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 (I c ) and to determine the irreversible limits for I c degradation in CC tapes. However, standards or codes for testing the electromechanical properties of hightemperature 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 I c and the reversible strain/stress limits for I c 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 I c during loading-unloading was measured when using the 99% I c (ε) recovery, 99% I c0 recovery and 95% I c0 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% I c (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 I c value, I c0 or determined using the reversible degradation of I c (ε) at residual strain, when unloaded.

The effects of low cyclic loading on the critical current, Ic, under uniaxial and transverse loadings, and bending deformations in GdBCO coated conductor (CC) tapes were evaluated. Under monotonic continuous bending deformation, CC tapes exhibit a high tolerance of Ic up to the lowest bending diameter of 12 mm using the Goldacker bending test rig. However, when the CC tape was subjected to alternate tension–compression bending, a lower irreversible bending strain limit was measured. This was also observed when cyclic bending was applied to the CC tapes which showed a significant decrease in Ic just after 10 cycles of alternate tension–compression bending at 20 mm bending diameter. Such different Ic degradation behavior under different bending deformation procedures gave insight into the proper handling of CC tapes from manufacturing, coiling and up to operating conditions. In the case of uniaxial tension, when electromechanical properties of CC tape were evaluated by repeated loading based on a critical stress level obtained under monotonic loading, Ic also did not show significant change in its degradation behavior up to the irreversible stress limit. The GdBCO CC tape adopted can allow cyclic loading up to 100 cycles without significant irreversible degradation below the monotonic irreversible limit. In the case of the transverse cyclic test, with regard to the large scattering of data especially in the tensile direction, a different cyclic loading procedure was established. For 10 repeated loadings, the mechanical and electromechanical properties of the GdBCO CC tapes showed similar values within the reversible range under the monotonic loading. Ic degraded abruptly indicating that no delamination occurred at the REBCO film during the subcritical cyclic loading. Different fracture morphologies were observed under cyclic loading depicting branch-like patterns of the remaining REBCO layer on the substrate of the CC tape.

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