Abstract:
This thesis is concerned with the interfacial and durability aspects of Extrinsic Fabry-
Perot Interferometric (EFPI) sensors embedded in carbon fibre reinforced composites.
Fibre optic sensors are being used in many long term applications and, as is the case for
all sensor types, the ability of the EFPI sensors to monitor accurately the measurands of
interest over the lifetime of the structure must be proved. Therefore, the aim of this
work was to examine the interface between the EFPI sensors and the structures, and
then to evaluate the durability of that interface and the sensors.
The first stage was an examination of the EFPI sensors including the method of
manufacture, interrogation option and inherent strength of the sensors. It was found that
the sensors have a very low tensile load to failure (~0.5 N). This was improved by
using a resin reinforcement, which was applied to the capillary ends. However, this had
implications for the overall sensor size and that influenced their embedment suitability.
The second stage was interfacial characterisation; this was achieved through the
examination of the surface energy of the sensors, carried out by contact angle
measurements; and the interfacial shear strength of the sensors to matrix, using a new
variation on the single fibre pull-out technique that involved the use of optical fibres
and composite prepreg. Overall, it was found that the silane treatment of the fibres
increased the surface energy but for the interfacial shear results the data was less
conclusive due to the scatter present within the results.
The durability of the sensors was examined through their embedment into carbon fibre
composite samples and exposure to tension/compression fatigue loading. From initial
quasi-static work it was found that the embedment of the sensors had no significant
effect on the composite samples. However, the sensors failed at a strain levels of 0.4%
in tension and at 1.1% in compression; the compression strain level was at the point of
composite failure. Under fatigue loading the sensors could survive a million cycles at
R=-1 at a max stress level of 156 MPa and maintain their reliability. If the tensile
loading was increased then the sensors would fail within a few thousand cycles.
However, if the compressive stress was increased the sensors survived but the reliability
was affected. Overall, it was felt that with some improvements to the sensor design
they should be able to survive and provided useful data when exposed to axial
tension/compression fatigue regimes.
