Strain Measurement on Quasi-Isotropic HTS Strand at 77 K by Fiber Bragg Grating and Extensometer Sensor

This article measures the strain response of quasi-isotropic strand at 77 K when subjected to uniaxial tensile loads by using extensometer sensor and simultaneously verifies the possibility of utilizing Fiber Bragg grating (FBG) sensor for the same measurements. Through matching the measured data from the two sensors, strain sensitivity of FBG sensor is calibrated and fitted to be 1.21 pm/με. In elastic deformation region, the stress-strain curve of the strand captured by FBG sensor coincides well with that from the extensometer sensor, but the range it could reach is only 0.31% strain (εtens ~0.31%) due to the limitation of the elongation. Through fitting the stress-strain curve obtained by the extensometer, elastic modulus 115.5 GPa and yield stress 471 MPa of the strand are, respectively, determined. Then, depending on the measured stress-strain results and the equation on critical current as a function of tensile strain, critical stress of the strand is estimated to be 464 MPa. Furthermore, a three-dimensional finite element method model of the strand established by Comsol software is employed to understand the testing results.