WO2009126991A1

WO2009126991A1 - Method and system for monitoring strain in a structure using an optical fibre

Info

Publication number: WO2009126991A1
Application number: PCT/AU2009/000452
Authority: WO
Inventors: Claire Elizabeth Davis
Original assignee: Swinburne University Of Technology
Priority date: 2008-04-14
Filing date: 2009-04-14
Publication date: 2009-10-22

Abstract

Some embodiments relate to a method of monitoring strain in a structure. The method comprises transmitting light from a light source along an optical fibre having a wavelength-specific highly reflective grating, wherein the optical fibre is arranged at an area of interest of the structure so that strain experienced by the structure in the area of interest is transferred to the grating; receiving reflected light that is reflected from the grating; and monitoring a power of the reflected light to determine a change in the power over time.

Description

METHOD AND SYSTEM FOR MONITORING STRAIN IN A STRUCTURE

USING AN OPTICAL FIBRE

TECHNICAL FIELD

The described embodiments relate to methods and systems for monitoring strain or other environmental conditions in a structure using an optical fibre. In particular, the optical fibre may employ a grating for reflecting a portion of light transmitted along the fibre.

BACKGROUND

The transmission of light through optical fibres is affected by a number of physical environmental factors that can cause changes in the refractive index of one or more parts of the fibre. It is the changes in spatial distribution of refractive index that primarily affect the travel of light through optical fibres.

SUMMARY

Certain embodiments relate to a method of monitoring strain in a structure, the method comprising:

transmitting light from a light source along an optical fibre having a wavelength- specific highly reflective grating, wherein the optical fibre is arranged at an area of interest of the structure so that strain experienced by the structure in the area of interest is transferred to the grating; receiving reflected light that is reflected from the grating; and monitoring a power of the reflected light to determine a change in the power over time.

The grating may comprise a Bragg grating, and the optical fibre may be adhered to or embedded in the structure. The grating may be equal to or more than 90% reflective at the specific wavelength. The grating may be equal to or more than 97% reflective. Other embodiments relate to a system for monitoring strain in a structure, comprising: an optical fibre having a wavelength-specific highly reflective grating, wherein in use of the system the optical fibre is arranged at an area of interest of the structure so that strain experienced by the structure in the area of interest is transferred to the grating; a light source coupled to the optical fibre for transmitting light along the optical fibre; a photodetector coupled to the optical fibre for receiving reflected light that is reflected from the grating; and at least one processor to receive signals corresponding to the reflected light received by the photodetector and configured to monitor a power of the reflected light to determine a change in the power over time.

The change in the power may comprise a reduction or increase in the power. The processor may be further configured to determine that a localised strain concentration exists in the area of interest based on the change in the power. The localised strain concentration may correspond with a joint, such as a scarf repair joint, or an area near a crack, for example.

The power of the reflected light may be determined by the processor based on output signals of the photodetector that correspond to the reflected light received by the photodetector. The grating may be equal to or more than 90% reflective at the specific wavelength. The grating may be equal to or more than 97% reflective.

The grating may comprise a Bragg grating. The optical fibre may be adhered to or embedded in the structure. The system may further comprise a bi-directional coupler for directing light from the light source into the optical fibre and for directing light reflected from the grating to the photodetector.

Some embodiments relate to a monitoring system comprising a plurality of strain sensors and at least one processor, wherein each strain sensor comprises: an optical fibre having a wavelength-specific highly reflective grating, wherein in use of the system the optical fibre is arranged at an area of interest of a structure so that strain experienced by the structure in the area of interest is transferred to the grating; a light source coupled to the optical fibre for transmitting light along the optical fibre; a photodetector coupled to the optical fibre for receiving reflected light that is reflected from the grating; and wherein the at least one processor is configured to obtain signals corresponding to reflected light received by the photodetector of each strain sensor, and to monitor a power of the reflected light to determine a change in the power over time for each strain sensor.

Some embodiments relate a method of monitoring strain in a structure, the method comprising using any of the systems described above to determine that a localised strain concentration exists in the area of interest based on the change in the power over time.

Some embodiments relate to a method of monitoring an environmental condition, comprising: transmitting light from a light source along an optical fibre having a wavelength- specific highly reflective grating, wherein the optical fibre is arranged at an area of interest of the structure so that an environmental condition experienced by the structure in ^■ the area of interest is transferred to the grating; receiving reflected light that is reflected from the grating; and monitoring a power of the reflected light to determine a change in the power over time.

Some embodiments relate to a system for monitoring an environmental condition in a structure, comprising: an optical fibre having a wavelength-specific highly reflective grating, wherein in use of the system the optical fibre is arranged at an area of interest of the structure so that an environmental condition experienced by the structure in the area of interest is transferred to the grating; a light source coupled to the optical fibre for transmitting light along the optical fibre; a photodetector coupled to the optical fibre for receiving reflected light that is reflected from the grating; and at least one processor to receive signals corresponding to the reflected light received by the photodetector and configured to monitor a power of the reflected light to determine a change in the power over time.

Some embodiments relate to a monitoring system comprising: at least one processor; a light source; a wavelength division multiplexer (WDM) coupled to receive light from the light source; a plurality of optical fibres coupled to receive light from the light source via the WDM, each optical fibre having a wavelength-specific highly reflective grating formed therein and being bonded to a respective area of interest of a structure; and a plurality of photodetectors coupled to the at least one processor and the WDM to each receive via the WDM reflected light that is reflected from a respective grating; wherein the at least one processor is configured to monitor a power of the reflected light received at each photodetector to determine a change in the power over time.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are described in further detail below, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a graph illustrating a longitudinal strain distribution along the surface of a typical scarf repair joint; Figure 2 is a graph showing the change in the reflected wavelength band from a highly reflecting Bragg grating placed at the edge of a scarf repair joint and subjected to a range of applied loads;

Figure 3 is a plot of wavelength versus load, illustrating variation in the peak wavelength of the Bragg reflected bands shown in Figure 2, as a function of applied load; Figure 4 is a plot of reflected power versus load, illustrating a change in the total reflected power contained in the Bragg reflected bands shown in Figure 2 as a function of applied load;

Figure 5 is a schematic representation of a sensor system for monitoring strain in a structure using an optical fibre; Figure 6 is a graph of the output of a photodetector over time, as disbond of the scarf repair increases over time, where the Bragg grating is placed over the scarf repair joint;

Figure 7 is a flow chart of a method for monitoring strain in a structure using an optical fibre;

Figure 8 is a schematic diagram of an alternative sensor system using a Bragg grating;

Figure 9 is a graph of reflection spectra from a FBG sensor bonded to a representative intact specimen as a function of applied load; Figure 10 is a graph illustrating reflection spectra at different crack lengths under zero load for the same specimen as Figure 9;

Figure 11 is a graph illustrating percentage increase in reflected intensity from temporarily and permanently bonded FBG sensors for loaded and unloaded cases;

Figure 12 is a graph illustrating reflected power in response to applied load at room temperature and at an elevated temperature of 50 °C;

Figure 13 is a block diagram of an alternative sensor system; and

Figure 14 is a block diagram of a further alternative sensor system.

DETAILED DESCRIPTION

Optical fibre sensor systems and methods for monitoring the health of structures with existing or potential localised strain concentrations, for example such as composite scarf repairs or cracks, are described. The described approach is not only inexpensive and lightweight but also self-diagnostic, in the sense that failure of the sensor is readily detectable. The basic sensor consists of a light source and a photodetector, connected to a fibre Bragg grating (FBG) by a bi-directional coupler, as shown in Figures 5 and 8.

An FBG is a region of modulated refractive index written into the core of an optical fibre. Bragg gratings typically reflect light over a narrow wavelength range and transmit all other wavelengths. The reflected wavelength X_B is determined by the pitch of the index difference Λ and the effective refractive index in the core n_ef_j and is given by the following equation:

Λ^λ .

The pitch of the index difference is sensitive to temperature and strain (and to a lesser extent pressure) and hence by monitoring the reflection and/or transmission spectra one can measure any one of these parameters.

Finite element modelling predicts a large strain gradient at the tip of a scarf repair. The longitudinal surface strain distribution along the length of the joint under a typical tensile loading condition is shown in Figure 1. It can be seen that there is a significant strain gradient at the tip of the scarf joint under load. The presence of a 2 mm debond at the tip of the scarf joint reduces the local strain significantly compared to the undamaged state. Under load, when the scarf joint is intact, a high reflectivity grating surface mounted in this region will experience a spectral broadening due to this strain gradient and hence an increase in reflected intensity.

The response of a highly reflecting grating to a strain gradient associated with a scarf repair joint is shown in Figure 2. This shows that the reflected band tends to broaden as a function of applied load, leading to an increase in the total reflected intensity. Figure 3 shows that the reflected wavelength increases linearly with applied strain, as expected. However, the total reflected intensity from the grating also increases approximately linearly, as shown in Figure 4. A change in the ambient temperature may shift the peak reflected wavelength position, but should not affect the total reflected power, rendering the measurement insensitive to environmental fluctuations. Note that the reflected intensity for the zero load condition is not zero. This gives the system a "normally on" property that provides the self-diagnostic capability of the sensor system. Any failure in the sensor system will result in a drop in reflected intensity, which can be detected by the crossing of a set threshold. This threshold may be a percentage of a long-term average of the reflected intensity, for example.

The monitoring system has been experimentally validated on several test specimens using a mechanical testing machine in cyclic tensile loading. The results show the ability of the sensor to detect disbond growth at the scarf joint as a reduction in applied load and hence reflected intensity.

Although a finite element model is used to determine the optimum sensor locations, it is not required for the purpose of damage identification. Instead, the sensor output can be continually sampled over time using the measurement at a known healthy state as the reference for comparison. This makes the technique usable for any structure, even complex constructions for which accurate finite element models are difficult to develop, as long as a significant change in localised strain or temperature distribution can be identified as a result of change in, or damage to, the structure.

Optical fibre strain sensors have several attractive features, including immunity to electromagnetic interference, compatibility with composite structures, multiplexing capability and potentially a wider operating range than conventional electronic strain gauges.

Some embodiments relate to a novel optical fibre sensor system, such as is shown in Figures 5 and 8, to monitor structural health in a bonded repair joint. The system is elegantly simple, compact, robust, self-diagnostic and temperature insensitive and can monitor structures at relatively high frequencies without the need for a separate wavelength-to-optical-power conversion filter. These features, together with the generic advantages of optical fibre sensors, are unique in a low cost package. This system has demonstrated potential to detect disbond growth in composite repairs where there is a region of substantial strain gradient at the repair tip.

The described embodiments have application to a wide range of structures (including complex structures) where a significant change in localised strain or temperature distribution can be identified, for example as a result of deterioration or damage to the structure. Such structures may include, for example, aircraft, bridges, railway lines, pipelines, concrete and other composite structures.

A sensor system 100 is shown schematically in Figure 5. The reflected spectrum from the Bragg grating is monitored using a photodiode detector 140. The electric current output from the photodetector 140 is converted to a photo voltage, which is measured by processor 150 to determine the power.

Under load there is a significant strain gradient at the tip of the scarf repair. For a highly reflecting grating (eg. >15 dB), the strain gradient will induce an increase in spectral width without a substantial decrease in peak intensity, which results in an increase in reflected power. In this context, gratings of 90% (10 dB) or greater are considered to be suitably highly reflective. Gratings of 97% (15 dB) or greater are considered to be more effective for determining the power of the reflected light. Gratings of 99%, 99.7% or higher are also considered to be suitable. Expressed in other terms, the grating may be described as a saturated grating, in which substantially all of the light at the Bragg wavelength is reflected prior to the transmitted light reaching the end of the grating.

Where the light source 130 is sufficiently broadband (such as a light emitting diode or an amplified stimulated emission source), the system 100 becomes generally insensitive to environmental temperature changes, as these will typically generate a uniform change across wavelengths rather than a gradient. Thus, the reflected power can be used as a measure of an applied load. For example, if the scarf repair joint forming part of a structure 115 starts to disbond, it is likely that the disbond will occur in this region and its response to load will reduce as the disbond grows under the grating 110.

In the sensor system 100 shown in Figure 5, the light source 130 is coupled into a fibre segment 135 that is received at a bi-directional optical coupler 120. A further fibre segment 145 is coupled between the bi-directional optical coupler 120 and the photodetector 140 for providing a return path for the reflected light. The main optical fibre segment 105, having the highly reflective Bragg grating 1 10 formed therein, receives light from the light source 130 via optical coupler 120. Part or all of optical fibre segment 105 may be fixed in position, for example by bonding, including embedment, adhesion or other fixative arrangement, relative to the structure having the existing or potential localised strain concentration which is to be monitored. In some applications of the sensor system 100, the localised strain concentration of structure 1 15 is associated with a scarf repair joint. Accordingly, the segment of optical fibre 105 in which the Bragg grating 110 is formed is positioned to physically lie over and/or coincide with the localised strain concentration (e.g. scarf repair joint) of structure 115. In other applications, the localised strain concentration 115 is due to a crack or potential crack in the structure being monitored.

Optical fibre 105 is terminated on the far side of the grating 110 by an index matching gel 125 to disperse the transmitted light without back-reflection as the light transmitted by the grating is not of interest in performing the strain monitoring function.

Processor 150 may provide on/off control to light source 130 and may provide wavelength tuning, depending on the kind of light source and whether any such tuning would be desired. Processor 150 receives electrical signals output from photodetector 140 and uses these electrical signals to make a relative determination of the optical power received by photodetector 140. An analog-to-digital conversion circuit, either as an on- chip function of processor 150 or as a separate circuit or device, may be used to convert analog outputs from the photodetector 140 to digital signals, if necessary. Alternatively, or in addition, photodetector 140 may comprise an output circuit for converting the photocurrent of the photodetector 140 into an output voltage to be provided to processor 150. The output circuit may comprise a load resistor coupled across the photodetector 140.

Processor 150 processes the electrical signals (e.g. an analog or digitized voltage signal) that are received to determine whether a change in the optical power (as a function of a change in photocurrent at photodetector 140) of the reflected light has occurred. In particular, processor 150 is configured to determine whether a decrease or increase in the optical power of the reflected light is indicated by the output signals from photodetector 140. If a change is large enough to exceed a predetermined threshold (which may be determined as a proportion of a fixed amount or an average or moving average of historical data) but still non-zero, the processor 150 may determine that this change, indicates that substantial structural change or defect in the structure has occurred. If the power decreases to zero (or close enough to be practically zero), processor 150 may determine that there is a failure of the sensor, for example because of damage to the fibre. Once such a change determination is made, processor 150 may record the determination and subsequently alert another system, such as a general monitoring system 170 that tracks outputs from a number of different sensors. For this purpose, processor 150 may be in communication with such a general monitoring system 170 over a network (not shown) and may communicate with that network in a wired or wireless fashion.

Processor 150 has access to a memory (not shown) that stores computer program instructions readable by the processor 150 for executing the monitoring and control functions described herein. Processor 150 may be comprised in a computer system, such as a personal computer (PC), for example, or may be part of a stand-alone processing device. Although processor 150 is described in the singular, it is intended that more than one processor can be used instead, where desirable, to accomplish the various functions of processor 150 described herein.

The different steps of the disbond growth are depicted as Sections I to VI in Figure 6. These are interpreted as follows, using visual confirmation of the crack growth as a confirmation:

I : intact

II : first sign of disbond initiation but no significant decrease in intensity III : continued disbond growth with decreasing intensity

IV : disbond growth substantially under the grating with low reflected light intensity

V : disbond growth almost fully under the grating with very low reflected light intensity VI : disbond growth has fully passed under the grating.

With these results, the self-diagnostic capability of the sensor is evident. Even at zero load applied to the structure, the photodetector shows a base non-zero level (see Figures 2 and 4). This is a big advantage compared to previous approaches. The "normally on" behaviour of the sensor continuously provides the observer with information about the present condition of the bond. Sensor breakage can immediately be detected if there is no output signal.

Referring now to Figure 7, there is shown a method 700 of monitoring strain in a structure using one or more sensor systems 100 or 800 (Figure 8) and/or monitoring systems 170. Method 700 begins at step 705, in which the sensor system 100 or 800 is positioned in relation to the structure to be monitored. This positioning includes positioning the grating

110 formed in fibre segment 105 to overlie or otherwise physically coincide with the part

(i.e. scarf repair joint) of the structure 1 15 to be monitored. The fibre segment 105 is directly or indirectly bonded to the structure 115 at an area of interest to extend on each side of the (existing, possible or predicted) localised strain concentration so that strain experienced by the structure 115 in the area of interest is transferred to the grating 1 10.

Once the sensor system 100, 800 or 1300 (Figure 13) is in position, then at step 710 light is transmitted along optical fibre 105 from light source 130 (via segment 135 and optical coupler 120). The light is then partially reflected by Bragg grating 1 10. The reflected light travels back in the opposite direction along optical fibre 105 to bi-directional optical coupler 120, where the reflected light is coupled into fibre segment 145 and subsequently received at photodetector 140 at step 720. The optical power of the reflected light is converted into a photocurrent by photodetector 140 and then into a photovoltage that can be sampled or continuously received by processor 150. The photovoltage may be amplified by an amplification circuit comprised in the photodetector 140 or in a separate amplification circuit 860, if necessary.

At step 730, processor 150 determines the optical power of the reflected light based on the output photovoltage signals from the photodetector 140. The output signals may be converted from analog to digital signals, if necessary. During step 740, processor 150 compares the determined power of the reflected light to one or more thresholds and/or historical data as part of a process of monitoring the power of the reflected light.

At step 750, the processor 150 determines whether there has been a sufficient change in power (i.e. where a threshold is passed or where no output signal is detected) to warrant generation of an alert. If there is no such change in power, then method 700 returns to step 710 and the method is repeated, either continuously or periodically. If at step 750 a change in power is determined, then processor 150 may provide an alert signal to an external system, such as monitoring system 170 in step 760, after which method 700 is continued by returning to step 710.

Optionally, processor 150 may periodically provide an output to the monitoring system 170 indicative of an output of the fibre sensor, regardless of whether there is a sufficient change in reflected light power. The monitoring system 170 may then process such signals from processor 150 to determine whether an alert condition may exist.

For fibre 105, a single mode optical fibre (SMF-28) with a 25 μm polyimide coating may be used for the Bragg grating sensors. In order to minimize surface flaws on the glass and maintain the strength of the optical fibre, it may be chemically stripped using a heated concentrated sulphuric acid bath prior to hydrogenation. The gratings may be fabricated using a standard phase mask exposure technique. For crack detection applications, the gratings may be fabricated to be about 5 mm long with a Blackmann-Harris apodisation profile. For scarf repair disbond detection applications, the gratings may be about 2 mm long with a Sine-squared apodisation. Both grating designs may have a reflectivity of at least 23 dB or at least 25 dB.

To address some of the issues associated with ease-of-handling and strain transfer for the fibre grating sensors, a package may be used to encase the optical fibre 105 in a nylon tape impregnated with an epoxy resin. The tape may be cured under vacuum at 175°C to form a lightweight, flexible tape which can be fabricated in a variety of widths, thicknesses and lengths to suit the application. Alternatively, the optical fibre 105 may be packaged within a protective sleeve, such as a tube. All components of the sensor and packaging may be designed to withstand sustained temperatures of up to 13O⁰C.

Samples of such tape with embedded FBGs were bonded to a number of aluminium test coupons for experimental testing. The tape was bonded to the aluminium specimens using a similar process and materials to that currently employed for the application of standard electrical resistance foil gauges. The strain response of both the FBG tape and a foil gauge co-located on the opposite side of the specimen demonstrated the effective strain transfer of the packaging and bonding process. A second re-useable adhesive was also trialled to adhere the tape to the specimen surface; this method allowed the sensing tape to be easily removed and re-used. As expected, this method of adhesion gave significantly reduced strain transfer to the grating, however as this particular sensing approach is essentially qualitative rather than quantitative it may be appropriate for short- term applications where the environment permits and the cost of the sensor is a factor.

Figure 8 shows a schematic diagram of a sensor system 800 as an alternative to sensor system 100. Sensor system 800 comprises the same or similar elements and functions as sensor system 100, except that it also has an optical feedback loop 845. A fibre-coupled 20 mW super luminescent light emitting diode (SLED) with a centre wavelength of 1548.4 nm and a relatively flat (0.13 dB variation) spectral profile over a 5 nm range on either side of the centre wavelength may be used as the light source 130 provided to optical coupler 120. To compensate for source fluctuations, 1% of the source 130 may be tapped off into a fibre segment 852 using a fibre optic coupler and monitored using a reference photodiode 855. The remaining 99% of the light is transmitted to grating 1 10, which may be embedded in a composite tape as described above and subsequently bonded to the structure 115 at the area of interest. The reflected intensity from grating 110 is recorded using a second photodiode 140 referred to as the signal photodiode. The input from both photodiodes 140, 855 are converted to a photo voltage and amplified by a conversion and amplification circuit or device 860 before being passed to a two channel data acquisition system (including processor 150 and/or monitoring system 170) to record both inputs in real time. The data may be post-processed by taking a ratio of the two channels to provide a signal output which is compensated for source fluctuations.

Cracking is considered one of the most common mechanisms for fatigue-related structural failure in metals and usually results in a localised strain concentration in the vicinity of the crack. In order to determine the effectiveness of this type of sensor and interrogation technique for detecting crack growth, a series of Al 7075 pre-notched specimens was subjected to low-frequency cyclic tensile loading to initiate crack growth from the notch tip. Twenty specimens with dimensions of 220 mm x 70 mm in two different thicknesses (1.6 mm and 3.2 mm) were tested. Each specimen had a 6 mm long machined side notch along the long edge. The packaged FBGs were bonded to the specimens with the optical fibre running parallel to the long edge of the specimen approximately 20 mm from the notch-tip.

Table 1

Table showing minimum and maximum values of sinusoidal loading forces applied to notched aluminium specimens

The specimens were loaded in tension using a 50 kN mechanical test machine. Sinusoidal loading at frequencies ranging from 5-10 Hz was applied. Table 1 shows the loading forces for each specimen thickness. The specimens were also statically loaded prior to fatigue cycling and at regular intervals of 1000 cycles. At each static load the reflection spectrum from the Bragg grating 110 was measured using an optical spectrum analyser to characterise the grating response to strain for the intact case and as the crack grew towards the sensor. The crack length was measured at each cycle interval under load using a micro gauge which was bonded to the specimen immediately below the notch.

Figure 9 shows the modelled reflection spectra from the grating 1 10 on a representative specimen as a function of applied load prior to the initiation of the crack. As expected, for the intact specimen there is no significant increase in spectral power or change in profile under the different loads which indicates that the tensile loading does not induce a strain gradient in the structure. There is a shift in the Bragg peak wavelength which is linearly proportional to the applied load. As the cyclic loading progresses, cracking initiates from the notch tip and Figure 10 shows the modelled change in reflection spectra as a function of the crack length under zero load. This data shows that as predicted there is a broadening in the grating reflection spectrum without any substantial decrease in the main Bragg reflection peak as the crack grows towards the sensor. The amount of reflected power is a function of the proximity of the crack tip to the FBG sensor.

Figure 11 shows the percentage increase in reflected intensity as a function of crack length for two different specimens. One specimen has a permanently bonded FBG sensing tape, the other specimen has a removable sensing tape. Both tapes contain FBGs of similar design. A crack length of 20 mm indicates that the crack has reached the sensor position. Each specimen shows an increase in reflected intensity as a function of crack length, however the distance from the crack tip at which the sensor begins to respond is significantly reduced for the case of the removable sensor and the amount of reflected power is also reduced. This is to be expected due to the reduced strain transfer properties of the removable adhesive. For both methods of adhesion, the amount of reflected intensity (and hence the degree of strain gradient) is increased when the specimen is under load as the crack will open and induce a larger strain gradient. However the sensor does also respond under zero load, although in this case the sensor has to be a lot closer to the crack -tip. This result is significant as it indicates that there is a strain gradient induced by ^■ the crack which is present even in the absence of any applied load (probably due to the plastically deformed region with precedes the crack-tip), meaning structures can be continuously monitored for cracks without a requirement for in-service loading. However, the results also indicate that the sensor has to be located in close proximity to the crack as the induced strain field is highly localised.

Using these results a simple system for crack detection can be designed using three basic states to infer the health of the structure and/or system as outlined in Table 2. Under normal operating conditions, the system 100, 800, 1300 (Figure 13) or 1400 (Figure 14) will observe a baseline signal which is representative of a healthy structure; any operational loading should not affect the reflected intensity from the sensor under these conditions. If there is a loss of optical power from the sensor this implies that there has been a failure in one of the components of the sensing system. If the sensor indicates a significant increase in reflected power (beyond a threshold level which may depend on the application), this may be indicative of the emergence of a region of localized strain concentration which was not previously present and suggests the appearance of a crack in the region surrounding the sensor.

Table 2

Definition and interpretation of three output states using FBG sensor for crack detection.

Scarf joints are commonly used in aircraft structures, particularly for structural repairs. The integrity of the bondline is vital for the load-carrying capacity and fatigue performance of the joint and even a small disbond must be identified and treated in order to maintain safety. Under load, there is a significant strain concentration at the tip of a scarf repair, making it another potential application on which to trial the sensor. Contrary to the crack detection application, where any increase in detected intensity implies the emergence of a region of stress concentration associated with a crack, for scarf joints, a reduction in signal intensity would imply the relaxation of a previously existing stress concentration, and hence initiation of a disbond at the tip of the scarf repair .

Further experiments were conducted in applying the described sensor system to detecting disbonding of scarf repair joints. The experimental test specimen was a bonded carbon/epoxy scarf joint. The sensor system was validated on a series of six scarf joints constructed using Cycom 970/T300 carbon/epoxy prepreg tape with a quasi-isotropic lay- up of [45 0 -45 90]_2s. The scarf regions were machined using a computer numerically controlled router and bonded with FM73 epoxy film adhesive. The overall dimensions of the specimens were 250 mm x 25 mm x 3.2 mm.

The gratings were surface-mounted on the top surface of each repair specimen using epoxy adhesive and the transmission side of the grating was fusion spliced to an angle- polished connector to minimise broadband back-reflection. The gratings applied to each coupon were 2 mm long with a Sine-squared apodisation profile and had a reflectivity of at least 25 dB. The bondline of the scarf joint was painted with a white indicator coating to provide an independent visual measure of the degree of crack growth. Once full cure of the adhesive had been achieved, the specimens were mounted in a 50 kN mechanical test machine.

The specimens were loaded in tension from 0 to 10 kN in 1 kN intervals and the reflection spectrum was recorded at each loading using an optical spectrum analyser. The response to loading was calculated by integrating the area under each spectrum. This analysis was repeated at an elevated temperature (50 ⁰C), which was achieved using halogen heat lamps. The surface temperature at the specimen was measured using a thermocouple that was placed in direct thermal contact. After characterisation of the grating response, cyclic loading was applied to the specimen at an amplitude of 12 kN and frequency of 2 Hz to induce fatigue failure of the scarf joint. This process was repeated for six specimens. The specimens were loaded until there was no further response from the sensor to cyclic loading (i.e. the disbond had grown fully under the grating, which reduced the level of strain transfer to the sensor).

The reflected power from the grating (as calculated by integration of the spectrum from the optical spectrum analyser) for a representative specimen is shown in Figure 12 as a function of applied load. The optical power response to applied load at both temperatures is consistent, confirming the relative temperature insensitivity of the measurement. The monotonic relationship between reflected power and applied load implies that this simple sensor arrangement can provide qualitative monitoring of the integrity of the scarf repair. Figure 6 shows a sequence of 2 second intervals from the output signal of the photodetector, plotted after every 5,000 cycles during the 12 kN cyclic loading. The amplitude of the signal can be used to monitor the actual condition of the bond. The different steps of the disbond growth depicted in the graph as sections I to VI are described above.

When complete load shedding has been achieved there is still a -0.5 V signal from the photodiode, which allows the bond failure to be distinguished from a sensor failure where there would be a null signal. After indication from the sensor that the disbond had reached 2 mm, the specimen continued to carry load for some time before final failure. This shows that the monitoring system is effective and can provide advance warning of an impending structural failure when the structure is subjected to a characteristic loading condition.

A novel structure monitoring technique has been presented, which exploits the unique reflection properties of highly reflective (saturated) Bragg gratings. The approach relies on the fact that the amount of total reflected energy increases for saturated gratings which experience a change in pitch profile. This means that the emergence (or disappearance) of a region of localized strain concentration can be inferred directly from an intensity measurement without the need for an optical filter. The approach is ambient temperature insensitive as ambient temperature changes apply globally to the grating and do not produce any local change in pitch.

Two potential applications were investigated using this sensing approach; the detection of cracks in metallic coupons and disbonds in scarf repair joints. For both applications, the sensing approach gave a clear indication of the evolving damage. In the case of the scarf repair, testing was carried out during thermal cycling and confirmed the relative temperature insensitivity of the measurement.

While these results show the merit of this approach for certain applications, it should be noted that this technique is a qualitative rather than quantitative approach to structural health monitoring. In addition, the localised nature of the sensor means that it would be best suited to locations that have been already identified as likely sites for damage initiation. Cracking and corrosion are considered the most common mechanisms of structural failure in metals. Delamination and debonding are also common failure mechanisms in composite structures. There are many non-destructive techniques available to detect for these failures, however these techniques usually require the structure to be taken out of service during inspection. Structural changes due to corrosion, cracking and delamination often result in regions of stress concentration.

The sensor system described herein can be located in these "critical" areas and monitored in real time using low-cost interrogation equipment. Any increase in detected intensity would imply the emergence of a region of stress concentration that was not present initially and merit further investigation of this area. Alternatively, a permanent reduction in signal intensity would imply the relaxation of a previously existing stress concentration, as is the case for a scarf repair joint disbond, for example. Consequently, this sensor may offer a robust and affordable solution to structural health monitoring in many applications of practical interest.

Depending on the specific structure or quality which the described sensor system is used to monitor, different assembling frequencies may be used. For example, for structures that are expected to experience changes in strain or temperature over a relatively long period of time, the sampling frequency of processor 150 may be relatively low, in order to avoid capturing a large amount of redundant information. On the other hand, where changes in strain and/or temperature may occur relatively quickly or often, the sampling frequency of processor 150 of the photo voltage may be commensurately higher in order to ensure that pertinent information regarding changes in the strain and/or temperature is not missed.

Although the embodiments described above (and experiments conducted) are concerned ■ with monitoring changes in strain for a localised strain concentration of the structure, the principles described herein are also applicable to monitoring highly localised temperature gradients and other environmental factors, conditions or parameters that can cause a localised pitch change of the grating 110 by at least partial transfer of the environmental factors, conditions or parameters to grating 1 10.

While the sensor systems described herein are generally insensitive to changes in ambient temperature that affect the whole system, a high temperature gradient experienced over part or all of grating 110 would be detectable in the same way as a high strain gradient.

That is, the reflected light would become spectrally broadened, which is detectable as a change in optical power of the reflected light reflected from grating 1 10. In embodiments of the sensor system that are used for sensing changes in highly localised temperature gradients, the physical characteristics of the system 100 or 800 generally remain the same, although different packaging materials and/or arrangements for bonding grating 110 to the structure may be employed, if necessary.

Whether the sensor system 100 and/or monitoring system 170 is used to monitor strain or temperature changes, the arrangement of the sensor in relation to the structure to be monitored should be such that the strain or temperature experienced by the fibre along the grating 1 10 is substantially the same as or within a small variation of the strain or temperature experienced by the structure. For some applications in which highly localised temperature gradients are desired to be monitored, there may be no underlying structure to which the fibre is bonded. Alternatively, the fibre may not need to be bonded to the structure to monitor the localised temperature gradient of interest, so long as the grating 110 experiences the localised temperature gradient in a manner sufficient to enable its detection as a function of a change in optical power of light reflected from the grating 1 10.

In further embodiments, such as are depicted in Figure 13, a sensor system 1300 may be provided, with the fibre 105 having more than one grating 1 10 formed along its length and each such grating having a different wavelength at which it reflects light. Thus, sensor system 1300 may be the same as system 100 or 800, except that it uses multiple gratings 110 arranged in series. Although this arrangement would not allow processor 150 to determine exactly which grating was experiencing a change in strain and/or temperature gradient, it would still be able to generate an alert signal indicative of the need for inspection and/or maintenance of the structure. In such embodiments, each grating 110 may be bonded to a respective area of interest of one or more structures and/or parts of structures and the optical power of the reflected light from the multiple gratings would need to be more closely monitored to detect changes in the optical power of the reflected light from any single one of the gratings 110.

In still further embodiments, such as are depicted in Figure 14, a sensor system 1400 may be provided, with multiple fibres 105 and gratings 110 coupled to a single source 130 via a wavelength division multiplexer (WDM) 1420, which may be a passive optical component or a switched device. Reflected light from the various different wavelength- specific highly reflective gratings 1 10 may be received back from the WDM 1420 at separate photodetectors 140, with each photodetector 140 being coupled to processor 150 to provide a photovoltage indicative of the optical power of the signals reflected from a specific one of the multiple gratings 110. Sensor system 1400 thus operates in a similar manner to sensor system 100, except that multiple sensors are provided within the one system by using WDM 1420.

Throughout this specification and the claims which follow, unless the context requires , otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Claims

CLAIMS:

1. A method of monitoring strain in a structure, the method comprising: transmitting light from a light source along an optical fibre having a wavelength- specific highly reflective grating, wherein the optical fibre is arranged at an area of interest of the structure so that strain experienced by the structure in the area of interest is transferred to the grating; receiving reflected light that is reflected from the grating; and monitoring a power of the reflected light to determine a change in the power over time.

2. The method of claim 1, further comprising determining that a localised strain concentration exists in the area of interest based on the change in power.

3. The method of claim 1 or claim 2, wherein the change comprises an increase or a reduction in the power.

4. The method of any one of claims 1 to 3, further comprising bonding the optical fibre to the structure so that the grating is at the area of interest.

5. The method of any one of claims 1 to 4, further comprising determining the existence of a defect in the structure based on the change in power.

6. The method of any one of claims 1 to 5, further comprising determining that a sensor failure has occurred when the power of the reflected light falls below a predetermined threshold.

7. The method of any one of claims 1 to 6, further comprising determining the power based on output signals of a photoelectric device that receives the reflected light.

8. The method of any one of claims 1 to 7, wherein at least part of the optical fibre is embedded in the structure.

9. The method of any one of claims 1 to 7, wherein at least part of the optical fibre is bonded to a carrier which is bonded to the structure.

10. The method of any one of claims 1 to 9, wherein the light source is a broadband light source and a reflectivity of the grating is equal to or greater than 90%.

11. The method of claim 10, wherein the reflectivity is equal to or greater than 97%.

12. The method of any one of claims 1 to 11, further comprising generating an alert signal responsive to determination of a change in the power over time.

13. A system for monitoring strain in a structure, comprising: an optical fibre having a wavelength-specific highly reflective grating, wherein in use of the system the optical fibre is arranged at an area of interest of the structure so that strain experienced by the structure in the area of interest is transferred to the grating; a light source coupled to the optical fibre for transmitting light along the optical fibre; a photodetector coupled to the optical fibre for receiving reflected light that is reflected from the grating; and at least one processor to receive signals corresponding to the reflected light received by the photodetector and configured to monitor a power of the reflected light to determine a change in the power over time.

14. The system of claim 13, wherein the change in the power comprises an increase or a reduction in the power.

15. The system of claim 13 or claim 14, wherein the at least one processor is further configured to determine that a localised strain concentration exists in the area of interest based on the change in the power over time.

16. The system of any one of claims 13 to 15, wherein at least part of the optical fibre is bonded to the structure so that the grating is at the area of interest.

17. The system of any one of claims 13 to 16, wherein the at least one processor is further configured to determine the existence of a defect in the structure when the power of the reflected light decreases.

18. The system of any one of claims 13 to 17, wherein the power of the reflected light is determined by the at least one processor based on output signals of the photodetector that correspond to the reflected light received by the photodetector.

19. The system of any one of claims 13 to 18, wherein, in use of the system, at least part of the optical fibre is embedded in the structure.

20. The system of any one of claims 13 to 18, wherein at least part of the optical fibre is bonded to a carrier which is bonded to the structure.

21. The system of any one of claims 13 to 20, further comprising a bi-directional coupler for directing light from the light source into the optical fibre and for directing light reflected from the grating to the photodetector.

22. The system of any one of claims 13 to 21, wherein a reflectivity of the grating is equal to or greater than 90%.

23. The system of claim 22, wherein the reflectivity is equal to or greater than 97%.

24. The system of any one of claims 13 to 23, wherein the light source comprises a broadband light source.

25. The system of any one of claims 13 to 24, further comprising means for compensating the fluctuations in the light source.

26. The system of any one of claims 13 to 25, wherein the optical fibre has a plurality of wavelength-specific highly reflective gratings arranged in series along the fibre, each grating reflecting light in a different wavelength range and each grating being arranged at a different area of interest of the structure.

27. A monitoring system comprising a plurality of systems according to any one of claims 13 to 26.

28. A monitoring system comprising a plurality of strain sensors and at least one processor, wherein each strain sensor comprises: an optical fibre having a wavelength-specific highly reflective grating, wherein in use of the system the optical fibre is arranged at an area of interest of a structure so that strain experienced by the structure in the area of interest is transferred to the grating; a light source coupled to the optical fibre for transmitting light along the optical fibre; a photodetector coupled to the optical fibre for receiving reflected light that is reflected from the grating; and wherein the at least one processor is configured to obtain signals corresponding to reflected light received by the photodetector of each strain sensor, and to monitor a power of the reflected light to determine a change in the power over time for each strain sensor.

29. A method comprising providing the system of any one of claims 13 to 26 for use in determining that a localised strain concentration exists in the area of interest based on the change in the power over time.

30. A method of monitoring an environmental condition, comprising: transmitting light from a light source along an optical fibre having a wavelength- specific highly reflective grating, wherein the optical fibre is arranged at an area of interest of the structure so that an environmental condition experienced by the structure in the area of interest is transferred to the grating; receiving reflected light that is reflected from the grating; and monitoring a power of the reflected light to determine a change in the power over time.

31. A system for monitoring an environmental condition in a structure, comprising: an optical fibre having a wavelength-specific highly reflective grating, wherein in use of the system the optical fibre is arranged at an area of interest of the structure so that an environmental condition experienced by the structure in the area of interest is transferred to the grating; a light source coupled to the optical fibre for transmitting light along the optical fibre; a photodetector coupled to the optical fibre for receiving reflected light that is reflected from the grating; and at least one processor to receive signals corresponding to the reflected light received by the photodetector and configured to monitor a power of the reflected light to determine a change in the power over time.

32. A monitoring system comprising: at least one processor; a light source; a wavelength division multiplexer (WDM) coupled to receive light from the light source; a plurality of optical fibres coupled to receive light from the light source via the WDM, each optical fibre having a wavelength-specific highly reflective grating formed therein and being bonded to a respective area of interest of a structure; and a plurality of photodetectors coupled to the at least one processor and the WDM to each receive via the WDM reflected light that is reflected from a respective grating; wherein the at least one processor is configured to monitor a power of the reflected light received at each photodetector to determine a change in the power over time.