Development of fiber Bragg grating strain sensor with temperature compensation for measurement of cryogenic structures

It is important to discriminate between mechanical strain and thermal output (apparent strain) in fiber Bragg grating (FBG) strain sensors, due to their sensitivity to both mechanical strain and thermal output. Additionally, when FBG sensors are directly bonded on monitored structures, the transverse effect and the strain-transfer error must be considered to correctly measure the strain. The objectives of this study are to develop pre-tension packaged FBG strain sensors with temperature compensation for use in cryogenic environments and to clarify the performance of the developed sensors in the range from room temperature to 4 K. A developed FBG sensor comprises sensor mounting parts and two FBG that are placed in a row on a single fiber. The one of the FBGs is a strain sensor and the other is a temperature compensation sensor. The developed FBG sensor intrinsically prevents both the transverse effect and the strain transfer error. Experimental investigations are conducted to clarify the performance in the temperature range from mom temperature to 4 K for the developed FBG strain sensors. Results confirm that the developed FBG strain sensor demonstrates superior gauge factor stability and a reasonable temperature compensation function. Moreover, the compressive fatigue strength of the developed FBG strain sensor is investigated due to its importance in cryogenic structures that are subjected to cyclic compressive deformations by the repeated operation of cooling and warming processes.