Reducing Residual Strain in Fiber Bragg Grating Temperature Sensors Embedded in Carbon Fiber Reinforced Polymers

The temperature response of fiber Bragg gratings (FBGs) embedded in carbon fiber reinforce polymers (CFRPs) is investigated in this paper. To provide strain-free temperature measurements, two pieces of anti-sticking materials are placed at both sides of the embedded optical fiber and between carbon fiber prepregs; thus, providing slippery surfaces that minimize the strain transfer to the FBG sensor. In particular, the impact of different anti-stick materials on the temperature and residual bending strain response of the embedded FBGs is experimentally investigated. Results demonstrate that although some materials can allow for minimum residual strain being transferred to the FBG, their thermal conductivity does not always fulfill the requirements for reliable temperature sensing. It is found out that, among the tested materials, aluminum and copper foils can provide both reliable temperature response (with negligible delay and bias) and minimum residual strain. Using such anti-stick materials, the error induced by the residual strain on FBG temperature measurements is also experimentally evaluated by applying temperature and bending loads (strain) simultaneously to the CFRP packaging. While the study is here performed for FBG-based point sensors, most of the results and conclusions are also expected to be valid for applications of embedded distributed optical fiber sensors being affected by strain-temperature cross-sensitivity issues.