This PDF file contains the front matter associated with SPIE Proceedings Volume 10170, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

This paper presents an overview of recent developments on predictive simulation of guided wave structural health monitoring (SHM) with piezoelectric wafer active sensor (PWAS) transducers. The predictive simulation methodology is based on the hybrid global local (HGL) concept which allows fast analytical simulation in the undamaged global field and finite element method (FEM) simulation in the local field around and including the damage. The paper reviews the main results obtained in this area by researchers of the Laboratory for Active Materials and Smart Structures (LAMSS) at the University of South Carolina, USA. After thematic introduction and research motivation, the paper covers four main topics: (i) presentation of the HGL analysis; (ii) analytical simulation in 1D and 2D; (iii) scatter field generation; (iv) HGL examples. The paper ends with summary, discussion, and suggestions for future work.

Composite materials are becoming ever more popular and being used in an increasing number of applications. This because, to meet the users' demand, it is possible to create a new material of given characteristics in a quite simple way by changing either the type of matrix, or reinforcement. Of course, any new material requires characterization for its appropriate exploitation. In this context, infrared thermography (IRT) represents a viable means since it is non-contact, non-intrusive and can be used either for non-destructive evaluation to detect manufacturing defects, or fatigue induced degradation, or else for monitoring online the response to applied loads. In this work, IRT is used to investigate different types of composite materials which are based on either a thermoset, or a thermoplastic matrix, which may be neat, or modified by addition of a percentage of a specific compatibilizing agent, and reinforced with carbon, glass, or jute fibers. IRT is used with a twofold function. First, to non-destructively evaluate, with the lock-in technique, materials before and after impact to either assure absence of manufacturing defects, or discover the damage caused by the impact. Second, IRT is used to visualize thermal effects, which develop when the material is subjected to impact. The obtained results show that it is possible to follow the material bending, delamination and eventual failure under impact and get information, which may be valuable to deepen the complex impact damaging mechanisms of composites

Composite materials find wide ranging applications due to their high strength-to-weight ratio. Due to this increasing dependence on composite materials there is a need to study their mechanical behavior in case of damage. There are several ENDT and SHM methods for the assessment of the mechanical properties each with their set of advantages and disadvantages. The paper presents a comparative study of three distinct damage detection methods (infrared thermography, neutral axis method based on optical strain sensor measurements and terahertz spectroscopy) for the detection of delamination in a simple GFRP beam-like structure.

Each of the methods have their own set of advantages and disadvantages. The terahertz spectroscopy is a specialized technique suitable for detecting deterioration inside the structure, but is not suitable for in performance monitoring. Similarly the infrared thermography technique in the active domain may be used for in situ monitoring but not in in-service assessment. Both methods allow the visualization of the internal structure and hence allow identification of the type and the extent of damage. Fibre optic sensors (especially FBG) due to their small diameter and no need of calibration can be permanently integrated within the sample and applied for continuous dynamic strain measurements. The measured strain is treated as an input for neutral axis (NA) method, which as a damage sensitive feature may be used for in-service monitoring but gives absolutely no information about the type and extent of damage.

The results for damage detection based on proposed comparative studies give a complete description of the analyzed structure.

Understanding the shear strain, viscoelastic response, and onset of damage within bonded composites is critical to their design, processing, and reliability. This presentation will discuss the multidisciplinary research conducted which led to the conception, development, and demonstration of two methods for measuring the shear within a bonded joint – dualplane digital image correlation (DIC) and a micro-cantilever shear sensor. The dual plane DIC method was developed to measure the strain field on opposing sides of a transparent single-lap joint in order to spatially quantify the joint shear strain. The sensor consists of a single glass fiber cantilever beam with a radially-grown forest of carbon nanotubes (CNTs) within a capillary pore. When the fiber is deflected, the internal radial CNT array is compressed against an electrode within the pore and the corresponding decrease in electrical resistance is correlated with the external loading. When this small, simple, and low-cost sensor was integrated within a composite bonded joint and cycled in tension, the onset of damage prior to joint failure was observed. In a second sample configuration, both the dual plane DIC and the hair sensor detected viscoplastic changes in the strain of the sample in response to continued loading.

In this paper, a full-field ultrasonic guided wave method is proposed to inspect a composite sandwich specimen made for an aircraft engine nacelle. The back skin/core interface of the specimen is built with two fabricated disbond defects (diameters of 12.7 mm and 25.4 mm) by removing areas of the adhesive used to bond the back skin to the core. A laser ultrasonic interrogation system (LUIS) incorporated with a disbond detection algorithm is developed. The system consists of a 1-kHz laser ultrasonic scanning system and a single fixed ultrasonic sensor to interrogate ultrasonic guided waves in the sandwich specimen. The interest area of 400 mm × 400 mm is scanned at a 0.5 mm scan interval. The corresponding full-field ultrasonic data is obtained and generated in the three-dimensional (3-D) space-time domain. Then, the 3-D full-field ultrasonic data is Fourier transformed and the ultrasonic frequency spectra are analyzed to determine the dominant frequency that is sensitive to the disbond defects. Continuous wavelet transform (CWT) based on fast Fourier transform (FFT) is implemented as a single-frequency bandpass filter to filter the full-field ultrasonic data in the 3-D space-time domain at the selected dominant frequency. The LUIS has shown the ability to detect the disbond with diameters of 11 mm and 23 mm which match to the pre-determined disbond sizes well. For future research, a robust signal processing algorithm and a model-based matched filter will be investigated to make the detection process autonomous and improve detectability

The behaviour of eco-composites, when subjected to laser or mechanical impact loadings, is not well known yet. A research was proposed looking at the behaviour of ‘green’ and synthetic composites under impact loading. The study was focused on composites reinforced with short, medium and long fibres. Short fibre composites were made of spruce fibres and ABS. The fibres were used both as received and after a thermal treatment. Another set of samples was made of 60 mm-long flax fibres. Two types of thermoplastic polymers were used as matrices: polypropylene and polylactide. Also a woven eco-composite was investigated. It was made of plain woven hemp fabric impregnated with epoxy resin. A fully synthetic woven composite, used as reference laminate for comparison with ‘green’ composites, was prepared by using a plain weave woven glass fabric impregnated with epoxy resin. Mechanical impacts were performed by means of a falling dart impact testing machine. The specimens were tested at different impact energy levels (from 1J to 5J) by keeping constant the mass of the impactor and varying the drop height. Laser impact tests were performed by means of a high power laser shock facility. All the samples were tested at six different laser shock intensities, keeping constant the shock diameter and the pulse duration. Six assessment techniques were employed in order to analyse and compare impact damages: eye observation, back face relief, terahertz spectroscopy, laser vibrometry, X-ray micro-tomography and microscopic observations. Different damage detection thresholds for each material and technique were obtained.

Currently there are numerous phased array techniques such as Full Matrix Capture (FMC) and Total Focusing Method (TFM) that provide good damage assessment for composite materials. Although, linear methods struggle to evaluate and assess low levels of damage, while nonlinear methods have shown great promise in early damage detection. A sweep and subtraction evaluation method coupled with a constructive nonlinear array method (CNA) is proposed in order to assess damage specific nonlinearities, address issues with frequency selection when using nonlinear ultrasound imaging techniques and reduce equipment generated nonlinearities. These methods were evaluated using multiple excitation locations on an impacted composite panel with a complex damage (barely visible impact damage). According to various recent works, damage excitation can be accentuated by exciting at local defect resonance (LDR) frequencies; although these frequencies are not always easily determinable. The sweep methodology uses broadband excitation to determine both local defect and material resonances, by assessing local defect generated nonlinearities using a laser vibrometer it is possible to assess which frequencies excite the complex geometry of the crack. The dual effect of accurately determining local defect resonances, the use of an image subtraction method and the reduction of equipment based nonlinearities using CNA result in greater repeatability and clearer nonlinear imaging (NIM).

Composite materials are commonly used in many branches of industry. One method to join or repair CFRP parts is by the use adhesive bonding. There is a search of effective methods for pre-bond assessment of bonded parts and post-bond inspection. Research reported here focuses on post-bond inspection of bonded CFRP plates. In this paper we reported results of two methods. We used noncontact ultrasonic testing (UT) technique as reference method. Ultrasonic testing was made in an immersion tank using phased-array probes. The second method was the electromechanical impedance (EMI). A piezoelectric sensors were surface mounted on each of the samples. Due to piezoelectric effect the electrical response of the sensor is related to mechanical response of the structure to which the sensors is bonded to. Measurements were conducted using HIOKI Impedance Analyzer IM3570. In order to perform a detailed study three samples of each kind were tested. There were three reference samples. The samples with modified adhesive bonds had three levels of severity, so there were three samples with each level of modification. The ultrasonic testing was focused on C-scan analysis taking into consideration the amplitude and time of flight (TOF). Two probes were used, one with 5 MHz frequency, second with 10 MHz. The EMI spectra were gathered up to 5 MHz and they were processed with signal processing algorithms in order to extract differences between reference samples and samples with modified bonds. The UT results provided relevant information about the investigated samples, while the EMI showed sensitivity to the level of adhesive bond modification.

A great deal of research has been devoted to controlling the dynamic behaviors of phononic crystals and metamaterials by directly tuning the frequency regions and/or widths of their inherent band gaps. Here, we present a novel approach to achieve extremely broadband flexural wave/vibration attenuation based on tunable local resonators made of piezoelectric stacks shunted by hybrid negative capacitance and negative inductance circuits with proof masses attached on a host beam. First, wave dispersion relations of the adaptive metamaterial beam are calculated analytically by using the transfer matrix method. The unique modulus tuning properties induced by the hybrid shunting circuits are then characterized conceptually, from which the frequency dependent modulus tuning curves of the piezoelectric stack located within wave attenuation frequency regions are quantitatively identified. As an example, a flexural wave high-pass band filter with a wave attenuation region from 0 to 23.0 kHz is demonstrated analytically and numerically by using the hybrid shunting circuit, in which the two electric components are connected in series. By changing the connection pattern to be parallel, another super wide wave attenuation region from 13.5 to 73.0 kHz is demonstrated to function as a low-pass filter at a subwavelength scale. The proposed adaptive metamaterial possesses a super wide band gap created both naturally and artificially. Therefore, it can be used for the transient wave mitigation at extremely broadband frequencies such as blast or impact loadings. We envision that the proposed design and approach can open many possibilities in broadband vibration and wave control.

Acoustic zero index metamaterials such as density-near-zero metamaterials have received increasing attention due to their potential applications on beam forming, cloaking, wave tunneling, and imaging. High transmission resulted by impedance matching of such zero index metamaterials and surrounding media requires the effective density and inverse bulk modulus to be simultaneously zero. Metamaterials possessing this property are called double zero index metamaterials. The design of double zero index metamaterials needs scatterers with sound speed lower than the background medium, which is extremely challenging for air acoustics because the air sound speed is among the lowest. This challenge can be solved for high order waveguide mode by designing structures with larger thickness. An experimental scan of the pressure field inside our design metamaterial excited by a point source reveals the existence of a Dirac cone at the Brillouin zone center. The measured envelope of the propagating wave inside the metamaterial shows double negative, double positive, and double zero properties below, above, and at the Dirac point,respectively. This result is confirmed by the measured acoustic beam out of the metamaterial. A gapless transition between double negative and double positive acoustic metamaterials is realized. The development of this double zero index metamaterial provides new routes to broaden practical applications of acoustic metamaterials.

One of the most daunting limitations of phononic crystals and acoustic/elastic metamaterials is their passivity: a given configuration is bound to display its phononic properties only around its design point, i.e., working at some pre-determined operating conditions. In the past decade, this shortcoming has inspired the design of phononic media with tunable wave characteristics; noteworthy results have been obtained through a family of methodologies involving shunted piezoelectric elements. Shunting a piezoelectric element means connecting it to a passive electric circuit; tunability stems from the ability to modify the effective mechanical properties of the piezoelectric medium by modifying the circuit characteristics. One of the most popular shunting circuits is the resistor-inductor, which allows the patch-and-shunt system to behave as an electromechanical resonator. A common motif among the works employing shunted piezos for phononic control is periodicity: the patches are typically periodically placed in the domain and the circuits are identically tuned. The objective of this work is to demonstrate that the wave attenuation performance of structures with shunted piezoelectric patches can be improved by leveraging notions of organized disorder. Based on the idea of rainbow trapping broadband wave attenuation obtained by tuning an array of resonators at distinct neighboring frequencies we design and test an electromechanical waveguide structure capable of attenuating waves over broad frequency ranges. In order to emphasize the fact that periodicity is not a binding requirement when working with RL shunts (which induce locally resonant bandgaps), we report on the performance of random arrangements of patches. In an attempt to demonstrate the tunability attribute of our strategy, we take advantage of the reconfigurability of the circuits to show how a single waveguide can attenuate both waves and vibrations over different frequency ranges.

This work reports on the wave transmission characteristics of a hybrid one dimensional (1D) medium. The hybrid characteristic is the result of the coupling between a mechanical waveguide in the form of an elastic beam, and an electrical network. The network configuration investigated is an LC high-pass, consisting of a series of capacitors connected in series through grounded inductors. The capacitors correspond to a periodic array of piezoelectric patches that are bonded to the beam thus coupling the two waveguides. The coupling is characterized by a coincidence frequency/wavenumber corresponding to the intersection of the dispersion curves. At this coincidence frequency, the hybrid medium features attenuation of wave motion as a result of the energy transfer to the electrical network. This energy exchange is depicted in the dispersion by eigenvalue crossing, a particular case of eigenvalue veering. This paper presents the numerical investigations of the wave propagation in the considered medium, and validates the numerical findings with experimental evidence of the wave transmission characteristics. Moreover, the dispersion properties of the electrical network are further studied by varying the inductances thus exploiting the tunability of the periodic electrical domain, i.e: monoatomic and diatmomic unit cell configurations. The LC high-pass network offers several advantages over other configurations, from ease of implementation as the piezoelectric elements are not grounded, to a smaller inductance values to achieve attenuation at a given frequency. Such media could be interfaced with more complex electrical networks to create a new type of smart materials.

We investigate wave dynamics in origami-based mechanical metamaterials composed of bellows-like origami structures, specifically the Tachi-Miura Polyhedron (TMP). One of the unique features of the TMP is that its structural deformations take place only along the crease lines, therefore the structure can be made of rigid plates and hinges. By utilizing this feature, we introduce linear torsional springs to model the crease lines and derive the force and displacement relationship of the TMP structure along the longitudinal direction. Our analysis shows strain softening/hardening behaviors in compression/tensile regions respectively, and the force-displacement curve can be manipulated by altering the initial configuration of the TMP (e.g., the initial folding angle). We also fabricate physical prototypes and measure the force-displacement behavior to verify our analytical model. Based on this static analysis on the TMP, we simplify the TMP structure into a linkage model, preserving the tunable strain softening/hardening behaviors. Dynamic analysis is also conducted numerically to analyze the frequency response of the simplified TMP unit cell under harmonic excitations. The simplified TMP exhibits a transition between linear and nonlinear behaviors, which depends on the amplitude of the excitation and the initial configuration. In addition, we design a 1D system composed of simplified TMP unit cells and analyze the relationship between frequency and wave number. If two different configurations of the unit cell (e.g., different initial folding angles) are connected in an alternating arrangement, the system develops frequency bandgaps. These unique static/dynamic behaviors can be exploited to design engineering devices which can handle vibrations and impact in an efficient manner.

Steel structures including bridges are susceptible to cracking, particularly due to fatigue-sensitive details found in older designs. Therefore, one of the major challenges to keep those steel bridges in service is to rehabilitate existing and potential fatigue damage. There are several conventional approaches to extend the fatigue-life of damaged steel members, e.g., drilling a crack stop-hole to reduce the stress concentration at the crack tip as well as welding and bolting of steel plates or adhesive-bonding of fiber-reinforced polymers (FRP) to reduce the overall stresses. Improvement in material properties of FRP and adhesives make them a viable candidate to apply for extending the fatigue-life of steel members. However, drawbacks include the potential for debonding of the adhesive layer and/or interfaces between adhesive and adherents as well as difficulty in monitoring fatigue crack growth after rehabilitation. In this research, a holistic approach is proposed and evaluated for simultaneous extension of fatigue-life and monitoring by integrating a carbon nanotube (CNT)-based sensing layer with an adhesively-bonded FRP reinforcement. CNT-based sensing layers have a nerve-like electric resistance network, which enables distributed sensing capabilities to monitor stress levels, crack growth, and damage progression. Using laboratory-scale experiments, the simultaneous fatigue-life extension and crack monitoring capability of multifunctional CNT-based composites was evaluated. This paper introduces the fundamental concept of integrated fatigue-rehabilitation and monitoring of steel members, presents a laboratory-scale experiment to demonstrate the feasibility and effectiveness, and discusses challenges for implementation in real structures.

In this presentation, we propose a novel design of elastic metamaterial that possesses unique anisotropic mass density and hyperbolic dispersion, which enables subwavelength-scale flexural wave manipulation. The metamaterial unit cell is inspired by kirigami, an ancient art of paper cutting and folding. A three-dimensional kirigami microstructure can be obtained by simply cutting and folding a thin metallic plate. By attaching the resonant kirigami microstructures periodically on the top of a host plate, a hyperbolic metamaterial plate can be manufactured without any perforation that degrades the strength of the pristine plate. A theoretical model based on the classic plate theory and mass-spring model is developed to understand the working mechanism of the elastic metamaterial. Dispersion curves are obtained by using an extended plane wave expansion method. An anisotropic effective mass density tensor is retrieved based on effective medium theory, which explains the different couplings between the local resonance of kirigami microstructure and the global flexural wave propagations in the host plate along two in-plane principal directions. Finally, numerical simulation on an elastic hyperlens is conducted to demonstrate the subwavelength-scale flexural wave control and super-resolution imaging abilities. The advantages of the proposed kirigami-based elastic hyperbolic metamaterial are twofold: (i) simple manufacturing process without perforation in the pristine plate and (ii) subwavelength flexural wave manipulation providing a high signal-to-noise ratio in plate-like engineering structures. Therefore, the proposed elastic hyperbolic metamaterial could be highly promising for high resolution damage imaging in nondestructive evaluation and structural health monitoring.

A novel class of acoustic metamaterial is proposed for directional collimation of a cylindrical source into a plane wave beam. The effect is based on transformation acoustics which retains the exact form of the wave equation under conformal mapping from a circular region to a triangular area. The transformation is adjustable, allowing the acoustic energy to be equally radiated in three directions, or preferentially in a single direction. Importantly, the material properties in the physical domain are isotropic and therefore practically realizable. Two example devices are proposed using cylindrical elastic shells in water as the metamaterial elements and demonstrated using full wave simulations. This approach has potential applications beyond acoustic antenna design in beam-steering and wavefront manipulation.

When a set of resonators is attached to a master structure, a bandgap opens in the vicinity of the resonance frequency. Then, using piezoelectric circuitry for instance, the spring constant coupling the resonators to the structure can be tuned thus allowing to actively control the resonance frequency and subsequently the position of the bandgap. In this study, we investigate the consequences of dynamically changing the resonance frequency of a resonant metamaterial on its dispersion diagram. In particular, the resonance frequency is modulated periodically in space and in time at a uniform speed in a wave-like fashion and at low frequencies of the same order of magnitude of the resonance frequency itself. A two-scale asymptotic homogenization approach shows that the modulated resonant metamaterial effectively behave as another resonant metamaterial with a different set of resonance frequencies. Changing the modulation speed reveals interesting effective dynamics whereby the bandgaps of the original metamaterial split, move, condense and merge to form new band structures. The results are illustrated and exemplified through the analytical study of a onedimensional elastic medium coupled with a continuous distribution of spring-mass oscillators resonating at low frequencies. The conclusions point towards possible applications in breaking time-reversal symmetry, active wave control and filtering.

Damping in miniature resonators is a consequence of many factors, one of which is due to interaction with the substrate to which the resonator is mounted. It is common practice to create a model of the resonator that includes a small segment of the substrate plate with a finite element (FE) software in conjunction with absorbing boundary elements. As an alternative to implementing absorbing boundary elements, semi-analytical methods have been developed in which such elements are replaced by analytical expressions for Lamb waves. This approach requires the specification of a harmonic load and the determination of the subsequent harmonic response at a point on the resonator. The modal frequency and damping can then be estimated from the computation of the frequency response function on a frequency grid. In this paper, the approach is demonstrated for single and double cantilever configurations on a plate in the case of plain strain. The influence of the number of selected Lamb modes, mesh density and the size of the modeled plate segment is investigated through parametric studies. Moreover, it is shown that the semi-analytical results are in good agreement with those from conventional transient finite element simulations.

Distributed Point Source Method (DPSM) was developed by Placko and Kundu¹, as a technique for modeling electromagnetic and elastic wave propagation problems. DPSM has been used for modeling ultrasonic, electrostatic and electromagnetic fields scattered by defects and anomalies in a structure. The modeling of such scattered field helps to extract valuable information about the location and type of defects. Therefore, DPSM can be used as an effective tool for Non-Destructive Testing (NDT). Anisotropy adds to the complexity of the problem, both mathematically and computationally. Computation of the Green’s function which is used as the fundamental solution in DPSM is considerably more challenging for anisotropic media, and it cannot be reduced to a closed-form solution as is done for isotropic materials. The purpose of this study is to investigate and implement DPSM for an anisotropic medium. While the mathematical formulation and the numerical algorithm will be considered for general anisotropic media, more emphasis will be placed on transversely isotropic materials in the numerical example presented in this paper. The unidirectional fiber-reinforced composites which are widely used in today’s industry are good examples of transversely isotropic materials. Development of an effective and accurate NDT method based on these modeling results can be of paramount importance for in-service monitoring of damage in composite structures.

In this paper, the wave field has been computationally modeled in the anisotropic plate. The Christoffel’s equation is solved to calculate the phase velocity of different wavefronts present at any point in the problem geometry. The calculated phase velocity is then used to develop the Green’s function. The anisotropic Green’s function was developed by implementing Radon transform and Spectral theorem into governing elastodynamic equation. Then the wave behavior in the anisotropic plate is simulated and studied by instigating the calculated Green’s function into the computational non-destructive evaluation (CNDE) technique, distributed point source method (DPSM). The wave propagation with different actuation angles for the transducer are simulated in plate with various forms of anisotropy such as transversely isotropic, orthotropic and, monoclinic plate. The study was performed such that the different anisotropic wave behaviors can be understood to the maximum extent in simplest form. The actuation angle used is normal incidence angle from the perpendicular to problem geometry surface with the actuation frequency of 1MHz. MATLAB is used for coding and simulation.

Carbon nanotube-based composites have been deeply investigated in recent years. CNTs due to their unique physical properties have been employed for various applications in different disciplines of science and engineering. Due to the remarkable electrical conductivity of CNTs, one of practical applications is related to the development of strain sensing smart coatings. Up to now, high sensitivity strain sensors for micro- and macro-scale applications were proposed. However, controlling electro-mechanical properties of those devices is still a challenging issue.

In order to facilitate the design process and to investigate multi-domain relationships between sensor's parameters and its properties, numerical models and simulations of CNT-based structures have been carried out with the primary focus on investigation of electrical conductivity for various concentrations of CNTs within the composite material. More accurate and detailed studies include analysis of the influence of deformation on changes in conductivity. However, due to significant microstructural complexity of the system (i.e. large number of CNTs within the structure) multiscale modeling and analysis approach must be employed.

The main objective of this paper is to outline an relationships between micro- and macro-structural properties of CNT-based strain sensors and to discuss guidelines for a multiscale electro-mechanical model based on the Representative Volume Element (RVE) concept. The device employs the change in electrical conductivity of a CNT-based nanocomposite under applied deformation. The study investigates the impact of various micro-scale model parameters (e.g. size of an RVE, CNTs parameters etc.), on the macro scale model. Model parameters convergence studies are performed for different geometrical properties of CNTs and for various sizes of RVEs - revealing their critical mutual relationships. Also, the impact of boundary conditions at the micro-scale RVE structure is discussed.

Acoustic angiography is a contrast-enhanced, superharmonic microvascular imaging method. It has shown the capability of high-resolution and high-contrast-to-tissue-ratio (CTR) imaging for vascular structure near tumor. Dual-frequency ultrasound transducers and arrays are usually used for this new imaging technique. Stacked-type dual-frequency transducers have been developed for this vascular imaging method by exciting injected microbubble contrast agent (MCA) in the vessels with low-frequency (1-5 MHz), moderate power ultrasound burst waves and receiving the superharmonic responses from MCA by a high-frequency receiver (>10 MHz). The main challenge of the conventional dual-frequency transducers is a limited penetration depth (<25 mm) due to the insufficient receiving sensitivity for highfrequency harmonic signal detection. A receiver with a high receiving sensitivity spanning a wide superharmonic frequency range (3rd to 6th) enables selectable bubble harmonic detection considering the required penetration depth. Here, we develop a new dual-frequency transducer composed of a 2 MHz 1-3 composite transmitter and a polyvinylidene fluoride (PVDF) receiver with a receiving frequency range of 4-12 MHz for adjustable harmonic imaging. The developed transducer was tested for harmonic responses from a microbubble-injected vessel-mimicking tube positioned 45 mm away. Despite the long imaging distance (45 mm), the prototype transducer detected clear harmonic response with the contrast-to-noise ratio of 6-20 dB and the -6 dB axial resolution of 200-350 μm for imaging a 200 um-diameter cellulose tube filled with microbubbles.

Piezo devices made of lead-zirconium-titanate (PZT) are known for driving mechanical device for positioning control and vibration actuation. Here we present a new rapid prototyped PZT actuator for potential 2D scanner application. The proposed 5-μm thick film PZT actuator is made by directly deposited on a thin 100 μm thick stainless steel substrate by using an aerosol deposition (AD) method. The actuator features a stable linear vibration and frequency response. Fabrication results, electrical impedance and mechanical response will be presented and discussed.

Force plates are used to detect static and dynamic reaction forces due to presence of stationary or moving objects as well as the location of applied forces. The application of force plates in various biomechanical fields, such as gait analysis, has been widely suggested and investigated in the past. Several sensor technologies like piezoelectrics, capacitance gauges, and piezoresistive sensors are utilized to develop force plates with special characteristics. Among the technologies employed in force plate designs, piezoelectrics present the ability of providing a self-powered sensory system. Recently, it has been suggested to implement piezoelectric transducers as sensors in the tibial bearing of total knee replacement (TKR) implants in order to transform the knee bearing into a force plate with the ability to detect force and contact point location for in vivo knee load analysis. Considering this application, a simplified design of a force plate instrumented with six piezoelectric transducers is presented in this study. The force plate is modeled using a finite element (FE) model to investigate the sensing performance of the system. In order to validate the simulation, a prototype force plate is fabricated and tested under the same loading condition applied on the FE model. The results are presented in terms of measured location and amplitude of applied force measured by the piezoelectric transducers. For the FE simulation, the deviation of the measured location of the applied force from the actual location is obtained as 0.62 mm in the x-direction and 0.13 mm in the y-direction, and the error in the amplitude of the measured force is 0.03% of the applied force. On the other hand, the deviation in the measured location of the force from the experimental test is 0.53 mm in the x-direction and 0.1 mm in the y-direction, while the error in force is 3.6% of the applied force. The small quantities of error in both sensed location and amplitude of applied force obtained from the FE simulation and experimental test results demonstrates the potential of the proposed design to be utilized as the sensor in the knee bearing of TKR implants.

Acoustic radiation force (ARF) is commonly used in ultrasound (US)-based elastography to generate shear waves deep within soft tissue. These waves can be detected with different methods, e.g. contact conventional ultrasound imaging probes or contact free magnetic resonance or optical coherence tomography (OCT). For many clinical applications, however, for instance the eye, a totally non-contact system for generation/detection of mechanical waves is needed. Here, we present a method for efficient non-contact excitation of broadband transverse mechanical waves in soft media. The approach is based on pushing the medium under study with a 1 MHz chirped US wave focused to its surface from air. The US beam reflected from the air/medium interface provides the ARF force to the medium surface launching a transient mechanical wave in the transverse (lateral) direction. The design and performance of the air-coupled transducer is discussed. The focal zone, peak pressure and acoustic intensity are measured for transducers with different numerical apertures. Time and frequency characteristics of the propagating mechanical waves, generated in soft tissue, are tracked with a phase-sensitive ultra-fast frame rate OCT imaging system. Application of the proposed method for non-contact, non-invasive, sub-mm resolution elasticity measurement in soft tissue is proposed.

One of the critical requirements for ensuring the safety of a steam pipe is to monitor the condensed water level under operation. For this objective, the authors initially developed methods and obtained preliminary test results based on the use of ultrasonic pulse-echo transducers and enhanced signal-processing tool; however, the methodology needed further development in order to obtain measurements in turbulent dynamic flow conditions. To improve the reliability of measurements taken in turbulent flow conditions, an experimental system was developed using multiple transducers driven by a multiplexer, and a data acquisition module capable of operating in any flow conditions. The system consists of a simulation testbed, which allows testing the performance over a range of flow rates and water levels and to observe flow conditions and patterns, as well as measure actual water level, flow velocities, wave conditions, etc. In this paper, we present the development details that include description of the testbed for simulating the flow of condensed water, the multiple transducers arrangement, the signal processing method, and the test results of both steady state and turbulent flow.

Secondary cooling presents control challenges. Conventional feedback control systems cannot be used effectively due to the insufficient reliability of temperature sensors. Here we propose a simple optical technique in placing intensity based portable optical sensor for a rapid nondestructive calibration of water distribution and pressure on dummy bar from nozzles in the secondary cooling system. For non-destructive testing, the proposed system would make use of the data collected by the optical sensor array to help calibrate the water nozzles in temperature control in cooling system. In this paper, we will present the design and characterization a new optical water pressure sensor. Initial field test shows this NIR intensity sensor is capable of operating in the specified range with good repeatability and low hysteresis. Experiment result shows that the impedance of sensing material changes relative linearly with relative humidity, and also features fast response (~4s). The overall sensing range of the sensor is from 20 to 80% and the sensitivity is about 2%.

High temperature (HT) environment in liquid metal cooled reactors poses major challenges towards development of ultrasonic transducer which is a key enabling technology for safety of reactors. In the current work, BS-PT (BiScO3- PbTiO3) piezoelectric material based ultrasonic transducer is proposed for the structural health monitoring at HT. Physics based model using finite element method simulates effect of temperature increase on the transduction ability of the BSPT piezoelectric material. Pulse-echo contact measurements are performed up to 260C which is the hot stand by temperature for liquid metal cooled reactors, to study the performance of the acoustic coupling agent and the BS-PT piezoelectric material bonded to a low-carbon steel sample. Experimental contact measurements indicate 6dB reduction in amplitude of the first backwall echo from 20C to 260C. Also, 0.1 MHz reduction in the fundamental and third harmonic resonance is observed in the spectral analysis of the first backwall echo.

Noise is unavoidable and ever-present in measurements. As a result, signal denoising is a necessity for many scientific and engineering disciplines. In particular, structural health monitoring applications aim to detect often weak anomaly responses generated by incipient damage (such as acoustic emission signals) from background noise that contaminates the signals. Among various approaches, stochastic resonance has been widely studied and adopted for denoising and weak signal detection to enhance the reliability of structural heath monitoring. On the other hand, many of the advancements have been focused on detecting useful information from the frequency domain generally in a postprocessing environment, such as identifying damage-induced frequency changes that become more prominent by utilizing stochastic resonance in bistable systems, rather than recovering the original time domain responses. In this study, a new adaptive signal conditioning strategy is presented for on-line signal denoising and recovery, via utilizing the stochastic resonance in a bistable circuit sensor. The input amplitude to the bistable system is adaptively adjusted to favorably activate the stochastic resonance based on the noise level of the given signal, which is one of the few quantities that can be readily assessed from noise contaminated signals in practical situations. Numerical investigations conducted by employing a theoretical model of a double-well Duffing analog circuit demonstrate the operational principle and confirm the denoising performance of the new method. This study exemplifies the promising potential of implementing the new denoising strategy for enhancing on-line acoustic emission-based structural health monitoring.

Elastic metamaterials are sub-wavelength structures with locally resonant components that contribute to the rise of tunable stop bands, i.e. frequency ranges within which waves do not propagate. A new approach is presented here to quantify this stop band behavior by evaluating structural vibrational power in the different constituents of locally resonant metamaterials undergoing axial excitations. It is shown that the power flow patterns match wave propagation information extracted from the dispersion analysis of the metamaterial unit cell, and can thus be used to develop an algorithm that numerically predicts stop band frequencies for finite realizations with given dimensions and a known number of cells.

Heterogeneity in a lattice system has gained continued attention from researchers due to its ability to support interesting localized dynamics and engineering applications. Most studies on the influence of the defects have been done in a one-dimensional monoatomic chain with both linear and nonlinear interactions. However, analysis of defect dynamics in a lattice under on-site potential is still a rare finding. Recently, extreme wave propagation has been demonstrated theoretically and experimentally on a bi-stable lattice with magnetic inter-site force, featuring quartic on-site potential. In this work, the nonlinear dynamics of introducing engineered defects in the form of mass impurities and inter-site forcing disparities on lattices of bi-stable elements are studied. We investigate the effect of the defect presence on the local wave propagation speed and identify the critical conditions that governs the stable propagation of transition waves. With the control of damping, we further observe a special satellite region, where stable transition of wave with intermediate jumps between the stable states of the local unit cell occurs.

Damage imaging of structural components in the field of Non Destructive Evaluations (NDE) and Structural Health Monitoring (SHM) using ultrasonic waves is usually performed by conventional imaging techniques, such as DelayAnd-Sum (DAS), by back-propagating the recorded waveforms to identify locations and size of defects and damages. This technique results in sidelobes and artifacts that worsen the accuracy of the damage identification. Here we propose a novel imaging approach that derives from the well-known technique of Matched Field Processing (MFP), often used in underwater acoustics and seismology. In MFP, the source or damage is located by a matching procedure between measurements (“data vector”) and expected responses (“replica vectors”) computed for each point of the imaging volume. In this work, we apply this matching approach only to selected features extracted from the recorded waveforms. These features, for example time-of-flights or amplitudes, will be selected for multiple modes of propagation of the ultrasonic waves (longitudinal and shear in bulk waves, multiple guided modes in waveguides). By considering multiple features and multiple wave modes, it is possible to increase the performance of this matching procedure, which can be possibly further improved by also combining different signal frequencies and excitation sources in analogy with biomedical ultrasonic imaging. A correlation metric showing high computational efficiency in the image reconstruction process will be tested as matching coefficient. Applications of this imaging approach to a metallic plate with holes and simulated defects will be shown.

In this paper we present a structural health monitoring (SHM) paradigm based on the simultaneous use of ultrasounds and electromechanical impedance (EMI) to monitor waveguides. The paradigm uses guided ultrasonic waves (GUWs) in pitch-catch mode and EMI simultaneously. The two methodologies are driven by the same sensing/hardware/software unit. To assess the feasibility of this unified system an aluminum plate was monitored for varying damage location. Damage was simulated by adding small masses to the plate. The results associated with pitch-catch GUW testing mode were used in ultrasonic tomography, and statistical analysis was used to detect the damages using the EMI measurements. The results of GUW and EMI monitoring show that the proposed system is robust and can be developed further to address the challenges associated with the SHM of complex structures.

In this paper problem of guided wave damping in composite materials is investigated. Material damping is estimated from experimental measurements based on energy of propagating guided waves. Simply Rayleigh damping is introduced into the model in the form of damping matrix proportional to the mass matrix. The numerical model is based on Spectral Element Method (SEM). Numerical model includes the piezoelectric transducer and bond layer between actuator and the host structure. In this paper each ply of composite laminate is simulated by separate layer consisting of 3D brick spectral elements. Numerical results are experimentally validated using Scanning Laser Doppler Vibrometry (SLDV). Guided waves are excited using piezoelectric transducer and registered using non-contact device – the laser vibrometer. Validation is based on signals gathered in dispersed points as well as on full wavefield measurements. The full wavefield measurements are conducted on dense grid of points. In this paper results for simple carbon fiber reinforced polymer are presented. Paper presents result for composite structure for damaged case. Investigated damage is in the form of delamination.

Structural Health Monitoring deals mainly with structures instrumented by secondary bonded or embedded sensors that, acting as both signal generators and receivers, are able to “interrogate” the structure about its “health status”. This innovative approach to the damage analysis is particularly promising for reducing the maintenance costs and eventually the weight of aerospace composite structures, without any reduction of the safety level required. These structures are currently designed and employed with significant reduction of the pristine material allowables to account certain failure mechanisms frequently inducing relatively small hidden damages called Barely Visible Damages, consisting among others in delaminations and/or debondings and being detectable only by specific instruments operated by trained personnel. It has been proved that the propagation of guided waves is affected by the presence of such type of damages, but their effective identification and localization depends on the accurate “tuning” of the wave characteristic (frequency, amplitude, velocity, mode) as well as on the proper selection of the best parameter of the specific wave mode selected and data analysis algorithm. The intent of this paper is to summarize the experiences gained by the authors in selecting the most sensitive parameters according to the type of damage to be investigated in several typology composite plate-like structures.

Internal resonators in lumped spring-mass elastic metamaterials reveal unique wave dispersion characteristics. Using the Bloch-wave analysis and the Transfer Matrix Method (TMM), the band structure of a unit cell of a locally resonant metamaterial shows a band gap (region of near-perfect wave attenuation) despite the lack of any damping elements. This paper presents an analytical closed-form model of a finite metamaterial structure comprising a chain of unit cells to try to understand the band gap behavior. The poles and zeros of the derived transfer functions explain the formation mechanism of the band gap and ties the band structure predictions of the single cell to the structural dynamics of the resultant metamaterial.

We have previously demonstrated an acoustic vortex wave antenna (VWA) based on a metamaterial aperture. This system produced arbitrary angular mode number waveforms,¹ but did not produce pure integer mode vortex waves. In this work we extend our previous result to a design which radiates pure integer vortex modes. By combining an acoustic leaky wave antenna with a ring resonator waveguide we produce integer mode acoustic vortex waves. Further, we computationally show that this spatial mode can be transferred between two opposing acoustic vortex wave antenna in a "pitch - catch" configuration.

The design and modeling of a bar made of functionally graded materials are studied in this paper. The material properties of the bar are harmonically varied along the bar. The bar is mathematically modeled and its governing equation is derived. The mode shapes and the natural frequencies of the system are calculated analytically using the boundary conditions at the two ends of the bar. The wave propagation in the bar is studied and the frequency band gap of the system is found. It is shown that by varying the material properties of the bar, we can design a system with a desired frequency band gap.

The lifetimes of sub THz acoustic phonon modes determine the intrinsic quality factor of nanomechanical resonators, and control the ultimate limits to sensing mass change, liquid density, charge and temperature with such devices. Recent experiments have provided direct measurements of longitudinal acoustic phonon lifetimes in the higher GHz to THz regime for silicon. However, the results do not definitively resolve the relative contributions of intrinsic mechanisms (such as Akhiezer) versus extrinsic mechanisms (such as boundary scattering), particularly at the higher frequencies. This work focuses on understanding how these mechanisms influence phonon transport through acoustic measurements in nanostructures with well-characterized surface morphologies. We employ a femtosecond laser pump-probe setup to excite and measure the lifetimes of longitudinal acoustic phonons in ultrathin silicon membranes with thicknesses down to 36 nm. We show that the phonon lifetime for membranes thicker than 200 nm is limited intrinsically by Akhiezer mechanism. In thinner membranes, boundary scattering is the most dominant dissipation mechanism. We use a surface specularity parameter based on Kirchhoff’s approximation to correctly predict the observed trend. Our results provide insights to understanding thermal and acoustic transport in nanostructures.

Kirigami is the ancient Japanese art of cutting and folding paper, widespread in Asia since the 17th century. Kirigami offers a broader set of geometries and topologies than classical fold/valleys Origami, because of the presence of cuts. Moreover, Kirigami can be readily applied to a large set of composite and smart 2D materials, and can be used to up-scaled productions with modular molding. We describe the manufacturing and testing of a topology of Kirigami cellular structures defined as Open Honeycombs. Open Honeycombs (OHs) can assume fully closed shape and be alike classical hexagonal centresymmetric honeycombs, or can vary their morphology by tuning the opening angle and rotational stiffness of the folds. We show the performance of experimental PEEK OHs with cable actuation and morphing shape characteristics, and the analogous morphing behavior of styrene SMPs under combined mechanical and thermal loading. We also show the dynamic (modal analysis) behavior of OHs configurations parameterized against their geometry characteristics, and the controllable modal density characteristics that one could obtain by tuning the topology and folding properties.

We designed and analyzed a piezo-lens to focus flexural waves in thin plates. The piezo-lens comprises a host plate and piezoelectric patches bonded on the surfaces of the plate. The piezoelectric patches are shunted with negative capacitance circuits. The effective refractive indexes inside the piezo-lens are designed to fit a hyperbolic secant distribution by tuning the negative capacitance values. A homogenized model of piezo-mechanical system is adopted in the designing process of the piezo-lens. The wave focusing effect is studied by the finite element method. Numerical results show that the piezo-lens can focus flexural waves by bending their trajectories, and is effective in a large frequency band. The piezo-lens has the ability to focus flexural waves at different locations by tuning the shunting negative capacitance values. Potential applications of the piezo-lens include energy harvesting, SHM, etc.

Localized and distributed guided ultrasonic waves array systems offer an efficient way for the structural health monitoring for large structures. The detection sensitivity for fatigue cracks depends on the orientation of the crack relative to the location of the sensor elements. Crack-like defects have a directionality pattern of the scattered field depending on the angle of the incident wave relative to the defect orientation and on the ratio of the defect depth and length to the wavelength. From FE simulations it has been shown that for cracks and notches almost no energy is scattered in certain directions from the defect, i.e., the data processing algorithm must take into account that for some transducer combinations no change in the signal even for a significant defect will be detected. The scattered wave field directionality pattern for an incident low frequency A0 Lamb wave mode was predicted from 3D Finite Element simulations and verified from experimental measurements at machined part-through and through-thickness notches using a laser interferometer. Good agreement was found and the directionality pattern can be predicted accurately. The amplitude of the scattered wave is quantified for a systematic variation of the angle of the incident wave relative to the defect orientation, the defect depth, and the ratio of the characteristic defect size to the wavelength. Based on these results the detection sensitivity for crack-like defects in plate structures using guided wave sensors arrays can be quantified.

Guided-wave based Non-Destructive Evaluation (NDE) and Structural Health Monitoring (SHM) systems validation under realistic conditions or environment requires complex setups. Numerical or theoretical approaches are useful to save time and cost associated with experimental tests but the interaction with realistic geometrical (rivets, thickness changes, stiffeners, extrusions) or damage features (fatigue cracks, fillet cracks, delaminations, disbonds) must be accurately captured in order to be representative. In this paper, an experimental methodology is presented for estimating the far-field scattering of geometrical or damage features. The principle is based on the use of a Hankel transform of the measured 3D velocity field in order to evaluate with precision the scattered pattern using a spatially averaged method. Application to scattering of a hole with simulated machined and real fatigue cracks is proposed. It is observed that the simulated machined crack generally used as a reference standard can only model accurately the transmission behaviour while the scattering patterns are only similar when the wavelength is about the size of the crack, limiting the practical use of machined cracks for experimental validation of SHM or NDE systems.

The increasing usage of Carbon Fiber Reinforced Plastics (CFRP) for primary aerospace structures involves dealing with the principal susceptibility of composite laminates to impact loads as well as the occurrence of barely visible impact damages. One special case among the variety of impact sources is the so called blunt impact, which may cause primarily damage to the internal structure. Thus, the assessment of debonding of stiffening elements in CFRP structures poses an attractive application case for Structural Health Monitoring by Guided Ultrasonic Waves. Wave propagation phenomena at impact damages as well as the utilized signal processing to extract a damage related feature (i.e. damage index) contribute to the sensitivity and thus to the reliability of SHM systems. This work is based on data from the EU-funded project SARISTU, where a generic CFRP door surrounding fuselage panel with an integrated sensor network has been built and tested by introducing a large number of impact damages. Wave interaction of stringer debondings of different size and morphology in omega-stringer stiffened structures are examined to highlight the factors contributing to the sensitivity. Common damage indicator formulations for use with imaging algorithms, such as the Reconstruction Algorithm for the Probabilistic Inspection of Damage (RAPID), are applied on data from various damage cases. Furthermore, the difference in detectability of debondings and delaminations as well as the implications on imaging algorithms are examined.

Lamb waves propagating in thin plates and shells are being widely studied for their potential applications in nondestructive inspection of large-scale structures. These structures are generally characterized by the presence of geometrical discontinuities such as stiffeners, mechanical joints or variable thicknesses that affect the propagation characteristics of Lamb waves that can be very similar to those from defects occurring in service (delamination, disbond, etc.). Therefore, the knowledge of the effects of such discontinuities on the propagation of guided waves is essential to avoid their false identification as defects. In this work Lamb waves propagating in a metal plate with a downward step are studied through laboratory experiments. A single 10 mm piezoceramic disk (PZT) bonded to the host structure using cyanoacrylate gage adhesive is utilized for Lamb waves generation and the responses are measured at multiple locations, along a line crossing the step, using a scanning laser Doppler vibrometer (LDV). The interaction of the fundamental Lamb mode A0 with the geometrical discontinuity in the isotropic plate is investigated and discussed.

Nonlinear features of elastic wave propagation have attracted significant attention recently. The particular interest herein relates to complex wave-structure interactions, which provide potential new opportunities for feature discovery and identification in a variety of applications. Due to significant complexity associated with wave propagation in nonlinear media, numerical modeling and simulations are employed to facilitate design and development of new measurement, monitoring and characterization systems. However, since very high spatio- temporal accuracy of numerical models is required, it is critical to evaluate their spectral properties and tune discretization parameters for compromise between accuracy and calculation time. Moreover, nonlinearities in structures give rise to various effects that are not present in linear systems, e.g. wave-wave interactions, higher harmonics generation, synchronism and | recently reported | shifts to dispersion characteristics. This paper discusses local computational model based on a new HYBRID approach for wave propagation in nonlinear media. The proposed approach combines advantages of the Local Interaction Simulation Approach (LISA) and Cellular Automata for Elastodynamics (CAFE). The methods are investigated in the context of their accuracy for predicting nonlinear wavefields, in particular shifts to dispersion characteristics for finite amplitude waves and secondary wavefields. The results are validated against Finite Element (FE) calculations for guided waves in copper plate. Critical modes i.e., modes determining accuracy of a model at given excitation frequency - are identified and guidelines for numerical model parameters are proposed.

Nonlinear damage in the composite materials is developed with the growth of damages in the material under fatigue loading. Nonlinear ultrasonic techniques are sensitive to early stage damages such as, fiber breakages, matrix micro-cracking, and deboning etc. Here, in this work, early stage damages are detected in Unidirectional (UD) carbon fiber composite under fatigue loading. Specimens are prepared according to American Society for Testing and Materials (ASTM) standard. Specimens are subjected to low cycle high load (LCHL) fatigue loading until 150,000 cycles. Sensors are mounted on the specimen used for actuation and sensing. A five count tone burst with low frequency (fc =375 kHz) followed by high frequency (fc =770 kHz) signal, was used as actuation signal. Pitch-catch experiments are collected at the interval of 5,000 cycles. Sensor signals are collected for various excitation voltage (from 5V to 20V, with 5V interval). First Fourier Transform (FFT) of the sensor signals are performed and side band frequencies are observed at around 770 kHz. Severity of damages in the material is quantified from the ratio of amplitude of side band frequencies with the central frequency. Nonlinearity in the material due to damage development is also investigated from the damage growth curve obtained at various excitation amplitude. Optical Microcopy imaging were also performed at the interval of 5,000 to examine developments of damages inside the material. This study has a good potential in detection of early stage damages in composite materials.

Extending a two-dimensional analytical framework previously developed for understanding contact acoustic nonlinearity (CAN) in a beam-like structure bearing a contact crack^[1], this study reports an analytical model for interpreting CAN induced due to the modulation from a “breathing” crack in a plate-like structure on propagating guided ultrasonic waves (GUWs) in a three-dimensional (3-D) scenario. The “breathing” crack is considered, in a 3-D manner, as a second source to excite additional wave fields. Thorough investigation of the interaction between the probing GUWs and the “breathing” crack leads to explicit, analytical and full-field description of additional wave fields. In this study, influences of reflected and diffracted waves by the crack on the motion of crack surfaces are scrutinized, yielding a depiction of the “breathing” behavior of the crack, beneficial for quantifying the crack-induced source at double frequency, with which the crack-induced nonlinearity (i.e. second harmonic) can be evaluated quantitatively, in conjunction with the use of an elasto-dynamic method. A nonlinearity index is consequently defined to represent the severity of the “breathing” crack. Results obtained from the 3-D model are compared with those from a finite element simulation, to affirm good agreement. This model does not request a benchmarking process against baseline signals for evaluation of damage.

Due to crack-induced nonlinearity, ultrasonic wave can distort, create accompanying harmonics, multiply waves of different frequencies, and, under resonance conditions, change resonance frequencies as a function of driving amplitude. All these nonlinear ultrasonic features have been widely studied and proved capable of detecting fatigue crack at its very early stage. However, in noisy environment, the nonlinear features might be drown in the noise, therefore it is difficult to extract those features using a conventional spectral density function. In this study, nonlinear spectral correlation is defined as a new nonlinear feature, which considers not only nonlinear modulations in ultrasonic waves but also spectral correlation between the nonlinear modulations. The proposed nonlinear feature is associated with the following two advantages: (1) stationary noise in the ultrasonic waves has little effect on nonlinear spectral correlation; and (2) the contrast of nonlinear spectral correlation between damage and intact conditions can be enhanced simply by using a wideband input. To validate the proposed nonlinear feature, micro fatigue cracks are introduced to aluminum plates by repeated tensile loading, and the experiment is conducted using surface-mounted piezoelectric transducers for ultrasonic wave generation and measurement. The experimental results confirm that the nonlinear spectral correlation can successfully detect fatigue crack with a higher sensitivity than the classical nonlinear coefficient.

This paper presents a numerical investigation of the nonlinear interactions between multimodal guided waves and delamination in composite structures. The elastodynamic wave equations for anisotropic composite laminate were formulated using an explicit Local Interaction Simulation Approach (LISA). The contact dynamics was modeled using the penalty method. In order to capture the stick-slip contact motion, a Coulomb friction law was integrated into the computation procedure. A random gap function was defined for the contact pairs to model distributed initial closures or openings to approximate the nature of rough delamination interfaces. The LISA procedure was coded using the Compute Unified Device Architecture (CUDA), which enables the highly parallelized computation on powerful graphic cards. Several guided wave modes centered at various frequencies were investigated as the incident wave. Numerical case studies of different delamination locations across the thickness were carried out. The capability of different wave modes at various frequencies to trigger the Contact Acoustic Nonlinearity (CAN) was studied. The correlation between the delamination size and the signal nonlinearity was also investigated. Furthermore, the influence from the roughness of the delamination interfaces was discussed as well. The numerical investigation shows that the nonlinear features of wave delamination interactions can enhance the evaluation capability of guided wave Structural Health Monitoring (SHM) system. This paper finishes with discussion, concluding remarks, and suggestions for future work.

In this study the amplitude and the phase of the structural response of samples of Single Lap Joint (SLJ) subjected to ultrasonic harmonic excitation was evaluated to identify and characterize the defects within the bonded region. Different parameters such as frequency, shape, and amplitude of the response signal coming back from the adhesive joint are key criteria for understanding the quality of the adhesion. Different metallic samples with the same geometry were experimentally tested: the defects were artificially introduced bonding partially two plates and changing the extension of the debonded region: two piezoelectric sensors (one exciting, one receiver) were attached on each of the two bonded plates. In this way, different experimental tests were carried out in order to study the influence of debonded regions on SLJ structural behavior. The structural dynamic response of the debonded samples was investigated and compared with the predictions of numerical models, for each SLJ, introducing viscoelastic properties for the adhesive layer, and applying the harmonic excitation. Moreover the numerical modal analysis was used to understand the experimental results by a proper description of viscoelastic tape behavior. The numerical simulations were used to find correlation between the content of the acquired signals and the defects of adhesion.

Parallel implementation of the time domain spectral element method on GPU (Graphics Processing Unit) is presented. The proposed spectral element method implementation is based on sparse matrix storage of local shape function derivatives calculated at Gauss–Lobatto–Legendre points. The algorithm utilizes two basic operations: multiplication of sparse matrix by vector and element-by-element vectors multiplication. Parallel processing is performed on the degree of freedom level. The assembly of resultant force is done by the aid of a mesh coloring algorithm. The implementation enables considerable computation speedup as well as a simulation of complex structural health monitoring systems based on anomalies of propagating Lamb waves. Hence, the complexity of various models can be tested and compared in order to be as close to reality as possible by using modern computers. A comparative example of a composite laminate modeling by using homogenization of material properties in one layer of 3D brick spectral elements with composite in which each ply is simulated by separate layer of 3D brick spectral elements is described. Consequences of application of each technique are explained. Further analysis is performed for composite laminate with delamination. In each case piezoelectric transducer as well as glue layer between actuator and host structure is modeled.

Due to their excellent strength-to-weight ratio, honeycomb sandwich panels are being increasingly used in lightweight structures, in particular in aircraft and aerospace industry. Delaminations of individual plies in the composite skins or disbonds of a layer in the multi-layer plate structures often remain undetected during visual inspection. Using guided ultrasonic waves, such hidden defects can be detected. For the successful application of ultrasonic nondestructive testing methods, however, wave propagation characteristics have to be well-understood. Recently developed semi-analytical techniques allow for the calculation of dispersion characteristics for many materials. However, the elastic material behavior is often simplified for these calculations. For example, woven composite laminates are modeled as a homogeneous, transversely isotropic plate. While these simplifications only lead to minor errors, the modeling of aluminum honeycomb core sandwich panels with homogeneous, transversely isotropic layers has yet to be validated. In this paper, an efficient numerical approach is used to determine the dispersion characteristics of a honeycomb core layer with and without simplified material behavior. A full 3D-model, including the honeycomb cells, of a small representative volume element of the material is generated using finite elements, and the resulting dispersion curves are compared to the ones obtained from simplified models. In addition to dispersion curves, the displacement fields of the waves are also analyzed.

Nonlinear ultrasonic techniques provide improved damage sensitivity compared to linear approaches. The combination of attractive properties of guided waves, such as Lamb waves, with unique features of higher harmonic generation provides great potential for characterization of incipient damage, particularly in plate-like structures. Nonlinear ultrasonic structural health monitoring techniques use interrogation signals at frequencies other than the excitation frequency to detect changes in structural integrity. Signal processing techniques used in non-destructive evaluation are frequently supported by modeling and numerical simulations in order to facilitate problem solution. This paper discusses known and newly-developed local computational strategies for simulating elastic waves, and attempts characterization of their numerical properties in the context of linear and nonlinear media. A hybrid numerical approach combining advantages of the Local Interaction Simulation Approach (LISA) and Cellular Automata for Elastodynamics (CAFE) is proposed for unique treatment of arbitrary strain-stress relations. The iteration equations of the method are derived directly from physical principles employing stress and displacement continuity, leading to an accurate description of the propagation in arbitrarily complex media. Numerical analysis of guided wave propagation, based on the newly developed hybrid approach, is presented and discussed in the paper for linear and nonlinear media. Comparisons to Finite Elements (FE) are also discussed.

Low-frequency ultrasonic collimated beam generation from radial modes of piezoelectric disc transducers is studied using a coupled electromechanical finite element approach. First, resonance and vibration characteristics of the radial modes of the disc transducers are obtained using an eigenfrequency analysis. The vibration patterns obtained from numerical simulation are compared with those obtained from experiments and are in good agreement. Next, ultrasonic beam profiles in water generated from the radial modes of a piezo-disc are studied. It was found that a free piezo-disc generates a Bessel-beam with multiple side-lobes. In contrast, clamping the lateral edges of the piezo-disc results in a well-collimated central beam with reduced side-lobes. This provides a novel transducer design for low-frequency collimated beam generation for imaging through highly attenuating materials

Phononic crystals (PCs) have attracted plenty of attention during the past two decades, and a lot of work has been devoted to the numerical, theoretical and experimental analysis of the band gaps of the PCs with 1D, 2D and 3D structures, respectively. The band gaps have been found to be related to the topology of the unit cell, filling ratio, contrast of the material properties between matrix and inclusion, and so on. However, they are fixed when the fabrication of corresponding devices is finished in most cases. Usually, biasing fields (e.g. initial stress, initial deformation, pre-existing electric field, external electric field and magnetic field, etc.) can be utilized to tailor the band gaps in flexible and reconfigurable ways. Recently, the instability-induced deformations triggered by external mechanical loadings have been found to be an effective and reversible way to tune the band gaps and the directionality of PCs made from soft materials, such as silicon and rubber. In this project, a novel design of PCs will be proposed, which consists of perforated plate with some individual beams fixed on the boundary of internal holes. When the external mechanical loading applied on the PCs reaches a threshold value, instability-induced buckling will be triggered and the internal beams might be in contact with each other, which will significantly alter the topology of PCs, and therefore effectively tune the band gaps of PCs. A systematical analysis will be carried out to study the influences on the tunability of PCs with different designs through finite element methods (FEM).