Structural sandwich materials composed of triaxially braided polymer matrix composite material face sheets sandwiching a foam core are being utilized for applications including aerospace components and recreational equipment. Since full scale components are being made from these sandwich materials, it is necessary to develop proper inspection practices for their manufacture and in field use. Specifically, nondestructive evaluation (NDE) techniques need to be investigated for analysis of components made from these materials. Hockey blades made from sandwich materials were examined with multiple NDE techniques including thermographic, radiographic, and laser based methods to investigate the manufactured condition of blades and damage induced from play. Hockey blades in an as received condition and damaged blades used in play were investigated with each technique. NDE images from the blades were presented and discussed. Structural elements within each blade were observed with radiographic imaging. Damaged regions and some structural elements of the hockey blades were identified with thermographic imaging. With shearography, structural elements, damaged regions, and other material variations were detected in the hockey blades. Each technique's advantages and disadvantages were considered in making recommendations for inspection of components made from these types of materials.

The objective of this research was to develop a practical integrated approach using extracted features from electrical resistance measurements and coupled electromechanical models of damage, for in situ damage detection and sensing in carbon fiber reinforced plastic (CFRP) composite structures. To achieve this objective, we introduced specific known damage (in terms of type, size, and location) into CFRP laminates and established quantitative relationships with the electrical resistance measurements. For processing of numerous measurement data, an autonomous data acquisition system was devised. We also established a specimen preparation procedure and a method for electrode setup. Coupon and panel CFRP laminate specimens with several known damage were tested and post-processed with the measurement data. Coupon specimens with various sizes of artificial delaminations obtained by inserting Teflon film were manufactured and the resistance was measured. The measurement results showed that increase of delamination size led to increase of resistance implying that it is possible to sense the existence and size of delamination. Encouraged by the results of coupon specimens, we implemented the measurement system on panel specimens. Three different quasi-isotropic panels were designed and manufactured: a panel with artificial delamination by inserting Teflon film at the midplane, a panel with artificial delamination by inserting Teflon film between the second and third plies from the surface, and an undamaged panel. The first two panels were designed to determine the feasibility of detecting delamination using the developed measurement system. The third panel had no damage at first, and then three different sizes of holes were drilled at a chosen location. Panels were prepared using the established procedures with six electrode connections on each side making a total of twenty-four electrode connections for a panel. All possible pairs of electrodes were scanned and the resistance was measured for each pair. The measurement results showed the possibility of the established measurement system for an in-situ damage detection method for CFRP composite structures.

The present research investigates the complex phenomena of wave scattering in bolted joint. The goal is to develop an understanding of the attenuation behavior of propagating waves, through the structure, as the bolt is subjected to different torques. This is a first step towards developing a structural health monitoring technique for detecting torque loss at a bolted joint. To simulate the local effects of the bolt, a micromechanics based model has been developed to model the scattering and attenuation behavior due to a single fiber in a matrix with a circumferential interface crack. A slicing approach is used to account for the effect of multiple interfacial cracks at different orientations through the depth of the structure, to simulate the global effects of the bolt. The change in wave attenuation as a function of bolt location, at different depths in a plate, is studied. Next, the variation in wave scattering as the bolt, which is now fully embedded in the plate, is subjected to different torque is investigated. The local stress fields that develop in the plate due to the torque are treated as a pre-stress condition and their effect on the resultant wave scattering is investigated using the developed model. The resultant attenuation accounts for the combined effect of the geometrical attenuation and the attenuation due to the pre-stress. Numerical results obtained show small but steady increase in the attenuation with the applied torque. Experiments conducted to validate the developed model show similar trends.

Novel techniques for generating robust and accurate meshes based on 3-D imaging data have recently been developed which make the prediction of macro-structural properties of composite structures based on micro-structural composition straightforward. The accuracy of reconstructions is a particular strong point of these new techniques with geometric accuracy only contingent on image quality. Algorithms developed and used are topology preserving, volume preserving and multi-part geometric models can be handled straightforwardly. In addition to modeling different constituent materials as separate mesh domains, material properties can be assigned based on signal strength in the parent image thereby providing a way of modeling continuous variations in properties for an inhomogeneous medium. These new techniques have been applied to the analysis of a ceramic matrix composite which was micro-CT scanned and the influence of imaging parameters on both predicted bulk properties and localized stresses has been explored. This paper utilizes the Computed Tomography (CT) as the NDE technique to characterize the initial matrix porosity's locations and sizes in a Ceramic Matrix Composites (CMC) test specimen. Further, the Finite Element (FE) method is applied to calculate the localized stress field around these pores based on the geometric modeling of the specimen's CT results, using image analysis, geometric modeling and meshing software, ScanIP/ScanFE [1]. The analyses will simulate experimental loading conditions where scanned specimens are then tensile tested to a 0.07 % total strain to identify the matrix cracking locations in relation to the original pores. Additional work is carried out combining the image processing and finite element to investigate the applicability of some novel meshing techniques. Finally, the calculated Finite Element [2-4] localized stress risers are compared with the observed matrix cracking locations. This work is expected to show that an FE model based on an accurate 3-D rendered model from a series of CT slices is an essential tool to quantify the effects of internal macroscopic defects of complex material systems such as CMCs.

This paper examines damage monitoring for woven graphite/epoxy laminate by means of an electrical resistance change method. The method has been proposed by the authors and successfully applied to cross-ply and quasi-isotropic laminates; the method has yet to be applied to woven laminates. Therefore, a woven graphite/epoxy composite is selected for the target material of the electrical resistance change method to identify the damage. Beam type specimens consisting of woven laminates are the focus of this paper. The influence of a different electrical property of woven laminate upon the electrical resistance change is investigated both analytically and experimentally, and the condition of the electrical contact between the electrode and the specimen is investigated experimentally. For the purpose of identification, the response surface is adopted as a solving method for the inverse problem. As a result, the method shows excellent performance for estimating delamination locations and sizes.

Impact damage in CFRP was monitored by ultrasonic inspection method using small-diameter fiber Bragg grating (FBG) sensors. The FBG ultrasound detection system consisted of broadband light source, FBG sensor and tunable optical filter. Broadband light was launched into the FBG sensor. Light reflected from the FBG sensor was transmitted through the tunable optical filter whose transmissive wavelength range is comparable to the reflected wavelength range of the FBG sensor. The operating wavelength of tunable filter was set to optimize the sensitivity of ultrasound detection. Ultrasound vibration was converted into change in intensity of light transmitted through the filter. A cross-ply carbon fiber-reinforced plastic (CFRP) plate was used as a test specimen for impact damage monitoring. A 6.3 X 9mm² impact damage was introduced by ball dropping. Both FBG ultrasound sensor and piezoelectric ultrasound transmitter were attached on the CFRP surface. The change in responses to ultrasound excited by either spike signal or toneburst signal before and after impact damage was investigated. In response to ultrasound excited by spike signal, the response after impact damage showed a scattered behavior where the period of response signal got longer. In response to ultrasound excited by toneburst signal, damage signal features scattered and distorted waveform. Experimental results proved that the FBG inspection system could monitor a 6.3 X 9mm² impact damage in CFRP.

Investigating material behavior under complex stress states is often done using in-plane biaxial loading approach. Utilizing such techniques requires using cruciform type specimens fabricated from plate material tested by gripping the specimen at four locations and loaded along two orthogonal axes. Servohydraulic systems are generally used in this application which is similar to those used for uniaxial testing. These kind of testing capabilities are currently being conducted at NASA Glenn Research Center via a new in-house testing facility. This is in support of the development of major components for the Stirling Radioisotope Generator (SRG). It is also used to assist in the generation of an analytical life prediction methodology [1] and to experimentally verify the flight-design component's life. Further, this work is intended to carry the immediate goal of developing a specimen design that is fully compatible with the in-plane biaxial testing systems installed at NASA Glenn Research Center [2]. Thus, details of the specimen design and its applicability to the ongoing experimental activities are being reported and discussed. Finite element analyses were carried out to optimize the geometry of specimen and to evaluate the stress response under biaxial loading conditions [3, 4]. The material of interest used in this research is nickel based superalloy. The data presented concluded that the specimen can be used to investigate the deformation behavior under general forms of biaxial loading. The provided measurement and observation are limited to 1-in [2.54 cm] diameter circular region at the specimen center.

We investigate the use of a vibrational approach for the detection of barely visible impact damage in a composite UAV wing. The wing is excited by a shaker according to a predetermined signal, and the response is observed by a system of fiber Bragg grating strain sensors. We use two different driving sequences: a stochastic signal consisting of white noise, and the output from a chaotic Lorenz oscillator. On these data we apply a variety of time series analysis techniques to detect, quantify, and localize the damage incurred from a pendulum impactor, including classical linear analysis (e.g. modal analyses), as well as recently developed nonlinear analysis methods. We compare the performance of these methods, investigate the reproducibility of the results, and find that two nonlinear statistics are able to detect barely visible damage.

Carbon-fiber-reinforced-polymer (CFRP) composites represent the future for advanced lightweight aerospace structures. However, reliable and cost-effective techniques for structural health monitoring (SHM) are needed. Modal and vibration-based analysis, when combined with validated finite element (FE) models, can provide a key tool for SHM. Finite element models, however, can easily give spurious and misleading results if not finely tuned and validated. These problems are amplified in complex structures with numerous joints and interfaces. A small series of all-composite test pieces emulating wings from a lightweight all-composite Unmanned Aerial Vehicle (UAV) have been developed to support damage detection and SHM research. Each wing comprises two CFRP prepreg and Nomex honeycomb co-cured skins and two CFRP prepreg spars bonded together in a secondary process using a structural adhesive to form the complete wings. The first of the set is fully healthy while the rest have damage in the form of disbonds built into the main spar-skin bondline. Detailed FE models were created of the four structural components and the assembled structure. Each wing component piece was subjected to modal characterization via vibration testing using a shaker and scanning laser Doppler vibrometer before assembly. These results were then used to correlate the FE model on a component-basis, through fitting and optimization of polynomial meta-models. Assembling and testing the full wing provided subsequent data that was used to validate the numerical model of the entire structure, assembled from the correlated component models. The correlation process led to the following average percent improvement between experimental and FE frequencies of the first 20 modes for each piece: top skin 10.98%, bottom skin 45.62%, main spar 25.56%, aft spar 10.79%. The assembled wing model with no further correlation showed an improvement of 32.60%.

Purpose of the paper is to present an innovative application inside the Non Destructive Testing field based on vibrations measurements, developed by the authors during the last three years, and already tested for analysing damage of many structural elements. The proposed new method is based on the acquisition and comparison of Frequency Response Functions (FRFs) of the monitored structure before and after an occurred damage. Structural damage modify the dynamical behaviour of the structure such as mass, stiffened and damping, and consequently the FRFs of the damaged structure in comparison with the FRFs of the sound structure, making possible to identify, to localize and quantify a structural damage. The activities, presented in the paper, mostly focused on a new FRFs processing technique based on the determining of a representative "Damage Index" for identifying and analysing damage both on real scale aeronautical structural components, like large-scale fuselage reinforced panels, and on aeronautical composite panels. Besides it has been carried out a dedicated neural network algorithm aiming at obtaining a "recognition-based learning"; this kind of learning methodology permits to train the neural network in order to let it recognises only "positive" examples discarding as a consequence the "negative" ones. Within the structural NDT a "positive" example means "healthy" state of the analysed structural component and, obviously, a "negative" one means a "damaged" or perturbed state. From an architectural point of view piezoceramic patches have been tested as actuators and sensors. Besides it has been used a laser-scanning vibrometer system to validate the behaviour of the piezoceramic patches.

Vibration based non-destructive evaluation shows promise for damage detection in metal-to-metal adhesive joints. This research investigates an experimental technique to diagnose damage in single-lap adhesive joints subject to cyclical tensile loading. Vibration analysis reveals that damage can be correlated with changes in identified modal damping ratios. Constant amplitude forcing functions are employed to eliminate amplitude-dependent nonlinearities in the dynamic response profiles. Damping estimates obtained from time-domain analyses correlate well with damage magnitudes. Finite element modal analysis of the lap joints supports the experimental results.

We present progress we have made in developing a structural acoustic-based methodology allowing interior fault detection and localization in plate-like structures using only processed vibration data readily available on the structure's surface. Our methods use measurements of surface displacement associated with vibration of the structure caused by externally applied forces. These forces can be created simply by a local actuator in direct contact with the structure or in some cases by an incident airborne acoustic wave. The measured normal surface displacements, u_z(x, y), are then inverted locally using various mathematically optimized algorithms in order to obtain a desired material parameter, for example, the elastic modulus, whose spatial variation then serves to detect and localize the fault. This structural acoustic approach is not limited to any particular length scale requiring only that the structure be mechanically excited at frequencies for which the structural wavelength is within an order of magnitude of the fault dimension and that the dynamic surface displacements be mapped with a spatial resolution smaller than the fault size. We present the results of deploying the structural acoustic technique in the US Capitol Building to locate faults within plaster walls and ceilings bearing large expanses of precious nineteenth century frescoes, in composite airframe skins in laboratory experiments to detect and locate de-bonding of thin (~1mm) stiffeners and frames, and in micro-structures to detect and locate faults in silicon micro-oscillators and their supporting structures with resolutions approaching 1μm.

Health management development for advanced propulsion systems and ultrasafe engine technologies continues to be among the NASA's aviation safety program goals. Health management attempts to predict, detect, and prevent safety-significant propulsion malfunctions. The primary goal is to minimize the number of propulsion system faults that leads or contribute to civil aircraft accidents. Health monitoring of essential and key components in aircraft engines such as rotors continues to interest engine makers and aviation safety government institutions to improve safety and to lower maintenance costs. Having reliable diagnostic tools for damage detection and health monitoring of rotating components is important to maintain engine safety and reliability. This paper presents finite element analyses as a means to study the durability issues of a propulsion component such as a rotor disk. The analyses are carried out under representative engine loading conditions to further investigate the application, the performance, and the functionality of a crack detection system. Rotational speeds in the range of 2000 to 10000 rpm are used. Several key design parameters such as center of mass shift, induced cracks that ranged in length from a minimum of 0.508 cm (0.2 inches) to a maximum of 5.08 cm (2.0 inches), attachment blades and typical holes within the disk are all being explored to study their influence on the crack detection system performance. Results showing relevant influence of these parameters on the performance of the disk and the crack detection systems are presented.

The use of time reversal methods for localization and characterization of damages in plates is usually combined with high frequency guided waves in a local elastic wave propagation formulation. In such a situation, pulses and echos may be clearly separated in time. As a consequence, the diffracted field on a damage with large geometrical dimensions compared to the wavelength used for wave propagation allows to consider the structure itself as "near infinite" because the modal behavior is not apparent. However, those high requencies may not be required and in the presented approach, medium frequencies are used and boundary conditions need to be considered. The interest of this frequency range is in using lightweight signal processing devices limited to low data transfer rates as expected for in flight fuselage skin inspections. It also allows to filter artifacts like very small damages in the structure. This study focuses on the case of wavelengths which are in the order of the largest geometrical dimension of the cracks. In the paper, a modelling tool is first extended to describe the vibration behavior of pristine and damaged finite thin plates in the low and medium frequency range below 50 kHz. The proposed analytical model employs a Hierarchical Trigonometric Functions Set (HTFS) to characterize homogeneous plates with through cracks. To approximate the effect of a small crack in a plate for all combinations of classical boundary conditions, high order approximation functions are required. The proposed approach takes the advantage of the stability of the HTFS for these high orders. A notable advantage of this model is that it does not require a dense uniform meshing of the plate, with a minimum of 10 nodes per wavelength, as most finite element models require. The time reversal concept introduced before is thus validated with this model for a finite plate with known boundary conditions. Experimental validation of the model is conducted in the time domain for pristine and cracked plate structures and shows great potential for crack detection.

This study develops a technique to decompose a multi-mode, transient Lamb wave signal into individual Lamb mode signals. The previously-proposed technique (presented at the SPIE NDE conference 2005) showed an encouraging efficiency for numerically-simulated signals, but suffered when evaluating real experimental signals due to its high sensitivity to noises and experimental errors. The improved technique starts with the same assumption of known Lamb wave propagation characteristics (known propagation dispersion curves). However, a highly-strict signal model governing the development of the previously-proposed technique is relaxed to tolerate unavoidable presence noises and errors, and the problem is re-formulated. For actual experimental signals, some additional signal processing techniques are introduced in signal pre-conditioning. The entire implementation of the improved technique is first tested with simulated signals, and then applied to actual experimental signals. The final results with real experimental are presented and discussed.

Components used for thermal protection have not previously been interrogated with the impedance method. In this study, structures are fabricated to represent typical thermal protections systems. The replicas are designed to simulate actual protection systems in use. Observations are made into the verification of the impedance method in effectively monitoring complex thermal protection systems from non-optimal sensor placement locations. The thermal protections systems are damaged in a way to represent typical damage mechanisms. The sensitivity of the impedance method to various types of damage in representative structures will also be discussed. Different operational conditions, including high temperatures, are also included in the experimentation.

Inflatable-rigidizeable composite space structures are an emerging technology that could revolutionize the design of large on-orbit satellites. These structural systems have the advantages of low mass, high packaging efficiency, low life cycle cost, low part counts, and high deployment reliability. As they are rigidized on-orbit, they do not depend on internal pressure to maintain their shape once deployed. However, as thin-walled structures, micrometeoroids and orbital debris (MMOD) are still a potential threat to their structural integrity. Such impacts will create punctures on the structure of varying sizes related to the size and kinetic energy of the debris/meteorite. For closed-cell geometries, such as booms or struts, MMOD objects can penetrate the outer wall twice, once on initial impact and once upon exiting the structure. As impact damage and structural degradation will be cumulative over time, being able to monitor the structural integrity of these satellites would be of great interest. Impedance-based structural health monitoring schemes using distributed piezoelectric transducers are one possible approach. In this study, several Macro-Fiber Composite (MFC) piezoelectric devices were installed on a representative space-inflatable rigidizeable composite boom and used in ground tests as collocated sensor-actuators for detecting and assessing simulated micrometeoroid/orbital debris strike damage. Electrical impedance signatures were compared before and after application of the simulated damage to determine the extent of the damage sustained. Both small and large footprint MFC piezocomposite sensor/actuators were shown to be effective in characterizing simulated MMOD punctures along the entire length of the boom.

Metallic surface roughness in a nominally smooth surface is a potential indication of material degradation or damage. When the surface is coated or covered with an opaque dielectric material, such as paint or insulation, then inspecting for surface changes becomes almost impossible. Terahertz NDE is a method capable of penetrating the coating and inspecting the metallic surface. The terahertz frequency regime is between 100 GHz and 10 THz and has a free space wavelength of 300 micrometers at 1 THz. Pulsed terahertz radiation, can be generated and detected using optical excitation of biased semiconductors with femtosecond laser pulses. The resulting time domain signal is 320 picoseconds in duration. In this application, samples are inspected with a commercial terahertz NDE system that scans the sample and generates a set of time-domain signals that are a function of the signal reflected from the metallic surface. Post processing is then performed in the time and frequency domains to generate C-scan type images that show scattering effects due to surface non-uniformity.

The aim of this study is to investigate dynamic failure initiation and failure modes of insulation panels of LNG carriers. Insulation panels of LNG cargo tanks may include mechanical failures such as cracks as well as delaminations within the layers due to impact sloshing loads and fatigue loadings, and these failures cause a significant decrease of structural integrity. In this study, a structural health monitoring system, employing fiber optic sensors is developed for monitoring various failures that can occur in LNG insulation panels. Fiber optic sensors have the advantage of being embedded inside of insulation panels. The signal of embedded fiber optic sensors is used to calculate the strain of insulation panels and is processed by digital filtering to identify damage initiations. It has been observed that the presence of defects and delaminations produce noticeable changes in the strain measurement in a predictable manner. In addition, fiber optic sensors are used to measure static and dynamic strain variations of insulation panels with and without damage. It is expected that this study will be used as a fundamental study for the safety assessment of the LNG insulation panels.

One of the greatest challenges in deploying structural health monitoring (SHM) systems is the need to manage the continuous stream of measurements obtained from tens or hundreds of installed sensors. In a practical system the analysis of these measurements must be performed in an automated and robust manner and be completed in real-time. As the first stage in this process, a neural computing based novelty detection system has been developed which is capable of modelling the basic behaviour of a structure and subsequently isolating noteworthy measurements. In this article we examine the trade-off between the system's need to adapt to normal changes in a structures behaviour over the long-term, with the need to maintain a reliable reference model so as to identify important events when they occur. It is demonstrated that extending the existing basic neural processing system, by introducing a 'mixture of experts' approach, can address the contradictory needs of adaptability and model stability. In addition, it is shown that this approach provides a means of incorporating detection of both short-term and long-term phenomena into a single integrated processing system.

This work describes the experimental and theoretical approach used for the development of a structural reliability model for the upper anchorage of a cable-stayed bridge. Experimental and field analysis are used to establish the statistical models for uncertainty in material constitutive laws, fracture toughness, external loads (traffic and wind), and material defects (inclusions and pores). A standard reliability method is used for the evaluation of probabilistic characteristics of parameters and for the failure probabilities. Finite element analysis is used within two levels; the first level considers the model of the whole bridge to calculate the loads on the anchorages under different external load conditions. The second level considers the detailed model for the anchorage to calculate stresses on critical points. A particular semi-empirical model is proposed to analyze fatigue and to predict structural life. The reliability model is evaluated through the simulation of different scenarios of load conditions.

To be able to offer services to the world's largest container vessels, commonly known as Post-Panamax vessels, the Halifax Port Authority launched a dredging project to increase the berthing depth at its Nova Scotia facility. To monitor the movement of the existing pier during and after construction, sensors were installed in the sub-sea foundation 14 m below mean sea level. Vibrating wire strain gauges were attached directly to steel sheet piles (positioned adjacent to the existing concrete cribs) and both Bragg grating and Fabry-Perot fibre optic sensors were attached to anchor bolts used to attach the sheet piles to the cribs. Strain monitoring over approximately an eight month period indicated that the sub-sea shoring of the crib foundation has become engaged to resist the slight settlement of the cribs. The resulting stresses are very low, however, confirming that the cribs are performing well within design and functional limits. The project also confirmed the durability and effectiveness of the use of fibre optic sensors in marine environments.

Damage induced within the deck of a bridge superstructure produces concomitant changes to its vibration characteristics (notably its natural frequencies and mode shapes). Vibration-based damage detection (VBDD) methods exploit these changes to infer information regarding the nature of the damage. This paper focuses on interpreting the spatial patterns of changes produced in the fundamental mode shape with the goal of determining whether the presence of damage can be reliably detected. The study was carried out under the constraint that mode shapes are derived from limited data, available only at a relatively small number of measurement points on the surface of the bridge deck. A detailed finite element (FE) model of a two-span, slab-on-girder, integral abutment bridge was developed and calibrated to match the measured natural frequencies and mode shapes of a structure located in Saskatoon, Canada. This model was used to simulate the dynamic response of the bridge as various states of small-scale damage were induced at different locations on the deck. The variation of the change in the fundamental mode shape along three longitudinally oriented lines was studied to identify patterns that would allow a reliable determination of whether damage is present and in what region of the bridge it might be located. It is shown that normalizing mode shapes along individual lines separately, rather than along all three lines simultaneously, emphasises localized changes caused by damage, but also magnifies the influence of random measurement noise, making it more difficult to recognize the global spatial patterns indicative of damage.

There is a growing need for designing and constructing innovative concrete bridges using FRP reinforcing bars as internal reinforcement to avoid the corrosion problems and high costs of maintenance and repair. For efficient use and to increase the lifetime of these bridges, it is important to develop efficient monitoring systems for such innovative structures. Fabry-Perot and Bragg fibre optic sensors (FOS) that can measure the strains and temperature are promising candidates for life-long health monitoring of these structures. This article reports laboratory and field performance of Fabry-Perot and Bragg FOS sensors as well as electrical strain gauges in static and dynamic strain monitoring in concrete bridge decks. The laboratory tests include tensile testing of glass FRP bars and testing of full-scale concrete bridge deck slabs reinforced with glass and carbon FRP bars under static and cyclic concentrated loads. The field tests include static and dynamic testing of two bridges reinforced with steel and glass FRP bars. The obtained strain results showed satisfactory agreement between the different gauges.

Fiber Bragg grating sensing is a relatively mature fiber optic sensor technology currently being used in structural health monitoring systems. Therefore, there are significant benefits to using this technology as a platform for other sensing modalities. In this work, a side polished fiber Bragg sensor is described for sensing refractive index changes. The effective refractive index of a fiber Bragg grating is a function of the refractive index of the media surrounding it, and its sensitivity may be optimized with appropriate design. As the external refractive index changes, the wavelength at which incident light experiences a maximum reflection from the grating will shift. The sensitivity of a fiber Bragg grating to external refractive index changes increases when the grating is polished on one side. This side-polishing technique enables the Bragg grating to preserve a greater portion of its mechanical strength compared with other techniques such as chemical etching. This work utilizes side-polished fiber Bragg grating sensors centered at a 1542.9 nm wavelength with cladding thickness values of approximately 1-2 μm. The response of these sensors to small refractive index changes was studied. Previous work on fiber Bragg grating sensors has shown that the peak wavelengths can be measured with 3 pm repeatability. With this repeatability, this study demonstrated that a 0.001 refractive index change can be observed. By using materials that change index with moisture or pH, this technique can be used to construct both pH and moisture sensors.

To obtain a better knowledge of existing structures behaviour monitoring can be used. The use of monitoring in bridge structures by the use of instruments to assess the integrity of structures is not new and there are reports from structures tested as early as in the 19th century according to ISIS Canada¹ However, the term SHM (Structural Health Monitoring) is relatively new to civil engineering and the driving force to implement SHM comes from recognising the limitations of conventional visual inspections and evaluations using conservative codes of practice. The possibilities to monitor existing structures with help of the rapidly evolving Information Technology are to day carried out. The objective of SHM is to monitor the in-situ behaviour of a structure accurately and efficiently, to assess its performance under various service conditions, to detect damage or deterioration, and to determine the health or condition of the structure1. In Sweden strengthening and periodic monitoring of a large freivorbau bridge (pre-stresed concrete box girder bridge) has been carried out, the Gr&diaero;ndals Bridge. The bridge is located in Stockholm and is approximately 400 m in length with a free span of 120 m. It was opened to tram traffic in year 2000. Just after opening cracks were noticed in the webs, these cracks have then increased, the size of the largest cracks exceeded 0.5 mm, and at the end of year 2001 the bridge was temporarily strengthened. This was carried out with externally placed prestressed steel stays. The reason for cracking is quite clear but the responsibility is still debated. Nevertheless, it was evidently that the bridge needed to be strengthened. The strengthening methods used were CFRP plates in the Service Limit State (SLS) and prestressed dywidag stays in the Ultimate Limit State (ULS). The strengthening was carried out during year 2002. At the same time monitoring of the bridge commenced, using LVDT crack gauges as well as optical fibre sensors. This monitoring was carried out during the summer period. In addition to this a winter monitoring was carried out in the beginning of 2005. This paper presents the background to strengthening and a comparison between summer and winter monitoring where the strengthening behaviour between the two seasons is enlightened. The result from the monitoring is very interesting; it would have been preferable to strengthen the bridge during the winter.

Over the past decade the interests in upgrading, assessment and maintenance of our ageing infrastructure has grown avalanche-like. The main reason is economical aspects but also reasons due to accessibility environmental consideration play a vital role. Recently the Swedish and Norwegian Railway Association decided to upgrade the Iron Ore Line "Malmbanan", a railway line for transportation of iron from northern Sweden to the coasts of Norway and Sweden. Here the owner wanted to increase the axle loads from 25 to 30 tons to reduce the transportation costs. In one of the cases, the Luossajokk Bridge, a recalculation according to design codes showed that the increased axle loads would exceed the yield limit in the reinforcement. Before any decision was taken regarding strengthening or replacing the bridge an assessment with probabilistic methods was used. It appeared that the bridge could carry the higher load with a safety index β ≥ 4.7 for reasonable assumptions of the load distributions. A measurement system was installed to check the real worst placement of the new iron ore locomotive (IORE), and the actually level of strains in the reinforcement for the worst load case¹. It was shown that the strain level was far from critical and that the evaluated worst placement of the locomotive was almost correct. To assure a reliable transportation a long term monitoring program was arranged to check the development of strains with time. Examples from the probabilistic evaluation and the monitoring of the bridge are given and discussed.

The draw-wire displacement sensors to monitor the longitudinal displacement of the main girder of Jiangyin Bridge were introduced in the upgrading projects of the structural health monitoring system. The draw-wire displacement sensors were placed at the two ends of the main girders. The longitudinal displacements at the end of the main girder are being real-time measured when the draw-wire moves with the main girder. The sensors were installed on the cushion caps at the southern and northern ends of the main girder. One sensor was installed at each eastern and western side of per cushion cap, so the total sensors were four. The signal of the sensors was transmitted through the communication cables. The sampling frequency is 50Hz. The displacement changes between the two ends of main girder and the synchronism of displacement at the eastern and western sides are acquired by analyzing statistically the measured data. On the accident of the impact by ship, the displacement response and the excited status of the bridge were collected to evaluate the damage of the structure. The research results show that the variation value of the daily displacement of the main girder is small but the frequency of the movement is high, the displacement at the eastern and western sides has a good synchronism. The high-frequency vibration of the structure occurred after the impact of ship but the amplitude of vibration was relatively smaller than the daily displacement variation, so the impact has small influence on the structure and the structural rigidness of the main girder has no obvious deterioration after six-year services.

In this paper, full implementations of structural health monitoring systems for long-span bridges and large-span domes are introduced. The frameworks of the health monitoring systems are introduced. The types and locations of sensors are also presented. The data acquisition system, including scheme of data acquisition system, strategies of collecting data, instrument and software used in the data acquisition system, is described. The data transmitting system, data management system and warning system are also designed. Based on the data collected by the structural health monitoring systems, response and dynamic properties of the structures, and the loads are statistically analyzed. Finite element (FE) model is updated based on the measured data by structural health monitoring.

We introduce a phenomenological model, based on steady state analytical solution adapted to transient regime through modification of the Brillouin spectrum with the pulse spectrum. This model can accurately de-convolve the strain profiles from measured spectra. The model includes experimental parameters such as the electro-optic modulator Extinction Ratio, the pulse width, pulse and pump powers, position and sensing fibre length. The pulse base is treated as pure steady state contribution. A systematic numerical analysis has been carried out and the results are qualitatively matched with our experimental results. The experimental results have been used to validate the model and evaluate its limitations. Within this context, the approach has been applied to experimental data obtained under well-controlled laboratory conditions. The agreement is good and reflects the Brillouin frequency and then the strain distribution along the fibre. The approach is also successful when used to deconvolve the main strain contributions of a pipe subjected to a compression stress. The strength of the model lies in its simplicity of implementation because it is quasi-analytical and is not restricted to short fibre lengths.

Intelligent sensing technologies have developed rapidly recent years, which meet the requirement of structural health monitoring (SHM). A number of types of intelligent sensing technologies have been developed in the mainland of China, such as optical fiber sensing technology, piezoelectric sensing technology, self-sensing smart materials, wireless sensors and sensor networks, CCD, GPS and so on. In this paper, various optical fiber sensors are introduced, including optical fiber sensors, six kinds of optical fiber Bragg grating (OFBG)-based sensors, fiber reinforced bars embedded with OFBG sensors (FRP-OFBG), OFBG-based smart cables, OFBG-based weighbridge, PVDF-based strain gauge and crack meter, shape memory alloy-based displacement transducer, self-sensing cement-based strain gauge, wireless accelerometers and sensor networks, wireless strain sensors and sensor networks, and GPS. The performance of various sensors mentioned above is also experimentally investigated, in particular sensing property, durability, fatigue and corrosion resistant performance. Additionally, applications of the sensors have also been carried out in the mainland of China. The full implementation of sensors in SHM systems for offshore platforms, long-span bridges, large-span domes, tall buildings and so on are also introduced in this paper.

The Institute for Scientific Research (ISR) and the Naval Research Laboratory (NRL) will build and operate portable real-time fiber Bragg grating interrogator systems for monitoring strain in ISR's Multi-Modal Sensor (MMS) uninhabited aerial vehicle (UAV). ISR's UAV is constructed of fiberglass composites with aluminum stiffeners. The cargo bay and on-board electronics are intended to accommodate a variety of compact sensors. Because of the small size of the UAV, weight and volume are restricted, necessitating considerable redesign of laboratory interrogators to meet UAV constraints. NRL will be supplying a multiplexed interrogator for monitoring structural response rates in the UAV up to about 2 kHz, while ISR will develop an optical frequency domain reflectometer (OFDR) for measuring lower frequency response of large numbers of gratings below about 100 Hz. The OFDR system will test a special differencing technique to separate strain induced signals from environmentally induced signals. A National Instruments CompactRIO system with a 3 million gate FPGA and a 200 MHz Pentium processor is being used for real-time data acquisition and onboard signal analysis. The CompactRIO system weighs about 1.6 kg, measures 18cm x 9cm x 9cm, consumes less than 5 W of power, and withstands over 50g of shock. Lithium polymer batteries will be used to power the system for flight times up to about one hour in the present configuration. While the near-term objective of this project is to overcome the challenges of applying fiber-optic strain monitors to aerial vehicles, the longer-term objective is to develop a system for detecting damage in aerial vehicles using chaotic attractor based methods. One of the key issues in damage detection by this means revolves around the ability to use the chaotic excitation of the airframe from random aerodynamic vortices to detect the onset of composite degradation. There is evidence that attractor based methods applied to these ambient chaotic vibrations will provide a very sensitive indication of damage.

Eddy current sensing has been successfully used in various applications from testing heat exchange tubes for nuclear power plants to assessing dielectric thickness on printed circuit boards. However, in civil infrastructures cosmetic or cementitious surface material often keeps the probe or reader coil from accessing conductive medium inside the structure, resulting in reduced coupling as the distance increases between the DUT (device under test) and probe. Thus, the direct application of existing eddy current sensing technique is not very useful to detect flaws in civil infrastructures. To address this weak coupling problem, a simple scheme is proposed in which a resonant passive repeater tag is placed between the reader coil and the conducting test target. In this paper, the feasibility of detecting defects like cracks or fractures in conductive medium using a passive resonant tag and remote inductive pick-up as a method of interrogation is shown. Experimental data taken from simple setups to demonstrate the advantage of the proposed scheme are presented.

Sensor development and signal processing are two key issues in structural health monitoring (SHM). A process of PVDF annular array sensor design via guided ultrasonic wave propagation, excitation, and wave damage interaction modeling is presented in this paper. A sample problem to monitor the occurrence and progression of simulated corrosion damage in an aluminum plate is studied. An effective guided wave mode for easy corrosion depth assessment was selected based on guided wave propagation analysis. Three dimensional finite element method (FEM) analyses were performed to study the wave field excited from PVDF annular arrays and sectioned annular arrays in the aluminum plate. Annular arrays with enhanced mode selection capabilities were permanently bonded to a 1mm aluminum plate. Simulated corrosion damage with progressive corrosion depths was successfully detected in the structure using wave mode based signal analysis and feature extraction. The relation between the damage depth and the reflection wave amplitude from the signal were studied with both numerical simulation and experiments.

In this work the Dempster-Shafer (DS) theory has been used for fusing nondestructive inspection (NDI) data. The success of a DS-based method depends on how the basic probability assignment (BPA) or probability mass function is defined. In the case of nondestructive inspection of aircraft lap joints, which is of interest here, the inspection data is presented in raster-scanned images. These images are discriminated by iteratively trained classifiers. The BPA is defined based on the conditional probability of information classes and data classes, which are obtained from ground truth data and NDI measurements respectively. Then, the Dempster rule of combination is applied to fuse multiple NDI inputs. The maximum mass outputs determine the final classification results. In this work, conventional eddy current (ET) and pulsed eddy current (P-ET) techniques were employed to inspect the fuselage lap joints of a service-retired Boeing 727 aircraft in order to map corrosion sites. Estimation of the remaining thickness from the inspection data is the aim of this work. The ground truth data was obtained by teardown inspections followed by a digital X-ray thickness mapping technique, which provides accurate thickness values. The experimental results verify the efficiency of the proposed method.

A new in-house test capability has been developed at the NASA Glenn Research Center to conduct highly critical tests in support of major and significant components of the Stirling Radioisotope Generator (SRG). It is to aid the development of analytical life prediction methodology and to experimentally assist in verifying the flight-design component's life. Components within the SRG such as the heater head pressure vessel endure a very high temperature environment for a long period of time. Such conditions impose life-limiting failure by means of material creep, a slow gradual increase in strain which leads to an eventual failure of the pressure vessel. To properly evaluate the performance and assist in the design of this component, testing under multiaxial loading setting is essential, since the heater head is subjected to a biaxial state of stress. Thus, the current work undertakes conducting analytical studies under equibiaxial and non-equi-biaxial loadings situations at various temperatures emulating creep environment. These analytical activities will utilize the finite element method to analyze cruciform type specimens both, under linear elastic and creep conditions. And further to calibrate the in-plane biaxial-test system. The specimen finite element model is generated with MSC/Patran [1] and analytical calculations are conducted with MARC and ANSYS finite element codes [2-3]. Complementing these calculations will undertake conducting experimental tests. However, only results pertaining to the analytical studies are reported and their impact on estimating the life of the component is evaluated.