Investigation of transverse stress measurements by using embedded fiber Bragg grating sensors subjected to host Poisson's effect

In many situations, it is desirable to measure the load acting in a specific direction by measuring the strain induced by Poisson effects in a direction perpendicular to the load direction. For this to be possible, a fixed relationship between the strains in both directions must be known. This can be useful, for example, when the geometry is such that there is not sufficient room to locate a strain gauge parallel to the load direction but a gauge can be placed in a transverse plane. In this paper, we investigate the use of a fiber Bragg grating in such an arrangement with the fiber embedded within the host material. The investigation is done by theoretical, numerical and experimental approaches and we concentrate on two aspects: (a) the non-uniform. strain transfer, particular in axial strains, due to shear-lag effects, and (a) the effect of induced birefringence in the optical fiber due to a load cross to its axis. The results of these approaches indicate that the strains of an embedded fiber sensor subjected to transverse loads are dependent on the location of the embedded sensor and the material properties of the host material. The results also show that when the Young's modulus of the host material is much less than the modulus of the embedded sensor, the Bragg spectrum broadening due to induced birefringence is not significant. However, a lower host Young's modulus also results in longer sections on non-uniform axial strain near the ingress and egress sections of the optical fiber. These two factors must be balanced if we desire to use conventional methods of Bragg grating interrogation that measure only the central wavelength of the Bragg grating's spectrum. In the case investigated (Host Young's modulus of 4.83 GPa) full strain build-up requires similar to 4mm of fiber length at each end. Likewise, the transverse stress coupling into the fiber modifies its wavelength-shift-to-axial-strain-coefficient by about 6%.