There has been considerable discussion in the technical community on a number of questions concerned with smart materials and structures, such as what they are, whether smart materials can be considered a subset of smart structures, whether a smart structure and an intelligent structure are the same thing, etc. This discussion is both fueled and confused by the technical community due to the truly multidisciplinary nature of this new field. Smart materials and structures research involves so many technically diverse fields that it is quite common for one field to completely misunderstand the terminology and state-of-the-art in other fields. In order to ascertain whether a consensus is emerging on a number of these questions, the technical community was surveyed in a number of ways including via the Internet and by direct contact. The purpose of this survey in the final analysis was to better define the smart materials and structures field, its current status and its potential benefits. Results of the survey are presented and discussed.

Acceptance of advanced composites in both retrofit applications and new construction necessitates quality control (QC)/quality assurance (QA) and in-service monitoring techniques to ensure the performance and serviceability of the composites. A technology that enhances the inspectability of the advanced composites is the particle tagging method. The concept of particle tagging involves embedding micron-sized particles into materials, such as composites, or adhesive layers to make them an integral part of the host material. When interrogated by suitable instrumentation, the embedded particle sensors interact with their host material and generate certain types of measurable signatures. The signatures can be correlated with the material and structural conditions, such as internal stress states, voids, inclusions, state-of- cure, and delaminations. This paper presents a technology assessment of the current and emerging state-of-the-art tagging methods. The objective of this review is to provide a framework for further understanding the physics and mechanisms involved in using the tagging method for in-field nondestructive evaluation (NDE) of composite materials. Information concerning the development of this technique has been obtained from patents, publications, databases within Newman Library at Virginia Polytechnic Institute and State University (Virginia Tech), and previous work and in-progress research at the Center for Intelligent Material Systems and Structures (CIMSS) at Virginia Tech. This paper includes a historic review of tagging techniques, a description of the tagging systems used for in-situ NDE of composites, and experimental results to date. The current and potential applications of this technique are also addressed. The analysis presented in this paper will assist in further efforts to develop the tagging approach that may be used as a new NDE technique for quality assurance of advanced composites.

A general method to monitor the structural integrity of prismatic structures composed of simple isotropic linear elastic materials is described and justified by a detailed mathematical model. This technique is distinct from usual approaches in that a small number of distributed sensors is shown to be sensitive to the presence of damage throughout the entire volume of a structure without determining its location. In addition, this approach inherently operates as a binary sensor thus alleviating the need to collect a large amount of data. Potential extensions to more general structural geometries and materials are also described.

Large, photoinduced refractive index changes on the order 10^-2 can now be achieved in hydrogen-loaded, GeO₂-doped optical fiber. Much of this work has focused on the research and development of Bragg gratings fabricated by exposing the core of fiber to the intensity pattern of two interfering plane waves. Unlike interferometrically formed Bragg gratings, we have exposed optical fibers to a single focused 244 nm laser beam to obtain two Fresnel reflections from each exposure site. Reflectors formed in this way are broadband and have application as markers in OTDR-based distributed sensing systems and as resonant cavities for localized intrinsic Fabry-Perot interferometric strain and temperature sensors.

In this paper a straightforward method is proposed to determine the MPD within a given multimode fiber. The method combines Gloge's mode loss theory with new assumptions about the MPD, as well as a simple experiment which involves measuring the amount of power which survives a series of increasingly sharp bends. Using this technique, the most likely MPD within a fiber can be derived before and after the addition of a sensor.

A new approach to a fully distributed fiber optic sensing system is presented for the first time. This sensing approach will measure the transverse normal strain components, and the axial strain component at any point along the specially designed sensing fiber. This paper presents the theoretical modeling of strain-optic behavior and sensitivity, as well as general system requirements.

One of the goals of smart structures research is the integration of functions such as sensing, actuation and control into structures themselves. Smart structures research activity to date has largely focused on process control or integrated structural health monitoring. Other functions can also be integrated into structures. In this paper, results of an experimental demonstration integrating fluid level sensing with the structural support of a nonconductive tank are reported. The system utilized a composite capacitance fuel probe permanently bonded into the tank walls as a structural support. The probe was connected in series with a resistor and inductive coil forming a resonant RLC circuit. Non-contact excitation of the circuit through the tank wall and measurement of the resonance frequency allowed the fluid level to be determined. The theoretical basis of the concept is provided and compared with experimental results. Potential applications of the technology are also discussed.

Rotating components, such as beams and blades, experience vibrations while rotating. Many blades, such as the ones used in rotorcraft, machine tools and turbines, experience fatigue failure due to the vibration of the rotating blade; therefore, optical sensors could indicate when the strain increases to unsafe limits. This paper describes two techniques of implementing optical fibers on rotating components. The first technique to be examined uses an optical slip ring to transmit the light signal from the rotating frame to a stationary photodetector demodulation circuit, that will determine the strain in the beam. The second technique uses an integrated chip mounted on the rotating frame, that records the signal. The signal recorded from the sensor during rotation is downloaded once the rotation has stopped. Preliminary results from the optical slip ring are presented. These results show difficulties in the alignment of such a device.

A modified design of the extrinsic Fabry-Perot interferometric (EFPI) optical fiber sensor, for complete characterization of multi-component strain fields, is proposed. The novel EFPI includes a sensor head with two input fibers such that the respective reflections from the output multimode fibers end face are in quadrature. Any strain field possessing a component along the line joining the axes of the two fibers causes the initial phase difference to be modulated. The measured changes in phase difference is employed to determine this transverse component of the strain field. Absolute measurements are possible using the AEFPI sensing system. Applications of the modified sensor-head to smart materials and structures are discussed.

Light reflected from chirped fiber Bragg gratings possess both phase and amplitude information which is unique. In this paper we report for the first time the use of the relative phase to determine the strain-induced linear chirp. By measuring this intramodal (chromatic) dispersion induced on the incident wave, the wavelength dependent penetration depth and therefore the strain gradient can be found by using a closed-form approximation. Moreover, the error can be reduced by selecting the appropriate grating properties like the writing induced chirp and the index variation parameters.

By looking at the full reflection spectrum of a Bragg grating strain sensor, one cal tell when strain gradients are presented along the sensing length and estimate their magnitude. Furthermore, if certain conditions of monotonicity of the strain profile along the sensor are met, complex strain profile shapes can be interpreted from the grating's spectrum. We show how this technique can be applied to the study of the strain transfer at the tip of an embedded fiber. A fiber embedded in epoxy was loaded up to 2.5% strain, which induced fiber end-zone damage. The strain profile along the fiber, estimated from the reflection spectrum of the grating, gives details about the failure process which could not be obtained by any other known method.

This paper describes an experimental and finite-element study of the optomechanical response of a diametrically loaded Bow-Tie high-birefringent optical fiber. The thermomechanical finite-element analysis performed in this study uses PATRAN as the solid modeler and ABAQUS as the analysis package. The purpose of the study is to determine the effect of a diametrical load on the optical phase generated by polarimetric sensors as a function of polar angle. This is done in order to understand the behavior of structurally embedded polarimetric sensors, and to determine the effect of load-induced rotation of principal optical axis in a Bow- Tie fiber so as to assess the response of the lead-insensitivity of lead-insensitive embedded polarimetric sensor configurations. The results indicate that the stress concentrations produced by the stress applying parts are responsible for a polarimetric phase sensitivity that is a function of the applied load direction. The results further indicate that the diametric loads do not significantly alter the principal optical axes in the lead-in fiber.

Hidden and inaccessible corrosion in aircraft structures is the number 1 logistics problem for the Air Force, with an estimated maintenance cost of greater than one billion dollars per year. The smart aircraft fastener evaluation (SAFE) system is being developed to detect and characterize corrosion factors in hidden locations of aircraft structures. The SAFE concept is a novel `in-situ' measurement approach that measures and autonomously records several environmental factors (i.e., pH, temperature, chloride) associated with corrosion. The SAFE system integrated an electrochemical-based microsensor array directly into the aircraft structure to measure the evidence of active corrosion as an in-situ measurement without reducing aircraft structural integrity. The long term-payoff for the SAFE system will be in predictive maintenance for fixed and rotary wing aircraft structures, industrial tanks, and fugitive emissions applications such as control valves, chemical pipeline vessels, and industrial boilers. Predictive maintenance capability, service and repair will replace the current practice of scheduled maintenance to substantially reduce operational costs.

The effective guarding of machines against human injury is an enormous problem already with many available solutions. Guarding procedures generally fall into one of three classes: (1) Physical guards which prevent the placing of limbs and personnel in potential crush points. (2) Those devices which require the removal of the human from the device so that they can be activated, e.g. switches for activation. (3) Sensor, such as light sheets which shut down the machinery when the presence of an object, such as a hand, is detected in a hazardous position. We have developed a novel machine guarding technique based on the measurement of electromagnetic field distortion. There are no physical connections between the operator and the machine. The machine guarding is accomplished by merely sensing operator presence/absence through the capacitance change caused by the operator themself. As such we see few easy ways in which such a machine guard may be deactivated.

An important opportunity recently transpired at Northrop Grumman Corporation to instrument an F/A - 18 E/F titanium bulkhead with broad band acoustic emission sensors during a scheduled structural fatigue test. The overall intention of this effort was to investigate the potential for detecting crack propagation using acoustic transmission signals for a large structural component. Key areas of experimentation and experience included (1) acoustic noise characterization, (2) separation of crack signals from extraneous noise, (3) source location accuracy, and (4) methods of acoustic transducer attachment. Fatigue cracking was observed and monitored by strategically placed acoustic emission sensors. The outcome of the testing indicated that accurate source location still remains enigmatic for non-specialist engineering personnel especially at this level of structural complexity. However, contrary to preconceived expectations, crack events could be readily separated from extraneous noise. A further dividend from the investigation materialized in the form of close correspondence between frequency domain waveforms of the bulkhead test specimen tested and earlier work with thick plates.

Experiments have been conducted to assess the feasibility of autonomously detecting crack growth in an aircraft lap-joint using acoustic emission (AE) sensors. In a lap-joint the crack must be detected early and reliably so that a repair can be made at a reasonable cost, and unnecessary inspections can be eliminated. The location of the crack initiation is known to within a few fastener holes, making detection using AE possible. Representative waveforms from cracked lap-joint specimens, as well as results from a simpler specimen are presented. A method for distinguishing crack waveforms from fretting waveforms and noise sources is discussed. A self-organizing map is investigated as a means to identify crack signals. Lessons learned, and suggestions for future work are presented.

This paper describes several alternative solutions to the problem of controlling sound transmission through rectangular plates radiating into an enclosed cavity. The specific problem examined is how adaptive filtering compares with feedback control techniques for reducing the level of sinusoidal disturbances transmitted by the plate. In a smart structure with embedded sensors and actuators, it will be likely that feedback control is the preferred option. The feedback controllers examined are LQG/LTR, H(infinity) , internal disturbance model pole placement, and internal disturbance model LQG. These controllers are compared to the filtered-x LMS technique, which is the standard method of dealing with sinusoidal disturbances for this type of problem. All the controllers are examined using simulation and then implemented on a test structure.

A smart structure based on fiber optic sensor technology is used to provide nondestructive evaluation of fluids. Acoustic Resonance Spectroscopy techniques are used to extract fluid signatures using the proposed smart structure. These signatures are then presented to a nonparametric classifier either in their raw format or in a processed format based on autoregressive modeling techniques. Three types of nonparametric fluid classifiers are examined in this study. The performances of these classifiers are compared and the effects of autoregressive modelling on the classification performance are examined.

The requirements of electronic instrumentation for smart structures are similar to those of data acquisition systems at our national particle physics laboratories. Modern high energy and heavy ion physics experiments may have tens of thousands of channels of data sources producing data that must be converted to digital form, compacted, stored and interpreted. In parallel, multiple sensors distributed in and around smart structures generate either binary or analog signals that are voltage, charge, or time like in their information content. In all cases, they must be transmitted, converted and preserved into a unified digital format for real-time processing. This paper will review the current status of practical large scale electronic measurement systems with special attention to architectures and physical organization. Brief surveys of the current state of the art will include preamplifiers and amplifiers, comparators and discriminators, voltage or charge analog-to-digital converters, time internal meters or time-to-digital converters, and finally, counting or scalar systems. The paper will conclude by integrating all of these ideas in a concept for an all-digital readout of a smart structure using the latest techniques used in physics research today.

We recently disclosed that the shape of the intensity spectrum reflected from a Bragg grating can serve as an indicator of strain gradient across this grating. However, in principle, to map the strain field distribution, both the intensity spectrum (Iota) ((lambda) ) and the phase spectrum (Psi) ((lambda) ) are needed. In this paper, we propose a method based on phase measurements. It relies on the correlation between the wavelength-dependent penetration depth of light in a nonuniform (chirped) grating and the phase delay of the reflected wave. If we know that the nonuniform strain field to be measured is a monotonic function of position along the grating (which is the most practical case), then the measured grating phase-spectrum (Psi) ((lambda) ) can be used to evaluate the penetration depth and consequently the distribution of the grating optical pitch-lengths, i.e. the strain distribution over the grating. Theory reveals that the phase slope spectrum d(Psi) ((lambda) )/d(lambda) induced by a grating can directly yield the strain field profile. For example, a linear phase slope spectrum indicates a linear chirp in the grating due to a constant strain gradient; and the strain gradient can be easily obtained from the dispersion constant d²(Psi) ((lambda) )/d(lambda) ². The validity of this method is also discussed in this paper.

Civil underground works offer good opportunities for fiber optic displacement sensors. This communication presents an overview of possible applications in this field. Low-coherence interferometry appears to be the ideal technique for the long-time monitoring of such structures. The principle of the portable measuring system developed at our institute and based on this technique is presented along with different possible multiplexing techniques. The presented experimental results confirm the precision of 10 micrometers found under laboratory conditions and the immunity to drift for at least one year. A description of the techniques used to install optical fibers in anchorage cables and piles, as well as the lessons learned from these field tests are also presented.

Optical fiber Bragg gratings, first demonstrated by Hill in 1978, will see increasing deployment as sensors in smart structure applications due to their minimal insertion loss and small profile. Bragg grating sensors provide narrowband reflections from a periodic index discontinuity. Strain and temperature changes occurring on the grating induce a shift in the reflected wavelength. Currently, the most basic Bragg grating demodulation system consists of monitoring the reflected wavelength with an optical spectrum analyzer, but use of these devices is limited due to high costs and poor frequency response. In this paper, we propose using dual-mode optical fiber as an intensity based interrogation system. The modal birefringence of the fiber is wavelength dependent and therefore, the phase relationship between two modes changes as a function of the Bragg grating reflected signal. We describe the optical architecture for this demodulation system and provide experimental data for temperature and vibration response. It is predicted that with an optical source operating at 300 microwatts, less than 1 microstrain resolution is achievable.

Fiber Bragg grating (FBG) arrays have provided the Smart Structures community with a powerful means for real-time and absolute point measurements of strain throughout extended structures. The mechanical properties of these optical fiber sensors uniquely suit them for unobtrusive and reliable incorporation into composite materials, while wavelength encoding of the measurand enables independent interrogation of many devices distributed along a single fiber. In view of recent progress towards practical and economical FBG array fabrication via the computer-controlled in-line writing process during fiber draw, there is a complementary need for economical instrumentation which makes efficient use of the reflected signals. This is an important issue, especially since the reflectivity of narrow-line, type I gratings produced during fiber draw is typically less than 5%.

Extrinsic Fabry-Perot interferometric (EFPI) sensors have previously been demonstrated for relative strain and temperature measurements for smart structure applications. Inherent difficulties in the signal processing of these devices has created the need for absolute measurement capabilities. In this paper, we present an absolute measurement technique based upon white-light interferometric path matching. The system matches a reference gap to the sensing gap of an EFPI. When the difference of these two lengths is within the coherence length of the source, an intensity envelope is created in the system output. Determination of the corresponding path mismatch indicates the size of the sensor gap and hence strain can be determined. This measurement technique is capable of multiplexing an array of EFPI sensors and data will be presented demonstrating four multiplexed devices. Theoretical considerations for system optimization are also presented.

In this paper we have investigated the cross-talk and noise issues that arise in serial multiplexing of In-Line Fiber Etalon (ILFE) strain sensors using three different topologies. First, the broadband source is numerically modeled to preserve all the artifacts in the line structure. Second, the effect of beam expansion in the non-guiding ILFE cavity is investigated. The two models are combined to calculate the fringe contrast, signal and background power, the shot-noise and cross-talk noise in the system. Also the signal extinction is studied as a function of mismatch between the ILFE gagelength and path-matching interferometer gagelength. Finally, suggestions are made to enhance the fringe contrast in ILFE based systems.

A nine element fiber Bragg grating sensor array is demonstrated utilizing a combination of wavelength and time division addressing techniques. This system utilized a pulsed broad-band source and a scanning Fabry-Perot element to recover sensor strain information from each of the FBG elements. By combining two multiplexing techniques, the potential number of FBG sensor elements that can be addressed is significantly increased.

We describe an interrogation technique for fiber Bragg grating sensors based on serially arranged set of receiver FBG elements, which are used to decode strain induced wavelength shifts of six FBG sensor elements. Each of the receiver elements is wavelength-matched to one of the FBG sensor elements allowing for wavelength discrimination and passive demodulation of individual sensor strain information.

An optical fiber strain-rate sensor based on in-line Doppler velocimetry is developed using a 3 X 3 coupler with a large path imbalance Mach-Zehnder interferometer for passive demodulation of Doppler wavelength shifts. A model is developed that relates the strain-rate in an optical fiber to the corresponding Doppler wavelength shifts. The strain-rate obtained by differentiating strain from a Michelson strain sensor mounted on the PZT cylinder shows good agreement with the strain-rate directly obtained from the Doppler-based strain-rate sensor.

The effect of prescribed delamination on natural frequencies of laminated composite beam specimens is examined both experimentally and theoretically. Delamination in composite laminate is of particular interest because they can cause catastrophic failure of the composite structure. One consequence of delamination in a composite structure is a change in its stiffness. This change in stiffness will degrade the modal frequencies of the composite structure. Modal testing of perfect beam and beams with different size of delamination is conducted using PVDF sensors and piezoceramic patch with sine sweep actuation. Model testing of beams is also conducted using PVDF sensors and instrumented hammer excitation. The results of instrumented hammer excitation and piezoceramic patch excitation are discussed. The experimental modal frequencies are compared with the results obtained using a simplified beam theory. Also, backpropagation neural network models are developed using the results from the simplified beam theory and used to predict delamination size. The effect of learning rate and momentum rate on neural network performance are discussed. Modal frequencies can be easily and accurately obtained with piezoceramic patch excitation and PVDF sensing. There is good agreement between modal frequencies from modal testing and those from the simplified beam theory. The developed neural network models successfully predict delamination size. Prediction errors varied from 0.25% to 19%.

The use of neural networks for recognizing and measuring objects is proposed. A review of the state-of-the-art of neural network systems and communicating sequential process formalizing was made, in order to select network type and topology. After a standard image acquisition step, the developed system prototype was implemented on parallel architecture, based on Transputer network following the same distribution of the nodes, that constitute the activation functions typical of all types of neural network. The realization of the prototype was carried out in C++ language,l for its portability to different platforms. Then, the developed algorithm was optimized by the translation into the original OCCAM language. The study constitutes the first effort of a research activity, intending to link recognition and measuring of objects to vectorial imagery, using different image acquisition systems (including those working in the infrared band) and the developed system prototype, with software dedicated to industrial drawing applications.

The goal of this work is to perform physical fault detection on a multicomputer by using interferometric optical sensors to measure applied strain on interconnection data links. The main interest in research of this sort is to provide reliable optical interconnections between computers that may operate in harsh environments. An example of such a system would be found on an aircraft or other military vehicle which, by nature of its function, is expected to undergo some form of structural trauma in its operating lifetime. This work will examine an application of the problem of dynamic fault tolerance, and provide an example of a practical solution. Damage will be inflicted upon a data link of an operational hybrid communication/sensing multicomputer and detected with the use of optical sensors. It will be shown that the multicomputer can reroute all communications functions and reallocate computational tasks around this faulty data link with no loss of information or functionality. It will also be shown that the multicomputer can accurately convert the output of an extrinsic Fabry-Perot interferometer into a strain value and compare this calculated value to a predetermined maximum strain value, by use of an algorithm that continuously calculates strain based on the sinusoidal output of the interferometric sensor

In this paper, an efficient rapid convergent control algorithm will be developed and will be compared with other adaptive control algorithms using an experimental active noise control system. Other control algorithms are Widrow's finite impulse response adaptive control algorithm, and a modified Godard's algorithm. Comparisons of the random noise attenuation capability, transient and convergence performance, and computational requirements of each algorithm will be made as the order of the controller and relevant convergence parameters are varied. The system used for these experiments is a test bed of noise suppression technology for expendable launch vehicles. It consists of a flexible plate backed by a rigid cavity. Piezoelectric actuators are mounted on the plate and polyvinylidene fluoride is used both for microphones and pressure sensors within the cavity. The plate is bombarded with an amplified random noise signal, and the control system is used to suppress the noise inside the cavity generated by the outside sound source.

In this paper we describe the non linear error signal extraction, which is an efficient and robust technique for the automatic control of optical interferometers. It represents a global solution to the problem of the longitudinal error signal extraction also when the uncontrolled optical system spans hundreds of fringes. This technique basically uses classic modulation techniques (phase modulation, mechanical modulation, etc.), but extends their range of validity using also the information available at the output photodiode. We have digitally implemented such technique following modular hardware and software architectures. In fact, the hardware consists of commercial VME boards for A/D and D/A conversion and processing, housed in standard VME crates, while the software algorithm is written in standard C language for portability and easy integration within digital control architectures.

This paper describes a comparative study of in-situ cure monitoring and cure modelling by three methods: (a) evanescent wave spectroscopy, (b) refractive index change, (c) near- infrared spectroscopy. Optical fibers were embedded into aerospace epoxy resins during the manufacturing process of the composite. The cure characteristics were then tracked in real- time during the processing of the material via evanescent wave interaction. This technique is based upon monitoring of characteristic infrared absorption bands of the resin system to find the concentration of the epoxy and amine hardener as a function of cure time. Hence this technique is suitable for on-line process monitoring and optimization. Results obtained from the optical fiber sensors were used to model the curing behavior of the resin system. The results were compared with near-infrared spectroscopy and differential scanning calorimetry experiments carried out under similar conditions. The feasibility of utilizing refractive index changes to monitor the extent of cure has also been demonstrated.

In order to reduce costs and develop new functionalities of continuous fiber reinforced metals, fiber/matrix interphase is developed by the original low cost fabrication method. Optical fiber can be embedded in stainless steel fiber or SiC fiber reinforced aluminum composites by the interphase forming/bonding method. Deterioration of optical fiber during composite fabrication by this method is small enough to connect it to external fibers leading to LD light source and power meter with rubber joints. The composites can be bent without fiber fracture as secondary forming by local melting of the interphase. At lower temperatures, if the SiC fiber/matrix interface adhesion is improved, fracture of SiC fiber is considered to become earlier than that of optical fiber, where it will work as health monitor.

A phase-conjugate mirror was used in the amplifier section of a KrF excimer laser to fabricate fiber Bragg gratings in single mode Ge-doped silica core fibers with an uncompensated interferometer. Reflectivities as high as 26% and FWHM linewidths of 5.5 GHz at 0.817 micrometers were obtained in as-drawn fiber with a single pulse exposure.

This paper reviews the progress of an ongoing research program at Thomson-CSF and Bertin & Cie which addresses an optical fiber system dedicated to the assessment of impact induced damages on airborne multilayered structures. The method is based on the use of embedded high birefringence optical fiber sensors and distributed white light interfero-polarimetry. The first part is devoted to the transduction process efficiency within optical fibers depending on the applied force intensity, direction versus the fiber eigen axes and the interaction length. To understand the behavior of these optical fibers and calibrate the detection system, experiments have been conducted on elliptical core fibers, `bow-tie' fibers and side-hole fibers and showed a wide range of available sensitivities. The second step is related to the inclusion of optical fibers in a sandwich structure representative of an airborne dome, and composed of foam between glass/epoxy composite skins. Different designs of grooves in the foam and tube sheathings have been investigated to support and protect the optical fiber. Impacts have been performed on the structure in the 1 to 10 Joules energy range. Experimental impact location and energy measurements have been achieved for a variety of stress fields.

A method of determining the location and extent of impact induced damage in isotropic plates is investigated numerically and experimentally. A computer simulation of impact to this model plate provided information in the form of frequency response data and strain traces. These data sets were inputs into two different backpropagation neural networks. The first neural network uses FFT of strain data, which is split into real and imaginary parts and integrated over the first ten harmonic natural frequencies for each of four sensors (8 inputs into neural network). This particular network has been trained on a set of 63 simulated impacts, with the network solving for the location of impact sites within an average of 8 millimeters per impact. The second network uses the same FFT of strain data (8 inputs), but additionally uses the impact coordinates as inputs (total of 10 inputs). The network has been trained on a set of 800 simulated impacts, and solves for magnitude of the impact events with an average error of 7.08%. Experimental impact location determination has been accomplished by using a set of four strain gage sensors mounted in the corners of the isotropic aluminum plate. Data from these sensors were post processed as mentioned above, and used to train a backpropagation neural network. A twenty impact subset of a 72 impact grid was trained to locate impacts with an average RMS error of 1.019 radial centimeters.

Location of impacts on an anisotropic polymer matrix composite panel was demonstrated by using a neural network to process the outputs of embedded fiber optic strain sensors. Three extrinsic Fabry-Perot interferometer sensors were embedded in a graphite/bismaleimide composite with a unidirectional lay-up. The location of an impact can be calculated directly by triangulation from the difference in arrival times of the impact-generated stress waves at the embedded sensors. A data set of 132 experimental results was generated by impacting the panel at evenly spaced locations, and measuring the time differences. A back-propagation neural network was simulated using commercial software, and the data set was used to train the network. Following training, the network was capable of determining the location of randomly located impacts with an accuracy of a few centimeters. These results were comparable to the accuracy achieved for impact location on an isotropic aluminum plate, indicating that the neural network performance is independent of material anisotropy.

Filament wound tubes are currently being used extensively in service because of their superior specific properties and the relatively simple manufacturing technique involved in their properties. However, the reinforcing fibers can suffer from low velocity impact damage (approximately 10 ms^-1) during service. Such damage can result in poor post- impact properties which in certain applications can reduce the burst strength below safe working levels. This paper discusses the use of optical fiber sensors, embedded during the filament winding process, to provide information on specified levels of impact damage incurred by the tube during service. The sensors being developed use silica based optical fibers in composites made from E-glass reinforcing fibers and high temperature cure epoxy resins. Various methods of damage detection are being evaluated to select the optimum sensor arrangement. These systems detect changes in the transmission characteristics of the optical fiber. The objective being to produce a working damage detection system which provides sensitive, cheap, accurate and reliable information about the levels of impact damage sustained by the tube. This paper presents initial results from the impact damage detection systems being evaluated for use in filament wound tubes. Issues relating to chemical compatibility between optical fiber sensors and the epoxy resin system were also investigated as part of this study. These results aid selection of the correct optical fiber properties to achieve reliable and sensitive systems. The advantages of using a new profile sensor compared to an optical fiber are also discussed.

This paper mainly describes a methodology of finding appropriate optical fiber sensors and associated potential demodulation techniques that have the capability to measure impact induced high-strain rate events in graphite/epoxy panels. The capacity of the sensor's dynamic range has to accommodate the average failure strain of about 9,000 (mu) (epsilon) and center frequency of about 40 Hz for a typical low velocity impact event for [90₂/0₄/OF(0)/0₄/90₂] and [(45/-45)₃/OF(0)/(-45/45)₃] layups. Three potential demodulators are investigated. These include the synthetic heterodyne with differential-cross-multiplier, single channel phase tracker, and sin((phi) )/cos((phi) ) based analog phase tracker all using a 10 KHz sinusoidal modulation. The maximum sensor lengths for different sensor types have been calculated based on the Fourier spectrum of a typical impact event and maximum detectable phase vs. frequency of these demodulators. As a result, a localized and reflective version of a In-Line Fiber Etalon sensor with a maximum cavity length of 73 micrometers for 1.3 micrometers wave-length of a light source is selected for impact testing. The strain responses from three different demodulators are comparable to that of a surface mounted resistance strain gage.

An experimental study if conducted to evaluate the performance of different types of strain gauges for impact detection in eight layer symmetric cross-ply Graphite/Epoxy composite plates. The plates were impacted with rigid spheres using the drop-weight technique. The impacts did not produce apparent damage in the composite plate. The responses of calibrated surface-mounted extrinsic Fabry-Perot fiber optical sensors, a metalized polyvinylidene fluoride foil sensor, and a conventional electrical resistance rosette are compared. Characteristics of signal signatures from each sensor are demonstrated. Advantages and disadvantages of utilizing each type of strain sensor in the composite structure are discussed. The experimental strain responses are presented for various impact energies. The significant features of the strain response are contact duration, peak strain, strain rise time, and full width at half maximum. All of the sensors exhibited reproducible and similar results. The extrinsic Fabry-Perot fiber optical sensor performed superior with respect to sensitivity and accuracy of point measurement.

This work is an investigation into the feasibility of using liquid core optical fibers for the detection and self repair of cracking in cement or polymer materials generated by dynamic or static loading. These experiments rely on our current research sponsored by the National Science Foundation. It combines that work on the concept of internal adhesive delivery from hollow fibers for repair with nondestructive fiber optic analysis of the crack localization and volume within the same system. The need to monitor the internal state of civil structures and materials is great. Existing instrumentation techniques that mainly rely on magnetism, electricity, or stress gauges are limited if used for remote measurements in concrete or composites. They are sensitive to electrical magnetic noises and they degrade in the environment over time. Optical fibers are attractive because they are immune to electromagnetic interference and are sensitive over long distances. The combination of the ability to remotely measure crack occurrence in real time and determine the location and volume of crack damage in the matrix is unique in the field of optic sensors (or any sensors in general). The combination of this with crack repair, rebonding of any detached or broken fibers, and replenishment of liquid core chemicals, when necessary, make this a potentially powerful sensing and repair tool. Work on this research topic of the combination sponsored by the University of Illinois, looks very promising as a rapid innovative advance.

This work examines the effects of different manufacturing techniques of fiber optic intracore Bragg gratings on their sensing and system parameters. Specifically, we have looked at tolerance with respect to center wavelength, strain, and temperature sensitivity. Our test results indicate a slight variation in the strain and temperature sensitivity of different gratings. In an attempt to quantify the effect of transverse loading on fiber gratings, three differently manufactured sensors were diametrically loaded and their spectrums studied. Our experimental results indicate a splitting of the grating spectrum with small loads for an optical fiber grating manufactured in either boron doped fiber or hydrogen loaded fibers. On the other hand, very small transverse sensitivity was observed for a fiber optic grating formed in a bend insensitive fiber suggesting some degree of control on the transverse sensitivity of these sensors.

The detection of fatigue damage within composite materials is vital for the safe use of these materials in engineering structures. Embedded fiber-optic sensors have been identified as suitable for damage detection in these materials, as they have minimal interference with bulk composite properties. The relationship between fatigue damage within a composite material and the associated moduli changes have been used to characterize damage development and progression. An intensity based fiber-optic strain sensor has been designed and evaluated for damage detection in composite laminates. This intensity sensor has the ability to monitor rapid strain changes without loss of the reference level. This sensor utilizes relatively simple and inexpensive instrumentation. In this study the fiber-optic strain sensor has been embedded within a 16 layer cross-ply carbon/epoxy laminate. The response of the sensor has been compared to that obtained from strain gauges and an extensometer. The composite coupons with the embedded sensors have also been subjected to tension-tension and tension- compression fatigue. The stiffness decay of the composite during fatigue loading was monitored in real-time.

In the design of aerospace materials, developmental testing is conducted to characterize the behavior of the material under severe environmental conditions of high stress, temperature and vibration. Such materials are designed to withstand these environmental extremities without undergoing mechanical failure. Of importance is the performance of the materials at elevated temperatures, particularly the yield strength, which must be known so that the component can be designed to operate below such limits. Currently, instrumentation used in high-temperature tests must rely on long throw instrumentation, such as a knife-edged extensometer, to probe the surface of the specimen. This often results in reduced accuracy and limits the flexibility available for material evaluation. In this paper, we present the development of a fiber optic strain gage which was used for creep measurement of a stainless steel specimen (Type 304) heated to a temperature of 1400 degree(s)F (760 degree(s)C) during tensile loading tests. Surface attached fiber optic strain gages designed for a .3% dynamic range were completely immersed in a 1400 degree(s)F (760 degree(s)C) furnace throughout the duration of the test. The yield strength of the steel specimen evaluated with optical strain sensors agreed well with the manufacturing specifications. In addition, strain measured with the optical fiber strain sensor showed a difference of less than 0.01% compared to a ceramic knife-edged extensometer.

When a Rectangular tension test specimen made of carbon steel undergoes yield stress on the occasion of a tensile test, striped patterns or `Stretcher Strains' appear on the surface affected. These patterns are easily discernible by the naked eye. Thus the patterns function as a visual sensor for yield stress. In order to utilize this phenomenon as a yield stress sensor one need only polish the surface to a fine smoothness. In the following the basic data on the use of `Yield Stress Patterns' or Stretcher Strains as a yield stress sensor will be presented. An example will be put forth in order to make an analysis of the use of yield stress patterns as a stress sensor.

A procedure is developed for mapping strain sensor readings into displacements. Optimal sensor layouts are determined by examining predicted mode shapes from synthesized strain data. The singular values of the transformation matrix bound the error in the inferred displacements. Issues of spatial aliasing as well as sensor spacing are also addressed. The methodology was validated by comparing both static and dynamic shape estimations with experiments.

Remote detection of natural corrosion of aluminum alloy has been demonstrated using optical fiber Fourier transform infrared evanescent wave absorption spectroscopy. This technique is a particular example of Attenuated Total Reflectance (ATR) spectroscopy. Significant absorption features in the spectral range from 3300 to 3700 wavenumbers have been recorded. For comparison, spectra of the same corrosion samples of aluminum have also been collected using a zinc selenide crystal as the ATR element. Both optical fiber and crystal ATR evanescent wave absorption spectra of pure aluminum hydroxide were recorded for reference. Formation of aluminum hydroxide is a major product of corrosion in aluminum and aluminum alloys. A four wavenumber resolution Michelson interferometer spectrometer was combined with a seven bounce zinc selenide crystal to make the ATR spectral reference measurements. The same interferometer spectrometer was used with 200 micrometer core diameter multimode chalcogenide optical fibers to make the evanescent wave absorption measurements simulating the remote detection process. The data was collected with one or two meter lengths of 500 micrometer diameter fibers which were decoated for 20 cm of their length. The optical fiber absorption measurements are very sensitive and can detect a very small amount of material which is in contact with the decoated fiber core. The qualitative results reported here demonstrate that the technique of optical fiber evanescent wave absorption spectroscopy may be used to monitor industrial corrosion of aluminum alloys and that quantitative analyses of this type of spectral data are possible.

A coupled micro-mechanical approach is employed to model an optical fiber in a reinforced composite. As a result, the opto-thermo-mechanical behavior of the optical fiber is determined in such a way that the phase-strain relationship is based on the micromechanical behavior of the host media. The microstructures of the carbon fiber reinforced composite and the optical fiber inclusion are analyzed, and their mutual influence on the performance of the entire composite structure is estimated. Different cases of laminated composite plates subjected to several cases of external loadings and temperature fields are studied. In-plane strains at the optical fiber, as well as optical phase changes are presented.

Carbon-carbon composite are used in many applications including aircraft disk brakes, space shuttle leading edges, rocket inlet nozzles and exit cones, and are currently being considered for future aircraft structural components, because of their mechanical stability under adverse temperature conditions. Unfortunately, the processing of carbon-carbon composites, especially the first carbonizations, has a relatively low yield, because of in-process interply failure leading to catastrophic delaminations. Considerable amount of man-power, equipment time and material are lost. Here we report on the progress in the development of sensors for monitoring these catastrophic events during the first carbonization. Specifically, acoustic emission sensors and gas sensors are singled out as the most promising ones for monitoring the evolution of micro and macro-structural changes during the first carbonization. Analysis of the high temperature waveguide was undertaken to determine the optimum waveguide geometry so that only a single mode is allowed to propagate in the waveguide.

This paper relates to the design of neural prostheses to restore motor functions, that is of artificial and/or externally controlled human extremities. Development of these neural prostheses depends partly on the availability of adequate sensors and actuators. The specific nature of the application imposes several limitations when selecting sensors and actuators. Most of limitations are related to the size, weight, power consumption, invasivity, reliability, stability, and reproducibility of the system. In order to explain the requirements of a motor neural prosthesis the very nature of biological sensors and actuators has been analyzed. Characteristics of biological sensors and actuators are discussed in the first part of the paper, while the second and third part deal with presently available artificial systems. A specific emphasis is on possible uses of biological transducers and real time processing of the information recorded from peripheral nerves.