Discriminated measures of strain and temperature in metallic specimen with embedded superimposed long and short fibre Bragg gratings

We propose a superimposed fibre Bragg gratings device to measure, localize and discriminate strain and temperature effects simultaneously for structural health monitoring. Long period grating (LPG) and fibre Bragg grating (FBG) exhibit different responses to an applied perturbation; thus, strain and temperature influences can be determined separately by measuring the corresponding wavelength shifts. In this paper, we present a configuration based on the use of these two grating types: a LPG and a FBG written in the same fibre section which allows us to discriminate the contributions of these two main perturbations. The sensor is calibrated in a temperature range from 22 °C to 120 °C and in a strain range from 0 to 1400 με. The reported errors are estimated to be within ± 0.4 °C and ±5 με respectively. Our sensor is compared to those suggested in the literature for the discrimination between strain and temperature with Bragg gratings. We propose a parameter E which allows us to compare the relative uncoupling efficiency of those techniques. These sensors were embedded and tested in metallic material plates for the purpose of validating structural health monitoring.