Multiplexable fiber Bragg grating temperature sensors embedded in CFRP structures for cryogenic applications

This paper presents the development of a special fiber Bragg grating temperature, FBG-T, sensor that can be embedded in carbon/epoxy composite material, CFRP, and operates at cryogenic temperatures. The temperature sensitivity of the developed sensor at 20K, which is almost zero in standard uncoated FBG-T sensors, has been improved to about 1,5 pm/K by covering the sensor with a miniature steel tube, so enhancing its thermal expansion behaviour. The performance of the FBG-T sensor in comparison with applicable theoretical calculations is also given. Two designs for the structural integration of the sensor are presented: first, a miniature CFRP tube where the sensor ends inside, and second, a stainless steel tube that enables the multiplexing of the embedded sensor in line with other following FBG strain or temperature sensors. The developed FBG-T sensor and the multiplexed sensor design have been embedded in a technological demonstrator that is representative for the wall structure of a liquid hydrogen tank for reusable launch vehicles and have been functionally tested at 20K.