Smart structures as an identified engineering concept emerged in Europe in the middle to late 1980s (though European structures were initially intelligent rather than simply smart . . .). This talk will reflect on the early history of smart structures in Europe and speculate upon where the ideas have progressed.

For very high-speed events, such as ballistics testing, strain measurement speed is not limited by the response of the fiber grating sensor, but rather the demodulation system used. This paper focuses on a current 10 kHz fiber grating demodulator used to support impact and ballistics testing of a composite panel. It also explores the next generation demodulator, pushing the emits of speed upwards of 3 Mhz.

Mode couplings between x and y-polarized modes in a birefringent optical fiber are strongly influenced by transverse loads. Then, it is considered to develop miniature load sensors by using a birefringent optical fiber. This sensor can be integrated into a serial-multiplexed fiber optic sensor system. Some fundamental experiments to obtain the design concept of load sensors were conducted. Experimental results showed that a relation between a mode coupling and a transverse load was almost linear on a logarithmic graph. It was shown that the linear elastic coating was necessary for reproducible measurements. The model couplings strongly depended upon the loading angle and the loading section length. From these experimental results, the design concept of stable, reproducible and quantitative sensors was obtained. A module model of load sensor was developed under this concept and demonstrated for verification of the concept. It was found that the sensor module had good performance for quantitative measurement of transverse loads. For practical use, three types of miniature load sensor modules, whose size and shape of the load sensor were improved for integration into a smart structure system, were developed. It was demonstrated that these senors had capability of quantitative measurement.

In this paper we present the results of an automated instrumentation system that we have designed for temperature gradient characterization in composite materials using the optical fibers embedded in it to construct a multichannel interferometer. The objective is to develop a specific automated measurement system that is able to interrogate different interferometric channels and electronic sensors at the same time. It is also of main concern the study of the interferometric signal processing and the disturbance analysis of such technique applied to this multichannel approach. Synchronous differential optical phase measurements have been used for both characterizations of common phase errors and spatial temperature gradient. Details of the performance, the system design and the experimental results obtained are given.

This paper investigates a method of producing internal mirrors for Intrinsic Fabry-Perot Sensors that have the potential for improving sensor strength. This approach is based on deposited mirrors on only the core of the fiber. Finite Element stress analysis and reflectivity modeling were used to determine appropriate coating thicknesses. TiO₂ sol-gels coatings were then developed to deliver the exact thicknesses and optical properties needed for a thin film partially reflective mirror. A reverse photo resist masking method was developed in order to selectively deposit the TiO₂ on the core. The results presented in this paper suggest that the core mirror concept shows promise, but that much work is still required to complete its development.

The response of a new type of hydrogen sensors based on electroplated palladium on Fiber Bragg Gratings (FBG) is demonstrated. Sensor response is characterized from 35 degrees to 95 degrees Celsius at 0.6% hydrogen in nitrogen and from 0.1% to 4% hydrogen at 95 degrees Celsius with air purging between each hydrogen cycle. The Bragg wavelength shift at saturation and the response times were used to analyze the data. The results showed that heat treatments in flowing air at 95 degrees Celsius to 150 degrees Celsius greatly improved the sensor response. A comparison of the experimental response times to those predicted by diffusion equations implied that temperature had a profound effect on hydrogen surface reaction probability.

Studies of the effects of various parameters like pre-stress, input azimuth, fiber turns etc. on the polarimetric fiber optic sensing system for smart structure applications are presented. The presence of the smart structure modifies the output characteristics of the highly birefringent fiber due to elastic properties of the structure. Experimental procedures are repeated for Single Mode and the bow-tie HiBi fibers with different optical configurations. Both carbon fiber reinforced (CFRP) and glass fiber reinforced (GFRP) specimens are studied in detail. These experimental evaluation clearly point out that the sensitivity of such embedded polarimetric sensors are affected by the above mentioned parameters. Hence these parameters are optimized for making the smart sensing systems with specific objectives. Keeping the modulated signal at the optimum sensitivity range for the realization of a smart weighing machine and an efficient design of the sensor to increase its range of sensitivity by optimizing the parameters for defect detection in the composite laminates will be the topics of discussion in this paper.

The fact that the wavelength shift in fiber Bragg grating sensors is insensitive to intensity fluctuations has made them superior to other intensity-based or phase-based sensors. However, the intensity variations in the light source, the bending on lead/in-out fibers, and a tunable filter can cause signal deviations when a tunable filter scans the Bragg grating sensors. In this paper, the spectrum analysis is used in the Bragg grating sensing system for characterizing the signal deviation by intensity effects. The intensity effects are quantitatively analyzed by assuming approximate Gaussian distributions of spectra in a Bragg grating and a single-loop transmitted mode of a tunable Fabry-Perot filter in the spectrum analysis. It is concluded that the intensity effects from the light source or the macrobending on lead-in/out fibers need to be considered in high-frequency harsh environments. Thus, a cancellation method is introduced to compensate intensity variations in the Bragg grating sensing system. Experimental data shows a good agreement compared with the analytical predication in compensating intensity variations. A cancellation technique, therefore, can be effectively applied in the high-frequency harsh environments, especially in explosive tests.

A novel technique (Ionic Self-Assembled Monolayer Process) to build up coatings on optical fibers is presented. With this technique is possible to control the thickness of the coating on the nanometer scale. Optical fiber sensors and other devices such as optical filters are experimentally demonstrated.

The number of advanced senors such as optical fiber sensors has exhibited tremendous growth in recent years. Optical fiber sensors, which are dielectric and chemically inert, possess characteristics that are attractive for aerospace applications. These characteristics include small size, immunity to Electromagnetic (EM) and radio frequency (RF) interference, solid state electronics reliability, geometric versatility and flexibility, and multi-parametric sensitivity. In this paper in-fiber Bragg grating fiber optic sensors are employed as strain sensors. The output of the fiber optic sensor is evaluated against currently employed sensors, resistance strain gauges and a photoelastic coating, for sensitivity, accuracy, reliability, and fatigue durability. This experimental study, which employs a tensile specimen with bonded sensors and sensors embedded between the host material and a photoelastic coating, also illustrates the reduced intrusiveness of embedded optical fiber sensors as compared to resistance strain gauges.

A fiber optic sensor using the first and the second order fiber Bragg grating spectra to simultaneously measure temperature and strain is investigated. A theoretical model for estimating the wavelength and the reflectivity of the second order Bragg resonance in a fiber grating is developed, and the results are compared with the experimental results in Corning SMF 28 fiber. Moreover, technical issues regarding the sensor design such as the spectral characteristics of the Bragg gratings, bending loss in optical fibers and the higher order propagation modes are investigated. Experiments are conducted to measure the strain and temperature response of the grating sensors and compared to conventional sensors with encouraging results.

As a potential biomaterial for many medical applications, NiTi alloy derives its good biocompatibility and corrosion resistance from a homogeneous and protective oxide layer, mainly composed of TiO₂, with little concentration of nickel. However, during corrosion testing at high potential, NiTi is susceptible to pitting corrosion, which may affect the amount of ions (nickel and titanium) released by the alloy and thus, may affect its biocompatibility. As a passivating treatment, electropolishing (EP) was demonstrated to decrease the amount of nickel on the surface and to remarkably improve the corrosion behavior of the alloy. After sterilization by ethylene oxide (EO), no modification of the promising corrosion behavior of electropolished NiTi were observed, although some surface modifications were reported. The corrosion resistance of ethylene oxide sterilized and electropolished samples ranked between that of the commonly used Ti6A14V and 316L (0.4 less than 1 less than 1.4 mV/SCE) implant alloys.

An immunosensor, utilizing immobilized antibody, is a promising sensing device for its high specificity and broad spectrum for detecting object. But physical adsorption is often an obstacle for its luck of enzymatic reactions. Fluorescence anisotropy immunosensor is a method which is, in principle, free from interference of physical adsorption. In this study 3 reagents, FITC, DNS-Cl and PAS, were employed for fluorescent labeling of antibody and lifetime of fluorophore was investigated to achieve optimum sensitivity. PAS, fluorophore with longest lifetime of up to 100 ns, showed the highest sensitivity which is in accordance with the correlation time of rotational relaxation of antibody, suggesting the importance of fluorescence lifetime being comparable with the correlation time of anisotropy decay. Immobilization procedure of antibody was also investigated to reduce interference of physical adsorption. Antibody immobilized on hydrophobic surface showed substantial anisotropy change by adsorption of non-antigenic protein but antibody on hydrophilic substrate showed no detectable anisotropy change. Further improvement of experimental condition will lead to application for microanalysis or implantable sensor. For practical use of this method, fluorescence measurement with higher S/N ratio is still to be attained.

A simple procedure is developed for the measurement of the differential quadratic electro-optic coefficient, R₃₃, by two-beam polarization (TBP) interferometry. It is shown that a TBP interferometer can be used for measuring the Kerr coefficient of a thin film with a strong Fabry-Perot effect. The measured values of the differential effective Kerr coefficient, R₃₃ of lead zirconate titanate 52/48 thin film lie inside the interval between -0.5 * 10^-18 m²/V² and +1.7 * 10^-18 m²/V² for the external DC field from -160 kV/cm to 160 kV/cm, in agreement with the known data. The correlation between differential electro-optic coefficients and field-induced birefringence is discussed.

Amine/epoxy based thermosets are used as matrices in a wide variety of advanced fiber reinforced composite structures. Thermosets can be formulated for specified processing routes, for example, pultrusion, prepregging, resin transfer moulding and filament winding. In general, the resin system has two or more components, which have to be mixed prior to use. Issues of concern in this area are (1) the homogeneity of the mixed resin systems; (2) the stoichiometry of the reagents and (3) the chemical stability of the individual components as a function of storage conditions prior to mixing. The availability of an on-line 'resin quality' sensor system could overcome some of the above mentioned problems. This paper reports on an on-line monitoring system for determining the amine concentration in an amine/epoxy-based thermoset, which is used for filament winding. The system is based on a dual- wavelength optical fiber sensor design. One light source at 1548 nm corresponds to the amine (N-H) absorption band and the second is centered around 670 nm. The latter serves as a reference to compensate for the scattering effects. The proposed system is capable of quantitatively determining amine concentrations during processing and offers the option of on- line process optimization for multi-component resins.

The processing of advanced fiber reinforced composites requires accurate data on the temperature and chemical composition of the resin system as a function of time. This paper reports on a preliminary study to evaluate two multi- functional sensor arrangements to facilitate in-situ chemical and temperature monitoring in epoxy resin-based thermosets. Sensor arrangements evaluated for chemical analysis include evanescent wave and transmission-based near-infrared spectroscopy; and for temperature monitoring optical fiber Bragg gratings. A comparative analysis was undertaken to ascertain the relative merits of each sensor configuration.

Layerwise processing methods allow parts to be built with sensors placed within the structure and fully embedded. Blocks of epoxy resin have been formed with embedded optical fibers. The fiber can be used to monitor curing and water uptake of the epoxy using ambient light which passes through the resin, is collected by the fiber and analyzed in a near-IR spectrometer. Piezoelectric polymer films have also been embedded in epoxy and used to monitor curing by changes in response to an external stress pulse. In the long run, it would be desirable to form parts containing many sensors with sensitivity differing environmental variables. Epoxy parts have been freeformed with lines of conducting carbon-filled polymer written into the structure during forming. Where they are at the surface of the part, these materials respond to solvent exposure by a resistance change. Parts have been made with sensors distributed across the surface and their ability to sense gradients of solvent vapor, and so direction to a source, is being tested.

This paper presents a novel electrically passive level gauge suitable for use under industrial and other extreme conditions. The method exploits acoustics resonance of one- dimensional gas resonator. A low cost interferometric fiber- optic microphone is proposed as an acoustic pressure sensor and the acoustic power is delivered to the resonator by the acoustic waveguide. The distances over 120 m between the electrically passive sensor and the electrically active controller were experimentally demonstrated. The proposed method proves to be very robust and useful under heavy operating conditions.

Fiber Bragg gratings were fabricated in commercially available photosensitive optical fiber, using a phase mask and UV irradiation at 245.6 nm and 266 nm. Prior to irradiation, the acrylate buffer coating was removed from the fiber using different stripping methods: chemical solvent, hot-acid and heat. The tensile strength of the fiber was measured at various stages of the fabrication process: with buffer coating intact, with the buffer coating chemically removed, and following UV irradiation. A Fabry-Perot demodulation system monitored the wavelength shift of the reflected peak from the FBG during the tensile loading, from which the strain to failure of FBG sensors has been determined. Each set of strength data has been plotted by applying the Weibull statistics. The results indicate that the removal of the buffer coating degrades the strength of optical fibers. The Weibull analysis indicates two failure mechanisms. Bare fibers show a high Weibull modulus, above 24, if no mechanical flaws have been introduced, while the fibers with surface flaws fail at low stress with the Weibull modulus as low as 2.78. UV irradiation has been found to further reduce the fiber strength. Most irradiated fibers fractured within the sensor gauge length. It is believed that a stress concentration or surface stress is induced by the UV energy, which results in mechanical degradation.

Using optical fiber sensors to study the drag force in molding flow is presented. This research have proved that this novel method can be used to study the drag force of the molding flow in IC packaging over a perfect elastic circular cylinder. The result of this study is found to be useful for characterizing the behavior of the wire sweep in encapsulation of semiconductor chips. An in-line fiber etalon (ILFE) as a sensor is designed, constructed, and implemented in the middle of the optical fiber for accurate strain measurement. This is done by first laying an optical fiber in the mid-plane of a simple rectangular mold cavity, and then measuring the strain at the mid-span of optical fiber subjected to the flow of homogeneous fluid. For a given flow field, several drag force models have been tried to calculate the drag force on the optical fiber, and the resulting strain of the optical fiber has been calculated by FEM and compared with the experimental results. From the comparison, a Takaisi's model to calculate the drag force exerted on the optical fiber by the flow of homogeneous fluid is modified according to the experimental data. Accordingly, the modified Takaisi's model for the drag force can be applied to study the wire sweep in the encapsulation of semiconductor chips. It is concluded that this measurement system can be used to verify the drag force model for a molding flow on a perfect elastic circular cylinder. Even more complex conditions in molding flow, the proposed sensing system can also be used to measure the axial strain accurately. In addition, the transient deformation of the optical fiber characterized by the ILFE sensor provides important information for developing the drag force model that can be used in IC packaging.

Intracore fiber Bragg gratings have been extensively used as longitudinal strain sensors both bonded and embedded in numerous applications, fulfilling the same task as conventional resistive strain gages. Comparative results obtained in composite laminates with both types of sensors show an excellent correlation in those places where the strain distributions are smooth. In this case, optical sensors offer additional advantages: unnecessary calibration, multiplexing capability, small size, embedding ability, etc. But optical sensors reveal all their potential in those locations where the anisotropy of the composite structural element promotes strong strain distributions, and complex stress fields. In these cases, the analysis of the distorted spectrum of a grating submitted to an intense strain gradient offers a big amount of information, even without using spectrum integration methods. Furthermore, the knowledge of the optical behavior of fiber Bragg gratings submitted to transverse loads allows having additional information about the residual stress field promoted during the manufacturing process, and the stress release due to the machining of the part. This paper demonstrates theoretically and experimentally that fiber Bragg gratings can be valuable tools not only to monitor complete composite structures in service, but to analyze the stress and strain state of those particular configurations in which any kind of information, due to their complexity and high requirements, is essential.

This paper investigates the use of embedded optical fiber Bragg gratings to measure strain near a stress concentration within a solid structure. Due to the nature of a stress concentration (i.e. the strong non-uniformity of the strain field) the assumption that the grating spectrum in reflection remains a single peak with a constant bandwidth may not be valid. Compact tension specimens including a controlled notch shape are fabricated with embedded optical fiber Bragg gratings at identical locations but with different gauge lengths. The spectra in transmission varies between such specimens for given loading conditions. This variation is shown to be due to the difference in gauge length. By using the strain field measured on the specimen surface with electronic speckle pattern interferometry and a discretized model of the grating, the spectra in transmission are then verified analytically. Thus, by considering the non-uniformity of the strain field, the optical fiber Bragg gauge functions well as an embedded strain gauge near the stress concentration. Due to the distributed nature of the measurements within a specific gauge length, the optical fiber Bragg gauge has a large potential to measure debonding in fiber reinforced composites.

With the increasing recognition that optical fiber-based sensor systems are ideal for structural health monitoring, there is a demand for a low-cost sensor. This paper reports on recent progress in the design, manufacture and evaluation of an intensity-based optical fiber strain sensor. The proposed sensor is referred to as the 'profile' sensor and it is made by deforming (tapering) a section of optical fiber using a standard fiber fusion splicer. Up to three profiles were made on a single fiber length and the attenuation during this process was monitored. The sensors were photographed to estimate the dimensions of the profile and then tensile tested by attaching the profile sensor to a micrometer stage. The sensors were strained via the micrometer stage in an incremental manner and the light transmission was monitored during this operation. An increase in the light transmission characteristics was observed during tensile loading. A good correlation was obtained between the experimental results and the predicted values.

The air pressure in the tires of a vehicle affects its stability, handling and braking and may contribute to causing an accident. Under-inflated tires increase fuel consumption. Existing measurement systems for the monitoring of the tire pressure use active sensors which need a battery or bulky energy transmission. This work shows a new approach: Quartz crystals as sensors can operate passively, without energy supply, by giving an echo to a stimulus pulse. Strain influences the otherwise extremely stable natural frequency of a quartz crystal which is therefore ideally suited for pressure measurements. As the natural frequency lies in the Megahertz range, stimulation and response can be transmitted by a pair of small antennas. A wireless measurement system has been built with excellent accuracy and resolution and a lightweight sensor which is very reliable and in principle maintenance-free.

Fiber optic data links and embedded sensors, such as Fabry- Perot and Mach-Zehnders, are important elements in smart structure architectures. Unfortunately, one problem with optical fiber is the inherent limit through which fibers and cables can be looped. A revolutionary, patented technology has been developed which overcomes this problem. Based on processing the fiber into low loss miniature bends, it permits routing the fiber to difficult areas, and minimizing the size of sensors and components. The minimum bend diameter for singlemode fiber is typically over two inches in diameter, to avoid light attenuation and limit stresses which could prematurely break the fiber. With the new miniature bend technology, bend diameters as small as 1 mm are readily achieved. One embodiment is a sub-component with standard singlemode fiber formed into a 180 degree bend and packaged in a glass tube only 1.5 mm OD X 8 mm long, Figure 1. Measured insertion loss is less than 0.2 dB from 1260 nm to 1680 nm. A final processing step which anneals the fiber into the eventual curvature, reduces the internal stress, thereby resulting in long life expectancy with robust immunity to external loading. This paper addresses the optical and physical performance of the sub-component. Particular attention is paid to attenuation spectra, polarization dependent loss, reflectance, thermal cycle, damp heat, and shock tests. Applications are presented which employs the bend technology. Concatenating right angle bends into a 'wire harness' demonstrates the ability to route fiber through a smart engine or satellite structure. Miniature optical coils are proposed for sensors and expansion joints.

We examine the effect of fiber surface oxyfluorination on the interfacial bonding between acrylic coated optical fiber and a cementitious matrix and polyester resin respectively. In this investigation, the optical fiber is surface oxyfluorinated through a gas reaction process at room temperature in which elemental oxygen and fluorine are introduced into the acrylic molecular chain to replace the hydrogen atoms partially. After surface fluorination, the wettability of the optical fiber improves. The surface of the unmodified and the surface oxyfluorinated optical fiber were observed by scanning electron microscopy. Through an optical fiber embedment pull-out test, it is found that the surface oxyfluorination treatment improves the adhesional shear bond strength between the optical fiber and the cementitious matrix by 22%. It is also found that this surface treatment improves the interfacial bonding between the optical fiber and the polyester resin matrix by 67%. The debonding of the unmodified optical fiber and the polyester matrix exhibits catastrophic failure characteristics, whereas that of the oxyfluorinated optical fiber with the polyester matrix is more gradual and there is still a substantial bond at the interface after the interfacial adhesive bond is broken. The mechanisms for the increase in interfacial bonding between the oxyfluorinated acrylic coated optical fiber and the cementitious and polyester resin matrices are proposed.

Plastic optical fibers are multi-mode fibers and can be used as a damage sensor by measuring optical power loss. In this work, plastic optical fibers were embedded in unidirectional and cross-ply GFRP laminates and tensile tests and three- points bending tests were conducted. The relations among the optical power, the strain/deflection and the number of cracks were studied. Transverse cracks in GFRP laminates were observed by a video microscope. As the strain increased, the optical power decreased linearly before the initiation of transverse cracks, but nonlinear decrease appeared after that. There was no damages of the optical fiber before final failure of a specimen. Therefore, it is concluded that the nonlinearly change of the optical power loss was affected by the transverse cracks. Then, it is considered that plastic optical fibers have much potential to be used as transverse crack detecting sensors for smart composites. The optical power loss by a single transverse crack in tension was simulated by using three-dimensional ray tracing model. Simulated results were compared with the experimental results, and it was found that the nonlinear change of the optical power loss was strongly affected by the deformation near a crack.

This paper describes recent developments of a practical, low cost embedded plastic optical fiber (POF) wear sensor system for the condition based maintenance of external outboard water lubricated bearings aboard U.S. Navy Ships. The benefit of this measurement system over the status quo is the ability to remotely monitor bearing wear. The Embedded Wear Sensor system (Navy invention disclosure #78,570) features a sacrificial wear fiber embedded into the nitrile rubber bearing. This fiber may also act as a conduit for the transmission of pressure and temperature data that may be resolved into alignment data. The authors selected a commercially-off-the- shelf plastic fiber for the sensor because of its material compatibility with the nitrile rubber bearing staves in terms of flexural modulus and wear properties. Presented herein is a description of the system concept, the results of non-linear finite element analysis, market survey of POF, mold studies, small scale prototyping and abrasive wear testing. A description of the sensor concept and the results of the preliminary finite element analysis of the bearing stave geometry are presented. Preliminary results of molding and glue bonding POF in nitrile rubber and then abrasive wear testing indicate that this is a viable concept.

In this paper we present the results that have been obtained with an automated electronic instrumentation system for monitoring of strain and vibrations in a composite material board with embedded optical fibers. The objective of this study is to provide an electronic instrumentation reference for the interferometric measurements using optical fiber sensors. The main features of our system are the simultaneous multipoint strain and vibrations measurements which allows us real time data acquisition and processing. In this work we provide results about the natural frequency and damping coefficient of the composite material board.

Navy administrators estimate that hundreds of tons of hazardous material (HAZMAT) are being wastefully discarded due to premature disposal. Currently, HAZMAT items are coded when they are brought into the DoD supply system to indicate their storage lifetime. However, this process has come under criticism for generating shelf life codes that are too conservative and can not account for the varying storage conditions experienced by an individual item. Naturally, a detailed laboratory examination could determine when an time has reached the end of its useful shelf life, but this logistically clumsy approach is rarely undertaken, and thus HAZMAT is wastefully discarded. An ideal inspection method would be fast, reliable and non-invasive. We have investigated the development and use of an inexpensive nondestructive sensor to actively assess the shelf state of a ubiquitous HAZMAT, Silicon Alkyd Haze Gray paint. A simple sensor was designed to measure six features of the paint: ultrasonic velocity and attenuation, electric mobility and polarization and temperature and thermal diffusivity. To simulate the storage environment, an accelerated environmental degradation procedure was implemented to force the paint along realistic and prominent failure modes. During this degradation process, the material was monitored with the sensor and a set of standard laboratory measurement techniques. Pattern recognition techniques were applied to identify key characteristics of the data and to design a classifier to discriminate between different classes of the aged samples. Issues of sensitivity, uniqueness and indeterminacy in the problem were considered. Based on these results, a prototype sensor for shelf life management of hazardous silicon alkyd paint appears to be promising.

Optical Fiber Bragg Grating (FBG) strain and temperature sensors were embedded into four carbon/epoxy, filament-wound 5.75' diameter Standard Testing and Evaluation Bottles (STEBs). These sensors were used to monitor temperature and strain during cure and pressurization of the pressure vessels. Preliminary to this work, micrographs were made of embedded fiber, showing good incorporation of the fiber into the material and no degradation of the optical fiber's acrylate coating. A survey was also made of different ingress/egress techniques to protecting the fiber as in enters the bottle and preventing attenuation and power fluctuation, with Tefzel tubing proving to be the most effective method. The FBGs were embedded parallel to the reinforcing fibers, in the hoop and helical directions, and also in the axial direction. The sensors showed close agreement with surface-mounted Resistance Strain Gages (RSGs),as well as finite element modeling. Sensors in the hoop direction embedded at mid-cylinder showed the closest agreement (-1.2%), while agreement for hoop- direction sensors embedded near the ends of the bottle (11%) was not as close. The agreement was also better for helically directed sensors embedded at mid-cylinder (-1.6%?) than for those embedded near the ends (-24%). Some preliminary impact testing was conducted that indicated FBG sensors would be appropriate for sensing impact damage.

We report a new addressing mechanism for quasi-distributed absorption sensors based on the frequency modulated continuous wave (FMCW) method. The sensor units consist of open-path micro-optic cells constructed from GRIN lenses, each of differing lengths. Coherence addressing of the cells using FMCW is achieved by the interferometric mixing of two signals originating from each cell (from the glass/air interfaces). The time delay between the two reflections, along with the linear frequency ramp of the source, gives rise to beat frequencies in the mixed output which are different for each cell. The connecting fiber length between two successive sensor cells is chosen to be much greater than the coherence length of the source so that the reflections from different cells do not interfere. The interference patterns of all sensor cells add up at the detector whereby each individual sensing cell is identified by its power spectrum in the frequency domain. We show theoretically and experimentally how individual cells can be addressed and the measured signals obtained by suitable choice of cell length, proper modulation of the source and appropriate signal processing.

Composite materials are vulnerable to damage, and this can result in conservatism in design and increased maintenance costs. One way of reducing these costs is by integrating a system into the composite material which can sense the damage condition and provide an assessment of its size, location and significance. This is a difficult task, and one of the main challenges is to develop sensor systems that can detect damage reliably while not compromising the material properties of the composite or significantly increasing its weight. A sensor system will be described which fulfills some of these requirements. It is based on the use of a highly birefringent optical fiber. The whole length of the fiber is used as the sensor, which minimizes additional weight. It also enables information to be obtained on the position of the damage along its length. The mechanical properties of the composite material are not degraded when the fiber is embedded, as long as some simple precautions are taken. The sensor utilizes the propagation of low coherence polarized light in highly birefringent optical fiber. Light is launched into one of the polarization states of the fiber and is coupled into the orthogonal state in the presence of damage. The sensor is interrogated interferometrically. Its operation will be described in detail and results will be presented which illustrate its perforce in detecting impact induced damage in a number of different composite material systems. Benefits of the system will be described together with areas which need further development.

The authors demonstrate that several optical fiber Fabry-Perot sensors can be multiplexed in series for axial strain monitoring at each individual sensor. White light interferometry was employed using the laser-referenced Michelson interferometer of a standard Fourier transform spectrometer as a receiving (interrogating) interferometer. The primary aim was to demonstrate that at least six fiber Fabry-Perot transducer interferometers (sensors) can be multiplexed in series provided that each sensor has a unique optical cavity length within the multiplex. The resulting differing optical path differences at each fiber Fabry-Perot sensor give rise to sharp correlation features (side-bursts) at unique positions in the time domain as observed in the interferogram. An optical cavity length change due to an axial strain perturbation is observed as a change in the position in the time-domain of the side-burst feature associated with the fiber Fabry-Perot sensor. This paper demonstrates that multiplexed strain metrology in the quasi-static regime using fiber Fabry-Perot sensors is possible with a measurement range of typically 0 to 4000 microstrain and a strain resolution of better than 10 microstrain.

A distributed feedback (DFB) fiber laser sensor for simultaneously measuring strain and temperature has been developed. The DFB fiber laser consists of a single fiber Bragg grating written in a low birefringent rare-earth doped fiber. By measuring the rf beat frequency between the two orthogonal polarized lasing modes and the absolute wavelength of one mode, both strain and temperature can be determined simultaneously to an accuracy of plus or minus 3 (mu) (epsilon) and plus or minus 0.04 degrees Celsius. Multiplexing capabilities make this sensor ideal for monitoring several locations within a civil engineering structure. Three gauge protection systems were developed to prevent damage to the fiber during embedment and insulate it from the high alkaline environment of the concrete. This sensor is easy to install, provides excellent strain transfer from the concrete to the optical fiber and is thin enough not to degrade the concrete structure.

We report recent work on acoustic measurements using a Bragg grating based Fabry-Perot sensor system. A single Fabry-Perot sensor using a path matched Michelson interferometer was developed, and a digital demodulation scheme based on the phase stepping technique was used to measure acoustic sound pressure from 100 Hz to 600 Hz. This sensor is designed to work in a multiplexed architecture to provide inputs to a feed-forward adaptive control system. This control system will be used to actively control the sound pressure level within an enclosure. A series of experiments were performed to investigate the possibility and potential use of this sensor system for acoustic noise detection. In this paper, we present initial test data from the prototype optical sensor microphone. We also illustrate the envisioned multiplexed sensor scheme and control system.

Distributed strain sensors based on Brillouin scattering in optical fibers are attractive as structural monitoring systems due to their unmatched measurement flexibility. Despite their potential, very little research has been conducted on these sensors under the types of conditions that would be experienced in practical applications. This paper presents the results of a simple study in which a Brillouin sensor was used to measure the strain along the length of a cantilever beam subjected to two loading patterns. A spatial resolution of 400 mm was used to perform the measurements and a precision of approximately plus or minus 50 (mu) (epsilon) was achieved. The experimental results were found to be in excellent agreement with theoretical predictions, demonstrating that this type of sensor system is well suited to use in structural monitoring applications.

Distributed fiber optic sensors based on Brillouin scattering are capable of measuring the strain on an arbitrary fiber section. Their small cross section and ability to sense over a long distance makes them ideally suited for use as a sensing component in smart civil and aerospace structures. Over the past few years, the sensing range and spatial resolution of Brillouin systems have been improved considerably. It has been speculated that linewidth broadening and diminishing signal strength for optical pulses less than 10 ns would limit the spatial resolution of a Brillouin sensor to about 1 m. While this is suitable for some applications, others would benefit from improved spatial resolution. Through numerical simulation we have determined the contributions that linewidth broadening and reduced signal strength have on sensing accuracy. Experimentally, we have discovered that while the signal strength does decrease linearly with pulse widths, the linewidth does not increase correspondingly. Instead, it was observed that at pulse widths below about 5 ns the linewidth decreases dramatically. By improving the signal to noise ratio in our system we have achieved a spatial resolution of 100 mm. At this resolution the Brillouin linewidth is approximately 50 MHz, about the same as the steady state linewidth.

Recent improvements to Brillouin scattering based distributed sensors have reduced both the spatial and strain resolutions to the point where they are acceptable for many smart structures applications. This type of optical fiber sensor can measure both strain and temperature as both parameters produce a change in the optical fiber's Brillouin frequency. Since both measurands have the same observed effect it is impossible to determine which measurand is responsible for the shift in frequency. This problem must be overcome for these sensors to be suitable for many smart structures applications. Techniques have recently been developed for Brillouin scattering based distributed sensor systems to separate strain and temperature information. However, these methods are limited theoretically to spatial resolutions approaching 5 - 10 meters. This paper reports on a new technique that was used at a shorter spatial resolution. The Brillouin loss spectrum peak power was determined as a function of strain and temperature at a spatial resolution of 3.5 meters. By combining this information with the conventional Brillouin frequency measurement, strain and temperature were successfully differentiated.

This paper presents novel fully distributed forward propagating system, suitable for use with microbend sensors. The presented principle bases on selective launch of modes in specially designed multimode fiber called OFDA (Optical Fiber for Dispersion Addressing). At the input of the OFDA fiber short, pulse is launched in fundamental mode. In presence of microbend disturbance located down the sensing fiber, light couples from fundamental to higher order modes that propagate at different group velocity as fundamental mode. The position of measurand is then determined on the basis of time delay between pulse carried by fundamental mode and by pulse carried by higher order modes. The difference of group velocities is maximized by proper construction of refractive index of the OFDA fiber profile. Experimentally produced fibers exhibited difference of group velocities in range over 1%. This allows for easy reconstruction of position and amplitude of microbend deformations located down the sensing fiber.

This paper presents a multimode fiber optic sensor system suitable for large smart structures. By many observations of speckle patterns from different multimode fibers, it seems that each speckle pattern has its own signature and uniquely defining its fiber. This prompted us to superimpose (multiplexed) the speckle patterns generated by two different fibers, and detect the resultant pattern. A two-stage feature extraction algorithm is then applied on the image, which reduces the dimension of the pattern vector. Next, a backpropagation neural network with single hidden layer is trained in mapping the feature vectors to the stress applied on each fibers. Series of experiments are conducted on two fiber sensors glued on a square shape plate under different point loads. The sensors successfully estimated that which fiber was under stress with light or heavy weight.

Automatic monitoring techniques are a means to safely relax and simplify preventive maintenance and inspection procedures that are expensive and necessitate substantial down time. Acoustic emissions (AEs), that are ultrasonic waves emanating from the formation or propagation of a crack in a material, provide a possible avenue for nondestructive evaluation. Though the characteristics of AEs have been extensively studied, most of the work has been done under controlled laboratory conditions at very low noise levels. In practice, however, the AEs are buried under a wide variety of strong interference and noise. These arise due to a number of factors that, other than vibration, may include fretting, hydraulic noise and electromagnetic interference. Most of these noise events are transient and not unlike AE signals. In consequence, the detection and isolation of AE events from the measured data is not a trivial problem. In this paper we present some signal processing techniques that we have proposed and evaluated for the above problem. We treat the AE problem as the detection of an unknown transient in additive noise followed by a robust classification of the detected transients. We address the problem of transient detection using the residual error in fitting a special linear model to the data. Our group is currently working on the transient classification using neural networks.

This paper describes ongoing work into the use of acoustic Lamb waves for composite material characterization. Particular emphasis will be placed on the use of optical fiber based sensing techniques. Previous work described by the authors has concentrated on the utilization of optical fibers for the detection of acoustic waves in composite plates, and the subsequent evaluation of the acoustic signature to gauge damage in the material. In this paper we intend to explore materially integrated techniques for effective acoustic wave generation.

On September 18, 1997, Honeywell Technology Center (HTC) successfully completed a three-week flight test of its rotor acoustic monitoring system (RAMS) at Patuxent River Flight Test Center. This flight test was the culmination of an ambitious 38-month proof-of-concept effort directed at demonstrating the feasibility of detecting crack propagation in helicopter rotor components. The program was funded as part of the U.S. Navy's Air Vehicle Diagnostic Systems (AVDS) program. Reductions in Navy maintenance budgets and available personnel have dictated the need to transition from time-based to 'condition-based' maintenance. Achieving this will require new enabling diagnostic technologies. The application of acoustic emission for the early detection of helicopter rotor head dynamic component faults has proven the feasibility of the technology. The flight-test results demonstrated that stress-wave acoustic emission technology can detect signals equivalent to small fatigue cracks in rotor head components and can do so across the rotating articulated rotor head joints and in the presence of other background acoustic noise generated during flight operation. During the RAMS flight test, 12 test flights were flown from which 25 Gbyte of digital acoustic data and about 15 hours of analog flight data recorder (FDR) data were collected from the eight on-rotor acoustic sensors. The focus of this paper is to describe the CH-46 flight-test configuration and present design details about a new innovative machinery diagnostic technology called acoustic fault injection. This technology involves the injection of acoustic sound into machinery to assess health and characterize operational status. The paper will also address the development of the Acoustic Fault Injection Tool (AFIT), which was successfully demonstrated during the CH-46 flight tests.

Reductions in Navy maintenance budgets and available personnel have dictated the need to transition from time-based to 'condition-based' maintenance. Achieving this requires new enabling diagnostic technologies. Stress-wave acoustic emission technology has shown promise for the early detection of helicopter rotor head dynamic component faults. In September 1997, Honeywell Technology Center (HTC) successfully completed a three-week flight test of its rotor acoustic monitoring system (RAMS) at Patuxent River Flight Test Center. This flight test was the culmination of an ambitious 38-month, proof-of-concept effort directed at demonstrating the feasibility of detecting crack propagation in helicopter rotor components. Honeywell is presently developing a time- frequency-based real-time processing algorithm under internal research efforts to automate the fault-detection process. The focus of this paper is to overview the CH-46 flight test and system configuration and present preliminary results of the time-frequency analysis of the flight-test dataset.

Though conventional time-of-flight ultrasonic sensor systems are popular due to the advantages of low cost and simplicity, the usage of the sensors is rather narrowly restricted within object detection and distance readings. There is a strong need to enlarge the amount of environmental information for mobile applications to provide intelligent autonomy. Wide sectors of such neighboring object recognition problems can be satisfactorily handled with coarse vision data such as sonar maps instead of accurate laser or optic measurements. For the usage of object pattern recognition, ultrasonic senors have inherent shortcomings of poor directionality and specularity which result in low spatial resolution and indistinctiveness of object patterns. To resolve these problems an array of increased number of sensor elements has been used for large objects. In this paper we propose a method of sensor array system with improved recognition capability using electronic circuits accompanying the sensor array and neuro-fuzzy processing of data fusion. The circuit changes transmitter output voltages of array elements in several steps. Relying upon the known sensor characteristics, a set of different return signals from neighboring senors is manipulated to provide an enhanced pattern recognition in the aspects of inclination angle, size and shift as well as distance of objects. The results show improved resolution of the measurements for smaller targets.

This paper investigates the use of shearography and waveform- based acoustic emission (AE) techniques to detect and assess damage in composite plates due to high velocity impact. Five 48-ply [0/+45/90/-45]6s laminated AS4/PEEK composite plates donated by Boeing Company in St. Louis were used as test specimens. Shearography images of all five test specimens were taken before impact testing to detect any pre- existing internal damage from fabrication. Three broadband AE sensors were mounted on the surface of the composite plates to measure AE signals due to impact. High velocity impact tests of plates with all four edges clamped were conducted using a gas gun facility. The AE sensor signals were instantaneously acquired during the impact tests and stored in a Pentium computer. The digitized AE signals were processed in time and frequency domains. The raw AE signals were preprocessed to remove reflections from the plate boundaries that distort the wave form and cause errors. The resulting damage due to impact was evaluated using shearography fringe patterns and AE sensor signal features. The results show a correlation of AE parameters such as AE energy, AE amplitude, AE count, and shearography fringe patterns with impact energy and impact damage of the composite plates. The AE signals show the presence of both extensional and flexural wave modes with flexural wave the dominant mode. There is quite a distinctive difference between shearography fringe patterns of undamaged and damaged composite plates.

Based on the example application of Emmenbridge, a newly built steel-concrete-composite bridge in Switzerland with 47 m long built-in carbon fiber reinforced polymer (CFRP) prestressing cables, we will present and analyze the process chain leading to a reliable surveillance of modern civil engineering structures with embedded fiber optical Bragg gratings. This consists first in the embedding of optical fibers and in-fiber Bragg gratings in long CFRP wires in an industrial environment, including fiber optical monitoring of the curing process. Then, various qualifying tests were done: annealing experiments for determining optical lifetime of the Bragg gratings used, dynamic and static tensile tests for estimating their mechanical lifetime under operation, push-out experiments to check adhesion of fiber/coating/matrix interfaces, and performance tests to determine strain and temperature sensitivity of the embedded Bragg gratings. Finally, the prestressing cables were equipped with the CFRP sensor wires and built into the bridge.

A novel antenna, known as a Good Impedance Matching Antenna (GIMA), has been developed for use in Ground Penetrating Radar (GPR) NDE of concrete structures. The requirements of a useful GPR antenna are that it provides sufficient penetrating depth in the concrete with sufficient resolution to determine the location and magnitude of the defects, such as deteriorations and delaminations. The GIMA antenna is designed to have a self-defined aperture that minimizes impedance mismatching at the aperture. This unique feature allows the antenna to be used in various frequency bands. The tested frequency range is from 500 MHz to 16 GHz. The antenna provided a high penetrating depth (more than 330 mm) and the sufficient resolution of the image that can recognize cracks up to 1 mm thick, with a radiation coefficient of about 99%. This paper will introduce the theory behind the design, as well as present experimental results. The characteristic parameters of the antenna, such as impedance matching status represented by SWR and Smith chart, as well as the power radiated using return loss. In addition, the aperture reflection is determined via the time-domain air shot reflections. Also presented are the results of the resolution test and penetrating test using concrete slabs.

This paper presents the recent research on impedance-based structural health monitoring technique at Center for Intelligent Material Systems and Structures. The basic principle behind this technique is to use high frequency structural excitation (typically greater than 30 kHz) through the surface-bonded piezoelectric sensor/actuator to detect changes in structural point impedance due to the presence of damage. Two examples are presented in this paper to explore its effectiveness to the practical field applications. First, the possibility of implementing the impedance-based health monitoring technique to detect damage on massive, dense structures was investigated. The test structure considered is a massive, circular, three-inch thick steel steam header pipe. Practical issues such as effects of external boundary condition changes and the extent of damage that could be detected were the issues to be identified. By the consistent repetition of tests, it has been determined that this impedance-based technique is able to detect a very small size of hole (4 X 20 mm), which can be considered the mass loss of 0.002% of entire structure. The second example includes the implementation of this technique in the high temperature applications. With high temperature piezoceramic materials, which have a Curie temperature higher than 2000 degrees F, experiments were performed to detect damage on the bolted joint structure in the temperature range of 900 - 1100 degrees F. Through the experimental investigations, the applicability of this impedance-based health monitoring technique to monitor such an extreme application was verified, with some practical issues need to be resolved. Data collected from the tests proved beyond a doubt the capability of this technology to detect both existing and imminent damage.

It is widely recognized that the dynamic behavior of the soil is essential to discuss the characteristics of ground behavior during earthquakes, and many research has been conducted for a long period. Many problems, however, remain unsolved about the dynamic property, which is usually treated as stress- strain relationship, of the soil. One of the reasons for this, is lack of a means to measure the strain in the real ground. This paper presents a system to measure the ground strain using the fiber Bragg grating (FBG) sensors. Employment of optical fiber sensor makes the device simple in mechanism and highly durable. Two types of prototypes of the ground strain measuring device are made and their applicability are examined in the dynamic shaking table experiments. Displacement values measured by the displacement meter and the presented system are compared. The experimental results indicate that it is possible to measure the ground strain by the presented system, although necessity to improve the accuracy is also recognized.

This paper describes ongoing research into the development of a fiber optic sensor for humidity sensing. Particular attention is paid to the compatibility of this fiber optic sensor with an existing system which is already in use for structural deformation monitoring. In order to achieve this, a special transducing coating induces a length variation of the optical fiber as a function of the surrounding humidity levels. An advantage of this setup is that the sensor can not only be read by the same reading unit but can also easily be multiplexed with other sensor types to form a multi functional sensor network. This is of a particular interest for monitoring materials such as reinforced concrete, where structural health assessment criteria include deformation, depassivation and humidity. Several sensor configurations have been tested using dry-wet cycles at room temperature. Through this testing a prototype humidity sensor has been developed which responds consistently to humidity. Using pH sensitive coatings, the same design could be used for a fiber optic pH sensor.

We report on an approach for reducing the effects of temperature in a fiber optic distributed sensor. This technique employs a sensing fiber and a Brillouin optical time domain reflectometer (BOTDR). The BOTDR has been proposed for measuring both strain and optical loss distribution along optical fibers by accessing only one end of the fiber. The BOTDR analyzes changes in the Brillouin frequency shift caused by strain. This device can measure distributed strain with an accuracy of better than plus or minus 60 X 10^-6 and a high spatial resolution of up to 1 m over a 10 km long fiber. However, temperature fluctuations have an adverse effect on the accuracy with which the Brillouin frequency shift can be measured because the shift changes with temperature as well as with strain. This has meant that both spatial and temporal fluctuations in temperature must be compensated for when a fiber optic distributed sensor is used for continuous strain measurements in massive civil structures. We describe a method for the simultaneous determination of distributed strain and temperature which separates strain and temperature in a fiber optic sensor.

This paper describes a single micro-optical fiber sensor capable of measuring three strains simultaneously in a composite structure. This single transducer is based on cascading four micro Fabry-Perot cavities to measure three normal strains and one shear strain in the plane of the optical fiber cross-section. The development of the sensor fabrication and signal processing techniques are discussed. This fabrication includes designing and fabricating new optical fibers, optical fiber circuit, and optical fiber multi-strain sensor head. This paper presents 2D and 3D finite element analysis and 2D closed form analysis to establish the transformation between fiber core and composite host strain stats. FEM is also used to design side-hole fiber for maximized sensitivity to transverse strains. Finally, analytical models are presented for expressing the desired strains in the host composite in terms of the measured optical phase shift in the fiber sensor.

A novel fiber Bragg grating based palladium tube sensor was designed for hydrogen leakage detection in aerospace vehicles. The sensor fabrication method was developed and the sensor response was characterized in terms of total wavelength change, response time and degassing ability. Several factors that influence the sensor performance, including the tube thickness, purging temperature, purging gas, hydrogen concentration, and operation temperature, were studied. The sensor response was improved by reducing the thickness of the palladium tube to around 33 micrometer, optimizing the operation temperature to 95 degrees Celsius, and thoroughly degassing the sensor in nitrogen at 95 degrees Celsius for 4 hours. At these conditions, the total wavelength change was about 0.6 nm, the response time (the time to reach a 0.05 nm wavelength change) was about 2 minutes for the four-hour 4% hydrogen tests.

Rockbolt anchors for tunnel or mine roofs are key elements during construction and operation. We report on the fabrication of glass fiber reinforced polymer (GFRP) rockbolts with embedded fiber optical Bragg grating sensors and their first field application in a test tunnel. Optical fibers and in-fiber Bragg grating sensors were embedded in GFRP rockbolts during a continuously ongoing pultrusion process on an industrial production machine. Depending on their outer diameter the rods equipped with fiber sensors serve as measuring rockbolts or as extensometric sensors for the motion of boulders in the tunnel roof. The adhesion and force transfer of different fiber coatings were tested by push-out experiments. By temperature and strain cycle tests the performance of the rockbolt sensors was evaluated. We will present these results and the measurements made during a first installation of fiber optical rockbolt sensors in a tunnel.

This paper reports a preliminary experimental investigation and characterization of an optical fiber based liquid level detection system for applications in cryogenic environment. The sensor system comprises a multiplexed array of point liquid probes. Two different designs of the probe were fabricated and tested. Probe tests were conducted in different liquid/vapor interfaces including water/air, liquid/gas nitrogen and liquid/gas hydrogen. Key performances of the liquid probe, such as vapor-liquid signal ratio, response speed were measured and compared experimentally. In addition, the multiplexing of multiple liquid probes using an OTDR device was successfully demonstrated. Finally, a novel liquid probe made by direct polishing the fiber tip is proposed, which may provide further miniaturization and performance improvements.

SOFO is a fiber optic sensor system that allows the monitoring of micrometer deformations over measurement bases up to a few meters. It is particularly adapted to measure civil structures built with conventional civil engineering materials (concrete, steel and timber). It has been successfully tested in different types of structures such as bridges, tunnels and piles. The application of the system is however limited in some case when unusual materials are used in the construction and in other cases by the dimensions of standard SOFO sensors. To extend the domain of application of the current system, special sensors have been developed. In this paper we present four special SOFO sensors: long, membrane, thin and stiff sensors. The long sensor has a measurement basis of several tenths of meters and its purpose is the measurement of deformations in massive and large structures (dames, tunnels). The membrane sensor is for use on laminated materials (e.g. membrane roofing) and it is easy to install by simply gluing it to the structure to be monitored. Since standard sensors can not be used for thin mortar layers because of their cross- section, a thin sensor has been developed, too. Finally, the aim of the stiff sensor is to determine the hardening (solidification) time of concrete. This time is determined by comparing the deformations of a stiff and a standard sensor, closely placed in the concrete at the very early age. The design of these sensors is presented along with significant application examples.