Simultaneous monitoring of strain and temperature embedded into composites using strength enhanced FBG sensors

Abstract

A sensor design suitable for simultaneous measurement of strain and temperature has been tested on an adaptive composite material. The sensor is formed by two FBGs, which are written in optical fibers with different dopants and then spliced close to each other. A sensing element results with two Bragg gratings having peak wavelengths around 1550 nm of similar strain sensitivity but different response to temperature. This is due to the dependence of the fiber thermo-optic coefficient on the core dopants and dopants concentration. Adaptive composite materials were then produced. The host was unidirectional Kevlar-epoxy prepreg material, in which several $150\um m$ diameter NiTiCu shape memory alloy (SMA) wires were embedded, together with one FBG-based sensor in the center. Strain and temperature during curing were obtained by the FBGs peak wavelength shifts. Finally, activation tests were performed on the finished composites by placing them in an oven up to $100 ^\circ C$, and monitoring the strain and temperature. The experimental strain-temperature curve compared well with an elastic force balance analysis, taking into account the hysteretic behavior of the wire. Also, several factors influencing the mechanical strength of fiber Bragg grating (FBG) sensors during their fabrication were examined and FBG sensors were made under the controlled UV laser condition. A chemical stripping technique to remove the acrylate coating of the fiber and $H-2$ loading treatment showed negligible impact to the mechanical strength of fiber optic sensor. UV laser beam width exposed on the fiber was controlled up to $60 \mu m$ so as to reduce damaged areas induced from laser exposure. Also the pulse fluence and the total dose of a pulsed KrF excimer laser were varied in order to investigate their influence on the mechanical strength. Finally FBG sensors with different reflectivity were fabricated with only slight strength degradation such that they could be used in a structure w...