A temperature-compensated strain-sensing scheme based on fiber Bragg gratings (FBGs) that is suitable for strain mapping applications is described. FBGs are bonded to a backing patch, together with an extra grating that is used for temperature compensation/measurement. The patch provides a simpler and more robust way of attaching the FBGs to a structure than directly mounting a bare fiber, though it was necessary to design it in such a way that there was minimal reduction in the strain transferred from the structure to the sensing fibers. Finite element (FE) analysis was used to help design the patch, which was then constructed accordingly. The authors have demonstrated experimentally that the use of the backing patch produces a reduction in strain sensitivity of only around 4%, which is slightly better than theoretically predicted values. The temperature measuring FBG had to be bonded in such a way that it experienced the changes in temperature, but not the strain, to which the structure was subjected. A design for doing this was developed and proven. The use of a backing patch to develop a rosette configuration of Bragg gratings, each having a different peak reflective wavelength, is described. The rosette configuration is one that is frequently used with electrical strain gauges and allows here to determine both the magnitude and the direction of strain.
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