SELF-HEATING EFFECTS ON STRAIN MEASUREMENTS PERFORMED BY EMBEDDED FIBRE OPTIC SENSORS UNDER CYCLIC LOADING

State of the art Fibre Bragg Gratings are nowadays more and more used for strain measurements in several applications for structural health monitoring. The mechanical strain variation, which is transmitted from the structure under investigation to the fiber sensor, induces a Bragg wavelength shift which represents the measured quantity. This Bragg wavelength shift depends not only on the strain level applied to the FBG sensor, but also on the temperature because of the temperature dependent change in the refraction index (thermo optic coefficient ξ). The total Bragg wavelength shift is therefore the sum of at least two terms, namely the ∆λB,S (strain induced wavelength shift) and ∆λB,T (temperature induced wavelength shift). where λB is Bragg wavelength of the FBG, peff is photo-elastic coefficient of the optical fiber and ∆T is the temperature change experienced by the FBG. Due to their very low intrusivity, FBG sensors can be embedded into composite materials, providing real-time monitoring of the structural component under working conditions. Furthermore, embedded FBG sensors can be used to predict fatigue failure