Low profile single-mode and multimode fibers with cladding sizes on the order of 35-40 microns have been fabricated for embedded sensor applications. Sensor performance of a single-mode Fabry-Perot configuration as well as a two-mode, elliptical-core, differential interferometric scheme shows sensitivities as high as those expected from conventional fiber sensors. Comparative effects of embedded fibers evaluated using simple c-scans and photomicrographs indicate the relatively less deleterious effects of low profile fibers. We present a prognosis of the lowest possible diameters that may be feasible in the future, from the handling ability and manufacturability standpoints.

Fabry-Perot fiber optic sensors embedded within composite macromodels are shown to be useful in measuring strain concentrations introduced by damage events. Fiber sensors effectively detect the signature of the fracture as well as provide accurate information about the state of the host material. We show that the results obtained from fiber sensors and electrical strain gauges validate newly developed micromechanical theories. Fiber sensors with gauge lengths of 4 mm are shown to be capable of resolving strains on the order of 1 micro-epsilon. The composite macromodel is described in detail and future issues are addressed.

The fundamental theory which relates optical phase changes to applied strain and temperature fields in structurally embedded interferometric optical fiber sensors of all types is described. The theory is developed in a unified manner through a careful discussion of basic assumptions and definitions. This theory is then applied to Mach-Zehnder, Michelson, intrinsic and extrinsic Fabry-Perot, polarimetric, and modal-domain sensors. Theoretical investigation of the phase-strain-temperature theory applied to all of these sensors embedded in transversely isotropic specimens under five different load conditions is used to investigate the importance of transverse strain terms.

The presented research addresses the interfacial behavior of polyimide-coated optical fiber sensors embedded in neat resin matrices and their use in gaining information for strain sensing applications. The interfacial shear strengths of glass fiber/coating/resin systems were examined via a single fiber pull-out test. The optical fiber strain sensor was developed in-house employing polyimide-coated, polarization-maintaining optical fiber and was characterized for sensitivity to strain and other strain related phenomena. Optical fiber strain experiments are compared to an established data base of single fiber pull-out test results. Initial experimental comparisons are presented.

This paper presents our preliminary results on the use of fiber optic sensors for impact detection on Kevlar-epoxy panels. Interfero-polarimetric measurements have been performed by using two different kinds of polarization-maintaining optical fibers. The sensitivity of the fibers to internal process stresses and to external impact stresses is discussed with respect to the coating nature. The effect of an over-coating on the fiber sensitivity is also reported. Impact-induced delaminations have been observed and the optical response has been adjusted by using an appropriate over-coating.

This paper discusses a novel approach for composite damage assessment with potential for DoD, NASA, and commercial applications. We have analyzed and modeled a two-dimensional composite damage assessment system for real-time monitoring and determination of damage location in a composite structure. The system combines two techniques: a fiberoptic strain sensor array and an optical neural network processor. A two-dimensional fiberoptic sensor array embedded in the composite structure during the manufacturing process can be used to detect changes in the mechanical strain distribution caused by subsequent damage to the structure. The optical processor, a pre-trained Kohonen neural network, has the capability to indicate the location of the damage due to its positionally associative architecture. Because of the parallel, all optical architecture of the system, the system has the advantages of having high resolution, a simple architecture, and almost instantaneous processor output. Results of the modeling and simulation predict a highly robust system with accurate determination of damage location. We are currently beginning work on a breadboard demonstration model of the system.

The mechanical interaction between an embedded optical fiber sensor and its host laminated composite is investigated in this paper with a combination of moire' interferometry and Fourier transform fringe interpretation in order to achieve sub-micron spatial resolutions. Typical strain distributions are presented for several coated and uncoated optical fibers embedded at the mid-plane of a 9012 graphite/epoxy laminated composite compression specimen. Descriptors indicating the effect which the fiber has on the host, such as ellipse of influence and strain concentrations, are presented. Instances of residual strain devolvement and total collapse of interfaces are also documented.

Optical fiber sensors coated with linear work hardening elastic-plastic materials are analytically explored to determine the effects which the coating properties have on the sensor performance. The optical fiber system is subjected to both an axial load and an arbitrary thermal gradient. An important consequence of the non-linear analysis is the discovery that this coated fiber system can be exploited to serve as an alternative to the conventional fiber breakage sensor for sensing impact damage. The non-linear analysis reveals a mechanism for designing coatings which provide a 'memory' to the fiber-optic sensor by undergoing permanent deformations in response to large thermal or mechanical strain excursions. Such sensors can be utilized to maintain a permanent record of the load/damage history of a loaded structure.

This paper shows a method to minimize the temperature sensitivity by coating an elliptical core fiber with an additional thin elliptical cladding. We analyze the temperature and the strain sensitivities of the fiber and discuss the design of such temperature-insensitive fibers with high strain sensitivities, suitable for smart structures and skin sensors.

A technique for embedding one or more optical fibers in a cast metal part or structure while maintaining optical transmission through the fiber is presented. This technique provides nondestructive monitor of internal perturbations of the structure. Application of the method to embedded fiber optic sensors in metallic structures and to fiber-embedded metal feedthrough are reported and the performances of temperature and ultrasound fiber sensor embedded in a cast aluminum block are demonstrated.

We report the use of a short-length, multimode sapphire rod as an extension to a Michelson configuration, but operated as a low-finesse Fabry-Perot cavity. We demonstrate the performance of such a device as an interferometric sensor, where the interference between the reflections from the sapphire-air interface and an air-metallic surface is observed for microdisplacement of the metallic surface which is placed close to the sapphire endface. We describe in detail the fabrication procedure and present results obtained from the detection of temperature changes, applied strain, and surface acoustic waves.

The current status and results of a cooperative program aimed at the implementation of a high-temperature acoustic/strain sensor onto metallic structures are reported. The sensor systems that are to be implemented under this program will measure thermal expansion, maneuver loads, aircraft buffet, sonic fatigue, and acoustic emissions in environments that approach 1800 F. The discussion covers fiber development, fabrication of an extrinsic Fabry-Perot interferometer acoustic sensor, sensor mounting/integration, and results of an evaluation of the sensor capabilities.

We report results from fiber optic sensor field tests on an F-15 mounted within a full-scale test frame for the purpose of fatigue testing at the Structures Test Facility, Wright Patterson Air Force Base, Ohio. Static and dynamic loading data obtained using multiple extrinsic Fabry- Perot fiber optic sensors is presented. The output fringes from two quadrature phase shifted Fabry-Perot sensors were linearized using computerized software. The results compare well with data obtained from conventional strain gauges located adjacent to the fiber optic sensors. Strain sensitivities on the order of 0.01 (mu) /m were observed.

We present the implementation of a coherence receiver that is able, in parallel, to demultiplex and measure the phase between up to seven interference signals generated in a "white light" polarimeiric quasi-distributed sensing system. It is based on a Michelson interferometer, one mirror of which has seven reflecting facets with differing thicknesses in its aperture. It was further used to measure the phase variation, between two polarization couplers, versus three point bending loading for optical fibres either surface bonded or embedded in GRP composite.In both cases, the sensitivity to the composite elongation is found in close agreennt with the sensitivity of the fibre itself.

Rayleigh backscatter from (near) singlemode fibers can be measured with centimeter-scale resolution using a photon-counting optical time domain reflectometer (OTDR) technique. It is shown that high resolution measurement of changes in backscatter signals can be constrained by spatially-varying polarization fluctuations in the backscatter signal, which can produce large changes in detected signals using a standard OTDR configuration. The magnitude of these effects is explored, and a means of minimizing their influence on the accuracy of loss measurement is described.

An integration of fiber sensor technology and photofluidic interfaces is proposed. A practical implementation of such an integrated system is described in which sensing is performed by employing an acceptable fiber optic method while the logic and feedback is performed using existing fluidic methods. The proposed closed-loop system needs no electrical conversion or feedback information. The capability of the integrated system to accurately track the vibrational behavior of a beam is demonstrated.

In this paper we demonstrate the measurement of fiber strain for use in embedded sensor applications. Both of these methods utilize source frequency interrogation to monitor fiber strain. The first system described provides absolute strain information from a single-mode interferometric sensor used to measure absolute surface strain induced by deflection of a cantilever beam. The second system presented demonstrates remote interrogation of a lead insensitive two-mode elliptical core fiber sensor to measure relative changes in fiber strain.

A possible line replaceable fiber optic sensor system compatible with future aircraft requirements is described which could be used to support health monitoring and damage assessment functions to augment survivability, repairability, and maintainability while enhancing performance and control systems by providing flexible sensor information channels. The architecture of an embedded sensor system is described, with attention given to the selection of sensors to support it.

This paper presents an overview of the engineering applications of smart structures technology to civil structures. The definition used for smart structures will include those cases where the smartness entails the ability of the structure to sense its present state and the external loads that are acting on it, as well as those cases where the structures are able to respond to external loads so as to ameliorate their effects. The first case will be referred to as `Sensing Structures.' The second case will be referred to as `Sensing and Reacting Structures.' The sequel contains a discussion of applications of sensing structures, sensing and reacting structures, and a brief section that speculates on future opportunities for smart civil structures technologies.

A collaborative European Programme N degree(s) RI 1B 0173-C(CD) under the auspices of BRITE (Basic Research in Industrial Technologies for Europe), jointly sponsored by the Commission of the European Communities and European Industry, was launched in 1988 to explore and develop an optical sensor network embedded in composite for measuring the strain and temperature distributions. Its objectives and first results were presented at the `Fiber Optic Smart Structures and Skins II' conference (Sept. 1989, Boston). This paper will describe the work and the main results which have been obtained since then. Three main areas have been covered which have concerned the implementation of a coherence based parallel quasi- distributed sensing system, the simultaneous measurement of temperature and strain and the mechanical properties of composite material with embedded sensor. All results have shown the high interest of such an optical sensing network for structure monitoring.

The use of embedded optical-fiber sensors in composite materials to measure strains and to detect delamination is investigated experimentally. The capability of embedded optical-fiber sensors to measure compression strain and to follow crack propagation in a composite material is demonstrated for carbon/epoxy and glass/epoxy composites. Details of the experiments and experimental results are presented.

Artificial neural networks (ANNs) and their ability to model and control dynamical systems for smart structures, including sensors, actuators, and plants, are directly applicable to the smart electromagnetic structures (SEMS) concept. The application of neural networks to the area of controls is being reported frequently. The ability of a structure to adapt to impinging electromagnetic (EM) energy will allow the structure to change its reflection characteristics and thus to change its radar signature. By embedding a control element in the structure of a single microstrip patch element, its electrical characteristics can be changed. If such an element can be controlled by a closed loop system the patch antenna element can be made to adjust its operating characteristics through the control algorithm. If the control algorithm can be implemented in a neural network, the system can be made to change its characteristics in response to the stimulus. This change can be used to alter the antenna's performance in real time. As part of our research, a model of the patch neural network antenna system is being developed and this analytical model, as well as experimental models of the antenna are being tested and compared. The neural network antenna model and prototypes are being taught to adapt to the magnitude and phase response of microstrip patch antennas to incoming signals. The response characteristics and speed are reported in this paper. We demonstrate that the patch can be given autonomous adaptive capabilities using neural networks. An array of such smart patches could be assembled to create an even more adaptable antenna system.

In relation to the concept of smart skins and structures two optical bow-tie fibers with different beat length have been investigated as sensors. For the particular fiber type a sensor system has been built up based on measuring the light intensity and calculating the phase difference in the fiber. The system is composed of standard optical components and constitutes the calibration unit for optical sensors. Experimental results using both fibers are presented, where the external actions are strain and temperature. The fibers are exposed to strain and temperature by a spindle device and an electrical furnace, respectively. This procedure is assessed by gluing the fiber onto a cantilever beam and surrounding the fiber by hot water. The experiments show a good linear relation between the optical phase difference in the fibers and the external perturbations, especially for the strain experiments.

Axial strain may be determined by monitoring the modal pattern variation of an optical fiber. In this paper we present the results of a numerical model that has been developed to calculate the modal pattern variation at the end of a weakly guiding optical fiber under axial strain. 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 modal phase term of the fields and the fiber output pattern are also modified. For multimode fibers, very complicated patterns result. The predicted patterns are presented, and an expression for the phase variation with strain is derived.

This paper reports on the development of a passive, fast demodulation system for use with in-fiber Bragg gratings used for strain or temperature measurements. This compact, potentially inexpensive self-referencing system permits absolute strain/temperature measurements over a wide dynamic range with fast temporal response by tracking the wavelength shifts of the narrow-band back-reflected Bragg spectrum. The wavelength, bandwidth and strain sensitivity of a Bragg sensor are discussed, and examples of both static and dynamic strain measurements are shown.

This paper will report on efforts directed toward the generation and detection of ultrasonic waves using optical fibers for the purpose of ultrasonic velocity determination. Interferometric optical fiber sensors are embedded within or bonded to the samples. Two different techniques, laser generated ultrasound and piezoelectric ultrasonics, were used in the study of pulse delay measurements in aluminum and in a room temperature cured epoxy. Group velocity in an aluminum sample and in a cured epoxy sample was determined with both techniques. The application of these two methods to epoxy cure monitoring is discussed.

This paper considers two important application issues of fiber optic sensors in aircraft structures. The first concerns the interfacing of optical fibers with composite material structures: concepts have been developed for realizing a reliable exit point for the embedded fiber from a composite aircraft component, by means of single-mode connector assemblies embedded in composite material structures. The second issue relates to the temperature stability of the sensors: temperature characteristics of two-mode fiber optic strain sensors were investigated, sensors made of elliptical core and bow-tie fibers are compared, and the trade-offs among strain sensor range, sensitivity and temperature stability are discussed.

The paper reports progress in three areas essential to avionics composite-embedded data links: evanescent coupling and signal distribution, embedded connectors, and embedment of high temperature optical fibers in metal matrix composites. The objective of this research is improving signal transmission capabilities and reducing weight through the use of composite-embedded optical fibers in airborne electronic packaging. Guidelines for future work in these areas are presented.

A Michelson Fibre optic sensor (MFOS) is described for in-situ strain and vibration monitoring as well as acoustic emission detection in composite material structures. The phase sensitive fibre optic sensor is localised, all-fibre, and intrinsic. The MFOS was successfully embedded in Keviar/epoxy and graphite/epoxy thermosets as well as graphite/PEEK thermoplastic in order to perform local strain and vibration measurements at the lamina level. A technique allowing acoustic emission detection in parallel with strain and vibration monitoring is illustrated.

The potential suitability of the Fabry-Perot and Bragg grating fiber optic sensors will be considered against a set of criteria for practical use with Smart Structures. Particular attention will be given to a number of key issues, such as: interrupt immunity, apparent strain, sensitivity, response time, structural integration, optical interfacing minimal perturbation and sensor performance lifetime. Some of our development of fiber optic sensors for use with Smart Structures will be reviewed in the light of these considerations.

The intrinsic Fiber Fabry-Perot (FFP) sensor is being considered for strain sensing in smart structure applications. The current art of FFP sensor fabrication as published in the literature is reviewed. A pseudo-heterodyne demodulation technique is presented and performance of the system detailed. The issue of thermally-induced apparent strain as it relates to fiber-optic strain sensors will also be introduced.

A method is demonstrated for demodulating the signal from a fiber-optic Fabry-Perot (FFP) strain gauge using white light interferometry. In particular, a Michelson interferometer with an identical path imbalance to the sensing FFP interferometer is used to recombine the sensing and reference components. Far-field interference from the Michelson interferometer, with one tilted mirror, is then projected onto a CCD camera where the fringes can be captured and analyzed on an accompanying microcomputer. The advantages of the system include the absolute measurement capability and the potential for multiprocessing.

Smart Structures and Materials technology will undoubtedly yield a wide range of new materials plus new sensing and actuation technologies and this will have a radical effect on current approaches to structural design. To meet the multi-disciplinary research challenge posed by this technology, the Smart Structures Research Institute (SSRI) has been established at the University of Strathclyde, Glasgow. This paper describes the background, current and planned activities and progress made in developing this new and very promising technology.

A programme of mechanical testing has been carried out to determine the effect of a variety of embedded optical fibres on the properties of carbon/epoxide composite systems. The properties of interest were longitudinal and transverse tension, longitudinal compression, inter- laminar and in-plane shear. The variables investigated included the diameter of the silica cladding and the thickness and type of jacketing material. It was found that both the polyimide and acrylate coated fibres with diameters of approximately 100 micrometers had little adverse effect on the mechanical properties of any of the composites, except in longitudinal compression where up to 26% reduction in strength was seen in some systems.

A technique based upon the differential sensitivities of dual mode and polarimetric sensing schemes is shown to be capable of resolving simultaneously temperature and strain variations to within 20 micro-epsilon and 1 K over a strain and temperature excursion of 2 micro-epsilon and 45 K. The technique is evaluated experimentally over an 80 cm sensing length of unembedded optical fiber and in an 8 ply unidirectional carbon/epoxide laminate subject to temperature and strain cycling. A comparative analysis of the performance of the embedded and the unembedded fiber sensors is presented.