Distributed pressure measurements from the lifting surfaces of aeronautical platforms provide valuable information to validate model predictions. These measurements are typically made on scale models of the structure in wind or water tunnels and require the sensors to be integrated into the model with minimal intrusion on the surface profile so as not to disrupt the flow. The small footprint and capacity for multiplexing of optical fibre-based sensors offer potential for successful application in this area. This paper reports on an experimental assessment of three distributed fibre optic pressure sensor designs intended for integration with the lifting surfaces of composite marine propeller blades.
Conventional non-destructive inspection approaches can be costly, require physical access to the subject and some of the established inspection methods are more difficult to implement on polymer composite materials. This has driven a growing interest in the use of embedded sensors. The physical form of optical fibres means they are well suited to embedment in fibre reinforced composites however there are technical challenges associated with their use.
The non-uniform geometry of woven fabric composite materials can induce localised macro bending in embedded optical fibre Bragg grating (FBG) sensors when they are compacted between layers during the lay-up process. This leads to a non-uniform strain profile along the optical fibres which can limit the efficacy of conventional peak tracking algorithms for demodulating strain.
This paper investigates the effect of gauge length on sensor response for FBGs of different length embedded in a woven glass fibre reinforced composite coupon. The experimentally measured FBG reflection spectra were compared to model predictions for the unloaded state assuming an FBG bend radius of similar dimensions to the weft of the fabric. Through thickness fibre optic strains under four point loading conditions were compared to side-imaged thermoelastic response measurements. The results show that the ratio of the gauge length to the curvature radius of the macro bending is critical with the optimal gauge length being a compromise between FBG reflectivity and sensor response.
Corrosion is a multi-billion dollar problem faced by industry. The ability to monitor the hidden metallic structure of an aircraft for corrosion could result in greater availability of existing aircraft fleets. Silica exposed-core microstructured optical fiber sensors are inherently suited towards this application, as they are extremely lightweight, robust, and suitable both for distributed measurements and for embedding in otherwise inaccessible corrosion-prone areas. By functionalizing the fiber with chemosensors sensitive to corrosion by-products, we demonstrate in-situ kinetic measurements of accelerated corrosion in simulated aluminum aircraft joints.
Lamb-wave based structural health monitoring (SHM) approaches are typically constrained to operate below the first cut-off frequency to simplify the interpretation of the wave field in the time-domain. However from a diagnostic perspective, it is desirable to unlock the additional information encoded in the higher-order Lamb wave spectrum. Wave-mode decomposition is necessary for the extraction of useful information from multi-modal acoustic wave fields, which requires spatially dense sampling over the field. The instrument of choice for this task is the laser Doppler vibrometer, which is capable of producing detailed spectral decompositions. However vibrometry is not suited to in-situ measurement for SHM. Fibre Bragg gratings (FBGs) are capable of sensing Lamb waves and detection of higher order modes using FBGs has been previously demonstrated. The ability to multiplex multiple short-length gratings along a single fibre to create an FBG array gives rise to an in-situ sensor with sufficiently dense spatial sampling of an acoustic wave field to perform useful wave-mode decomposition. This paper explores some of the fundamental limits to modal decomposition resolution and bandwidth that exist for such sensors. Potential sources of noise and distortion encountered due to limitations of the sensor fabrication and interrogation methods are also discussed. In addition, modal decomposition of Lamb waves with frequencies up to 1.25 MHz is demonstrated in a laboratory experiment using an array of sixteen ~1 mm long gratings bonded to an aluminium plate. At least four modes are distinguishable in the resulting spectral decomposition.
This paper reports on a series of tests investigating the performance of Draw Tower Gratings (DTGs) combined with
custom-designed broad area packaging and bonding techniques for high-strain sensing applications on Defence
platforms. The sensors and packaging were subjected to a series of high-strain static and cyclic loading tests and a
summary of these results is presented.
This paper outlines the various steps considered in the design, development and application of a network of 294 optical
fibre based strain sensors on a glass fibre reinforced rudder for a mine counter measures vessel. The sensing array is
designed for use together with a vibration-based analysis tool to be implemented as an in-service structural health
assessment system.
The Structural Irregularity and Damage Evaluation Routine (SIDER) is a broadband vibration-based technique that uses
features in complex curvature operating shapes to locate damage and other areas with structural stiffness variations. It is
designed for the inspection of large-scale composite structures not amenable to more conventional inspection methods.
The current SIDER methodology relies on impact excitation at a series of grid points on the structure and records the
response using a small number of accelerometers to determine the operational curvature shapes.
This paper reports on a modification to the SIDER technique whereby the acceleration measurements are replaced with
in-plane strain measurements using Fibre Bragg Gratings (FBGs). One of the major challenges associated with using
Bragg gratings for this type of response measurement is that the strains induced by structural vibrations tend to be low,
particularly at higher frequencies. This paper also reports on the development of an intensity-based, swept wavelength
interrogation system to facilitate these measurements.
The modified SIDER system was evaluated on an E-glass/vinyl ester composite test beam containing a machined notch.
The measurements accurately detected the presence and location of the notch.
The distributive capacity of FBGs means that these sensors have the potential to replace the excitation grid with a
measurement grid, allowing for single point or environmental excitation. The spatially separated measurements of strain
can be used to provide the curvature shapes directly. This change in approach could potentially transition SIDER from an
interval-based, broad-area inspection tool to an in-service structural health monitoring system.
A method for fabricating glass exposed-core microstructured optical fibre is demonstrated. This fibre design consists of a
jacketed suspended optical nano/micro-wire that is partially exposed to the external environment, which is particularly
useful for sensing. These fibres allow for strong evanescent field interactions due to the small core size, while also
providing real-time and potentially distributive measurements. We compare experimentally the exposed-core fibre and
an equivalent enclosed-core microstructured fibre in terms of their performance as evanescent field sensors, and
demonstrate that the exposed-core fibre can provide at least an order of magnitude faster measurement response time.
A simple interrogation technique is presented which relies on a characteristic specific to saturated fibre Bragg gratings
(i.e. gratings where most of the energy at the Bragg wavelength has been reflected prior to the incident light reaching the
far end of the grating). In this regime, when the grating is illuminated by a broadband source a change in pitch within a
region of the grating will result in the emergence of reflected energy in other spectral regions without significant loss in
power from the main Bragg peak. Hence there will be an increase in the overall integrated power reflected from the
grating, which is a function of the degree of strain gradient experienced by the grating. This allows the degree of strain
gradient to be directly converted to an intensity measurement without the need for an optical filter. Because
environmental temperature effects would generally not be localised along the short physical length of the grating, any
temperature changes will typically shift the reflection spectrum in the wavelength domain rather than alter the amount of
reflected light, which renders the measurement effectively temperature-insensitive.
Experimental data is presented demonstrating the application of this sensing approach to the detection of growth of
cracks in metallic structures and disbonds in composite repairs. Some of these experiments were carried out during
environmental thermal cycling to demonstrate the temperature independence of the measurement technique.
This paper introduces the concept of a fail-safe sensor to monitor the structural health of a composite repair. The low-cost
fiber Bragg grating (FBG) sensor system consists of a light source, two specially designed fiber Bragg gratings and
a photodiode detector. This system is applied to a typical bonded composite scarf joint often employed in aerospace
structures. A finite element model is developed to assess the change in strain distribution as the result of a structural
debond. The proposed monitoring system will be validated through an experimental investigation.
This paper reports on aspects of the design and validation of a Bragg grating sensor for acoustic plate wave detection. Advanced numerical modelling and an alternative experimental approach using Laser Vibrometry is used to validate the response of the Bragg measurements across a range of acoustic frequencies in a metal plate.
This paper reports on the development of an optical fibre sensor comprising an array of uniformly distributed
Bragg gratings that are configured to allow for the detection and modal decomposition of structural plate waves.
Aspects of the design, fabrication and validation of the sensor are discussed. Laser vibrometry (LV) and advanced
numerical modeling are used to demonstrate the fidelity of dynamic strain measurements furnished by a fibre
Bragg grating. The sensor is applied in an experimental study involving a metal plate where it is shown that
mode conversion of Lamb waves caused by a structural inhomogeneity is robustly measured.
Structural health monitoring (SHM) technology may be applied to composite bonded repairs to enable the continuous
through-life assessment of the repair efficacy. This paper describes an SHM technique for the detection of debonding in
composite scarf repairs using fibre optic Bragg grating strain sensors. A typical composite sandwich structure with a
scarf repair on one surface is examined in this paper. A finite element study was conducted which showed that the strain
in the debonded region changed significantly compared to the undamaged state. A differential strain approach was used
to facilitate the detection of debonds, where two sensors were strategically positioned so that their strain differential
increased as the damage propagated. With the use of matching gratings, this technique greatly reduced the support
equipment requirement by converting the spectral information into an intensity-modulated signal, thus allowing a
compact photodetector to be used for sensor interrogation. An experimental investigation was conducted to validate the
theoretically predicted results. The experimental measurements agreed well with the numerical findings qualitatively,
indicating that the proposed scheme has great potential as a simple and effective monitoring technique for composite
scarf repairs.
Structural health monitoring (SHM) technology may be applied to composite bonded repairs to enable the continuous through-life assessment of the repair efficacy. This paper describes an SHM technique for the detection of debonding in composite bonded patches using fibre optic Bragg grating strain sensors. A two sided doubler repair is examined in this paper. A finite element study was conducted which showed that the strain in the debonded region changed significantly compared to the undamaged state. A differential strain approach was used to facilitate the detection of debonds, where two sensors were strategically positioned so that their strain differential increased with the disbond length. With the use of matching gratings, this technique greatly reduces the interrogation equipment requirement by converting spectral information into an intensity-modulated signal, thus allowing a threshold value to be set to indicate imminent critical repair failure. An experimental investigation was conducted, using carbon/epoxy patches to carbon/epoxy substrates, to validate the theoretically predicted results. The experimental measurements agreed well with the numerical findings, indicating that the proposed scheme has great potential as a simple monitoring technique for composite bonded repairs.
The economic implication of fleet upgrades, particularly in Australia with military aircraft such as the F-111 and F/A-18, has led to an increasing reliance on composite repair technology to address fatigue and corrosion-affected aircraft components. The increasing use of such repairs has led to a research effort to develop various in-situ health monitoring systems that may be incorporated with a repair. This paper reports on the development of a theoretical methodology that uses finite element analysis (FEA) to model the strain profiles which optical sensors, on or within the patch, will be exposed to under various operational scenarios, including load and disbond. Numerical techniques are then used to predict the fibre Bragg grating (FBG) reflections which occur with these strain profiles. The quality of these reflection are a key consideration when designing FBG based structural health monitoring (SHM) systems. This information can be used to optimise the location of both surface mounted, and embedded sensors, and determine feasibility of SHM system design. Research was conducted into the thermal residual strain (TRS) within the patch. A finite element study revealed the presence of significant thermal residual strain gradients along the surface of the tapered region of the patch. As Bragg gratings are particularly sensitive to strain gradients, (producing a result similar to a chirped grating) the strain gradient on the composite at potential sensor locations both under load, and in the event of disbond was considered. A sufficiently high gradient leads to an altered Bragg reflection. These spurious reflections need to be considered, and theoretically obtained reflections can provide information to allow for load scenarios where the Bragg shift is not a smooth, well defined peak. It can also be shown that embedded fibres offer a higher average thermal residual strain reading, while being subject to a much lower strain gradient. This particularly favors the optical disbond detection system that is being developed. While certification concerns exist with embedding sensors in repairs, this study shows that embedded optical fibre sensors may provide for a health monitoring system with enhanced reliability and sensitivity.
As the replacement costs of military aircraft escalate, there is an increasing trend to operate existing aircraft well
beyond their original design life. As the fleet ages, structural problems such as airframe corrosion and cracking are becoming significant issues. In recent years, bonded composite patches or doublers have been developed to repair or reinforce defective regions of the airframe. However certification concerns have limited most application of these bonded composite repairs to secondary structures. In order to alleviate certification concerns, and thus facilitate the implementation of this repair technology to critical damage in primary structure, the 'smart patch' approach has been proposed. This approach involves incorporating sensors into the composite patch to self-monitor patch health. This paper describes the use of optical fibre Bragg gratings to measure the changes in thermal residual strain that occur when a composite patch starts to disbond from the parent structure. Conventionally, the Bragg sensing mechanism relies on a shift in reflected wavelength, which requires the use of costly optical measurement tools. A modified sensing arrangement is proposed, which incorporates two Bragg gratings, and a fibre optic coupler. The reflection from the first Bragg grating acts as a reference source for an active Bragg grating on the patch. This modified arrangement allows a relative wavelength shift to be translated into a change in the optical power, which can be measured easily using a low cost interrogation system. The modified sensing arrangement also allows us to more readily miniaturise the opto-electrical interrogation system, thus enabling these systems to be more easily implemented on operational aircraft.
Spatially resolved, non-contact, displacement measurements are reported from the membrane surface of a piezoelectric-driven micropump. The measurement system uses a fibre optic interferometric technique which incorporates an air path to the pump in the signal arm, allowing measurements to be made remotely. The interferometer operates at 1523nm, has a bandwidth of 200Khz, a focussed spot size of 22(mu) m and a noise equivalent displacement of 0.36nm. Membrane displacement profiles while pumping air and water have been obtained using custom designed automated fringe counting and interpolation software to interpret the digitised fringe patterns from the interferometer. Measurements show significant differences in membrane velocity, displacement and settling time between the two different pumping media. Transient underdamped vibration of the membrane surface was also detected in the rapid excursion and recursion phases of the pump cycle while pumping air. Analysis of the vibration transients allowed the resonant frequency and damping ratio of the system to be calculated. In addition, the amplitude of the membrane displacement was demonstrated to be dependent on the pumping frequency when pumping air. Analysis of the driving voltage and displacement profiles indicated that this frequency dependent relationship was primarily due to two effects: insufficient settling time between pump cycles and capacitive loading of the driving voltage at high pumping frequencies.
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