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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