Study On Electro-mechanical Properties Of High-temperature Superconducting Coated Conductor Tapes Under Combined Tension-bending

Rare-earth barium copper oxide(REBCO)high-temperature superconducting coated conductor(CC)tapes with high critical current density and strong mechanical properties is ideal conductors for the application in high-field magnets.In an operating high-field magnet at extremely low temperatures,REBCO CC tapes are not only subjected to bending loads during coil winding but also axial tension loads under hoop strong electromagnetic forces.The critical current(I_c)of REBCO CC tapes is at a risk of irreversible degradation under these combined tension-bending loads.Therefore,the electro-mechanical properties of REBCO CC tapes under combined tension-bending loads are systematically studied in this paper.In this paper,a three-dimensional elastoplastic finite element model is established to analyze the thermal residual stress/strain accumulated in REBCO CC tapes during the fabrication-cooling process,and the mechanical behavior of REBCO CC tapes is analyzed by taking the thermal residual stress/strain as the initial stress/strain state under axial tension loads,bending loads and combined tension-bending loads at 77 K.According to the mechanical results,a critical current phenomenological theoretical model combined the Ekin power law formula with the Weibull distribution function based on the internal strain in REBCO films is developed to analyze the electro-mechanical properties of REBCO CC tapes under axial tension loads,bending loads and combined tension-bending loads at 77 K.The results show that the thermal residual strains accumulated in REBCO films at 300 K and 77 K are-0.17%and-0.25%,respectively.The calculated results are in good agreement with the experimental results.Under axial tension loads,when the internal strain in REBCO films arrives at 0.45%,the I_c of REBCO CC tapes begins to degrade irreversibly,which is consistent with the experimental results.Due to the Poisson effect of bent shells,the axial strain of REBCO CC tapes and the internal strain in REBCO films show non-uniform distribution along the width direction of the tapes.Thus,the irreversible degradation of critical current occurs at the boundary of the tapes.The electro-mechanical properties of REBCO CC tapes under bending loads analyzed by the I_c model based on the internal strain in REBCO films is verified by experimental results.In this paper,the electro-mechanical properties of REBCO CC tapes under combined tension-bending loads is analyzed by the I_cmodel based on the internal strain in REBCO films.It is found that the bending mode of REBCO CC tapes has a significant influence on the electro-mechanical properties under combined tension-bending loads;when the deformation of the Hastelloy substrate is linear elastic,the internal strain state of REBCO films and the electro-mechanical properties of tapes are almost not affected by the different tension and bending loading paths of the combined tension-bending loads;the increasing winding tension reduces the reversible stress/strain limit of REBCO tapes under combined tension-bending loads.Based on the investigation of the electro-mechanical properties of REBCO CC tapes under combined tension-bending loads,an effective suggestion for the structural design and winding method of REBCO high-field magnets is provided.