JEC COMPOSITES MAGAZINE - Issue #94 - January/February 2015 - 42

monitoring
optical fibre and the baseline ones. In some cases, those with
the optical fibre had a longer life than those without. The
failed beams were examined to determine the origin of the
delaminations. These did not appear to be associated with
the presence of the optical fibre. The fibre optic sensors also
were durable enough to measure the strain levels throughout
the duration of the test. The full results were reported at the
SAE's Aero Tech Conference held in Montreal in September
2013.

Fig. 3: Tidal Energy Limited, DeltaStream turbine at Pembroke dock
(Picture: ©TEL)

bending. Microbending occurs if the structural fibres in the
surrounding laminates press on the optical fibre so that it
is bent back and forth as it passes over them. If this occurs,
light will escape and that returning to the interrogator can
be reduced to a level where its wavelength can no longer be
measured. Also the exit of the optical fibre from the structure needs to be engineered to avoid the risk of fibre damage.
The beams were subjected to a fatigue test that replicated
the cf and bending strains that would be experienced in service. The repeated flexing of the beam was sufficient to raise
its temperature by several degrees Celsius during the test.
There were two side benefits of the embedded sensor. The
first was that, during cure, it was possible to detect when
the process became exothermal. This feature could be used
to help control the cure. The second effect was that it was
possible to measure the strains locked into the component
during cure. In this case, some 450 µε of compression had
been locked in at the end of the cure process.

Embedded sensors did not bring any premature
failure
The results showed that there was no measurable difference in fatigue life between the specimens with embedded

Multiplexing Fibre Bragg Gratings

Multiple FBGs can be written into a single fibre. There are two
commonly used ways to read the wavelength of the returning
pulse from the individual FBGs on a fibre. One, Wave Division
Multiplexing (WDM), assigns a specific waveband to each FBG.
The other, Time Domain Multiplexing (TDM), uses the different
times that the pulse returns to distinguish between each FBG.
WDM has the advantage that the FBGs can be placed close
together but waveband limits the number of FBGs that can be
read, whereas TDM requires a finite length of fibre between
each FBG but is capable of reading far more. It also has the
advantage that all the FBGs are the same wavelength whereas
for WDM each needs to have a unique wavelength band.

For many applications, the most practical way to install the
sensors is in the form of pre-cured composite patches that
can be bonded to the surface of the structure. Each patch can
incorporate one or more sensors. The patches can then be
supplied as a fully tested array, complete with cable and connectors ready for installation.

Composite patch sensors
A typical recent application was for monitoring the rotor
blades and base structure of the DeltaSteam tidal turbine developed by Tidal Energy Limited (TEL) and due for deployment off the coast of Wales in the next few months. Arrays
of sensor patches were supplied to Design Craft, who was
responsible for building the composite blades. The patches
could then be installed by their own personnel as required
with minimal delay to the production process and without
the need for repeated site visits by Epsilon Optics' engineers.
Tidal energy turbines are subjected to fast tidal flows which
create huge drag forces on the structure. TEL's DeltaStream
turbine relies on its own weight to keep it in place. There
is therefore a requirement to monitor the steel base structure in order to assess how it interacts with the sea bed. In
this case, composite patches were also used, bonding them
directly to the steel surface of the rock feet at each corner
of the structure. Both the composite and the optical fibre
array have excellent resilience to long-term immersion in
seawater. Even though the composite will absorb some water,
this has little or no effect on the fibre optics or measurement
accuracy.
Fibre-optic sensing and composites are highly compatible
and complementary technologies. Provided the correct application techniques are used, fibre optic sensors can be incorporated into almost any composite structure and provide
a reliable, accurate and cost-effective means of monitoring
strains, loads and deflections throughout the service life of
the structure. n

42 jec composites magazine / No94 January - February 2015

More information:
www.epsilonoptics.com
Contact:
info@epsilonoptics.com


http://www.epsilonoptics.com

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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