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1
Development of a Practical Optical Fibre System for
Health Monitoring Composite Structures
Dr. Mark Volanthen*
Insensys Ltd., 6 & 7 Compass Point, Ensign Way, Hamble, Southampton, SO31 4RA, UK
Dr. Peter Foote†
BAE Systems Advanced Technology Centre, Sowerby Building,, PO Box 5, Filton, Bristol, BS34 7QW, UK
and
Dr. Kalliopi Diamanti‡
Hexcel Composites Ltd., Duxford, Cambridge, CB2 4QD, UK
This paper reports important advances in the technology required for monitoring
aircraft structural loads using optical fibre sensors embedded in composite airframe
structures. New, compact, lightweight instrument technology based on Bragg grating
optical sensors (FBGs) is reported. Also included are reports of design and testing of a
new system for embedding robust fibre connectors as an integral part of aerospace
composite structures, and methods for easing the handling of the optical fibre during the
manufacturing process. The combination of the three elements results in a practical
system for the use of FBGs to monitor composite structures. This has led to an increasing
number of applications including the use of composites to carry FBGs to monitor metallic
structures and to provide robust patches for surface bonding to all forms of structures.
I. Introduction Monitoring the usage of aircraft structures by measuring loads at key points on an aircraft allows the
consumption of fatigue life and hence the remaining life of an airframe to be accurately determined. Systems
based on strain gauge measurement or calculation from recorded flight parameters (or a mix of the two) is
becoming established as essential technology to underpin the fleet management of modern military jets.
With an eye to future systems, significant strides to evolve the sensor technology from conventional
electrical strain gauges to fibre optic strain sensors have been made in recent years. The technology especially
that associated with the Optical Fibre Bragg Grating (FBG) is rapidly maturing with a burgeoning commercial
supplier base and the size, weight and cost of equipment dramatically falling.
Optical fibre sensors for measuring strain and temperature offer a number of advantages over their electrical
strain gauge counterparts. Optical fibre sensors are lighter, smaller, easier to deploy and immune to electrical
interference. The benefits are best summarised by the example of single optical fibre, weighing a few milligrams,
containing a string of more than 100 strain sensors, all accessed via a single connector. This is now a reality with
fibre optics and is impossible to achieve with electrical strain gauges.
Perhaps one of the most significant attributes of fibre optics is that the small size and mass also allows these
sensors to be embedded into structural composites enabling ‘smart structures’ 1,2
. With this capability, the
sensors cease to be an add-on feature and become part of the structure itself. As an integral element of the
structure the sensors are also able to play a key role in validating the design and for other functions such as
damage detection.
A number of recent developments have combined to bring the technology to a point where it is now poised
to enter the aerospace domain and this paper reports on three of the most significant:
1) The development of robust, lightweight systems for interrogating multiple FBGs on a single fibre
2) A solution to the problem of forming robust connections between fibre optic sensors embedded in
carbon fibre composite structures and external cables 3-5
3) A technique that significantly increases the robustness of the optical fibre and eases the problems of
handling them during the manufacture of the composite component.
The paper also provides a number of examples where these developments are being used. This experience
further demonstrates the maturity of the technology and its increasing suitability for aerospace use.
*Director, Insensys Ltd, 6 & 7 Compass Point, Hamble, Southampton, SO31 4RA, UK † Optics & Laser Technology Department, BAE Systems plc, Advanced Technology Centre, Sowerby Building,
Filton, Bristol, BS34 7QW, UK ‡ Process Technologist, Prepreg and Adhesives Process Development, Hexcel Composites Ltd,
47th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Confere1 - 4 May 2006, Newport, Rhode Island
AIAA 2006-2116
Copyright © 2006 by Insensy Ltd, BAE Systems plc and Hexcel Composites Ltd. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
2
II. Development of Lightweight and Robust FBG Interrogation System FBGs are entirely passive, in-fibre devices that are manufactured within a host optical fibre through
exposure of the silica core to intense radiation from an ultraviolet laser 3-5
. In their simplest form they can be
considered as miniature in-line mirrors, which can strongly reflect optical signals over a narrow (user definable)
range of wavelengths, yet cause little attenuation to the transmission of signals at all other wavelengths. This
versatile optical filtering property has meant that FBGs have been very successfully employed in a range of
signal conditioning applications within telecommunications, but ever since their initial discovery the devices
have also been noted for their excellent sensing properties. An increase of either the ambient temperature or the
external strain that is applied to an FBG will result in a shift of the optical filtering profile of the device towards
longer wavelengths. As such, by illuminating an FBG using a broadband optical source and measuring the
wavelength of the resulting reflection signal, an assessment of the physical conditions experienced by the device
is possible 6,7
. When operated in this manner FBGs form highly sensitivity, passive, all-optical sensors that
allow remote measurement of temperature or strain.
Furthermore, FBGs also have the advantage that they can be used to form the basis of more complex
transducers, which provide measurement of other environmental, physical, chemical and electrical variables.
Some examples of applications that have employed such sensors have included the detection of gases and liquids 8, the measurement of strain
9, temperature
10, salinity
11 and corrosion
12 and the analysis of pressure
13, vibration
14 inclination
15 and fluid flow
16.
However, it is not only the diversity of measurements recorded using FBGs that make them important
devices for optical fibre sensing; one of the most fundamental benefits is their inherent potential for
multiplexing. Since FBGs are typically employed as wavelength selective reflectors, multiple transducers can be
independently operated within a single length of optical fibre. The only requirement for this mode of operation
is that the wavelength interrogation equipment is able to identify the individual reflection signals returned from
each sensor.
A. Wave Division Multiplexing
Numerous techniques have been proposed that have the potential for multiplexing FBG sensors within a
single fibre, but of these by far the most commonly exploited is wavelength-division-multiplexing (WDM) 6,7
, as
shown in Figure 1. This approach employs application-specific sensor arrays, which contain FBGs that are each
manufactured to operate within a different wavelength window. The sensors are typically illuminated from a
single continuous wave, broadband optical source, such that the reflections from all sensors are returned
simultaneously to the wavelength measurement system. Since each of the reflection signals is of distinct
wavelength the identification of individual sensors can occur with relative ease if the correct detection
components are chosen. Such WDM systems can be developed with common bench-top laboratory equipment
and can provide adequate performance for many research applications.
Figure 1. The general arrangement for a wavelength-division-multiplexed fibre Bragg grating
sensor interrogation system
Unfortunately however, despite the simplicity of the WDM technique, there are a number of restrictions that
limit its performance and commercial viability. Since each of the FBGs is required to operate within a distinct
optical window, the maximum number of sensors that can be multiplexed onto a single optical fibre is often
fewer than 10; the finite operating bandwidth of both the optical source and the wavelength measurement
components means that a trade-off exists between the measurement range of each sensor and the total number of
sensors that can be used. The requirement for bespoke array design, such that each FBG is of a unique
wavelength (chosen according to the desired sensor range) also causes considerable increases in costs; sensor
yields are reduced while the fabrication time is greatly increased as a result of continuous tooling changes, stock
inventories become more complex and delivery lead-times are increased. Finally, the WDM requirement for
Broadband
optical
source 50:50 fibre
coupler
Fibre Bragg grating sensors
External parameters being measured Multi-wavelength
detection system
λ
PIncident
optical signal
λ
P
Resultant
reflection
signal
λ
P
λ
PIndividual
reflection
signals
λ
P
Results
3
individual identification of multiple, simultaneously received reflection signals places a limit on the variety of
wavelength measurement techniques that can be employed; some of the lower cost, passive approaches cannot
be used 17, 18
.
B. Time Division Multiplexing
As a result of the limitations of the WDM technique, several other approaches have been proposed for
multiplexing FBG sensors, of which time-division-multiplexing (TDM) is perhaps the most common 6,7
. In
contrast to WDM, which identifies each of the sensors by their unique wavelength, TDM employs sensors that
are all at the same nominal wavelength and of low reflectivity. Individual sensors are uniquely identified by
measuring the time of flight of the reflections generated from illumination of the sensors by a pulsed optical
source. The sensors at more distance positions in the sensor array will have reflections that arrive at the
wavelength interrogation system later in time, as shown in Figure 2.
Figure 2. The general arrangement for a time-division-multiplexed fibre Bragg grating
interrogation scheme
The TDM approach has a number of key advantages that arise from its use of identical wavelength, low
reflectivity sensors. Firstly, the maximum number of sensors that can be supported is far higher than for WDM
systems, as the bandwidth of the source and detector do not have to be divided between the sensors. Bespoke
sensor array design is not required as all sensors can operate with the full working range of the system. The
actual cost of array manufacture is therefore significantly cheaper since no tooling changes are needed and only
low reflectivity FBGs are required; such factors remove the requirement for expensive manual intervention,
allow the use of through-coating fabrication 19
and greatly increase yield. Large spools of identical sensors can
be manufactured in a single operation and can then be used for many different installations. This approach
exploits economies of scale, enables a reduction in the diversity of the stock inventory and decreases the project
lead-time.
Insensys has developed an interrogator unit that uses unique optoelectronic architecture capable of high
resolution over a wide operating range (normally ± 5000 micro strain) and has demonstrated that it can
interrogate over 100 gauges, each to their full range, on a single fibre. The interrogator unit is small, occupying
a single integrated circuit board, light weight at 22oz (for aerospace applications this can be further reduced) and
requires only some 3 watts of electrical power. The unit can either be packaged as a standalone unit or
integrated into a larger Line Replaceable Unit (LRU).
As a standalone unit it has already been deployed to measure strain in the offshore oil and gas industry and
to provide dynamic control in wind turbines. A system has also been fitted to a naval frigate where it is being
employed for the long term measurement of hull loads. The system will shortly be flown in a UAV and a
number of other aerospace applications are currently being considered.
III. A New Approach to Embedding Fibre Optic Connectors in Aerospace Carbon Fibre
Composite Materials. Aerospace carbon composites are typically high modulus thermoset material manufactured from pre-
impregnated laminates, laid up into shaped components on a tool face. The assemblies are then cured at
temperatures up to 180 C and pressures of 6.5 bar in autoclaves. Optical fibres containing sensors such as Bragg
gratings are laid between the laminate plies creating virtually no disruption to the surrounding material (primary
jacketed optical fibres are typically only 130 - 150 microns in diameter). In the past, the point where the fibre
emerges from the laminate has created a vulnerable interface both for the fibre and the composite material. The
easiest solution is to allow the fibre to emerge from the edges of the laminate. However, this approach means
that no machining of the laminate can be performed at those edges where optical fibres emerge. This is often an
Pulsed
tuneable
laser source 50:50 fibre
coupler
Identical low reflectivity FBG sensors
External parameters being measured
Very fast gated
photodetector and
signal processing
t
PTiming of
incident signal
t
P
Timing
of resultant
reflections
t
P
t
PTiming of
individual
reflections
t
P
Results
λ
P
The tuneable laser optical source produces a series of very
short pulses as it is scanned across the operating
wavelength range of the fibre Bragg grating sensors. High
speed gating in the optical detector system is then used to
distinguish between the reflections from individual sensors
using time-of-flight measurement.
4
unacceptable constraint on manufacturing so an alternative arrangement must be found in which the fibre
emerges through the surface of the laminate, away from edges. Until the development reported in this paper, the
harsh manufacturing conditions and vacuum bag curing arrangements imposed severe limitations on the
practicality of embedding fibre optic in this way.
BAE Systems and Insensys working with UK connector company Deutsch Ltd have evolved an entirely
new connector concept that overcomes these limitations by adapting connector design to the composite lay-up
and manufacturing process. This was achieved by designing a multipart connector comprising a partially
embedded portion with a fully terminated length of optical fibre containing sensors. The fibre and the connector
base are incorporated into the surface plies of a carbon fibre structures as the laminates are being laid. The
connector can be placed anywhere on the composite surface. Once embedded, the connector base is encased in
customized tooling designed to seal outside the vacuum bag assembly while curing takes place in the autoclave.
When the manufacturing cycle is complete, the connector tooling is removed along with the bagging
material. The composite components can then be machined and trimmed in the normal way while the connector
base is still protected with a protective cap. Once the machining of the structural item is complete, the protective
cap is removed and the upper half of the connector is assembled as shown in Figure 3. The upper portion of the
connector incorporates components and processes that have already been qualified to military aerospace
standards and embodied in aerospace products currently supplied by Deutsch. The connectors are easily mated
and de-mated using a standard threaded coupling mechanism.
A prototype design for a surface emerging connector / plug assembly was used in conjunction with custom
tooling in manufacturing trials within BAE Systems in the UK. High strength, thermoset carbon fibre material
was used to manufacture structural test components.
The components containing embedded connectors were then subject to a series of tests in a preliminary
evaluation of performance. These tests were:
1) Repeated mating / demating of the connector
2) Combined thermal and humidity cycling between limits of –55° C and 120°C and 0 – 90% humidity
3) Vibration of 10g between 5Hz and 2 kHz (sine wave excitation) at ambient temperature and at 120°C
During all of tests, the optical power transmission and optical reflected power (return loss) was measured
continuously.
Figure 3. Embedded optical fibre connector assembly showing surface-emerging, partially
embedded female connector and mating plug connector and cable.
The results obtained for optical power loss following termination and embedding of single-mode fibre was
comfortably within the target design limits for load monitoring systems, as was repeatability measurements and
environmental conditioning for temperature, humidity and vibration. The full characterisation of these and
derivative developments is the subject of ongoing work.
In addition to the surface mounted connector detailed above, an edge connector has also been developed.
5
IV. Development of Manufacturing Compatible Optical Fibre Carrying System Hexcel’s development has focused on taking the Insensys optical fibre sensors and processing them to
provide simple and effective ways of deploying them in composite structures, Figure 4. The team has
successfully developed techniques for embedding sensors in new composite structures, using Hexcel’s prepreg,
and for retrofitting optical sensors into existing structures, using surface bonding techniques. The various
options are described below.
Figure 4. Prepreg tapes and towpreg products carrying optical fibres.
A. Prepreg tape
The ready sensored optical fibres are introduced with the fibre tows in the initial stage of prepreg
manufacturing passing through sets of rollers to achieve the optimum fibre spreading. The spacing of the optical
fibres is set according to the required width of prepreg tapes. The resin films are then fed on top and bottom and
pass through consolidating rollers to form the pre-impregnated material. Finally the prepreg is slit into tapes of
the required width before being rolled on the spools.
Another option is to embed the optical fibres centrally between two prepreg webs. The prepreg is slit into
tapes of the required width before being rolled on the spools. The number and spacing of optical fibres can vary
according to the final structure requirements.
B. Towpreg
A Solution Dip process is used for this product form. The ready sensored optical fibre is incorporated into
fibre tows positioned centrally in the width and thickness of the final product. The bundle of fibres passes
through a set of rollers controlling the width of the tow. It is then submerged in resin to hold them in place. A set
of metering bars is located after the resin bath to control the resin content. The impregnated tow is then fed into
drying ovens. The initial product form has been presented with a poly interleave though this may not be essential
on further development work.
C. Cured laminates
This product form can be made from either woven or UD products, glass or carbon and any combination of
these materials. The optical fibre is positioned on the surface of one ply of the composite product. The other ply
is then put on top of this. The lay-up is then fully cured maintaining appropriate protection of the exposed ends
of the optical fibres.
A smart towpreg product has been manufactured for use in composite pipes. The pipes were manufactured
using the filament winding process. The towpreg protected the optical fibre permitting easy, quick and accurate
placement on the pipe. Bare optical fibre would have to be placed by hand as it would be too fragile to be applied
by the filament-winding machine. Any length of towpreg can be provided. The customer can use the required
amount, cut it from the bobbin and retrieve the optical fibre ends to make the electrical connections required.
The sensors are marked on the optical fibre and the spacing can vary according to application. The different
products, Figure 5, that have been manufactured are described in Table 1. Any combination of fibre and resin
can be used to match the specific composite structure requirements.
6
Figure 5. Different towpreg products with glass (a,c) and carbon (b) tows, single (a,b) or double (c)
optical fibres.
Description Figure
600tex E glass tow with single optical fibre 6 (a)
1600 tex carbon tow with single optical fibre 6 (b)
1530 tex glass with 2 optical fibres 6 (c)
Table 1 Towpreg product variants that have been produced
Preliminary mechanical test data shows that there is no significant effect on the tensile strength of the
laminate from the embedded optical fibre. Towpreg with SMF optical fibre of 250 microns diameter was
embedded in a UD laminate of the same material (fibres and resin). The results from both the control specimen
and the one with the embedded towpreg are presented in Table 2. The microscope pictures in Figure 6 show a
good fibre bond interface in both carbon and glass fibre prepregs.
Material control embedded
HexPly®
M9.6F/35%/600/HS 1859 ± 73 1788 ± 33
HexPly® M9.6/32%/1500/G 1138 ± 47 1148 ± 76
Table 2 Tensile strength (MPa)
Figure 6. Optical fibre embedded in (a) HexPly® M9.6F/35%/600/HS and (b) HexPly®
M9.6/32%/1500/G prepregs.
(a) (b) (c)
(a) (b)
7
V. Examples of Recent Applications A. Embedment in Composite Structure – Large Transport Winglet Spar
A number of FBGs on a single optical fibre were embedded in both the web and flange of the forward spar
of a development composite winglet for a large civil transport aircraft. The installation used the Deutsch
connector to facilitate the manufacture of the spar. Using the TDM interrogation system the FBGs were
successful in measuring the strain in the spar including the shock loads induced by both lightning and bird strike
tests. The ability to detect strain during the lightning strike demonstrated the systems resistance to the effects of
high voltage electrical pulses. Figure 7 shows the recording from one FBG during the bird strike test.
Figure 7. Bid strike impact on a composite winglet demonstrator.
Figure shows the output from three FBGs mounted on the front spar
B. Composite Carrier Used to Measure Loads in a Steel Oil and Gas Riser Pipe
In deep water operations tidal currents can cause vortex induced vibrations in the riser pipes that bring the
oil and gas from the sea bed to the surface. These vibrations induce strains in the pipe, which can result in
premature fatigue failure. The requirement was to measure the strain induced in a 6 inch diameter steel riser
pipe and to use the resultant data to help predict the residual life remaining in the riser. The solution was to
embed an optical fibre in a composite half section tube that could then be strapped to the pipe in such a way as to
follow the shape of the pipe as it flexed. The optical fibre was placed so that FBGs were positioned alone the
both edges and the centre line of the half pipe. The resulting system was able to detect strains down to 1
microstrain in the riser pipe. The TDM interrogator was mounted in a container fitted to the half pipe, with data
being transferred to the surface using an electrical umbilical. The system has been successfully deployed at
depths of 6000 feet on an oil platform in the Gulf of Mexico, Figure 8, and has been in operation for many
months.
Figure 8. Oil and gas riser pipe strain monitor
Optoelectronics Deployed Sub-sea
Sensors delivered as single robust unit
capable of detecting strain in pipe down to 1
microstrain
Electronic Interface to Rig
System clamped to pipe to ensure compatibility with
existing equipment and methods
8
C. Composite Riser Pipe
The Oil and Gas Industry’s Joint Industrial Project “Deep Star” is investigating the problem of vortex induced
vibrations on riser pipes with the aim of improving the understanding of this phenomenon. As part of that
project there was a requirement for an instrumented pipe to collect data to verify the predictive models. The
pipe, 500feet long and 1.3 inches in diameter, was manufactured from composite material. Two hundred and
eighty FBGs were embedded using the Hexcel towpreg during the spiral winding manufacturing process. The
sensors were orientated axially in groups of 4 (1 in each quadrant) every 7 feet down the length of the pipe. By
measuring the differential strain across the pipe at each measurement location, the complete shape of the pipe at
any instant was determined. The system enabled the curvature of the pipe to be measured continuously in two
axes along its whole length with very high resolution and bend radii out to 10.25 miles were detectable.
Figure 9. Deployment of Deep Star Riser Pipe for Vortex Induced Vibration Trials The pipe was towed through the water to create the vortices the resultant strains in the pipe being
measured using sensors embedded during manufacture.
D. Surface Bonded Composite Patches to Provide Data for the Active Control and Health Monitoring of
Wind Turbines.
Due to their proximity to the ground, wind turbines operate in turbulent conditions and are subject to wind
shear. This results in the blade when it is in the upper quadrant experiencing high wind speeds than when it is in
the lower quadrant. In turbines where all blades are set to the same pitch this results in an out of balance load on
the main bearings and a consequential reduction in operating life. To overcome this problem composite patches
containing a number of FBGs, see Figure 9, are bonded to the internal surface of the blade to measure strain and
hence the loads. A single TDB Interrogator is mounted in the rotating hub and monitors all three blades with a
reading every 3º of rotation. This data is then used to determine the lift generated by individual blades. The
pitch of each blade is then adjusted as they rotate so that the loads in all 3 blades are in balance. The system is
designed for a life of 20 years and has shown both a significant increase in the lives of the rotating components
and improvements in power output. Additional sensors, monitored by the same interrogator, are fitted to monitor
the structural health of the blades.
9
Figure 10. Typical Sensor Patch for Wind Energy Turbine Blade Active Control System
The patch contains two FBGs at 0º and 90º orientation with third FBG for temperature compensation
VI. Conclusions With the three advances detailed above and the confidence gained from applications in the offshore oil and
gas, and wind energy sectors, FBG based strain and temperature measurement has now reached at a state of
maturity where it is a practical proposition for aerospace applications.
The development of a TDM based interrogator unit which is capable of monitoring large numbers of FBGs
on a single optical fibre greatly simplifies the installation as it is now possible to achieve a comprehensive
coverage of major structures with a small number of fibres. The reduced number of fibres also reduces the
number of connections required. The development of connector technology suitable for embedding in high
quality aerospace grade carbon fibre composite structures complete with the processes and tooling that is
compatible with current composite lay-up and other manufacturing process further reduces the task. By
providing a method for carrying those fibres, which is robust, the difficulty and cost of inserting the fibres into
the structure during manufacture is also greatly reduced.
The low power requirement and weight of TDM based interrogator is a further advantage in that it is easy to
integrate into the avionics system. The low power requirement has a further potential benefit in that it makes it
practical to monitor structural elements using battery power and hence detects events that occur when the aircraft
is on the ground.
Insensys together with BAE Systems, Deutsch UK and Hexcel are currently examining a number of
aerospace applications.
Acknowledgments The Authors thank:
Insensys Ltd, 6 & 7 Compass Point, Ensign Way, Hamble, Southampton, Hants, SO31 4RA, UK
BAE Systems Advanced Technology Centre, PO Box 5, Filton Bristol, BS34 7QW, UK
Hexcel Composites Ltd, Duxford, Cambridge, CB2 4QD, UK
Deutsch UK Ltd, Castleham Industrial Estate, 4 Stanier Road, St Leonards on Sea, E. Sussex, TN38
9RF, UK
GKN Aerospace, Whippingham Road, East Cowes, Isle of Wight, PO32 6LR, UK
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