Future avionics data links will require active couplers to achieve smart processing. This effort reports progress toward achieving smart processing in composite-embedded avionics data links. The specific objective is to develop a symmetric evanescent coupler which achieves electronic switching by means of a poled electro-optic polymer.

Applications of polymeric adhesives in joining different materials have necessitated quantitative health inspection of adhesive joints (coverage, state of cure, adhesive strength, location of voids, etc.). A new in-situ sensory method has been proposed in this paper to inspect the amount and distribution of the critical constituents of polymers and to measure the characteristic parameters (complex Young's modulus and damping). In this technique, ferromagnetic particles have been embedded in a polymeric matrix, similar to a particle- reinforced composite. The dynamic signatures extracted from the tests as a result of magnetic excitation of the embedded ferromagnetic particles are used to evaluate the complex Young's modulus of the host polymers. Moreover, the amplitude of the frequency response is utilized to identify the amount and distribution of embedded particles in polymeric materials or adhesive joints. The results predicted from the theoretical model agree well with the experimental results. The theoretical analyses and the experimental work conducted have demonstrated the utility of the sensory technique presented for in-service health interrogation.

One definition of a smart structure is a structure with a miniaturized control system embedded in or attached to the material for improved performance of the structure. The feasibility of embedding piezoelectric sensors and actuators has been demonstrated. Current research is focusing on the development of the support electronics for these sensors and actuators. In this paper we discuss miniaturized power conversion/amplifier electronics for these actuators suitable for these smart structure applications. State-of-the-art dc to dc power converters with a 2 square inch footprint and a 1/4 in profile with power densities up to 50 W/in³ with efficiencies above 80% are described. The power converters to minimize thermal dissipation. The power bus requirements are also discussed.

A new fiber optic magnetostrictive phase modulator/magnetometer utilizing the highly magnetostrictive rare-earth iron compound Tb_xDy_1-xFe₂ is demonstrated to have a high sensitivity, dynamic range, minimum field detectability, and a better structural suitability than metallic glasses. The experimental linear phase shifting coefficient was 1.102 rad/Vp-m with the corresponding phase shift normalized to the magnetic field of 0.306 rad/G- m. A minimum detectable phase shift of better than 1 (mu) rad was obtained, which corresponds to 2 (mu) G/m for a 4 Hz bandwidth and 1 (mu) G/Hz^1/2 per meter of fiber interaction length. The projected minimum detectable magnetic field for 30 meters is thus seen to be 3 pT/Hz^1/2.

In the development of gas turbine aeroengines, the high cost of development testing and market lead pressure to reduce program timescales has led to increasing use of advanced instrumentation. A growing interest has been the integration of sensors into the engine components, both to look outward at the behavior of the flow over the component, and to look inward at the temperature and strain of the component itself. This paper describes a range of pieso-resistive and thermal sensor applications developed by Rolls-Royce concluding with a view of this industry's future needs and sensor design challenges.

Optical fiber sensor elements were embedded in ceramic matrix composite (CMC) specimens fabricated at the Oak Ridge National Laboratory using a rapid chemical vapor infiltration (CVI) process. The silica and sapphire optical fiber sensors were placed between adjacent layers of interwoven Nicalon^R fibers during the stacking of a preform. This preform was then coated with pyrolytic carbon, used as an interface layer, and then densified with additional silicon carbide through CVI. This paper discusses the survivability of both the silica and sapphire optical fiber sensor elements, and suggests the possibility of using embedded optical fiber sensor elements inside high temperature composites for both fabrication monitoring and subsequent in-service lifetime monitoring at high temperatures.

Optical fibers were embedded into graphite-epoxy polymer matrix composites using a variety of geometries and insertion techniques to evaluate different manufacturing methods for embedding sensors in composite materials. Fiber durability during processing and post- processing handling was examined. In general, it was found that the fibers survived the processing step, even when efforts were made to induce failure. Handling during the demolding and trimming operations did cause fiber fracture. Protection techniques designed to improve the fiber survival rates were developed. These techniques were demonstrated to improve the survival rate. Both edge and surface exit embeddment techniques were developed.

We describe the development and testing of intrinsic and extrinsic sapphire fiber-based sensor designs intended for use in high temperature environments. The first is a sapphire-fiber-based intrinsic interferometric sensor. In this sensor, a length of uncoated, unclad, structural-grade multimode sapphire fiber is fusion spliced to a singlemode silica fiber to form a Fabry-Perot cavity. The reflections from the silica-to-sapphire fiber splice and the free endface of the sapphire fiber give rise to the interfering fringe output. This sensor has been demonstrated for the measurement of temperature above 1510 degree(s)C, and a resolution of 0.1 degree(s)C has been obtained. The second is an extrinsic sapphire fiber-based Fabry-Perot interferometer. Based on the intrinsic sapphire fiber sensor, another sapphire fiber is placed end-to-end to the first sapphire fiber and the two adjacent endfaces of the two sapphire fibers form a Fabry- Perot cavity. This sensor has been shown to be useful for strain and temperature measurement.

This paper describes the detailed design of an acoustic emission-based micromachined sensor for smart structures applications to assess in-flight structural integrity. This unique sensor approach combines silicon sensor micromachining, conformal optical waveguide technology, and high-speed optical interconnects to measure acoustic emission event data related to structural failure. A micron-level dual-element micromachined cantilever beam architecture design is described which incorporates conformal on-chip polyimide optical waveguide interconnects to optically sense structural integrity acoustic parameters and route optical power to the sensor assembly. High-speed optical interconnects are used to transfer sensor data to avionic interfaces for debrief and detailed analysis. A detailed design of the optical waveguide sensor design and the significant payoffs and benefits of these sensors are discussed.

This paper examines the detection of laser generated surface acoustic waves (SAWs) by fiber optic sensors. A Nd:YAG laser is used to illuminate the surface of an aluminum sample in various ways. Elastic waves are launched into the bulk and along the surface of the sample. The emphasis of the work presented is the performance of the fiber optic sensor as compared to theoretical predictions and that of a conventional sensor. By such analysis, the efficiency and limits of the fiber optic sensors for the interrogation of surface acoustic waves are investigated.

A surface acoustic wave (SAW) sensor based on an extrinsic Fabry-Perot interferometer is described. A single-mode fiber, used as the input/output fiber, and a multimode fiber, used solely as a reflector, form an air-gap that acts as a low-finesse Fabry-Perot cavity. The Fresnel reference reflection from the glass/air interface at the front of the air-gap interferes with the sensing reflection from the air/glass interface at the far end of the air-gap. Strains in the silica tube housing the two fibers change the air-gap length, thereby altering the phase difference between the reference and sensing reflections. A theoretical analysis of the interaction between the strain induced by elastic stress wave fields and the fiber sensor is presented. A dual optical wavelength stabilization technique is used to minimize signal drifts. Signal to noise ratios on the order of 39 dB are obtained with a sensitivity of 4 degree(s)/microsecond(s) train cm-1 for strain measurements. The sensor was also attached to a steel cantilever beam and submerged in liquid nitrogen. The sensor was used to measure strain at liquid nitrogen temperatures with typical errors of less than 10%.

Optical fibers are attractive candidates for sensing applications in near-term smart materials and structures, due to their inherent immunity to electromagnetic interference and ground loops, their capability for distributed and multiplexed operation, and their high sensitivity and dynamic range. These same attributes also render optical fibers attractive for avionics busses for fly-by-light systems in advanced aircraft. The integration of such optical fibers with metal and composite aircraft and aerospace materials, however, remains a limiting factor in their successful use in such applications. This paper first details methods for the practical integration of optical fiber waveguides and cable assemblies onto and into materials and structures. Physical properties of the optical fiber and coatings which affect the survivability of the fiber are then considered. Mechanisms for the transfer of the strain from matrix to fiber for sensor and data bus fibers integrated with composite structural elements are evaluated for their influence on fiber survivability, in applications where strain or impact is imparted to the assembly.

A major opportunity for smart structures research is the information processing challenge. Massive amounts of sensor information are required to monitor the health and integrity of any system, mechanical or biological. A prohibitively large computer is needed, if conventional methods are used to monitor a large number of sensors. We have addressed this complex problem by developing a neural network/expert system (NN/ES) hybrid. The NN portion is used for data acquisition and sensor processing. The ES portion monitors the results of the NN to determine if there are anomalies. Anomaly detection feedback is provided to the NN. This unique feedback situation provides our smart structure system with the capability to identify damage which it has never been trained with and which it has never seen. The NN/ES health monitoring capability has been used to detect both temporary and permanent damage introduced into a portable table-top test article. In addition to damage detection and health monitoring, the NN/ES can also provide simulated estimates of maintenance schedules and performance envelopes.

A process which uses electroplating methods has been developed to fabricate metal coated optical fiber sensors. The elastic-plastic characteristics of the metal coatings have been exploited to develop a sensor capable of `remembering' low velocity impact damage. These sensors have been investigated under uniaxial tension testing of unembedded sensors and under low velocity impact of graphite/epoxy specimens with embedded sensors using both Michelson and polarimetric optical arrangements. The tests show that coating properties alter the optical fiber sensor performance and that the permanent deformation in the coating can be used to monitor composite delamination/impact damage.

The increasing complexity of large space structures further emphasizes the need for multi-use single element spacecraft components and sensors. Sensing of the vibrations of components and/or sections of a large space structure is necessary for ultra-high precision pointing and tracking as well as general platform stability. At the same time, the quest for a method of detecting spacecraft impacts with micrometeorites or debris continues. In this paper a method for using a single optical fiber for simultaneous impact detection and vibration sensing is described. Laboratory studies of these techniques using composite material panels are presented.

The ability of the extrinsic Fabry-Perot interferometric strain sensor (EFPI-SS) to detect the onset of damage in composite laminates was investigated. Composite specimens with EFPI-SS embedded near a local stress concentrator (0.25 in. diameter hole) were subjected to quasi- static and constant amplitude cyclic loading. The test results showed that the particular EFPI- SSs used in this study were inadequate for structural health monitoring purposes. Many sensors did not survive the specimen fabrication process, and the surviving sensors degraded at an applied gross stress of approximately 30 ksi. The degradation seems to be due to the failure of the adhesive bond between the single-mode optical fiber lead and silica glass tube. Nevertheless, the data shows that improved EFPI-SSs can be used to monitor local strain changes in composites during service usage.

The use of fiber-optic strain gauges in realistic smart structures applications is contingent on the availability of a number of sensors embedded or adhered to the structure at a cost comparable to that of existing on-board diagnostic sensor systems. The most effective way of implementing such a sensor topology is to share the most expensive components of the system among many sensors. A cost-intensive component in a fiber-optic strain gauge system is the laser diode and its associated drive electronics. Parallel multiplexing allows many gauges to share a common source. Design and performance issues related to parallel multiplexed fiber Fabry-Perot strain gauge systems are discussed. A commercial four-channel parallel multiplexed strain gauge system is described.

A system has been developed to linearize the output of most interferometric optical sensors from dc to high frequency. The system employs a broad band light source with an unbalanced Mach-Zehnder interferometer to cancel phase changes induced by the sensor. An analog circuit is employed to produce the phase tracking signal which is linearly proportional to sensor phase. The system has demonstrated a phase resolution of 1.92 degree(s) when used to track an extrinsic Fabry-Perot strain sensor. This corresponds to a strain resolution of 0.35 (mu) (epsilon) . Frequency response is estimated to be greater than 100 kHz for the current system and above 100 MHz for an integrated optic embodiment. In addition, the system is insensitive to polarization and intensity changes induced by source drift or lead fiber perturbations.

A multiplexed configuration of the fiber optic extrinsic Fabry-Perot interferometer is demonstrated by using two sensor heads on a single directional coupler in a split-cavity cross- coupled extrinsic fiber interferometric (SCEFI) arrangement. The provision of four-beam interference along with a spectrum analysis detection scheme devised for the SCEFI eliminates the need for quadrature phase biasing and leads to independent measurements of the phase shift in the two sensor heads. Good agreement between the experimental results and the model for interference is demonstrated.

The `smartness' of the optical position sensitive detector (PSD) lies in its capability to read the centroid position of the incident optical intensity distribution, independent of total intensity. Most measurands of interest to the smart structures community can be made to appear as changes in spatial distributions of optical intensity, such as interference pattern shifts, motion of reflected spots from moving or deforming objects, or changes in relative intensity among two or more light sources. In this work we demonstrate a technique to make a PSD smarter, in this case capable of reading centroid positions of multiple light intensity distributions which are modulated at different frequencies. This capability can be exploited in multi-degree-of- freedom position sensors as well as for multiplexing outputs from sensors distributed over a smart structure or skin. In this novel operating mode, the PSD behaves as a transmission line with detectors at either end and a current source at the centroid. Measurements of phase difference versus spot position for different modulation frequencies show good agreement with a superposition-based numerical model. Multiple light distributions incident on the same PSD can be frequency multiplexed. Spatial resolution increases with modulation frequency for positions near the center of the working range.

Single-ended optical fiber sensors, for which the same fiber length serves as lead-in and lead- out paths of the optical signal, offer many practical advantages over dual-ended sensors (particularly in their installation and interconnection). However, and this is especially relevant to optical fiber strain gauges, high interfacial stresses can develop at the tip of the sensor when the host material is under load. This can result in debonding of the sensor and, consequently, the loss of the sensor's function. The tip region can also be the site of initial crack formation in the host material. Using a shear-lag model and FEM analysis, we show how the use of coatings with appropriate properties can reduce or eliminate these problems. This analysis brings to attention important properties of the glass/coating and coating/resin interfaces that need to be determined from experiments. The conclusions from this analysis are discussed in the context of the broader issue of sensor/host interaction and sensor life-performance.

Understanding elastic interactions that occur between an embedded optical fiber and its host matrix is important for implementation of optical strain sensors into aerospace structures. A previously developed analytical model predicted the effects of fiber coating thickness and elastic modulus on the strain transfer from an isotropic matrix to an embedded coated fiber optic. To verify this, a series of tungsten coatings were magnetron sputtered onto bare intrinsic Fabry-Perot sensors that were embedded into a neat epoxy resin block. The block was then compression loaded in the transverse direction to the fiber, and the optical strain response was measured. Comparisons between the responses of both coated and uncoated sensors indicated that the experimental measurements of strain transfer in embedded fiber-optic devices yielded good agreement with the analytical model.

Surface mounted optical fiber strain sensors have been extensively used in smart structure sensing and actuator applications in the recent past. The performance of such sensors depends strongly on the conditions of bonding. Optimum static strain sensitivity of an extrinsic fiber optic interferometric strain gauge surface mounted on a cantilever beam was obtained by comparing several bonding techniques to each other, by comparing the outputs of the fiber sensors and resistance foil gauges, and by comparing the strain calculated from applied end displacements and elastic beam theory. The extrinsic fiber interferometer was formed between the ends of a single mode fiber (SMF) and a multimode fiber (MMF) inserted into a hollow core fiber (HCF) from opposite ends. Resistance foil and optical interferometric measurements were consistently lower than theoretical values. The closest agreement with theoretical values was realized by a configuration in which the SMF end was free to move within the HCF. For a sensor completely embedded in epoxy, optical strain measurements were 12.2% below theoretical values while the foil gauge readout was 18.1% lower.

Smart structure technology could revolutionize engineering by the 21st Century. Improvements in safety, reductions in cost, and enhancement of the environment are some of the expected benefits of this technology. A major impediment to the implementation of this technology in practical structures is going to be extracting the optical information from the structurally integrated sensing system. The answer to this problem lies in: demodulating, processing, and multiplexing the raw optical data from an array of sensors and transmitting these data from the structure in a single channel by means of an unobtrusive optoelectronic interface that is part of the structure. The single output channel from this optoelectronic smart structure interface could take several user-friendly forms, the most versatile involves free- space propagation and leads to the `optical synapse' concept. We have demonstrated that intracore Bragg grating laser sensors could form the basis of an OSSI and have shown that optical multiplexing is possible in the case of Bragg grating fiber laser sensors.

The desire to counteract impact loads on laminated structures in order to reduce ply damage has recently led to research using smart materials in laminated composites. The ability to sense the onset of these impact loads using piezoelectric material is the first step in the damage control process. This research focuses on smart laminated plates under impact and the electric field created by piezoelectric patches in the structure. A modified version of an existing finite element program is used to numerically simulate a laminated plate under a low velocity impact. Linear constitutive relations are used in the program which calculate the electric field generated by the in-plane strains on the piezoelectric plies. Several studies are made comparing the electric field generated by patches of piezoelectric material of varying size, shape, and location within the plate. The average electric field generated by larger patches which contain more oscillations of electric field filters the high frequency components. The distance between the impacting point and the piezoelectric sensor is found to be directly related to the lag time between the impact and generation of an electric field.

Axial strain may be determined by monitoring the phase shift of a single mode optical fiber. Whenever an optical fiber is under stress, the optical path length, the index of refraction, and the propagation constants of each fiber mode change. In consequence, the phase term, (beta) (Zeta) , is also modified. In this paper, a relationship for the phase shift, due to axial strain, of the hybrid modes HE_1n and EH_1n is presented using the exact field solution. This relation is applied to single mode fibers in order to determine phase sensitivity characteristics.

Smart structures technology is being increasingly applied to civil structure applications. In particular, development of health monitoring for bridge structures is of considerable importance. In order to explore the possibility of developing such a system, an investigation was carried out on a scale model steel bridge element using an attached sensor system consisting of two point sensors (piezoelectric accelerometers) and one integrating sensor (fiber optic modal sensor). The model element was selectively configured to produce the equivalent of a number of damage conditions. For each condition, it was physically perturbed. The sensor outputs were then used as inputs to a neural net which then provided an estimate of structural damage. A reasonable correlation between net output and actual damage indicated that this type of health monitoring system offers potential for practical application on full scale bridge structures.

An array of optical fibers can be applied to smart skin sensing applications, which include stress monitoring, fatigue detection, temperature sensing, etc. Although a single light source can be used in a fiber array sensor, the input light power is divided by a sensing fiber array and each fiber may require a detector. Thus as the channel number of a fiber array increases, the number of detectors also increases and the signal intensity of each fiber channel decreases, which decreases the signal-to-noise ratio and increases the complexity and the cost of the system. In this paper, we analyze the Hadamard transform to enhance the signal processing of the fiber array sensor. The apparent advantages are: only one detector is needed and the signal-to-noise ratio of the fiber array sensor can be improved by a factor of (root)N/2, where N is the number of sensing fibers. Analysis for an array of 127 sensing fibers is illustrated, in which we have shown that the calculated results are in agreement with the analytical model.

An intracore Bragg grating and a broadband reflector are used as cavity end mirrors of a tunable Erbium-doped fiber laser where the Bragg grating serves the dual purpose of the tuning element and the sensor. This fiber laser sensor (FLS) is integrated with a wavelength demodulation system (WDS) to form a practical strain and temperature sensor. Laser tuning with both temperature and strain have been achieved and the data is presented. To further promote practical applications of the FLS, a two element fiber laser sensor array has also been designed and developed. Preliminary data indicates excellent stability and no cross-talk.

This paper describes two optical fiber waveguide modification methods which allow the synthesis of effective modal filters and sensors for the analysis and active feedback control of vibrating structures. Both methods allow the sensitivity of the fiber to be varied along its length when used as a long gauge length integrating sensor. The performance of these sensors is similar to that of shaped piezoelectric film elements which have been demonstrated for several years for similar applications as modal filters, except the fiber sensors allow low- profile geometries in both dimensions orthogonal to the fiber axis. The first method uses modal birefringence between the LP₀₁ and LP₁₁ modes in two-mode fibers modified by tapering the waveguide during or subsequent to fabrication, with the sensitivity of a differential section of fiber along its length determined by the local core radius. By adjusting the taper geometry as a function of position along the fiber, individual vibration modes in a structure may be measured, with information concerning other orthogonal modes suppressed. The second method uses spatially weighted photorefractive index gratings in similar fibers to achieve modal selectivity. For both cases, we describe new methods for generating tapers and two-mode gratings, and present theoretical data and experimental results for such sensors embedded in composite laminates.

A highly accurate linear method of static phase shift measurement based on the concurrent utilization of signal fading, a tan^-1 algorithm and photovoltage spectrum analysis in a fiber optic interferometric strain sensor is presented. The static phase shift is superimposed on a controlled dynamic phase modulation derived from an optical phase modulator in one arm of the interferometer. Good agreement is demonstrated between the phase shift obtained through experiment and that from the theoretical analysis based on 1/f noise. This detection scheme has wide applications to static and dynamic phase shift measurements in fiber optic interferometric sensors used in several smart structure configurations.

We have developed a mathematical model of a technique in which a pyramidal arrangement of wavelength-selective optical detectors can be used to determine the identity (wavelength), intensity, and direction of arrival of laser irradiation. The advantages of this technique are that only unobtrusive, skin-like structures are required and that the large collection areas provide high sensitivity. The disadvantage is that the angular resolution (approximately 5 degrees in a 60 degree field of view) is less than that which can be achieved using methods requiring thicker structures. The detector elements are large-area, polyvinylidine fluoride, pyroelectric devices with wavelength selective coatings. We situated four identical arrays of these elements on the top and sides of a frustrated, three-sided prism. Because only the relative orientations are significant, we are able to use selected regions on the surface of an existing structure in an application of this work. Our efforts included developing the mathematical description of the system and, using coated detectors whose performance was experimentally verified, modeling the response to in-channel, monochromatic radiation. Although we limited our model to a simple, two-channel system, the concept and algorithm can easily be extended to a system of any reasonable number of nonoverlapping channels.

A reconfigurable antenna whose size, shape, and polarization can be dynamically modified is a desirable component for today's communication and surveillance systems. To date most proposed reconfigurable antenna concepts have used a matrix of conducting elements connected by electronic/optoelectronic switches with the on-off pattern of the switches defining the size or shape of the antenna. An alternative approach for realizing this concept is to exploit the photoconductive effect in various semiconductor materials. This paper reviews the general theory of photoconductivity in semiconductors emphasizing the parameters relevant to this application. The experimental arrangement and the initial results for our proof- of-concept demonstration are then presented. In this experiment a bow-tie shaped transient carrier distribution was created in a 4 inch diameter, high resistivity (6 - 8 k(Omega) -cm) Si wafer through illumination with a pulsed Nd:Glass laser. The voltage amplitudes measured by the transient Si bow-ties (three different wafers) were 45 - 50% of those received in the same environment by an equivalent copper bow-tie antenna designed to operate from 1.4 - 2.1 GHz. Analysis of this initial data indicates that an optimized design incorporating improved semiconductor processing techniques, an optimized feed/balun, soldered interconnects, and impedance matching would significantly improve performance.

A preliminary investigation was conducted to experimentally examine stress relaxation in optical fiber packs wound under tension. In the experiment, two high strength fibers, comprised of different buffer coating materials, were each wound under two different winding tension distribution scenarios to study the effects of both fiber winding tension and buffer coating properties on the mechanical stability in fiber packs during storage and under rapid deployment conditions. A strain sensing optical time domain reflectometer (SSOTDR) was used to measure the time-dependent strain changes in test fibers that were coiled on thermally compensated spools. The strain measurements obtained during the winding process and during temperature cycling are presented and discussed in this paper. Description of the SSOTDR, the test methodology, and an analytical fiber pack mechanics model is provided. Model predictions are used in concert with data discussion to supply theoretical versus experimental consideration.

A parametric design model has been created to optimize the sensitivity of the sensing cable in a distributed sensing system. The system consists of electrical time domain reflectometry (ETDR) signal processing equipment and specially designed sensing cables. The ETDR equipment sends a high-frequency electric pulse (in the giga hertz range) along the sensing cable. Some portion of the electric pulse will be reflected back to the ETDR equipment as a result of the variation of the cable impedance. The electric impedance variation in the sensing cable can be related to its mechanical deformation, such as cable elongation (change in the resistance), shear deformation (change in the capacitance), corrosion of the cable or the materials around the cable (change in inductance and capacitance), etc. The time delay, amplitude, and shape of the reflected pulse provides the means to locate, determine the magnitude, and indicate the nature of the change in the electrical impedance, which is then related to the distributed structural deformation. The sensing cables are an essential part of the health-monitoring system. By using the parametric design model, the optimum cable parameters can be determined for specific deformation. Proof-of-concept experiments also are presented in the paper to demonstrate the utility of an electrical TDR system in distributed sensing applications.

A non-intrusive low-profile strain monitoring system using an optical fiber time domain (OFTD) sensor has been developed. The OFTD monitoring system is used to determine the strains of specimens during static tension and fatigue tests. Quasi-isotropic E-Glass/epoxy composite specimens with and without embedded optical time domain fiber sensors were fabricated. The optical fiber (OF) sensors were placed in the mid surface of the composite laminate. Each specimen of the embedded polyimide-coated OF sensor consisted of a 100/140 micrometers core/cladding diameter fiber, which was segmented with two reflective sites. Each reflective site consisted of a physical interruption of the optical fiber achieved by cleaving and realigning the fiber. These reflective sites were spaced 30 cm from each other, defining the length over which strain was measured. The dimensions of the reflector are 1.0 cm length and 1.0 mm diameter. Stainless steel tubes 1.0 mm in diameter were fitted over the fibers at the area where they exited the laminate to connect with the optical processor. These tubes protected the fiber optic (FO) from breaking during fabrication and prevented gripping pressure during testing.

An experimental study was conducted to determine the utility of in-line optical fiber-based extrinsic Fabry-Perot interferometers (EFPIs), in a fatigue environment typical of aircraft structures. Metallic and composite coupons with EFPIs attached to and embedded within were tested in constant amplitude cyclic fatigue at room temperature. An additional composite coupon was tested similarly at an elevated temperature. For the consideration of composite material applications the objectives were to determine the durability of the sensor and its ability to measure strains accurately, even when the EFPI sensor was embedded at an angle with respect to the principal adjacent reinforcing fibers of the composite. For metals, in addition to durability considerations, research was conducted as to how the EFPI sensor may be used to detect crack initiation and growth. The results of the test program have established the excellent durability of the EFPI sensor element for fatigue loading up to 50,000 cycles at R equals 0.1 (tension-tension fatigue) with a maximum strain level of 3,500 microinch/inch, for both attached and embedded sensors, once the optical fiber and sensor survived the composite laminate panel curing process.

This paper presents an experimental technique of magnetically exciting embedded ferromagnetic particles in epoxy to facilitate active inspection and interrogation of bond integrity. The proposed concepts involve biasing an alternating magnetic field and optimizing the activating location to maximize and linearize the exciting force on the particles. The methods are analytically and experimentally proven to be effective in amplifying the force and reducing the harmonic distortion resulting from the nonlinearity of the system. The effects of material properties and the geometry of the embedded particles on the activation force are also extensively studied. The particle's geometry is found to affect the activation significantly. The active tagging technique is thus pushed one more step forward on the way to practical application.

During the past number of years, research has been underway in Vermont investigating development and implementation of smart civil structures. During the course of this effort, numerous fiber optic and conventional sensor techniques and designs have been developed and tested. Laboratory studies have led to field testing at numerous sites and types of installations ranging from residential homes, conventional multi-story buildings, pedestrian footbridges, interstate highway road surfaces and railway bridges to high performance structures such as a hydroelectric dam. A description of the efforts in sensor design as well as multiplexing and multi-use sensors and systems with particular emphasis on smart civil structures applications is given.

Experimental results are presented on the embedding of optical fiber cables into cement specimens in order to determine the feasibility and limitations of using said fibers as sensors for the measurement of internal stresses as well as the evaluation of structural integrity. Pull- out tests have revealed that the surface bonding between the plastic jacket of a fiber cable and the cement matrix is poor and inadequate for an effective load transfer. Experiments using loaded cement specimens with embedded fiber cables inside suggest that the various protective layers present in the cable's construction prevent the fibers from properly sensing any external perturbations up to the specimen's failure and, in some instances, even after failure. Therefore, use of optical fiber cables as direct sensing elements for stress/strain measurements is not recommended for most applications, due to the lack of an appropriate load transfer mechanism and sensitivity. However, they can instead be used as leads to communicate the actual embedded sensors with the outside world.

This paper reports the use of short gauge length optical fiber sensors for the quantitative measurement of strain in reinforced concrete and the direct comparison between the performance of these sensors and co-located foil strain gauges. Pairs of fiber sensor elements and reference foil gauges were mounted to specific rebar elements which together form the three-dimensional reinforcement cage in a cement cross-beam specimen. The geometry of the specimen sections, approximately 20 cm square and 2.5 meters along each axis, is intended to model typical construction of post and beam-reinforced concrete structures. The results of this work are that (1) the robust fiber sensors survived the embedding process, (2) their calibrated output signals accurately match with those of co-located reference foil strain gauges, (3) they may be used to extract quantitative strain information from a complicated reinforced concrete specimen, and (4) they may be used to monitor crack progression.

Installation of fiber optic sensors into large civil structures entails either embedding or attaching the sensors to some structural member. While we have performed sensor installations on numerous types and sizes of civil structures, this process is frequently quite labor- and time-intensive -- both of which can potentially conflict with construction site practices. In instances where it is desirable to install many sensors in close proximity, it may prove very beneficial to prefabricate panels of sensors and then install the panel into the structure. However, issues such as the effect of the panels on the structural integrity of the building may arise. In this paper we discuss this matter as well as the panel interconnect and sensor interrogation issues. Examples of prefabricated sensor panels designed for installation into a new wing of the Medical Center Hospital of Vermont are then presented.

One of the important tasks in sensor design is the development of a model for a sensing phenomena. Artificial neural networks are ideal for such a task because of their capability for representation of the mapping functions describing the processes and phenomena which are mathematically difficult or even intractable. We examined a radial basis function (RBF) neural network for modeling of acoustical properties of colloidal TiO₂ slurry. The colloidal slurry is a very complex multiphase medium. The RBF network with a set of local Gaussian functions was trained using the data from the earlier developed physical model of TiO₂ slurry. Next the TiO₂ neural model was used for a prediction of the TiO₂ particle size distribution. The resulting prediction accuracies of the RBF network were 99.8% for the data used in the training process and 88% for the data not used in the training. Compared to other available techniques neural networks can offer an effective and time efficient approach for the modeling of complex materials.

An integrated phase modulator/signal multiplexer has been demonstrated that can be used to demodulate a fiber optic interferometer's output. The phase demodulator/optical switches are monolithically integrated with an active 1 X 10 photonics polarization preserving coupler. This device was fabricated with rib waveguides and etched-facet turning mirrors. Active multiplexing and demultiplexing of a fiberoptic interferometer has also been demonstrated using the active photonics chip. The active photonics demodulation/multiplexing system offers the advantage of nearly lossless splitting and functionality in an extremely small package size.

A commercial instrument for distributed temperature measurements has been used with germanium doped silica optical fiber to investigate the technique and test the accuracy and precision of the technique. Slightly more than 1.2 km of fiber was used, 25 to 35 meters of which could be uniformly heated to approximately 550 degree(s)C in an oven. The ratio of the anti-Stokes to Stokes intensities was used to determine the temperature along the fiber length.

Research into the development of smart structures and materials has begun to coalesce from a number of subfields into a distinct discipline of its own. A common philosophical framework for the field has yet to be achieved, however, due to the interdisciplinary nature of the activities involved. In this paper, an exploration of such a framework is carried out and related to the development of instrumentation architecture development for future smart structures.