Optical fibre Bragg grating sensors have been field-trialed for the monitoring of dynamic loading of fish-plates, stretcher
bars and switchblades on a tram network, with the aim of developing a condition monitoring system. This paper provides
preliminary data showing the ability to identify changes in track/component condition.
A chirped long period grating (LPG) is used as a directional flow sensor by monitoring the change in the transmission
spectrum upon immersion in an oil of refractive index higher than that of the fibre cladding. The change in profile of an
attenuation band is shown to depend on the direction of immersion, indicating the possibility of using such devices in
applications such as resin flow sensing in the manufacture of fibre reinforced plastic composite structures.
A cascaded long period grating Mach-Zehnder interferometer is used to monitor the change in refractive index of a UV
cured epoxy resin over a cure cycle. Fourier techniques are used to calculate the phase shift and frequency spectral
amplitude of the associated fringe pattern during the cure. The results are compared with the refractive index change
during cure calculated using a Fresnel reflection based technique.
Optimum performance from advanced composites requires careful control of the resin matrix during cure. This is to
ensure there are no cure induced voids and to minimise the build up of internal stresses. Careful control of the process is
also necessary to reduce wastage. Traditional resin inspection techniques are bulk or sample oriented and thus cannot
provide data about critical component parts. Optical fibre based sensors however, allow for in-situ monitoring techniques
to be deployed in components without effecting their structural integrity.
In this work, two fibre optic grating techniques are demonstrated as process monitoring sensors and are compared with a
Fresnel refractometric method. The change in refractive index of a resin has previously been used as a means for
assessing the degree of cure. The central wavelength of an attenuation band of a long period grating (LPG) was
monitored during the cure of a resin. In parallel the spectral resonances of a tilted fibre Bragg grating (FBG) are also
monitored. The two techniques are shown to correlate well with the Fresnel based method in both detecting the resin and
monitoring the state of cure, indicating the potential of the techniques for online production monitoring.
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