Establishing a test procedure for evaluating the electromechanical properties of practical REBCO coated conductor tapes by the uniaxial tension test at 77 K

Abstract

Nowadays, the superior electrical and mechanical properties of second generation (2G) practical coated conductor (CC) tapes make them viable options for various applications such as power cables, magnets and coils. The CC tapes are composite conductors where the constituent layers are directly affected by the mechanical and the electromechanical properties of the superconducting layer under operating conditions. It is therefore important to investigate the strain effect on the critical current (Ic) and to determine the irreversible limits for Ic degradation in CC tapes. However, standards or codes for testing the electromechanical properties of high-temperature superconductors are lacking. Therefore, the establishment of a test method or procedure for determining the electromechanical properties of 2G CC tapes in both engineering and practical aspects is needed. This study is a standardization activity, wherein the strain/stress sensitivity of Ic and the reversible strain/stress limits for Ic degradation of practical CC tapes using uniaxial tension tests were evaluated at 77 K and self-field. The reversible strain/stress limits of CC tapes were defined and the reversible recovery of Ic during loading–unloading was measured when using the 99% Ic (ε) recovery, 99% Ic0 recovery and 95% Ic0 retention criteria. The electromechanically determined irreversible limits were also compared to the mechanically determined stress and strain at the yield point. The obtained reversible stress limits showed a similar value regardless of criteria adopted, excepting Sample 2. However, the irreversible strain limit value obtained using the 99% Ic(ε) recovery criterion showed the largest value as compared to the cases using other criteria. Through repeated tests, the electromechanical test procedure of the CC tapes was evaluated based on statistical estimates. The obtained results can provide a fundamental basis for establishing test procedures comparing the criterion that should be considered in determining irreversible limits, whether normalized to the original state Ic value, Ic0 or determined using the reversible degradation of Ic(ε) at residual strain, when unloaded.