This PDF file contains the front matter associated with SPIE Proceedings Volume 10600 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Advanced composite materials have gained popularity in high-performance structural designs applications that require light weight 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. Damage can appear by external impacts or internal failures. The structural damage can occur due to many factors which are difficult to predict in advance, e.g.: sudden impact loads. An external impact can cause an internal damage with no visible marks on the external surfaces of the element. The possible hidden damage can be the source of a further mechanical deterioration of the composite structural element. Recently one of the promising monitoring method is based on Fiber Bragg Grating (FBG) sensors. It is due to the advantages of FBG sensors such as small size and weight, high corrosion resistance or no calibration requirements. Also it is very easy to multiplex FBG sensors and make rosettes arrays that can be implemented on/ into structures. The goal of the research is to analyse impact detection capability of different FBG strain rosettes types. Additionally the sensitivity and area of working of an individual rosette will be considered. The experiment will be carried out on a sandwich plate with the use of impact hammer. The proposed impact detection method by a rosette network can be applied on structures like plane wings, turbine blades or ship masts. The locations of detected impacts can be further analysed by more sophisticated method (terahertz spectroscopy or infrared thermography).

High-frequency ultrasonic sensors are an important sensing technology in structural health monitoring applications. Compared with the traditional PZT transducer as ultrasonic sensors, novel ultrasonic sensors based on optical methods such as micro-ring resonators have gained increased attention. These micro-rings can be as small as a few microns in diameter, which improves their sensitivity to high-frequency ultrasound. In principle, acoustic waves irradiating the micro-ring induce strain, changing the dimensions and refractive index of the waveguides via the elasto-optic effect. This leads to a change of the guided whispering gallery modes (WGMs), which are extremely sensitive to change in the ring radius induced by the ultrasound strain field. Based on our prior research, here we present an integrated high-frequency ultrasonic sensor array based on optical micro-ring resonator array fabricated by direct laser writing. The fabrication has been optimized to provide high optical quality factor to ensure high detection sensitivity. The experiments demonstrate the potential of the polymer micro-ring resonator working as a high-performance ultrasonic sensor. Applications of the integrated ultrasonic sensor array for acoustic-emission ultrasound detection are shown.

Electro-mechanical impedance based damage detection has been shown to detect small changes in structure. Due to the large frequency band which may be assessed with the EMI technique, it has been shown to detect very local damage irrespective of the change in the boundary conditions. Studies have also been carried out to compensate for ambient temperature effects. Unfortunately, the drawback of the local nature of the EMI approach is the relatively low range of sensing. As a result most of the studies using EMI approach are limited to applications where the sensitive region is apriori known. This is not always the case, and thus the study of the technique at the array level is necessary. Thus, the present study tries to establish the approach for optimization of the array of PZTs for EMI based approach. The primary task is to establish the range of the sensors to damage in an anisotropic GFRP plate. The range of the sensors and their directionality then may be used to optimize the sensor placement to ensure the maximum coverage of the plate for damage detection. Once the cost function of the optimization has been established, genetic algorithm (GA) is employed for optimization. GA offers several advantages over brute force based methods as well as other optimization approaches. GA is ideally suited for multi-objective optimization which then paves the way for incorporating other optimization objectives in the search.

Hole-edge damage of joint structure is one type of the most closely watched damages for ultrahigh buildings, bridges, aircrafts, etc., due to stress concentration at the bolt hole and complex load conditions. This paper employs an eddy current array sensor, made by flexible printed circuit technology, bonded on the bolt screw to monitor the growth of hole-edge damage. Coil winding configuration of the eddy current array sensor made by one actuation coil throughout all depth and several sensing coils along the depth is proposed to quantify the damage growth, especially the damage depth. Simulation and experimental study were conducted to verify the ability of quantitatively monitoring the hole-edge damage growth.

The paper presents a damage characterization framework based on a simulation library and matching pursuit algorithm to estimate damage features in typical aerospace structures. The large damage database is generated by numeric simulation. The recent development of the University of Michigan’s Local Interaction Simulation Approach (UM/LISA) is an ideal tool for generating such a damage database in a very efficient manner. It includes capability for piezoelectric coupled field simulation, non-reflective boundary techniques, and contact penalty method for nonlinear guided wave simulation, and can execute on multiple-GPU platform for fast computation. The selected damage identification problem in the paper is modeled as contact interface and simulated in UM/LISA using the contact penalty method. The process first populates a library of possible damage signals using UM/LISA by varying damage parameters, such as crack length, depth and orientation. The matching pursuit decomposes the damage difference signals into atoms and the atom parameters are used as signal features. Then the algorithm evaluates a matching merit metric and its special distribution provides parametric regions of damage presence. A representative model of fatigue cracks on aluminum plate considering various crack scenarios is investigated to test the effectiveness of the algorithm. The relation between the crack features and signal features provides better understanding of the nonlinear interactions between the guided waves and fatigue cracks. The matching quality plots demonstrate that the framework can provide good estimation of the crack parameters.

A nondestructive testing approach capable of evaluating high temperature hydrogen attack (HTHA) damage in carbon steel pressure vessels is presented. The approach, involving non-collinear wave mixing of ultrasonic waves, is applied to a test sample extracted from a retired pressure vessel. Nonlinear ultrasonic results are consistent with tensile test results obtained using specimens extracted throughout the thickness of the pressure vessel, and with damage observed using scanning electron microscopy micrographs. Results show that the nonlinear ultrasonic approach has the potential of being capable to detect and assess HTHA damage through the thickness of pressure vessels. The method only requires access to the vessels’ outside surface, which makes it very attractive for field inspections.

Based on the two-dimensional (2D)¹ and three-dimensional (3D)² analytical models previously developed for interpreting the contact acoustic nonlinearity (CAN) generated due to the modulation from a “breathing” crack in solid media on propagating guided ultrasonic waves (GUWs), this study proposes a new characterization approach, able to orientate a fatigue crack, even when the crack is at its embryo stage. CAN embodied in the scattered Lamb waves and shear horizontal (SH) waves converted from incident GUWs is extracted upon interaction with fatigue cracks, and the unique scattering pattern of CAN is associated with crack slant via the 3D analytical models, whereby the orientation of a fatigue crack can be pinpointed, without making a reference to the baseline signal. Experimental validation of the characterization approach is implemented, in which an undersized fatigue crack is orientated accurately and visualized in the image.

This paper presents the investigation of nonlinear scattering features of guided waves from fatigue cracks. The fatigue cracks nucleated from a rivet hole are studied as the representative case. A small-size numerical model based on the Local Interaction Simulation Approach (LISA) is introduced, which enables the efficient analysis of the Contact Acoustic Nonlinearity (CAN) of guided waves. Fatigue tests on a thin aluminum plate with a rivet hole is conducted to induce cracks in the specimen. An active sensor array surrounding the crack zone is implemented to generate and receive ultrasonic guided waves in various directions. Several distinctive aspects of the nonlinear scattering phenomenon are discussed: (1) the directivity and mode conversion features, which addresses the scattering direction dependence of fundamental and superharmonic wave mode components; (2) the amplitude effect, which stems from the rough crack surface condition with initial openings and closures; (3) the nonlinear resonance phenomenon, which maximizes the nonlinear response during the wave crack interactions at certain excitation frequency ranges. All these features may provide insights and guidelines for nonlinear guided wave based Structural Health Monitoring (SHM) system design. The numerical studies are compared with experimental data. The paper finishes with discussion, concluding remarks, and suggestions for future work.

TN32 casks are multi-layer cylindrical structures used for storage of nuclear spent fuel. The National Center for Physical Acoustics at the University of Mississippi has manufactured a scaled down model of the TN32 cask. To identify the most relevant nondestructive evaluation parameters, which will be useful while doing experiments on real TN32 casks, a series of experiments have been conducted on TN32 cask model. This paper discusses the data analysis of the experiments conducted on the cask model and the conclusions based on those experiments. Elastodynamic waves are generated in the cask model by pencil lead break and hammer hit excitation and the waves in the cask at certain locations are sensed using piezoelectric wafer active sensors (PWAS). The waveforms and frequency spectrums of waveforms arriving at PWAS are studied. There are two types of joints on the cask model: structures joined using adhesives and structures joined using press fit. The effects of various joints in the structure on elastodynamic wave propagation are also studied. Pitch catch experiments on the cask was also done using in plane excitation using PWAS. The most sensitive frequency for the cask model was identified from the frequency response spectrum obtained from a wide band chirp excitation. The influence of various joints on the frequency response spectrum is also studied. Analytical modeling of cask geometry for a given excitation is done using Normal Mode Expansion (NME) technique. Prediction of wave propagation through the scaled down model is done based on the theoretical expression derived.

Carbon fiber laminate composites provide good strength to weight ratio for aerospace applications. Manufacturing imperfections and impact during the operation and servicing of the aircraft can lead to barely visible and difficult to detect damage. Impact can lead to delaminations and matrix or fiber cracks, reducing the load carrying capacity of the structure. Both ultrasonic and X-ray techniques have a good track record for the nondestructive testing of composite structures. Immersion ultrasonic C-scans were performed to measure the delamination extent for impact damage in a cross-ply composite specimen. Guided ultrasonic waves propagating along composite plates were employed for defect imaging. The first antisymmetric A0 Lamb wave mode was excited experimentally using piezoelectric transducers and measured using a laser vibrometer. X-ray imaging was used for the detailed visualization of the damages in the composite material. Application examples include carbon fiber composite plates with barely visible impact damage and manufacturing defects. The composite specimens and damage were characterized using ultrasonic measurements. Guided ultrasonic wave and X-ray imaging were used and the respective sensitivity for damage detection in composite panels is discussed.

Structural Health Monitoring deals mainly with structures instrumented by secondary bonded or embedded sensors. Sensors, acting passively or actively as both signal generators and receivers, are able to “listen” to any event happening in the structure (passive SHM) and to “interrogate” the structure to check its “health status” (active SHM). Structures embedded with sensors appear promising for reducing the maintenance costs and the weight of aerospace composite structures, without any reduction of the safety level required. Among many actuators/sensors technologies under investigation for active SHM systems, the combination of piezoelectric patches employed as guided wave exciters or impact sensors and optical fiber Bragg gratings (FBG) as stress wave detectors look promising for their distributed sensing capability as well as weight reduction compromise in a so-called “hybrid structural component”. FBGs have been employed only recently as stress ultrasonic wave sensors due to the reduced number of high-frequency optical interrogators available. One such device, a multi-channel fiber optic acoustic emission (FAESense™) system developed by Redondo Optics, has been employed by the authors for this purpose. Hybrid SHM systems employing FBGs as sensor arrays could provide more distributed data about the local integrity of the structure with less weight addition compared to other sensor types. Typical diameter of fiber optics could allow the embedding of sensor arrays within the composite laminate. Finally, FBGS can provide simultaneously high frequency data characterizing guided wave propagation as well as low frequency local deformations permitting an SHM approach combining global and local impact and damage detection. Intent of this paper is to summarize the first experience gained by the authors in developing SHM systems for composite plate-like hybrid structures for impact detection.

For the purpose of nondestructive testing (NDT), guided waves can be transmitted into a structure, and any defects or anomalies in the waves’ path modify the measured waves. Signal processing methods can be used to extract information about these features. In this work, an NDT method is demonstrated based on laboratory experiments for the case of a flat, rectangular, aluminum plate, which has a stiffener mounted underneath along the middle axis, such that the stiffener cannot be seen from the upper “outside” surface. Piezoelectric transducers are set up in a pitch-catch arrangement on this surface with the assumption that the location of the stiffener is unknown. When guided waves are induced in the plate by one of the transducers, the waves that are received by the other carry the information of the stiffener, as well as any defects in or boundaries of the structure. By transmitting from different points on a grid on the plate, the location and size of any geometry or material discontinuities can be identified. Hence, the developed algorithm reverse engineers the plate by mapping its edges and identifying the region of the stiffener.

Lead Zirconate Titanate (PZT) transducers are commonly used to generate guided acoustic waves for health monitoring of structures made of isotropic and anisotropic materials. Therefore, characterization of PZT transducers is very important not only for reliability but also for the accuracy of data. In this research, Scanning Acoustic Microscopy (in reflection mode) at 30 MHz excitation frequency is conducted for determining the quality of disc type guided wave transducers. Multiple samples are scanned but only four transducers are selected for further investigation. Guided waves are dispersive in nature and multiple guided wave modes can coexist at single frequency. It has been already reported in the literature that good bonding condition between transducer and testing material is very important to restrict other factors that can affect the strength of recorded signals. Chirp signal is excited in the specimen in a single sided excitation/detection setup to investigate the effects of transducer quality on monitoring plate like structures. Sideband peak count technique is used for comparative study of transducers.

In the photovoltaic industry monocrystalline silicon wafers are employed for the manufacture of solar panels with high conversion efficiency. The cutting process induces micro-cracks on the thin wafer surface. High frequency guided ultrasonic waves are considered for the structural monitoring of the wafers and the nondestructive characterization of the micro-cracks. The material anisotropy of the monocrystalline silicon leads to variations of the wave characteristics depending on the propagation direction relative to the crystal orientation. In non-principal directions of the crystal, wave beam skewing occurs. Selective excitation of the fundamental Lamb wave modes was achieved using a custom-made angle beam transducer and holder to achieve a controlled contact pressure. The out-of-plane component of the guided wave propagation was measured using a noncontact laser interferometer. Artificial defects were introduced in the wafers using a micro indenter with varying loads. The defects were characterized from microscopy images to measure the indent size and combined crack length. The scattering of the A₀ Lamb wave mode was measured experimentally and the characteristics of the scattered wave field were correlated to the defect size. The detection sensitivity is discussed.

The paper presents a preliminary study about a de-icing system using ultrasonic waves. The activity has been developed within the project “SMart On-Board Systems” (SMOS), which is part of Italian Aerospace National Research Program, funded by the Italian Ministry of Education and Research and coordinated by CIRA. Conceived for an aircraft wing leading edge, the system shall be extended to other aircraft components, once its efficiency and reliability will be demonstrated. Herein, the results of a preliminary numerical work on a NACA 0012 profile are presented. Guided waves are generated by a piezoelectric transducer bonded on the structure and they cause shear stresses that induce ice delamination and fracture. The investigation is focused on the selection of most suitable excitation frequency for the actuator. Finite element analyses are performed to demonstrate the effectiveness of this approach.

Glass fibre reinforced plastic (GFRP) composites are finding increasing application in aerospace structures. The monitoring of these structures is not only necessary but also mandatory by the safety codes. The present state of the art allows isolation of damage (level II) and the quantification of damage (level III) is the next challenge. The quantification of the damage may allow for better maintenance scheduling and as a result lower downtime for airplanes, yachts and wind turbine which makes it significant in the different disciplines. The paper presents a comparative study of three distinct damage detection methods on a sample of GFRP composite. The aim of the research is to compare the performance of the three methods for the assessment of the deterioration of the composite samples due to the influence of moisture. The electromechanical impedance (EMI) and guided waves (GW) based methods have been shown to be sensitive to moisture induced deterioration. The dynamic strain based damage detection using neutral axis (NA) as a damage sensitive feature is sensitive to moisture induced deterioration as well. In addition to the detection of deterioration, the use of measured strains provides an intuitive way for the quantification of the moisture induced deterioration in the sample. Thus, the present study allows the calibration of the NA based structural health monitoring (SHM) technique using already established SHM methods like EMI and GW based techniques. Hence, it may be seen as a necessary step for the standardization, validation and development of the strain based method for SHM.

Ultrasonic rail inspection is the most commonly implemented method for detecting internal rail defects. While the conventional ultrasound wheel probe has gained its popularity within rail maintenance community, it suffers from the limited test speeds (25 mph at most). This paper presents the state-of-the-art developments in ultrasonic rail inspection technique that utilizes non-contact receivers and no active transmitters. The transfer function between two points of the rail is reconstructed by deconvolutions of multiple pairs of receivers that sense the acoustics naturally excited in the rail by the running wheels. The deconvolution process eliminates the random effect of the excitation to reconstruct a stable acoustic transfer function of the rail. A fixed bulk delay and frequency selection technique are introduced to facilitate the power spectral density estimation for robust transfer function reconstruction. A multivariate analysis based on selected features extracted from various frequency bands is conducted on the signals recorded by multiple sensor pairs respectively. Furthermore, damage index traces based on data from different sensor pairs provide system redundancy for improved reliability with the voting logic for damage detection. The proposed approach lends itself to extremely high testing speeds (as fast as the service train speed, e.g. 60 mph and above), that would enable the real-time evaluation of rail track integrity at train operational speeds. A prototype based on this passive-only inspection idea has been constructed and tested with the DOTX216 testing vehicle of the Federal Railroad Administration at the Transportation Technology Center (TTC) in Pueblo, CO in September 2016. Test runs were made at various speeds from 25 mph to 80 mph (the maximum speed allowed on the test track). The results show the feasibility of stable reconstruction of the transfer function from the random wheel excitation, as well as the detection of joints and welds present in the track. Some tests were also conducted on TTC Defect Farm showing the potential for defect defection.

Earthquake can cause severe damage to structures, the assessment of structural performance, before, during, and after an extreme event, is critical for ensuring their safe operations and resiliency to potentially catastrophic events. However, the conventional monitoring methods need complicate sensors, acquisition devices and data transmission system, it’s difficult to obtain the real-time response of the structures during earthquake. Moreover, current damage assessment always relies on the numerical simulation, the monitoring data is rare to apply on the damage detection due to the difficult SHM system implementation during earthquake. Furthermore, the displacement was particular difficult to be monitored. In this work, the objective was to extract the damage features such as the modal frequencies variation and residual displacement using smartphone data in a three-story steel frame structure subjected to shaking table earthquake excitations, and study the acceleration integration method in frequency domain to obtain the displacement more convenient and quickly with higher precision. First, a discussion of experimental details, including test structure, test plan and damage cases was introduced. Then the modal frequencies variation and residual displacement in different cases were obtained with the conventional and smartphone monitoring data. Third, the integration techniques for obtaining displacements from acceleration raw data based on one-story measured displacement were investigated to reduce the errors caused by uncertain cut-off frequencies.

Phase-based motion estimation and video magnification are non-contact and target-less efficient approaches that are being used to extract the operating deflection shapes of vibrating structures. In recent years, Operational Modal Analysis (OMA) and Experimental Modal Analysis (EMA) as the most well studied structural dynamics identification tools have benefited from the unique advantages that phase-based motion estimation and video magnification can offer. Within this study, the phase based motion estimation and video magnification techniques are adopted to perceive the operational deflection shapes and dominant vibration patterns in a Wind Turbine Blade (WTB) cross-section. Moreover, the estimated resonant frequencies are validate by the commercial accelerometer measurements as well that indicates the reliability of the results provided by phase-based motion estimation and video magnification. The identified vibrational operating deflection shapes and the resonant frequencies can be valuable information for design enhancement, as well as finite element model updating and model validation. Moreover, subcomponent structural studies can utilize the results obtained from the vision-based OMA test on the cross-section of the wind turbine blade.

MRI myocardium T*₂ measurement is a very important task in MRI for the detection, for example, of myocardial iron overload. Generally, T*₂ values are obtained by a T*₂ multiecho MRI. In particular, signal intensities of selected ROIs on different TE images are evaluated and the signal -TE relation is used in order to estimate the T*₂ . In order to correctly estimate the T*₂ , it is important that the different selected ROIs correspond to the same anatomical pixels. In this paper, a new PCA-based recognition algorithm is presented in order to recognize and quantify the same anatomical pixels on different TE images of a multiecho sequence. The algorithm was implemented in Matlab. In order to test the algorithm and to obtain preliminary results, a group of 10 patients, referred to MRI with presumptive, clinical diagnosis of myocardial iron overload, was examined in order to test the algorithm. All patients showed no myocardial iron overload with a T*₂ >20ms.To assess intra- and interobserver variability, two observers blindly analyzed the data by delimiting myocardial region. A good intra- and inter-observer reproducibility was obtained, in fact the mean difference between the two observer measurements was 0.8 ms and the 95% limits of agreement on the Bland-Altman plot were -4.8 to 6.5 ms.

In this paper, a semi-analytical finite element (SAFE) approach is presented to model the guided-wave propagations in composite plates. The theoretical framework is formulated using finite element method (FEM) to describe the material variation along the thickness direction and assuming analytical solutions in the wave propagation direction. As with any finite element approach, the convergence study is first performed to ensure the accuracy of the solution. Then, the dispersive curves are obtained in terms of phase velocity, group velocity, and steering angle. In general, a wave packet in composite plates with anisotropic characteristics does not travel in the same direction as the phase velocity, and the difference is defined as steering angle. Knowledge of these properties in composite plates is important in guided waves based SHM applications. Finally, it is experimentally validated using the scanning laser Doppler vibrometer (SLDV) measurements of guided wave packets generated by a piezoelectric wafer active sensor (PWAS) in a unidirectional carbon fiber reinforced polymer (CFRP) composite plate. It will be shown that the SAFE approach achieves a good agreement with experimental results.

In recent years, mesh-free semi-analytical technique called distributed point source method (DPSM) has been increasingly used in computing ultrasonic wave field. A generalized computational method has been used for simulation in generic anisotropic plate using a comprehensive formulation of Green’s function. The Green’s function required for the implementation of DPSM is formulated using Fourier Transform method. Wave fields in unidirectional composite materials with 0 and 90-degree fiber orientations are reported using DPSM followed by generalized mathematical formulations. DPSM is implemented for multilayered anisotropic plates. Applying generalized mathematical formulations, NDE of multilayered anisotropic plate is simulated and reported in this article. Virtual NDE experiments of anisotropic plates in Pulse-Echo mode are simulated using a circular transducer of central frequency ~1MHz. Wave fields inside both the fluid and the solid media were calculated.

NDE flaw detectability is given by flaw size such as crack length. Flaw size parameters or flaw response parameters are used to assess crack detectability. Normalized exposure is used to calculate the flaw size parameter in x-ray radiography. Detector response is used to calculate more complex flaw response parameters. The flaw size parameter provides crack indication width and crack indication amplitude parameter such as void path length ratio parameter. X-ray flaw size parameter model, given here, uses ray tracing model in two dimensions to compute flaw size parameters such as the indication width and the void path length ratio parameter. Results from an analytical model previously published by the author were compared with the ray tracing model. Two runs of ray tracing model are provided. These runs include normal and oblique incident angle x-ray. Since the revised analytical model provides better agreement with the ray tracing model for indication width and normalized exposure, it was used to provide flaw detectability assessments using contrast-to-noise ratio and net unsharpness. Film and digital detector responses are modeled in the analytical model. Modulation transfer function (MTF) is also modeled in the analytical model based on measured basic detector resolution SRb as given in ASTM E2033. Using the detector response and detector MTF, flaw response parameters are computed. Several runs of analytical model were conducted. These runs included zero and oblique incident angle x-ray; simulated film detector, simulated digital detector, micro-focus source and conventional (0.4 mm focal spot) source. Part thickness was also varied. Surface plots with crack depth on x-axis and flaw width on y-axis are provided for MTF. These runs provide plots of MTF, flaw size parameter width, void path length amplitudes, MTF accounted amplitude parameter, simulated detector signal, revealing effect of various quantities on the flaw detection parameters and crack detectability using ASTM E2698 contrast sensitivity, contrast-to-noise ratio and net unsharpness. The paper provides a method to assess and optimize capability of x-ray technique to detect cracks or cracklike flaws reliably through simulation and analysis.

Advances in non-destructive testing and evaluation has given rise to the use of embedded structural health monitoring (SHM) for space structures. Space venturing companies and governments are looking to utilize the SHM technology to improve their missions and capabilities of future space vehicles. The need for modeling and analyzing data for SHM purposes increases with the increased interest in the field. With many different entities pursuing many different avenues to perform SHM it is important to create tools that can guide practical SHM applications. Piezoelectric transducers have seen much use in the field due to their ability to perform as sensors and actuators. The transducers can be used as active components to determine damage to a structure using electro-mechanical impedance methods. The research presented insights into creating tools for real-time electro-mechanical impedance SHM. The electro-mechanical impedance tool investigates the use of a circuit simulation to model the electro-mechanical response of a piezoelectric and a structure. Various elements of the circuit modeling are discussed and practical applications to modeling both sensor and structure are given.

Osseointegration of a prosthesis offers a novel approach to enhancing the quality of life of an amputee because it makes an artificial limb an integral part of their body. While osseointegrated prostheses offer amputees many benefits, long-term health of the prosthesis fixture in the host bone is a concern. In particular, overloading of the fixture can result in damage to the host bone including bone fracture. This study offers a novel sensing strategy implemented on the percutaneous end of an osseointegrated prosthesis. Piezoelectric actuators are used to generate elastic stress waves in the prosthesis to interrogate the integrity of the prosthesis-bone interface. In this study, flexural mode Rayleigh waves are introduced in the prosthesis to identify the existence and location of fracture in the host bone. A prosthetic model consisting of a titanium rod implanted in a synthetic sawbone with piezoelectric wafer elements bonded to the rod surface is used to validate the proposed approach. The work reveals the waveforms associated with flexural wave modes are directly correlated to bone fracture occurring at the prosthesis-bone interface with fracture location identifiable in the reflect wave features.

Skin imaging is a powerful, noninvasive method used with potential to aid in the computer-assisted diagnosis of numerous dermatological diseases and assess overall skin health. By tracking the evolution of various skin features, we can monitor skin health. One interesting feature is known as the “microrelief,” which are the fine micrometer scale furrows and ridges that appear like irregular geometric patterns on the skin surface. However, it is difficult to accurately observe the microrelief structure and its evolution over time due to the micrometer dimensions of the microrelief and the 3D non-rigid nature of the body. Registration and matching of the same skin region are further complicated by noisy and distorted optical images. We have designed a high resolution, handheld optical system to image the skin microrelief. The device has potential to be used in clinical settings since it is small and lightweight. With proper experimental design, we are able to acquire repeatable images of a selected skin patch to monitor over time. Additionally, we have developed methods for registration of skin patches and analyzing skin feature stability. Using real and synthetic skin images, we demonstrate that we can accurately and robustly register large area skin images and identify skin pattern correspondences. Essentially, through repeatable, high resolution imaging, we can monitor the microrelief structure in select individuals over a period of 1-2 years. This has interesting applications because we can use the microreliefs for health monitoring and as a map for the body since we notice that these features are stable over time in healthy individuals.

While high-intensity focused ultrasound (HIFU) is already being used for the ablation of tissue near the skin, such as in the case of prostate cancer, targeting tissue deeper inside the body remains challenging due to the increased obstruction and scattering of ultrasonic waves. In this work, the partial and complete obstruction of the ultrasonic beam path from a HIFU transducer operating at 670 kHz by bone phantom is imaged in laboratory experiments to visualize wave transmission and reflection at solid-fluid interfaces. Ultrasonic wave-scattering under such conditions has scarcely been the focus of previous ultrasound visualization studies. Thus, this work provides a qualitative visual reference for focused waves scattering at water-bone interfaces. A diffraction-based shadowgraph technique is used for the ultrasound visualization. The ultrasonic waves are imaged in water with no obstruction, with varying partial obstruction, and with complete obstruction by a thin fiber-filled epoxy plate mimicking bone tissue. Experimental findings are compared to those obtained through finite element simulations, showing good agreement. Furthermore, it is found that in certain partial obstruction cases, the waves scatter in such a way that the destructive interference between the transmitted waves lead to a significantly reduced maximum pressure at the focal point. Overall, the results of this study can provide a visual framework for future research in the field of therapeutic ultrasound.

Ultrasound (US) is a widely used modality for medical imaging, since it is non-invasive and relatively inexpensive. The ability of US imaging to detect internal structures, like tissue interfaces or lesion sites makes it a promising candidate for dental imaging. So far, the inherent technical difficulties of US imaging in tissues and organs with very heterogeneous (acoustic) properties and limited access have prevented its widespread use. In this study, we characterized the acoustic properties of sectioned teeth by scanning acoustic microscopy in reflection and transmission modes. The spatial distribution of sound velocity was measured in sections of extracted human teeth by use of a scanning acoustic microscope (SAM). Freshly extracted teeth were fixed in 4% formaldehyde solution and embedded in a polymer block (PMMA). Sections of approximately 1 mm thickness were cut along the coronal-apical axis. Radio frequency (RF) data of teeth were collected in a scan region of 15 × 15 mm² by a SAM operating at 30 MHz with a lateral step size of 50 µm and a sampling rate of 500 MSa/s. Sound velocity was determined from the time resolved reflection and transmission signals. Values for sound velocity from transmission mode were about 20% lower than that from the reflection mode, if thickness information from reflection mode was used. If thickness was determined from the transmission mode, sound velocities from transmission were very close to those obtained from the reflection mode. Transmission mode is less sensitive to artifacts caused by the inclination of the specimen.

The human body is comprised of a variety of networks that can be monitored and used as body positioning systems. Furthermore, structural changes observed in these networks have clinical significance as they can aid in disease diagnosis and determining the overall health of an individual. One such network is the superficial vascular structure. As the primary network supplying blood to the body, observing the vein structure gives insight into the cardiovascular health and hydration levels of an individual. Additionally, because of the uniqueness of the network, there is growing interest in using veins as a biometric for identification and mapping. However, because vasculature is difficult to image and existing imaging technology is expensive, the potential for superficial vascular structure to map the body and provide insight into overall health has not been well studied. Furthermore, given the 3D nature of the body, registering and matching corresponding vascular regions proves to be quite challenging. In order to address these needs, we have designed a near-infrared (NIR) imaging system to image the superficial vascular structure. It is compact, easily integrated into any computer system, and cost-effective, thereby having the potential to be used in clinical settings. By carefully designing the image acquisition system and developing registration and matching algorithms, we can robustly image and extract the vascular structure. By extracting the vascular structure from certain limbs, we show the potential for using vasculature as a body map. Additionally, we demonstrate the uniqueness of the vascular structure and its potential to be used as a biometric identifier.

Piezo-composite transducers have been widely used for medical applications such as the medical imaging, brain stimulator, blood flowmeter and humidity sensor due to its merits of high sensitivity and broad bandwidth. Conventional ultrasonic transducers and arrays were mostly developed with a rigid flat or curved front with a fixed curvature. However, most parts of the human body would have curved or irregular shapes such as a human skull or chest. Thus, a flexible ultrasound transducer may be preferred in the medical diagnosis devices. Recently, the flexible sensors or transducers are of great interest and the associated progress has been made for the medical imaging. However, published works have not provided an appropriate solution to overcome the limitations of metal-type electrodes such as cracking or delamination at the presence of transducer surface bending. In this work, we have developed a flexible piezo-composite transducer composed of the active piezoelectric material (PZT-5H) and passive polymer matrix (PDMS) to achieve sufficient flexibility, sensitivity, and bandwidth for the medical applications. In addition, the flexible electrodes composed of silver nanowires (AgNWs) and PDMS were deposited on the transducer using the spray coating method. AgNW/PDMS electrode is a promising alternative to metal-type electrodes such as Au to possess a reliable durability to the cracks from the strained fatigue while providing a sufficient conductivity as an electrode. The prototyped transducers can be applicable to the curved or irregular surface of the target structure for detecting any acoustic variation with high sensitivity and good matching contact.

Background: Although ultrasound imaging has been widely applied in medical diagnosis for decades, the data processing remains primitive. Traditional B-mode ultrasound imaging exhibits the amplitude of the scattered ultrasonic signals as brightness of the images, neglecting rich information delivered by the frequency modulation of the signals. While the doctors diagnose upon the whole image with their experiences, the limited local information poses significant difficulties on computer diagnosis. Methods: Ultrasonic harmonic imaging employs multiple frequency bands in the imaging strategy, indicating that the frequency variation in the spectrum contains nonlinear vibrations which are specific for given biological tissues. We hypothesize that detailed analysis and characterization of the spectrum enable the software to recognize the signals from different organs or from diseased regions. Wavelet transform was utilized to exhibit the ultrasonic signal in both time and frequency domain, followed by the principal component analysis which extracted the feature of the frequency. Pseudo colors, red, green, and blue, were associated with the first 3 principal components as a colorized augmentation of the ultrasound imaging. Results and Conclusion: In the preliminary test, each pixel of the image distinguished itself by frequency characteristics in the wavelet transform. Principal component analysis recognized the major characteristics and presented them in pseudo color images. The hypodermic layers, the kidney, and the surrounding tissues distinguished themselves clearly from one another by the color association. The ultrasonic spectral analysis and augmented visualization technique pioneered the way to intelligent ultrasonic imaging systems and computer-aided diagnosis.

In the recent years, various membrane-type acoustic metamaterials were developed for low frequency sound absorption. However, a membrane absorber usually requires a large back cavity to achieve low frequency sound absorption and on the other hand, in order to guaranty a multiple peaks absorption decorated membrane resonators or membrane with multiple magnetic negative stiffness cell shall be considered. This research proposes a new concept of membrane-type metamaterial which can achieve multiple peaks and broadband absorption at low frequencies. The basic concept behind the design of the elementary cell is associated to the vibro-acoustic behavior of the structure. In fact, the maximum sound absorption is related to the symmetrical mode of the membrane, so playing with the geometry, the mass and the stiffness of the membrane the eigenfrequencies can be tuned easily in the prescribed frequency range. At same time local increase of strain energy around geometrical discontinuity or around discontinuity associated to the material properties may lead a gain in sound absorption. A mono-layer membrane structure is presented where the geometrical shape and material properties distribution in terms of density and stiffness in the elementary cell are optimized in order to manipulate the vibro-acoustic properties and maximize the absorption at required frequencies. To optimize the geometry and the vibro-acoustic properties of the proposed metamaterial, finite element simulation were carried out. The numerical model was then validated using experimental measurements. A preliminary prototype was tested into an impedance tube test ring and the normal sound absorption was measured following the transfer function approach and compared with the numerical results

Dynamic materials are materials whose constitutive properties vary at a rate comparable to the frequency of waves traveling within. Also referred to as spatio-temporal composites or modulated phononic crystals, they constitue a playground for remarkable non-standard wave phenomena. Of most interest in the currently trending context of non-reciprocal and non-time-reversible wave motion, is their ability to block wave forms traveling in a given direction while transmitting the same wave forms if incident in the opposite direction. This asymmetry between left and right is visible in the dispersion diagram where aligned left and right gaps, blocking left- and right-going waves, get tilted and become misaligned, thus blocking either left- or right-going waves. Band tilting in dynamic materials is expected to be a function of the time profiles of the constitutive properties and of their rate of change. Interestingly, it is proven that the ratio tilt to rate only depends on a number of well-identified qualitative parameters and is insensitive to the detail of said time profiles. Specifically, the tilt-to-rate ratio turns out to be a robust (topological) perturbation-immune quantity quantized in units of length of the unit-cell. The proof makes use of two quantum mechanical tools, namely the adiabatic theorem and Berry’s phase, generalized and adapted to the case of elastic waves. The example of a modulated 3-periodic spring-mass lattice whose spring constants are being slowly and periodically modulated in time is thoroughly treated so as to illustrate theoretical findings.

Considerable research attention has been recently devoted to the study of periodic structures given their unique wave dispersion. Phononic crystals and acoustic metamaterials have emerged as two main categories of such periodic structures that can exhibit radically different band gap characteristics. Here, we present a novel configuration that combines hybrid wave attenuation attributes culminating in enhanced metadamping and energy dissipation properties. The results are compared with a benchmark example from literature to show the potential of the new design.

The limitations on resolution due to the effects of diffraction have presented a significant barrier to generating and observing small features with acoustic or electromagnetic waves. Previously proposed methods to overcome this limit, and therefore achieve superresolution, have largely been restricted to operating within the near-field region of the aperture. In this work, we will describe how acoustic helicoidal waves generated using a phased acoustic aperture (such as a traditional phased array or acoustic metasurface) can create acoustic vortices that are well below the resolution limit, and how this can enable far-field superresolution acoustic imaging. The acoustic vortices generated in this manner propagate from the near-field into the far-field through an arrangement of stable integer mode vortices, thereby enabling the generation of far-field superresolved features in the acoustic pressure field. Through the use of non-axisymmetric vortex beam distributions, splitting of the on-axis vortex occurs. This leads to arbitrary off-axis arrangements of vortices, enabling more complicated superresolved structures to be created such as squares, triangles and multi-point stars. In this paper, theoretical and numerical results will be presented for an acoustic aperture which is capable of generating superresolved far-field features in the radiated acoustic pressure, and results will be shown illustrating the superresolution capability of this novel technique.

We have succeeded in designing a one-dimensional two-way unidirectional acoustic filter in the linear regime for the first time by combining the concept of functionally graded phononics and the free vibration characteristic of a finite-sized phononic structure. Such a design also enables the realization of a truly one-dimensional and linear bi-directional acoustic diode. The key to this design is to locate a natural frequency precisely in a band gap, and we show theoretically here that this frequency should be associated with an edge mode. The efficiency of the acoustic diode is also discussed and optimized.

We investigate nonlinear wave dynamics in origami-based mechanical metamaterials composed of origami-based structures, specifically the Triangulated Cylindrical Origami (TCO). The TCO structure shows coupling behavior between longitudinal and rotational motions. One of the unique features of the TCO is that the unit cell can exhibit mono- or bistable behavior selectively, which is determined by initial configurations such as height and rotational angle. In this study, we first fabricate physical prototypes made of paper sheets, and conduct compression tests on the prototypes to verify this unique tunable mono-/bistable features. By utilizing this tunability, we design a 1D chain of the TCO unit cells in which mono-/bistable behaviors of each unit cell can be altered by geometric parameters. Then, we analyze wave propagation in this origami-based system numerically by applying impact to the end of the chain. When the monostable configuration is selected for all of the unit cells, our numerical analysis shows that the application of compressive impact creates a tensile solitary wave propagating ahead of the initial compressive wave. In addition, the wave speed of this tensile solitary wave can be manipulated by the configurations of the TCO unit cells. These unique tunable static/dynamic behaviors can be exploited to design engineering devices which can mitigate impact in an efficient manner.

Spring-mass lattices constitute an accessible model for the understanding of various physical phenomena. Here, they are used to probe fundamental aspects of mechanical topological insulators. First, in gapped one-dimensional 2-periodic lattices, a simple interpretation of Zack’s phase and of the associated integer winding number is provided based on the stiffness coupling two consecutive masses. Nearest neighbor and non-nearest neighbor interactions are explored so as to generate more diverse winding numbers. Lattices with different winding numbers are shown to be topologically distinct. In that case, the difference in winding numbers is interpreted as a count of edge modes localized at the interface between the two topologically distinct lattices. The existence of these edge modes is verified through numerical modal analysis and through homogenization-type asymptotic analysis. The study is extended to two-dimensional systems. Although a visualization of the winding number, also known as a Chern number in this context, is harder, various aspects remain unchanged. Most importantly, an interface separating two topologically distinct gapped lattices will carry a number of edge modes. Last, robustness and immunity to back scattering of localized interface modes against defects is assessed for different systems.

Recent development of artificially engineered metamaterial has significantly widened the range of acoustic responses found in nature. Propagation of elastic waves through such composite materials unveils many applications most of which are acoustic analogue of electromagnetic metamaterials. While holographic imaging using electromagnetic metamaterials is visually indistinguishable from original object, hologramic acoustic imaging is still in a trivial stage. In this article, a conceptual design of butterfly shaped engineered metamaterial consisting of an array of full ring resonators at multiple-length scales embedded in a polymer matrix is reported. Wave propagation in the proposed media is largely affected by the geometrical anisotropy and the anisotropic constituent materials. Introduction of local anisotropy made this engineered structure a suitable candidate for ultrasonic wave bifurcation and convergence. A numerical simulation confirms the negative refraction phenomenon near 37.3 kHz and explores the superlensing capability. Wave dispersion and transmission were analyzed, which showed the formation of acoustic image at a distance away from the source. While superlensing capability is found primarily in electromagnetic metamaterials, local anisotropy in this butterfly design causes negative refraction that results in acoustic hologramic image formation. As the negative refraction leads to a richness of diversified material properties, the proposed acoustic metamaterial will have important applications in biomedical devices, ultrasonic imaging, wave guiding and marine transportations.

In this work, perfect elastic wave mode transmission and isolation in an anisotropic Pentamode Material (PM) slab sandwiched between two semi-infinite isotropic solids are first explored. Selective elastic wave mode filtering is achieved in a broad frequency range. Furthermore, elastic wave experiments are conducted, which are in good agreements with the full-wave finite element simulations. As a result, the anisotropic PM greatly expands the horizon of subwavelength elastic wave control with broadband advantage. Such filtering abilities can be very useful for underwater noise isolation and elastic wave manipulation devices.

Although the ultrasonic nonlinearity has been studied to evaluate the material degradation, it does not tell intuitively about the degree of material degradation so that the yield strength obtained from the destructive tensile test is still widely used. This study proposed a new algorithm to measure the linear and nonlinear elastic moduli based on the linear and nonlinear ultrasonic techniques, which is reduced to estimate the 0.01% offset yield strength by through the reconstruction of the tensile stress-strain curve in the form of quadratic polynomial within the elastic range. In order for demonstration, the heat-treated Al6061-T6 specimens were prepared, and the 0.01% offset yield strengths were estimated by the proposed algorithm to compare with those obtained by the tensile test. Results showed good agreement. This method can be used to evaluate the degradation of yield strength quantitatively in isotropic material.

In the nonlocal theory of peridynamic the partial derivatives that appear in the classical (local) continuum mechanics are replaced with integral equations. This is an important feature of peridynamic theory allowing it to be easily applied to problems where partial derivatives of the displacement field may not exist (e.g. sharp corners, bifurcation) inside an elastic continuum medium. Crack edge is an example where displacement field is not continuous and hence partial derivatives are undefined. In the past decade peridynamic theory has attracted researchers in modeling crack initiation and propagation, specifically phenomena like crack branching and multiple micro-crack interactions where other classical (local) theories may experience challenges. Despite its remarkable results peridynamics is still a relatively new topic and it has room for development. One area of development is coupling the peridynamics theory with the traditional multibody dynamics. This will provide a useful simulation tool in damage prediction of rotating parts such as wind turbines or helicopter rotor blades. In this paper, a coupled formulation of peridynamics and flexible multibody dynamics is presented. A floating frame of references (FFR) approach is taken to capture the large rotation and translation of a body that itself is modeled by using peridynamic theory.

We study a continuous phononic elastic structure capable of reconfigurable topological edge states. The occurrence of edge states is due to a mechanism that can be considered as the acoustic analogue of the quantum valley Hall effect. By assembling two lattices having broken space-inversion-symmetry we induce gapless edge states at the corresponding domain wall, that is the interface between the lattices. The spatial symmetry of the phononic lattice as well as the topological transition are controlled by an externally imposed strain field. The underlying physical mechanism controlling the propagation behavior in such phononic structure is investigated by a combination of theoretical analyses and numerical simulations. Results show that chiral edge states can be obtained at the domain wall and that the strain field enables their direct tuning. Although this approach produces only a weak topological material in which time-reversal symmetry is still intact, the edge states supported at the domain wall prove to be very robust against back-scattering, even in presence of strong lattice disorder.

The discovery of topological insulators in materials science revolutionized the concept of wave propagation by giving rise to the existence of edge modes that are immune to backscattering. Similarly, the tunability in waveguiding – including in-situ frequency modifications and path designation – can be highly useful in manipulating energy flow, which still remains an open challenge. Here we investigate topologically tunable mechanical metamaterials based on the quantum valley hall effect (QVHE) by utilizing the bi-stable Stewart platform (SP). Generally, topologically protected wave propagation can leverage two physical mechanisms: the quantum hall effect (QHE) and the quantum spin hall effect (QSHE). Compared to the QHE and the QSHE, the QVHE exploited in this study maintains the time reversal symmetry and can be achieved by using a relatively simple, passive system with one degree-of-freedom. The tunable system we propose and investigate in this study is made of a two-dimensional hexagon crystal and is composed of SPs at nodes connected by linear springs. Each building block can exhibit one of the two stable states of the SP, so that the C6 inversion symmetry of the lattice is broken while C3 symmetry is reserved. By changing the sequence of the bi-stable state in the SP, we can formulate two kinds of unit cells – marked as A and B – with different topological properties. Berry curvatures as well as corresponding eigenmodes are obtained to demonstrate the topological conversion between the two lattices. Then we conduct super-cell analysis by forming a 1-by-20 array of A and B unit cells. Band structure of the super-cell indicates the existence of edge modes over the while band gap, which appear at the interface of A and B unit cells. Based on this tunable property of bi-stable SP, we can easily form S-type and L-type (60 and 120 degree bents) topological paths in the 40-by-40 lattices without breaking the original geometry parameters. We then conduct the numerical simulations with these topological wave guides to verify the topological protection of the valley hall edge states from backscattering. The tunable system we proposed in this paper may pave a possible way to achieving tunability of topological metamaterials.

In this paper, a nonlinear elastic metamaterial (NEM) is presented for broadband wave attenuation by incorporating strongly nonlinear elements in a triatomic microstructural design. The nonlinear elements are considered between the primary and secondary orders of the triatomic model where the primary focus is the influence of damping between the secondary and lowest orders of the triatomic microstructure, respectively. The NEM with both weak and strong damping is investigated for efficient attenuation of transient blast waves. The 4th order Runge-Kutta numerical method is used for obtaining the attenuation, transmission, and reflection coefficients of the NEM. It is found that the NEM can expand the bandwidth of the bandgap and enhance the absorption of elastic waves compared with a purely linear elastic metamaterial. This work provides a novel model for efficient energy absorbing materials capable of suppressing blast induced shock waves or impact generated pulses capable of causing severe local damage to nearby structures.

We consider the problem of avoided crossings (level repulsion) in phononic crystals and suggest a computationally efficient strategy to distinguish them from normal cross points. This process is essential for the correct sorting of the phononic bands and, subsequently, for the accurate determination of mode continuation, group velocities, and emergent properties which depend on them such as thermal conductivity. Through explicit phononic calculations using generalized Rayleigh quotient, we identify exact locations of exceptional points in the complex wavenumber domain which results in level repulsion in the real domain. We show that in the vicinity of the exceptional point the relevant phononic eigenvalue surfaces resemble the surfaces of a 2 by 2 parameter-dependent matrix. Along a closed loop encircling the exceptional point we show that the phononic eigenvalues are exchanged, just as they are for the 2 by 2 matrix case. However, the behavior of the associated eigenvectors is shown to be more complex in the phononic case. Along a closed loop around an exceptional point, we show that the eigenvectors can flip signs multiple times unlike a 2 by 2 matrix where the flip of sign occurs only once. Finally, we exploit these eigenvector sign flips around exceptional points to propose a simple and efficient method of distinguishing them from normal crosses and of correctly sorting the band-structure. Our proposed method is roughly an order-of-magnitude faster than the zoom-in method and correctly identifies > 96% of the cases considered. Both its speed and accuracy can be further improved and we suggest some ways of achieving this. Our method is general and, as such, would be directly applicable to other eigenvalue problems where the eigenspectrum needs to be correctly sorted.

We design and acoustically simulate additively manufactured, flat acoustic membranes (also called metasurfaces) which can be reconfigured into 3-dimentional solids. Using finite element simulations, we design frequency selective acoustic ‘window’ membranes. These transmit narrow frequency bands near flexure resonances. The frequency range of coverage was chosen to be in the audible range and spans from 2,500Hz to 10,000Hz with first order resonances only. We demonstrate selective, non-overlapping acoustic transmission through each membrane window in its flat configuration, and directional selectively when the flat metasurface is folded into the truncated-octahedron with an omnidirectional microphone placed on the interior of the solid form. This work was supported by the Office of Naval Research.

The design of smart metamaterials for vibration control is usually based on the use of Bloch theorem which considers a single cell with adequate boundary conditions. These boundary conditions correspond to the infinite repetition of the unit cell in 1D, 2D or 3D. Complex geometries and composite systems can then be designed using this approach with finite elements. The control of the elastic waves can be performed by combining Bragg’s (wave interferences), resonant’s (resonance of a component embedded in the unit cell), damping and/or active control. The energy can then be reflected, transmitted, damped, focused or confined in a specific zone of the structure. However, the practical realization of real-life 2D or 3D finite systems may lead to some situations where energy transfers are not in accordance with those predicted by the infinite system considered in the design, because of reflections on the boundary conditions of the finite structure. The behavior of the system may be simulated by full system modelling, but this is time consuming and may lead to huge calculation costs. In this paper, we propose an extension of the Bloch approach to handle finite system boundary conditions in order to be able to identify situations in which energy transfer may arise because of reflections on the border of the elastic domain. Calculations are performed on 2 cells with adequate boundary conditions. The methodology is described and validated using full finite model and experimental tests on a 2D metamaterial structure.

We examine the propagation of a large-amplitude wave in an elastic one-dimensional medium that is undeformed at its nominal state. In this context, our focus is on the effects of inherent nonlinearities on the dispersion relation. Considering a thin slender rod, where the thickness is small compared to the wavelength, we present an exact formulation for the treatment of a nonlinearity in the strain-displacement gradient relation. As an example, we consider Green Lagrange strain. The ideas presented, however, apply generally to other types of nonlinearities. The derivation starts with an implementation of Hamilton’s principle and terminates with an expression for the finite-strain dispersion relation in closed form. The derived relation is then verified by direct time-domain simulations examining both instantaneous dispersion (by direct observation) and short-term, pre-breaking dispersion (by Fourier transformations), as well as by perturbation theory. The results establish a perfect match between theory and simulation. A method is then provided for extending this analysis to a continuous thin rod periodic layering (phononic crystal) or with periodically embedded local resonators (elastic metamaterial). The method, which is based on a standard transfer matrix augmented with a nonlinear enrichment at the constitutive material level, yields an approximate band structure that accounts for the finite wave amplitude. The effects of the nonlinearity on the subwavelength band gap, among other intriguing outcomes, are highlighted. This work provides insights into the fundamentals of nonlinear wave propagation in solids, both natural and engineereda problem relevant to a range of disciplines including dislocation and crack dynamics, geophysical and seismic waves, material nondestructive evaluation, biomedical imaging, elastic metamaterial engineering, among others.

Identifying delaminations at the early stage is crucial to ensure the integrity and safety of carbon fiber reinforced polymer (CFRP) laminated plates. To precisely localize incipient delaminations relying on mode shapes, very small spatial sampling intervals matching the size of a delamination are required; however, noise components inevitably involved in densely-sampled mode shapes can cause intense noise interference, masking actual delamination-caused changes. Under this situation, developing noise-robust methods for identifying incipient small-sized delaminations in CFRP laminated plates is one of the current research interests. To this end, a noise-robust damage index is formulated in this study for damage identification in plates under noisy environments. The damage index relies on the 2D multiresolution modal Teager-Kaiser energy of measured and reconstructed mode shapes. The capability of the damage index for identifying incipient delamination is experimentally validated on a CFRP laminated plate with an incipient delamination, whose mode shapes are acquired via the non-contact measurement using a scanning laser vibrometer. The experimental result show that the damage index can effectively designate the presence and location of the incipient delamination in the CFRP laminated plate under noisy environments.

The objective of this study is to investigate the effect of nonlocal precursor damages through modulated constative properties on the Guided wave propagation in composite materials. To understand the effect of lower scale damage on the interaction of wave propagation in composite materials, all the constitutive coefficients need to be evaluated. Hence, a method is developed to investigate the effective material properties of damaged composite materials using the representative volume element (RVE) model. To calculate the full matrix of constitutive coefficients, periodic boundary conditions were applied on the RVE and average stresses and strains were evaluated using a finite element model. In this study, the effect of different percentages of void contents on effective material properties is presented. Further, the effect of modified material properties on the Guided wave propagation in a transversely isotropic composite plate was investigated.

Acoustic emission (AE) source localization in plate-like structures with geometric features, such as stiffeners, usually requires a large number of sensors. Even without any geometric feature, such approaches are usually accurate only within the convex area surrounded by sensors. This paper proposes a deep learning approach that only requires one sensor and can localize acoustic emission sources anywhere within a metallic plate with geometric features. The idea is to leverage the edge reflections of acoustic waves as well as their multimodal and dispersive characteristics. This deep learning approach consists of three autoencoder layers and a regression layer. The input to the first autoencoder layer is the continuous wavelet transform of AE signals and the output of the regression layer is the estimated coordinates of AE sources. To validate the performance of the proposed approach, Hsu-Nielsen pencil lead break tests were performed on an aluminum plate with a stiffener. The results show that the proposed approach has no blind zone and can localize AE sources anywhere on the plate.

This work is concerned with the mechanical characterization of bistable composite plates in order to investigate their nonlinear behavior dependence on mechanical factors, e.g. strain, stress trends and potential energy. The bistable laminates have two stable shapes that are actuated by a variety of mechanisms (piezoelectric ceramic based actuators, shape memory alloys or thermal actuation) to induce “snap-through” between states. These composite structures are receiving interest in several aeronautic applications such as shape changing applications without the need of servoactivated control systems. Scope of the work is to describe the “0” strain-stress status of the asymmetric bistable laminates, immediately after the manufacturing process. An experimental testing is carried out with the purpose of collecting enough data for the numerical and analytical analyses. Numerical simulations based on Finite Element Models (FEM) are used to study strain and stress fields of the laminates and successively to validate semi-analytical results. By the Classic Plate Lamination Theory (CLPT), an analytical model is developed to provide an interpretation of the bistability phenomenon. The experimental results, with FE and CLPT models, help to understand the relation between the mechanical features of the composite laminate and the bistability phenomenon. This paper reports on detailed nonlinear characterization of bistable plates using numerical, analytical and experimental data in order to provide a starting point for future works characterizing bistable strain-stress evolution over the time.

Composite elements are recently commonly used in many industrial branches like aviation, marine or civil engineering. One of the problems that can influence on material durability are contaminations (e.g. water, oil, dust) that can be introduced into elements structure during manufacturing process or can diffuse into its internal part during exploitation. Such intrusions locally change material characteristics, affect its durability and can be a damage origin significantly decreasing mechanical properties of a composite object. Due to this it is important to have a non-destructive method to inspect an internal structure of elements just after manufacturing or in intervals during its exploitation. One of the techniques is THz spectroscopy that can be applied for non-conductive materials. This method can be used for identification of discontinuities or material structural disintegrations that results in changes of absorption, refractive index or scattering of THz waves propagating throughout the material structure. The paper presents an application of THz spectroscopy for detection and localization of different contaminations (like water, oil or dust) that were introduced into composite material during its manufacturing process or were an effect of exploitation faults. During analysis the limitations of proposed method will be determined.

This work presents an effort to understand the evolution of Bragg scattering band gaps in the context of the transfer functions of finite Phononic Crystals (PCs). Following the dispersion analysis of an infinite PC based on a single unit cell, an analytical derivation of the natural frequencies of a finite PC with a given number of cells is presented. Next, the transfer function between the tip displacement of a finite PC and a force exerted at the other end is derived in closed-form, and used to establish an understanding of the band gap formation in the finite setting. The analysis reveals that the phenomenon can be attributed to the split of poles around the center of the band gap and the absence of any poles within it. The formation mechanism is then discussed in light of several numerical examples with different combinations of system parameters and number of cells.

The unique phenomena in acoustic metamaterial at the Dirac-like cone, and at the exceptional spawning ring could transform the field of engineering with multiple new applications that was never possible before. Formation of localized conical dispersion (Dirac cone) at the Brillouin boundaries are the well-known facts, which exhibits many intriguing phenomena. However, Dirac cone like dispersion at the center of Brillouin zone (k ⃗=0) is rare and only happens due to accidental degeneracy at finite frequencies in two-dimensional periodic crystals (PCs), with or without microarchitectures. Additionally, a possible deformation of a Dirac cone instigates a degenerated state called spawning ring or exceptional ring, where two resonant modes coincides over a zonal wave numbers. The point of exceptional ring is called exceptional point, and known as parity-time symmetry breaking point. Exploiting the behaviors of Dirac cones and spawning rings at the origin and boundaries of the Brillouin zone, a directional and bifurcation lens were designed which will propagate sound wave in specific directions at multiple frequencies. In this article, PCs having a square array of cylindrical polyvinylchloride (PVC) inclusions in air media are studied numerically, that exhibits Dirac-like points and exceptional points simultaneously at k ⃗=0 by modulating the physical parameter of the cylindrical inclusions (PVC) in fixed lattice constant. Detailed numerical study of 2D PCs showed that by adjusting the system parameter, an accidental triple degeneracy of dispersion at Γ point can be achieved. The authenticity of the claim is demonstrated by simulating the phenomena in a designed zero-refractive index material.

We design and experimentally demonstrate a linear active elastic metasurface for real-time and simultaneously multifunctional wave control on a steel plate. The metasurface consists of an array of circuit-controlled piezoelectric patches bonded on the plate separated by thin slots for active wave phase modulations. Our experiments illustrate that by properly programming digital circuits of metasurface unit cells, wave steering directions and paths can be arbitrarily tuned in real-time, which also has an excellent agreement with numerical simulations. We further explore that multiple wave control functions can be integrated into one within the circuits to achieve a simultaneously multifunctional wave control device by using only one metasurface layer. Our numerical results prove the feasibility of the design for broadband and oblique incident applications. The active metasurface breaks the time-revisal symmetry and behaves nonreciprocal propagations of elastic waves. Our design can be simply extended for other elastic wave mode control and wave mode conversion. We believe that the proposed active elastic metasurface could open new avenues for novel and unconventional real-time elastic wave control applications.

In this paper we use level set topology optimization to reveal novel phononic crystal topologies which give rise to metamaterial properties including negative and singular effective properties. The level set formalism has been developed on the basis of polynomial functions, the locations of whose zeros control the distribution of material phases. This significantly reduces the number of design variables involved and allows us to search very large design spaces using global optimization techniques. Optimization process reveals that a 2-phase unit cell in which one of the phases is simultaneously lighter and stiffer than the other results in dynamic behavior which has all the attendant characteristics of a locally resonant composite. This behavior is further explored through the use of mode shape analysis. Results presented in this paper are also an example of how purely computational techniques can illuminate novel physical phenomenon.

Phononic crystals and acoustic metamaterials enable the precise control of elastic properties, even in ranges inaccessible to traditional materials, making them useful for applications ranging from acoustic waveguiding to thermoelectrics. In particular, surface phononic crystals (SPCs) consisting of periodic nanolines on a semi-infinite substrate can be used to generate narrow bandwidth pseudosurface acoustic waves with exquisite sensitivity to the elastic properties of the underlying substrate. Tuning the period of the surface phononic crystal tunes the penetration depth of the pseudosurface wave, and thus selectively probes different depths of layered substrates. In our experiments, we use ultrafast near infrared laser pulses to excite these waves in the hypersonic frequency range by illuminating absorbing metallic nanolines fabricated on top of complex substrates. We probe the nanoscale dynamics launched by our SPCs via pump-probe spectroscopy where we monitor the diffraction of ultrafast pulses of extreme ultraviolet light generated via tabletop high harmonic generation. We then extract the mechanical properties of the substrate by comparing our measurements to quantitative finite element analysis. Utilizing this technique, we characterize the effective elastic and thermal transport properties of 3D periodic semiconductor metalattices.

This paper presents a numerical approach based on spectral methods for the computation of guided ultrasonic wave modes in stressed elastic plates and rods. The approach is applicable to Lamb modes in anisotropic plates and longitudinal modes in isotropic rods under uniform stress. The proposed approach computes the modeshapes and the full complex dispersion spectrum (real frequency, complex wavenumber), accommodating both propagating (real wavenumber) and non-propagating (complex wavenumber) modes. Numerical results are presented for plates composed of fiber-reinforced graphite/epoxy (GREP) and plates and rods composed of Hecla 17 steel. The results are used to investigate and compare the effect of stress on the dispersion curves for plates and rods, while demonstrating the computational efficiency of spectral methods. The convergence rate is demonstrated, showing spectral convergence.

In this work, a composite pipe is subjected to multiple cycles of mechanical loading/unloading in a three point bending configuration. The specimen is instrumented with eight piezoelectric wafer active sensors (PWAS), used as passive receivers of acoustic emission signals during loading. Amplitude-frequency and amplitude-duration analysis of the signals allows detection of different damage mechanisms. Active monitoring is done using PWAS successively as transmitters and receivers of guided waves, in a pitch-catch configuration. Signals are collected for each chosen excitation frequency in the range 200-300 kHz. Scans are recorded between successive loadings of the specimen to assess the state of damage at each stage, and compare against the ‘pristine’ condition. The axisymmetric L(0,2) mode at 230-250 kHz is shown to be attractive for long distance propagation between axially aligned sensors. Cross-comparison of tuning curves obtained from the pristine condition and test data show attenuation in amplitude of the L(0,2) mode, respectively. Based on this, a damage index is proposed.

Tasmania eucalyptus nitens is one of the most important plantation hardwood species used for paper production. Forest growers and wood processing companies have recently considered it for the production of high quality sawlog. The high quality sawlog, however, can be produced from pruned plantation eucalyptus niten as the unpruned one contains several knots and cracks which lessen the quality of the log. Thus, it is vital for forest growers to deliver pruned log to wood processing companies. The pruned log, however, could not be discriminated from unpruned stems by harvester within the plantation plot due to self (natural) pruning process of unpruned tree. This leads to the delivery of the pruned log to the processors challenging. Although wood processors use large x-ray image machines during processing to optimise wood recovery, high costs are incurred from transporting poor quality, knotty timber following the harvest. In this paper, a 17 year old eucalyptus nitens has been considered for non-destructive evaluation. The aim is to investigate the effects of the defects including knots and cracks on the ultrasonic wave. 12 samples from different parts of trunk have been selected and conditioned at the forest moisture content of 120% (70% water content). The samples were scanned by ultrasonic waves at every 10 cm distance in longitudinal direction and at every 45 degree spacing in circumferential direction along the samples. Results show that there is a significant difference between recorded ultrasonic waveforms propagated through unpruned billets and pruned ones. The unpruned billets had a larger effect on ultrasonic waves while the waves are relatively steady when pruned billets are tested.

Theoretically, resonance-based metamaterials offer strong benefits in the realm of passive vibration control, given their potential to generate low-frequency band gaps. However, these concepts have seen little implementation in industrial applications due to a number of limitations. In vibration of heavy machinery, practical limitations are introduced by the combination of the low amount of space available, the high static stiffness of supports required for the mass, and low frequencies of operation. This makes the implementation of metamaterial concepts as a retrofit to existing support structures challenging. In analyzing elastic metamaterials, typically an infinitely repeating unit cell is assumed, but in practical implementation as few as two or three unit cells may be all that can be accommodated, significantly reducing their efficacy. Several different resonance based metamaterial concepts have been analyzed, using analytical models and finite element analysis, as candidates for retrofits or replacements to typical elastic supports for heavy machinery, specifically large generators, that avoid these problems. One concept, using grounded resonators, reduces vibration at the frequency of interest by creating a mechanical high pass filter. Another concept has been used to target a significantly contributing rotational mode of the generator, to reduce longitudinal vibration of the supports, instead of targeting the longitudinal vibration of the supports directly. Off-resonance operating frequencies of the generator have been targeted and successfully reduced using these concepts. Results from these simulations show that resonance-based metamaterial concepts can be used to reduce vibration transmission at operating frequencies of massive machinery.

In this presentation, a lightweight metastructure is designed based on the prismatic tensegrity structure which enables uniquely coupled compressional and torsional waves. A prismatic tensegrity structure consists of elastic bars and cables with pre-stress to provide its stiffness and therefore, has very high strength-to-weight ratio. A theoretical model with coupled compressional-torsional stiffness matrix is developed to study the band structure of the proposed metastructure. Microstructure designs based on both Bragg scattering and local resonance mechanism are investigated for vibration isolations in different targeted frequency ranges. It is noticed that unit cell with opposite chirality can lead to broadband isolation for both compressional and torsional vibrations. Interesting wave mode mixing and selective wave mode transmission phenomena are also studied based on the proposed theoretical model. Moreover, tunable wave propagations and vibration suspension are achieved by two approaches: (i) harnessing the geometrically nonlinear deformation of the periodical tensegrity prisms under global torsional or/and compressional loads to achieve large-range and coarse adjustment of the band structure; (ii) modifying the pre-stress in the tension cables with active components, such as hydraulic actuators, for small-range and fine adjustment of the band structure. The proposed tensegrity metastructure could be useful for various engineering applications in the fields of space and civil engineering where high strength-to-weight ratio as well as broadband vibration suspension are in a high demand.

The existing concepts of non-reciprocity in propagation of acoustic or elastic waves are based either on nonlinear effects, or on local circulation of linear elastic fluid that leads to red or blue Doppler shift, depending on the direction of sound wave. The same concepts exist for electromagnetic non-reciprocity, where external magnetic field may produce the effect similar to local rotation of the medium. These two concepts originate from two known methods of breaking a time-reversal symmetry (T-symmetry), that is necessary for observation of nonreciprocal wave propagation. Both concepts require additional electrical or mechanical devices to be installed with their own power sources. Here we propose to explore viscosity of fluid as a natural factor of the T-symmetry breaking through energy dissipation. We report experimental observation of the nonreciprocal transmission of ultrasound through a water-submerged phononic crystals consisting of several layers of aluminum rods arranged in a square lattice. While viscous losses break the T-symmetry, making the wave propagation thermodynamically irreversible, the transmission remains reciprocal if the scatterers are symmetrical. To generate different energy losses for opposite directions of propagation, the P-symmetry of the crystal is broken by using asymmetric scatterers. Due to asymmetry, two sound waves propagating in the opposite directions produce different distributions of velocity and pressure that leads to different local absorption. Dissipation of acoustic energy occurs mostly near the surface of the scatterers and it strongly depends on surface roughnesses. Using two phononic crystal with smooth and rough aluminum rods we demonstrate low (2-5 dB) and high (10-15 dB) level of non-reciprocity within a wide range of frequencies, 300-600 kHz. Experimental results are in agreement with numerical simulations based on the Navier-Stokes equation. This nonreciprocal linear device is very cheap, robust and does not require energy source.

In this paper results of numerical and experimental analysis of guided wave mode conversion phenomenon are presented. Research presented in this paper is focused on S0/A0’ mode conversion caused by discontinuities in the form of notches and delaminations. In the numerical research Spectral Element Method is utilized for modelling of elastic wave propagation. Two kinds of structures are investigated: aluminium beam with notch and composite panel with delamination. In both cases influence of symmetrical and non-symmetrical (in respect to the thickness of structure) location of discontinuities on S0/A0’ mode conversion is investigated. Numerical results lead to the conclusion that necessary condition for mode conversion is the non-symmetric location of a discontinuity in respect to the thickness of the structure. Experimental research is based on scanning laser Doppler vibrometry and full wave-field measurements. In this approach, guided wave generation is conducted based on piezoelectric transducer while sensing process is performed for a dense mesh of points that span over an investigated area of the composite part. Only composite panel with teflon inserts with different shapes is investigated in experimental research. Moreover, only symmetric locations of teflon inserts are investigated. Mode conversion S0/A0’ was noticed clearly for the symmetrical location of teflon inserts (in respect to the panel thickness). Experimental results lead to the conclusion that in real condition always a small distortion of symmetry exists. In reality, perfect symmetry is extremely rare, so it is expected that in real-world damage scenarios mode conversion will always occur.

This work aims at presenting techniques for the damage identification in single lap joints (SLJs). The two proposed experimental approaches, exploiting particular interactions of the structure with vibrational waves produced by piezoelectric sensors, allow to perform a Structural Health Monitoring (SHM) without a baseline. The first technique involves the excitation of the structure by means of stationary sinusoidal waves: the presence of a subharmonic in the frequency response spectrum at a receiver point indicates the presence of damage in the joint. In addition, through a simplified analytical model it is possible to relate the frequency of this subharmonic to the size of the damage. The second technique is based on the use of a tone burst: the exciting sensor sends this transient signal that travels through the bonded area and is subsequently read by the receiving sensor; the information received is the result of an interaction between the sent wave and the reflection of the boundaries, sensitive to possible damages. The attenuation of the burst, studied through the wave equations, gives indications on the size of the damage. Both experimental campaigns were carried out on aluminum SLJs bonded with acrylic adhesive, using piezoelectric sensors (one exciting and one receiving). Simplified analytical models were used to validate the experimental results. The good analytical-experimental correlation confirms the validity of the proposed approaches.

The A22 Colle Isarco Viaduct is one of the most important infrastructural links in Italy, of strategic importance on the European route E45, connecting Northern Europe to Italy. A disruption of this bridge caused by a damage event would result in a critical increase in traffic congestion, with negative consequences for users and environment. To optimize its management after a possible damaging event, we developed an innovative decision support system (DSS), based on the data from a multi-technology structural monitoring system, which includes a robotized topographic system, a fibre optic sensor network and a thermometer network. The DSS analyses the monitoring data, assesses the probabilities that the bridge is damaged or not by using formal Bayesian inference, and identifies the optimal action according to the axioms of expected utility theory (EUT). This DSS is one of the first of its kind developed in Europe and can help in optimizing the traffic management along the A22 highway while enhancing users’ safety and reducing the bridge maintenance costs. It highlights in real time abnormal states of the bridge and allows the owner to act promptly with inspection, maintenance or repair, only when strictly necessary. We developed this DSS in collaboration with Autostrada del Brennero SpA, and although designed for a specific case study, its scope is very broad and can be applied to any problem of infrastructure management which requires optimal decision based on uncertain information under safety and economic constraints.

Only very recently our community has acknowledged that the benefit of Structural Health Monitoring (SHM) can be properly quantified using the concept of Value of Information (VoI). The VoI is the difference between the utilities of operating the structure with and without the monitoring system, usually referred to as preposterior utility and prior utility. In calculating the VoI, a commonly understood assumption is that all the decisions to concerning system installation and operation are taken by the same rational agent. In the real world, the individual who decides on buying a monitoring system (the owner) is often not the same individual (the manager) who will actually use it. Even if both agents are rational and exposed to the same background information, they may behave differently because of their different risk aversion. We propose a formulation to evaluate the VoI from the owner’s perspective, in the case where the manager differs from the owner with respect to their risk prioritisation. Moreover, we apply the results on a real-life case study concerning the Streicker Bridge, a pedestrian bridge on Princeton University campus, in USA. This framework aims to help the owner in quantifying the money saved by entrusting the evaluation of the state of the structure to the monitoring system, even if the manager’s behaviour toward risk is different from the owner’s own, and so are his or her management decisions. The results of the case study confirm the difference in the two ways to quantify the VoI of a monitoring system.

Safety of the pressure-retaining equipment is a matter of cardinal significance in power plants. On-line monitoring of pressure-retaining equipment in power plants is a useful way to keep their safety. In the present research, some on-line fatigue monitoring systems that have been put into use in the plants are summarized, and the reason that possibly leading to accidents in plants has been analyzed. Moreover, the development of the structural integrity of the pressure-retaining equipment has been studied based on papers published in Engineering Village TM database during the recent 10 years. And the research subjects are statistically investigated and enumerated. According to the key research subjects, a new method of on-line damage monitoring of the pressure-retaining equipment is put forward. And the key technology problems in this new method are discussed.

In order to monitor the strain of concrete which caused by freeze-thaw (F-T) cycles, a novel type of white light interferometer (WLI) sensor is designed and tested. The single-mode optical fiber is embedded in the cement mortar ring as strain gauge, after that, poured the cement mortar ring into concrete cylinder. To study the feasibility of the proposed measurement, two concrete cylinders are exposed in a temperature chamber which uses to simulate the F-T environment. The strain is monitored at 15°C after every 10 cycles. Temperature compensation method is conducted to eliminate the optical fiber strain which caused by temperature. The results indicate that the monitoring method is stable and the residual strain increases with F-T cycles

Laser ultrasonic techniques (LUTs) perform an inspection based on raster scanning pattern to obtain three-dimensional (3D) ultrasonic signals for damage detection in mechanical structures. Even though the raster scan-based in LUTs provides full-field ultrasonic data with high spatial resolution, the scan process consumes substantial time and generates redundant ultrasonic data in many applications. In this paper, statistical damage detection based on the full-field covariance of circumferential scanning is proposed to accelerate the damage detection process using LUTs. A laser ultrasonic interrogation method based on a Q-switched laser scanning system was used to interrogate 3D ultrasonic signals in a 6-mm aluminum plate with four square through-thickness at four different depths. The circumferential scans at a given radius were obtained from the 3D ultrasonic wavefield and represented in a two-dimensional (2D) matrix, angle-time (θ-t) domain. The proposed method was tested at three different circumferences where the defects were located right on, outside, and inside the area of the scan circumference. The covariance matrix, Cθ, of the vector variables in θ-direction was calculated and represented as a covariance image. The covariance image of Cθ demonstrated the ability to detect the defects at these three different circumferences. Hence, the covariance map of an ultrasound circumference can facilitate the existing LUTs to determine the damage existence instead operate in raster scanning mode.

Non-contact optical measurements are being more commonly used in numerous industrial and research domains to obtain reliable displacement measurements, due to their unique advantageous over other instrumentations. Phasebased motion estimation and motion magnification are target-less methods to process the sequence of images captured from vibrating structures. Within this study, a new hybrid computer vision algorithm is proposed to process the motion-magnified sequence of images in an automatic fashion. Particle filter enhanced point-tracking method is employed to track the feature points in the motion-magnified videos and k-means clustering algorithm as an autonomous statistical learning method is utilized for the segmentation of the particles. The performance of the proposed algorithm is investigated on the experimental data acquired from a cantilever beam subject to vibration. By applying the proposed algorithm, human interference and supervision can be decreased dramatically compared to the state of the methods such as canny edge detectors. Therefore, the sensitivity of the structural dynamics identification is improved and the ODS vector estimation procedure can be completed in shorter time.

In this paper, an integrated nondestructive evaluation / structural health monitoring (NDE / SHM) system based on the use of acoustic emission (AE), electromechanical impedance (EMI) and guided ultrasonic waves (GUWs) is presented. The system is integrated into a single hardware/software unit and is driven by a few graphical user interfaces created in the laboratory. The feasibility of this multi-modal monitoring approach is assessed by monitoring an aluminum plate with an array of six wafer-type piezoelectric transducers. AE events are generated with the pencil-lead break technique whereas damage is simulated in the form of permanent magnets attached to the plate. The waveforms associated with the AE are processed using a source localization approach, whereas the GUWs and EMI data are processed using simple metrics based on cross-correlation. The results presented here show that the proposed system is robust and the three NDT methods complement each other very well.

The impedance/admittance measurements of a piezoelectric transducer circuit bonded to or embedded in a host structure can be used as damage indicator, since damage will introduce notable impedance shifts. When a credible model of the healthy structure, such as the finite element model, is available, using the impedance/admittance change information as input, it is possible to identify both the location and severity of damage. In this research we cast the damage identification problem into a many-objective optimization framework through impedance response calibration using Gaussian Process. With damage location and severity as unknown variables, the objective functions are response surfaces calibrated using emulated damaged scenarios assisted by Gaussian Process. Subsequently, a ε - dominance enabled many-objective algorithm based on multi-objective Simulated Annealing is devised to facilitate the many-objective optimization. The proposed approach yields high-quality results that can be further investigated for model updating.

The fundamental shear horizontal (SH₀) wave has been regarded to be specially useful in guided wave-based detection techniques since it is the only non-dispersive wave mode in plate-like structures. In structural health monitoring (SHM) applications, it is required that the transducers can generate and receive guided wave omni-directionally to detect defects in all directions. However, developing omnidirectional SH wave piezoelectric transducer (OSH-PT) has always been a challenge. In this work, we firstly proposed an OSH-PT based on a thickness-poled piezoelectric ring. By dividing the ring into twelve sectors and applying the electric field circumferentially, a new thickness shear (d₁₅) mode was formed. Both finite element simulations and experiments indicated that the proposed OSH-PT can excite and receive single-mode SH₀ wave in a wide frequency range with uniform sensitivities. Then the performance of the proposed OSH-PT is demonstrated by defects localization in a 1000 mm×1000 mm×2 mm aluminum plate. Both single defect and multiple defects were detected and the defects localization algorithm was also presented. Results indicated that the proposed SH₀ wave-based SHM scheme can locate defects in high resolution with the position error less than 10 mm. Considering its excellent performances, low cost and easy fabrication, the proposed OSH-PT is expected to be widely used in SHM of plate-like structures in near future.

Public demands for renewable energy generation are the driving factor for advancements in wind energy, with ever larger wind turbines erected in remote sites. However, regular physical inspections of these structures, as defined by international standards and guidelines, contribute a significant share to the total operation and maintenance expenditures. Efficient structural health monitoring (SHM) systems are key technologies for reducing these costs by enabling maintenance actions according to the true structural state and preventing dramatic failures. This paper presents an enhanced methodology for classifying structural damages using optimally projected multivariate damage sensitive features (DSFs) extracted from vibration signals. Sequential projection pursuit is employed for obtaining low-dimensional transformations of DSFs with the help of an advanced evolutionary strategy. The classification algorithm is based on Bayes’ theorem and an advanced multivariate statistic. A stochastic objective function is defined according to this algorithm. The optimal number of transformation vectors is found using a fast-forward selection. The approach is applied to DSFs defined by the coefficients of vector autoregressive models, which are estimated from multivariate acceleration response signals of an experimental wind turbine blade. Small masses were added to the blade to simulate different damage scenarios non-destructively. Wind-like excitations were applied using a pedestal fan. The results demonstrate that the proposed procedure can reduce DSF dimensionality and, at the same time, preserve damage classification accuracies with respect to the original DSFs. The outcomes are promising for future developments of enhanced vibration-based SHM techniques.

In this study, the surface response to excitation method (SuRE) is investigated using a data-driven method for diagnostics and prognostics of applied load on a plate structure. The SuRE method is an emerging approach in ultrasonic wave-based structural health monitoring (SHM). In this method, high-frequency, surface-guided waves are excited on the structure using piezoceramic elements. The waves propagate and interact with internal or surface damages on the structure. State of heath is evaluated by monitoring changes in the measured frequency transfer functions. Reliability and computational efficiency of the SuRE method has been verified for several diagnostic and structural health monitoring applications. In this paper, the effectiveness of the SuRE method for prognostics and health management (PHM) of a composite plate under applied load is studied. Two piezoelectric elements are attached on the surface of a carbon fiber reinforced polymer (CFRP) composite plate. Sweep excitation-generated (150-250 kHz), surface-guided waves and the transmitted waves were monitored at the sensory position. The reference data set comprised of characteristic transfer functions was generated. SHM data using the SuRE method was captured for eleven locations of applied load between the sensor and exciter. Four data-driven prognostic models, using Gaussian Processes Regression (GPR), were qualified by interval-averaging features extracted from the spectrums and predicted the location of load. During this study, a new approach based on SuRE method is proposed for identifying the location of applied load on a composite and the optimum parameters of the study were evaluated to enhance the performance of GPR identified the optimum parameters number of SuRE method and selected features for most accurate predictions.