JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 41

Fig. 1: Rotor head

reduction in structural properties has been observed. However, it is worth relating an example of one such investigation
into a composite component which is required to withstand
a fairly extreme combination of strain and fatigue cycles.
Recently, Epsilon worked with Bell Helicopters on a project
funded by the Vertical Lift Consortium to better understand
the effect of incorporating optical fibre within a composite
component.
Helicopter rotor heads are subjected to very high levels of
load. Bell uses composite beams that flex to provide the necessary freedom and to withstand centrifugal loads generated
by the spinning rotor blades. These beams predominantly
carry a tension load due to the centrifugal forces (cf ) but
also twist and bend. As a result, they experience strains on
the top and bottom surface in excess of 15,000 µε. These are
well beyond the limit for conventional strain gauges. The
only way to overcome this has been to attach gauges on the
edge where they are closer to the neutral axis and therefore
experience lower strains. However, the width of the gauges
complicates the calculation and even in this position, the

strains are such that bond failures are frequent. Optical fibres are well able to withstand strains in excess of 15,000 µε
and, using composite patches, can be bonded to the surface
and endure very high numbers of loading cycles. However,
the wavelength shift associated with these high strains presents a problem in measuring using existing interrogators.
In a previous programme, attaching the sensors at an angle
to the main load direction to reduce the strain was tried.
However, even though fibre optic sensors do not suffer from
Poisson's ratio effects, the complicated strain pattern meant
that it was very difficult to separate out the strains due to the
different forces. However, if a sensor could be placed in the
direction of the predominant cf force within the structure
closer to the neutral axis, it would solve this problem. The
answer was to use optical fibre sensors embedded within the
flex beams. Buried in the flex beam, the sensor would also be
protected from damage.
With Epsilon's assistance, Bell manufactured a number of
trial flex beams. In half of them, optical fibres were embedded within the ply stack approximately midway between the
neutral axis and the surface. The test beams were made using
prepreg material in a closed mould under significant pressure. Epsilon was responsible for developing the technique
for embedding the fibre optic sensor. This needed to ensure
that the optical fibre was fully bonded in, but that this was
achieved in such a way as to minimise the risk of micro-

More information ...

FBGs are produced using a UV laser to modify the refractive
index of the glass in the core of the optical fibre to form an
interference pattern. This can be pictured as a series of stripes
over a length of typically 5 to 10 mm of the optical fibre core
with around 1300 "stripes" per mm. If a broad band pulse of
light is sent down the fibre, the interference pattern will allow
most wavelengths of light to travel freely down the fibre while
strongly reflecting one particular wavelength. This wavelength
is equivalent to twice the pitch of the interference pattern so
that any strain induced in the optical fibre will increase the pitch
of the interference pattern and, therefore, the wavelength of
the reflected light. In this way, the FBG provides a direct and
virtually ideal means of measuring strain.

Fig. 2: Installation of sensor patches using epoxy and vacuum bagging
(Pictures: © Magma Structures)

No94 January - February 2015 /

jec composites magazine 41



JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015

Cover
Edito
Point of view: Raw materials
Contents
Companies & Business
In brief
Resins - lnterview
Agenda of Events
Applications
Aerospace
Defense
Sustainability
Market
Overview 2014
China - Interview
FEATURE CARBON
Carbon
Training
Investment - Interview
Simulation
CFRT
Nano
Equipment
Marine
Research & Development
Monitoring
LCM
Technology & Innovations
Manufacturing
Pultrusion
NDT
Matrix
Simulation
Index
In the world
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Cover
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 2
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Edito
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Point of view: Raw materials
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 5
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Contents
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 7
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - In brief
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 9
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 10
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Resins - lnterview
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 12
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 13
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 14
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Agenda of Events
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Ad1
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Ad2
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Aerospace
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 17
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Defense
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Sustainability
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Overview 2014
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 21
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - China - Interview
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 23
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Carbon
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 25
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Training
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Investment - Interview
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 28
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Simulation
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 30
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 31
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - CFRT
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 33
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Nano
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 35
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 36
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Equipment
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Marine
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 39
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Monitoring
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 41
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 42
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - LCM
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 44
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 45
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Manufacturing
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 47
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Pultrusion
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 49
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - NDT
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Matrix
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 52
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 53
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Simulation
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 55
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 56
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - Index
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - In the world
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 59
JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 60
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