Corrosion develops due to adverse environmental conditions during the life cycle of a range of industrial structures, e.g., offshore oil platforms, ships, and desalination plants. Both pitting corrosion and generalized corrosion leading to wall thickness loss can cause the degradation of the structural integrity. The nondestructive detection and monitoring of corrosion damage in difficult to access areas can be achieved using high frequency guided waves propagating along the structure from accessible areas. Using standard ultrasonic transducers with single sided access to the structure, guided wave modes were generated that penetrate through the complete thickness of the structure. The wave propagation and interference of the different guided wave modes depends on the thickness of the structure. Laboratory experiments were conducted and the wall thickness reduced by consecutive milling of the steel structure. Further measurements were conducted using accelerated corrosion in a salt water bath and the damage severity monitored. From the measured signal change due to the wave mode interference the wall thickness reduction was monitored. The high frequency guided waves have the potential for corrosion damage monitoring at critical and difficult to access locations from a stand-off distance.

Structural Health Monitoring (SHM) based on Lamb waves, a type of ultrasonic guided waves, is a promising technique for in-service inspection of composite structures. This study investigates the attenuation mechanisms of Lamb wave propagation fields. The attenuation of an anisotropic plate is experimental measured with air-coupled ultrasonic scanning techniques and analytical modeled using higher order plate theory. Based on the experimental and analytical data the various attenuation mechanisms are characterized for the fundamental Lamb wave modes.

The interest in composite repair technologies has been recently increased following the wide applications of com- posite materials in aerospace industry. Bonded repair patch technology provides an alternative to mechanically fastened repairs with signi cantly higher performance. The work here demonstrates two case studies: a woven composite panel repaired with an external composite patch and a woven composite panel with a scarf repair that minimises discontinuities and provides smoother surface. In the rst case, piezoelectric transducers were utilized in order to excite low frequency Lamb waves while the panel was tested under static tensile loading. In the sec- ond case, piezoelectric (PZT) transducers were surface bonded and the propagation of the rst anti-symmetric Lamb waves mode was investigated while the panel was subjected to fatigue loading. Along with Lamb wave testing, additional techniques were used to monitor the extent of the developed damage such as 3-dimensional Digital Image Correlation, X-ray radiography and optical microscopic analysis. All signals in both cases were further processed with outlier analysis and linear and non linear principal component analysis in order to develop an appropriate damage prognosis strategy. The current work identi es the problems of the traditionally used novelty detection approaches and proposes solutions through the concept of principal curves and appropriate feature extraction techniques.

Composite materials are being used increasingly in advanced aircraft and aerospace structures. Despite their many advantages including high strength to weight ratio, formability and low coefficient of thermal expansion, composites are often susceptible to hidden damage that may occur during their manufacturing and/or service of the structure. Safe operation of composite structures requires careful monitoring of the initiation and growth of such defects before they grow to a critical size resulting in possible catastrophic failure of the structure. Ultrasonic methods using guided waves offer a reliable and cost effective method for defects monitoring in advanced structures due to their long propagation range and their sensitivity to defects in their propagation path. In this paper some of the useful properties of guided Lamb type waves are investigated in an effort to provide the knowledge base required for the development of viable defects monitoring systems in composite structures. Some of our recent research in this area is presented in this paper. The research includes laboratory experiments using a pitch catch method in which a pair of moveable transducers are placed on the outside surface of the structure for generating and recording the wave signals. The recorded signals are analyzed to construct the dispersion and other relevant properties of the guided waves. Theoretical simulations using analytical and numerical methods are carried out and compared with the experimental results. The specific cases considered include an aluminum plate, a woven quasi-isotropic composite panel and an aluminum honeycomb panel with woven composite face sheets. The agreement between the experimental and theoretical results are shown to be excellent in certain frequency ranges, but not for others, providing a guidance for the design of effective inspection systems.

Composite materials such as carbon fiber reinforced panels offer many advantages for aerospace applications, e.g., good strength to weight ratio. However, impact during the operation and servicing of the aircraft can lead to barely visible and difficult to detect damage. Depending on the severity of the impact, fiber breakage or delaminations can be induced which reduce the functionality of the structure. Efficient structural health monitoring of such plate-like components can be achieved using guided ultrasonic waves propagating along the structure and covering critical areas. However, the guided wave propagation in such anisotropic and inhomogeneous materials needs to be understood from theory and verified experimentally to achieve sufficient coverage of the structure. Using non-contact laser interferometer measurements the guided wave propagation in carbon fiber reinforced panels was investigated experimentally. Impact damage was induced in the composite panels and the guided wave scattering at the damage measured and quantified. Good agreement with theoretical wave propagation predictions was found and barely visible impact damage in composite panels detected.

Due to the aging global civil infrastructure (e.g. bridges), there is a critical need for monitoring and assessing structural integrity of large scale structures. According to the ASCE, in 2008, the average bridge in the U.S.A. was 43 years old and 161,892 bridges were structurally deficient or obsolete. Currently, bridge health is assessed primarily using qualitative visual inspection, which is not always reliable because some damage is difficult to detect, quantify visually, or is subject to human interpretation. Traditional sensors such as strain gages, and displacement sensors, have been recently used to monitor bridges. These sensors only measure at discrete points or along a line, making it difficult to detect damage that is not in the immediate vicinity of the sensor or is difficult to interpret. To address these issues, this paper investigates the use of three-dimensional (3D) digital image correlation (DIC) as a sensing approach for improved bridge structural health monitoring. 3D DIC is a non-contact, full field, optical measuring technique that uses digital cameras to measure surface geometry, displacement, and strain. It is proposed that DIC can be used for monitoring by imaging a bridge periodically and computing strain and displacement from images recorded at different dates or operating conditions. In this paper, DIC is shown to locate non-visible cracks in concrete, quantify spalling, and measure bridge deformation. These techniques are first demonstrated in the laboratory. Field measurements are also made on three full-scale bridges. This paper discusses challenges and solutions to implementing DIC on large structures in the field. The results reveal that DIC is an effective approach to monitor the integrity of large scale civil infrastructure.

There is an increased interest in sensing technologies to meet the challenges in the engineering and construction fields. These technologies will help us better monitor and evaluate structural performance and health. This research presents a framework for a stochastic design of an early warning system for buildings by introducing a risk measure as the reference variable that encapsulates the different effects retrieved by the monitoring instruments. Bayesian Network was implemented to develop the proposed early warning system. Within a decision-making framework, the risk measure serves as the index for defining the system warning thresholds. In order to develop the framework, it is necessary to build a sensor equipped monitoring system first. This paper addresses development of a structural health monitoring system for Katherine Harper Hall at Appalachian State University, NC, USA as a very useful approach to improve the safety of the building. It proposes a framework for a stochastic design of an early warning system to avoid, or at least mitigate the impact posed to building occupants by a threat related to live loads.

Ultrasonic surface wave generating inter-digitized transducers (IDTs) have been designed, fabricated and applied to a simulated critical component of a bridge structure. The goal of the current investigation using a simulated gusset plate was to find out the minimum number of IDT sensors required to achieve the maximum monitoring coverage over a given gusset plate, as well as the size and the optimum operating frequency for the sensors. These sensors can be customized depending on the design of gusset plate to include the number of bolt holes or welds, thickness, and overall dimensions. Various sizes of sensors ranging from several centimeters to several millimeters with resonance frequencies between 100 KHz and 3 MHz were tested. IDT sensors operating at 1 MHz with the overall physical dimensions of 15 mm x 17 mm were found to give the best coverage with a minimum number of sensors. A second test component investigated in this work was a dog-bone shaped fatigue test component to simulate fillet welds of bridge structures. The main goal for this fatigue testing was to find how early in the fatigue process a fillet weld failure can be detected using surface waves. Changes in the amplitude of surface waves were monitored in-situ for this study and it was found that the fillet welds tested on a fatigue loading frame showed an indication of weld failure at an early stage of the fatigue crack initiation.

Reusable Launch Vehicles are often used in space applications to guarantee space exploration with reduced costs. These structures often use components from newly developed materials. It is inevitable that reliable inspection methods will be required for quality control and maintenance of such structures to avoid potential damage. This paper describes some initial results from evaluation tests based on Lamb waves for damage detection of Reusable Launch Vehicle composite components. Low-profile, surface-bonded piezoceramic transducers were used for Lamb wave generation. Non-contact measurements of Lamb wave responses were taken by a laser vibrometer. The results presented in this paper demonstrate the great potential of the method for quality inspection and structural damage detection of space composite structures.

Composite structures require a rigorous program of nondestructive inspection and maintenance to detect and characterize hidden defects at an early stage of their occurrence so that preventive measures can be taken before the structure loses its load carrying capacity or suffers from catastrophic failure. Current methods for defects detection in large aircraft and aerospace structures are slow, labor intensive and costly. This is especially true for composite structures where conventional techniques are often ineffective. Ultrasonic guided waves offer an attractive complementary tool for improving inspection techniques in relatively large plate-like structural components due to their large propagation range and sensitivity to defects in their propagation path. Since the waves are affected by the geometrical structural features (e.g. stringers) as well as harmful defects (e.g. delaminations), the application of guided waves in the NDE or SHM of real structures requires a good understanding of these interaction effects. This will help identify the defects from their distinguishing features in the signal in structural components with complex geometry. In this paper a detailed study of the interaction of guided waves with defects in an aluminum plate and a honeycomb composite sandwich structure is carried out using numerical simulations and laboratory experiments. The simpler aluminum plate is used for model validation and understanding the basic characteristics of the interaction phenomena. The agreement between the simulated waveforms and those measured from the experiments are found to be excellent in both cases indicating the possibility of applying guided wave based techniques to more realistic structures.

This paper discusses shear horizontal (SH) guided waves that can be excited with shear type piezoelectric wafer active sensors (PWAS). The paper starts with a review of the state of the art in SH waves modeling and their importance in non-destructive evaluation (NDE). This is followed by basic sensing and actuation equations of shear-poled PWAS transducers with appropriate electro-mechanical coupling coefficients. The electro-mechanical impedance of the SHPWAS transducer is studied. The equations for shear stress transfer between PWAS and the structure are developed. The amplitudes of shear horizontal wave modes are normalized with respect to the wave power; normal mode expansion (NME) method is used to account for superpositioning multimodal SH waves. Modal participation factors are presented to show the contribution of every mode. Model assumption includes: (a) straight crested guided wave propagation; (b) evanescent waves are ignored; and (c) ideal bonding between PWAS and structure with shear load transfer concentrated at PWAS tips. Power and energy transfer between PWAS and the structure is analyzed in order to optimize sensor size and excitation frequency for maximum wave energy production for a given source. The paper ends with summary, conclusion and suggestion of future work.

A new temperature compensation technique combining optimal baseline selection and the filter based on Adaptive Linear Neuron Network was developed to enhance the robustness and effectiveness of guided Lamb wave-based damage detection. This paper focuses on three main issues for practically implementing the proposed method: (a) Establishment of temperature compensation standard; (b) Parameters design of compensation filter; (c) Determination of temperature gradient of baseline signals. Experiments were conducted on two stiffened composite plates to demonstrate the feasibility of proposed method under a temperature range from -40°C to 80°C for compensating temperature effects. Results showed that a reasonable temperature step for providing good temperature compensation can be up to 20°C in a baseline dataset.

An advanced signal processing methodology is being developed to monitor the height of condensed water thru the wall of a steel pipe while operating at temperatures as high as 250°C. Using existing techniques, previous study indicated that, when the water height is low or there is disturbance in the environment, the predicted water height may not be accurate. In recent years, the use of the autocorrelation and envelope techniques in the signal processing has been demonstrated to be a very useful tool for practical applications. In this paper, various signal processing techniques including the auto correlation, Hilbert transform, and the Shannon Energy Envelope methods were studied and implemented to determine the water height in the steam pipe. The results have shown that the developed method provides a good capability for monitoring the height in the regular conditions. An alternative solution for shallow water or no water conditions based on a developed hybrid method based on Hilbert transform (HT) with a high pass filter and using the optimized windowing technique is suggested. Further development of the reported methods would provide a powerful tool for the identification of the disturbances of water height inside the pipe.

Robust damage detection algorithms are a fundamental requirement for development of practical structural health monitoring systems. Typically, structural health-related decisions are made based on measurements of structural response. Data analysis involves a two-stage process of feature extraction and classification. While classification methods are well understood, feature design is difficult, time-consuming, and requires application experts and domain-specific knowledge. Genetic programming, a method of evolutionary computing closely related to genetic algorithms, has previously shown promise when adapted to problems involving structured data such as signals and images. Genetic programming evolves a population of candidate solutions represented as computer programs to perform a well-defined task. Importantly, genetic programming conducts an efficient search without specification of the size of the desired solution. In this study, a novel formulation of genetic programming is introduced as an automated feature extractor for supervised learning problems related to structural health monitoring applications. Performance of the system is evaluated on signal processing problems with known optimal solutions.

Health and usage monitoring systems (HUMS) are being incorporated into an increasing number of applications, e.g. monitoring safety critical components in civil, aerospace, mechanical and other structures. A good example is the use of HUMS in monitoring transmission and drive train components on rotary-wing aircraft. These transmission HUMS have enjoyed success in predicting the deterioration of components, however, current system implementations rely on highbandwidth hardwired sensors and significant data processing capability to perform feature extraction and classification, limiting the locations where they can be installed. To extend HUMS capability into new application areas, such as wind turbine blades or helicopter rotor head components, and other applications impossible to hard wire, the functionality of HUMS needs to be implemented within a wireless sensor network (WSN). The power, processing and packaging constraints of a WSN present many challenges. This paper initially considers the performance requirements of a conventional wired HUMS and contrasts this with that available from state-of the art WSN components. Technical issues related to power supply, sensor technologies, signal conditioning, damage detection and prognostic algorithms for low power microprocessors, robustness and data integrity on wireless radio are discussed. The paper further considers different approaches reported in the literature to overcome system limitations, such as the use of intelligent sensor nodes which perform local signal processing and transmit only a reduced dataset. Finally, simple statistical measures are executed on a low power microcontroller to demonstrate the potential of such devices for damage diagnostics.

Reliable damage detection and quantification is a difficult process because of its dynamic and multi-scale nature, which combined with material complexities and countless other sources of uncertainty often inhibits a single non-destructive testing (NDT) technique to successfully evaluate the extension of deterioration in critical structural components. This paper presents an integrated non-destructive testing approach (INDT) for effective damage identification relying on the intelligent integration of the Acoustic Emission (AE), Guided Ultrasonic Waves (GUW) and Digital Image Correlation (DIC) methods. The proposed system has been utilized to identify wire breaks in seven-wire steel strands and crack initiation and development in masonry concrete walls and is based on the cross-correlation of heterogeneous damage-related NDT features. Conventional AE monitoring relies on damage monitoring by evaluating multiple extracted and/or computed features as a function of load/time. In addition, advanced post-processing methods including mathematical algorithms for statistical analysis and classification have been suggested to improve the robustness of AE in damage identification. Unfortunately, such approaches are often found to be unsuccessful, due to challenging environmental and operational conditions, as well as when used on actual civil structural components, such as bridge cables and masonry walls. This paper presents the framework for successful correlation of AE features with GUW and mechanical parameters such as full field strain maps, which can provide a route towards actual cross-validated damage assessment, capable to detect the initiation and track the development of damage in structures. The presented INDT approach could lead to reliable damage identification approaches in mechanical, aerospace and civil infrastructure applications.

Simultaneous localization and mapping (SLAM) is a process wherein a robotic system acquires a map of its environment while simultaneously localizing itself relative to this map. A common solution to the SLAM problem involves the use of the extended Kalman filter (EKF). This filter is used to calculate the posterior probability of the robot pose and map given observations and control inputs. From the EKF, one estimates the mean and error covariance of the robot pose and map features by using nonlinear motion and observation models. In this article, the conditions required for the convergence of the errors in the EKF estimates obtained by linearizing the nonlinear system equations are studied and applied to the SLAM problem. In particular, the observability condition of the system describing the typical SLAM problem is studied. Numerical studies are carried out to compare the accuracy of the EKF estimates for a representative SLAM formulation which is not observable with a SLAM formulation that satisfies the observability condition.

Propagation of nonlinear guided waves is a field that has received an ever increasing interest in the last few decades. They are excellent candidates for nondestructively interrogating long waveguide like structures since they conveniently combine high sensitivity to structural conditions (typical of nonlinear parameters), with large inspection ranges (characteristic of wave propagation in bounded media). Nonlinear wave propagation in solids has been classically studied using finite strains theory. According to this framework a system of nonlinear PDEs is required to mathematically describe nonlinear phenomena such as acoustoelasticity (wave speed dependency on state of stress), wave interaction, wave distortion, higher harmonics generation, and so on. This work introduces a novel physical model aimed at predicting nonlinearity in constrained waveguides characterized by infinitesimal (ideally zero) strains subjected to thermal variations. Interatomic potentials are employed to explain the origin of nonlinear effects under constrained temperature changes. These potentials highlight at least a cubic dependence on strain of the residual strain energy that is stored in the material due to the prevented thermal expansion. The cubic relationship between strain energy and strain produces second-harmonic generation of propagating elastic waves. This principle is validated experimentally for longitudinal bulk waves propagating in a steel block under constrained thermal excursions.

Glass fiber reinforced cement (GRC) is a Portland cement based composite with alkali resistant (AR) glass fibers. The main drawback of this material is the ageing of the reinforcing fibers with time and especially in presence of humidity in the environment. Until now only destructive methods have been used to evaluate the durability of GRC. In this study ultrasonic guided wave inspection of plate shaped specimens has been carried out. The results obtained here show that acoustic signatures are capable of discerning ageing in GRC. Therefore, the ultrasonic guided wave based inspection technique is a promising method for the nondestructive evaluation of the durability of the GRC.

This paper presents a fatigue crack detection technique based on nonlinear wave modulation created by mixing two ultrasonic guided waves. Two independent input signals are generated using two surface-mounted PZT transducers; a high-frequency probing signal and a low-frequency pumping signal. Corresponding guided wave responses are measured by additional PZT transducers installed on a specimen. The presence of a system nonlinearity, such as a crack formation, can provide a mechanism for nonlinear wave modulation, and create spectral sidebands around the frequency of the probing signal. A signal processing technique combining linear response subtraction (LRS) and synchronous demodulation (SD) is developed to extract the crack-induced spectral sidebands. The proposed crack detection method is successfully applied to the identification of actual fatigue cracks grown in metallic plates. Finally, the effect of the pumping and probing frequencies on the amplitude of the first spectral sideband is investigated using the first sideband spectrogram (FSS) obtained by sweeping both pumping and probing signals over specified frequency ranges.

The generation of cumulative second harmonic ultrasonic guided wave modes is analyzed with respect to their applications for nondestructive evaluation (NDE) and structural health monitoring (SHM). Due to the multimodal nature of guided waves, the selection of a primary wave mode that will generate a cumulative second harmonic is a critical first step for NDE and SHM applications. Thus, the nonlinear boundary value problem that must be solved by perturbation analysis is summarized and the results are tabulated for steel plates and circular cylindrical shells (pipes). The analysis includes shear-horizontal and Lamb waves in plates and axisymmetric torsional and longitudinal waves in pipes. Nonlinear finite element analyses that include kinematic and material nonlinearities are conducted for plate and pipe geometries. An excitation is applied to both plate and pipe samples by a simulated interdigitated transducer; SH1 for the plate and T(0,2) for the pipe. In each case a second harmonic mode having an orthogonal polarization is generated; S1 for the plate and L(0,4) for the pipe. In both cases the second harmonic grows linearly with propagation distance, and therefore is cumulative. A third example simulation is presented that demonstrates mode mixing in a pipe. T(0,3) and L(0,2) primary modes traveling in opposite directions intersect and generate significant harmonics at the sum and difference frequencies. Mode mixing provides a great opportunity to expand the potential of harmonic generation for NDE and SHM.

In this paper, we investigate the scattering behavior of defects in composite plates. Scattering coefficients are of great importance for the estimation of wave-damage interaction, for the interpretation of recorded Lamb wave signals, and for the development of novel signal processing strategies. The anisotropy of composite laminates makes modeling of wave propagation and the numerical estimation of scattering coefficients particularly challenging. The paper studies numerical models of wave propagation in composites, and evaluates their predictive ability. These evaluations rely of full wave field measurements on selected composite plate specimens, through which information on dispersion and directionality of propagation are conveniently extracted. In addition, filtering in the frequency-wavenumber domain allows the extraction of scattered wave fields and the estimation of the scattering coefficients. Comparisons between numerical and experimental data highlight modeling challenges, illustrate mesh-driven directional propagation, and suggest an effective strategy for the estimation of the scattering coefficients through tests and FE modeling.

This paper presents a basic idea of using computer controlled blocks to build an autonomous system of blocks capable of assembling themselves into various configurations. At the beginning, mechanical components are briefly described. The focus of this paper is on the control of a single block, and the interaction between two blocks. Different processes with different circuit diagrams are shown in details. Several pictures are taken under testing condition and indicate the processes and the results.

Structural dynamic characterization is important for ensuring reliability and operability of spacecraft payloads in harsh environments. During the launch, a structure experiences dynamic loads, including acoustic excitation. Conventional sensors are used to infer structural dynamic characteristics. Limitations of conventional strain sensors include low frequency band, susceptibility to electro-magnetic interference, and use of multiple wires. To mitigate these deficiencies, an innovative fiber optic strain measurement system is considered to obtain strain distribution at specific locations on a payload. Theoretical models are suggested and compared with results of experimental testing. Limitations of analytical models are discussed and comparisons with numerical models are presented. The research addresses the usability of presented models in determining the dynamic response of a payload and variation due to distribution of components. It is proposed that discussed experimental and theoretical procedures can be used in determining structural performance for a variety of missions.

Life cycle health monitoring technology for composite airframe structures based on strain mapping is proposed. It detects damages and deformation harmful to the structures by strain mapping using fiber Bragg grating (FBG) sensors through their life cycles including the stages of molding, machining, assembling, operation, and maintenance. In this paper, we firstly carried out a strain monitoring test of CFRP mock-up structure through the life cycle including the stage of molding, machining, assembling, and operation. The experimental result confirms that the strain which arises in each life cycle stage can be measured by FBG sensors embedded in molding stage and demonstrates the feasibility of life cycle structural health monitoring by using FBG sensors. Secondly, we conducted the strain monitoring test of CFRP scarf-repaired specimen subject to fatigue load. FBG sensors were embedded in the scarf-repaired part of the specimen and their reflection spectra were measured in uni-axial cyclic load test. Strain changes were compared with the pulse thermographic inspection. As a result, strain measured by FBG sensors changed sensitively with debonded area of repair patch, which demonstrates that the debondings of repair patches in scarf-repaired composites due to fatigue load can be detected by FBG sensors.

An optical-fiber sensor system for structural health monitoring of concrete elements such as beams and columns is presented. The system employs arrays of conventional optical fibers embedded in the concrete elements as crack sensors. Twelve types of optical fibers as well as several embedding techniques have been tested for this role. The survival rate of optical fibers embedded in concrete could be as high as 80%. The loss of fibers during the embedding process was acceptable provided that the number of fibers in the array had redundancy. The optical transmission of all fibers in the array was monitored in a time-division multiplexed mode at a high repetition rate, in the kHz range. The monitoring scheme allowed a quasi-continuous data acquisitions of large optical fiber arrays. A sharp decrease in the optical transmission of one or more optical fibers was a clear indicator of the development of cracking in the element subjected to flexural loads. The system was successful in detecting not only the initiation but also the propagation of cracks in concrete elements subjected to incremental flexural loading. In this work, the relation between the mechanical properties of the optical fibers and their behavior for the described application is discussed. Also, considerations towards a rational design of the system are proposed. The damage detection system may be used for the mapping and monitoring of cracks in concrete elements. The simplicity of the operation and relatively low cost of the proposed system make it a great candidate for applications in structural health monitoring of critical elements in civil infrastructure.

Ultrasonic probes were designed, fabricated and tested for high temperature health monitoring system. The goal of this work was to develop the health monitoring system that can determine the height level of the condensed water through the pipe wall at high temperature up to 250 °C while accounting for the effects of surface perturbation. Among different ultrasonic probe designs, 2.25 MHz probes with air backed configuration provide satisfactory results in terms of sensitivity, receiving reflections from the target through the pipe wall. A series of tests were performed using the airbacked probes under irregular conditions, such as surface perturbation and surface disturbance at elevated temperature, to qualify the developed ultrasonic system. The results demonstrate that the fabricated air-backed probes combined with advanced signal processing techniques offer the capability of health monitoring of steam pipe under various operating conditions.

Investigations with the aid of longitudinal guided waves in cylindrical structures have been regularly carried out for nondestructive evaluation (NDE) and structural health monitoring (SHM). While earlier works concentrated on the amplitude reduction of the propagating waves due to structural anomalies in this work the change in time-of-flight is investigated. Longitudinal (axisymmetric) modes are excited by a PZT (Lead Zirconate Titanate) transducer for detection of any fluctuation or change in the surface of a steel pipe. Propagating waves are analyzed after proper signal processing. To observe the small change in TOF due to lamination on the surface of a steel pipe, cross-correlation technique is used to attain a higher temporal resolution. The experimental technique is discussed and the obtained results are presented in this paper.

In this work a pulse-echo procedure suitable to locate defect-induced reflections in irregular waveguides is proposed. In particular, the procedure extracts the distance of propagation of a guided wave scattered from a defect within the echo signal, revealing thus the source-defect distance. To such purpose, first, a Warped Frequency Transform (WFT) is used to compensate the signal from the dispersion of the guided wave due to the traveled distance in a portion of the waveguide that is assumed as reference. Next, a pulse compression procedure is applied to remove the additional dispersion introduced by the remaining irregular portion of the waveguide. Thanks to this processing the actual distance traveled by the wave in the regular portion of the irregular waveguide is revealed. Thus the proposed strategy extends pulse-echo defect localization procedures based on guided waves to irregular waveguides. Since the processing is based on Fast Fourier Transforms, the algorithm can be easily implemented in real time applications for structural health monitoring. The potential of the procedure is numerically demonstrated by processing Lamb waves propagating in an irregular waveguide composed by aluminum plates with different thicknesses and tapered portions.

This paper proposes an adaptive Unscented Kalman Filter (UKF) algorithm for Acoustic Emission (AE) source localization in plate-like structures in noisy environments. Overall, the proposed approach consists of four main stages: 1) feature extraction, 2) sensor selection based on a binary hypothesis testing, 3) sensor weighting based on a well-defined weighting function, and 4) estimation of the AE source based on the UKF. The performance of the proposed algorithm is validated through pencil lead breaks performed on an aluminum plate instrumented with a sparse array of piezoelectric sensors. To simulate highly noisy environment, two piezoelectric transducers have been used to continually generating high power white noise during testing.

In this paper the investigation of a structural health monitoring method for thin-walled aircraft part is presented. The concept is based on the guided elastic wave propagation phenomena. This type of waves can be used in order to obtain information about structure condition and possibly damaged areas. In reported investigation piezoelectric transducer was used to excite guided waves in chosen structural element. Specimen was a riveted panel from an aircraft structure. Dispersive nature of guided waves results in changes of velocity with the wave frequency, therefore a narrowband signal was used to minimize the dispersion phenomenon. The generated signal was amplified before applying it to the transducer in order to ensure measurable amplitude of excited guided wave. Measurement of the wave field was realized using laser scanning vibrometer that registered the velocity responses at a points belonging to a defined mesh. This non-contact tool allowed to investigate phenomena related to wave propagation in considered aircraft element. Due to high complexity of the element baseline measurements were taken before measurements for component with the introduced discontinuity. Signal processing procedures were developed in order to visualize the interaction of elastic waves with specimens components (rivets, etc.). In the second stage of research the signals gather by laser vibrometry method were input to the damage detection algorithms. Signal processing methods for features extraction from signals were proposed. These features were applied in order to detect and localize the presence of damage. In the first step damage detection was based on full wavefield measurements. In this way it was possible to obtain amplitude contrast between region with discontinuities and without them. In the second step a point-wise damage detection was conducted. It was based on several laser measurement points treated as sensors. The signal processing was conducted in MATLAB with the procedures developed by authors. The results of damage detection were compared with each other and conclusions were drawn.

In this paper, we present a non-destructive inspection method for immersed waveguide. A laser operating at 532 nm is used to excite leaky guided waves on an aluminum plate immersed in water. The plate has a few artificial defects. An array of immersion transducers is used to detect the propagating waves. A signal processing based on continuous wavelet transform is utilized to extract a few damage-sensitive features that are used in an outlier analysis and in a probabilistic-based imaging method. The experimental results show that the proposed system can be used for the inspection of underwater waveguides.

One of the major concerns in structural health monitoring (SHM) for aerospace systems is the impact localization in plate-like structures. The aim of this paper is to develop a miniaturized, self-contained and ultra-low power device for automated impact detection that can be used in a distributed scheme to control the structural integrity of large isotropic plates, such as those that can be found on an aircraft, without central coordination. The proposed system is based on a geometric composition of 4 different conventional piezoelectric transducers connected to a STM32F4 board equipped with an ARM Cortex-M4 microcontroller and a IEEE802.15.4 wireless transceiver. The processing framework and the algorithm are implemented on-board and optimized for speed and power consumption. The difference in travelled distances (DDOA) and the localization of the impact point are obtained by cross-correlating the signals related to the same event acquired by the different sensors in the warped frequency domain. The performance of the proposed SHM system is analysed in terms of DDOA accuracy and power consumption, showing the effectiveness of the proposed implementation.

Damage monitoring is of great concern to manufacturers as well as maintenance personnel for significantly improving safety and reliability of aircrafts. Delamination and corrosion are among the most interested types of damage which the industries want to monitor for composites and metals correspondingly. In plate-like structures, the aforementioned damage can all practically be treated as 2-dimensional damage. Many progresses have been made on monitoring the location of the damage; however, to monitor the size of the damage is still very challenging. It is known from dynamic theory that elastic waves will be scattered and reflected at the interface of two different media. Thus scattered and reflected waves will be generated at the boundaries of the damage. By analyzing these scattered and reflected waves, the boundaries of the damage can be determined, then, not only the location, but also the size of 2-dimensional damage can be given. In this study, to get as exact monitoring results as possible, two types of locating curves are used: one type acquired by pitch-catch mode and the other type by pulseecho mode. By taking the inner most locating curves, the boundaries of the damage can be given. Experimental results showed that the size of ahole damage can be monitored quite well by the envelop locating curves method.

Various PZT/epoxy 1-3 composites were investigated for high power applications. “Hard” lead zirconate titanates (PZT4 and PZT8) were chosen for active piezoelectrics owing to their high mechanical quality factors, Q_ms, while the passive polymers were selected based on the desired properties for high power composites - low elastic loss, low elastic modulus and high thermal conductivity. The results demonstrated that the composites with high thermal conductivity polymers generally have degraded electromechanical properties with significantly decreased mechanical quality factors, whereas the composites filled with low loss and low moduli polymers were found to have higher Q_ms with higher electromechanical coupling factors k_t: Qm ~ 200 and k_t ~ 0.68 for PZT4 composites; Qm ~ 400 and k_t ~ 0.6 for PZT8 composites. The effects of high drive field on the behavior of 1-3 composites were further investigated by varying active and passive components. Improved high power characteristics of 1-3 piezoelectric composites were achieved by selection of optimized composite components, with enhanced electromechanical efficiency and thermal stability under high drive conditions.

High resolution ultrasound medical imaging requires high frequency transducers, which usually are known with decreased penetration depth because of high loss in two-way-loop at high frequencies. To obtain high resolution imaging at large depth, a dual frequency transducer was designed for contrast imaging. Specifically, a 35 MHz receiving transducer with aperture of 0.6 mm x 0.6 mm was integrated into a 6.5 MHz transmitting transducer with aperture of 0.6 mm x 3 mm. High pressure ultrasound at low frequency was generated by the transducer to excited microbubbles in tissue. High frequency component of the nonlinear response from microbubbles were received by the 35 MHz transducer for high resolution imaging at a relatively large depth. The prototyped transducer showed the ability of transmitting about 2 MPa pressure at 6.5 MHz, under an input of 5-cycle burst at 250 Vpp, which is high enough to generate nonlinear oscillation of microbubbles. The pulse-echo test showed that the -6 dB bandwidth of the 35 MHz transducer is 34.4% and the loop sensitivity is -38.3 dB. The small aperture, dual frequency ultrasound transducers developed in this paper are promising for high resolution ultrasound medical imaging.

In this paper, a novel imaging technique is assessed with a structural health monitoring (SHM) system based on a sparse array of piezoceramic sensors and actuators for in-plane inspection. The imaging approach used in this system is based on the Time-of-Flight (ToF), and the knowledge of the velocity of ultrasonic waves in the structure. While this technique assumes non-dispersive wave propagation, the proposed imaging technique exploits the dispersion of waves as it is based on the phase velocity. The signal measured at a given sensor is correlated with a theoretical prediction of a propagated burst in the structure and, combining the results for multiple sensors, an image of the reflectors in the structure is obtained. This paper presents the implementation of the novel imaging technique in an existing system, including considerations for physical access to the signals and their conditioning. The performance of the existing imaging approach is compared with the novel imaging technique proposed for two test cases. The first assessment is conducted on a simple aluminum plate where magnets are used to simulate a defect. Then, the assessment of the novel imaging techniques is conducted on riveted plates with simulated cracks of different lengths. Imaging results are presented for a number of damage detection scenarios on these structures. The novel imaging technique is shown to improve imaging localization, resolution and robustness, while allowing fast implementation.

Rapid diagnostics and virtual imaging of damages in complex structures like folded plate can help reduce the inspection time for guided wave based NDE and integrated SHM. Folded plate or box structure is one of the major structural components for increasing the structural strength. Damage in the folded plate, mostly in the form of surface breaking cracks in the inaccessible zone is a usual problem in aerospace structures. One side of the folded plate is attached (either riveted or bonded) to adjacent structure which is not accessible for immediate inspection. The sensor-actuator network in the form of a circular array is placed on the accessible side of the folded plate. In the present work, a circular array is employed for scanning the entire folded plate type structure for damage diagnosis and wave field visualization of entire structural panel. The method employs guided wave with relatively low frequency bandwidth of 100-300 kHz. Change in the response signal with respect to a baseline signal is used to construct a quantitative relationship with damage size parameters. Detecting damage in the folded plate by using this technique has significant potential for off-line and on-line SHM technologies. By employing this technique, surface breaking cracks on inaccessible face of the folded plate are detected without disassembly of structure in a realistic environment.

Non-destructive testing methods for rapid and reliable corrosion detection in complex metallic assemblies are an ongoing challenge due to practicalities of inspection and geometric complexity. Corrosion damage, unlike the fatigue damages are almost impossible to determinate where or when it will affect the structures, the current engineering methodology can only determinate the susceptible areas for corrosion. In this scenario, it is very difficult to chose the structures to monitor. Placing sensors on all susceptible areas is not practical. This work demonstrates the evaluation of the Lamb Waves approach in order to detect and locate simulated damages in aluminum alloys placed orthogonally from the sensor network surface. The tests were performed using a typical aeronautical specimen configuration and Direct Image Path from Acellent Technologies. The experimental results indicate the Lamb Waves technique is highly accurate and it has become promising application to detect corrosion damage. This study is part of a set study of several SHM Technologies, like CVM (Comparative Vacuum Monitoring), EMI (Electro-Mechanical Impedance), AE (Acoustic Emission), LW (Lamb Waves). Those studies are under EMBRAER's RandD program.

Growing demands in self-powered, wireless Structural Health Monitoring (SHM) systems has placed a particular attention on energy harvesting products. While most of works done in this domain considered directly coupled active materials, it may be preferential to use seismic (or indirect-coupled) harvesters for maintenance issues. With a seismic type harvester, a model considering constant vibration magnitude excitation is no longer valid as electrical energy extraction from mechanical vibration leads to a reduction of the vibration magnitude of the harvester because of electromechanical coupling effect. This paper extends a Single Degree of Freedom (SDOF) model with a constant force or acceleration excitation to a Two Degree of Freedom (TDOF) approach to describe the tradeoff between the damping effect on the host structure and the harvested power due to the mechanical to mechanical coupling effect. When the harvester mass to host structure mass ratio is around 10^-3, the maximal power is obtained and the host structure has then a sudden displacement reduction due to the strong mechanical to mechanical coupling. Its application to self-powered SHM will be also introduced in the paper.

This paper presents the current investigation in UCSD on the feasibility of using an impedance-based Structural Health Monitoring (SHM) technique in monitoring the Continuous Welded Rail (CWR). Being welded to form uninterrupted rails that are several miles long, the CWR has been widely used in the modern rail industry since 1970s. However, the almost total absence of joints for expansion of CWR would create the potential of buckling with high temperature and breakage in cold environment due to the rail thermal stresses. The objective of this research is to utilize the capability of the impedance method in identifying the neutral temperature or zero-stress state in CWR. The principle of Electromechanical Impedance (EMI) is to utilize high frequency structural vibration through a piezoelectric transducer to detect changes in structural point impedance due to the presence of change of structural integrity or in-situ stress. In practical CWR monitoring, the rail track structure being monitored is undergoing changes due to the effect of thermal stress and the environmental factors. Based on this assumption, three sets of experiments were conducted: the influence of axial stresses on the EMI signature was studied with an axial loading test on a rectangular section of steel milled from a 136lb RE rail; the temperature effect on the proposed method was investigated with heating-cooling cycle test on an unconstrained 136lb RE rail; the third test to simulate the monitoring scenario as expected in the field was conducted with heating-cooling cycle test on constrained 136lb RE rail testbed in UCSD. During the analysis, both the real and imaginary parts of the EM signatures were studied since both the stress and temperature would have different influence on the signatures compared with defect detection. The temperature effects on the piezoelectric materials and structures were investigated.

Making the use of the electromechanical characteristics of piezoelectric, admittance-based structure health monitoring method is an effective method to detect damages. The experiment study on the admittance-based damage detection method with high-order resonant circuit is presented in this paper, including comparisons with first-order resistive circuit, and second-order inductive circuit on the aspects of signal-to-noise ratio and damage detection sensitivity. The results show that the designed damage detection method with high-order resonant circuit has higher signal-to-noise ratio and higher sensitivity to damages than the first-order resistive circuit and the second-order inductive circuit. Optimal electrical components, i.e., resistors, inductors, and capacitors, are found in the experiment study. It also indicates that the admittance change due to damages is very sensitive to the inaccuracies of electrical components. We find that the damage detection method with high-order resonant circuit has very high requirement for the resistance in the circuit, that is to say, even a small resistance (for example, 0.5 Ohm) may cause too large damping for the system.

In this paper a geometric multiscale finite element method (GMsFEM), recently developed by the authors, is applied to the analysis of wave propagation in damaged plates. The proposed methodology is based on the formulation of both two- and three-dimensional multi-node (or multiscale) elements capable of describing small defects without resorting to excessive mesh refinements. Each multiscale element is equipped with a local mesh that is used to compute the interpolation functions of the element itself and to resolve the local fluctuations of the solution near the defect. The computed shape functions guarantee the continuity of the solution between multiscale and conventional elements. This allows using an undistorted discretization in the uniform portion of the domain while limiting the use of multiscale elements only in the vicinity of the defects. In this article the method is applied to evaluate the reflection coefficients due to cracks of different size and orientation in an otherwise homogeneous plate. Also, numerical simulations of wave-damage interaction are used to compute the scattering coefficients associated to three-dimensional defects in isotropic plates.

Recently there has been an increased utilization of composite structures in aerospace and other industries due to their superior physical attributes compared to traditional metallic structures. This has spurred the need for structural health monitoring (SHM) systems to support structural integrity. Guided wave (GW) based techniques for health monitoring have shown to be reliable and promising. The local interaction simulation approach (LISA), a finite difference based numerical method, has been proven to be efficient in modeling GW propagation in isotropic and composite plate structures. Piezoelectric actuators are traditionally used to generate GW in structures. In this work, iterative equations which form the basis of the LISA method are derived for a generic orthotropic laminated structure with a piezoelectric actuator on top. The piezoelectric actuator is modeled by considering the coupled electromechanical equations.

This paper presents an analytical approach to modeling guided Lamb waves interacting with linear and nonlinear structural damage. The active sensing process using piezoelectric wafer active sensors (PWAS) was modeled in the following four steps: (1) guided waves generation by transmitter PWAS (T-PWAS); (2) Lamb wave multi-mode dispersive propagation in the host structure; (3) linear and nonlinear interaction between Lamb waves and damage; (4) guided waves detection by receiver PWAS (R-PWAS). Structural damage was modeled as a new wave source, where guided waves are transmitted, reflected, and mode-converted. In addition, when guided waves interact with nonlinear damage, nonlinear higher harmonics will also be present. Real time sensing signal at R-PWAS was obtained, as well as the time-space wave field and the frequency-wavenumber representation. A Graphical User Interface (GUI) called WaveFormRevealer (WFR) was developed based on this analytical model. High frequency guided wave propagation in thick plates was done first. Beside fundamental modes (S0 and A0), higher wave modes were also observed. These analytical results were verified by experiments. Analytical simulation of linear interaction between Lamb waves and a notch was done next and compared with experiments. New wave packets due to mode conversion at the notch were observed. Subsequently, the nonlinear interaction between Lamb waves and a breathing crack was investigated using a contact finite element model (FEM). Distinctive nonlinear effects were noticed in both FEM simulation and analytical solutions. The paper finishes with summary, conclusions, and suggestions for future work.

The increasing applications of guided waves in the field of ultrasonic Non-destructive Evaluation (NDE) testing leads to the necessity of a mathematical tool able to extract dispersive data for waveguides of general geometrical and mechanical characteristics. Well stated analytical and numerical methods are represented, respectively, by the Matrix family methods and the Semi Analytical Finite Element (SAFE) methods. However, while the former are generally limited to simple geometries, the latter are usually not efficient in problems involving energy losses due to leakage of bulk waves. In this paper, a coupled SAFE-BEM formulation is proposed for the dispersion data extraction of leaky guided waves propagating in waveguides of arbitrary cross-section. To this end, the SAFE is used to efficiently model both the mechanical and geometrical characteristics of the waveguide, while the BEM is used to represent the external (unbounded) isotropic domain. Complex geometries with boundary corners, as well as Cauchy singular integrals typically involved in BEM formulations, are treated using a regularization procedure. The final equation for the wave propagation problem is configured as a nonlinear eigenvalue problem in which the complex wavenumbers appear implicitly. This problem is efficiently solved by means of a Contour Integral Algorithm, that does not require initial guesses of the solutions and derivative operations, providing at the same time good performances in parallelized environments. The reliability of the proposed method is demonstrated by means of a numerical application of practical interest.

Today a steadily growing interest in on-line monitoring of structures is seen. Commonly referred to as structural health monitoring (SHM), the basic idea of this technique is to decrease the maintenance costs based on a continuous flow of information concerning the state of the structure. With respect to the aeronautic industry increasing the service time of airplanes is another important goal. A popular approach to SHM is to be seen in ultrasonic guided wave based monitoring systems. Since one focus is on typical lightweight materials elastic waves seem to be a viable means to detect delimitations, cracks and material degradation. Due to the complexity of such structures efficient numerical tools are called for. Several studies have shown that linear or quadratic pure displacement finite elements are not appropriate to resolve wave propagation problems. Both the mesh density and the spatial resolution needed to control the numerical dispersion are prohibitively large. Therefore, higher order finite element methods (p-FEM, SEM) are considered by the authors. One important goal is to simulate the propagation of guided ultrasonic waves in carbon/glass fiber reinforced plastics (CFRP, GFRP) or sandwich materials. These materials are typically deployed in aeronautical and aerospace application and feature a complex micro-structure. This micro-structure, however, needs to be resolved in order to capture effects like transmission, reflection and conversion of the different wave modes. It is known that using standard discretization techniques it is almost impossible to mesh the aforementioned heterogeneous materials without accepting enormous computational costs. Therefore, the authors propose to apply the finite cell method (FCM) and extend this approach by using Lagrange shape functions evaluated at a Gauss-Lobatto-Legendre grid. The latter scheme leads to the so called spectral cell method (SCM). Here, the meshing effort is shifted towards an adaptive integration technique used to determine the cell matrices and load vectors. Hence, a rectangular Cartesian grid can be used, even for the most complex structures. The functionality of the proposed approach will be demonstrated by studying the Lamb wave propagation in a two-dimensional plate with a circular hole. The perturbation is not symmetric with respect of the middle plane in order to introduce mode conversion. In the paper, an efficient method to simulate the elastic wave propagation in heterogeneous media utilizing the finite or spectral cell method is presented in detail.

A wave propagation based approach for the detection of damage in components of structures having periodic damage has been proposed. Periodic damage pattern may arise in a structure due to periodicity in geometry and in loading. The method exploits the Block-Floquet band formation mechanism , a feature specific to structures with periodicity, to identify propagation bands (pass bands) and attenuation bands (stop bands) at different frequency ranges. The presence of damage modifies the wave propagation behaviour forming these bands. With proper positioning of sensors a damage force indicator (DFI) method can be used to locate the defect at an accuracy level of sensor to sensor distance. A wide range of transducer frequency may be used to obtain further information about the shape and size of the damage. The methodology is demonstrated using a few 1-D structures with different kinds of periodicity and damage. For this purpose, dynamic stiffness matrix is formed for the periodic elements to obtain the dispersion relationship using frequency domain spectral element and spectral super element method. The sensitivity of the damage force indicator for different types of periodic damages is also analysed.

Physics-based computational models play a key role in the study of wave propagation for structural health monitoring (SHM) and the development of improved damage detection methodologies. Due to the complex nature of guided waves (GWs), accurate and efficient computation tools are necessary to investigate the mechanisms responsible for dispersion, coupling, and interaction with damage. In this paper, a fully coupled electromechanical elastodynamic model for wave propagation in a heterogeneous, anisotropic material system is developed. The final framework provides the full three dimensional displacement and electrical potential fields for arbitrary plate and transducer geometries and excitation waveform and frequency. The model is validated theoretically and proven computationally efficient. Studies are performed with surface bonded piezoelectric sensors to gain insight into the physics of experimental techniques used for SHM. Collocated actuation of the fundamental Lamb wave modes is modeled over a range of frequencies to demonstrate mode tuning capabilities. The effect of various actuation types commonly used in numerical wave propagation models on Lamb wave speed are studied and compared. Since many studies, including the ones investigated in this paper, are difficult to perform experimentally, the developed model provides a valuable tool for the improvement of SHM techniques.

We fabricated and tested (in a laboratory configuration) a flexible insert for wireless strain sensing in an intramedullary tibial nail. The nail is a thick-walled titanium tube with an anatomic bend, and our flexible insert was designed to be advanced into the hollow of the nail by a surgeon during the late stages of the fracture fixation process. The flexible insert is a stainless steel rod, 170 mm long, with a part-circular section, roughly 4 mm wide and 2 mm deep that is slightly smaller than the hollow. A lithium niobate SAW device developed by our research team, with an operating frequency of 468 MHz, is bonded to the insert and demonstrates wireless strain sensing when the insert is bent. We describe the mechanics of the flexible insert, which can be used for structural monitoring applications where conventional strain gauge installation and wiring would be impractical. Because of its flexibility, the insert can be advanced into an irregular channel, such as the hollow of a tibial nail, or a narrow access path drilled into a solid. The insert will bend elastically into a configuration with at least three points of contact with the host body. The insert will be prestrained (in bending) during installation and need not be anchored or bonded to the host body other than through contact. Bending strain in the insert will vary as the host body deforms. We discuss possible application questions, and we demonstrate strain sensing in a laboratory specimen.

Viscosity sensors based on mechanical resonance, such as piezoelectric resonators, semiconductor resonators, vibrating wire resonators and our previously developed optical-fiber based resonator [1] have gained popularity due to their simplicity of design and operation. They detect viscosity by submerging an oscillating probe in the fluid of interest. Viscosity is measured by using the fact that the vibration amplitude of the probe is inversely proportional to the viscosity of the fluid. In practice, the vibration amplitudes are always kept sufficiently small in order to avoid nonlinear vibration effects. In this paper, however, nonlinear vibration is intentionally excited to improve the sensitivity of viscosity measurements, in particular super harmonic resonance case. The results show that a nonlinear effect drastically improves sensitivity to viscous damping. Experimental results and several applications are also presented.

Experimental results on the propagation of guided waves through a bolted joint under various bolt load values are presented. Piezoelectric wafer active sensor (PWAS) transducers are used for the generation and reception of the guided waves. Two specimen types are used, a strip lap joint and a plate lap joint. The signals measured under various bolt load values and frequency values are studied in order to identify relevant features that change drastically with bolt load. We found that some of the signals, especially those at S0 tuning frequency of 320 kHz, were very much simplified by the change from strip to plate conditions. The main contribution of this paper to the advancement of the state of the art consists in highlighting the need for removing the confounding effects of the strip side reflections on the correct interpretation of guided wave changes as they travel through a lap joint with various fastener loads.

Electrospun fibers have been applied in numerous areas. In many of the applications, aligned fibers are desired for their specific properties. Recently, many different techniques have been developed to create aligned electrospun fibers. The direction of the fibers as well as the distribution of fiber orientations, in general, are analyzed by SEM which requires significant processing, thus increases time, and therefore costs. Currently we are examining these materials with forward light scattering in order to obtain fiber alignment and the distribution of fiber orientations. The proposed technique is slightly modified in the hardware and software from previous small angle scattering techniques to provide a more accurate and detailed mapping of the fibrous structure.

In this work, sensing capability of gold nanoparticles embedded in polydimethylsiloxane (PDMS) based localized surface plasmon resonance (LSPR) was investigated. A simple, stable, and low-cost sensor is presented. Various samples at different initial thickness of gold films were prepared and used to investigate the thickness effect on their optical properties. For optical characterization, a UV-Vis spectrophotometer was used in order to measure absorption spectra of the prepared samples. The UV-Vis spectrophotometer results showed that thicker initial gold film produces a wider and red shift absorption spectrum compared to the thinner gold film. Besides, the spectroscopic ellipsometry was used to investigate the sensitivity based LSPR of the proposed structure. Hence, the proposed sensor could be good as a cost-effective scheme for biosensing applications.

Attenuation of Lamb waves, both fundamental symmetric and anti-symmetric modes, propagating through carbon fiber reinforced polymer (CFRP) was modeled using the multi-physics finite element methods (MP-FEM) and compared with experimental results. Composite plates typical of aerospace applications were used and provide actuation using integrated piezoelectric wafer active sensors (PWAS) transducer. The MP-FEM implementation was used to combine electro active sensing materials and structural composite materials. Simulation results obtained with appropriate level of Rayleigh damping are correlated with experimental measurements. Relation between viscous damping and Rayleigh damping were presented and a discussion about wave attenuation due to material damping and geometry spreading have been led. The Rayleigh damping model was used to compute the wave damping coefficient for several frequency and for S0 and A0 mode. The challenge has been examined and discussed when the guided Lamb wave propagation is multimodal.

This paper examines the mechanics of delamination, ply variation, and the fabrication on the sensing ability for magnetostrictive particles embedded in a carbon fiber reinforced polymer laminate. An analytical method is used to determine how delamination and ply variation affect the mechanical state and magnetic properties of the embedded terfenol-d particles. Numerical models are also used to simulate the effect of delamination and ply variation on the mechanical state. For the analytical method, the mechanical properties observed are the net strain and stress in a local particle section resulting from magnetostriction. A one dimensional load line equation and material property data are used to obtain approximate solutions. The magnitudes of the stress and magnetostriction drop in the laminates are observed. Based on the local mechanical and magnetic state, the magnetic permeability can be selected from experimental data. The analytical method reveals that the effect of a delamination is to reduce the resistance to particle actuation in a local area, which allows for variation in stress and magnetostriction magnitudes in damaged areas vs. nondamaged areas. This variation in the mechanical state subsequently affects the magnetic permeability, which changes the reluctance in the local particle layer. These results are compared to a numerical model of terfenol-d embedded in carbon fiber reinforced polymer laminate, which reveal a drop in stress and increase in magnetostriction in the delamination region. Finally, these results are projected to experimental results from health monitoring scans of carbon fiber reinforced polymer laminates with varied ply count and a delamination.

Continuous Welded Rail (CWR) is used in modern rail construction including high-speed rail transportation. The absence of expansion joints in these structures brings about the risk of breakage in cold weather and of buckling in warm weather due to the resulting thermal stresses. The University of California at San Diego (UCSD), under a Federal Railroad Administration (FRA) Office of Research and Development (RandD) grant, is developing a system for in-situ measurement of the rail Neutral Temperature in CWR. Currently, there is no well-established technique able to efficiently monitor the rail thermal stress, or the rail Neutral Temperature (rail temperature with zero thermal stress), to properly schedule slow-order mandates and prevent derailments. UCSD has developed a prototype (Rail-NT) for wayside rail Neutral Temperature measurement that is based on non-linear ultrasonic guided waves. Numerical models were first developed to identify proper guided wave modes and frequencies for maximum sensitivity to the thermal stresses in the rail web, with little influence of the rail head and rail foot. Experiments conducted at the UCSD Largescale Rail NT Test-bed indicated a rail Neutral Temperature measurement accuracy of a few degrees. The first field tests of the Rail-NT prototype were performed in June 2012 at the Transportation Technology Center (TTC) in Pueblo, CO in collaboration with the Burlington Northern Santa Fe (BNSF) Railway. The results of these field tests were very encouraging, indicating an accuracy for Neutral Temperature measurement of 5°F at worst, on both wood ties and concrete ties.

This paper examines the current challenges of using Lamb wave interrogation methods to localize fatigue crack damage in a complex metallic structural component in the presence of temperature variations. The goal of this research is to improve damage localization results for a structural component interrogated at an unknown temperature by developing a probabilistic and reference-free framework for estimating Lamb wave velocities. The proposed approach for temperature-independent damage localization involves a model that can describe the change in Lamb wave velocities with temperature, the use of advanced time-frequency based signal processing for damage feature extraction, estimation of the actual Lamb wave velocities from transducer signals, and a Bayesian damage localization framework with data association and sensor fusion. The technique does not require any additional transducers on a component and allows the estimation of the velocities for the actual Lamb waves present in a component. Experiments to validate the proposed method were conducted using an aluminum lug joint interrogated with piezoelectric transducers for a range of temperatures and fatigue crack lengths. Experimental results show the advantages of using a velocity estimation algorithm to improve damage localization for a component interrogated at both known and unknown temperatures.

Laser ultrasonic line sources have been used to study the ultrasonic properties of nuclear graphites. These materials exhibit varying degrees of porosity and texture that relate directly to the conditions imposed during material processing-extruded materials display significant texture while the anisotropy of molded materials is significantly lower. Both the texture (related to grain orientation) and porosity impact the long term performance of graphite under service conditions and methods are needed to assess the microstructural states of these materials during service. Laser ultrasonic measurements can be used to assess aspects of material microstructure by measuring longitudinal and shear wavespeeds as a function of propagation direction and polarization. While porosity-related effects are independent of propagation direction for materials with spherical pores, material texture (related to preferred grain orientation) produces anisotropic wave propagation effects. In particular, propagation perpendicular to extrusion directions can produce shear wave birefringence effects that can be used to assess texture. Ultrasonic measurements in this work were made using laser ultrasonic methods that yield waveforms that can be interpreted using elastodynamic models for wave propagation in anisotropic materials. In particular, models for laser ultrasonic line sources in transversely isotropic materials have been used to simulate laser sources in nuclear graphites. The effects of optical penetration (related to material porosity) have been incorporated to produce synthetic waveforms that can be used to extract modulus information from experimental measurements. Current results hold open the opportunity for porosity and texture assessment using limited sets of ultrasonic measurements.

Although damage detection using Lamb waves has been investigated for many years, real engineering applications are limited due to practical aspects related to implementation. Temperature effect is one of the major problems. It is well known that temperature variations influence Lamb wave propagation response parameters. In practice it is important to compensate for this effect. Experimental tests are often required to understand how temperature influences wave propagation. Numerical simulation can ease this task preventing many time-consuming experiments. Simulated Lamb wave responses can be used to develop new methods for temperature compensation. The effect of temperature variations on piezoceramic transducer responses is investigated using finite element modelling. The model takes into account temperature-dependent physical properties of low-profile PZT transducers and transducer bonding layers. The model is used to predict the S₀ and A₀ Lamb response in aluminium plate for the temperature range from -60 to +40°C. The study shows relevant changes in Lamb wave amplitude response caused by temperature fluctuations. This approach can provide the basis for temperature compensation in ultrasonic guided wave damage detection systems used for structural health monitoring applications.

The paper demonstrates how to remove the undesired temperature effect from Lamb wave data in order to detect structural damage more precisely and reliably. The method used is based on the cointegration technique and wavelet analysis. The former is built on the analysis of non-stationary behaviour whereas the latter brings the concept of multiresolution decomposition of time series. Instead of directly using Lamb wave data for damage detection, three approaches are used: (1) analysis of the variance of wavelet coefficients of Lamb wave responses before cointegration, (2) analysis of the cointegrating residuals obtained from the cointegration process of Lamb wave responses, and (3) analysis of the variance of wavelet coefficients of Lamb wave responses after cointegration. These approaches are tested on undamaged and damaged aluminium plates that have been exposed to temperature variations. The experimental results show that the first approach still exhibits temperature variability and damage cannot be detected. In contrast the second and third approaches can isolate damage-sensitive features from temperature variations, detect the existence of damage and classify its severity.

Experimental findings of non invasive in-vivo monitoring are essential to study the diversity and evolution of musculoskeletal kinematics. In this paper, results obtained from the uni-axial monitoring of the quasi-static dynamics of the biceps muscle-belly are reported. Monitoring of the belly diameter is based on a custom developed ultrasonic caliper combined with the synchronously recorded applied external force and joint angle variations detected with a custom build ultrasonic force sensor and a resistive angle decoder respectively. The monitored muscle action includes the processes of active muscle contraction and relaxation in a closed path starting with an initial isotonic contraction followed by eccentric (spring like) stretching. The technology applied here allows for observations of those processes and registration of their paths in the length-forceangle parameter space. That way of presentation reveals that at some conditions the closed-loop human cycles follow in close approximation characteristic lines of well identifiable elementary processes. The presentation of these processes in the length-force parameters space allows for discussion of the mechanical energy expenditure during different muscle actions. Comparative studies of identical closed-loop muscle actions and the joint angle-force-length relationships of the muscle-tendon complex are presented. This synchronous monitoring also allows quantifying the joint torques and positions with high accuracy for living person.

An image acquisition utilize a 2D piezoelectric actuated optical scanner is presented. The scanner is consisted of a chemically etched tapered single mode optical fiber (SM600) mounted on a X-Y coupled piezoelectric bimorphs operating at the actuators’ resonant frequencies of 2800 Hz and 70 Hz, that generates a40 lines raster scanning light pattern around the area of interest. The image is acquired by a nearby photodetector based on the reflected intensity generated by the scanner. . The scanner achieves a 64.5 μm vertical and 7.6 μm horizontal tip displacements with a + 18V input. Initial 1D image acquisition shows the system is capable of resolving a line pattern of 85μm with a gap space of 100μm with a S/N ratio of 10 dB.

This paper presents a dedicated Finite Element approach for quantitative time-transient simulations of stress and pressure waves propagation in biological structures as human bones. The tool, starting from a magnetic resonance image (MRI) as the one of a human leg, builds a three-dimensional finite element (FE) mesh by converting voxels into elements. This step does not require any segmentation or further geometric interpretation of the tissue structure, only the mechanical properties have to be provided via Hounsfield (HU) number density mapping. The proposed tool improves upon the usually adopted models taking into account the irregular geometry of the bone as well as the soft tissues and their damping role, typically neglected. The tool code can handle models of hundred millions of elements in a standard PC desktop, exceeding thus capabilities of commercially available FEM codes. Here, an application on a human leg is proposed to show the potential of the proposed tool. The results of the time-transient simulations are next exploited to validate the use of guided waves models for the non invasive ultrasonic diagnosis of elongated bones. In particular, the recorded time-waveforms are analyzed via the 2D Fast Fourier Transform and the frequency-wavenumber energy content of the propagating waves is extracted. Such information is compared with the guided waves dispersion curves predicted, considering a representative cross-section of the tibia, via a Semi Analytical Finite Element (SAFE) formulation. Some final considerations on the comparison of the extracted and predicted dispersion curves close the paper.

We report an electromagnetic inductance/coil-based non-destructive method to target distal screw-holes in an intramedullary interlocking-nail surgical operation for fixing a long-bone fracture. The method is a radiation-free approach addressing the over-exposure issue of radioactivity caused by the typical X-ray-imaging approach. According to the method, we fabricate a targeting-system consisting of an internal inductance, external coil, guiding-mechanism, and driving/measurement electronics. When a voltage is applied to the internal inductance embedded in one of the distal screw-holes of a nail inserted in a bone, a directional magnetic flux is generated by the internal inductance due to the electromagnetic induction. Subsequently, the directional magnetic flux penetrates the nail and bone. When the external coil outside the bone scans along the axial and angular directions of the nail/bone, different amount of the generated magnetic flux is detected by the coil and consequently corresponding voltage response is induced in the coil due to the electromagnetic induction. In contrast to the magnetic flux generated and detected by the inductance and coil, respectively, we also investigate the reverse physics-behavior of the flux transmission (i.e., flux generated and detected by the coil and inductance) in order to improve the approach. Finally, by correlating the induced-voltage responses with the scanned axial-locations along the nail/bone, correlation curves are plotted. Through analyzing the curves, a criterion for predicting the location of the screw-holes of the nail is established. When compared the predicted location with the actual location of the screw-hole, the maximum targeting error is 2 mm for locating a screw-hole with a diameter of 5 mm. The result shows the targeting-method is accurate, fast, and easy for the surgeons and significantly simplifies the existed interlocking-nail surgical procedures.

One of the significant engineering applications of the elastic metamaterial is for the low-frequency vibration absorption because of the existence of low-frequency bandgaps. However, the forbidden gap from existing elastic metamaterials is of narrow bandwidth which limits their practical engineering application. In this paper, a chiral-lattice- based elastic metamaterial beam with multiple resonators is suggested for the broadband vibration suppression by overlapping their resulting bandgaps. First, a theoretical modeling of the metamaterial beam with periodically multiple resonators is performed for bending wave propagation. The wave interaction between the multiple resonators is found to generate new passbands, which is a barrier to form a complete bandgap. To address this issue, a section distribution of the multiple resonators is suggested to diminish the interaction. Finally, the chiral-lattice-based metamaterial beam is fabricated and experimental testing of the structure is conducted to validate the proposed design. This work can serve as a theoretical and experimental foundation of the broadband vibration suppression by using the metamaterial structure in practical engineering applications.

Stiffness and damping are conflicting requirements in many material systems. High stiffness is required in a wide range of structural components to provide sufficient robustness under demanding loading conditions. Simultaneously, a structure should be able to effectively mitigate shock and vibrations dynamically transmitted to it by the environment. While most conventional structures currently exhibit limited adaptability and damping capabilities, design strategies to simultaneously endow structural assemblies with high stiffness and high damping performance are proposed in this work. To this aim, a backbone structure suitable to meet stiffness requirements is combined with metamaterial inclusions able to provide fully-passive shock and vibration absorption. Viscoelastic resonant lattices with chiral topology are employed as inclusions, whose aim is to confine vibrational energy, pump it away from the backbone structure, and dissipate it through viscoelastic damping. The lattices are composed by an elastomeric matrix with the desired chiral shape, and stiff resonating inclusions are inserted at nodal locations. Both finite element simulations and experimental tests demonstrate that periodic chiral assemblies give rise to wide frequency bandgaps. Low-frequency tuning of the assembly for effective suppression of the first resonant mode of a backbone structure represented by an aluminum box-beam is demonstrated both numerically and experimentally. The considered lightweight inclusion is a chiral matrix realized with castable rubber, featuring graded cylinder mass insertions. The proposed design methodology can be flexibly tailored to various frequency ranges and is applicable to both existing and novel structural components at different scales.

A novel metamaterial using split ring resonator is envisioned. Unlike traditional metamaterials, multiple, uniquely designed split rings and elliptical full rings, embedded in polymer matrix are proposed. Purpose of such metamaterial is not only creating simultaneous negative effective mass density and negative bulk modulus but also use them to control structural vibration. Eigen-frequency analyses are performed to find multiple resonant frequencies and corresponding multiple acoustic band gaps. The dispersion equation of the periodic media is solved numerically using finite element method. The acoustic wave modes corresponding to both low and high frequency phonons are obtained. The proposed media was considered to be a periodic media consists of periodic array of unit cells, each cell containing specific geometry of split rings. Each unit cell is made of a metal sphere enclosed in metal circular ring. A pairs of circular split rings are placed symmetrically at the center of the unit cell. In addition to circular split rings, another set of elliptical split rings are also positioned to enclose the aforementioned assembly. To understand the effect of elliptical split rings on band gaps, split rings are then replaced by full rings and dispersion of similar acoustic wave modes are compared.

Here we present methods for modeling, analysis, and design of metamaterial beams for broadband vibration absorption/isolation. The proposed metamaterial beam consists of a uniform isotropic beam with many small spring-mass- damper subsystems integrated at separated locations along the beam to act as vibration absorbers. For a unit cell of an infinite metamaterial beam, governing equations are derived using the extended Hamilton principle. The existence of stopband is demonstrated using a model based on averaging material properties over a cell length and a model based on finite element modeling and the Bloch-Floquet theory for periodic structures. However, these two idealized models cannot be used for finite beams and/or elastic waves having short wavelengths. For finite metamaterial beams, a linear finite element method is used for detailed modeling and analysis. Both translational and rotational absorbers are considered. Because results show that rotational absorbers are not efficient, only translational absorbers are recommended for practical designs. The concepts of negative effective mass and stiffness and how the spring-mass-damper subsystems create a stopband are explained in detail. Numerical simulations reveal that the actual working mechanism of the proposed metamaterial beam is based on the concept of conventional mechanical vibration absorbers. It uses the incoming elastic wave in the beam to resonate the integrated spring-mass-damper absorbers to vibrate in their optical mode at frequencies close to but above their local resonance frequencies to create shear forces and bending moments to straighten the beam and stop the wave propagation. This concept can be easily extended to design a broadband absorber that works for elastic waves of short and long wavelengths. Numerical examples validate the concept and show that, for high-frequency waves, the structure’s boundary conditions do not have significant influence on the absorbers’ function. However, for absorption of low-frequency waves, the boundary conditions and resonant modes of the structure need to be considered in the design. With appropriate design calculations, finite discrete spring-mass- damper absorbers can be used, and hence expensive micro- or nano-manufacturing techniques are not needed for design and manufacturing of such metamaterial beams for broadband vibration absorption/isolation.

In this paper, a method to focus flexural Lamb waves to a local area by mounting elastic metamaterials (EMMs) on the surface of the plate is proposed. The EMM consists of silicon rubber and lead connected in series bonded vertically on an aluminum plate. A simplified effective mass-“spring”-mass model is used to study the EMM plate. The frequency-dependent effective mass density of the EMM plate is determined with the aid of the numerically based effective medium method. By making use of the low locally resonant frequency of the EMM plate, the EMM plate is carefully designed with different dimensions to attain high effective mass densities. The effective mass density can be assumed to dominate the change of wave velocity and propagation direction in the EMM plate. An effective mass density profile is then employed along the transverse direction of wave propagation to achieve focusing. Finally, numerical simulation with finite element method (FEM) is utilized to investigate the focusing phenomenon of the A₀ mode Lamb waves at 30 kHz and the out-of-plane displacement response beyond the EMM region. Numerical simulation results have shown that focusing the low frequency A₀ mode Lamb waves using EMMs is feasible. The focusing may have potential applications in structural health monitoring by manipulating Lamb waves through controlling and focusing Lamb waves to any arbitrary location of the plate with amplified displacement and yet largely retained five-peaked toneburst waveform.

This study investigates the stability of guided wave signals generated and recorded from bonded piezoelectric sensor packages proposed for use in structural health monitoring systems. The study considers the effects of both actuation cycling and thermal exposure on the piezoelectric sensors. The tests are performed using polyimide film encapsulated piezoelectric sensors bonded to aluminum plates and a titanium wing attachment lug, with the testing specifically designed to avoid causing any damage to the host structure. Stability is quantified by computing a correlation coefficient between a reference signal and each test signal. The reference signal is recorded under the initial healthy condition, so any potential changes in the correlation coefficient value are attributed to aging effects. The effects of possible timing differences causing decreases in the correlation coefficient values are reduced by a jitter correction algorithm. The first set of experiments uses four aluminum plates held at constant temperature; four piezoelectric actuators on each plate transmit to a centrally located piezoelectric sensor. To investigate the effect of accumulated actuation cycles on the transducers, different numbers of actuation signals are applied to each set of four actuators. Each test is conducted for 150 blocks, with each of the four actuators producing either 1000, 500, 250, or 100 signals per block. Results from the testing are mixed, with some excitation/sensing paths remaining stable over the duration of all blocks while other paths show substantial changes, including clear trends of decreasing correlation coefficient values. In a second experiment, sensors on a titanium wing attachment lug are exposed to relatively benign levels of thermal exposure. Each test starts with the temperature at a selected baseline value of 120°F. A series of ten elevated temperature exposures are applied with the exposure temperature increasing in 10°F increments to 220°F. The reference and test signals are collected after returning to the baseline temperature. As in the first set of experiments, results from the testing are mixed. Some excitation/sensing paths remain stable over the duration of the test, while others are substantially degraded. For both tests, the exact mechanism causing the instabilities remains unknown. However, the mixed outcomes suggest that the signal changes observed over the course of the collections may be due to flaws within the piezoelectric or electrode material of a specific sensor, or involve the adhesive bond between a particular sensor and the structure.

Assessing the robustness of a sensor system and the related predictions is a key step in structural health monitoring (SHM). In this paper, the SHM system under consideration uses macro fiber composite (MFC) sensors to generate ultrasonic guided waves for inspection of a composite bolted joint. Bolt bearing failure is introduced to a target hole through tensile loading. The MFC sensor-actuators are configured in a circular array around the target hole and used to send and receive the ultrasonic waves from many different directions. This strategy facilitates a scattering matrix approach to identifying the most effective actuation and sensing angles for damage detection. Multiple interrogation wavelengths are also utilized to study the effect of the wavelength relative to the size of hole being monitored. However, various other factors are expected to impact the ultrasonic signal and therefore the damage detection results. Particularly, the position and orientation of each sensor is precisely measured through image processing techniques in order to quantify the effects of sensor misalignment. Finally, the sensitivity of the ultrasonic inspection technique to variation in each phenomenon is compared to better understand the most significant effects on damage detection performance.

In this paper, a guided wave-based pressure mapping system is presented for medical and touch-screen applications. Piezoceramics are used as actuators and to measure the reflected waves due to an added mass or local pressure, responsible for a change of local surface impedance. SHM imaging algorithms are implemented in order to obtain cartography of the reflections and deduce the presence, localization and intensity of pressure spots. Analytical and numerical models are validated on an aluminum plate prototype instrumented using 1 actuator and 2 sensors. It is observed that imaging of single or multiple pressure spots is achieved using both A0 and S0 modes around 300 kHz with a resolution of 0.5 mm.

A novel Compressed Sensing (CS) procedure is presented in this study for dispersive guided wave propagation analysis in passive structure health monitoring applications. The proposed approach combines theWavelet Packet multiresolution analysis, best basis selection and coefficients thresholding to generate a sparse but accurate time-frequency representation of the acquired dispersive signal, with the CS framework to efficiently compress Lamb waves signals. This approach is tested on experimental data obtained by passive excitation in a 1 m square aluminum plate and acquiring the dispersive signal with a conventional piezoelectric sensor. The proposed algorithm performance is analysed in term of compression ratio and percent residual difference. Results show the improvement in signal reconstruction with the use of the modified CS framework respect to the EZW coding, and the robustness of the proposed approach to additive noise in transmission.

In this work, we present our study of Lamb wave crack detection using wavenumber analysis. The aim is to demonstrate the application of wavenumber analysis to 3D Lamb wave data to enable damage detection. The 3D wavefields (including v_x, v_y and v_z components) in time-space domain contain a wealth of information regarding the propagating waves in a damaged plate. For crack detection, three wavenumber analysis techniques are used: (i) time-space Fourier transform which can transform the time-space wavefield into frequency-wavenumber representation while losing the spatial information; (ii) short space Fourier transform which can obtain the frequency-wavenumber spectra at various spatial locations, resulting in a space-frequency-wavenumber representation; (iii) local wavenumber analysis which can provide the distribution of the effective wavenumbers at different locations. All of these concepts are demonstrated through a numerical simulation example of an aluminum plate with a crack. The 3D elastodynamic finite integration technique (EFIT) was used to obtain the 3D wavefields, of which the v_z (out-of-plane) wave component is compared with the experimental measurement obtained from a scanning laser Doppler vibrometer (SLDV) for verification purpose. The experimental and simulated results are found to be in close agreement. The application of wavenumber analysis on 3D EFIT simulation data shows the effectiveness of the analysis for crack detection.

In this paper, the authors propose a finite element model of a simple single bolt joint that undergoes loosening in order to verify characteristic changes due to bolt loosening and develop a loose-bolt detection system. The model is created using 3D solid elements and surface-to-surface contact elements between head/nut and flange interfaces. Pretension effects and contact behavior between flanges to be joined are also taken into account. In order to validate the finite element model by experiment, vibration testing method based on non-contact impulse excitation by high-power YAG pulse laser which can produce an ideal impulse is conducted. The characteristic changes due to the bolt loosening in high frequency region can be extracted by the present laser excitation system. Finally, an approach of loose bolt detection is demonstrated by applying statistical evaluation of Recognition-Taguchi (RT) method to a six bolt cantilever which has loose bolt.

The paper presents a method to locate discontinuities in form of transversal cracks in beams, based on vibration measurements. Patterns characterizing frequency changes of the first ten weak-axis bending vibration modes are determined for all possible locations on the structure, using a relation contrived by the authors. It base on the correlation between the strain energy stored in a segment of the beam, which is proportional with the square of the mode shape curvature of the considered vibration mode at that location, and the frequency change for this mode by the occurrence of a discontinuity on that segment. The patters consist from a series of ten values representing the normalized relative frequency shifts for the first ten vibration modes. For a structure similarly supported, by continuous or periodical measurements, potential frequency changes can be detected. By processing these data the so-called damage location index for that crack is found out, also as a series of ten values representing the relative frequency shifts of the ten vibration modes. To precise locate the crack a pattern matching method involving the database with all possible patterns and the damage location index is used. Knowing the location, it is easy to determine by analytic calculus the crack depth. The method is easy to be used, provide accurate results, demands modest computational effort and has the advantage that the measurements may be carried out in situ with rather simple equipment. The method was validated by experiments.

This paper presents dynamic characteristics and damage inspection of beams with actual fatigue cracks by using a boundary-effect evaluation method (BEEM) to perform space-wavenumber analysis of operational deflection shapes (ODSs) and a conjugate-pair decomposition (CPD) method to perform time-frequency analysis of dynamic responses of some points to a harmonic excitation. BEEM is for locating and estimating small structural damage by processing ODSs measured by a full-field measurement system (e.g., a scanning laser vibrometer or a camera-based motion measurement system). BEEM is a nondestructive spatial-domain method based on area-by-area processing of ODSs and it works without using any structural model or historical data for comparison. CPD uses adaptive windowed regular harmonics and function orthogonality to perform time-frequency analysis of time traces by extracting time-localized regular and/or distorted harmonics. Both BEEM and CPD are methods for local spectral analysis based on local, adaptive curve fitting. Numerical simulations and experimental results show that dynamic characteristics of beams with actual fatigue cracks are different from those of beams with open artificial cracks. Moreover, results show that the combination of BEEM and CPD for space-wavenumber and time-frequency analysis provides an accurate tool for damage inspection of thin-walled structures with actual cracks.

Patients who suffer from unstable pelvic fractures are usually implanted with internal fixations which provide structural rigidity as well as allow sufficient contact between fracture edges for healing to occur. A 12 week post-operative period of immobility is typically enforced on the patient to ensure this healing process is not hindered. This extended period of restricted movement could cause muscle wastage which results in additional rehabilitation time. It is therefore highly beneficial to develop a non-invasive method which can be used in-situ to monitor the healing of the pelvis in hopes of allowing patients to undertake earlier weight bearing activities and reduce muscle degradation. This paper studies the dynamic behaviour of a fixated synthetic pelvis by monitoring its response over varying stages of stiffness recovery. The synthetic pelvis was cut at the sacrum araldite was applied over the cut site and allowed to cure over a one hour period. Excitation signals were introduced to the synthetic structure by means a shaker using the fixation screws as wave guides. Transfer functions obtained from an array of sensors bonded to the pelvis and the fixation demonstrate significant changes occurring over the stiffness recovery period due to glue curing. If these changes can be quantified, future research could lead to the development of smart fixations which can monitor the state of healing in the pelvis.

Frequency response related quantities are widely used for damage detection and structural health monitoring (SHM) because of their computational robustness and clear physical interpretations. In reality, there is uncertainty due to noise or other operational variability involved in any feature evaluation, which makes it hard to select a robust and sensitive SHM feature. Two specific spectra are considered in this paper, namely, frequency response function (FRF) and transmissibility, while FRF includes system resonances and transmissibility only has system zeros. A Bayesian model selection framework is adopted by comparing the Bayes factor of using either feature in structural health monitoring applications, and suggests which is better with regard to plausibility. This framework is implemented with data acquired from a lab-scale plate structure, and to be more realistic, external artificial noise is contaminated to the data imitating a more stringent test condition.

Calibration of a finite element model based on measurement-data for complex structures is usually costly and sometimes not applicable. In this article, a methodology for detecting abnormal behavior including slow aging degradations of a structure solely based on historical patterns of the measurement data will be introduced. In the first step, principal components of the truck load test measurement data - that is centered and scaled - are calculated. In the second step, unsupervised classification is applied to the score data that is regenerated based on the major principal components. The same algorithm is applied to the measurement data of the bridge response to the sharp temperature change as well. Finally, the specified algorithm is applied separately to the collected static data from the Jeremiah Morrow Bridge (more than four years) using the calculated truck load test principal components. The optimized clustering model detected the outliers that are caused by heavy truck loads; clustering result is detailed. In summary, a simple data model that is able to find any known data signature such as truck load test in the daily measurement data is proposed. The proposed method is part of an ongoing effort in University of Cincinnati Infrastructure Institute to use the correlation between collected readings from different members of a bridge in order to interpret abnormal trend changes in the measurement data.

The main objective of this study is to present a novel method for damage detection in plate-type structures using twodimensional (2D) continuous wavelet transforms. For this purpose, the 2D Mexican wavelet is employed to remold the equation of motion for transverse vibration of a plate. The remolded vibration equation of a plate can serve as a multiscale damage detection scheme that characterizes damage using an indicator of multiscale pseudo-load. Effects of multiscale pseudo-load can pinpoint the location of the damage as well as revealing its configuration; moreover, the strong solid mechanics foundation of the method results in the identified damage with an explicit physical implication. The performance of the proposed technique is validated through an experimental program of using a scanning laser vibrometer (SLV) to measure the transverse deflection of an aluminum plate bearing a cross-like notch and an added small mass. The results confirm the robustness and superior capability of the proposed method in detecting damage in plate-type structures.

There is increasing use of spectroscopic techniques, such as high-resolution NMR spectroscopy, to examine variations in cell metabolism and/or structure in response to numerous physical, chemical, and biological agents. In these types of studies, in order to obtain relative quantitative information, a comparison between signal intensities of control samples and treated or exposed ones is often conducted. The methods thus far developed for this purpose are not directly related to the overall intrinsic properties of the samples, but rather to the addition of external substances of known concentrations or to indirect measurement of internal substances. Another possibility is to estimate, by an opportune algorithm, a normalization constant which takes into consideration all cell metabolites present in the sample. Recently, a new normalization algorithm, based on Principal Component Analysis (PCA), was presented. PCA is a well-known statistical technique for analysis of large, multivariate datasets, which extracts the basic features of the data. The PRICONA (PRincipal COmponent Normalization Algorithm) algorithm use PCA in a new totally different manner: PCA is, in fact, used to normalize spectra in order to obtain quantitative information about the treatment effects. In this paper, it is shown that PRICONA can be used in the time domain, that is on NMR FIDs (Free Induction Decay) instead of on NMR spectra. That is advantageous because NMR FIDs do not require any operator dependent manipulation. The algorithm was tested by Monte Carlo simulations of NMR FIDs.

Vibration-based nondestructive damage detection relying on modal curvatures has been investigated in various applications. An intrinsic deficiency of a modal curvature is its susceptibility to the noise inevitably present in a measured mode shape. This adverse effect of noise is likely to mask actual features of damage, resulting in false results of damage detection. To circumvent this deficiency, a Teager energy operator (TEO), aided by a wavelet transform, is adopted for the treatment of mode shapes to produce a new algorithm for damage identification. After wavelet-transform- based preprocessing to separate the effective components of modal curvatures from noise interference, a TEO is employed as a singularity detector, acting on the separated effective components, to reveal and characterize the features of damage. The capacity of the TEO is demonstrated analytically in cases of cracked beams. The applicability of the algorithm is experimentally validated using a scanning laser vibrometer to acquire mode shapes of an aluminum beam bearing a crack. The analytical and experimental results show that the TEO, aided by wavelet transforms, has stronger sensitivity to slight damage and greater robustness to noise than modal-curvature- and wavelet-transform-based damage detection methods.

Guided Lamb waves in the ultrasonic range have potential for structural health monitoring of thin structures, e.g. for the detection of impact damages which may cause delamination in carbon fiber reinforced materials. For the emission of guided waves piezoelectric transducers can be used which are applied to the surface of the structure. By using a phased array of transducers a directivity pattern for the inspection of a limited area on the structure can be created with common beam forming algorithms. Line arrays require only a small number of transducers but the main lobe is generated on both sides of the array which means an excitation towards an unwanted direction is produced. In this contribution a 2D array design is introduced which tends to emit only one main lobe towards the direction of interest. The concept basically utilizes two parallel line arrays. Both arrays emit signals with a single burst. The signal emitted by the second line array is meant to suppress the unwanted lobe of the main array by out-of-phase superposition. This requires an appropriate timing of the emission of the signals of the single transducers. The feasibility of the concept has been studied by simulation. Practical experiments on CFRP (carbon fiber reinforced polymer) sheets have been carried out with an array layout with eight single piezoelectric transducers. A PC-controlled electronics system has been used for the actuation of the transducers. Emission and directional behavior of the Lamb waves on the structure has been monitored with a Laser scanning vibrometer.

Tactile perception of different types of tissue is important in order for surgeons to perform procedures correctly and safely. This is especially true in minimally invasive surgery (MIS) where the surgeon must be able to locate the target tissue without a direct line of sight or direct finger touch. In this study, tissue characterization using an acoustic wave tactile sensor array was investigated. This type of tactile sensor array can detect the acoustic impedance change of target materials. Abnormal tissues can have different Young’s moduli and shear moduli caused by composition change compared to those of healthy tissues. This also leads to a difference in acoustic impedance which can be detected using our sensor array. The array was fabricated using a face-shear mode PMN-PT piezoelectric resonator which is highly sensitive to acoustic impedance load. Gelatin and water mixtures with weight concentration of 5 wt % - 30 wt % were prepared as tissue phantoms. The shear modulus of each phantom was measured using bulk face-shear mode crystal resonators, and it was found that shear modulus change from 120 kPa to 430 kPa resulted on 30 % electrical impedance shift from the resonator. Imaging display of elastic properties of prepared phantoms was also tested using the fabricated sensor array. The proposed tissue characterization technique is promising for the development of effective surgical procedures in minimally invasive surgery.

Integration of multi-type senor data, which is mutual complementary, is a potential way to improve the accuracy and robustness of structural damage detection method. However, the effect of damage diagnosis based on multi-sensor data integration depends on the optimal sensor placement for multi-sensor data integration. Therefore, in the paper, an innovative optimal sensor placement based on sensitivity is proposed to determine the number and locations of three kinds of sensors including accelerometers, Fiber Brag Gauges (FBG), which are generally applied in vibration tests, and piezoelectric (PZT) sensors, which are commonly used in active sensing-based structural health monitoring. With considering the boundary effect and uncertainty caused by environment, the sensitivity-based object function to detect every possible damage location is established. Computational simulation on a fixed-supported steel thin plate-like structure is implemented to evaluate minimum sensor number of accelerometers, FBG and PZT according to methods above. After that, the optimal locations and number for three kinds of sensors are calculated.

Fatigue is one of main failure forms that the steel structure loses its functions under fluctuating loads. Take a container crane for example, the randomness of lifting load and trolley reaching positions, namely load probability and position probability, are the most obvious characteristic when loading and unloading containers on the ship. Correspondingly load spectrum and position spectrum is constructed to estimate the crack growth remaining life by using Paris’ Law and then the periodic inspection interval is proposed to conduct the structural risk management. A real case is analyzed in accordance with the risk assessment process mentioned in the paper and successfully explains which members to inspect, where to inspect, and how to inspect. At last the inspection interval is recommended based on the field structural monitoring data and it is economical and feasible.

This study presents a damage detection method for transmission towers based on vector auto-regressive (VAR) models. The vibration signals obtained from both baseline and unknown conditions of the structure are divided into multiple data segments, respectively, and each segment is then modeled as a VAR time series. The diagonal elements of the VAR coefficient matrices series are extracted, and that vector’s Mahalanobis distance (MD) is used as a damage-sensitive feature. At the sensor locations where damage is introduced, the mean and variance of MD distribution will change from their values under baseline condition. Thus, the area under a receiver operating characteristic (ROC) curve and deflection coefficient of MD distribution are used as the decision metric for damage detection, localization, and severity. The method’s effectiveness is assessed on a 6 degree-of-freedom mass-spring simulation system and a transmission tower model. The results confirm the high potential and effectiveness of this method for data-driven damage assessment.

The civil engineering community is becoming increasingly interested in monitoring structural behavior of civil infrastructure and in evaluation of the structural performance. The demand has largely been driven by deficiencies in structural performance due to the aging of the infrastructure, excessive loading, and natural disasters such as an earthquake, a landslide, a typhoon and a tsunami. In this study, a structural health monitoring methodology using acceleration responses is proposed for damage detection of a three-story prototype building structure during shaking table testing. A damage index is developed using the acceleration data and applied to outlier analysis, one of unsupervised learning based pattern recognition methods. A threshold value for the outlier analysis is determined based on confidence level of the probabilistic distribution of the acceleration data. The probabilistic distribution is selected according to the feature of the collected data.

We proposed a type of piezoelectric composites for energy harvesting from low-frequency noise environment. The composites are composed of metallic rings and discs placed on two surfaces of a clamped PVDF film. An equivalent mass-spring model is used to estimate the range of operating frequencies. Acoustic-structure coupled simulation is conducted to calculate electricity generated from PVDF films and the vibration effects of the central discs and the outer rings areanalyzed. Numerical results show that the vibration modes can be tuned by changing the sizes of discs and rings. The first peak in harvested electricity at a lower frequency corresponds to the resonant mode in which the ring and the disc vibrate in unison, while the second peak at a higher frequency is due to the resonant mode in which the larger block (the ring or the disc) vibrates while the other remains almost motionless. The two types of electrodes, namely the S-type and the U-type, are designed according to the two vibration modes. The experimental result shows that the proposed composite with optimal structure parameters can improve the harvesting efficiency more than 100% in the band of 50- 800Hz.

The intrusion of unwanted sound is a matter of increasing concern worldwide as growing sound pollution impacts human health, productivity and wellbeing. Lower frequency noise is particularly important, as this is where the human ear is most sensitive. Irritating acoustic intrusion increasingly occurs in buildings, yet sound insulation at low frequencies is challenging and expensive. Meta-materials o er a relatively new approach to achieving sound and vibration isolation. This approach is being used to develop panels with internal resonant structures that are only a few centimetres thick yet strongly interact with acoustic waves. These structures can yield signi cantly greater transmission loss than conventional insulation systems. Numerical models based on networks of single-degree of freedom oscillators were used to understand how the components of the locally resonant structure (LRS) can be manipulated to generate sound transmission loss (STL) performance spectrums. Designs with the desired STL characteristics were then examined in detail and samples were fabricated using industry-standard materials and processes. This paper focuses on the acoustic testing of these LRS samples at low frequencies. Comparisons were made between, numerical predictions and experimental results (small scale (plane wave) to large scale (di use eld) conditions). The highest performing network arrangements combined layers of resonators with multiple resonances to increase system bandwidth. At frequencies below 1 kHz the samples yielded large attenuation gains with peaks of 80dB under normal incidence, and good correspondence to modelling predictions. In di use eld conditions the samples still showed signi cant STL improvements above that of a conventional panel over bandwidths in the order of 300 Hz. The resulting systems have the potential to provide signi cantly higher transmission loss at low frequencies than conventional wall systems of similar size and weight.