Electromechanical Behavior Of High Temperature Superconducting Coated Composite Tapes Under Combined Tension-Torsion Deformation

High-temperature superconducting REBCO coated composite tapes can realize superconductivity state in liquid nitrogen environment and have broad application prospects in the development and utilization of new energy,modern transportation,electric power transportation and other technology frontier fields because of their excellent high critical magnetic field and high critical current.However,due to the special multi-layer composite structure of REBCO conductors and the extremely low temperature,the superconducting composite tapes show complex stress-strain characteristics under external loads.The analysis of the mechanical behavior of the tapes under complex loading,especially the stress/strain distribution in superconducting layer,is beneficial to reveal the electromechanical behavior of tapes.A systematical experimental and numerical study was carried out on the mechanical behavior of HTS REBCO coated composite tapes under uniaxial tension,torsion and tension-torsion deformation and the superconducting current carrying properties.In the experiment study,a low-temperature tension-torsion combined deformation device was designed to test the electromechanical properties of the REBCO coated composite tapes under uniaxial tension,torsion and tension-torsion deformation.In this device,the stable low-temperature environment was realized by the way of liquid nitrogen immersion,and the low-temperature resistance strain gauges were used to measure tape's deformation under tensile load with high accuracy.The synchronous acquisition of the mechanical signal can be realized,through the experimental test system of superconducting mechanics in low temperature/variable temperature environment.Firstly,we carried out the tensile experiment during loading and unloading processes.The results showed that with the increase of the tensile strain,the degradation behavior of the critical current(I_c)presented two obvious stages:reversible stage and irreversible stage.While the uniaxial strain reached 0.7%,I_c did not degenerate,and can be restored to 99%of I_c0.When the tensile strain was larger than0.7%,I_c degraded sharply,and only partially returned to the I_c0 after unloading.Additionally,the behavior of I_c was similar with above mentioned,during loading-unloading experiments of pure torsional and tension-torsion combined deformation.For example,when the strain was small,I_c had no degradation,and I_c could recovery to the original value after unloading.While the strain was large,I_c dramatic degraded and after unloading,I_c cannot be recovery to the original value.In the numerical simulation study,based on the elastic theory,the 2D multi-layer finite element model of HTS REBCO coated composite tapes was constructed with COMSOL.The simulation results of stress/strain distribution under torsion deformation were compared with the 3D finite element model(FEM).The validity of the model was verified by comparing with the full 3D FEM.The results showed that the 2D multi-layer shell model performed sufficient accuracy and much higher computational efficiency than the full 3D model.Subsequently,the 2D multi-layer shell FEM of REBCO superconducting with all intact materials was established,and the stress-strain characteristics of each constituent layer under tensile,torsional and tension-torsion combined deformation were numerically simulated.Furthermore,based on the damage theory of brittle fiber reinforced metal matrix composites and Weibull's distribution function,I_c degradation behavior with uniaxial tension,torsion and tension-torsion combined deformation was analyzed.The calculated results showed that the center area of the width direction of REBCO superconducting tape was compressed and the edge of both sides was strained under torsion action.When the tape was tensile under pre-torsion,the degradation curve of I_c tended to be more gentler than that under pure tensile deformation.In addition,the tension-torsion combined deformation under the small torsion transformation was similar to I_c degradation behavior under the pure tensile deformation,and the difference is that there was a gradual change process before I_cdegenerates sharply.The numerical simulation results were in good agreement with the experimental data.The experiments and numerical simulations studied in this paper can provide effective methods for predicting the electromechanical behavior of HTS composite materials under complex loads and deformation in practical application,and have certain engineering guiding significance.