This PDF file contains the front matter associated with SPIE Proceedings Volume 7648, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Fiber Bragg gratings (FBGs) are wavelength selective optical reflectors with excellent strain sensitivity and small sensing footprint, which makes them suitable as diagnostic sensors for structural health monitoring applications. In this work, we explore the narrowband wavelength selectivity of FBGs for optical feedback in a tunable fiber ring laser. The fiber ring laser consists of an erbium doped fiber laser that is pumped with a Raman laser (980 nm) to produce population inversion and amplified spontaneous emission (ASE) in the C-band. The ASE light is used to illuminate a FBG sensor connected to the ring, and the reflected light from the sensor is fed back into the laser cavity to produce stimulated emission at the instantaneous center wavelength of the sensor. As the wavelength of the sensor shifts due mechanical or thermal strains, the wavelength of the optical output from the ring laser shifts accordingly. By combining the ring laser with a dynamic spectral demodulator for optical readout, the instantaneous wavelength of the ring laser is tracked with high temporal resolution. The fiber ring laser system offers several potential advantages in the diagnostic sensing of mechanical strains for SHM applications including, fully integrated laser and sensor system, high source power levels at the sensor wavelength, narrow spectral line-width, coherent spectral demodulation, and low system costs. In this work, we present experimental results that detail the feasibility of dynamic spectral tuning of the fiber ring laser at frequencies up to hundreds of kilohertz using a single FBG sensing element. Using multiple sensing elements, the fiber ring laser system would allow for active monitoring of dynamic strains in a multi-point sensor array configuration, which is particularly suitable for the localization of high frequency mechanical strains produced by impact loading and cracking events in structures.

This paper presents a means for the high repetition rate interrogation of fiber Bragg gratings (FBG's). The new system highlights a method that allows a tradeoff between the full spectrum capture rate and the wavelength range and/or the spectral resolution of the technique. Rapid capture of the entire reflection spectrum at high interrogation rates shows important features that are missed when using methods that merely track changes in the peak location of the spectrum. The essential feature of the new system is that it incorporates a MEMs tunable filter driven by a variable frequency openloop sinusoidal source. The paper demonstrates the new system on a laminated composite system under impact loading.

In this study we evaluate the measurements of fiber Bragg sensor spectra from a sensor embedded in a composite laminate subjected to multiple low velocity impacts. The full-spectral response of the sensor is interrogated in reflection at 100 kHz during the impact events. The measurement of the time dependent spectra features are compared with previous results obtained at a 534 Hz interrogation rate. With the increased interrogation rate, we can observe a smooth transition in the full-spectra response of the sensor between strikes and the presence of peak-splitting due to transverse compression from the beginning of the laminate lifetime. Finally, at the 100 kHz acquisition rate, it is possible to determine the maximum wavelength and accurately determine the duration of the impact event for all of the strikes.

We present very simple and sensitive techniques capable to interrogate ultra-weak Bragg gratings written in a long SMF-28 fiber. The techniques are suitable for distributed detection and localization of alarm conditions in early warning systems. Also, a high multiplexing capability was demonstrated in a multi-point measuring system utilizing an array of identical FBGs. This technique is based on measuring correlation between the probe and reflected signal. A DFB laser operating in a CW regime was used as a light source. We present results of experimental verification of the techniques in different sensor configurations for static strain and vibration measuring. Multipoint sensor using Bragg gratings with reflectivity of 0.01% printed in a 3-km long fiber was demonstrated.

Thermal strain measurements by fiber Bragg grating (FBG) sensors mounted onto different host materials are demonstrated for low coefficients of thermal expansion (CTE). Such low CTEs are typically found in carbon fiber reinforced plastics (CFRP). This work has application potential for FBG sensor networks in the highprecision control of thermal deformations in structures or in curing monitoring. For this purpose, a thermal error model of the FBG sensor, which accounts for the thermo-optic coefficient and the thermal expansion of the FBG, was characterized experimentally. The error-model characterization method is based on reference measurements of FBGs bonded to ZERODUR ceramics. Using this error model, thermal strain can be measured by surface-mounted FBGs on any given host structure using an external temperature reference and the FBG's wavelength shift. This method is demonstrated successfully for unidirectional layers of CFRP with a CTE of -0.4 · 10^-6 1/K in fiber direction and for steel (316 Ti), which is commonly used in cryogenic applications. Measurements are performed for temperatures from 100K to 320K and the results are verified by high-precision dilatometer measurements. Accuracy limits of the FBG-based thermal strain measurements are discussed, as well as the minimization of errors induced by the FBG's structural interface. Further, the reduction of errors in the adhesive bonding is discussed. This work expands the understanding of the separation of thermal and mechanical effects in the signals obtained by FBGs.

A recently developed technique is presented for thermographic detection of flaws in composite materials by performing temperature measurements with fiber optic Bragg gratings. Individual optical fibers with multiple Bragg gratings employed as surface temperature sensors were bonded to the surfaces of composites with subsurface defects. The investigated structures included a 10-ply composite specimen with subsurface delaminations of various sizes and depths. Both during and following the application of a thermal heat flux to the surface, the individual Bragg grating sensors measured the temporal and spatial temperature variations. The data obtained from grating sensors were analyzed with thermal modeling techniques of conventional thermography to reveal particular characteristics of the interested areas. Results were compared with the calculations using numerical simulation techniques. Methods and limitations for performing in-situ structural health monitoring are discussed.

The measurement of pressure is essential for the design and flying controlling of aircraft. In order to measure the surface pressures of the aircraft, the common pressure tube method and Pressure sensitive paint measurement method have their own disadvantages, and are not applicable to all aircraft structures and real time pressure monitoring. In this paper, a novel thin film pressure sensor based on Fiber Bragg Grating (FBG) is proposed, using FBG measuring the tangential strain of the disk sensing film. Theoretical circle strain of the disk sensing film of the pressure sensor under pressure and temperature variation are analyzed, and the linear relationship between FBG center wavelength shift and pressure, temperature variation is gotten. The pressure and temperature calibration experiments prove the theoretical analysis. But the calibration sensing parameters are small than the calculating ones, which is caused by the constraint of optical fibre to the thin sensing film.

The use and viability of fiber Bragg grating sensors in sandwich composite structures for the purpose of structural health monitoring under low velocity impact. Initially, a group of twelve specimens were tested to characterize the impact response of sandwich composite structures. Each specimen test consisted of repeated impacts at a constant impact energy to measure and observe damage progression. Once this was completed, a single optical fiber with a fiber Bragg grating was embedded in the structure between the core and the faceplate to and measured using a laser. The shift and deformation of the reflected spectrum from the fiber Bragg grating sensor resulting from each strike was analyzed and the corresponding strain was measured. The peak wavelength shift measurements did not have a strong correlation to the accumulation of damage in the sandwich laminate. However, the spectral distortion did evolve throughout the initial accumulation of damage in the laminate. Further analysis of the spectrum is needed to correlate the spectral response to the damage modes.

High powered microwave weapons use electric fields to overload electronics. We developed a non-intrusive sensor using a technology based on slab coupled optical sensing (SCOS). Each sensor detects the electric field component normal to the surface of the slab. By mounting two of these sensors orthogonally to each other, a more complete image of the electrical field can be obtained. One of the major hurdles of creating a multi-axial SCOS is keeping the size of the sensor small. The size is limited by (1) the size of the sensing material and (2) the ability to package the sensor to maintain its structural integrity and orientation. Good sensitivity is attained with SCOS with a length less than 3 mm and the D-fiber platform has a small core which allows for much less bending loss than standard single mode fiber. We have developed a mounting system that is heat resistant and structurally robust to protect the sensor that is extremely small when compared to traditional electric field sensors.

One limitation of piezoelectric impedance/admittance approach is that the sensor is permanently fixed after it is bonded/embedded into the mechanical structure to be monitored. Recently, the magnetic transducer, which is essentially an electrical coil inserted with a permanent magnet, is explored for impedance/admittance-based damage detection. Since there is no direct contact between the magnetic sensor and the host structure, the magnetic impedance/admittance approach is capable of online health monitoring of structures with complicated geometries and boundaries. Also, the magnetic impedance/admittance sensor is moveable above the structure surface, which may reduce the number of sensors needed to cover a large structural area. In an earlier study a new magnetic impedance sensing scheme with circuitry integration is proposed, which can greatly enhance the signal-to-noise ratio and amplify the damage induced admittance change. In this research, we systematically study the sensor location on the performance of the magnetic impedance/admittance-based damage detection scheme with circuitry integration. By examining the resonant peaks in the circuitry impedance curves, the damage-induced change of circuitry admittance and the two-way magneto-mechanical coupling, the different amplification effects of the magnetic sensor on the dynamical responses around mechanical modes is investigated. The criteria of tuning the capacitance of the tunable capacitor to achieve significantly amplified admittance changes in a wide frequency range are also developed. Correlated numerical and experimental studies are carried out to validate our proposed tuning criteria.

Characteristics of Metal Oxide Semiconductor Field Effect Transistor (MOSFET) magnetic sensors have been investigated using a three-dimensional physical simulator which accurately couples the magnetic field equation and the carrier transport equations. The effects of the device geometric parameters, the bias conditions, and the magnetic field on the relative sensitivity of a split drain magnetic sensors are accurately determined. The MOSFET magnetic sensor capability is further enhanced by suggesting an integrated smart structure which is able to fully detect the magnetic field variations in two-directions. The current deflection and relative sensitivity for the suggested magnetic sensor under different operating conditions are finally investigated with the present efficient physical simulator.

Interferometric fiber optic accelerometers constitute a high-responsivity, high-resolution sensing architecture, with achievable sensitivities of several rad/g and resolutions in the micro-g range, depending on the specific configuration. Fiber Bragg grating (FBG) optical accelerometers offer ease of multiplexing but are inherently less sensitive than their interferometric counterparts. Fiber-based accelerometers have the usual optical advantages of being lightweight, electromagnetically immune, and non-spark emitting over traditional (piezo-electric) accelerometer architectures. Among fiber optic sensing methodologies, both interferometric and FBG accelerometers can be interrogated using phase-based demodulation, which offers advantages over intensity-based sensing schemes such as increased linearity, repeatability, and insensitivity to extraneous measurands. The performance of an accelerometer is often characterized in terms of its bandwidth, sensitivity, and resolution, all of which depend on the specific transducer design (the mechanical architecture) as well as the optical interrogation architecture. For a given optical interrogation architecture, a fundamental tradeoff exists in accelerometer transducer design between bandwidth and sensitivity; attempts to increase bandwidth will generally result in a decrease in sensitivity. This paper investigates the frequency and displacement characteristics that govern this tradeoff for several transducer configurations, in order to determine a pair of configurations that offer the greatest sensitivity for a given optical interrogation methodology (interferometric or FBG), at a prescribed bandwidth. The feasibility of several mechanical architectures is assessed based on the physical dimensions required for a given configuration to achieve a primary resonance of at least 15 kHz. The deflection of those configurations under their own self-weight is then considered a measure of accelerometer sensitivity in the measurement band below primary resonance. This paper has been reviewed by Los Alamos National Laboratory and received the following release number: LA-UR 10-00671.

In this paper, a low-cost digital image correlation-based constitutive sensor with a novel identification algorithm that is deployable and scalable in the field is proposed. The term 'constitutive sensor' is coined herein to describe a sensor that is capable of determining the target material constitutive parameters. The proposed method is different from existing identification methods in that it does not need to solve boundary value problems of the target materials using updated parameters. Since the development of the digital image correlation (DIC) technique in the 1980s, the DIC technique has been broadly evaluated and improved for measuring full-field displacements of test specimens, mainly in laboratory settings. Although its potential in damage and mechanical identification is immense, the high cost of current commercial DIC systems makes it difficult to apply the DIC technique to in-field health monitoring of structures. To realize a first ever application of DIC in the field, a prototypical low-cost sensing unit consisting of a high performance embedded microprocessor board, a low-cost web camera, and a communication module is suggested. In the proposed constitutive sensor, DIC displacement fields considered as true values are used in computing stress fields satisfying the equilibrium condition and strain fields using finite element concepts. The unknown constitutive law is initially assumed to be fully anisotropic and linear elastic. A steady state genetic algorithm is utilized to search for the material parameters by minimizing a cost function that measures energy residuals. The main features that allow the sensor to be deployable in the field are introduced, and a validation of the proposed constitutive sensor concept using synthetic data is presented.

Gas cylinders are used in many different situations, such as in research, in industry, in healthcare, and even in the home. Due to demand in such a wide variety of circumstances, there is the inevitable ambition of gas suppliers to improve the efficiency of their business. To this end, a prototype inventory management has been implemented in order to provide such improved efficiency whilst also integrating sensors in order to monitor gas cylinders from a safety perspective. The prototype system is presented in this paper and its operation described in detail. Preliminary results from the prototype system are also shown and the sensors implemented for demonstration are discussed. Future work to be conducted is also alluded to.

This article presents experimental demonstrations of a self-writing waveguide in a photopolymerizable resin system. The waveguide will be embedded in a structure and serve as a self-repairing strain sensor. The sensor would fabricate via lightwaves in the ultraviolet (UV) wavelength range and operate as a sensor in the infrared (IR) wavelength range. Optimized self-written waveguides are obtained by varying the input UV laser power and testing the repeatability of the waveguide fabrication between two optical fibers. An IR laser output is then transmitted between two MM fibers during the fabrication process to quantify the response of the self-repaired optical sensor by measuring the transmitted IR power. The IR power is successfully transmitted through a self-written waveguide; however problems with optical fiber alignment and bending of the waveguide can induce loss of IR transmission.

This paper describes the fabrication of a novel type of pressure sensor based on optical feedback in a Vertical Cavity Surface Emitting Laser (VCSEL). The detection mechanism of the sensor is based on a displacement measurement through self-mixing interferometry in the laser cavity. Using this technique a sensing resolution of half the laser wavelength can be achieved. The use of unpackaged VCSELs allows the integration of the sensor in a flexible polymer material, which enables thin and ultimately bendable optical sensing foils. Moreover, the use of unpackaged dies limits the sensor dimensions and minimizes the distance between two sensing points. Consequently, dense matrices of highly accurate sensing points can be fabricated. A proof of principle set-up for this new sensing mechanism has been developed and a first demonstrator has been fabricated and characterized.

One of the key challenges in developing effective and scalable technologies necessary to realise future pervasive systems and ubiquitous computing is implementing a methodology that genuinely integrates embedded sensing and processing capabilities with everyday materials and objects. Embedding intelligent systems into polymer materials and using such "smart blocks" for constructing smart objects is a promising way to achieve the above. This work provides new solutions to challenges of realising a functional system, comprising sensing, processing and actuating components, fully encapsulated in a block of polymer material. The paper also investigates the possibilities of connecting arrays of such smart blocks in 1.5-D and 2.5-D arrangements to form a modular smart object. Experimental and numerical studies were conducted to establish a level of degradation in mechanical properties and strength of the plastic materials embedded with inserts. In current work, a bare cubic system and the system in a capsule-like package were realised and tested. The results of a full physical characterisation of both individual smart blocks and smart block arrays are presented.

Civil structures are important for any society and it is necessary to monitor their health condition in order to mitigate risks, prevent disasters, and plan maintenance activities in an optimized manner. Structural health monitoring (SHM) recently emerged as a branch of engineering with a great potential for addressing the above mentioned challenges. In spite of its importance and promising benefits, SHM is still relatively infrequently used in real structures. A possible reason for this is a lack of understanding of the SHM process, which is often considered to be a supplemental activity that does not require detailed planning. However, the opposite is true - only proper and detailed development and implementation of each SHM step can ensure its successful and maximal performance. The aim of this paper is to present the SHM process through more than 350 projects. Basic concepts are introduced, and the purpose, requirements and benefits of SHM are discussed. The importance of monitoring over a life span is highlighted. Core activities such as creating monitoring strategy, installation and maintenance of hardware, and data management are presented and discussed. The involved parties are identified and their interaction with the monitoring process is analyzed. Finally, important SHM challenges are identified.

Civil Engineers have used fiber reinforced polymer (FRP) with high axial strength as an effective and economical alternative to steel in harsh corrosion environments. However, the practical applications of FRP are limited by the tendency of FRP materials to fail suddenly under lateral pressure and surface injury. For example, shear stresses result from the bonding effect between the FRP material and the structure of the anchorage system due to the lower shear strength of FRP. This paper proposes a novel smart FRP anchorage system with embedded optical fiber Bragg grating (FBG) sensors to monitor the axial strain state and accordingly the interfacial shear stress, as well as the interfacial damage characteristics of FRP anchorage. One FBG sensor was embedded in an FRP rod outside the anchorage region to evaluate the properties of the material, and seven FBG sensors were distributed along the rod in the anchor to monitor the axial strain variations and study the interfacial mechanical behaviors of the smart FRP anchorage under a static pulling load. The experimental results agreed well with theoretical predictions. The smart FRP anchorage system with optical FBG sensors proves effective and practical for monitoring the long-term mechanical behavior of FRP anchorage systems.

Active sensing methods use actuators/sensors permanently attached to the structure to generate guided waves and measure the arrival waves propagating through the structures. The damage diagnosis is performed through the examination of the arrival waves carrying structural features. Since direct sensory data in guided wave interrogation are implicit with damage related information, advanced signal processing is necessary to extract damage related features for damage diagnosis. Array signal processing is an approach that can map the structure being interrogated with propagating guided waves, producing a visual indication of damage presence, location, and size for crack damage. The arrays can be configured with sensors closely placed or spatially distributed and used in pitch-catch mode. In this paper, we first studied guided wave excitation on isotropic plates and the capability of using piezoelectric wafer active sensors to selectively excite a certain mode in the structure. Then several algorithms for imaging with different types of arrays were investigated. The algorithms were applied to isotropic specimens including thin aluminum plates with hole and crack damage, and thick steel plates with hole damage. The resolution (minimal detectable damage size) was also investigated and compared to the resolution of a linear phased array. Image post processing was used to yield an estimation of the damage size.

There are many applications of ultrasound in the field of material properties' evaluation and structural health monitoring. Here we will consider the detection of broadband laser generated ultrasound taking as an example acoustic emission as simulated by the pencil break test. In this paper three optical methods of detecting these ultrasound signals are compared; these are polarimetry, fibre Bragg gratings and vibrometery. Of these, the first two involve the bonding of a fibre sensor to the sample, whilst the vibrometer is a non-contact instrument that measures out-of-plane displacements. FBGs respond to the inplane strains associated with an ultrasound wave whilst the polarimeter detects birefringence produced by pressure waves acting normal to the fibre. The sensitivities of the systems are compared and their relative merits are discussed. It will also be shown that the polarimetric responses of symmetric and antisymmetric Lamb waves differ, which opens up the possibility of learning more about the nature of an acoustic signal using this technique than can be determined simply from the measurement of in-plane or out-of plane displacements alone.

This work focuses on fatigue crack detection, crack tip localization and quantification in plate like structures using a reference-free method. In many practical applications the environmental conditions in which a structure is operated do not remain same over time. Sensor signals, thus, collected for the damaged state cannot be compared directly with the baseline because a change in the signal can be caused by several factors other than a structural damage. Therefore, reference-free methods are needed for damage detection. Two methods have been discussed in this paper, one with collocated sensors and the other using matching pursuit decomposition (MPD) to detect waves undergoing mode conversion from fatigue crack tip. The time of flight (TOF) of these mode converted waves along with their respective velocities are further used to localize the crack tip and ultimately find the extent of crack. Both these approaches were used to detect fatigue cracks in aluminum plates made of 6061 alloy. These samples were instrumented with collocated piezoelectric sensors and tested under constant amplitude fatigue loading. Crack tip localization was done from the TOF information extracted for mode converted waves using MPD. The crack lengths obtained using this reference-free technique were validated with experimental crack lengths and were found to be in good agreement.

The need for custom-designed sensor networks, tailored to the specific SHM task for practical application of guided waves, is constantly growing. As a prerequisite for a successful development of different monitoring concepts the transducers wave excitation and receiving properties have to be known. The more exactly they are understood the more reliable monitoring concepts are possible. Nowadays different piezoelectric transducer concepts, with varying acting principles having their specific advantages, are used in SHM applications. Strongly unequal properties concerning source density and directivity patterns are revealed. Hence, a method is introduced by which different transducer types can be compared in respect to their efficiency in exciting and receiving guided waves. Furthermore a reciprocity-based model for the estimation of the maximum transducer to transducer distance is applied and discussed.

Flexible ultrasonic transducers (FUTs) which have the on-site installation capability are presented for the non-destructive evaluation (NDE) and structural health monitoring (SHM) purposes. These FUTs consist of 75 μm thick titanium membrane, thick (> 70 μm) thick piezoelectric lead-zirconate-titanate (PZT) composite (PZT-c) films and thin (< 5 μm) thick top electrodes. The PZT-c films are made by a sol-gel spray technique. Such FUT has been glued onto a steel pipe of 101 mm in diameter and 4.5 mm in wall thickness and operated up to 200°C. The glue served as high temperature ultrasonic couplant between the FUT and the external surface of the pipe. The estimated pipe thickness measurement accuracy at 200°C is 34 μm. FUTs also were glued onto the end edge of 2 mm thick aluminum (Al) plates to generate and receive predominantly symmetrical and shear-horizontal (SH) plate acoustic waves (PAWs) to detect simulated line defects at temperature up to 100°C. FUTs glued onto a graphite/epoxy (Gr/Ep) composite are also used for the detection of artificial disbonds. An induction type non-contact method for the evaluation of Al plates and Gr/Ep composites using FUTs is also demonstrated.

New processes introduced by nano science into much more conventional industrial applications require fast, robust and economical reasonable inspection methods for process control and quality assurance. Developed for semiconductor industries the methods available for thin film characterization and quality control are often complex and require highly skilled operation personnel. This paper presents a new concept based on high frequency eddy current spectroscopy that allows reliable and robust thickness measurements of thin conducting films on silicon or insulation substrates with a thickness resolution of about 2.5 nm. The transmission mode sensor configuration is a more practical method for inlinemonitoring of thin film characterization. Due to the insensitivity of the transmission mode to dislocations or slight tilting of the sample the high frequency eddy current method is a practical method for thin film characterization in the industrial environment.

Identification of the source mechanism and measurement of source strength are important requirements for wider field application of the acoustic emission technique. It is difficult to relate a given source event to resulting acoustic emission waveforms in experimental results. However, it is practical to simulate such source events using numerical simulations and examine the resulting waveforms. The present paper uses such an approach to identify the patterns embedded in the waveforms and their variation with relative positions of the source and sensor. Important elements in the waveforms are shown to have strong variation with respect to the relative positions of the source and sensor. The resulting amplitude variations should be taken into account in the measurement of acoustic emission source strength. In addition, it is shown that the shear horizontal wave has a prominent component in the normal stresses in the radial direction. Acoustic emission waveforms obtained from the numerical simulations were also used to demonstrate pattern classification of these waveforms and identify the source mechanisms. The three elements of the waveforms, So, Ao, and Shear, were considered as the basic elements of the waveform. These elements have different frequency bandwidths that are directly related to the impulse duration of the incremental crack growth. Correlation coefficients between these elements and the acoustic emission waveforms were used as a means for identifying the source type.

Many optimal sensor placement methods for structural health monitoring establish performance metrics based on the detection of a limited set of damage states and locations. In guided wave-based inspection, however, monitoring is carried out over a continuous region with a continuous distribution of possible damage locations, types, sizes, and orientations. Here, traveling waves are excited and then received by a set of transducers with the intent of detecting and localizing previously unobserved scattering sources that are associated with damage. To measure sensor network performance in this application, we implement a Bayesian experimental design approach by computing the total posterior expected cost of detection over the entire monitoring region. Since the optimization usually must be carried out using a computationally expensive meta-heuristic such as a genetic algorithm, efficient modeling of the interrogation process is key to solving this distributed sensor placement problem. In this work, we implement a previously developed semi-analytical modeling approach for wave scattering within our Bayesian probabilistic framework in order to optimally place active sensors for detecting cracks of unknown location, size, and orientation. This involves assuming a set of a priori probability distributions on the three unknowns and defining spatial distributions of cost associated with type I and type II detection error. These parameters are driven by the geometry, material, in-service structural loading, and performance requirements of the structure. Through a set of sensor placement examples, we demonstrate how changes in the probability and cost distributions will dramatically alter the optimal layout of the transducer network.

Automated detection of damage due to impact in composite structures is very important for aerospace structural health monitoring (SHM) applications. Fiber Bragg grating (FBG) sensors show promise in aerospace applications since they are immune to electromagnetic interference and can support multiple sensors in a single fiber. However, since they only measure strain along the length of the fiber, a prediction scheme that can estimate loading using randomly oriented sensors is key to damage state awareness. This paper focuses on the prediction of impact loading in composite structures as a function of time using a support vector regression (SVR) approach. A time delay embedding feature extraction scheme is used since it can characterize the dynamics of the impact using the sensor signal from the FBGs. The efficiency of this approach has been demonstrated on simulated composite plates and wing structures. Training with impacts at four locations with three different energies, the constructed framework is able to predict the force-time history at an unknown impact location to within 12 percent on the composite plate and to within 10 percent on a composite wing when the impact was within the sensor network region.

Various densities of optical fibers are embedded into a total of eighty woven, graphite fiber-epoxy composite laminates, for which the response to low velocity impacts are evaluated. The goal of this work is to determine the role of hostsensor interaction on the lifetime of the host material system. The woven composites are subjected to multiple impacts at 14.5 J until perforation of the specimen. We obtain the energy dissipated by the laminate and the maximum contact force between the impactor laminate for each strike. From these experimental data we calculate the statistical distribution of the total energy dissipated at failure as a function of embedded optical fiber density. The total dissipated energy, a measure of the specimen lifetime, decreased with increasing embedded optical fiber density, however remained constant after a threshold density was reached. The total maximum contact force per specimen, a measure of the specimen stiffness, continued to decrease with the number of embedded optical fibers.

Considering the choice of sensors in optical fiber temperature sensing field, temperature measurement experiments on fiber Bragg grating, optical fiber Brillouin scattering, Raman Scattering Fiber Optic sensors which are based on three different theories were carried out, so temperature measurement results are obtained, meanwhile, the temperature characteristics these three kinds of sensors can get known by comparison between sensing principle and experimental data. This can provide a reference for application of fiber optic temperature sensor in the temperature monitoring field.

Whilst many Wireless Sensor Network (WSN) applications remain in the research domain, there is increased effort in some circles to apply the concept and related technology to industrial purposes. This study experimentally tests how low power sensor devices perform in simulated industrial scenarios in terms of communication with a particular focus on metallic environments, where radio frequency devices tend to fare badly. The study covers experimentation in a number of different physical environments, as well as with varying materials which may be found in typical industrial situations. The study also considers two popular operating frequencies for comparison: 915MHz and 2.45GHz. The aim of this study is to gauge the effect that the environment has on a low power sensor device, as this is important when considering their constrained operating parameters. In doing this it will be possible to ensure that WSN are practical for industrial deployment and potentially suggest ways in which improvements could be made.

As a new and sensitive sensing element, OFBG(Optical Fiber Bragg Grating) has been widely used in aerospace engineering and civil engineering. The sensing mechanism and properties have been widely studied by lots of researchers, but the sensing properties of OFBG under large negative strain are still destitute. In this paper, with the aids of large shrinkage performance of PP(polypropylene) during its curing, we gained about -13000 με's strain changes by embeding bare OFBG inside the PP bar to study the sensing properties of OFBG in this strain level. The results show that OFBG can remain its sensing properties well---- linearity, repeatability and the shape of centre wavelength are both reasonably. And the strain sensitivity coefficient of PP-OFBG is about 0.85 pm/με, this is very near with that of calculating results considering strain transmission between PP and OFBG. Which are all helpful and useful for further use of OFBG in other applications.

The traditional wired structural monitoring systems often suffer of various problems mainly related to the cabling which limits their applicability. Then wireless monitoring system is needed. Most of the current wireless SHM systems are mainly based on single-hop, and the network can only support a small number of nodes. For the realization of multi-hop, low complexity and low power requirements, we introduce a system based on ZigBee protocol built on the IEEE802.15.4, then design and implement the corresponding hardware and software of wireless sensor. The desired features are validated by experiments, including relatively high network capacity, low power consumption, and moderate data rate.

Now a day, cable plays a more and more important role in civil engineering. As an effective construction member, cable is used in many long-span spatial structures. The cable tension measurement is required in the construction control, assessment and long-term monitoring of cable-supported structures. Mostly, the detection uses the Fourier Transform to get the frequencies of the cable, and then applies the vibration-based cable tension theory to evaluate the cable tension. As a conventional method for cable tension measurement, the Fourier Transform can only be used in the static cable tension force test, but not dynamic cable tension test. The cable dynamic tension describes the load-deformation behavior of cables subjected to dynamic loading. It represents the intrinsic dynamic properties of cables. In order to get the dynamic cable force, time-frequency analysis must be done. In this paper, wavelet transform tool is used to analyze the signal, and obtain the cable tension dynamic change along with the time.

In this paper, a distributed structural damage detection approach is proposed for large size structures under limited input and output measurements. A large size structure is decomposed into small size substructures based on its finite element formulation. Interaction effect between adjacent substructures is considered as 'additional unknown inputs' to each substructure. By sequentially utilizing the extended Kalman estimator for the extended state vector and the least squares estimation for the unmeasured inputs, the approach can not only estimate the 'additional unknown inputs' based on their formulations but also identify structural dynamic parameters, such as the stiffness and damping of each substructure. Local structural damage in the large size structure can be detected by tracking the changes in the identified values of structural dynamic parameters at element level, e.g., the degrading of stiffness parameters. Numerical example of detecting structural local damages in a large-size plane truss bridge illustrates the efficiency of the proposed approach. A new smart wireless sensor network is developed by the authors to combine with the proposed approach for autonomous structural damage detection of large size structures. The distributed structural damage detection approach can be embedded into the smart wireless sensor network based on its two-level cluster-tree topology architecture and the distributed computation capacity of each cluster head.