This conference focuses on sensory mechanisms in smart material systems, methods for the instrumentation of integrated smart structural systems, and techniques for the characterization of smart or designed materials that are substantially different from methods used to characterize more conventional materials. In this paper, an overview of the purpose of the conference will be provided describing its place within smart structures research, the conference structure will be discussed, and some of the conference papers of note will be previewed.

The objective of this paper is to describe the performance of optical fiber sensors that have been embedded within polymer matrix composites (PMCs) for more than 15 years. This paper was included in a session concerning the history of sensors for smart materials and structures, and was intended as an overview of early work in this area. Our group at Virginia Tech has been involved in the use of embedded fiber sensors since 1978 when R. Claus, then at the NASA Langley Research Center, helped embed sensors in PMCs to monitor cure and post-cure strain and temperature. Our oldest surviving cross-ply laminate composite specimen with embedded fiber sensors dates from 1982, and was fabricated on campus using a hot platen press. We have recently physically examined this specimen to study possible degradation of the material in the vicinity of the embedded fiber elements, and interrogated the embedded sensors using intensity, modal, interferometric and time-of-flight measurement systems. The basic conclusions of this work thus far are 1) the sensor fibers are still functional, 2) the sensor leads have not been sheared off the specimen after 15 years of use, 3) the composite specimen has not delaminated or otherwise showed signs of degradation, and 4) problems concerning the motion of sensor elements within curing systems, the interconnect problem, and cross-sensitivities that were difficulties in the early 1980s remain key issues today.

McDonnell Douglas began efforts on fiber optic sensor technology in 1977 that resulted directly in the development of a technology base that was used in the mid 1980s to implement fiber optic nervous systems that later would be termed fiber optic smart structures. This paper overviews some of the early history associated with this program as well as a story of how the field of fiber optic smart structures got its name.

This review and orientation to strain gage technology presents the historical development of the bonded foil resistance strain gage as well as modern design and selection criteria for the successful use of strain gages in smart structure technology. Discovered in 1865 by Lord Kelvin, the principle of using resistance change in conductors to measure strain has existed in its present form of the foil strain gage since the late 1950s. Since then, advances in manufacturing and applications techniques have made the bonded foil resistance strain gage one of the most economical and easy to use sensors available for direct measurement and transducer applications alike. Millions are produced and use each year throughout the world. The factors affecting strain gage design and construction include of foil alloy, backing, pattern, and temperature characteristics. Over 250,000 unique strain gage constructions are possible, each particularly suited to a specific set of application conditions. The effect of each of these factors on smart structure performance is considered.

Critical civil and military structures require 'smart' sensors in order to report their strain histories; this can help to insure safe operation after exposure to potentially damaging loads. A passive resetable peak strain detector was developed by modifying the mechanics of a differential variable reluctance transducer. The peak strain detector was attached to an aluminum test beam along with a bonded resistance strain gauge and a standard DVRT. Strain measurements were recorded during cyclic beam deflections. DVRT output was compared to the bonded resistance strain gauge output, yielding correlation coefficients ranging from 0.9989 to 0.9998 for al teste, including re-attachment of the DVRT to the specimen. Peak bending strains were obtained by the modified peak detect DVRT to the specimen. Peak bending strains were obtained by the modified peak detect DVRT and this was compared to the peak bending strains as measured by the bonded strain gauge. The peak detect DVRT demonstrated an accuracy of approximately +/- 5 percent over a peak range of 2000 to 2800 microstrain.

Self-monitoring of static/fatigue damage and dynamic strain in a continuous crossply carbon fiber polymer-matrix composite by electrical resistance (R) measurement was achieved. With a static/cyclic tensile stress along the 0 degree direction, R in this direction and R perpendicular to the fiber layers were measured.Upon static tension to failure, R in the 0 direction first decreased and then increased, while R perpendicular to the fiber layers increased monotonically. Upon cyclic tension, R decreased reversibly, while R perpendicular to the fiber layers increased reversibly, though R in both directions changed irreversibly by a small amount after the first cycle. Upon fatigue testing at a maximum stress of 57 percent of the fracture stress, R irreversibly increased both in spurts and continuously, due to 0 degree fiber breakage, which started at 15 percent of the fatigue life, while R irreversibly increased both in spurts and continuously, due to delamination, which started at 33 percent of the fatigue life. The peak R in a cycle irreversibly decreased, while the minimum R at the end of a cycle irreversibly increased during the first 0.1 percent of the fatigue life, due to irreversible increases in the degree of 0 fiber alignment. R became noisy starting at 87 percent of the fatigue life, whereas R became noisy starting at 50 percent of the fatigue life. For a unidirectional composite, R increased reversibly upon tension and decreased reversibly upon compression in the 0 direction, due to piezoresistivity.

The role of cation transportation under the effect of an electric field on the actuation of the ionic polymeric- platinum composite artificial muscles is studied. Nafion-117 was platinized by different methods including in situ chemical reduction, physical vapor deposition and electroplating. The strips of membranes platinized by different methods were tested under the effect on an electric field either in the air or water. The extent of the strip bending was correlated with the history of the membrane platinization. Three different mechanisms that could explain the bending of ionic polymeric-platinum composite artificial muscles were reviewed. The history of the membrane platinization together with the extent of bending observed when an electric field was applied identified the electrostatic between the membrane and the platinum particles as the main contributor to the actuation capability in the platinized membrane under the effect of an electric field.

A novel interferometric modulation technique for optical thin film testing using GaAs:Cr adaptive photodetectors based on the effect of the non-steady-state photoelectromotive force is presented. The technique needs no special vibroinsulation and automatically adjusts and keeps the operation point of the interferometer. Two different interferometric setups with GaAs:Cr adaptive photodetectors are reported. A modified Mach-Zehnder interferometer with adaptive photodetector is used to study piezoelectric coefficient, d$_33) of the thin film and the influence of the bending motion of the substrate. A two beams polarization interferometer with adaptive photodetector is developed for effective differential Pockels coefficient, r_c equals r₃₃-(n₀/ne)$=3)r₁₃, measurement. It is shown that the proposed two beams polarization technique allows measurement of the Pockels coefficient of thin films with a strong Fabry-Perot effect usually present in ferroelectric thin film. Strong hysteresis effect with a slightly asymmetric form of the hysteresis loop was observed at the dependence of the d₃₃ and r_e coefficients of the PZT thin film from the DC electric field. The values of d₃₃ and R_e are in agreement with known data.

In this paper, principle and system fabrication for concrete health monitoring based on embedded distributed piezoelectric sensing network are presented. Piezoelectric circular plates are embedded in concrete and the electrodes of the piezoelectric plates are wired together according to some order to form a passive stress-sensory network. For a passive piezoelectric sensory device only senses dynamic physical observables, to test the structural state in concrete, which is usually determined by static parameters, the structure is mechanically or acoustically excited to cause impact wave or acoustic wave propagation within concrete. The state parameters will modulate the caused wave and the modulated wave propagation is sensed by the piezoelectric distributed sensing network embedded in concrete. As the piezoelectric distributed sensing network can work without a power supply and outputs of all sensory elements in the network are passed through limited signal channels by addressing them, it is feasible to build up a sensing network distributed over a range of the concrete structure in large dimensions. Compared with the ultrasonic testing technique of concrete, the concrete structural health monitoring technique based on the passive piezoelectric distributed sensing network will exactly determine the structural state by capturing parameters at multiple points and avoid the uncertainty in ultrasonic testing caused by uncertain coupling condition of transmitters and receivers. The varied phase and frequency in sensing output, compared with the exciting signal, reflect the state in concrete. It is required to address one element in the sensing network in a sustained duration to get continuous output for analysis. Here, readout method of continuous output for each element in the sensing network is described in detail.

We report that anew ionically self-assembled monolayer (ISAM) method for thin-film deposition can be employed to fabricate materials possessing the noncentrosymmetry that is required for a second order, (Chi) (²), nonlinear optical response. Using several different commercially- available polyelectrolytes and additional precursors designed and fabricated in our laboratories, we have produced ISAM nonlinear optical thin films with (Chi) (²) films self-assemble into a noncentrosymmetric structure that has exhibited no measurable decay of (Chi) (²) at room temperature over a period of more than four months. The (Chi) (²) of ISAM thin-films has been examined by second harmonic generation using a fundamental wavelength of 1200 nm. The second harmonic intensity of the films exhibits the expected quadratic consistent with orientation of the chromophobe dipole moment perpendicular to the substrate. We describe the potential application of such NLO thin-film materials in field sensing elements and support instrumentation systems.

Rate controlled sintering (RCS) was applied to nano-sized BaTiO₃ powder compacts. The starting average particle size was measured to be 270 nm. Resulting microstructures were compared to those from constant heating rate sintering (TCS). Specimens were heat treated under a series TCS and RCS schedules until reaching 1350 degrees C. Grain sizes and porosities were compared using digital analyses of SEM microstructures. The more rapid TCS and RCS rates resulted in smaller average grain sizes and greater volume percentages of porosity. Specimens were also sintered under a constant RCS rate to successive degrees of densification. A dramatic increase in average grain size was observed between shrinkage of 18.7 percent, and 19.1 percent. This excessive grain growth was attributed to the final stages of RCS heat-treatment where specimens were exposed to elevated temperatures in the range of 1250-1350 degrees C. These immoderate temperatures resulted from the RCS control algorithm's response to the specimens having reached near or complete termination of densification where they could not follow a rapid setpoint densification rate.

Conventional ultrasonic Lamb wave testing of materials often relies on the efficient generation and detection of a single Lamb wave mode that propagates in a relatively non- dispersive region of its characteristic. Such an approach allows identification of defect sites within the material by looking for distinct pulse reflections in the time domain. The fundamental symmetric mode is often utilized due to its low dispersion and relatively high velocity. However, this mode is restricted to a low frequency-plate thickness product where its wavelength may be several tens of millimeters with obvious implications for defect sensitivity. An alternative approach to single mode excitation is to utilize broadband Lamb wave measurements. Since a wider portion of the Lamb wave spectrum is now excited it is anticipated that defect resolution could be enhanced. We have investigated the propagation characteristics of ultrasonic Lamb waves in thin plates of metal and composite structure. Ultrasonic generation and detection was accomplished using a combination of non- contacting optical techniques, material integrated optical fiber sensor and conventional piezoelectric sources. Utilizing the broad optical techniques, material integrated optical fiber sensor and conventional piezoelectric sources. Utilizing the broad temporal and spatial bandwidth of a pulsed laser source combined with a point detector, it was possible to simultaneously measure the dispersion characteristics of several propagating Lamb wave modes in both aluminium and carbon fiber composite plates. An alternative approach to signal interpretation was undertaken by developing a NARMAX model of composite plates. An alternative approach to signal interpretation was undertaken by developing a NARMAX model of Lamb wave propagation in a sample plate and correlating properties of the model to damage in the samples. A possible extension to this techniques using a modulated laser diode for acoustic generation is also described. It is anticipated that these related techniques could be used to identify the presence of defects in addition to material ageing effects in composites.

A high frequency NDE technique has been under investigation at the Center for Intelligent Material Systems and Structures. Physical changes in the structure cause changes in the mechanical impedance. Due to the electromechanical coupling in piezoelectric materials, this change in structural mechanical impedance cause a change in the electrical impedance of the piezoelectric sensor. Hence, by monitoring the electrical impedance and comparing this to a baseline impedance measurement, we can determine when structural damage has either occurred or is imminent. However, there are still basic research issues that need thorough investigation before full-scale development and commercialization can take place. Included in these is the effect of temperature on this impedance based NDE technique. Since piezoelectric materials exhibit strong temperature dependency and change in temperature results in marked changes in the structural dynamic responses, any variation that is associated with a change in temperature may be confused as damage. In this paper we analyze temperature effects on the electrical impedance of piezoelectric materials and the structures. We have used an empirical approach due to the complexity of the thermo-electrical- mechanical constitutive models for piezoelectric materials. Through the experimental investigations, it was found that a change in temperature modifies both the magnitude and phase of the electrical impedance of the piezoelectric sensors. A computer algorithm was developed which incorporates temperature compensation into our health monitoring applications. This compensation technique minimizes the effect of temperatures on the electrical impedance of piezoelectric sensors bonded on the structure, in the range from 80 to 160 degrees Fahrenheit. In this paper, we show how it is applied successfully to a bolted pipe structure.

A nondestructive identification system for diagnostics of rubberlike materials has been developed. The system is based on the simultaneous contact impedance and compliance test conducted at two different sampling frequencies which correspond to the dynamic and quasistatic loadings, respectively. Measurements are taken by a piezoactive contact impedance tester which is a standing wave device registering the leakage of acoustic flow from the actuator via indenter into loaded specimens. Lame's elastic constants, density and rheological parameters of specimen are well correlated with a change in the output signal obtained from the sensor. Experimental data are processed by means of digital filtering, autoscaling and polynomial fitting. The LP filtered data, which correspond to the transient period of loading, are fitted with the dynamic model consisting of several Voigt's elements connected in series. Furthermore, the non-filtered data sampled at high frequency, which correspond to the quasistatic loading, are compared with the database.

This paper reports the research of a Fiber Optic Corrosion Sensor (FOCS) fabricated by electroplating an Fe-C alloy film onto an optical fiber core within the sensing region. Fabrication of the sensing film involves removal of the cladding, metallization of the optical fiber core, and electroplating of the Fe-C alloy layer. The initial results show that the sensor output power increases by about 35 percent when the film is corroded with the sensor of the film passivated, and that the time taken to rise to maximum output power is prolonged by almost 3 times, compared with that for non-passivated film. These result demonstrate the feasibility of using optical fiber corrosion sensors for monitoring corrosion of steel in civil structures.

The combination of chemically sensitive, swellable polymer materials with novel optical fiber cable designs to transduce the swelling activity into microbend loss enables a simple yet powerful sensor to be produced. Interrogating such cables with standard optical time domain reflectoctrometry (OTDR) instruments allows particular chemicals of interest to be detected and located along a cable which may extend to several kilometers. We report here on a sensor cable which uses a water swellable material, a hydrogel, to detect positions of water ingress, relative humidity level or pH value. In direct water ingress tests, wet sensor lengths as small as 5 cm in several hundreds of meters have been detected using conventional OTDRs. Following a review of the sensor design, we present the results of an investigation of the mechanical interaction between the hydrogel polymer and the optical fiber within the sensor. The behavior of the sensor is then characterized within environments of different relative humidity levels from 70 percent to 100 percent at temperatures ranging from 0 to 60 degrees C. The sensor was initially designed for applications within civil engineering but can be applied to a much broader range of measurement requirements, for example soil moisture measurement. We will report details on experimental observations on concrete cure within reinforcing tendon ducts and soil humidity measurements within different soil types.

A fully remote all-optical technique for the generation and detection of ultrasonics in epoxy material. A sensitive wide-bandwidth optical fiber interferometer is used to remotely detect the arrival of these waves at the rear surface of the material. By monitoring the arrival time of these waves, the acoustic velocity can be determined. The technique has been used to monitor the acoustic velocity throughout the complete curing cycle of a rapid cure epoxy material.

Advance fiber reinforced composites are used extensively for primary and secondary load-bearing applications. In general, the resin systems used in the manufacture of fiber reinforced composites are multi-component. In other words, they generally contain a resin, hardener and a catalyst. The thermal, mechanical and chemical properties of these materials are strongly influenced by the extent of mixing and the cure schedule which is used to cure the resin. This paper presents preliminary results in the deployment of a static resin mixer system to ensure efficient mixing of the resin system. This paper also reports on initial result on the feasibility of using a simple optical fiber-based sensor system to (a) infer the extent of mixing in the multi- component resin, (b) monitor the chemical integrity of the resin. Differential scanning calorimetry was also use to assess the influence of mixing efficiency on the enthalpy of the cure reaction. The technology reported here has significant implication to composite manufacturing processes which involve the use of resin bath. The contribution of the proposed technology to Clean Technology is also discussed.

In this paper, the nondestructive, nonlinear distributed- strain sensing technique known as Brillouin Optical Time Domain Reflectometry (BOTDR) is described. The strain distributions along optical fibers wound in precision configurations are measured and analyzed using a recently developed commercial-off-the-shelf BOTDR system. Determinations of sensitivity and spatial resolution are made for the instrument. Measurement comparisons to an analytical model are made for the wound fiber packs. Characterizations of measurements of certain geometric configurations and deformity phenomena are expressed. Assessment of the applicability of the technique to reliability predictions for precision-wound fiber optic dispensers and fiber optic gyroscopes is made.

Microelectromechanical systems or MEMS are miniature devices that have several advantages over conventional sensing and actuating technology. MEMS devices benefit form well developed integrated circuit production methods which ensure high volume, high yield processes that create low-cost sensors and actuators. OPtical fiber interconnected MEMS will provide new functionality in MEMS devices such as multiplexed operation for distributed sensing applications. This paper presents approaches in optical fiber to MEMS interfacing and some preliminary results.

This paper describes a novel optical fiber microbend sensor architecture which may be utilized in distributed and quasi distributed measurement. The actual sensor element is graded index multimode fiber coupled to the measurand field through the usual microbend inducing structures. However, the feed to the sensing section is through a single mode fiber spliced to the multimode fiber to ensure that only the lowest order spatial mode is launched. Similarly the receiver is also coupled to the sensing element through a single mode fiber. The single mode within multimode fiber propagates with minimal mode coupling with source to receiver losses of typically 0.5 dB for short sensor ranging to approximately 0.3dB per each additional kilometer of sensing fiber. The sensitivity of this structure to microbend induced losses has been thoroughly characterized. Typically the optical power loss for a given microbend structure and force is about three to six times higher in this architecture than for conventional fully mode filled microbend sensor. Further since the microbend operates only on the lowest order mode of the sensor fiber operation of the sensors does not depend on initial modal distribution in sensing fiber. The structure is also almost totally insensitive to the macrobend induced losses and allows a variety of novel designs in microbend inducing structures. Additionally, the use of spatial mode filters allows effective control over concatenation effects that are common in microbend sensors.

A developed wavelength scanning fiber-optic interferometry has been studied in this paper. Using this method for distance/displacement measurement, we adopt a tunable external cavity semiconductor laser to simultaneously illuminate two Fabry-Perot interferometers, one as the sensing interferometer, the other as the reference interferometry. We analyze the characteristics of the scanning source and the interferometric signals, then illustrate the limitation of the measurement accuracy and resolution in terms of theory. The experimental results show that the accuracy of 0.05 micrometers and resolution of 0.01 micrometers are achieved, in the range of 1 mm.

We report test results using optical fiber sensor to measure dynamic strain and temperature on ceramic-matrix composite (CMC) specimens at temperatures up to 600 degrees C. For strain sensing we are employing extrinsic Fabry-Perot interferometric strain gages fabricated with gold-coated optical fibers and attached to the CMC specimens using high- temperature ceramic adhesive. For temperature measurements, specially fabricated Bragg and long-period grating sensors are being employed.

We will describe the construction of a fiber optic strain gauge rosette (FOSGR) using four in-line fiber Bragg gratings (FBGs). These FBGs are formed into a triangular- shaped loop structure. The first three FBGs are orientated at 0 degrees, 60 degrees, and 120 degrees. The final grating, which is at the end of the loop, is isolated from strain and acts as a temperature sensor. Using this independent temperature sensor, the temperature cross sensitivity of the strain sensing gratings can be factored out.Moreover, the inclusion of temperature sensing allows this optical strain gauge to be used on any structural material with arbitrary thermal properties. This FOSGR has been attached techniques. This paper describes experimental results obtained from a FOSGR that has been mounted on an aluminium test piece, which has been subjected to both compressive and tensile loads. Also shown are the temperature characteristics of a FOSGR.

In this paper, we demonstrate long-period grating fiber sensors incorporated with fiber Bragg grating demodulation. The sensor exhibit high sensitivity to temperature and strain and are able to respond to dynamic measurands in real-time. The simple demodulation makes long-period grating fiber sensor useful for strain and temperature sensing applications.

We report on the experimental demonstration of a switched frequency domain technique for multiplexing of fiber Bragg grating sensors using subcarrier intensity modulation. The technique can be used to address fiber Bragg grating sensor arrays consisting of multiple branches and is capable of multiplexing fiber Bragg gratings of either identical or different Bragg wavelengths. The crosstalk between sensors was found to be negligible.

The development of a new fiber Bragg grating (FBG) multiplexing technique is reported. The technique is based on a special 2D spectrometer and is potentially capable of interrogate near 2000 FBGs at high speed and in a random fashion, i.e. each FBG in the sensor array can be addressed at different sample rates ranging from 0 to several tens of kilohertz. The operating principle of the technique is presented with its potential performance indicators analyzed by theoretical calculation and computer simulation. Preliminary experimental result using an off-the-shelf spectrometer are also included in the paper.

Multiaxis fiber grating based strain sensors can be formed by writing one of more fiber gratings onto polarization preserving birefringent optical fiber. This paper describes these fiber sensors, how they are characterized and efforts that are being made to make their utilization practical.

Due to the small dimensions of the adhesive layer, the high non-uniformity of the strain field and the non linear elastic behavior of the adhesive material, the strain distribution at an adhesive joint can be predicted by FEM, but can not be experimentally obtained with classical approaches; only non standard procedures like Moire interferometry, or special artifacts like KGR extensometers may afford some insights on the behavior of the adhesive. Due to their small size, ensuring low perturbation of the strain field, and their innate ability to measure strain and strain gradient along the sensor, fiber Bragg gratings offer a good opportunity to solve this problem, and it is a good example of situations that may benefit from these new sensors. Fiber Bragg gratings may be placed or at the interface, within the adhesive layer, or embedded at the adherents, if these were made of composite material. Tests may be run at different temperatures, changing the adhesive characteristics from brittle to pseudoplastic without additional difficulties. When loading the joint, the strain field is obtained by analyzing the distorted spectrum of the reflected light pulse; the algorithm for doing it has already been published. A comparison with theoretical results is done, and the validity and utility of these sensors for this and similar applications is demonstrated.

Interferometric noise in fiber optic grating sensors is investigated. Interference between signal wave and reflected waves causes signal fluctuation in the output which limits the wavelength detection accuracy of the sensing system. The measurement error limited by interferometric noise are calculated for both reflective type and transmission type sensors.

In this paper, we present the experimental result of the effect of bending an over-written long-period fiber grating (LPG) on the transmitted power spectrum and total transmitted power of a light-emitting diode (LED). It is found that the total transmitted power through the LPG decreases linearly with bend curvature within the range from 0 to about 0.001/mm. The error for determining the bend curvature due to the actual non-linearity of the sensor is estimated to be +/- 2 X 10^-5/mm. Therefore, a simple power measurement scheme using an embedded LPG can be employed to measure the static bend curvature and dynamic process of structures.

Fiber optic distributed sensors based on Brillouin scattering can measure strain and temperature in arbitrary regions of a sensing fiber. The fiber optics group at the University of New Brunswick has recently developed an automated system for strain measurements in a distributed sensing system. Under a computer control program, strain measurements are taken using Brillouin Optical Time Domain Analysis. The computer takes a series of measurements of Brillouin loss in the fiber as a function of the frequency difference between the two lasers in the system. By fitting the returned data to a predetermined model, accurate determination of the Brillouin frequency and hence strain in the fiber can be made. An experiment was conducted to test the sensor system in which fiber was stretched by use of dead weights hanging on a system of pulleys. Determination of strain to within 17 (mu) (epsilon) was realized. Spatial resolutions of better than 1 m were obtained through standard BOTDA methods and resolutions of better than 500 mm were realized using our compound spectrum analysis method.

Long-range distributed strain and temperature measurements along an optical fiber is presented, using a novel optical sensor based on stimulated Brillouin scattering. The optical effect only depends on the fiber material, so that the bare fiber itself acts as sensing element without any special fiber processing or preparation. The sensor accuracy is +/- 1 degree C for temperature and +/- 20 (mu) e for deformation. The spatial resolution is 1 meter and the sensor range is more than 20 km. Successful monitoring of a concrete dam element has been performed using an embedded standard cabled fiber. The temperature dynamics of lake waters have been also observed by simply laying a cable over the lake bed.

A distributed strain and temperature sensor system based on the Brillouin loss principle has been constructed. Computer control software has been developed to automate the measurement process for this system. This paper describes the data processing performed by the software and the rationale behind its development. Several methods of measuring Brillouin loss signals are described with a discussion of the merits of each. Results from a simple experiment are presented to demonstrate the capabilities of the automated system.

This paper reports on a novel optical fiber sensor configuration for conducting simultaneous strain and temperature measurements. The sensor consisted of an optical fiber-based extrinsic Fabry-Perot interferometer (EFPI) with an integrated fiber Bragg grating (FBG). The FBG was located within a glass capillary which housed the EFPI sensor and was thus in a strain-free condition. The FBG is primarily sensitive to temperature, while the EFPI was sensitive to both strain and temperature. The integrated FBG/EFPI sensor was embedded in a carbon fiber reinforced composite and evaluated. The standard deviation of strain measurement was 36 (mu) e in the range 0 to 1200 (mu) e, and the temperature measurement had a standard deviation of 3.5 degrees C in the range 30 degrees to 70 degrees C. The thermal expansion of the cross-ply composite was investigated and was found approximately 4.05 X 10^-6 degrees C.

A novel technique for simultaneous measurement of static- strain, temperature and vibration for structural health monitoring is demonstrated using a wavelength-multiplexed in-fiber-Bragg-grating (FBG)/fiber-Fabry-Perot- interferometer (FFPI) sensor system, which combines the advantages of both FBGs and fiber-optic low-coherence interferometry. An experimental system, including two 1m long FFPIs with central wavelengths of 1531nm and 1534nm and a FBG with a central wavelength of 1555nm, is demonstrated. A static strain resolution of better than 1 (mu) (epsilon) , a temperature sensitivity of 0.1 degrees C and a vibration amplitude sensitivity of better than 1nm/(root) Hz have been obtained. The system cross-talk measured is less than -50dB.

This paper reports on a novel optical fiber-based sensing scheme for conducting simultaneous strain and temperature measurements. The sensor design involved the use of an extrinsic Fabry-Perot interferometric strain sensor and a rare-earth doped fiber fluorescence decay-time based temperature sensor. The combined sensors were embedded in a carbon fiber reinforced composite system and evaluated. The feasibility of using this embedded sensor configuration for simultaneous strain and temperature measurements was demonstrated.

In this paper, we present results of our automated data acquisition system for temperature and strain measurements in composite materials. The objective is to provide electronic instrumentation in a plate of carbon fiber and to automate the measurement process in order to compare and analyze the results obtained through interferometric sensors based on embedded optical fiber. The acquisition system is developed to operate in quasi-real time and connected to a PC for providing the temperature and strain maps in the material. Details on performance, system design and the results obtained are given.

This paper describes the development of an approach to estimate the applied stress sensed from a set of multimode fiber optic sensors which are laid on the surface of a smart structure. The estimation of the applied stress is based upon the discrimination between speckle patterns produced by different strain signals. Three approaches have been formulated to estimate/classify the applied stress from the speckle patterns: (a) neural network estimation, (b) Markov random field model classification, and (c) signature-based classification. In order to develop the neural network estimator which is trained to output an estimate of the applied strain signal vector, the dimension of the original input speckle vector is first reduced by estimating the entropy of each pixel and selecting the set of pixels which carry the most information in the training set. A statistical based clustering approach is formulated to reduce the dimension further by combining highly correlated pixels in the selected set. In the Markov random field model based approach, a Markovian model for texture is assumed to fit the speckle patterns. The model parameters, as estimated using maximum likelihood techniques, are used in conjunction with a nearest neighbor rule to classify the speckle images. The signature-based classification approach is a method which incorporates both dimensionality reduction and classification directly for the case when the reference speckle images from highly representative strain vectors are available.

A digital signal processing algorithm, Kwon's method, was developed for strain measurement by a passive quadrature demodulated fiber optic Michelson interferometric sensor. The fiber optic Michelson sensor, which is constructed of a 3 by 3 fiber optic coupler, can give the information about the magnitude and direction of the strain of structures. The beating, drifting, and noise, which are caused by the longitudinal strain and the lateral strain of the fiber, bring about the counting error of the phase differences. Kwon's algorithm is based on the reference line crossing count method and resets the reference line during the presence of the signal drifting. The accuracy of the strain calculation was confirmed by the various simulated fiber optic signals with signal beating, drifting and noise. A passive quadrature demodulated 3 by 3 fiber optic Michelson interferometric sensor was bonded on the cantilevered aluminum beam to experiment the strain sensing. The capability of the real-time processing was verified by the real fiber optic signals.

The Swiss Expo 2001 will be held in 2001 on the three lakes of Neuchatel, Bienne and Morat. The main events of this exhibition will take place on four platforms protruding from the seashore to the lake and with dimensions of about 400m X 100m. The covering 'deck' of these platforms will be created using the Tensegrity concept and will be dismantled and recycled after the event. This kind of structures is composed by a network of cables, struts and reinforced membranes. The resulting structural behavior is highly geometrically non-linear and is relatively complicated to calculate and simulate. This pointed to the necessity of extensive testing and permanent monitoring of the structures as well as to the introduction of active elements able to compensate for quasi-static variable loads such as temperature variations, snow on the membranes, successive construction phases and additional weight of scenic elements. This data will be continuously analyzed and, when corrective actions are necessary, hydraulic actuators placed at key locations will optimize the shape and the tensile state of the whole structure. Tests of this concept are now carried out on 1:10 scale models.

Global Positioning System (GPS) technology is an emerging tool for structural deformation monitoring applications. One key technique for achieving millimeter level accuracy with GPS is averaging data over time to reduce effects of the dominant error sources. Smart structure technology incorporating active control mechanisms, requires information about structural deformations in real-time, with minimal latency. This paper discusses the capabilities and limitations of GPS-based monitoring systems to deliver real- time structural deformation data with accuracies at the sub- centimeter to millimeter level. Two types of issues are addressed' system operational performance, and measurement accuracy.

Hidden and inaccessible corrosion in aircraft structures is the number one logistics problem for the US Air Force, with an estimated maintenance cost in excess of $LR 1.0B per year in 1990-equivalent dollars. The Smart Aircraft Fastener Evaluation (SAFE) system was developed to provide early warning detection of corrosion-related symptoms in hidden locations of aircraft structures. The SAFE system incorporates an in situ measurement approach that measures and autonomously records several environmental conditions within a Hi-Lok aircraft fastener that could cause corrosion. The SAFE system integrates a miniature electrochemical microsensor array and a time-of-wetness sensor with an ultra low power 8-bit microcontroller and 4- Mbyte solid-state FLASH archival memory to measure evidence of active corrosion. A summary of the technical approach and a detailed analysis of the KC-135 lap joint test coupon results are presented.

An epoxy-matrix composite with continuous crossply carbon fibers was found to be a semiconductor in the through- thickness direction, with a tunable energy gap of 10^-2-10^-1 eV. The higher the pressure during composite fabrication by lamination, the higher the interlaminar stress and the greater the energy gap, which is the activation energy for electron jumping from one lamina to the adjacent one in the composite. The semiconducting behavior involves the contact electrical resistivity between adjacent laminae in the composite decreasing reversibly with increasing temperature. The concept of optoelectronic and electronic devices made from carbon fiber polymer-matrix composites is provided. Devices include solar cells, light emitting diodes, lasers, infrared detectors and transistors.Thus, a new dimension is added to smart structures and a new field of electronics is born.

We report underwater explosion test results of the effect of shock wave pressure on an optical cable link in a fiber- optic velocimeter. The shock wave induced Doppler frequency shifts in the optical carrier transmitted in an optical cable link were measured. Two experimental configurations were tested: (1) optical cable arranged with its axis parallel to the wave front of the shock wave and (2) optical cable arranged with its axis perpendicular to the wave front of the shock wave. Results from both case indicated moments of signal fading due to dynamic flexing on the optical fiber resulting in optical power loss and polarization changes. In addition, the shock wave front moving along the fiber cable may have generated a dynamic axial strain on the optical fiber and caused a Doppler shift in the optical carrier similar to that from a moving target.

This paper discribes a new method for measuring rotation rate with laser interference technology.The methods of measuring angle before have two limitations.One is that the methods can not measure the instantaneous value of rotation rate,the other is that the accuracy ofrotation rate depends on the measuring accuracy oftime and rotation angle.This method in this paper overcomes these shortages,and fulfills dynamic contactiess angle measurement. This system consists of two elastic bodys that fix two pyramid prisms,a rotary table for giving angle, a laser,an optic measuring system and two photo detector received interference fringes.The beam from the laser is devided reflex beam and transmission beam by pyramid prism.The reflex beam is received by photo detector.The transmission beam is reflexed by pyramid prisms that rotate with rotary table,and is received by another photo detector.By comparing the interference fringes that two photo detector received,we can get Doppler frequency difference z f In addition,we can find the relation of the rotation rate and L f. Theoretical analyses and experimental testing fmd that the faster rotation rate is,the higher measurement resolution of rotation rate is.But the distance of elastic body is limited.So,experiment in this paper determines that the highest rotation rate of rotary table is 4.3rad/s.Moreover,in this paper,we analyse errores and give their values that influence experimental results. Keywords:rotation rate,interference,accuracy,measurement

Composite optical fiber cantilever micromechanical vibrated sensor is considered. Usual quartz fiber is supplemented by crystal sapphire fiber part which have high Q-factor of mechanical oscillations. Sensitivity of the Bragg-gratings with yr-phase shift at the end of the fiber cantilever are discussed. Keywords: fiber, cantilever, mechanical vibrations, fiber grating.

To compete globally in the next millennium, designers of new transportation vehicles will have to be innovative. Keen competition will reward innovative concepts that are developed and proven first. In order to improve reliability of aerospace platforms and reduce operating cots, new technologies must be exploited to produce autonomous systems, based on highly distributed, smart systems, which can be treated as line replaceable units. These technologies include photonics, which provide sensing and information transfer functions, and micro electro mechanical systems that will produce the actuation and, in some cases, may even provide a computing capability that resembles the hydro- mechanical control system used in older aircraft systems. The combination of these technologies will provide unique systems that will enable achieving the reliability and cost goals dictated by global market. In the article we review some of these issues and discuss a role of photonics in smart system for aerospace platforms.