The smart structures concept, in which the structure is instrumented with a network of sensors and computers to monitor the flight loads environment and structural integrity, and initiate corrective actions, will eventually replace the current individual aircraft tracking system. Herein, two smart structures concepts are discussed in detail and the required research and development indicated.

Future United States Air Force (USAF) and National Air and Space Administration (NASA) space systems such as Space Based Radar (SBR), Space Based Laser (SBL), and Space Station will incorporate smart structures/skins as part of their active vibration control system to sense, evaluate, and damp out any natural and spurious vibrations and health monitoring system to sense any degradation to the structure. The concept called smart structures/skins is identified as one of the Project Forecast II technology areas. A smart structure/skin is defined as the embedment of sensors, actuators, and microprocessors in the material which forms the structure. One particular sensor that is being studied in depth is fiber optic. Fiber optics are lightweight, immune to electromagnetic interference, and are easily embeddable into composite material. The Astronautics Laboratory (AL) is committed to develop and incorporate this technology into future space systems. This paper describes the current and future activities, both in-house and contractual at AL, in the area of smart structures/skins with emphasis placed on the role of fiber optics. Items to be discussed include, types of sensor and actuator systems, areas that future research and development need to address, and plans to incorporate smart structures/skins into future space systems.

The Air Force Project Forecast II identified a number of key technology initiatives for development. This paper addresses one such initiative, PT-16, Smart Skins. The concept of the Smart Skin is introduced by briefly highlighting its attributes and potential advantages over standard avionics packaging and maintenance, and then goes on to describe some of the key ingredients necessary for its development. Problem areas are brought out along with some of the required trades that must be made. Finally, a time phased development roadmap is introduced which shows Calspan's proposed sequence of technology development programs that can, in combination, lead to first functional Smart Skins implementations in narrowband form in the late 1990's and in wideband form in first decade of the twenty - first century. A Smart Skins implementation in integral aircraft skin structure form will take at least until 2010.

The original objectives of the Smart Skins technology program are reviewed, illustrating the proposed evolution of phased array antennas over the next three decades. In describing this program, particular emphasis is given to antenna performance capabilities, RF feeding, and beamforming. Various beamforming techniques employing phase shifters and optical fiber true time delays are examined. Other aspects of Smart Skin arrays to which optical technologies may apply are reviewed and current technical limitations are addressed.

The potential use of optical fiber sensors embedded, prior to curing, in reinforced concrete buildings and in structures such as bridges, dams and tanks is discussed with regards to the non-destructive measurement of internal strain, and the evaluation of structural integrity. Novel applications in the areas of structural monitoring, experimental stress analysis and in the management and control of service installations are presented. A discussion of the fundamental issues regarding the practical implementation of this technology is given.

Over the last few years, a significant amount of work has been conducted on developing fiber-optic sensors that can be used to monitor the performance of structures". At times, the development of sensors seems to have been done without consideration of the specific types of structures for which they are being developed. Typical aerospace structures are composed of strut, beam, plate, and shell components, which can be made of many different kinds of materials. Depending on the types of components used in a structure, the materials used, the expected operating conditions, and expected in-service damage, the requirements for the sensors can vary drastically.

Military aircraft design has been in a state of evolutionary development since its invention. After the first usage of aircraft for military applications in World War I, the military has been seeking ways of improving mission effectiveness to maintain military superiority. The military has been constantly searching for new methods and technologies to accomplish this goal. In the early days of aviation, there were clear distinctions and divisions between aircraft system functions. Although they all served to support the flight of the aircraft, they operated independently of each other. With the modern trend of developing a high degree of integration between the various aircraft systems, requirements are emerging for a group of new technologies to support this trend. One such group of emerging technologies is the combination of photonics, integrated optics and fiber optics. The integrated approach to the development of the avionics, non-avionics and airframe of the aircraft is part of the approach to improving aircraft mission effectiveness through enhanced mission function performance and associated susceptibility (reliability, availability, survivability). This paper will discuss the general evolution of aircraft and, in particular, the role of photonics, integrated optics and fiber optics on the avionics, non avionics and airframe of past and future aircraft. Included will be the relationship of these emerging optical technologies to the military programs involving smart structures and smart skins.

Structurally integrated arrays of fiber optic sensors could serve as an effective nervous system for future Smart Structures. The structural integrity of such structures would be monitored throughout their life making obsolete the catastropic failures that sometimes plague aircraft, trains, cars......today. In addition the strain, deformation, vibration and temperature state of these structures could also be monitored. Our research program is directed at both the development and application of this new technology. We have built and carefully characterized a localized, all-fiber, dual wavelength polarimetric fiber optic sensor. We have also developed a localized, all-fiber, Michelson fiber optic sensor that has measured the strain within a thermoplastic and detected the acoustic emission associated with delamination within a composite. It has also been used as the basis of an optical strain rosette . We have demonstrated that embedded optical fibers do not reduce the strength or damage resistance of composites but can detect load-induced growth of damage. Within the past week we have completed the first fabrication of an aircraft composite leading edge with a built' in fiber optic damage detection system.

Virginia Tech supports the largest university-based program in fiber optics and smart structures in the country. Fiber optics and fiber optics-related undergraduate and graduate lecture and laboratory courses serve more than 450 students each year in classes on campus and at several off-campus locations. The Fiber & Electro-Optics Research Center within the Bradley Department of Electrical Engineering currently is engaged in more than thirty separate fiber optics research programs sponsored by companies and government organizations. These include work in long distance and local area network fiber communications, fiber sensing including smart structures, fiber micro-optical devices, and fiber manufacturing and materials. The Smart Materials and Structures Laboratory within the Department of Mechanical Engineering currently is engaged in an additional twelve sponsored programs which focus primarily on actuators, shape memory composites, and active structural control. This paper reviews these combined activities.

This paper reviews fiber optic sensor research at Texas A&M University. This research, which began in 1987, is based primarily on the fabrication of internal dielectric mirrors in continuous lengths of silica fiber by fusion splicing. Application of these internal mirrors to reflectively monitored Fabry-Perot interferometric sensors are discussed and experimental results are presented.

This paper discusses the fundamental issues being investigated by Florida Institute of Technology (F.I.T.) to implement the technology of smart structural systems for DoD, NASA, and commercial applications. Embedded sensors and actuators controlled by processors can provide a modification of the mechanical characteristics of composite structures to produce smart structures^1-3. Recent advances in material science have spurred the development and use of composite materials in a wide range of applications from rotocraft blades and advanced tactical fighter aircraft to undersea and aerospace structures. Along with the advantages of an increased strength-to-weight ratio, the use of these materials has raised a number of questions related to understanding their failure mechanisms. Also, being able to predict structural failures far enough in advance to prevent them and to provide real-time structural health and damage monitoring has become a realistic possibility. Unfortunately, conventional sensors, actuators, and digital processors, although highly developed and well proven for other systems, may not be best suited for most smart structure applications. Our research has concentrated on few-mode and polarimetric single-fiber strain sensors4-7 and optically activated shape memory alloy (SMA) actuators controlled by artificial neural processors. We have constructed and characterized both few-mode and polarimetric sensors for a variety of fiber types, including standard single-mode, high-birefringence polarization preserving, and low-birefringence polarization insensitive fibers. We have investigated signal processing techniques for these sensors and have demonstrated active phase tracking for the high- and low-birefringence polarimetric sensors through the incorporation into the system of an electrooptic modulator designed and fabricated at F.I.T.. We have also started the design and testing of neural network architectures for processing the sensor signal outputs to calculate strain magnitude and actuator control signals for simple structures.

An in-situ fiber optic technique for determining the endpoint of cure of a thermoset polymer has been demonstrated and described previously [1]. The technique requires no calibration and is readily used as a cure detector in epoxy-based composite materials [2]. In brief, a short sensing fiber element is prepared from the thermoset resin of interest, and brought to a fully-cured state. In order to detect the endpoint of cure of this same resin when used as a matrix in a composite panel for example, the sensing fiber element is embedded within the panel during layup. Conventional multimode silica optical fibers are joined to each end of the sensing fiber with a small amount of epoxy. The silica fibers extend beyond the panel itself. Near infrared light from an LED is transmitted through the fiber arrangement, and the intensity of the transmitted light is measured with a photodetector. The fully-cured resin sensing fiber has a larger refractive index than uncured or partially-cured resin, and therefore acts as an efficient waveguide of varying numerical aperture when it is surrounded by the progressively curing matrix. When the surrounding resin becomes fully cured, the waveguiding capabilities of the sensor fiber are lost, and the transmitted light decays to zero, independent of the cure temperature. Thus, a null sensor output indicates that the resin in the composite panel is fully cured. This occurs at any temperature, without calibration, due to the self-referencial nature of the sensor.

The use of a multi-parameter fiberoptic sensing system for monitoring temperature, pressure, and strain within composite structures will be described. The system uses interferometric principles to measure a variety of physical parameters. Because of the sensor's micro-miniature size and inherent immunity to electromagnetic and radio frequency interference, the system is ideally suited to numerous composite monitoring applications.

Real time in situ monitoring of the chemical states of epoxy resins were investigated during cure in an autoclave using infrared evanescent spectroscopy. Fiber evanescent sensors were developed which may be sandwiched between the plies of the prepreg sample. In this work a short length of sapphire fiber as the sensor cell portion of the fiber probe was utilized. Heavy metal fluoride glass (HMFG) optical fiber cables were designed for connecting the FTIR spectrometer to the sensor fiber within the autoclave. The sapphire fibers have outstanding mechanical properties (Young's Modulus 65 = Mpsi) and outstanding thermal properties (T. = 2000°C) which should permit their use as an embedded link in all thermoset composites. The system is capable of operation at a temperature of 250°C (482 °F) for periods up to 8 hours without major changes to the fiber transmission. A discussion of the selection of suitable sensor fibers, the construction of a fiber optic interface, and the interpretation of in situ infrared spectra of the curing process is pre-sented.

Tactical aircraft and commercial and military space structures of the 21st century will employ embedded sensors and actuators to optimize their performance and survivability. As design engineers attempt to incorporate these devices into the new designs, they are faced with numerous additional variables but little criteria for making critical decisions. Textron Aerostructures and Tennessee Technological University have undertaken to develop the technologies necessary to design and build laminated composite structures incorporating a variety of embedded devices. Production techniques are being developed for embedding the devices using both manual and automated methods. Design guidelines to help establish the appropriate device, embedding location and installation methods are also being developed. The experience obtained through the fabrication of a variety of test panels will be discussed and shown. Photomicrographs will be used to illustrate a range of embedding techniques and to document the results. A variety of techniques to bring the optical fibers out of the laminate will also be illustrated and discussed. Conclusions, preliminary recommendations and future plans will also be discussed.

The development of thick composite structures for the underwater environment has led to a need for sensors that can be incorporated into composite structures so that through-the-thickness measurements can be obtained. By embedding fiber optic sensors into thick composite structures more detailed information on the material properties can be obtained, however there are still many technical challenges to be faced.

American Composite Technology has conducted an investigation of materials and processing methods suitable for the automated production of smart structures. This program differs from many other investigations of smart structures in that equal emphasis was placed on producibility issues and performance of the structural system. For the study, a two inch diameter graphite/epoxy tube was used as a baseline structure to represent a single component of a space truss. This baseline tube was designed to have "smart" control of both shape and damping characteristics. A wide variety of materials for use as sensors, actuators and communications networks were investigated, and a number of the most promising candidates were identified. Compatibility with the pultrusion process was a major criterion for the selection of optimum material combinations. Pultrusion is the composite processing technology identified as being the most suited for automating the production of tubular graphite/epoxy structural members while simultaneously integrating the smart hardware. Two alternative designs were studied. A near term solution employing current technology would use resistance strain gauges for sensors and piezoelectric crystals as actuators. A solution based on evolving technology might employ fiber optic sensors and shape memory metal actuators. Use of fiber optics for sensing would probably require significant advances in signal processing capability to allow the system to respond in real time.

A prototype fibre optic damage assessment system for an aircraft wing leading edge is described. This system is based on the fracture of embedded optical fibres. The sensor configuration was determined from tests conducted on small coupons. We report on the design and recent construction of an instrumented wing leading edge panel. The tests that will be carried out for its evaluation will be discussed.

An optical fibre Michelson interferometer is embedded in Kevlar-epoxy composites for acoustic emission detection. Application of this sensor system to damage detection in composites is demonstrated.

This paper describes a collaborative European Programme No. RI IB-0173-C(CD) under the auspices of BRITE (Basic Research in Industrial Technologies for Europe) which is jointly sponsored by the Commission of the European Communities and by European industry. The programme aims to explore the use of optical sensing techniques in composites. Several sensor methods (microbending, phase, polarimetric and multiplexing schemes) have been considered. The fabrication issues relating to moulding and filament winding of composite samples containing embedded sensors with emergent pigtails have been addressed. The effects of embedded fibre optics on the structural integrity of the composites have been investigated by both mechanical testing and by the use of two mathematical modelling techniques, a homogenization method based on continuum mechanics and finite element techniques. Through a suitable choice of sensor dimension, jacketing type and thickness the detrimental effects of the embedded inclusions on mechanical properties can be minimized. This has been verified by the experimental determination of deformation fields around optical fibres embedded in composite laminates.

We explore the application of the smart structures concept to fighter aircraft. The discussion addresses the issues of survivability of the structure during flight and combat, and the supportability of the aircraft as a weapon system. Smart structures concepts offer promise in both of these areas. We also present data from initial investigations of fiber optic sensors embedded within composite panels and bonded to aluminum specimens. The sensors detect the occurrence of impact and measure the strain.

The properties and durability of polymeric matrix composites are affected by the fabrication process and by the effects of their service environment. A dual-role, fluorescence based fiber-optic sensor was developed to monitor the composite curing process and subsequently to monitor the sorbed water content of the cured composite. An embedded sensor waveguide generates a signal which follows the chemorheological changes during cure of a carbon-epoxy laminate and can be used to control the proc-ess. The embedded sensor signal declines in proportion to the sorbed water content and increases when water is desorbed. The sensor can also monitor the processing variables of a carbon-PEEK thermoplastic composite. The potential of free-volume dependent fluorescence probes to monitor the physical aging of an epoxy resin was demonstrated pointing to the potential for an additional application for the sensor.

An elliptical core two-mode fiber can be used as an effective fiber sensor, due to the fact that interference between the LP01 and LP11 modes in an elliptical core fiber is sensitive to perturbations in the surrounding environment. A fiber optic sensor system using an elliptical core two-mode fiber has been developed for measuring strain and impact damage in composite materials. Various elliptical core fibers have been embedded in composite materials and the effects of the embedding process on the fibers have been investigated. The strain measurement was calibrated by measuring the beatlength of the two-mode fiber after embedding. Impact damage detection was performed by measuring the acoustic impact waves propagating in the composite material. The sensor system employs a laser diode whose wavelength can be adjusted to correct the offset of each individual sensor. The experimental results demonstrate the feasibility of using two-mode fiber optic sensors for measuring strain and detecting damage in composite materials.

The problem of development of smart structures and their vibration control by the use of piezoceramic sensors and actuators have been discussed. In particular, these structures are assumed to have time varying model form and parameters. The model form may change significantly and suddenly. Combined identification of the model from parameters of these structures and model adaptive control of these structures are discussed in this paper.

Embedded or internal actuators in a structure are being utilized to perform vibration attenuation, pointing maneuvers, damage control, and other adaptive tasks. The controls-structures-interaction (CSI) design process depends strongly on the coupled dynamic response of the structure and the actuators. A bond graph approach is used to guide this analytical study to uncover the effects of certain generic actuator types on the closed-loop, initial condition control of a single-mode structural model and an eighteen-state model of an active truss. So-called "flow" and "effort" source actuator types were studied and were shown to have significant differences in closed-loop reponse for ranges of initial conditions and sytem open-loop natural frequencies.

This paper presents an experimental and theoretical study of the dynamic and thermal characteristics of SHAPE MEMORY ACTUATORS (SMA). The actuators are made of a NIckel-TItanium (NITINOL) wires formed in the shape of helical coils that are capable of generating large deflections when subjected to low voltage excitations. The deflections produced are accompanied by significant forces resultingfrom the unique phase transformation of the NITINOL alloy as it is heated past its transition temperature. The effect of varying the wire diameter and length of the actuator on its dynamic and thermal behavior is measured both in the time and frequency domains. The actuators' behavior is monitored at different levels of excitation voltages as well as different heating and cooling strategies. The experimental results obtained are utilized to guide the development of mathematical models that describe the actuators' behavior as influenced by their design parameters and operating conditions. The experimental results and mathematical models presented are invaluable in guiding the selection of the design parameters of NITINOL actuators that are suitable for particular operating conditions.

A new generation of revolutionary multi-functional, dynamically-tunable, intelligent, ultra-advanced composite materials featuring electro-rheological fluids is proposed herein for the active continuum vibrational-control of structural systems. This paper reports on pioneering proof-of-concept experimental investigations focused on evaluating the elastodynamic transient and also the forced response characteristics of beams fabricated in this new class of materials. The results of these investigations clearly demonstrate the ability to dramatically change the vibrational characteristics of beam-like specimens fabricated in ultra-advanced composite materials by changing the electrical field imposed on the fluid domains. In addition, experimental results are presented which characterize the elastodynamic response of a connecting rod of a slider-crank mechanism fabricated in these ultra-advanced composite materials. Again, the combined forced and parametric responses are controlled by the voltage imposed on the electro-rheological fluid domain in the structure. The capability of these materials to interface with modern solid-state electronics can be exploited by extending the fundamental phenomenological work presented herein through the successful incorporation of intelligent sensor technologies and modern control strategies in order to significantly accelerate the evolution of these novel composite materials for the military and aerospace industries.

Shape memory alloy (SMA) reinforced composites are a relatively new class of adaptive materials. The composites employ shape memory alloy fibers or films as distributed actuators to achieve adaptive functions while in service. In order to use this material reliably and exploit its potential, it is necessary to understand several of its thermomechanical characteristics. In this paper two fundamental design issues of the SMA reinforced composites, i.e., mechanical response of the SMA reinforced graphite/epoxy under tensile loading and qualitative examination of the interfacial bond between actuator fibers and matrices, are briefly described. The experimental methodologies and test results are presented and discussed.

Neural networks have been shown to be useful in sensor pre-processing and dynamic system modeling and control, as well as various other signal analysis areas. Their application to the problem of developing a real-time smart structure is further indicated by the extremely fast post-learning speed of the neural network. Fiber-optic sensors also possess very high response rates, and sensitivities and are well suited to the detection of structural changes. The integration of these two technologies is a natural step toward a high-speed smart structure. A smart structure that is applicable to airfoil control surfaces as well as artificial human joints is being modeled and built at Florida Institute of Technology. The actuators (muscles) in our model are replaced with shape-memory metal strands. The joint-position sensors of the neuro-muscular system are modeled by fiber-optic sensors. The control (neural) circuits are replaced by an artificial neural network. We are investigating the suitability of each of these subsystems to the problem. The appropriate neural-network architecture, as well as the sensors and actuators, is under investigation, and a prototype system is being fabricated and tested. Results of the modeling, design, and performance will be discussed.

This paper is concerned with a discussion of recent research at VPI&SU in the area of active structural acoustic control. The material is divided into progress in the areas of structural acoustics, actuators, sensors, and control approaches. However, due to the coupled nature of the problem, considerable effort throughout the program has been given to the interaction of these areas with each other. The results presented show that significant progress has been made towards controlling structurally radiated noise by active/adaptive means applied directly to the structure.

Future spacecraft will have stringent performance requirements that need advanced vibration suppression systems. Examples of these spacecraft are an extended space station, Mission-to-Planet-Earth platforms, and military platforms. Operating conditions for these large space structures (LSSs), e.g., retargeting slew maneuvers and spacecraft docking, can induce unacceptable structural vibrations that take many minutes to naturally dampen out. Increased damping must be added to the structures. This added damping can be a passive mechanism, an active vibration control system, or a combination of passive and active systems. In this investigation, we consider how to satisfy the settling time requirement of a typical LSS. We attempt to determine the amount of vibration suppression or damping necessary to meet the stringent settling time requirement of a typical LSS and then to design, analyze, and experimentally verify an active control system to meet the damping requirement. We also study the effect of passive damping on the active damping system of an LSS to get an idea of what an optimal mix of both passive and active vibration suppression might depend on.

Few mode optical fibers have been shown to produce predictable interference patterns when strained. Here we describe the use of a modal domain sensor in a vibration control experiment. An optical fiber is bonded along the length of a flexible beam. A control signal derived from the output of the modal domain sensor is used to suppress vibrations induced in the beam. A distributed effect model for the modal domain sensor is developed. This model is combined with models of the beam and actuator dynamics to produce a system suitable for control design. Simulated results are presented.

Research in smart structures, especially the area of vibration suppression, has warranted the investigation of advanced computing environments. Real time PC computing power has limited development of high order control algorithms. This paper presents a simple Real Time Embedded Control System (RTECS) in an application of Intelligent Structure Monitoring by way of modal domain sensing for vibration control. It is compared to a PC AT based system for overall functionality and speed. The system employs a novel Reduced Instruction Set Computer (RISC) microcontroller capable of 15 million instructions per second (MIPS) continuous performance and burst rates of 40 MIPS. Advanced Complimentary Metal Oxide Semiconductor (CMOS) circuits are integrated on a single 100 mm by 160 mm printed circuit board requiring only 1 Watt of power. An operating system written in Forth provides high speed operation and short development cycles. The system allows for implementation of Input/Output (I/O) intensive algorithms and provides capability for advanced system development.

A Mach Zehnder interferometer has been constructed with each arm affixed to opposite sides of a vertically cantilevered test beam. Static deflection and tip velocity have been detected by processing and counting the fringes at the output. Tip velocity signal is used in a closed loop control scheme to dampen vibrations.

An optical-time-domain-reflectometry based shape sensor is presented. Fibers with air-gap splices at certain known positions are attached to the exterior of a flexible structure. The strain induced in each fiber section due to the bending of the structure is detected by the Optical Time Domain Reflectometer (OTDR) as shifts in the pulses reflected at the splices. The approximate shape of the structure is then determined using the strain information from each fiber section. Experimental results for simple shapes are given, and future directions for determining complex shapes are discussed.

Measurements of DC strain produced by transverse deflection of a fiber optic embedded graphite/ bismaleimide composite panel in cantilever geometry were made using a Mach Zehnder interferometer and active homodyne demodulation technique. The measured strain ranged from an average longitudinal strain of 2.0 x 10-4 to 6.6 x 10-7, corresponding to a maximum transverse deflection of 0.3 inches (3%) and a minimum transverse deflection of 0.001 inches respectively. We find that the strain optic coefficient of the embedded fiber ranges from 30% to 40% less than the value for uniaxial strain. In addition, these measurements reveal hysteresis and or creep during the loading and unloading cycle of the composite. Experiments were performed to determine sources of DC drift, and the main sources of DC drift identified. Calibration of the results is discussed as well as future modifications that will permit four orders of magnitude increase in tracking range, greatly improved interferometer stability, and elimination of the effects of piezoelectric hysteresis on the measurement of strain.

Recent interest in distributed fiber optic sensing systems utilizing optical reflectometry has generated a need for simulation tools to study the effects of the many system variables on performance, as well as assess the performance claims of various Optical Time Domain Reflectometry (OTDR) instruments and the suitability for their use in a particular sensing application. Computer simulation tools have been developed to enable the optoelectronics system designer to study the fundamental limitations of instrument and sensing system performance based on Rayleigh backscattering. A standalone program utilizing a three segment optical fiber model has been developed that allows calculation of the absolute backscattered power as a function of time, both the total power and the time dependent power from a particular fiber segment. In addition, comprehensive PC based spreadsheet tools have been developed to allow for modeling of signal to noise ratios,including receiver design, in all types of coherent and incoherent optical time domain reflectometry systems, for both single mode and multimode systems. Several examples of the applications of these programs are presented and discussed, with application to the design and development of a high spatial resolution measurement terminal for monitoring short structures. Finally, we present experimental data on a two sensor system based on periodic microbending that shows effects of sensor interaction, and demonstrates the need for careful interpretation of OTDR results as well as further experimental and theoretical work on the effects of microbending induced leaky mode coupling and backscattering in the presence of such modes.

Smart structures deployed in low earth orbit will be exposed to a hostile environment which will include temperature extremes during each orbit as the platform moves from the day to the night side. The stresses due to thermal cycling may compromise the performance of embedded fiber optic strain sensors because of differential thermal expansion of the fiber and the composite material. We have investigated the effects of elevated temperature and thermal cycling on the performance of a fiber optic strain sensor embedded in a graphite-epoxy tube for temperatures from 65 to 220 ° F. The results indicated that the fiber optic strain sensor measurements correlated well with conventional resistance strain gauges attached to the tube.

The design, phase stability and strain sensitivity of a fiber optic interferometric strain sensor are discussed. For this sensor, the laser diode is modulated with a pulsed 100 MHz RF signal. The phase difference between the modulation recovered from the sensing leg and the reference RF oscillation is proportional to strain. Strain sensitivities as low as + 4.5 x 10^-4 were measured.

Design flexibility is often touted as an advantage of optical fiber transducers. This advantage is exploited by formalizing the geometric design of interferometric optical fiber stress and strain sensors. The equations that govern the phase-strain correlation are used to define some basic design laws. The most common design goal is to separate strain or stress components from composite phase-strain data. This can be accomplished in a Mach-Zehnder format or in a format in which both the sensing and reference fibers are exposed to the strain field. The design laws together with simple configurations are used to devise several fiber transducers. Design flexibility exists because there are many solutions which satisfy the design objectives and constraints. The constant strain assumption is the basis of design. A comparison of the transverse sensitivity of resistance and optical fiber gages is presented.

Acoustic waveguides embedded within materials provide a rugged and simple means for sensing stress, strain and other phenomena within smart structures and skins. The waveguides which function well in hostile environments can be used in two signal monitoring modes - transmit/receive and receive (listen), which is helpful when monitoring in noisy environments. They can be applied for monitoring cure as a material is made and subsequently for NDE during the material lifetime. In this paper applications of acoustic waveguides for monitoring material cure, viscosity, strain, and other phenomena are outlined, and the likelihood of the merging of the technologies of embedded optical and acoustic fiber waveguides is discussed.

A planar flexible fiber-optic interferometric acoustic sensor has been developed by wrapping optimized single-mode fibers in a planar spiral form and then embedding the fiber in a thin polyurethane layer. The acoustic response of the element was studied and it was found to be high and frequency independent. The acoustic response was then interpreted in light of calculations based upon an approximation to the sensing structure, normally one in which the polyurethane layer is placed concentrically over the sensing fiber. Excellent agreement was found between experiment and analysis. The acceleration response of the element was also studied. The planar fiber acoustic sensor was compared to a PVF2 sensor of similar geometry. It was found that in an acceleration controlled environment the fiber sensor acoustic performance compared favorably to that of the PVF2.

Fiber optic smart structures can be used to perform a variety of functions ranging from monitoring the manufacturing process, performing nondestructive evaluation of components once the parts have been made, providing means to implement vehicle health monitoring and maintenance systems, and enabling systems such as actively damped structures and performance monitoring of aircraft engines. This paper describes a low cost approach to mirobending sensing and its application to feasibility studies for manufacturing and health maintenance.

A quasi-distributed polarimetric strain sensing system with minimized component count is described. The sensing fiber consisted of a number of discrete single mode optical fiber segments connected in series so that known backreflected energy was returned from the splice between each fiber segment. An rf ramped integrated optic Mach-Zehnder interferometer was used in the system in a bi-directional fashion as both a modulator and mixer. The beat signals produced in the mixing process were passed through an analyzer and detected by a low speed photodetector. Signal processing in the frequency domain then allowed the transverse strain on each fiber segment to be determined. A theoretical analysis of the technique is presented and compared to experimental results.

A phase sensitive fiber optic strain gauge that is both spatially local and unidirectional is described. The design consists of a path balanced lead-in/out Michelson interferometer using a single-mode directional coupler. It is tested as a surface adhered device in both uniaxial and rosette configurations. The device is integrated between plies of graphite/PEEK thermoplastic to provide lamina strain measurements. Performance parameters gauge factor, transverse sensitivity, and failure strainmare evaluated. The utility of such a strain gauge is discussed in the general context of smart structures and skins.

To be effective in high temperature environments, optical fibers require coating materials which maintain their integrity over their lifetimes of intended use. Polyimide-based fiber coatings are suitable for temperatures up to 350°C. These fibers, used as embedded sensor elements, would remain undamaged during the cure cycles of low temperature composites. To enhance the utility of such sensor elements, an understanding of the structure/property correlations related to adhesion and composite matrix integrity is essential. Several commercially available polyimide coating/glass fiber systems were compared in uniform adhesion tests. The primary concern was the characterization of adhesion between these materials and the surrounding epoxy matrix. Factors to be considered were the effect of coated versus uncoated fibers, the performance of cured versus uncured coatings, and the method of fiber pretreatment. Specific analytical techniques were used to determine the relationship between molecular structure and physical properties. Fiber pullout tests were performed using a tensile stage attached to a light microscope. Failure modes were verified via infrared microspectrometry (FTIRMA) and optical microscopy.

Refractory materials are examined for thermodynamic suitability for coating sapphire waveguides. The materials selected are silicon carbide, zirconia, silica and metal niobium. Experimental verification of chemical reaction in very low pressure and at 857C temperature is studied through X-ray diffraction patterns of the samples before and after heating. Electron microprobe analysis will provide valuable information on weight percentage of chemical reactions and SEM will provide some information on visible changes in the sample. Various methods are available for coating the fiber that would make it suitable for high temperature, pressure, vibration, strain and other mechanical sensing in harsh environments.

In recent years Optical Time Domain Reflectometry (OTDR) and, more generally, optical time delay methods, have been applied to measure optical fiber strain and elongation. These techniques have been recognized as having significant potential in the area of non-destructive testing and analysis of structures. However, limitations such as the lack of local strain monitoring capabilities and poor spatial resolution have kept OTDR based fiber optic sensors from becoming more popular in the Non-Destructive Evaluation (NDE) industry. Significant advances and improvements regarding these two main limitations have been achieved. Extremely fast OTDR systems which are capable of launching optical pulses with Full Width Half Maxima (FWHM) on the order of 100 ps and temporal stabilities of less than 2 ps have recently made spatial resolutions in the sub-millimeter range possible. Furthermore, techniques have been developed which allow quasi-distributed local strain sensing using a single optical fiber. Such systems rely on the physical segmentation of the fiber through the use of partially reflective air-gap splices which provide marker reflections. The positions in time of these reflections are individually monitored to obtain local strain measurements. In this report we present an analysis of such systems, and discuss experimental results obtained using a picosecond resolution OTDR. It is shown that it is possible to measure strain locally with a resolution on the order of 0.0001 m/m with a gage length as small as a few centimeters. A discussion on how to further improve upon the performance by taking advantage of reentrant fiber loop technology is also given.

A polarimetric sensor using high birefringent optic fibre is used to measure strain on the surface of a cantilevered beam. Localization of the sensor is accomplished with either a dual 45° splice or a single 45° splice with mirrored endface. Quadrature signal recovery is obtained using a dual wavelength approach. The sensor's performance characteristics are compared with those of an equivalent fibre optic interferometer. Applications of the polarimetric sensor are then discussed.

The feasibility of a power by light headset for use in aircraft communication has been explored. The research has focused on defining a system concept which optimizes the optical efficiency of the uplink and the headset "power-by-light" elements and minimizes the electrical power consumption of the microphone downlink. Breadboards and tests of the critical power-by-light power supply, earphone driver, and downlink elements have been performed. The data from the experiments are presented and analyzed in order to develop design rules and component requirements for the fiber optic, power by light headset design.

We report the detection of surface acoustic waves on metals using a Michelson-type fiber optic interferometric sensor. Surface acoustic waves generated along an aluminum bar are detected by a miniature fiber optic probe placed between an acoustic transmitter and a piezo-electric receiver. Extensions towards measuring acoustic wave velocities are considered.

Few-mode, elliptical core fibers embedded in graphite-epoxy composites have been shown to detect the strain and vibration effects on the host material. A theoretical description of the sensor operation is presented and the insensitivity of the lead-in and lead-out fibers to strain is investigated. A scheme using a single-mode elliptical-core fiber as the lead-in fiber and an offset circular core single mode fiber as the lead-out fiber is successfully implemented. The effect on the sensitivity of the system, when the fibers are embedded between different layers of the laminate, is evaluated.