experimental measurements are used to validate the analytical conclusions.

In this work, reduced order nonlinear state of Lamb wave propagation due to stress-relaxation of composites was experimentally observed. Residual stresses in the composites are developed under tensile-tensile fatigue loading, which reduce over time during relaxation process due to viscoelastic behavior of the polymer matrix. To investigate reduction in nonlinearity of Lamb wave during stress-relaxation, fatigue loading on the composite specimens were conducted at an interval of 75k until 225k cycles for different cyclic frequencies (i.e., 2Hz, 5Hz and 10Hz), and relaxation experiments were conducted for a duration of 8hrs between two successive fatigue loading sequence. Experimental results show a 6-20% reduction acoustic nonlinearity of Lamb wave during relaxation. Reduction in nonlinearity is mainly contributed by stress redistribution at fibers and recovery of plastic strain during relaxation. This technique is imperative to explore long-term performances and conditions of advanced composite structures.

Despite demonstrated effectiveness in characterizing material properties or defect, the evaluation of material acoustic nonlinearity is highly prone to measurement contaminations introduced by various practical factors and the low robustness restricts its application. In order to obtain a precise quantification of the material acoustic nonlinearity in a robust manner, an approach based on the thermal fluctuations in nonlinear features of ultrasonic waves is developed. In this approach, the influence of temperature and defect on the interatomic distance is scrutinized analytically, and on this basis, the nonlinear features of ultrasonic waves linked with the temperature and defect is ascertained explicitly, whereby a thermal sensitivity index is proposed. With this thermal sensitivity index, the material acoustic nonlinearity can be evaluated without being affected by contaminations from practical sources, and therefore the defect which intensifies the material acoustic nonlinearity can be identified in a robust manner. Experimental validation corroborates the theoretical prediction, demonstrating that the proposed thermal sensitivity-based approach is capable of enhancing the robustness of material acoustic nonlinearity evaluation and defect characterization.

Measured by distributed fiber Bragg grating (FBG) sensors, an application and strain analysis of real-time structural health monitoring system for muti-MW scale wind blades during the service process has been established and investigated. The experiments of the multi-MW scale wind turbine were performed at the top of the hill, near the sea and the FBG sensors were mounted at various locations on the internal surfaces of the rotating blades. The feasibility and effectiveness of the system were validated by continuously transmitting the optical signals between the FBG demodulator and the sensors. The internal dynamic strain of the blades during the environmental fatigue loadings were monitored, valuated and given crash alert with FBG sensors through the results of preliminary field tests. Through Hilbert-Huang transform (HHT), the strain data were decomposed into a series of intrinsic mode functions (IMF) and residual component by the empirical mode decomposition (EMD) method under different frequencies.

In general, the distance between the seismic stations is above 100 km and this distance is not small enough for monitoring the structural seismic response. As a result, crowdsourcing will be a good method to acquire the structural seismic response. Nowadays, the smartphone has been popularized and integrates different sensors. Based on the past work, a structure seismic response monitoring system named GroundEye based on smartphones is proposed. Here is the framework of the system. First, people can use software on the smartphone to acquire the structural seismic response parameters including acceleration, the inter-story drift, strain when the seismic comes. Beside the structural seismic response parameters mentioned above, the picture of the buildings and cracks after the seismic could also be taken so that the cracks could be recognized through the deep learning algorithms. Second, these data will be uploaded to the database to draw the intensity map. Third, these structural seismic response parameters monitored will be the database to assess the damage level of buildings. As mentioned above, the GroundEye can collect vast data in the earthquake monitoring fast within a large area with very low cost. And it will be very useful for identifying the health quality of the buildings.

Recently, severe earthquakes, such as the Great East Japan Earthquake in 2011 and the Kumamoto Earthquake in 2016, occur frequently in Japan, and it is urgent task to establish the simple system for evaluating the soundness of wooden houses. From this background, the purpose of this research is estimating maximum story drift angle by using only one accelerometer. As a previous research, there is the method based on pseudo response spectrum for estimating the maximum displacement response, and the possibility of estimating the maximum displacement response by using only one accelerometer is shown. However, the problem of this method is low accuracy of estimation due to the deficient consideration on how to calculate the natural frequency. Therefore, in this paper, in order to improve the accuracy of this method, the authors conducted two detailed evaluations on condition setting for natural frequency calculation, one is how to choose the necessary value from multiple calculation results and another is how to decide the intervals for analysis. The data the authors analyze is the shaking table test data of wooden house with two-by-four method. As a result, the effectiveness of this method is shown..

Leakage of oil and gas pipe systems, water pipes and other pipe networks can cause serious environmental, health and economic problems. In order to minimise the damages brought to the environment, human health and the economic issues, rapid non-destructive detection of pipeline leakage is imperative. In recent works, number of non-destructive testing (NDT) methods was used in detecting this defect in pipeline systems such ultrasonic, magnetic particle inspection, pressure transient and acoustic wave methods. In this study, the acoustic wave method and a modal frequency technique are used to detect leakage in pipeline system. Finite element analysis (FEA) was employed to simulate acoustic wave propagation in fluid-filled pipes with leakage. Furthermore, experimental testing was conducted to validate some of the numerical results. The experiment performed consisted of the measurement of acoustic wave propagation in a straight fluid-filled pipe. The FEA analysis of fluidfilled pipe can be used to simulate the acoustic wave propagation and acoustic wave reflectometry of a fluid-filled pipe with leakage of different using the ACAX element in order for accurate predictions. Also, the measured signal of acoustic wave propagation in pipeline from the experiment can be decomposed and de-noised to identify and locate leakages of different sizes.

In this paper, we report our study on using P(VDF-TrFE) to develop a piezoelectric yarn. A new type of needleless nozzle for electrospinning multiple piezoelectric fibers is developed. It is verified that the developed yarn can withstand a mechanical statin as high as 40%. After 2.5 hours alignment process, the yarn has better aligned fiber bundle and lower hysteresis. Furthermore, it could produce a more linear charge output under different strain ranges. This yarn can be applied to a flexible sensor for large strain sensing.

In the last few years, advancements made in cameras technology, optically-based systems, and computer-aided methods have made three-dimensional digital image correlation (3D-DIC) a robust tool for structural health monitoring (SHM) and extracting structural deformations and geometry profiles. To perform 3D-DIC measurements, the position of cameras relative to each other must be determined. It is achieved by taking several pictures of calibration objects to determine the camera’s extrinsic parameters (i.e., separation distance and orientation in space). This practice can be very cumbersome and cameras calibration difficult to perform for large-sized structures. This is especially true if data is to be acquired from multiple fields of view. This study describes the design of a MEMS-based sensor board to extend 3D-DIC’s capability and allow for easier calibration and measurement. The suggested system relies on a MEMS-based Inertial Measurement Unit (IMU) for determining the spatial orientation of the cameras (i.e., roll, pitch, and yaw angles) and a 77 GHz radar sensor for measuring the relative distance of the stereo cameras. Both systems are integrated on a commercially available microcontroller unit (MCU) that makes the system suitable for low-power applications. In this research, the efforts for programming the sensor board and the performance of the combined IMU-radar system in comparison with traditional instrumentation are described. To finish, the system is used for calculating the extrinsic parameters of a stereophotogrammetry system and results are compared with data obtained from a traditional calibration.

This paper presents the result of simulations performed on absorber geometries of a thermoresistive sensor using ANSYS thermal electrical module. Four absorber geometries with different serpentine structures are used to test the percentage resistance change per unit temperature change. Thermoresistive material poly (3, 4- ethylenedioxythiophene): poly (4-styrenesulfonate) (PEDOT: PSS) with temperature dependent negative temperature coefficient of resistance is used to model the absorber geometry placed on a substrate of BK7 Glass. A heat source at a constant temperature of 36 OC is placed just above the face of the absorber and proper radiative heat transfer boundary conditions were applied, it is similar to bringing your finger closer to the sensor. Simulation results showedthat the percentage resistance change per unit temperature rise is indifferent of the geometrical shape. Calculation of view factor for radiation heat transfer conveys more absorbing area will have more amount of heat transfer and analytical formulation for calculating final temperature change shows its dependency on geometry i.e. a serpentine shaped geometry will have more temperature rise than a perfect square-shaped geometry having same fill factor.

In this paper, a bandgap meta-surface is carefully designed for enhancing the identifiability of nonlinear ultrasonic superharmonics for fatigue crack detection. In the unit cell design stage, modal analysis with Bloch-Floquet boundary condition is performed to obtain the dispersion features of guided waves in the meta-surface. Then, a finite element model (FEM) for a chain of unit cells is simulated to verify the bandgap effect. In practice, due to the inherent nonlinearity from the electronic instrument and bonding adhesive, the corresponding weak superharmonic components will adversely affect the identifiability of the nonlinear characteristics raised by wave crack interactions. In the current approach, the guided waves generated by the transmitter propagate into the structure, carrying the inherent nonlinearity with them. Immediately afterwards, they pass through the meta-surface with optimized transmission of the fundamental excitation frequency and complete mechanical filtration of the second harmonic component. In this way, the appearance and amplitude of the second harmonic in the sensing signal become evidently indicative of the presence and severity of the fatigue crack along the wave path between the meta-surface and the receiver. The proposed method possesses great potential in future SHM and NDE applications. Nonlinear ultrasonic experiments with the designed meta-surface are conducted to verify the theoretical and numerical investigations as well as to demonstrate the practical application of metamaterial in SHM and NDE. The paper finishes with summary, concluding remarks, and suggestions for future work.

Accidental degeneracy is the only known reason behind the degeneration of 3 or more modes, giving a Dirac cone or Dirac-like cone, depending on the position of the occurrence. The generation of triply degenerate points at the center of the Brillouin zone (where the wave number k→ = 0) is rare and only happens accidentally. In this article, it is proposed to execute triple degeneracy using the simplest geometric microarchitecture of phononic crystals (PnCs). The modulation of the crystals can be performed to demonstrate multiple Dirac-like cones at Γ point by using the nondispersive deaf band obtained from the periodic structure. Thus, a deaf band based predictive model of PnCs can be realized, by proving the existence of the deaf band both numerically and experimentally. The claims have been proved and validated using a squared array of cylindrical polyvinylchloride (PVC) inclusions in an air matrix. This phenomenon yields multiple wave guiding patterns that can be practically used in many research fields.

Traditional Lamb wave structural health monitoring (SHM)/nondestructive evaluation (NDE) system employs contact type transducers such as PZT, ultrasonic transducers, and optical fibers. In application, transducer attachment and maintenance can be time and labor consuming. In addition, the use of couplant and adhesives can introduce additional materials on structures, and the interface coupling is often not well understood. To overcome these limitations, we proposed a fully non-contact NDE system by employing pulsed laser (PL) for Lamb wave actuation and scanning laser Doppler vibrometer (SLDV) for Lamb wave sensing. The proposed system is implemented on aluminum plates. The PL Lamb wave excitation is calibrated, and the optimal parameters are obtained. Lamb wave modes are then characterized through 1D wavefield analysis. With the calibrated and characterized system, defect detection and evaluation are achieved on aluminum plates with simulated defects (surfaced-bonded quartz rod, and machine milled crack) through 1D and 2D inspection in both time-space and frequency-wavenumber domains.

This paper presents a Lamb wave virtual time reversal algorithm with transducer transfer function compensation to eliminate the transducer influence for dispersive, multimodal Lamb waves. This virtual time reversal procedure builds upon a complete 2D analytical model for Lamb wave generation, propagation, and reception. The analytical solution shows that, with the transducer transfer function compensation, a perfect reconstruction of the original excitation waveform can be achieved for both symmetric and antisymmetric Lamb wave modes. In addition, the Finite Element Modeling (FEM) and experimental validations are further performed to verify the compensated virtual time reversal procedure. Finally, a time reversal tomography experiment is conducted with a piezoelectric transducer array for structural damage imaging. The Lamb wave virtual time reversal algorithm with transducer transfer function compensation can achieve more accurate and robust damage imaging results. The paper finishes with discussion, concluding remarks, and suggestions for future work.

In recent years, many researchers have explored the use of guided ultrasonic waves for nondestructive testing (NDT). When guided waves are transmitted into a structure, any geometric and material discontinuities in the waves’ path modify these waves. Using appropriate signal processing methods for the waves received at a sensor, information about these features can be extracted. However, little research has been conducted to locate the features and automatically generate maps without using a priori knowledge. For NDT of large-scale structures, such as the wings of an airplane, many (automated) measurements need to be conducted, and localization of identified features on a map is crucial for successful damage detection. Hence, in this work, methods to detect edges are investigated in an effort to generate a map of the structure using Lamb waves. Measurements are conducted with contact and air-coupled ultrasound transducers in laboratory experiments. While the used contact transducers do not exhibit any directional sensitivity, air-coupled transducers are only sensitive to incoming waves from one direction. Therefore, different data processing methods have to be applied, depending on the applied actuator or sensor technology. Even though the experiments are conducted for a pristine aluminum plate, an outlook for composite plates is given as well. In addition, it is explored whether guided-wave based methods also allow for the detection of other structural features, such as stiffeners. The accuracy of the applied identification methods is validated against the structures’ true dimensions. Even though substantial assumptions have to be made, the investigated methods show promise for successful application in real scenarios.

An integral aspect of modern infrastructural engineering is to constantly monitor the health of a structure either actively or passively in order to ensure its safe performance throughout the design life. For passive structural health monitoring, it is important to estimate the location of an acoustic source that may be caused by events such as impact of a foreign object with the structure, failure of a structural element, formation of cracks, etc. Such an acoustic source generates acoustic waves that propagate through the medium. These waves can be captured by ultrasonic sensors mounted on the structure at some pre-selected locations and, subsequently, analyzed to predict the location of the acoustic source. Over the years, several researchers have proposed techniques for acoustic source localization in both isotropic and anisotropic structures. While acoustic source localization in isotropic structures is relatively simple, introduction of anisotropy adds a layer of difficulty to the problem due to the fact that waves do not propagate with the same speed in all directions. This study presents acoustic source localization techniques for anisotropic plates based on the analysis of the wave front shapes typically observed in anisotropic plates and presents experimental verification of the techniques. Three different geometric shapes are considered as the assumed wave front shapes: a rhombus, an ellipse and a parametric curve. A slightly modified version of the rhombus-based technique from the original approach is proposed. The experimental study is performed on two plates with different degrees of anisotropy.

Numerical simulations such as Finite Element (FE) modelling allow the simulation and prediction of guided wave propagation and scattering. Specific considerations concerning the choice of mesh, element type, time step, and analysis method are required to ensure stable and accurate simulations. Modeling considerations and validation against experimental results are shown for two examples of increasing complexity. The approximation of complex geometries and the effects on guided wave propagation are discussed. Examples concerning guided wave scattering at defects in single and multiple metallic layers of plate structures were compared against experimental results.

Total Knee Arthroplasty (TKA) continues to be a common and important orthopedic procedure for many in the United States. Despite recent medical advancements and increasing knowledge in the orthopedic community, it has been determined that 20% of TKA patients are still dissatisfied with their knee replacements. Causes of this failure include septic loosening and wear on the bearing component of the implant. Another cause of failure that has received specific attention from the mechanical community is aseptic loosening, which has been attributed to unbalanced ligaments or misalignment of the implant components. Previous efforts have been made to detect loosening by using passive force sensors such as piezoelectric transducers or strain gauges to detect misalignment. An alternative to this is to perform active sensing or structural health monitoring to evaluate possible loosening before it becomes a critical concern to the patient. One method of structural health monitoring, called the electromechanical impedance (EMI) method, is particularly attractive as it can use a single, compact piezoelectric transducer to determine the state of the host structure. This work is intended to evaluate the ability of the EMI method in sensing loosening between the cement and bone of a TKA tibial tray. This work will utilize real tibial trays implanted into synthetic bone (Sawbone) to evaluate the feasibility of detecting loosening using the EMI method. The intention of this work is to serve as a foundation for further in-vivo and intraoperative studies.

The aim of this research study is to develop a flexible ultrasound transducer capable of determining the blood volume flow. Currently, there are a few different methods of measuring fluid flow inside a vessel using ultrasound. In Doppler shift and time transit flowmeters, a wedge has been used to mount a piezoelectric transducer in order to create a known angle between the direction of fluid flow and the direction of generated wave propagation. In general, the flat nature of piezoelectric transducers has restricted the application of this method to mounting surfaces with known geometry. However, in a recent study, a flexible piezo-composite ultrasonic transducer was developed using PZT-5H and a passive polymer matrix (PDMS). Due to the flexibility of this unique transducer, it can be mounted on surfaces of unknown and varying geometry. In the context of measuring the blood flow rate in a human vessel, the transducer can be integrated into a wearable device capable of determining the orientation and position of the vessel’s path using wave time of flight. In this article, we measured a flow speed using the flexible transducer embedded on a curved surface of a tissue-mimicking material, in which water flows through an artificial flow vessel aligned in a known angular direction. Then, the velocity of the flowing medium in the vessel is estimated by using the Doppler shift method. The experimental results will provide the fundamental background for application of the flexible transducer to the wearable device capable of measuring the blood flow and the pressure.

Total Knee Arthroplasty is an extremely common procedure carried out across the United States. However, despite the extensive research and testing leading to new methods of surgery and improved implant designs, approximately 20% of patients are dissatisfied with their knee replacements. Like any system, there are multiple factors that can lead to failure. These include wear and loosening, which can be caused by a misalignment during surgery or unbalanced ligaments. In order to detect loosening, there have been several attempts to utilize passive sensors, such as piezoelectric transducers and strain gauges, installed in artificial knee replacements to detect a shift in the proper alignment of the implant. There has also been recent work reported that utilized the active electromechanical impedance (EMI) sensing method, which uses a single PZT in order to detect a change in the state of the monitored system, to monitor knee replacements. However, the study isolated the system so that there was no external force applied to the PZTs while the testing was performed. This work is intended to evaluate the reliability of the EMI method for monitoring of total knee replacements under an applied force in order to determine whether or not this method can be utilized in-vivo to evaluate if a replacement has failed before it becomes hazardous to the patient. This work utilizes a rectangular block of artificial bone, a rectangular block of a similar metallic alloy used in tibial trays, and a polyethylene block made of material similar to the polymer used in bearings in order to simulate the tibia, tibial tray, and bearing, respectively. The artificial bone and metallic alloy components are bonded together with bone cement, and a PZT transducer is bonded to the alloy component using superglue. The polymer component is placed on top of the PZT transducer. The system is tested under static load to monitor several stages of artificial damage. This work is intended to serve as a foundation for further in-vivo and intra-operative studies.

The use of medical devices is essential for proper care and treatment of patients in hospitals. Many of these devices have also been the primary cause of pressure ulcers resulting in skin damage. Pressure ulcer formation is preceded by the impairment of cutaneous blood flow in the dermis. Endotracheal tube (ETT) support devices are one such category of medical devices which are attached on the face to secure ETT during invasive ventilation in intensive care units. In this paper we compare an existing design of ETT supporting device and investigate its effect on cutaneous blood flow (CBF) in skin. CBF is measured after it is removed, using Laser Doppler Vibrometry (LDV) measurement technique at multiple points on the upper surface of the hand. Additionally, we propose a novel design of an ETT support device to reduce the pressure ulcer formation. The aim of the proposed inflated support design is to reduce skin friction and the resulting damage, which was achieved by reducing the contact area and using a flexible lightweight material with a modular shape. The results observed on one such fabricated device have been provided here and are compared with the twill tape securement technique for transverse blood flow velocity and vibrations.

Optical stretcher is a tool in which two counter-propagating, slightly diverging, and identical laser beams are used to trap and axially stretch microparticles in the path of light. In this work, we utilized the dual-beam optical stretcher setup to trap and stretch human embryonic kidney (HEK) cells and mammalian breast cancer (MBC) cells. Experiments were performed by exposing the HEK cells to counter-propagating laser beams for 30 seconds at powers ranging from 100 mW to 561 mW. It was observed that the percentage of cell deformation increased from 16.7% at 100 mW to 40.5% at 561 mW optical power. The MBC cells exhibited significantly higher cell stretching compared to HEK cells at the same power (80 mW). Moreover, the minimum trapping power in HEK cells was 80.5mW as compared to 65.2mW in MBC cells. This study provides useful insights into the characterization of cytoskeletal elasticity in different cell types based on non-contact optical cell stretching.

In order to ensure the safety of construction, all kinds of construction machinery are widely applied to the construction site. Tower crane, as a material handling equipment, has the characteristics of wide operating range and large potential energy, and has become the core machinery in the construction site. The tower crane driver’s field of vision is often blocked, which seriously affects the safety of hoisting. To increase the view of tower crane drivers, most of the current monitoring systems will install a camera on the boom above the hook. But this camera can only view the situation around the hook, and it cannot be quantified. Based on this, this paper proposes a hoisting security detection technology based on deep learning. Firstly, the camera in the monitoring system is used to collect data sets. Secondly, the hook and workers are marked in the image. Then, Faster R-CNN is used to train and evaluate the data sets. The results show that the method has high recognition accuracy. However, the worker and the hook are not on a horizontal plane, so a verification test of the relationship between the height and the ratio of pixel length to true length was completed. The results show that the method can convert the ratio of the hook to the ratio of the worker, and then the real distance between the worker and the hook can be calculated.

A safe and healthy road condition plays a supporting role in the public travel and the national economy. Therefore, effective management and maintenance methods have become the key problems that the researchers and engineers are urgently solving, early damage detection and warning are also important for disaster emergency treatment, but some traditional road damage identification methods are often costly and need to be equipped with professional persons. Due to the complexity of pavement conditions, some existing defects datasets are not perfect, although the accuracy is high, they cannot be put into practical use. Based on the object detection technology of deep learning, the author introduced a novel method which is more effective and relatively cheap. In this paper, 5966 images with road damage of different angles and distances were collected, and the damage categories included Lateral Crack, Longitudinal Crack, Pothole and separation, Alligator Crack, and Damage around the well cover which had never been considered in the datasets in any researches. After training with GPU using convolutional neural network, the average precision can reach 96.3%.

Detecting surface damages is vital for keeping concrete bridges structurally healthy and reliable. Currently, most of imagebased detection techniques are based on handcrafted low-level features which make them less applicable to actual images taken under varying environmental conditions. Recent rapid advancement in convolution neural network has enabled the development of deep learning-based visual inspection techniques for detecting multiple structural damages without needing manually-crafted features. However, most deep learning-based techniques are built on two-stage, proposal-driven detectors using less complex image data, which is not promising for practical applications and integration within intelligent autonomous inspection systems. In this study, a faster, simpler single-stage detector is proposed based on the real time object detection technique, You Only Look Once (YOLOv3) for detecting multiple surface damages of concrete bridges. A large field inspection images dataset of bridge damage is used for training and testing of YOLOv3. The original YOLOv3 is further improved by introducing a novel transfer learning method, batch renormalization and focal loss. The results show that the improved YOLOv3 has a detection accuracy of up to 80% and its performance is about 13% better than the original YOLOv3.

Digital image correlation (DIC) is a non-contact, full-field optical measurement method that has been extensively used in various applications like structural health monitoring, material characterization, high temperature testing etc. However, most of the above applications are used in the laboratory, and rarely employed in the actual structure. This is mainly because of, firstly, DIC instruments are expensive and cannot be afforded by ordinary people. Secondly, the corresponding equipment is complicated to operate and cannot be widely used. The built-in camera of the smartphone is becoming more and more high-definition so that it has brought about an opportunity to solve this problem. From this point of view, this study monitored the deformation of compressive concrete blocks by smartphone and DIC technology, and analyzed the changes of displacement and strain field on the surface of concrete blocks under different loads. The location and shape of cracks in the displacement results were compared with those in the actual image as well. The results show the feasibility of using smartphones to monitor the strain of structures.

Sandwich-structured composites consisted of two outer skins and a lightweight internal core. Due to structural and geometrical properties, these composites are light, have a relatively high flexural strength and could have high thermal insulating characteristics. Therefore, such materials are widely used in many industrial applications (e.g. aeronautics, civil engineering, road vehicles, and ships) especially where high strength to weight ratio is required. In such applications, it is often important to control the loads and operation conditions e.g. temperature. Apart from strength requirements, due to the safety issues, there is a big need for equipping structures in structural health monitoring (SHM) systems. One of the promising monitoring methods base on Fiber Bragg Grating (FBG) sensors. It is due to the advantages of FBG sensors such as small size and weight, high corrosion resistance. The paper presents an application of FBG sensors for assessment of load and temperature influences on a composite sample. For this purpose glass fiber - foam sandwich-structured composite sample with embedded FBG sensors was handmade. A relationship between temperature and strain as well as a relationship between static point load and strain determined from FBG sensors measurements were analyzed. Structural responses for static point load applied in selected locations on the sample surface were studied.

In this study, the effect of different void sizes with different void contents are investigated on all coefficients of constitutive coefficients for unidirectional composites. The unidirectional composite can be assumed as a periodic structure. To fulfill this requirement, unit cells with different void contents and different void sizes are simulated. To capture the real effect of void sizes, the unit cells are modeled with different uniform void sizes with a fixed percentage of void content. To quantify all coefficients of material properties in presence of voids, the periodic boundary conditions are applied to the unit cells. The average stresses and strains are obtained using ANSYS interface. The results showed that in the fixed percentage of void content, constitutive coefficients degraded more with the smaller void sizes.

The Surface Response to Excitation (SuRE) method is a guided-wave based Structural Health Monitoring (SHM) technique. Up to date, no analytical model has been developed and validated for the SuRE method. This paper experimentally and analytically investigates the delamination between two plates using the SuRE method in conjunction with the COMSOL Multiphysics software. Simulation results are validated by experimental results. The results showed that the findings from the analytical approach correspond with the experimental results and can be effectively used for studying delamination. This approach can be utilized for different types of structures with similar conditions.