This PDF file contains the front matter associated with SPIE Proceedings Volume 9062 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

The drive for high spatial resolution (millimeters) distributed fiber sensors has renewed the interest in optical frequency domain reflectrometry (OFDR) systems. Because millimeters equivalent spatial resolution in optical time domain reflectrometry (OTDR) systems would require a data acquisition card with a bandwidth of 10 GHz and a sampling rate of tens of G Samples/s, such a digitizer or data acquisition card plus the pulse generator and detection system will make a distributed sensors very expensive, while a tunable laser with a wide tuning range can provide millimeters resolution with short sensing range (<100m). We developed a high precision temperature (0.1°C) and strain (1μ strain) resolution and 2.5mm spatial resolution over 180m range by auto and cross-correlation of OFDR in PMF. The dual modes of PMF allow the discrimination of the temperature and strain with distinct dependency. The application of this sensor for internal crack detection of concrete beam has been demonstrated. For distributed dynamic measurement, the upper frequency is limited by the repetition rate of the laser pulse in sensing fiber; in addition the weak Rayleigh scattering signal demands many averaging to improve SNR. The continuous wavelet transform approach has been introduced in phase OTDR sensor system to suppress random noise, and multiple vibration disturbances have been measured simultaneously for power generator monitoring. For the high frequency vibration detection, the coherent detection combined with polarization diversity scheme is implemented.

In a new distributed fiber optic sensing system, single-ended double-pulse input is used to strengthen Brillouin backscattered light by increasing the population of acoustic phonons. The first pulse (pump pulse), has a wide pulsewidth (tens or hundreds of us) and is used to generate a nonlinear population of acoustic phonons in the sensing fiber. Whereas the second pulse (probe pulse) has a different central wavelength and a much narrower pulse-width (several ns), and is emitted into the sensing fiber with a controlled time delay to absorb the generated abundant acoustic phonons, so that strong Anti-Stokes light can be generated. In this way, Brillouin backscattered light can be detected easily, leading to higher signal to noise ratio and better spatial resolution (less than 1 m), as well as good temperature and strain resolution, and longer sensing distance.

Reliable Thermal Protection System (TPS) sensors are needed to achieve better designs for spacecraft (probe) heatshields for missions requiring atmospheric aero-capture or entry/reentry. In particular, they will allow both reduced risk and heat-shield mass minimization, which will facilitate more missions and allow increased payloads and returns. For thermal measurements, Intelligent Fiber Optic Systems Corporation (IFOS) is providing a temperature monitoring system involving innovative lightweight, EMI-immune, high-temperature resistant Fiber Bragg Grating (FBG) sensors with a thermal mass near that of TPS materials together with fast FBG sensor interrogation. The IFOS fiber optic sensing technology is highly sensitive and accurate. It is also low-cost and lends itself to high-volume production. Multiple sensing FBGs can be fabricated as arrays on a single fiber for simplified design and reduced cost. In this paper, we provide experimental results to demonstrate the temperature monitoring system using multi-sensor FBG arrays embedded in small-size Super-Light Ablator (SLA) coupon, which was thermally loaded to temperatures in the vicinity of the SLA charring temperature. In addition, a high temperature FBG array was fabricated and tested for 1000°C operation.

In this paper we propose and demonstrate the scheme of vibration pattern recognition and classification in the OTDR based distributed optical-fiber vibration sensing system. We set up the engineering system with signal processing PC for perimeter security in some high-tech park in Nanjing. Three types of disturbing actions, including climbing up and kicking at the wall by a person, and watering on the sensing optical fiber cable same as the rain falling on, are implemented. By using level crossing rate (LCR), we can obtain their individual pattern features, so that the eigenvalue database for three disturbing actions can be built in the system. By comparing three types of vibrations, the differences among these can be given out. The results show three vibration patterns can be recognized and classified effectively.

We demonstrate the measurement of and applications for full-spectral measurements collected from FBG sensors in dynamic loading environments. The measurement of the dynamic response of a laminated plate to an impact event highlights the information gained during the event as compared to after the event. The measurement of damage induced spectral distortion in a thin plate during vibration loading demonstrates the capability of separating spectral distortion due to multiple effects, including damage and vibration loading. Finally, the measurement of the change in dynamic response of an adhesively bonded joint highlights the capability to measure the progression of fatigue damage. Confirmation that the change in FBG response is due to fatigue damage is performed through independent pulsed phase thermography imaging of the adhesively bonded joint.

This paper presents the application and validation of optical equipment suitable for high frequency guided wave and acoustic emission detection with fiber Bragg grating (FBG) sensors. Guided wave and acoustic emission (AE) measurements were compared between piezoelectric wafer active sensors (PWAS) and fiber Bragg grating (FBG) sensors embedded onto isotropic plates and beams with an emphasis on testing FBG ultrasonic wave propagation frequency characteristics. The use of an acousto-ultrasonic FBG ring sensor to eliminate FBG directional dependence is also discussed. Since FBG sensors only detect strain longitudinal to the fiber, unlike PWAS they cannot serve as omnidirectional guided wave and AE sensors. To overcome this limitation, the use of an acousto-ultrasonic ring sensor, designed to augment and enhance the performance of FBG sensors, is tested. The ring sensor uses mechanical amplification principles to force in-plane vibration of the ring to occur at a specific resonance frequency. In this study, a ring sensor is bonded onto an isotropic plate; incoming guided wave and AE measurements from an FBG bonded to the ring sensor were compared to measurements from an FBG bonded to the plate. Preliminary results show the use of the ring sensor nearly eliminated the directional dependence of the FBG; concurrently the FBG on the ring sensor sensed incoming guided waves and AE events near its resonance frequency and rejected phenomenon occurring at other frequencies.

A novel application of chemiluminescence resulting from the chemical reaction in a glow-stick as sensors for structural health monitoring is demonstrated here. By detecting the presence of light emitting from these glow-sticks, it is possible to develop a low-cost sensing device with the potential to provide early warning of damage in a variety of engineering applications such as monitoring of cracks or damage in concrete shear walls, detecting of ground settlement, soil liquefaction, slope instability, liquefaction-related damage of underground structure and others. In addition, this paper demonstrates the ease of incorporating wireless capability to the sensor device and the possibility of making the sensor system self-sustaining by means of a renewable power source for the wireless module. A significant advantage of the system compared to previous work on the use of plastic optical fibre (POF) for damage detection is that here the system does not require an electrically-powered light source. Here, the sensing device, embedded in a cement host, is shown to be capable of detecting damage. A series of specimens with embedded glow-sticks have been investigated and an assessment of their damage detection capability will be reported. The specimens were loaded under flexure and the sensor responses were transmitted via a wireless connection.

Researching high-sensitivity flexible ultrasonic sensor is important in the field of structural health monitoring (SHM). In this research, a novel ultrasonic sensor based on fiber ring laser with an in-built phase shifted fiber Bragg grating (PSFBG) is proposed and demonstrated. The first function of the PS-FBG is to determine the wavelength of the laser. Thus, this sensing system is robust to temperature change and quasi-static strain change because the PS-FBG is always illuminated. The other function of the PS-FBG is a sensor with ultra-steep slope and short effective grating length. It is beneficial for achievement of high-sensitivity and broad-bandwidth ultrasonic detection. The experimental evaluated sensitivity was 58.5±3 dB, which is 7.5 dB higher than traditional PZT sensor. This may be the highest sensitivity obtained by optical fiber sensing system. Because of the advantages including robustness, simple structure and low cost in addition to the high sensitivity and broad bandwidth, this sensing system has potential practical applications in ultrasonic SHM.

The development of a chemical sensor based on steering-wheel photonic crystal fiber (SW-PCF) and a NanoSpectrometer^TM from Nano-Optics Devices, LLC can benefit industrial process-monitoring and environmental sensing applications. This chemical sensor can potentially result in a compact, image-based sensor with enhanced spectral resolution (~0.15nm) for applications such as environmental monitoring of water quality or quality control of pharmaceutical production. A nanospectrometer is a planar spectrometer-on-chip that can be combined with a number of light sources. The chip diffracts incident light to a series of wavelength dependent spatially addressed units that can be imaged and collected with a CCD camera. It is compact in size (10 mm × 15 mm × 0.5 mm) and has a high spectral resolution of 2×10^-5um. This study is an extension of a previous investigation of water-filled SW-PCF spectroscopy. Instead of analyzing water samples fluorescent dyes were tested. Different types of dyes that absorbed and emitted light in the same spectral window as the chip were identified. Spectroscopy measurements for nile blue perchlorate dye are presented in this conference paper. A 70 mW laser at 637nm was employed to demonstrate the fluorescence spectroscopy capability of SW-PCF enhanced spectroscopy with a nanospectrometer. It was demonstrated that the SW-PCF is suitable for spectroscopy of dyes with a conventional optical spectrum analyzer and a nanospectrometer. A 5 microliter sample of dye was loaded into a 14cm long SW-PCF. The fluorescence-spectroscopic data was compared to an un-filled SW-PCF. Absorption and emission spectra for the dye were measured near 637nm.

An image-processing algorithm for use with a nano-featured spectrometer chemical agent detection configuration is presented. The spectrometer chip acquired from Nano-Optic Devices^TM can reduce the size of the spectrometer down to a coin. The nanospectrometer chip was aligned with a 635nm laser source, objective lenses, and a CCD camera. The images from a nanospectrometer chip were collected and compared to reference spectra. Random background noise contributions were isolated and removed from the diffraction pattern image analysis via a threshold filter. Results are provided for the image-based detection of the diffraction pattern produced by the nanospectrometer. The featured PCF spectrometer has the potential to measure optical absorption spectra in order to detect trace amounts of contaminants. MATLAB tools allow for implementation of intelligent, automatic detection of the relevant sub-patterns in the diffraction patterns and subsequent extraction of the parameters using region-detection algorithms such as the generalized Hough transform, which detects specific shapes within the image. This transform is a method for detecting curves by exploiting the duality between points on a curve and parameters of that curve. By employing this imageprocessing technique, future sensor systems will benefit from new applications such as unsupervised environmental monitoring of air or water quality.

In this paper, a novel design of microbending hetero-core fiber optic sensor for force and location sensing is proposed, and potential applications to home security systems are discussed. Force and location detection is done by using two different microbending fiber optic sensors. The main idea is, we have two unknowns, two different fibers, and two simultaneous intensity measurements. In order to demonstrate the location detection of the microbending fiber optic sensor, changes in the light intensity are examined with different force locations and forces magnitudes on the microbending fiber optic sensor. Several experiments are performed for different microbend sensors by varying periodicity, corrugation size, thickness of plates, and the configuration of optical fiber type. All experiments were done on a microbending sensor constructed from 62,5/125 μm multimode fibers and a microbending sensor constructed from 62,5-50-62,5/125 μm hetero-core fiber. For each case, the output light intensity is measured as a function of applied force. The characteristics of hysteresis, repeatability and location comparison are examined for each combination of microbending fiber optic sensors. Experimental results show that the sensitivity of the proposed microbending sensor constructed using hetero-core optical fiber having loops is the highest.

This paper presents the design of a novel periodic macrobending hetero-core fiber optic sensor embedded in textile for respiratory movements’ analysis. We report on several different designs based on textiles which have different loop periodicity and configuration of optical fiber types. In all experiments, the changes of textile elongation are measured during breathing movements. In order to demonstrate the superiority of the proposed sensor, experiments were done on a macrobending sensor constructed from 62.5-50-62.5 hetero-core fiber and a macrobending sensor constructed from 62.5/125 μm multi-mode fiber having different loops. Experimental results show that the sensitivity of the proposed macrobending sensor constructed using hetero-core optical fiber is much higher than the sensor constructed from plain multi-mode optical fiber. It is also shown that, the sensitivity of the sensor increases as the number of loops is increased. On the other hand, several experiments were performed for periodic macrobending sensors having different bending radius by changing the lengths of loops amplitude and period. We demonstrate that the sensors tested on different patients’ morphology can successfully sense respiratory movements.

This paper discusses the RF interferometry at millimeter-wave frequencies for sensing applications and reports the development of a millimeter-wave interferometric sensor operating around 35 GHz. The sensor is completely realized using microwave integrated circuits (MICs) and microwave monolithic integrated circuits (MMICs). It has been used for various sensing including displacement and velocity measurement. The sensor achieves a resolution and maximum error of only 10 μm and 27 μm, respectively, for displacement sensing and can measure velocity as low as 27.7 mm/s with a resolution about 2.7mm/s. Quick response and accurate sensing, as demonstrated by the developed millimeter-wave interferometric sensor, make the millimeter-wave interferometry attractive for sensing of various civil and mechanical structures.

In this study, three different CFRP specimens with internal artificial delaminations of various sizes and located at different depths were investigated by means of Pulsed Thermography (PT) under laboratory conditions. The three CFRP panels, having the same thickness and defects characteristics but with a different shape (planar, trapezoid and curved), were assessed after applying various signal processing tools on the acquired thermal data (i.e. Thermographic Signal Reconstruction, Pulsed Phase Thermography and Principal Component Thermography). The effectiveness of the above processing tools was initially evaluated in a qualitative manner, comparing the imaging outputs and the information retrieval in terms of defect detectability enhancement and noise reduction. Simultaneously, the produced defect detectability was evaluated through Signal-to-Noise Ratio (SNR) computations, quantifying the image quality and the intensity contrast produced between the defected area and the adjacent background area of the test panel. From the results of this study, it can be concluded that the implementation of PT along with the application of advanced signal processing algorithms can be a useful technique for NDT assessment, providing enhanced qualitative information. Nevertheless, SNR analysis showed that despite the enhanced visibility resulting from these algorithms, these can be properly applied in order to retrieve the best possible information according to the user’s demands.

In the present work, a novel method of infrared (IR) thermography called Thermo - Electrical Lockin Thermography (TELT) was developed for the characterization of subsurface defects in materials and structures. This new IR thermography method is based on the thermal excitation of materials under testing using a Peltier device and appropriate electronics allowing for accurate thermal cycling. Results from using this method were compared with different IR methodologies (i.e. Pulsed Phase thermography). It was found that Thermo - Electrical Lockin Thermography provides not only qualitative but also quantitative results.

This paper presents an overview of non-intrusive electric field sensing. The non-intrusive nature is attained by creating a sensor that is entirely dielectric, has a small cross-sectional area, and has the interrogation electronics a long distance away from the system under test. One non-intrusive electric field sensing technology is the slab coupled optical fiber sensor (SCOS). The SCOS consists of an electro-optic crystal attached to the surface of a D-shaped optical fiber. It is entirely dielectric and has a cross-sectional area down to 0.3mm by 0.3mm. The SCOS device functions as an electric field sensor through use of resonant mode coupling between the crystal waveguide and the core of a D-shaped optical fiber. The resonant mode coupling of a SCOS device occurs at specific wavelengths whose spectral locations are determined in part by the effective refractive index of the modes in the slab. An electric field changes the refractive index of the slab causing a shift in the spectral position of the resonant modes. This paper describes an overview of the SCOS technology including the theory, fabrication, and operation. The effect of crystal orientation and crystal type are explained with respect to directional sensitivity and frequency response.

Ion traps are widely used in the field of mass spectrometry. These devices use high electric fields to mass-selectively trap, eject, and count the particles of a material, producing a mass spectrum of the given material. Because of their usefulness, technology pushes for smaller, more portable ion traps for field use. Making internal ion trap field measurements not yet feasible because current electric field sensors are often too bulky or their metallic composition perturbs field measurements. Using slab coupled optical sensor (SCOS) technology, we are able to build sensors that are compatible with the spacing constraints of the ion trap. These sensors are created by attaching a nonlinear crystal slab waveguide to an optical fiber. When a laser propagates through the fiber, certain wavelengths of light couple out of the fiber via the crystal and create “resonances” in the output light spectrum. These resonances shift in proportion to a given applied electric field, and by measuring that shift, we can approximate the electric field. Developing a sensor that can effectively characterize the electric fields within an ion trap will greatly assist in ion trap design, fabrication, and troubleshooting techniques.

We present an optical fiber non-intrusive sensor for measuring high voltage transients. The sensor converts the unknown voltage to electric field, which is then measured using slab-coupled optical fiber sensor (SCOS). Since everything in the sensor except the electrodes is made of dielectric materials and due to the small field sensor size, the sensor is minimally perturbing to the measured voltage. We present the details of the sensor design, which eliminates arcing and minimizes local dielectric breakdown using Teflon blocks and insulation of the whole structure with transformer oil. The structure has a capacitance of less than 3pF and resistance greater than 10 GΩ. We show the measurement of 66.5 kV pulse with a 32.6μs time constant. The measurement matches the expected value of 67.8 kV with less than 2% error.

In this paper, we demonstrate a self-powered AC-current sensor using a piezoelectric connected-in-series approach to increase the sensitivity. The sensor consists of a CuBe-beam, piezoelectric-PZT-sheet, NdFeB hard-magnet, and mechanical-frame. When the sensor is placed in an alternative magnetic-field induced by an alternative current-carrying wire, the magnet fixed on the beam is subjected to an alternative magnetic-force produced by the magnetic-field. Therefore, the beam is oscillated. Consequently, the piezoelectric-sheet fixed on the beam is periodically deformed and continuously produces voltage-response. When beams are connected in-series, the total voltage-response is significantly enlarged while the background-noise remains the same. The experimental result shows the sensitivity of the sensor consisting 8 beams connected in-series under the magnetic-field generated by a wire of 8-Ampere from a breaker is enlarged from 130 mV/A to 640 mV/A.

The ability to monitor the structural health of the rotating components, especially in the hot sections of turbine engines, is of major interest to the aero community in improving engine safety and reliability. The use of instrumentation for these applications remains very challenging. It requires sensors and techniques that are highly accurate, are able to operate in a high temperature environment, and can detect minute changes and hidden flaws before catastrophic events occur. The National Aeronautics and Space Administration (NASA) has taken a lead role in the investigation of new sensor technologies and techniques for the in situ structural health monitoring of gas turbine engines. As part of this effort, microwave sensor technology has been investigated as a means of making high temperature non-contact blade tip clearance, blade tip timing, and blade vibration measurements for use in gas turbine engines. This paper presents a summary of key results and findings obtained from the evaluation of two different types of microwave sensors that have been investigated for possible use in structural health monitoring applications. The first is a microwave blade tip clearance sensor that has been evaluated on a large scale Axial Vane Fan, a subscale Turbofan, and more recently on sub-scale turbine engine like disks. The second is a novel microwave based blade vibration sensor that was also used in parallel with the microwave blade tip clearance sensors on the same experiments with the sub-scale turbine engine disks.

Generally, rotating engine components undergo high centrifugal loading environment which subject them to various types of failure initiation mechanisms. Health monitoring of these components is a necessity and is often challenging to implement. This is primarily due to numerous factors including the presence of scattered loading conditions, flaw sizes, component geometry and materials properties, all which hinder the simplicity of applying health monitoring applications. This paper represents a summary work of combined experimental and analytical modeling that included data collection from a spin test experiment of a rotor disk addressing the aforementioned durability issues. It further covers presentation of results obtained from a finite element modeling study to characterize the structural durability of a cracked rotor as it relates to the experimental findings. The experimental data include blade tip clearance, blade tip timing and shaft displacement measurements. The tests were conducted at the NASA Glenn Research Center’s Rotordynamics Laboratory, a high precision spin rig. The results are evaluated and examined to determine their significance on the development of a health monitoring system to pre-predict cracks and other anomalies and to assist in initiating a supplemental physics based fault prediction analytical model.

The Aeronautical Sciences Project under NASA’s Fundamental Aeronautics Program is interested in the development of novel measurement technologies, such as optical surface measurements for the in situ health monitoring of critical constituents of the internal flow path. In situ health monitoring has the potential to detect flaws, i.e. cracks in key components, such as engine turbine disks, before the flaws lead to catastrophic failure. The present study, aims to further validate and develop an optical strain measurement technique to measure the radial growth and strain field of an already cracked disk, mimicking the geometry of a sub-scale turbine engine disk, under loaded conditions in the NASA Glenn Research Center’s High Precision Rotordynamics Laboratory. The technique offers potential fault detection by imaging an applied high-contrast random speckle pattern under unloaded and loaded conditions with a CCD camera. Spinning the cracked disk at high speeds (loaded conditions) induces an external load, resulting in a radial growth of the disk of approximately 50.0-μm in the flawed region and hence, a localized strain field. When imaging the cracked disk under static conditions, the disk will be undistorted; however, during rotation the cracked region will grow radially, thus causing the applied particle pattern to be ‘shifted’. The resulting particle displacements between the two images is measured using the two-dimensional cross-correlation algorithms implemented in standard Particle Image Velocimetry (PIV) software to track the disk growth, which facilitates calculation of the localized strain field. A random particle distribution is adhered onto the surface of the cracked disk and two bench top experiments are carried out to evaluate the technique’s ability to measure the induced particle displacements. The disk is shifted manually using a translation stage equipped with a fine micrometer and a hotplate is used to induce thermal growth of the disk, causing the particles to become shifted. For both experiments, reference and test images are acquired before and after the induced shifts, respectively, and then processed using PIV software. The controlled manual translation of the disk resulted in detection of the particle displacements accurate to ~1.75% of full scale and the thermal expansion experiment resulted in successful detection of the disk’s thermal growth as compared to the calculated thermal expansion results. After validation of the technique through the induced shift experiments, the technique is implemented in the Rotordynamics Lab for preliminary assessment in a simulated engine environment. The discussion of the findings and plans for future work to improve upon the results are addressed in the paper.

Fibre reinforced plastic (FRP) rotors are lightweight and offer great perspectives in high-speed applications such as turbo machinery. Currently, novel rotor structures and materials are investigated for the purpose of increasing machine efficiency, lifetime and loading limits. Due to complex rotor structures, high anisotropy and non-linear behavior of FRP under dynamic loads, an in-process measurement system is necessary to monitor and to investigate the evolution of damages under real operation conditions. A non-invasive, optical laser Doppler distance sensor measurement system is applied to determine the biaxial deformation of a bladed FRP rotor with micron uncertainty as well as the tangential blade vibrations at surface speeds above 300 m/s. The laser Doppler distance sensor is applicable under vacuum conditions. Measurements at varying loading conditions are used to determine elastic and plastic deformations. Furthermore they allow to determine hysteresis, fatigue, Eigenfrequency shifts and loading limits. The deformation measurements show a highly anisotropic and nonlinear behavior and offer a deeper understanding of the damage evolution in FRP rotors. The experimental results are used to validate and to calibrate a simulation model of the deformation. The simulation combines finite element analysis and a damage mechanics model. The combination of simulation and measurement system enables the monitoring and prediction of damage evolutions of FRP rotors in process.

The characterization of the dominant fracture mode may assist in the prediction of the remaining life of a concrete structure due to the sequence between successive tensile and shear mechanisms. Acoustic emission sensors record the elastic responses after any fracture event converting them into electric waveforms. The characteristics of the waveforms vary according to the movement of the crack tips, enabling characterization of the original mode. In this study fracture experiments on concrete beams are conducted. The aim is to examine the typical acoustic signals emitted by different fracture modes (namely tension due to bending and shear) in a concrete matrix. This is an advancement of a recent study focusing on smaller scale mortar and marble specimens. The dominant stress field and ultimate fracture mode is controlled by modification of the four-point bending setup while acoustic emission is monitored by six sensors at fixed locations. Conclusions about how to distinguish the sources based on waveform parameters of time domain (duration, rise time) and frequency are drawn. Specifically, emissions during the shear loading exhibit lower frequencies and longer duration than tensile. Results show that, combination of AE features may help to characterize the shift between dominant fracture modes and contribute to the structural health monitoring of concrete. This offers the basis for in-situ application provided that the distortion of the signal due to heterogeneous wave path is accounted for.

The Impulse Excitation Technique (IET) is a useful tool for characterizing the structural condition of concrete. Processing the obtained dynamic parameters (damping ratio, response frequency) as a function of response amplitude, clear and systematic differences appear between intact and cracked specimens, while factors like age and sustained load are also influential. Simultaneously, Acoustic Emission (AE) and Ultrasonic Pulse Velocity (UPV) techniques are used during the three point bending test of the beams in order to supply additional information on the level of damage accumulation which resulted in the specific dynamic behavior revealed by the IET test.

This paper presents the development of optical equipment that is suitable for ultrasonic guided wave detection for active SHM in the hundreds of kHz range. In recent years, fiber Bragg grating (FBG) sensors have been investigated by many researchers as an alternative to piezoelectric sensors for the detection of ultrasonic waves. FBG have the advantage of being durable, lightweight, and easily embeddable into composite structures as well as being immune to electromagnetic interference and optically multiplexed. However, there is no commercially available product that uses this promising technology for the detection of ultrasonic guided waves because: (a) the frequency is high (hundreds of kHz); (b) the strains are very small (nano-strain); (c) the operational loads may also induce very large quasi-static strains (the superposition of very small ultrasonic strains and very large quasi-static strain presents a very significant challenge). Although no turn-key optical system exists for ultrasonic guided wave detection, we developed optical ultrasonic guided wave equipment using a tunable laser device. The measurement resolution and sampling speed were considered as the most important criteria in our test. We achieved high sensitive (nano-strain) and high sampling rate. Comparative measurements of low-amplitude ultrasonic waves have been done including FBG, strain gauge, and piezoelectric wafer active sensors (PWAS). Calibration and performance improvements for the optical interrogation system are also developed and discussed. The paper ends with conclusions and suggestions for further work.

We study theoretically the potentiality of dual phononic-photonic (the so-called phoxonic) crystals for liquid sensing applications. We investigate the existence of well-defined features (peaks or dips) in the transmission spectra of acoustic and optical waves and estimate their sensitivity to the sound and light velocities of the liquid environment. Two different sensors are investigated. In the first one, we study the in-plane transmission through a two-dimensional (2D) crystal made of cylindrical holes in a Si substrate where one row of holes is filled with a liquid. In the second one, the out of plane propagation is investigated when considering the transmission of the incident wave perpendicular to a periodic array of holes in a slab. Such ultra compact structure is shown to be a label-free, affinity-based acoustic and optical nanosensor, useful for biosensing applications in which the amount of analyte can be often limited.

Cortical bone is one of the most complex heterogeneous media exhibiting strong wave dispersion. In such media when a burst of energy goes into the formation of elastic waves the different modes tend to separate according to the velocities of the frequency components as usually occurs in waveguides. In this study human femur specimens were subjected to elastic wave measurements. The main objective of the study is using broadband acoustic emission sensors to measure parameters like wave velocity dispersion and attenuation. Additionally, waveform parameters like the duration, rise time and average frequency, are also examined relatively to the propagation distance as a preparation for acoustic emission monitoring during fracture. To do so, four sensors were placed at adjacent positions on the surface of the cortical bone in order to record the transient response after pencil lead break excitation. The results are compared to similar measurements on a bulk metal piece which does not exhibit heterogeneity at the scale of the propagating wave lengths. It is shown that the microstructure of the tissue imposes a dispersive behavior for frequencies below 1 MHz and care should be taken for interpretation of the signals.

Ultrasonic strain sensing performance of the large area PVDF with Inter Digital Electrodes (IDE) is studied in this work. Procedure to obtain IDE on a beta-phase PVDF is explained. PVDF film with IDE is bonded on a plate structure and is characterized for its directional sensitivity at different frequencies. Guided waves are induced on the IDE-PVDF sensor from different directions by placing a piezoelectric wafer actuator at different angles. Strain induced on the IDE-PVDF sensor by the guided waves in estimated by using a Laser Doppler Vibrometer (LDV) and a wave propagation model. Using measured voltage response from IDE-PVDF sensor and the strain measurements from LDV the piezoelectric coefficient is estimated in various directions. The variation of ℯ11 e at different angles shows directional sensitivity of the IDE-PVDF sensor to the incident guided waves. The present study provides an effective technique to characterize thin film piezoelectric sensors for ultrasonic strain sensing at very high frequencies of 200 kHz. Often frequency of the guided wave is changed to alter the wavelength to interrogate damages of different sizes in Structural Health Monitoring (SHM) applications. The unique property of directional sensitivity combined with frequency tunability makes the IDEPVDF sensor most suitable for SHM of structures.

The present study evaluates the potential of GPR for the inspection of pre-stressed concrete bridges and its usefulness to provide non visible information of the interior structural geometry and condition, required for strengthening and rehabilitation purposes. For that purpose, different concrete blocks of varying dimensions with embedded steel reinforcement bars, tendon ducts and fabricated voids, were prepared and tested by means of GPR in a controlled laboratory environment. 2D data acquisition was carried out in reflection mode along single profile lines of the samples in order to locate the internal structural elements. 3D surveys were also performed in a grid format both along horizontal and vertical lines, and the individual profiles collected were interpolated and further processed using a 3D reconstruction software, in order to provide a detailed insight into the concrete structure. The obtained 2D profiles provided the accurate depth and position of the embedded rebars and tendon ducts, verifying the original drawings. 3D data cubes were created enabling the presentation of depth slices and providing additional information such as shape and localization of the internal elements. The results obtained from this work showed the effectiveness and reliability of the GPR technique for pre-stressed concrete bridge investigations.

Prognosis regarding durability of composite structures using various Structural Health Monitoring (SHM) techniques is an important and challenging topic of research. Ultrasonic SHM systems with embedded transducers have potential application here due to their instant monitoring capability, compact packaging potential toward unobtrusiveness and noninvasiveness as compared to non-contact ultrasonic and eddy current techniques which require disassembly of the structure. However, embedded sensors pose a risk to the structure by acting as a flaw thereby reducing life. The present paper focuses on the determination of strength and fatigue life of the composite laminate with embedded film sensors like CNT nanocomposite, PVDF thin films and piezoceramic films. First, the techniques of embedding these sensors in composite laminates is described followed by the determination of static strength and fatigue life at coupon level testing in Universal Testing Machine (UTM). Failure mechanisms of the composite laminate with embedded sensors are studied for static and dynamic loading cases. The coupons are monitored for loading and failure using the embedded sensors. A comparison of the performance of these three types of embedded sensors is made to study their suitability in various applications. These three types of embedded sensors cover a wide variety of applications, and prove to be viable in embedded sensor based SHM of composite structures.

A flexoelectric bridge-structured microphone using bulk barium strontium titanate (Ba_0.65Sr_0.35TiO₃ or BST) ceramic was investigated in this study. The flexoelectric microphone was installed in an anechoic box and exposed to the sound pressure emitted from a loud speaker. Charge sensitivity of the flexoelectric microphone was measured and calibrated using a reference microphone. The 1.5 mm×768 μm×50 μm micro-machined bridge-structured flexoelectric microphone has a sensitivity of 0.92 pC/Pa, while its resonance frequency was calculated to be 98.67 kHz. The analytical and experimental results show that the flexoelectric microphone has both high sensitivity and broad bandwidth, indicating that flexoelectric microphones are potential candidates for many applications.

The chemically powered nanowire model consisting of a catalytic and a noncatalytic nanowire confined in a cubic box is investigated. The interactions between nanowire and solvent are simulated using hybrid molecular dynamics/ multiparticle collision dynamics. The motion of the nanowire is analyzed using rigid body dynamic. The effect of temperature and solvent concentration on the center-of-mass velocity of motor are provided. The center-of-mass velocity of the nanomotor along its axis increases with an increase of temperature and solvent concentration. The results are also compared with existed nanodimer model.

This paper proposes a damage assessment methodology for the non-structural elements, especially the ceiling, in cooperation with the smart sensors and the inspection blimp robot with the Wi-Fi camera. The developed smart sensors use the infrared LEDs in sending the measured data to the inspection blimp robot. The inspection blimp robot integrated in the proposed system has a Wi-Fi camera and an infrared remote control receiver for receiving the data from the smart sensor. In the proposed methodology, the distributed smart sensors firstly detect the damage occurrence. Next, the inspection blimp robots can gather the data from the smart sensors, which transmit the measured data by using an infrared remote control receiver and LED signals. The inspection blimp robot also can inspect the damage location and captures the photographic image of the damage condition. The inspection blimp robot will be able to estimate the damage condition without any process of engineers’ on-site-inspection involved. To demonstrate the effectiveness of the inspection blimp robot, the blimp robot is utilized to estimate the aging ceiling of a real structure. For demonstrating the feasibility or possibility of the proposed damage assessment methodology in cooperation with the smart sensors and the inspection blimp robot, the conceptual laboratory experiment is conducted. The proposed methodology will provide valuable information for the repair and maintenance decision making of a damaged structure.

This research aims in characterizing modified cement mortar with carbon nanotubes (CNTs) that act as nanoreinforcements leading to the development of innovative materials possessing multi-functionality and smartness. Such multifunctional properties include enhanced mechanical behavior, electrical and thermal conductivity, and piezo-electric characteristics. The effective thermal properties of the modified nano-composites were evaluated using IR Thermography. The electrical resistivity was measured with a contact test method using a custom made apparatus and applying a known D.C. voltage. To eliminate any polarization effects the specimens were dried in an oven before testing. In this work, the thermal and electrical properties of the nano-modified materials were studied by nondestructively monitoring their structural integrity in real time using the intrinsic multi-functional properties of the material as damage sensors.

Electromagnetic interference (EMI) immune and light-weight, fiber-optic sensor based Structural Health Monitoring (SHM) will find increasing application in aerospace structures ranging from aircraft wings to jet engine vanes. Intelligent Fiber Optic Systems Corporation (IFOS) has been developing multi-functional fiber Bragg grating (FBG) sensor systems including parallel processing FBG interrogators combined with advanced signal processing for SHM, structural state sensing and load monitoring applications. This paper reports work with Auburn University on embedding and testing FBG sensor arrays in a quarter scale model of a T38 composite wing. The wing was designed and manufactured using fabric reinforced polymer matrix composites. FBG sensors were embedded under the top layer of the composite. Their positions were chosen based on strain maps determined by finite element analysis. Static and dynamic testing confirmed expected response from the FBGs. The demonstrated technology has the potential to be further developed into an autonomous onboard system to perform load monitoring, SHM and Non-Destructive Evaluation (NDE) of composite aerospace structures (wings and rotorcraft blades). This platform technology could also be applied to flight testing of morphing and aero-elastic control surfaces.

A Lamb wave-based damage identification method called damage imaging method for composite shells is presented. A damage index (DI) is generated from the delay matrix of the Lamb wave response signals, and it is used to indicate the location and approximate area of the damage. A piezoelectric actuator is employed to generate the Lamb waves that are subsequently captured by a fiber Bragg grating (FBG) sensor element array multiplexed in a single fiber connected to a high-speed fiber-optic sensor system. The high-speed sensing is enabled by an innovative parallel-architecture optical interrogation system. The viability of this method is demonstrated by analyzing the numerical and experimental Lamb wave response signals from laminated composite shells. The technique only requires the response signals from the plate after damage, and it is capable of performing near real-time damage identification. This study sheds some light on the application of a Lamb wave-based damage detection algorithm for curved plate/shell-type structures by using the relatively low frequency (around 100 kHz) Lamb wave response and the high-speed FBG sensor system.

Fiber optic sensor systems can alleviate certain challenges faced by electronics sensors faced when monitoring structures subject to marine and other harsh environments. Challenges in implementation of such systems include scalability, interconnection and cabling. We describe a fiber Bragg grating (FBG) sensor system architecture based that is scalable to support over 1000 electromagnetic interference immune sensors at high sampling rates for harsh environment applications. A key enabler is a high performance FBG interrogator supporting subsection sampling rates ranging from kHz to MHz. Results are presented for fast dynamic switching between multiple structural sections and the use of this sensing system for dynamic load monitoring as well as the potential for acoustic emission and ultrasonic monitoring on materials ranging from aluminum and composites to concrete subject to severe environments.

Thermal diffusivity of anodic alumina (AAO) templated bismuth telluride nanowires has been measured using recently proposed frequency modulated thermal wave imaging (FMTWI). The technique provides a fast and efficient non-contact approach for in-plane thermal characterization of nanomaterials. An intensity modulated up-chirp signal is applied as photothermal excitation and the thermal response is monitored using an infrared (IR) thermography based temperature sensing system. Thermal diffusivity of the sample is experimentally assessed using the multiple phase information extracted from a single run of the experiment. This feature considerably reduces the operational time of the experiment as compared to similar lock-in thermography based approaches. This unique approach of solely using the phase information for thermal diffusivity measurements, allows the experiment to be more immune to the local variations in surface temperature and emissivity of the radiating surface. The experimental details of the technique are discussed, with practical measurement of thermal diffusivity of Bi₂Te₃/AAO nanocomposite in direction perpendicular to the nanochannel axis.