On-orbit structural health monitoring aboard space platforms requires the development of sensor systems for assessing impact damage from particles and debris, the effects of atomic oxygen erosion, and the integrity of power systems, storage tanks, pressure vessels, and major structural elements. The task of implementing such a smart structure diagnostic system during the initial phase of the NASA Space Station Freedom is evaluated, with a view to more complete smart structures implementation in the course of station evolution. The data processing/cataloguing task may ultimately require AI and neural networks.

Various sensor configurations useful in the prospective detection of perturbations in both polymeric composite and metallic materials are investigated. Attention is given to (1) a cross-point coordinate axis impact-and-location sensor, and (2) a strain gage-functioning tapered sensor for stress and crack detection. Crack initiation should be detectable if it occurs perpendicular to the sensor's line-of action, in the region between contact pads.

An evaluation is made of NASP vehicle features which bear on the need for, and the feasibility of, incorporating fiber-optic structural-state diagnostic sensors. The most likely application appears to be fiber-optic data links for high-bandwidth communications; this is followed in order of diminishing likelihood by distributed sensing systems, advanced optical transduction techniques, fiber-optic microphones, and nonintrusive measurements of flowfield density. Specific fiber-optic sensors will be employed where they are overwhelmingly favored by a cost-benefit analysis.

Smart skins technology is in its infancy, and significant workmust be accomplished to ensure that evolving fiber-optic sensor technology can be incorporated into smart skins concepts. This paper gives an overview of research into embedding optical fibers and sensors in composites. It also presents a concept for a fiber-optic sensor network embedded in aircraft skins to perform structural health monitoring and discusses issues, problems, and potential solutions encountered in bringing multiple technologies together.

This paper discusses several novel concepts being investigated in the Center for Fiberoptic Sensor Systems and Smart Structures at Florida Institute of Technology associated with fiberoptic sensors, actuators and processor technology, and efforts to integrate these components into distributed smart systems. Projects include: a polarimetric sensor with active phase tracking test set, a combination polarimetric/two mode sensor, an N-mode sensor with neural processor, damage assessment using embedded fiber-optic arrays and a neural processor, a pulsed interferometric sensor, neural network-processed polarimetric sensor signals, and optically-energized shape-memory alloy actuators.

Bragg gratings have been exposed into the core of optical fibers at specific locations to implement distributed, discrete gages for measurement of strain and temperature. Demonstrations using these devices to monitor composite specimen curing and bending are discussed. The sensor signals are also used in a control loop to drive an actuator for active damping of the test sample.

A linear elastic study is performed as a first order approximation to investigate the geometry of the resin-rich region observed around optical fibers embedded in laminated composites. The Rayleigh-Ritz method is employed with beam bending functions as assumed trial functions. The total potential energy is formulated in terms of unknown force distributions and the length of the resin pocket. The resulting system of coupled nonlinear equations is solved by the Levenberg-Marquardt algorithm to compute the shape and size of the resin pocket. Results of this analysis show the effect of laminate stacking sequence on the geometry of the resin pocket and are found to agree well with experimental observations.The computed geometry is automatically discretized for FEM analysis in order to obtain stress intensity information at the lateral ends of the resin pocket.

The importance of optical fiber coatings to the overall performance of structurally embedded optical fiber sensors is explored with closed form solutions. Particular attention is paid to the mechanical ramifications of material property and coating diameter choice when the fiber is embedded in a transversely isotropic host material. Analytical solutions are developed which show that strain concentrations near an embedded optical fiber are highly dependent on the coating material properties. Clear evidence is presented which indicates that there exist optimum coating material and radius combinations for a given host material. This optimum choice can minimize or even eliminate stress concentrations in the host material immediately surrounding the embedded fiber. Sensor phase sensitivity is explored for a wide range of coating properties.

This paper summarizes the reduction in mechanical properties of graphite/bismaleimide (Gr/BMI) laminates due to the presence of multiple embedded fiber-optic sensors. Previous work has shown that small quantities of optical fibers embedded parallel to the loading direction have only a negligible effect on the tension behavior of laminated structures. This work establishes the significance of larger quantities of embedded optical fibers on the tensile and compressive behavior of composite laminates. Experimental strength and modulus data from six test groups of seven tension and seven compression specimens are compared to a control group. All specimens were fabricated from G40-600/5245C Gr/BMI pre-preg tape to form a [°3/9o2/°]s stacking sequence. The quantity of optical fibers embedded parallel to the loading direction was varied in the different test groups. Uniaxial compression testing using an IITRI fixture and uniaxial tension testing were performed in accordance with ASTM Standards. Catastrophic failure was induced by fiber fracture. Results indicate that large quantities of embedded optical fibers reduce the static tensile strength up to 4% and stiffness up to 9%, while reducing the static compressive strength up to 24% and stiffness up to 20%. It is concluded that large quantities of embedded optical fibers result in significant degradation of the compressive strength of laminated composite structures, but do not significantly affect the tensile behavior.

The mechanical and optical interaction of an optical fiber strain sensor which is structurally embedded in a monolithic host material is explored with closed form solutions, finite element methods, and fiber interferometry. A stripped single mode optical fiber is embedded in a four-point bend specimen to investigate both the stress state induced in the specimen by the fiber acting as an elastic inclusion, and the phase-shift induced in the optical fiber by the specimen strain field. Generalized plane strain elasticity solutions are combined with a 3D phase-strain model developed for embedded sensors to predict the strain induced optical retardation of the light propagating in the fiber sensor. The closed form solutions are used in conjunction with finite element solutions to investigate the local stress state induced by the presence of the fiber.

A novel structure for a temperature-compensated fiber-optic strain sensor has been demonstrated both in reflection and in transmission configuration. In the transmission configuration the sensor consists of two identical (sensing and compensating) parts of a highly birefringent bow-tie polarization-maintaining fiber, spliced at 90 deg in relation to their polarization axes. If both parts remain at the same temperature, their total temperature-induced phase retardation will cancel out. The reflection configuration, incorporating the principle of polarization-rotated reflection, also has this structure but in addition requires a reflective mirror deposited at the end of the sensing section and a polarization-preserving coupler to separate the output signal. This sensor can operate using straightforward and cost-effective electronic signal processing and its gauge factor is at least 2 orders of magnitude greater than that of the most sensitive conventional strain gauges.

A strain sensor system for smart structures and skins has been developed. The sensor system uses a two-mode elliptical core fiber sensor and two diode lasers, operating at nominal wavelengths of 750 nm and 780 nm, respectively. The lasers are frequency-modulated such that periodic mode hopping is induced. Data is sampled for each laser mode. A least-absolute-deviation method has been devised for calculating the sensor phases at both wavelengths. The ambiguity arising from the periodicity of the interferometer response is resolved by comparing all possible optical path differences corresponding to the sensor phases at the two wavelengths. The system is able to measure 0.01 of a fringe over a range of ten fringes.

Performance improvements regarding high resolution optical time domain measurements recently achieved are reported. Newly developed segmented fiber sensors and fiber-reentrant-loop techniques involving optical pulse recirculation for the enhancement of resolution have been implemented on small scale flexible structures. A three-segment sensor has been tested in both tension and compression on a clamped cantilever beam. The resolution enhancement concept using fiber-reentrant loops has been verified through experiments conducted with fibers attached to the same beam. Speed limitations are investigated to identify potential hardware modifications necessary to make these techniques feasible in the near future.

The recent growth and interest in manufacturing and designing of health-monitoring, light-weight smart graphite/epoxy composite structures has generated a need for better understanding of the behavior of such materials in a severe structural environment. This paper reports on the damage of a smart structure caused by hail impact and tool-drop impact. Both types of impact were simulated and the resulting damage was investigated. These forms of impact are considered to represent the states of low mass and intermediate velocity and high mass and low velocity. The failure mechanisms of delamination and crack initiation due to successive impact damage were analyzed based on the signal variations produced by optical fiber sensors and microdamage evaluation. Microscopic examination of an impact smart graphite/epoxy composite structure reveals that the initiation of cracks and delamination did not occur in the high local strain near optical fiber. A very important contribution of this work is that two distinct types of single/multimode optical fiber sensors, straight and serpentine, were employed in each impact test. The level of sensitivity from each type of optical fiber sensor was evaluated. Comparison of the sensitivity concluded that the serpentine design of an optical fiber sensor produced significantly better results upon impact with respect to the straight fiber sensor.

An optical fiber, used as a leg of a Michelson interferometer, is cast into a long cylindrical bar of E-CAST F-82 epoxy. The bar can be selectively excited in any of its lowest flexural, torsional, and longitudinal modes. The interferometer is used to detect the resonant modes of the 'free-free' bar and from these modes both the Young's and shear elastic moduli are determined. The complex modulus is determined by measuring the quality factor Q for each resonant mode. The measurement technique is entirely nondestructive, and yields results of two independent moduli with the same transducers.

The relation between a three-dimensional state of strain and the optical phase retardation in a single mode optical fiber is formalized. Neumann's strain optic relations are combined with weakly guiding fiber theory to develop an integral which relates the optical phase shift in a structurally embedded interferometric optical fiber strain sensor to the induced three dimensional strain field. The phase-strain integral is general in that it governs the phase shift in an arbitrarily configured optical fiber sensor experiencing a spatially varying three-dimensional strain field. It is shown that under the correct assumptions Butter and Hocker's [1] equation can be recovered. The phase-strain model is then used to predict the phase shift produced in a straight fiber sensor embedded in a uniaxial tension specimen when the fiber is aligned parallel and perpendicular to the direction of load. The solutions are used to assess the influence of the waveguide dispersion on the total optical phase shift. This process leads to a previously undisclosed waveguide dispersion term which contributes on the same order to the total strain induced phase retardation as does Butter and Hocker's [111 original term. Still, however, waveguide dispersion effects are found to be negligibly small, even in three dimensional loading. Finally, it is shown that in certain cases, the Butter and Hocker's [1] equation and the complete phase-strain model developed herein can give very similar results when both are applied to embedded sensors. This anomaly can lead to the false conclusion that Butter and Hocker's results are equally valid for arbitrary embedded sensors.

Broadband ultrasonic stress waves have been generated and detected in solid resin specimens using embedded fiber elements. Generation was achieved by launching a short, high energy pulse into a large diameter fiber rod having one end embedded in the specimen. Absorption at an absorber location along the length of the fiber produced ultrasonic wave motion through rapid localized heating. The generated waves were detected using a stabilized Mach-Zehnder fiber interferometer with part of one arm embedded several centimeters from the generation site.

It is shown that both impact localization and impact magnitude determination can be achieved in rigid structures with embedded fiber-optic sensors. Impact-generated acoustic signals were detected by means of statistical mode fiber-optic vibration sensors that were embedded in a polymer matrix composite material panel. The 1-sigma error in inferred vs actual impact location, determined via relative-timing measurements, was found to be of about 4 cm.

This work investigates the development of Shape Memory Alloy (SMA) actuators and a fiber-optic Mach- Zehnder interferometric dynamic motion sensor as components of an active vibration control system. The test set-up consisted of a graphite-epoxy flexible cantilever beam with distributed SMA wires and optical fibers attached along both sides. A constant amplitude dead-band control algorithm was used to provide a rate feedback force to actively control transient vibrations. The SMA actuators were also used to demonstrate static shape control. The settling time of the beam was reduced by more than a factor of 24 through the use of the SMA actuators and fiber optic dynamic motion sensor. Analytical models were developed for the integrated structure/actuator/sensor system which helped understand the dynamic effects and the results correlated well with the experimental results. This investigation demonstrated the feasibility ofusing SMA actuators and fiber optic dynamic motion sensor for control of flexible structures. The work described in this paper was sponsored by the Astronautics Laboratory AL(AFSC) as a part of the "Advanced Composites with Embedded Sensors and Actuators (ACESA)" program.

A fiber optic sensor is used to selectively detect a particular vibration mode of a cylindrical rod (strut) made from castable epoxy. Two optical fibers, comprising the legs of a Michelson interferometer, are cast into a long rod (1.33 cm diameter, 35.1 cm length) at a radial distance of 5.3 mm. The free-free rod is then electrodynamically driven to excite the lowest flexural, torsional, and longitudinal modes of vibration. As expected due to the placement of the fibers, the fundamental flexural mode was easily detected and the torsional and longitudinal modes were highly suppressed since the latter modes produced common mode signals in each interferometer leg. At the fundamental flexural resonance of the rod, (158.5 Hz), the correlation between measured transverse peak displacement (14.6 microns) and the measured number of strain induced optical fringes (14.5 at a wavelength of 817 nm) was in good agreement with theoretical predictions.

For sophisticated smart structures where sensing and actuation is distributed over large areas or consists of dozens to thousands of discrete elements, the processing task is computationally intensive. Artificial neural networks offer an opportunity to implement a massively parallel architecture with near real time processing speed and the ability to learn the desired response. This overview of applied neural network processing projects at Florida Institute of Technology includes: processing polarimetric and N-mode strain sensor signals, damage assessment using embedded sensor arrays, and development of electrooptic neural networks

In this paper we discuss the use of a distributed-effect modal domain optical fiber sensor (MD sensor) as a component in an active control system to suppression vibrations in a flexible beam. We integrate the sensor model into the model for a flexible structure. Based on this system model, a control system with a dynamic compensator is designed to add damping to the low order modes of the flexible structure. To verify the modeling procedure a cantilevered beam was instrumental with a piezoelectric actuator and an MD sensor. It is shown that experimental responses closely match simulated responses for both open ioop and closed loop tests.

A twin-core optical fiber sensor was cured into a graphite reinforced PEEK composite tube (6 feetlong and 2 inches in diameter). Aftercuring, the sensorwas interfaced to an electro-optics package to measure static and dynamic strain. During dynamic testing, signals from the embedded sensor were introduced into a control system to drive a piezoelectric, mass-reaction actuator. The damping coefficient of the first fundamental mode ofvibration was improved by more than a factor of 7 for large transient disturbances. Even better performance was observed for low level disturbances. Predictions from an analytic system modelcorrespond well with thelaboratory system and provide insight into optimization of control system parameters. This paper presents the details of the demonstration effort.

Metal-matrix composite structural components' subsurface strain and temperature levels can be ascertained by means of embedded fiber-optic sensors, if these can be incorporated without degradation in the course of composite fabrication. Attention is presently given to a USAF program for the embedding of fiber-optic sensors in Ti-alloy matrix composites, whose severe conventional processing conditions are broken down into less severe steps. It is noted that the sensor fiber used should be of a composition optimized to minimize dopant migration, and should incorporate abusive processing-resistant coatings.

Two silica fiber strain sensors are presented and their capabilities at high temperatures are demonstrated. A silica fiber extrinsic Fabry-Perot strain sensor is demonstrated with nanometer resolution at temperatures up to 975 C. The second silica-based fiber sensor is an intensity-based strain sensor constructed by placing two multimode fibers inside a hollow silica tube. As the gap between the endfaces of the fibers changes, the intensity of the output optical power varies depending on the longitudinal misalignment. SiO2 was deposited using a CVD method to form a cladding on commercially-available sapphire rods. The sapphire optical fiber was used to build a displacement sensor.

An overview of our advances towards the development of fiber optic based Smart Structures will be presented, including our work on the development and testing of an aircraft wing leading edge with a structurally integrated fiber optic damage assessment system, the development and testing of a Fabry-Perot strain rosette, studies of acoustic emission within composites materials detected by embedded fiber optic sensors and a comparison of sensory and vision based structural deformation systems.

Mechanics issues are the principal focus of the intelligent structures research at the University of Maryland. The bulk of this activity is in describing the mechanical and optical interaction between structurally embedded optical fiber sensors and their host structures. Issues currently being addressed are the phase-strain theory for embedded optical fiber sensors, mechanically optimal coatings for embedded sensors, the micromechanics of passive and active instrumentation embedded in monolithic and composite host materials under static, fatigue, and dynamic loading.

A fiber-optic strain rosette is embedded in Kevlar/epoxy. The individual arms of the rosette are fiber Fabry-Perot interferometers operated in reflection-mode with gauge (i.e., cavity) lengths of approximately 5 mm. Procedures for manufacturing the cavities, and bending the fibers, to form a strain rosette are described. Experimental results showing 2D interlaminar strain-tensor measurement are presented. The sensor is also tested as a surface adhered device.

An interferometric fiber optic sensor using ordinary single-mode fibers is developed to detect acoustic emission (AE) for damage assessment of composite materials. This fiber sensor has been embedded in both graphite/epoxy and Kevlar/epoxy composite specimens and used to produce the fast direct correlation of acoustic emission with their concomitant forms of damage, such as matrix crack or material fiber rupture. Applications of the sensor for assessment of damage due to impact and out-of-plane loading are presented. Limitations of the sensor are also discussed.

The university-based program for fiber optic smart-structures technology development presently discussed gives attention to fiber production and embedding techniques, intelligent fiber-incorporation process monitoring, structural strain/temperature/vibration evaluation, and ultrasonic NDE of crack initiation and propagation in smart materials. Also being actively developed are shape-memory alloys, piezoelectric actuators, and real-time software for control and suppression of structural vibration.

The development status of the USAF Astronautics Laboratory's contractual and in-house investigations into 'smart' aerospace structures and skins is evaluated. Plans have been drawn up for the incorporation of smart-structures technologies into future satellite vibration active control systems capable of sensing, evaluating, and damping-out any natural and spurious vibrations. In addition, health monitoring to sense any major degradation of the structure will be incorporated. Attention is given to the major role played by fiber-optics.

A Kalman filter algorithm has been developed to estimate the amplitude, phase and their derivatives of the first four modes of vibration of a composite structural element. The filter input is provided by a single measurement of integrated strain within the beam, which is obtained from an embedded fiber optic interferomethc sensor. The time-consuming step -f miittrn 11vitd 1w intrMiwin iihnntimi1 ptimtinn ci1rnnthm with constant gain.

The goal of this project is to fabricate a chassis/circuit card demonstration entirely 'wired' with embedded and interconnected optical fibers. Graphite/epoxy Standard Electronic Module E (SEM-E) configured panels have been successfully fabricated. Fiber-embedded SEM-E configured panels have been subjected to simultaneous signal transmission and vibration testing. Packaging constraints will require tapping composite-embedded optical fibers at right angles to the direction of optical transmission.

An interferometric fiber optic sensor employing a light emitting diode (LED) as the optical source and two fiber Fabry-Perot interferometers (FFPI) which were fabricated in continuous length of a single-mode silica fiber is analyzed. The performance as a temperature sensor is demonstrated and the predicted behavior is confirmed.

Remote cryogenic temperature measurements can be made by inducing fluorescence in phosphors with temperature-dependent emissions and measuring the emission lifetimes. The thermographic phosphor technique can be used for making precision, noncontact, cryogenic-temperature measurements in electrically hostile environments, such as high dc electric or magnetic fields. The National Aeronautics and Space Administration is interested in using these thermographic phosphors for mapping hot spots on cryogenic tank walls. Europium-doped lanthanum oxysulfide (La2O2S:Eu) and magnesium fluorogermanate doped with manganese (Mg4FGeO6:Mn) are suitable for low-temperature surface thermometry. Several emission lines, excited by a 337-nm ultraviolet laser, provide fluorescence lifetimes having logarithmic dependence with temperature from 4 to above 125 K. A calibration curve for both La2O2S:Eu and Mg4FGeO6:Mn is presented, as well as emission spectra taken at room temperature and 11 K.

Two novel quasi-distributed optical fiber sensor systems are presented using 'Optical Coherence Domain Polarimetry'. This technology utilizes the two polarization axes of a single length of hi-bi fiber as two independent optical paths and detects the positions of the discrete sensing units along the fiber with 'white-length' interferometry. The two systems presented can be classified as intensity and interferometric type, respectively by the way the system measures the relevant measurand amplitudes. Experimental results with the two systems performed as a distributed pressure and temperature sensor, respectively are also given.

We used the analogy between a single-mode optical waveguide and a particle in a quantum mechanical potential well to calculate the effect of adsorption on the outer surface of a fiber on its modal propagation constant by time independent perturbation theory. The results shows that the change in it will be measurable in a straight fiber only if the surface is very close to the core. A very smooth and homogeneous self-developed plasma-etching technique is described to attain this condition. A different approach, which gave more positive results, used bent fibers. In this case, the influence of the adsorbed molecules on the phase of the transmitted wave could be measured, even on the full external diameter of the fiber.