This PDF file contains the front matter associated with SPIE Proceedings Volume 8348, including the Title Page, Copyright information, Table of Contents, and Introduction, and Conference Committee listing.

Ultrasonic load and structural health monitoring schemes, based on high temporal resolution of the transport times of ultrasound, have lately been refined such that the achievable temporal resolution can reach 1 ps even for center frequencies in the lower MHz-regime. Whereas this technique was initiated with signal processing based on digitization and subsequent correlation by software controlled digital processing of the data, equipment has lately been designed and manufactured, allowing rather universal processing of low level analog signals by analog based correlations, employing digital references converted to respective analog signals. The signal and data processing schemes are presented and illustrated with experimental results. The advantages and disadvantages of both methods are discussed, including the limiting effects concerning restrictions given by technological and theoretical aspects. Basic features are demonstrated based on the actually developed and adapted instrumentation including software based operations for both principles of signal and data processing. Even though these methods are also used in ultrasonic imaging, the range of applications presented here focuses dominantly on ultrasonic structural health and load monitoring with bulk and guided acoustic waves.

Dispersive and multimodal guided wave propagation environments are often avoided in Structural Health Monitoring (SHM) applications by single mode selection through narrowband excitation and wavelength tuning. However, scatterer-induced mode conversions are relevant to damage localization and characterization. A Bimodal Warped Frequency Transform (BWFT) is proposed to track the time-frequency structure of propagating waves undergoing mode conversion in dual-mode environments. The BWFT provides a two-dimensional representation of a recorded time-trace in a modal distance vs. total distance domain. Consequent "modal path history" retrieval can effectively improve damage detection and characterization in broadband, multimodal regimes, as demonstrated both numerically and experimentally.

This paper presents a method for Acoustic Emission (AE) source localization in isotropic plate-like structures based on the Extended Kalman Filter (EKF). The accuracy of the traditional triangulation methods depends on the time of flight (TOF) measurements and on the group velocity assumption so uncertainties in both should be taken into account and filtered out. An algorithm based on the Extended Kalman Filter (EKF), capable of filtering out these uncertainties, has been developed for the estimation: 1) the AE source location and 2) the wave velocity. Experimental tests have been carried out on an aluminum plate to show accuracy and robustness of the proposed approach.

Lamb wave method detects the defects from the propagation characteristics of the created brief harmonic signals. Generally, the defects are detected by analyzing the delays and amplitudes of the received waves. The envelopes of the sensory signals may be used to calculate the delays and amplitudes of the received signals. Sometimes, similar envelopes could be observed at different test conditions. Use of the time-frequency spectra of the s-transformation is proposed for distinction of the problems when the envelopes of the monitored signals are very similar. In the study, a beam was compressed from different points with a hydraulic crimping tool. In separate tests, the cross sectional area at the middle of the beam was reduced by opening slots. The envelopes and time-frequency spectra of the sensory signals were calculated by using the s-transformation. The difference of the time-frequency spectra successfully distinguished the test condition when the envelopes were very similar.

The structure of a wind turbine blade plays a vital role in the mechanical and structural operation of the turbine. As new generations of offshore wind turbines are trying to achieve a leading role in the energy market, key challenges such as a reliable Structural Health Monitoring (SHM) of the blades is significant for the economic and structural efficiency of the wind energy. Fault diagnosis of wind turbine blades is a "grand challenge" due to their composite nature, weight and length. The damage detection procedure involves additional difficulties focused on aerodynamic loads, environmental conditions and gravitational loads. It will be shown that vibration dynamic response data combined with AANNs is a robust and powerful tool, offering on-line and real time damage prediction. In this study the features used for SHM are Frequency Response Functions (FRFs) acquired via experimental methods based on an LMS system by which identification of mode shapes and natural frequencies is accomplished. The methods used are statistical outlier analysis which allows a diagnosis of deviation from normality and an Auto-Associative Neural Network (AANN). Both of these techniques are trained by adopting the FRF data for normal and damage condition. The AANN is a method which has not yet been widely used in the condition monitoring of composite materials of blades. This paper is trying to introduce a new scheme for damage detection, localisation and severity assessment by adopting simple measurements such as FRFs and exploiting multilayer neural networks and outlier novelty detection.

In this study, a relatively new non-destructive evaluation technique known as electro-mechanical impedance (EMI) method has been investigated on concrete structures with large surface area. Although various studies have been conducted on concrete components, a major issue still exists as when using EMI method, damage detection can be very difficult. Due to the property of concrete, only a minor variation in the impedance signature is noticed subjected to damage. Such small change can be hard for one to verify whether the host structure is damaged or not. Since other factors can also change the impedance signature including, temperature changes, humidity and aging of the piezo material, this can increase the uncertainties when differentiating an intact case from a damaged case. In this study, the effect of a technique to enhance the damage detection ability of EMI method is evaluated on concrete structures with the aforementioned problem.

Ultrasonic guided waves are routinely used to inspect pipes. The advantage of this technique is that it enables a fullyvolumetric screening of several metres of pipe from a single transducer location, resulting in substantial time and cost savings. However, it suffers from limitations such as relatively low damage sensitivity and difficulties in dealing with intricate pipe networks; furthermore, for a pipe that is buried, submerged or high up in a plant, access to even a single point can be prohibitively expensive. The use of permanently attached sensors can overcome these limitations since access needs to be obtained only once during installation and they enable the use of baseline subtraction, so that any reading from a sensor can be compared to previous readings. This paper discusses the advantages of baseline subtraction and the challenge of compensating for signal changes due to effects other than the growth of damage. It is shown that the use of baseline subtraction allows significant damage sensitivity improvements, particularly in the vicinity of large reflectors. Data from four years of field experience is backed up by accelerated laboratory testing.

Guided waves are becoming popular for pipeline monitoring because of its sensitivity to small defects and long sensing range. Several research groups have explored ultrasonic imaging techniques for pipeline monitoring by fully utilizing the advantageous characteristics of guided waves. They have generated torsional mode and measured torsional reflections from pipe defects using expensive shear-mode piezoelectric transducers or electromagnetic acoustic transducers (EMAT), and then extracted crack-induced flexural modes as a post processing step. In this study, these existing techniques are further advanced to perform ultrasonic imaging by measuring the crack-induced flexural modes only, eliminating the necessity of the torsional mode information. This enables us to replace the expensive shear-mode transducers used to measure the torsional reflections with inexpensive compression-mode macro fiber composite (MFC) transducers. First, a magnetostrictive transducer attached at one end of the pipe is used for the excitation of a pure torsional mode in the pipe specimen. Then, the torsional mode propagates through the pipe, and the reflections are created by the interaction of the incident torsional mode with the crack. Due to the sensing characteristics of MFC transducers, only the crack-induced flexural modes can be measured by multiple MFC transducers placed along the circumference near the excitation magnetostrictive transducer. Using the normal mode expansion technique, each individual flexural mode among all reflected signals is extracted. By propagating this crack-induced flexural mode back in the space considering its dispersion characteristics, the location of the crack is visualized.

This paper presents the development of a Bayesian framework for optimizing the design of a structural health monitoring (SHM) system. Statistical damage detection techniques are applied to a geometrically-complex, three-story structure with bolted joints. A sparse network of PZT sensor-actuators is bonded to the structure, using ultrasonic guided waves in both pulse-echo and pitch-catch modes to inspect the structure. Receiver operating characteristics are used to quantify the performance of multiple features (or detectors). The detection rate of the system is compared across different types and levels of damage. A Bayesian cost model is implemented to determine the best performing network.

Elastic properties of materials can be easily determined from the ultrasonic wave velocity measurement. However, material hardness cannot be obtained from the ultrasonic wave speed. Heat treatment and aging affect the microstructure of materials changing their hardness and strength. In this study it is investigated how the attenuation of ultrasonic guided waves is affected by the duration of heat treatment and varying material hardness. To this aim six identical Aluminum 2024 alloy plate specimens were subjected to different durations of heat treatment at 150°C and were inspected nondestructively propagating Lamb waves through the specimens. Attenuation of the Lamb wave was found to be inversely related to the hardness. Rockwell Hardness test was performed to corroborate the ultrasonic observations. In comparison to Rockwell hardness test the ultrasonic inspection was found to be more sensitive to the heat treatment duration. From these results it is concluded that guided wave inspection method is a reliable and probably more desirable alternative for characterizing the hardness of heat treated materials.

The high strength to weight ratio of fibrous composites such as glass-fiber reinforced polymers (GFRP) makes them prominent structural materials. However, their laminar nature is susceptible to delamination failure the onset of which traditional structural health monitoring (SHM) techniques cannot reliably and accurately detect. Carbon nano-tubes (CNT) have been recently used to tailor the electrical conductivity of polymer based materials that otherwise behave as insulators. The occurrence of damage in the polymer matrix produces localized changes in conductivity which can be tracked using electrical impedance tomography (EIT). This paper explores combining advances in composite manufacturing with EIT to develop a SHM technique that exploits anisotropic conductance monitoring for enhanced delamination and matrix crack detection.

The laminated construction of composite offers the possibility of permanently embedding sensors into structure, for example, ultrasonic transducers which can be used for NDE applications. An attractive and simple solution for probing embedded sensors wirelessly is via inductive coupling. However, before this can be achieved it is necessary to have a full understanding and proper design strategy for the inductively coupled system. This paper presents the developments of both system design procedure and a computer program for one dimensional inductively coupled transducer system mounted on a solid substrate. The design strategy in this paper mainly focuses on issues of localization of transducers, and optimizing the signal to noise level. Starting from a three coil equivalent circuit, this paper also explains how the measured impedance of a bonded piezoelectric disc is implemented into the system model representing a transducer bonded to an arbitrary solid substrate. The computer programme using this model provides immediate predictions of electrical input impedance, acoustic response and pulse-echo response. A series of experiments and calculations have been performed in order to validate the model. This has enabled the degree of accuracy required for various parameters within the model, such as mutual inductance between the coils and self-inductance of coils, to be assessed. Once validated, the model can be used as a tool to predict the effect of physical parameters, such as distance, lateral misalignment between the coils, and the coil geometry on the performance of an inductively coupled system.

An admittance-based damage detection method with a higher-order circuit is proposed and investigated in this paper, in order to increase the damage detection sensitivity. The change of the stiffness in the structure due to the damages can be detected by measuring the admittance from the piezoelectric transducer adhered on the host structure. It is known that an inductive circuitry of an R-L-in-series branch serially connected with the piezoelectric transducer can introduce an additional resonance to the mechanical system. With the piezoelectric capacitance, it is a second-order circuit system with two resonances. In this research, based on the electrical-mechanical analogy, a higher-order circuit is designed to improve the damage detection capability, i.e., increasing the admittance magnitude and the admittance sensitivity magnitude. Theoretical analyses and simulations are carried out. The results show that the admittance magnitude and the admittance sensitivity are both significantly increased by using the higher-order circuit when comparing with the second-order circuit, both without electrical damping.

Composite structures are being extensively used in modern industrial applications due to their superior physical attributes, thus necessitating the need for structural health monitoring (SHM) systems to ensure their structural integrety. Guided wave (GW)-based methods are an obvious choice because of their ability to travel long distances through the thickness of the composite structures. In this work, local interaction simulation approach (LISA), a finite difference (FD)-based numerical method, is used to study the GW propagation characteristics in laminated composite plates. The iterative equations, which form the core of the LISA method, have been derived for orthotropic materials with in-plane rotation. Simulation results for uni-ply and quasi-isotropic graphite/epoxy laminates are used to demonstrate the capabilities of the advanced equations.

Elastic waves are frequently employed in Non-Destructive Testing and Structural Health Monitoring systems for damage detection and evaluation. Accurate and fast simulation of structural responses - due to high frequency ultrasonic excitation - is possible by the application of graphical processing units (GPUs). The research work presented in this paper is motivated by high demands for model sizes and GPU memories. The paper presents new developments related to the simulation framework for elastic wave propagation. The proposed software environment uses the modified local interaction simulation approach and graphical processing units to model ultrasonic wave propagation in structures. The method has been enhanced by the application of multi-GPU workstations. This allows for effective simulations that can be performed in complex structural components. The multi-GPU architecture and decomposition approach are accompanied by absorbing boundary conditions for the model, enabling the reduction of model size and replacing missing parts of the model by infinite space.

Damage detection and localization on composites can be impaired by inaccurate knowledge of the mechanical properties of the structure. This paper demonstrates the feasibility of using a chirplet-based correlation technique, called Excitelet, to evaluate the mechanical properties of orthotropic carbon fibre-based composite laminates. The method relies on the identification of an optimal correlation coefficient between measured and simulated dispersed signals measured on a structure using piezoceramic (PZT) transducers. Finite Element Model (FEM) is first conducted to demonstrate the capability of the approach to evaluate the mechanical properties of a composite structure. Experimental validation is then conducted on a unidirectionnal 2.30 mm thick laminate composed of unidirectional plies and a 2.35 mm thick laminate composed of unidirectional plies oriented at [0, 90]_4s. Surface bonded PZT transducers were used both for actuation and sensing of guided waves bursts measured at 0° and 90° with respect to upper ply fibre orientation. The characterization is performed at various frequencies below 100 kHz using A₀ or S₀ modes and comparison with the material properties measured following ASTM standard testing is presented. The results indicate that large correlation coefficients are obtained between the measurements and simulated signals for both A₀ and S₀ modes when accurate properties are used as inputs for the model. Strategies based on multiple modes correlation are also assessed in order to improve the accuracy of the characterization approach. The results obtained using the proposed approach for the unidirectional plate and most of the results obtained using the proposed approach for the [0, 90]_4s laminate are in agreement with the uncertainty associated with ASTM tests results while the proposed method is non destructive and can be performed prior to each imaging processing.

The paper discusses basically a wave propagation based method for identifying the damage due to skin-stiffener debonding in a stiffened structure. First, a spectral finite element model (SFEM) is developed for modeling wave propagation in general built-up structures, using the concept of assembling 2D spectral plate elements and the model is then used in modeling wave propagation in a skin-stiffener type structure. The damage force indicator (DFI) technique, which is derived from the dynamic stiffness matrix of the healthy stiffened structure (obtained from the SFEM model) along with the nodal displacements of the debonded stiffened structure (obtained from 2D finite element model), is used to identify the damage due to the presence of debond in a stiffened structure.

In this research work, an imaging method of the nonlinear signature in a reverberant complex anisotropic structure with hysteretic behaviour is reported. The proposed technique relies on a combination of phase symmetry analysis with frequency modulation excitation and nonlinear time reversal, and it is applied to a number of waveforms containing the nonlinear impulse responses of the medium. Phase symmetry analysis was used to characterize the third order nonlinearity of the structure due to delamination and cracks, by exploiting its invariant properties with the phase angle of the input waveforms. Then, a "virtual" reciprocal time reversal imaging process, using only two sensors in pitch-catch mode, was used to "illuminate" the damage. Taking advantage of multiple linear scattering, this methodology allows achieving the optimal focalization at the nonlinear source by a compensation of the distortion effects in a dissipative medium. The robustness of this technique was experimentally demonstrated on a damaged sandwich panel undergone to low-velocity impact loading. The nonlinear source was retrieved with a high level of accuracy with little computational time (less than 1 sec). Its minimal processing requirements make this method a valid alternative to the traditional nonlinear elastic wave spectroscopy techniques for materials showing either classical or non-classical nonlinear behaviour.

This paper presents an ultrasonic method to evaluate the contact state of two solid boundaries using guided wave propagating between the joining surfaces. The main idea is that the guided wave travels with different wave velocity depending on the contact pressure of interface. Mathematical formulation for acoustic wave propagation at contacting solids was made to obtain the dispersive relation between acoustic wave and contact pressure. Three different kinds of steel block with cylindrical tip were machined and pressed together at various compression loads to form contact surfaces of different contacting condition. Guided wave reflected at the edge of the steel block was measured to determine wave speed using the time-of-flight, which increases sensitively with the load. Experimental results proved that contact states such as contact force and interfacial stiffness between two solid surfaces can be monitored by the acoustic wave speed of guided wave in the interface.

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 presents the development of mode selective actuator and sensor systems based on interdigital transducer (IDT) design. Various parameters such as wavelengths, number and apodization of electrodes as well as eigenfrequencies of the transducer are characterized. Therefore, an analytical model based on the theory of surface acoustic wave (SAW) filters is investigated in order to evaluate the acoustic response of the transducer. Furthermore, experimental tests on composite plates are performed.

Current limitations for diagnosing mineralization state of tooth enamel can lead to improper surgical treatments. A method is investigated by which the tooth health state is characterized according to its thermal response, which is hypothesized to be sensitive to increased porosity in enamel that is caused by demineralization. Several specimens consisting of previously extracted human teeth a re prepared by exposure to Streptococcus mutans A32-2 in trypticase-soy-borth supplemented with 5% sucrose at 37°C for 3 or 6 days to de-mineralize two 1×1mm²-windows on each tooth. One of these windows is then re-mineralized with 250 or 1,100ppm-F as NaF for 10 days by pH-cyclic-model. Pulse thermography is used to measure the thermal response of these sections as well as the sound (healthy) portions of the specimen. A spatial profile of the thermal parameters of the specimens is then extracted from the thermography data and are used to compare the sound, de-mineralized, and re-mineralized areas. Results show that the thermal parameters are sensitive to the mineralization state of the tooth and that this method has the potential to accurately and quickly characterize the mineralization state of teeth, thereby allowing future dentists to make informed decisions regarding the best treatment for teeth that have experienced demineralization.

Here we present the current status of our microfabricated SU-8 cantilever beam scanner for endoscopic examination. The current design has improved performance with the implementation of a MEMS based push-pull actuator. Fabrication of the SU-8 rib waveguide was measured to be ~5.0μm as compared to ~50μm in the previous design, further improves our spatial resolution. We have made it easier to couple an optical fiber into the device and achieved ~98% coupling efficiency by altering the system geometry. The proposed rib waveguide design also allows a relatively large waveguide cross section (4μm in height and 55μm in width) and broader band single mode operation (λ= 0.7μm to 1.3μm) with a minimum transmission loss (85% output transmission efficiency with Gaussian beam profile input). Our design provides a new means to create a 2-D raster scanning pattern, verified by transient finite element analysis (FEA). The scanner's line resolution and field of view (FOV) have been optimized through a parametric study using modal and harmonic analyses. This paper describes the fabrication and testing of our optical scanner, which may find application in the area of endoscopy, where there is a need for a minimally invasive device that reduces patient discomfort.

The force-length relation is one of the most important mechanical properties of skeletal muscular tissue. Due to the rather limited availability of non-invasive methods suitable to quantify the in-vivo biomechanical properties of activated human muscles and connected tendons, the quantification of the bio-mechanical properties is difficult. The measurement principle applied here is based on the detection of the dynamics of the muscle under observation by an ultrasonic caliper and monitoring of the externally present forces by a synchronously operated ultrasonic force sensor. The developed monitoring scheme is exemplified for gradual increasing voluntary isometric contraction (MVIC) of the gastrocnemius muscle up to maximum contraction, with the force sensor restricting the flexion of the joint. The temporal resolution for the monitoring is 0.01 s, relating to a monitoring rate of 100 Hz and is achieved with a spatial resolution concerning the observed lateral extension of the muscle of 0.01 mm. The employed low power, economic and non-intrusive detection scheme and respective instrumentation have the demonstrated potential to quantify the in-vivo hysteretic behavior of the observed force-length relation for MVIC of the human gastrocnemius muscle for the first time. The purpose of this study was to determine in-vivo the force-length relations for the human gastrocnemius and biceps muscles noninvasively by suitable experimental techniques with high temporal and spatial resolution concerning monitoring of the biomechanical relevant parameters involved in the dynamics of activated muscle. The data is collected and analyzed to derive quantitative information on force-length relations, essential for the analysis of muscle performance and interpretation by musculoskeletal models. The involved technologies are demonstrated and the respective results are presented and discussed.

Magnetic Resonance Imaging (MRI) is a medical imaging technique used in radiology to visualize the detailed internal structure and body soft tissues in complete 3D image. MRI performs best when optimal imaging parameters such as contrast, signal to noise ratio (SNR), spatial resolution and total scan time are utilized. However, due to a variety of imaging parameters that differ with the manufacturer, a calibration medium that allows the control of these parameters is necessary. Therefore, a phantom that behaves similar to human soft tissue is developed to replace a real human. Polymer gel is novel material that has great potential in the medical imaging. Since very few have focused on examining the behavior of polymer lesions, the motivation of this study is to develop a polymer gel phantom, especially for liver, with embedded lesions. Both the phantom and lesions should be capable of reflecting T₁ and T₂ relaxation values through various characterization processes. In this paper, phantom and lesion particles were fabricated with carrageenan as a gelling agent by physical aggregation. Agar was used as supplementary gelling agent and T₂ modifier and Gd-DTPA as T₁ modifier. The polymer gel samples were fabricated by varying the concentrations of the gelling agent, and T₁ and T₂ modifiers. The lesion particles were obtained by extracting molten polymer gel solution in chilled oil bath to obtain spherical shape. The polymer gel properties including density, elastic modulus, dielectric constant and optical properties were measured to compare with human tissue values for long period of time.

Fourier transform spectroscopy (FTS) is a potent analytical tool for chemical and biological analysis, but is limited by system size, expense, and robustness. To make FTS technology more accessible, we present a compact, inexpensive FTS system based on a novel liquid crystal (LC) interferometer. This design is unique because the optical path difference (OPD) is controlled by voltage applied to the LC cell. The OPD is further improved by reflecting the polarized incident light through the LC several times before reaching the second polarizer and measurement. This paper presents the theoretical model and numerical simulations for the liquid crystal Fourier transform spectrometer (LCFTS), and experimental results from the prototype. Based on the experimental results, the LCFTS performs in accordance with the theoretical predictions, achieving a maximum OPD of 210μm and a resolution of 1nm at a wavelength of 630nm. The instrumental response refresh rate is just under 1 second. Absorbance measurements were conducted for single and mixed solutions of deionized water and isopropyl alcohol, demonstrating agreement with a commercial system and literature values. We also present the LCFTS transmission spectra for varying concentrations of potassium permanganate to show system sensitivity.

Results achieved with the developed health and load monitoring schemes based on high resolution temporally resolved observation of the transport properties of guided ultrasonic waves are presented and discussed. The methods are illustrated and results covering monitoring of ablation and delamination, time resolved load detection, and monitoring of impact positions with axial resolution are presented. The detection schemes have been primarily developed for integral monitoring of the presence and occurrence of damages including scaling of damages respectively determination of the occurrence of overloading and impact. Sensitivities and methods suitable for damage scaling are presented and discussed. The developed schemes are as well suitable for on-ground as for in-flight monitoring, with the exception that the impact position detection requires continuous operation at least in a stand by mode. Concerning the rather compact size and moderate power consumption the developed instrumentation is already suitable for in-flight respectively on-ground hand hold and battery powered operation. The achievements and results are presented, explained, and discussed.

Based on mode selective monitoring of the transport of guided ultrasonic waves with high temporal resolution achieved by correlation techniques, a helicopter tail boom section, manufactured from aluminum, has been monitored concerning structural health and load including monitoring under shaker driven conditions. The characteristic sensitivities of the developed health and load monitoring schemes are presented together with methods suitable for the compensation of the influence of temperature variations during monitoring. Concerning the detection of loose joints, a novel monitoring scheme has been introduced capable to detect fluctuations in joints including riveted joints and connections by high-locks. The currently operated system is fully remote controlled and can be operated by internet. Concerning the rather compact size and power consumption, the developed instrumentation is already suitable for in-flight respectively for on-ground hand hold and battery powered operation. The observations, achievements, and results are presented, explained, and discussed.

The structural health monitoring literature has shown an abundance of features sensitive to various types of damage in laboratory tests. However, robust feature extraction in the presence of varying operational and environmental conditions has proven to be one of the largest obstacles in the development of practical structural health monitoring systems. Cointegration, a technique adapted from the field of econometrics, has recently been introduced to the SHM field as one solution to the data normalization problem. Response measurements and feature histories often show long-run nonstationarity due to fluctuating temperature, load conditions, or other factors that leads to the occurrence of false positives. Cointegration theory allows nonstationary trends common to two or more time series to be modeled and subsequently removed. Thus, the residual retains sensitivity to damage with dependence on operational and environmental variability removed. This study further explores the use of cointegration as a data normalization tool for structural health monitoring applications.

Features derived from estimations of transmissibility (output-to-output response) are now widely used in vibration-based structural health monitoring (SHM) applications. However, for realistic conditions, transmissibility measurements are always subject to environmental, operational and measurement variability, and the uncertainty will propagate through to any features derived from it, leading to misinterpretation and false alarms. This paper proposes a statistical model that quantifies that uncertainty so that confidence intervals on feature estimates are possible, leading to reduced false alarms and the possibility of a minimum detection performance calculation. The uncertainty quantification model is validated via a clamped plate structure, and for a stricter validation requirement, additive noise is contaminated to the lab measurements to simulate more realistic in-situ conditions. A good consistency is observed between the proposed statistical model and real test results, and leads to promising applications for structural health monitoring with quantified uncertainty.

The integrity monitoring of foam based thermal insulation systems is investigated under a load-temperature environment. Guided wave propagation studies are conducted on two specimens using the pitch-catch approach. To receive only the guided wave, the interval distance is 15mm and 20 mm between two adjacent sensors for specimens. The appropriate guided wave modes are generated by changing the excitation signal. To simulate the thermal and load environment of the thermal insulation system, the test piece is assembled on the load-temperature test machine, where the compression load gradually up to 3 tons is applied to the test piece along its axial direction. Different temperatures are applied to two sides of the test piece. The side without the foam is cooled using Liquid nitrogen to -196°C, while the other side with the foam is heated by the thermocouple to 120 °C. The guided wave signals are obtained before/after the experiment begins/ends, and also received periodically in the joint work process of the pressure and the different temperatures. Signals processing and damage imaging techniques are combined to demonstrate the possible disbond defect. Experimental results show that the disbond defect between the foam and the substrate can be qualitatively detected, and its expansion could be identified.

The scattering of Lamb waves from multiple corrosion pits on the surface of a plate is investigated. Previous work produced a model which solved for the scattered field when a Lamb wave is incident on a single corrosion pit, and we present an improvement on that model, and use it to study the case of multiple corrosion pits. The solution technique solves for the scattered field by replacing the corrosion pits with equivalent point loads applied to the surface of the plate, and the reciprocity theorem is used to calculate the scattered field. For the case of multiple corrosion pits, an implicit set of equations are derived using the self consistent method, and an approximation is advanced which results in an explicit set of equations which approximate the scattered field. The second order approximation to the multiple scattering problem is considered and solved semi-analytically for an ensemble of corrosion pits.

High-frequency guided ultrasonic waves allow for the non-destructive testing of aerospace structures. This type of structure often contains multi-layer components subjected to cyclic loading conditions, where fatigue cracks and localized disbonds can develop. Using standard ultrasonic transducers, high frequency guided wave modes were generated in a model structure consisting of two adhesively bonded aluminum plates. This type of waves propagates along the structure and penetrates through the complete thickness. The wave propagation along the specimen was measured experimentally using a laser interferometer. Good agreement with 2D finite element simulations was found. Two types of hidden defects were considered: localized lacks of sealant and small defects in the aluminum layer facing the sealant. The interaction of the high frequency guided waves with the hidden defects was investigated. Standard pulseecho measurements were conducted to verify the detection sensitivity and the influence of the stand-off distance predicted from the finite element simulation results. The high frequency guided waves have the potential for fatigue crack growth monitoring at critical and difficult to access fastener locations in aerospace structures from a stand-off distance.

Coulomb coupling has been applied for imaging of bulk and guided acoustic waves propagating in a 0.5 mm thick, z cut Lithium Niobate single-crystal. The excitation and detection of acoustic waves was performed by localized electrical field probes. The developed scheme has been applied to imaging of the transport properties of skimming longitudinal and guided acoustic waves. A short pulse of 20 ns has been used for the excitation of acoustic waves. Broadband coupling is achieved since neither mechanical nor electrical resonances are involved. The attenuation of acoustic waves in piezoelectric crystals is studied by this method. A thin film of conductive silver paint was deposited on the surface of the crystal acting as an acoustic attenuator inducing also mass loading effects and shortening of electrical fields. The group velocities of the propagating acoustic waves for both conditions, with and without the conductive silver paint film, are determined from the propagation of the acoustic wave fronts.

The frequency-based beam steering concept effectively supports Guided-Wave-based Structural Health Monitoring (SHM) by enabling directional waveguide inspection. This is implemented by acoustic transducers whose peculiar shapes provide different wavelength tuning in different directions. When these devices are used for guided wave (GW) sensing, spatial filtering of the propagating wavefield results in a prominent frequency component within the recorded signal spectrum, which can be uniquely associated with the direction of an incoming wave. A sensor geometry whose 2D spatial Fourier Transform produces a spiral-like distribution of maxima in the wavenumber domain allows for one-to-one frequency-angle correspondence in the [0°, 180°] range. Prototypes of this wavenumber spiral frequency steerable acoustic transducer (WS-FSAT) have been fabricated by patterning the electrodes' shape on a metallized polyvinylidene fluoride (PVDF) substrate through inkjet printing. Prototype testing in various pitch-catch configurations demonstrates accurate 2D localization of acoustic sources and scattering events by processing a single output signal. Extremely easy, quick and inexpensive fabrication approach, along with very low hardware and computational requirements make the proposed FSAT an ideal candidate for a wide range of in-situ, low-cost and wireless SHM applications.

In recent years, the interest in the research and development of "green energy" has increased dramatically, with numerous research grants and investment in the areas of wind power, solar power and fuel cell technology. We present results obtained from the evaluation of the acoustic properties of proton-exchange membranes used in hydrogen fuel cells, which relate directly to the microelastic properties of such membranes. These properties play an important role in the durability and applicability as well as the efficiency of such membranes. DuPont Nafion membranes are the most commonly used polymeric membranes in hydrogen/oxygen fuel cells and are therefore used as examples in this study. The microscope used in this non-destructive characterization study is a vector-contrast version of the scanning acoustic microscope which yields images in magnitude- and phase contrast.

In this paper, we present our preliminary results of high aspect ratio 3D PEDOT pillar study by drop-on demand (DOD) direct printing system. Design of the experimental setup and the fabrication of the DOD PEDOT pillar are introduced. Currently, the system can achieve a PEDOT pillar with a height of 300 μm and 80 μm in diameter. The proposed PEDOT 3D printing process has a wide range of potential applications in the eletronics and display industry.

Although there are many laser ultrasonic imaging techniques developed so far, it still remains challenging to create such images from a rotating object. In this study, an advanced laser ultrasonic imaging technique is developed so that wavefield images can be constructed from a rotating blade using an embedded piezoelectric sensor and a scanning excitation laser system. Here, the biggest challenge is to precisely estimate and control the exact excitation point when the wind blade is rotating with additional ambient vibration and having complex shapes. In this study, the laser excitation point is precisely estimated by computing the correlation values between the measured response signal and the ones in the training data sets. First, training ultrasonic signals are measured at the fixed sensing point by scanning the excitation laser over the target surface of the blade when the blade is in a stationary condition. Once the training is complete, an ultrasonic signal is generated for the rotating blade using the excitation laser and measured by the sensor. The correlation between the measured response and a training response is maximized when they correspond to the same excitation point. Finally, ultrasonic images are generated by scanning the excitation laser over the target surface of the blade. The effectiveness of the proposed imaging technique is investigated through experimental tests performed on a rotating blade specimen.

This paper presents a novel damage detection method which simultaneously updates the undamaged as well as damaged structure model in a multi-objective optimization (MOO) process. Structural health monitoring via analysis of modal data and model updating has received considerable attention in the previous decade. Such damage detection methods typically require an updated baseline model of the undamaged structure and the associated errors can become aggregated when this baseline model is subsequently used for damage detection. The use of multi-objective model updating alleviates those issues. A beam structure with and without damage has been used as an example and different noise levels have been added to the identified mode shapes. The results have been compared with single-objective model updating and it has been found that the proposed method is more efficient for accurate estimation of damage severity.

The paper presents a method to assess damages in beams, based on how natural frequencies of bending vibration modes change due to damages. The authors have contrived a correlation between the strain energy stored in a segment of the beam, which is proportional with the mode shape curvature of a considered vibration mode at that location, and the frequency change for this mode if damage appears on that segment. For a certain mode, damages placed on inflection points of the mode shape curvature, where the strain energy is null, will not produce changes in frequency, while damages placed on maxima will produce the highest changes in frequency. For other locations of damage, the frequency shift will be proportional with the mode shape curvature of the vibration mode at that location. We worked out a general relation, which gives the frequency shift of all bending modes, with one coefficient depending on the support type. To evaluate damages, we determine analytically the relative frequency shift as ratio between the frequency change and the natural frequency of the undamaged beam, for the first ten vibration modes, considering various damage depths and locations. Comparison of results with that obtained by measurements on the real beam permits detection, location and assessment of damages in beams with high accuracy. The method was validated by experiments.

The piezoelectric transduction mechanism has received a great attention in the energy harvesting field. Although there are many energy harvesting possibilities where piezoelectric transduction can be suitably employed, design of piezoelectric energy harvesters are typically restricted within the cantilever beams. As an example the energy scavenging from the wings of a Micro Air Vehicles (MAV) will certainly beneficial for its sustainable missions and which is predominantly a plate type harvester of arbitrary shape. In this paper a generalized multi directional and multimodal placement of piezoelectric layers on the substrate is discussed in order to maximize the power output. A theoretical study has been conducted using a mathematical model for the plate type harvesters and an optimization problem is solved to maximize the power output from the harvesters. This will provide harvesting of the energy from wide band of frequency and thus it is called energy scavenging. It is quite well known that the cantilever plate with straight edge has one directional bending whereas the wings of arbitrary shapes will have two directional bending in association with torsion. Thus potentially all these possible modes of bending can generate energy. A preliminary mathematical study is conducted in this paper.

Composite materials offer many advantages for aerospace applications, e.g., good strength to weight ratio. Different types of composites, such as non-crimp fabrics (NCF), are currently being investigated as they offer reduced manufacturing costs and improved damage tolerance as compared to traditional pre-impregnated composite materials. NCF composites are made from stitched fiber bundles (tows), which typically have a width and thickness in the order of millimeter. This results in strongly inhomogeneous and anisotropic material properties. Different types of manufacturing imperfections, such as porosity, resin pockets, tow crimp and misalignment can lead to reduced material strength and thus to defects following excessive loads or impact, e.g. fracture and delaminations. The ultrasonic non-destructive testing of NCF composites is difficult, as the tow size is comparable to the wavelength, leading to multiple scattering in this inherently three-dimensional structure. For typical material properties and geometry of an NCF composite, a full three-dimensional Finite Element (FE) model has been developed in ABAQUS. The propagation of longitudinal ultrasonic waves has been simulated and the effect of multiple scattering at the fiber tows investigated. The effect of porosity as a typical manufacturing imperfection has been considered. The potential for the detection and quantification of such defects is discussed based on the observed influence on the ultrasonic wave propagation and attenuation.

Vibration-based structural health monitoring could be a useful form of determining the health and safety of space structures. A particular concern is the possibility of a foreign object that attaches itself to a satellite in orbit for adverse reasons. A frequency response analysis was used to determine the changes in mass and moment of inertia of the space structure based on a change in the natural frequencies of the structure or components of the structure. Feasibility studies were first conducted on a 7 in x 19 in aluminum plate with various boundary conditions. Effect of environmental conditions on the frequency response was determined. The baseline frequency response for the plate was then used as the basis for detection of the addition, and possibly the location, of added masses on the plate. The test results were compared to both analytical solutions and finite element models created in SAP2000. The testing was subsequently expanded to aluminum alloy satellite panels and a mock satellite with dummy payloads. Statistical analysis was conducted on variations of frequency due to added mass and thermal changes to determine the threshold of added mass that can be detected.

Bi-stable composite tape springs present several volume efficient solutions for deployable structures in small satellites. Viscoelastic changes within the composite matrix of these materials caused by their long term storage and exposure to varying temperatures can negatively impact the ability to deploy the structure. This study investigates a method for developing an in situ sensor for structural health monitoring in space structures employing tape springs. A method is developed by employing a custom load cell to detect stress relaxation in a bent tape spring over a period of time and two tests of this method were conducted. Results from the first test reveal the correct trend for stress relaxation but with significant noise. The second test showed the cause of the noise to be material behavior changes due to temperature fluctuations. The results show the expected decreasing exponential trend in the strain data as stress relaxation occurs, proving the feasibility of the approach.

Current satellite transportation sensors can provide a binary indication of the acceleration or shock that a satellite has experienced during the shipping process but do little to identify if significant structural change has occurred in the satellite and where it may be located. When a sensor indicates that the satellite has experienced shock during transit, an extensive testing process begins to evaluate the satellite functionality. If errors occur during the functional checkout, extensive physical inspection of the structure follows. In this work an alternate method for inspecting satellites for structural defects after shipping is presented. Electro- Mechanical Impedance measurements are used as an indication of the structural state. In partnership with the Air Force Research Laboratory University Nanosatellite Program, Cornell's CUSat mass model was instrumented with piezoelectric transducers and tested under several structural damage scenarios. A method for detecting and locating changes in the structure using EMI data is presented.

The paper presents a discussion of the design, development, and assembly of Structural Health Monitoring (SHM) experiments launched in space on a sub-orbital flight. Onboard experiments were focused on investigating the utility of piezoelectric wafer active sensors (PWAS) as active elements of spacecraft SHM systems and the electro-mechanical impedance method as a promising SHM methodology for space systems. A Magneto-elastic active sensor (MEAS) was used to record in-flight dynamics of the payload. The list of PWAS experiments included a bolted-joint experiment, an adhesive endurance experiment, and an experiment to monitor PWAS condition during spaceflight. Electromechanical impedances of piezoelectric sensors were recorded in-flight at varying input frequencies using onboard microcontroller units. PWAS and MEAS data were recovered from the payload after landing. Details of the sub-orbital flight experiments are considered and conclusions pertaining to flight results are presented. The paper discusses issues encountered during design, development, and assembly of the payload and aspects central to successful demonstration of the SHM during sub-orbital space flight.

This contribution presents a novel strategy to achieve real-time prediction of non-penetrating impact-induced damage, especially delamination between plies of composite laminated structures. The proposed strategy aims to create an optimum-sized database of simulated damage information on a given laminated structure using numerical failure models and pattern recognition technique. A multi-stage data clustering algorithm is implemented to identify regions in the structural Finite Element (FE) model that have similar damage signatures. The generated database is linked to the real-time estimate of impact location and load-time history obtained from piezoelectric transducers based passive impact monitoring system. A composite stiffener panel is selected as a model problem and it is shown that the proposed strategy based on pattern recognition will result in large savings in computational cost for the database generation besides providing real-time damage diagnostic capabilities for in-service adverse impact events within certain confidence bounds.

A bonded joint between an aluminum plate and CFRP plate (7 plies) is considered using a titanium spar. The bonding is ensured by double sided adhesive that is prone to degradation with aging structures. The problem is to detect the disbond occurring at the CFRP plate/titanium spar interface using guided waves generated by piezoceramic transducers (PZT) bonded on the CFRP plate. The objective of the present work is to optimize the SHM configuration (PZT location, Lamb wave mode, size and shape of the PZT) for pitch and catch measurements within the bond. 1D, 2D and 3D numerical simulations of the instrumented structure were performed to optimize the SHM configuration. It appears that the rectangular shape can ensure a plane wave front within the bond, since the circular shape generates complex wave fronts. For experimental investigation, coupon structure was manufactured with synthetic damages inserted using two hemispherical Teflon tapes between adhesive and titanium spar. The structure was instrumented for inspection within the bond by using rectangular PZT. Experimental validation of propagation characteristics and damage sensitivity are performed using LDV measurement within the bond line. Damage detectability using rectangular piezoceramics in pitch-catch configuration within the bond is validated.

Lamb wave type guided wave propagation in foam core sandwich structures and detectability of damages using spectral analysis method are reported in this paper. An experimental study supported by theoretical evaluation of the guided wave characteristics is presented here that shows the applicability of Lamb wave type guided ultrasonic wave for detection of damage in foam core sandwich structures. Sandwich beam specimens were fabricated with 10 mm thick foam core and 0.3 mm thick aluminum face sheets. Thin piezoelectric patch actuators and sensors are used to excite and sense guided wave. Group velocity dispersion curves and frequency response of sensed signal are obtained experimentally. The nature of damping present in the sandwich panel is monitored by measuring the sensor signal amplitude at various different distances measured from the center of the linear phased array. Delaminations of increasing width are created and detected experimentally by pitch-catch interrogation with guided waves and wavelet transform of the sensed signal. Signal amplitudes are analyzed for various different sizes of damages to differentiate the damage size/severity. A sandwich panel is also fabricated with a planer dimension of 600 mm x 400 mm. Release film delamination is introduced during fabrication. Non-contact Laser Doppler Vibrometer (LDV) is used to scan the panel while exciting with a surface bonded piezoelectric actuator. Presence of damage is confirmed by the reflected wave fringe pattern obtained from the LDV scan. With this approach it is possible to locate and monitor the damages by tracking the wave packets scattered from the damages.

This paper presents a methodology for determining the existence of delaminations in complex composite structures. The changes in damage features due to changing temperature are investigated. A Lamb wave based active damage detection technique is used. The Matching Pursuit Decomposition (MPD), a time frequency based signal processing technique, is used for feature extraction. The signals from two different test structures, a healthy specimen and a specimen with seeded delamination, are compared to incorporate the effect of manufacturing variability. Tests are conducted under varying ambient temperature. The results obtained validate the effectiveness of this approach in detecting delamination.

Composite T-joints are commonly used in modern composite airframe, pressure vessels and piping structures, mainly to increase the bending strength of the joint and prevents buckling of plates and shells, and in multi-cell thin-walled structures. Here we report a detailed study on the propagation of guided ultrasonic wave modes in a composite T-joint and their interactions with delamination in the co-cured co-bonded flange. A well designed guiding path is employed wherein the waves undergo a two step mode conversion process, one is due to the web and joint filler on the back face of the flange and the other is due to the delamination edges close to underneath the accessible surface of the flange. A 3D Laser Doppler Vibrometer is used to obtain the three components of surface displacements/velocities of the accessible face of the flange of the T-joint. The waves are launched by a piezo ceramic wafer bonded on to the back surface of the flange. What is novel in the proposed method is that the location of any change in material/geometric properties can be traced by computing a frequency domain power flow along a scan line. The scan line can be chosen over a grid either during scan or during post-processing of the scan data off-line. The proposed technique eliminates the necessity of baseline data and disassembly of structure for structural interrogation.

Endoscopes have been utilize in the medical field to observe the internals of the human body to assist the diagnosis of diseases, such as breathing disorders, internal bleeding, stomach ulcers, and urinary tract infections. Endoscopy is also utilized in the procedure of biopsy for the diagnosis of cancer. Conventional endoscopes suffer from the compromise between overall size and image quality due to the required size of the sensor for acceptable image quality. To overcome the size constraint while maintaining the capture image quality, we propose an electro-optic beam steering device based on thermal-plastic polymer, which has a small foot-print (~5mmx5mm), and can be easily fabricated using conventional hot-embossing and micro-fabrication techniques. The proposed device can be implemented as an imaging device inside endoscopes to allow reduction in the overall system size. In our previous work, a single prism design has been used to amplify the deflection generated by the index change of the thermal-plastic polymer when a voltage is applied; it yields a result of 5.6° deflection. To further amplify the deflection, a new design utilizing a cascading three-prism array has been implemented and a deflection angle to 29.2° is observed. The new design amplifies the beam deflection, while keeping the advantage of simple fabrication made possible by thermal-plastic polymer. Also, a photo-resist based collimator lens array has been added to reduce and provide collimation of the beam for high quality imaging purposes. The collimator is able to collimate the exiting beam at 4 μm diameter for up to 25mm, which potentially allows high resolution image capturing.

The characterization of bones via axial ultrasonic transmission techniques can be fully exploited only once the complexities of guided wave propagation are unveiled. Generally, plate/cylindrical waveguide models, where the soft tissues and their damping role are generally neglected, are used to identify the propagating waves in the bone. Here, a numerical strategy for a more rigorous simulation of guided wave propagation in elongated bones is proposed. First, from a computed tomography image of a human leg a three-dimensional finite element (FE) mesh of the problem is built by converting voxels into elements. At this level, the mechanical properties of bones and soft tissues can be obtained converting the Hounsfield units. If necessary, the FE mesh can be enhanced by smoothing the outer surfaces of the bone and/or skin. Next, time-transient three-dimensional explicit FE simulations are performed to simulate the propagation of stress waves along the bone with and without the soft tissues. The propagative energy is revealed by processing the bone time-responses with a 2D-FFT transform suitable for guided waves extraction. Finally, a representative bi-dimensional cross-section of the bone only is used to set the guided wave equation by means of a Semi-Analytical Finite Element (SAFE) formulation. Via SAFE, the dispersion curves are obtained and compared with the 2D-FFT energy map. The proposed strategy can support the research on non-invasive techniques based on stress waves for the assessment of long bones.

The mechanical properties of cells reflect dynamic changes of cellular organization which occur during physiologic activities like cell movement, cell volume regulation or cell division. Thus the study of cell mechanical properties can yield important information for understanding these physiologic activities. Endothelial cells form the thin inner lining of blood vessels in the cardiovascular system and are thus exposed to shear stress as well as tensile stress caused by the pulsatile blood flow. Endothelial dysfunction might occur due to reduced resistance to mechanical stress and is an initial step in the development of cardiovascular disease like, e.g., atherosclerosis. Therefore we investigated the mechanical properties of primary human endothelial cells (HUVEC) of different age using scanning acoustic microscopy at 1.2 GHz. The HUVECs are classified as young (t_D < 90 h) and old (t_D > 90 h) cells depending upon the generation time for the population doubling of the culture (tD). Longitudinal sound velocity and geometrical properties of cells (thickness) were determined using the material signature curve V(z) method for variable culture condition along spatial coordinates. The plane wave technique with normal incidence is assumed to solve two-dimensional wave equation. The size of the cells is modeled using multilayered (solid-fluid) system. The propagation of transversal wave and surface acoustic wave are neglected in soft matter analysis. The biomechanical properties of HUVEC cells are quantified in an age dependent manner.

The proposed research is aimed at developing, fabricating and implementing a flexible fiber optic bend loss sensor for the measurement of plantar pressure and shear stress for diabetic patients. The successful development of the sensor will greatly impact the study of diabetic foot ulcers by allowing clinicians to measure a parameter (namely, shear stress) that has been implicated in ulceration, but heretofore, has not been routinely quantified on high risk patients. A full-scale foot pressure/shear sensor involves a tactile sensor array using intersecting optical waveguides is presented. The basic configuration of the optical sensor systems incorporates a mesh that is comprised of two sets of parallel optical waveguide planes; the planes are configured so the parallel rows of waveguides of the top and bottom planes are perpendicular to each other. The planes are sandwiched together creating one sensing sheet. Two-dimensional information is determined by measuring the loss of light from each of the waveguide to map the overall pressure distribution. The shifting of the layers relative to each other allows determination of the shear stress in the plane of the sensor. This paper presents latest development and improvement in the sensors design. Fabrication and results from the latest tests will be described.

Particle focusing has been numerically studied in a microchannel filled by an acoustic metamaterial fluid that possesses negative density, and under a pair of ultrasound incidences from the lateral boundaries. Acoustic metamaterial with negative density exponentially damps the ultrasound field along its propagation direction that forms a very low field at the center of the microchannel. Driven by the acoustic radiation force and dissipated by the fluid, the particles laterally vibrate in the microchannel and gradually aggregated in the vicinity of the channel center. A structural microchannel with acoustic resonant elements that generates equivalent negative density property for the fluid in the microchannel has been designed, which decays the ultrasound field in a similar way. Particle movement in the structural microchannel has also been investigated and particle focusing is also achieved. The merit of the proposed particle focusing method by metamaterial concept lies in its independence on the type of the incident wave and width or size of the microchannel.

In designing a periodic acoustic metamaterial it is possible to have one or more of the constituent material phases to be damped (i.e., lossy/dissipative), for example by using a viscoelastic material such as rubber. The presence of damping results in temporal attenuation of the acoustic/elastic waves as they freely propagate through space in the periodic media. In this work we develop Bloch wave propagation models for damped periodic acoustic metamaterials and study the effects of damping on the dispersion relation. We demonstrate several intriguing phenomena that get triggered due to the presence of inherent dissipation.

A numerical method for obtaining the effective anisotropic mass density of elastic composite with arbitrary periodic microstructure is presented and the effective anisotropic mass density is proved to be a second-order tensor. Using the proposed method, a new metamaterial plate with strong anisotropicity in mass density is obtained. Using 3-D elasticity theory, the metamaterial plate is modeled as a continuum medium with obtained effective material properties. The accuracy of the continuum model was evaluated by comparing the dispersion curves with those obtained by exact finite element analysis. Moreover, mode coupling and level repulsion in the anisotropic metamaterial plate are discussed. Finally, preferential directions of wave propagation and energy flow are studied through the comparison of the difference between the phase velocity and group velocity directions.

Monitoring data obtained at a seismically isolated building in Tokyo during the 2011 off the Pacific coast of Tohoku Earthquake are analyzed to investigate the changes in the modal parameters of the building, which are generally used as global soundness indices in structural health monitoring, in correspondence with the response amplitude of the building. The modal parameters are identified using AR models from each short segment of the record every tien seconds. The AR model orders are selected appropriately by the Bayesian framework. The results show that the natural frequency decreases as the response increases and then regains its value as the response fades, where the value at the end is lower than the initial value. The modal identification is also conducted for daily monitoring data, which are two minute ambient vibration response time histories recorded twice a day, showing that the reduction of the natural frequency is not temporary. This indicates that it is feasible to predict the amplitude of the building response during a severe earthquake from ambient vibration observation before and after the earthquake.

Earthquake early warning (EEW) systems are currently operating nationwide in Japan and are in beta-testing in California. Such a system detects an earthquake initiation using online signals from a seismic sensor network and broadcasts a warning of the predicted location and magnitude a few seconds to a minute or so before an earthquake hits a site. Such a system can be used synergistically with installed structural health monitoring (SHM) systems to enhance pre-event prognosis and post-event diagnosis of structural health. For pre-event prognosis, the EEW system information can be used to make probabilistic predictions of the anticipated damage to a structure using seismic loss estimation methodologies from performance-based earthquake engineering. These predictions can support decision-making regarding the activation of appropriate mitigation systems, such as stopping traffic from entering a bridge that has a predicted high probability of damage. Since the time between warning and arrival of the strong shaking is very short, probabilistic predictions must be rapidly calculated and the decision making automated for the mitigation actions. For post-event diagnosis, the SHM sensor data can be used in Bayesian updating of the probabilistic damage predictions with the EEW predictions as a prior. Appropriate Bayesian methods for SHM have been published. In this paper, we use pre-trained surrogate models (or emulators) based on machine learning methods to make fast damage and loss predictions that are then used in a cost-benefit decision framework for activation of a mitigation measure. A simple illustrative example of an infrastructure application is presented.

A data-driven approach for earthquake damage detection and localization in multi-degree of freedom (MDOF) system subjected to strong ground motion is proposed. The new method is based on the combination of wavelet analysis and fractal characteristics. The box counting method is employed to obtain the fractal dimension of the time frequency distribution within the first natural frequency. It is verified that the proposed fractal dimensions at each DOF of linear system are identical, while the fractal dimension at the DOFs with nonlinearity will be different from those at the DOFs with linearity. Therefore, the nonlinearity or weakness of the structure caused by strong ground motion can be detected and localized through comparing the fractal dimensions at the measured DOFs. The numerical simulation on a three-bay sixteen-story moment resist frame shows that the aforementioned approach is capable of detecting and localizing seismic damage.

By embedding appropriately designed chiral local resonators in a host elastic media, a chiral metamaterial with simultaneously negative effective density and bulk modulus can be achieved. In this work, an two dimentional (2D) ideal discrete model for the chiral elastic metamaterial is proposed. The discrete dynamic equation is derived and then homogenized to give the continuous description of the metamaterial. The homogenization procedure is validated by the agreement of the dispersion curve of the discrete and homogenized formulations. The form of homogenized governing equations of the metamaterial cannot be classified as a traditional Cauchy elastic theory. This result conforms the conscience that the Cauchy elasticity cannot reflect the chirality, which is usually captured by higher order theory such as the non-centrosymmetric micropolar elasticity. However when reduced to a (2D) problem, the existing chiral micropolar theory becomes non-chiral. Based on reinterpretation of isotropic tensors in a 2D case, we propose a continuum theory to model the chiral effect for 2D isotropic chiral solids. This 2D chiral micropolar theory constitutes a hopeful macroscopic framework for the theory development of chiral metamaterials.

In this paper, an elastic metamaterial made of lead cylinders coated with elliptical rubbers in an epoxy matrix is considered, and a new multi-displacement microstructure model is proposed to capture the dipolar resonant motion. In the formulation, additional displacement and kinematic variables are introduced to describe global and local deformations, respectively. For the chiral metamaterial, one more rotation variable is needed. The macroscopic governing equations of the two-dimensional elastic metamaterial are explicitly derived. To verify the multi-displacement model, the wave dispersion curves from the current model are compared with those from the finite element simulation for wave propagation. The good agreement is observed in both the longitudinal and transverse wave modes.

A thin-plate metamaterial made of a thin plate periodically attached with mass-spring oscillators is analyzed. Based on the analytic solutions of sound waves incident on the metamaterial, effective mass density can be defined. Approximate expressions of effective mass can be derived when the first-order vibration mode of the plate is considered. It is found that the Lorentz and Drude behavior of effective mass can be obtained. As an example of potential applications, the sound insulation effects of multilayered thin-plate metamaterials are studied. High transmission loss can be achieved in a finite-layered metamaterials at negative-mass frequencies. Their applications to noise control can be anticipated.

Wireless sensor network is one of prospective methods for railway bridge health monitoring. It has drawn much attention due to the long-term operation and low-maintenance performances. However, how to provide power to wireless sensors is a big issue. In railway health monitoring, the idea of converting ambient vibration energy from the vibration of railway track induced by passing train to electric energy has made it an efficient way for powering the wireless sensor networks. In this paper, a bimorph piezoelectric energy harvester from base excitation was investigated in the laboratory, and the energy output of the bimorph energy harvester was predicted by an equivalent single-degree-of-freedom (SDOF) model. Reasonable results have been found between the tested and predicted data. Based on the theoretical model, further works on optimization of the bimorph piezoelectric energy harvester will be performed in future.

The applicability of Electro-Mechanical Impedance (EMI) approach to damage detection, localization and quantification in a mobile bridge structure is investigated in this paper. The developments in this paper focus on assessing the health of Armored Vehicle Launched Bridges (AVLBs). Specifically, two key failure mechanisms of the AVLB to be monitored were fatigue crack growth and damaged (loose) rivets (bolts) were identified. It was shown through experiment that bolt damage (defined here as different torque levels applied to bolts) can be detected, quantified and located using a network of lead zirconate titanate (PZT) transducers distributed on the structure. It was also shown that cracks of various sizes can be detected and quantified using the EMI approach. The experiments were performed on smaller laboratory specimens as well as full size bridge-like components that were built as part of this research. The effects of various parameters such as transducer type and size on the performance of the proposed health assessment approach were also investigated.

Highway bridges are the backbones of a country's road network infrastructure. In order to efficiently maintain these important structures, structural health monitoring (SHM) can be implemented to impart a more deterministic management procedure. To date, many damage detection strategies have been developed and implemented on lab-scale or simple bridge structures however, damage detection research has rarely been conducted taking full scale in-service structures into account with ambient vibration. Among the different approaches modal flexibility method is one of the sensitive tools for damage detection which has been widely used over the last two decades. This paper presents a damage detection based on the stochastic damage locating vector (SDLV) method for an in-service highway bridge using ambient vibration data from long-term SHM. The target bridge was equipped with a long-term SHM system as a part of a research project of the University of Connecticut. The ambient vibration data during 2001 and 2005 are used to identify the damage on the highway bridge. Finally, the potential damage locations are determined using the SDLV method with the limited number of sensors.

The stiffness of a bridge span is evaluated by the dynamic displacement response corresponding to a three-axial vehicle load moving with constant speed. The dynamic displacement influence line obtained from the dynamic displacement time history was filtered by window smoothing and empirical modal decomposition (EMD) methods to acquire the quasi-static displacement influence line. The beam stiffness was obtained by dividing the moment diagram corresponding to a concentrated load applying on the measuring position with the curvature of the quasi-influence line. The effects of three-axial moving load, moving speeds, and measuring positions on the stiffness estimation are explored. The results show the window smoothing method is a better technique to obtain the quasi-static influence line. The only discrepancies in curvature for single and three-axial load cases are near both ends of the beam. A larger range of correct stiffness can be recovered for load moving with lower speed. Similar stiffness diagram can be obtained from the influence lines at different measuring positions.

In a wireless sensor network, data loss often occurs during the data transmission between wireless sensor nodes and the base station, which decreases the communication reliability in wireless sensor network applications. Errors caused by data loss inevitably affect the data analysis of the structure and subsequent decision making. This paper proposed an approach to recover lost data in a wireless sensor network based on the compressive sampling (CS) technique. The main idea in this approach is to project the transmitted data from x onto y, where y is the linear projection of x on a random matrix. The data vector y is permitted to lose part of the original data x in wireless transmissions between the sensor nodes and the base station. After the base station receives the imperfect data, the original data vector x can be reconstructed based on the data y using the CS method. The acceleration data collected from the vibration test of Shandong Harbin Sifangtai Bridge by wireless sensors is used to analyze the data loss recovery ability of the proposed method.

Innovative mechanoluminescent (ML) particles emit light repeatedly in response to small stresses applied, such as deformation, friction, or impact. When dispersedly coated on a structure, each particle acts as a sensitive mechanical sensor, while the 2-dimentional emission pattern of the whole assembly reflects well the dynamical stress distribution inside the structure and mechanical information around crack and defect. Thus, we have applied the remarkable strong points of ML sensing technique to a bridge in use as a real social structure for the first time. For the first ML monitoring test at bridge, we selected a relatively old bridge (established in 1954, 3-span continuous T-type RC bridge, length 24.4 m, width: 7.89 m). The ML sheet type sensors were put around the central area (700×400 mm) of the main girder, and ML images originated from dynamic load application via general traffic vehicles had recorded by using lab-made CCD camera under roughly dark condition. As the result, we successfully detected intense ML patterns not only along visible crack but also at round soundless part on the girder at a glance with responding ML intensity reflecting the crack mouth opening displacement (CMOD) of visible crack and invisible progressing microcrack.

Most of the nonlinear ultrasonic studies to date have been experimental, but few theoretical predictive studies exist, especially for Lamb wave ultrasonic. Compared with nonlinear bulk waves and Rayleigh waves, nonlinear Lamb waves for structural health monitoring become more challenging due to their multi-mode dispersive features. In this paper, predictive study of nonlinear Lamb waves is done with finite element simulation. A pitch-catch method is used to interrogate a plate with a "breathing crack" which opens and closes under tension and compression. Piezoelectric wafer active sensors (PWAS) used as transmitter and receiver are modeled with coupled field elements. The "breathing crack" is simulated via "element birth and death" technique. The ultrasonic waves generated by the transmitter PWAS propagate into the structure, interact with the "breathing crack", acquire nonlinear features, and are picked up by the receiver PWAS. The features of the wave packets at the receiver PWAS are studied and discussed. The received signal is processed with Fast Fourier Transform to show the higher harmonics nonlinear characteristics. A baseline free damage index is introduced to assess the presence and the severity of the crack. The paper finishes with summary, conclusions, and suggestions for future work.

This paper shows that time-frequency analysis is most appropriate for nonlinearity identification, and presents advanced signal processing techniques that combine time-frequency decomposition and perturbation methods for parametric and non-parametric identification of thin-walled structures and other dynamical systems. Hilbert-Huang transform (HHT) is a recent data-driven adaptive time-frequency analysis technique that combines the use of empirical mode decomposition (EMD) and Hilbert transform (HT). Because EMD does not use predetermined basis functions and function orthogonality for component extraction, HHT provides more concise component decomposition and more accurate timefrequency analysis than the short-time Fourier transform and wavelet transform for extraction of system characteristics and nonlinearities. However, HHT's accuracy seriously suffers from the end effect caused by the discontinuity-induced Gibbs' phenomenon. Moreover, because HHT requires a long set of data obtained by high-frequency sampling, it is not appropriate for online frequency tracking. This paper presents a conjugate-pair decomposition (CPD) method that requires only a few recent data points sampled at a low frequency for sliding-window point-by-point adaptive timefrequency analysis and can be used for online frequency tracking. To improve adaptive time-frequency analysis, a methodology is developed by combining HHT and CPD for noise filtering in the time domain, reducing the end effect, and dissolving other mathematical and numerical problems in time-frequency analysis. For parametric identification of a nonlinear system, the methodology processes one steady-state response and/or one free damped transient response and uses amplitude-dependent dynamic characteristics derived from perturbation analysis to determine the type and order of nonlinearity and system parameters. For non-parametric identification, the methodology uses the maximum displacement states to determine the displacement-stiffness curve and the maximum velocity states to determine the velocity-damping curve. Numerical simulations and experimental verifications of several nonlinear discrete and continuous systems show that the proposed methodology can provide accurate parametric and non-parametric identifications of different nonlinear dynamical systems.

This paper investigates the piezo-based nonlinear vibro-acoustic modulation technique for impact damage detection in composite structures. The method is based on combined low-frequency modal excitation and high-frequency ultrasonic excitation that lead to vibro-acoustic modulations in damaged specimens. The work presented focuses on sensor location analysis. Low-profile, surface bonded piezoceramic transducers are used for ultrasonic and modal excitation.. Modulated responses are acquired using laser vibrometry. Various areas of monitored composite structures are investigated to establish positions exhibiting the largest intensities of vibro-acoustic modulations resulting from impact damage. The study shows that sensor location in composite structures is important for efficient damage detection.

The interrogation systems based on fiber-optic sensors are very attractive for the practical applications in structural health monitoring owing to a number of advantages of optical fiber elements over their electronic counterparts. Among the fiber-optic sensors, the fiber Bragg gratings (FBGs) have their own unique features to be widely used for detection of acoustic emission. We have developed a dynamic strain sensing system by using a tunable single longitudinal mode Erbium-doped fiber ring laser to be locked to the middle-reflection wavelength of the FBG as the demodulation technique. A proportional-integral-derivative device continuously controls the laser wavelength that is kept at the FBG middle-reflection wavelength, thus stabilizing the operating point against quasi-static perturbation, while the high frequency dynamic strain shifts the FBG reflection profile. The reflected power varies in proportion to the applied strain which can be derived directly from AC photocurrent of the reflected signal. We have designed and assembled a fourchannel demodulator system for simultaneous high frequency dynamic strain sensing.

Digital image correlation (DIC) has been becoming increasingly popular as a means to perform structural health monitoring because of its full-field, non-contacting measurement ability. In this paper, 3D DIC techniques are used to identify the mode shapes of a wind turbine blade. The blade containing a handful of optical targets is excited at different frequencies using a shaker as well as a pluck test. The response is recorded using two PHOTRON™ high speed cameras. Time domain data is transferred to the frequency domain to extract mode shapes and natural frequencies using an Operational Modal Approach. A finite element model of the blade is also used to compare the mode shapes. Furthermore, a modal hammer impact test is performed using a more conventional approach with an accelerometer. A comparison of mode shapes from the photogrammetric, finite element, and impact test approaches are presented to show the accuracy of the DIC measurement approach.

A data fusion technique implementing the principles of acoustic emission (AE), ultrasonic testing (UT) and digital image correlation (DIC) was employed to in situ monitor crack propagation in an Al 2024 alloy compact tension (CT) specimen. The specimen was designed according to ASTM E647-08 and was pre-cracked under fatigue loading to ensure stable crack growth. Tensile (Mode I) loads were applied according to ASTM E1290-08 while simultaneously recording AE activity, transmitting ultrasonic pulses and measuring full-field surface strains. Realtime 2D source location AE algorithms and visualization provided by the DIC system allowed the full quantification of the crack growth and the cross-validation of the recorded non-destructive testing data. In post mortem, waveform features sensitive to crack propagation were extracted and visible trends as a function of computed crack length were observed. In addition, following a data fusion approach, features from the three independent monitoring systems were combined to define damage sensitive correlations. Furthermore a novelty detector based on the Mahalanobis outlier analysis was implemented to quantify the extent of crack growth and to define a more robust sensing basis for the proposed system.

As sensors become integrated in more applications, interest in magnetostrictive sensor technology has blossomed. Magnetostrictive materials have many advantages and useful applications in daily life, such as high efficient coupling between elastic and polymer material, large displacement, magnetic field sensors, micro actuator and motion motor, etc. The purpose of this paper is to develop a metal sensor which is capable of detecting different geometries and shapes of metal objects. The main configuration is using a Mach-Zehnder fiber-optic interferometer coated with magnetostrictive material. The metal detector system is a novel design of metal detector, easy to fabricate and capable of high sensitivity. In our design, metal detection is made possible by disrupting the magnetic flux density that encompasses the magnetostriction sensor. In this paper, experimental setups are described and metal sensing results are presented. The results of detecting complex metal's geometry and metal's mapping results are discussed.

In cylindrical structures such as pipelines and pressure vessels, cracks are most likely to occur along the longitudinal (axial) direction and they can be fatal to the serviceability of the structures. Unfortunately, the conventional ultrasonic crack detection techniques, which usually use longitudinal wave, are not very sensitive to this type of cracks. This paper focuses on the detection and monitoring of axial cracks in cylindrical structures using torsional wave generated by piezoelectric macro-fiber composite (MFC). The first order torsional wave is a kind of non-dispersive pure shear wave which propagates at a fixed wave speed. Torsional wave is utilized in this work because, intuitively, it is more sensitive to axial cracks than the family of longitudinal waves. Numerical simulation has been performed using ANSYS to show the effectiveness of torsional wave in detecting and monitoring axial cracks. The time of flight (TOF) of the waves is used to determine the crack position, while the crack propagation is monitored by measuring the variation in the crack induced disturbances. Experiments have also been conducted to investigate the feasibility of the proposed method. MFC transducers oriented at 45° against the axis of the specimen are used to generate and receive torsional waves. The experimental results demonstrated that the crack position can be indentified and its growth can be well monitored with the presented approach using torsional wave.

An ultrasonic guided wave based damage detection technique has been developed for structural health monitoring (SHM) of composite structures submerged in water. Specially designed guided wave transducers are utilized to selectively excite and receive guided waves with dominant shear horizontal particle displacements. It has been shown that the SH type waves are insensitive to water loading conditions. With appropriate water sealing, the transducers can be applied to composite structures submerged in water. The guided wave signals collected from an underwater composite structure are almost identical to the signals that are obtained before the structure is submerged. Experiments have been conducted to demonstrate the feasibility of damage detection in underwater composite structures. A thick carbon/epoxy composite beam is used as the test sample. Excellent damage detection results were obtained for both dry and underwater tests.

Cloud model is a new mathematical representation of linguistic concepts, which shows potentials for uncertainty mediating between the concept of a fuzzy set and that of a probability distribution. This paper utilizes cloud model theory as an uncertainty analyzing tool for noise-polluted signals, which formulates membership degree functions of residual errors that quantify the difference between the prediction from simulated model and the actual measured time history at each time interval. With membership degree functions a multi-objective optimization strategy is proposed, which minimizes multiple error terms simultaneously. Its non-domination-based convergence provides a stronger constraint that enables robust identification of damages with lower damage negative false. Simulation results of a structural system under noise polluted signals are presented to demonstrate the effectiveness of the proposed method.

Basalt fiber reinforced polymer (BFRP) is a structural material with superior mechanical properties. In this study, unidirectional BFRP laminates with 14 layers are made with the hand lay-up method. Then, the acoustic emission technique (AE) combined with the scanning electronic microscope (SEM) technique is employed to monitor the fatigue damage evolution of the BFRP plates in the fatigue loading tests. Time-frequency analysis using the wavelet transform technique is proposed to analyze the received AE signal instead of the peak frequency method. A comparison between AE signals and SEM images indicates that the multi-frequency peaks picked from the time-frequency curves of AE signals reflect the accumulated fatigue damage evolution and fatigue damage patterns. Furthermore, seven damage patterns, that is, matrix cracking, delamination, fiber fracture and their combinations, are identified from the time-frequency curves of the AE signals.

Most of the members of modern structures operate under loading conditions, which may cause damages or cracks. Interest in detecting structural damage, at the earliest possible stage (i.e. breathing crack), has always been a major issue in the structural health monitoring community. Among structural health monitoring techniques, Lamb waves is frequently used as diagnostic tools to detect damage in plate-like structures. The decomposition of the time reversal operator (DORT) method is a selective detection and focusing technique using an array of transmit-receive transducers. But there is little research on locating breathing crack on the plate using Lamb wave DORT method. In this paper, a proposed approach based on the DORT method is developed to locate a simulated breathing crack on the aluminum plate using Lamb wave through finite element simulation on the commercial finite element code ANSYS platform. The nonlinear superharmonic, generated by a breathing crack under the effect of an external Lamb wave interrogation signal, was related to the location of the crack. This method, which uses PZT array and operates under multi-modes pulse-echo mode, can estimate the position of the breathing crack in 2D image by numerically backpropagating selective eigenvector using the S0 Lamb wave propagation analytical solution. The numerical simulation results illustrate the proposed method.

An actual structure including connections and interfaces may exist nonlinear. Because of many complicated problems about nonlinear structural health monitoring (SHM), relatively little progress have been made in this aspect. Statistical pattern recognition techniques have been demonstrated to be competitive with other methods when applied to real engineering datasets. When a structure existing 'breathing' cracks that open and close under operational loading may cause a linear structural system to respond to its operational and environmental loads in a nonlinear manner nonlinear. In this paper, a vibration-based structural health monitoring when the structure exists cracks is investigated with autoregressive support vector machine (AR-SVM). Vibration experiments are carried out with a model frame. Time-series data in different cases such as: initial linear structure; linear structure with mass changed; nonlinear structure; nonlinear structure with mass changed are acquired.AR model of acceleration time-series is established, and different kernel function types and corresponding parameters are chosen and compared, which can more accurate, more effectively locate the damage. Different cases damaged states and different damage positions have been recognized successfully. AR-SVM method for the insufficient training samples is proved to be practical and efficient on structure nonlinear damage detection.

Weight reduction and maintenance simplification are high in the agenda of companies and researchers active in the aerospace sector. Energy harvesters are being investigated because they enable the installation of wireless sensor nodes, providing structural health monitoring of the aircraft without additional cabling. This paper presents both a weight-optimized composite wing structure and a piezoelectric harvester for the conversion of mechanical strain energy into electrical energy. Finite elements modelling was used for the minimum-weight optimisation within a multi-constraints framework (strength, damage tolerance, flutter speed and gust response). The resulting structure is 29% more compliant than the original one, but is also 45% lighter. A strain map was elaborated, which details the distribution of strain on the wing skin in response to gust loading, indicating the optimal locations for the harvesters. To assess the potential for energy generation, a piezoelectric harvester fixed to a portion of the wing was modelled with a multi-physics finite elements model developed in ANSYS. The time-domain waveforms of the strain expected when the aircraft encounters a gust (gust frequencies of 1, 2, 5 and 10 Hz were considered) are fed into the model. The effects of harvester thickness and size, as well as adhesive thickness, were investigated. Energy generation exceeding 10 J/m² in the first few second from the beginning of the gust is predicted for 100μ-thick harvesters. The high energy density, low profile and weight of the piezoelectric film are greatly advantageous for the envisaged application.

μCompact and lightweight energy harvesters are needed to power wireless sensor nodes (WSNs). WSNs can provide health monitoring of aircraft structures, improving safety and reducing costs by enabling predictive maintenance. A simple solution, which meets the requirements for lightness and compactness, is represented by piezoelectric generators fixed to the surface of the wing (i.e. the wing skin). Such piezoelectric patches can harvest the strain energy available when the wing is flexed, as occurs, for example, in the presence of gust loading. For this study, monolithic piezoelectric sheets and macro fibre composite (MFC) generators were fixed to plates made of two materials commonly used for aircraft wing skin: Al-2024 aluminium alloy and an epoxy-carbon fibre composite. The plates then underwent harmonically varying loading in a tensile testing machine. The power generation of the harvesters was measured at a selection of strain levels and excitation frequencies, across a range of electrical loads. The optimal electrical load, yielding maximum power extraction, was identified for each working condition. The generated power increases quadratically with the strain and linearly with the frequency. The optimal electrical load decreases with increasing frequency and is only marginally dependent on strain. Absolute values of generated power were highest with the MFC, reaching 12mW (330μW/cm²) under 1170μstrain peak-to-peak excitation at 10Hz with a 66kΩ load. Power generation densities of 600μW/cm² were achieved under 940μstrain with the monolithic transducers at 10Hz. It is found that MFCs have a lower power density than monolithic transducers, but, being more resilient, could be a more reliable choice. The power generated and the voltage outputs are appropriate for the intended application.

An identification method based on interval analysis for moving forces on cable-stayed bridge with uncertain parameters and noisy measurements is presented. Although there have been many reports on moving force identification methods on bridges, they did not pay much attention to the problem of identifying loads on cable-stayed bridge. And the uncertainties of measurements and bridge parameters were ignored in previous studies. The influence matrix model of cable-stayed bridge was established from the finite element model or in-field test in this study - if using in-field test only, the proposed method will be a model-free method. The upper and lower bound of the identified loads is analyzed based on the interval theory, which is caused by the uncertainties of two kinds: 1) noisy cable tension force measurements; 2) uncertain bridge parameters or influence line test noise. The numerical demonstration and validation are carried out based on Nanjing No.3 Yangtze River Bridge. The main contributions of this paper are: a. to propose an identification method for cable-stayed bridge with uncertain parameters and noisy measurements; b. the location, speed and magnitude of vehicle loads can be estimated simultaneously with the upper and lower bounds.

Compared with the conventional monitoring approach of separately sensing and then compressing the data, compressive sensing (CS) is a novel data acquisition framework whereby the compression is done during the sampling. If the original sensed signal would have been sufficiently sparse in terms of some orthogonal basis, the decompression can be done essentially perfectly up to some critical compression ratio. In structural health monitoring (SHM) systems for civil structures, novel data compression techniques such as CS are needed to reduce the cost of signal transfer and storage. In this article, Bayesian compressive sensing (BCS) is investigated for SHM signals. By explicitly quantifying the uncertainty in the signal reconstruction, the BCS technique exhibits an obvious benefit over the existing regularized norm-minimization CS. However, current BCS algorithms suffer from a robustness problem; sometimes the reconstruction errors are large. The source of the problem is that inversion of the compressed signal is a severely ill-posed problem that often leads to sub-optimal signal representations. To ensure the strong robustness of the signal reconstruction, even at a high compression ratio, an improved BCS algorithm is proposed which uses stochastic optimization for the automatic relevance determination approach to reconstructing the underlying signal. Numerical experiments are used as examples; the improved BCS algorithm demonstrates superior performance than state-of-the-art BCS reconstruction algorithms.

In this paper a displacement amplifier is designed for integrating an amplifier into an extensometer to improve precision and resolution of the extensometer for strain monitoring of high temperature components. Firstly the displacement amplifier is investigated and the requirements for displacement amplifiers applied for high temperature deformation monitoring is summarized. Secondly a lever-type mechanical displacement amplifier for the extensometer is designed and the amplification ratio is derived. At last, feasibility of the designed displacement amplifier is analyzed from loading force, amplification ratio and environmental temperature under harsh environment for online strain monitoring using FEA. Analyzed results show that the loading force coming from the torque moment of the flexure hinge can be forced by the extensometer rods, amplification ratio equation is proved correct, and the thermal effect on accuracy can be corrected in data processing.

In this paper, taking the cantilever beams as example, a novel and simple damage localization approach is proposed: the Curvature Difference Probability Method of Waveform Logarithms of Standard Deviation (CDPWLSD). Firstly, the feature, the common Logarithms of Standard Deviation (LSD) for the response signals before and after damage should be computed at every measured node. Then, the curvature changes of the waveform LSD are selected as candidates for the potentially damaged locations. Lastly, the probability of every potentially damaged element presenting in multiple identifications is considered to determine the finally damaged elements. Numerical results of both single and multiple damage cases show that the proposed approach can be used to locate damage very well. And it is still effective even if the noise level is up to 15%.