IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 1 Critical Current and AC Loss of Tapes Subjected to Tensile Stress Guo Min Zhang, D. N. Nguyen, A. Mbaruku, P. V. P. S. S. Sastry, and J. Schwartz, Fellow, IEEE Abstract—Practical applications of HTS tapes cannot avoid var- ious stresses and strains, including tension, bending and/or torsion. These loads directly affect the critical current and AC losses of HTS tapes. Thus the investigation of the effect of stress/strain on HTS tapes is important for practical applications. Here, the critical cur- rents and AC losses are measured at 77 K for different Bi2223/Ag composite tapes under different room temperature stress/strain. The strain dependence of the critical currents and AC losses of Bi2223/Ag composite superconducting tapes are studied. A com- parison is given for the measured results and the values calculated from the formulae developed from the Norris formula and critical current-strain relationship. Index Terms—AC losses, Bi2223/Ag tape, critical current, tensile strain. I. INTRODUCTION S UPERCONDUCTING applications are now being based on flexible long-length, high critical current density high temperature superconducting (HTS) wire. Presently, the long- length first generation superconducting wire with average engi- neering critical current density of (77 k, self field) can be manufactured by several companies commer- cially [1]. It enables power application of HTS in the near future. Because HTS tape is composed of brittle (Bi2223) filaments and sheathed by ductile Ag (or Ag-alloy), the transport properties of present HTS tapes are sensitive to me- chanical deformations. In practical applications, the HTS tapes experience different stress/strain which can result in degrada- tion of the critical currents and increase AC losses of the tapes. Thus, it is crucial to know the effects of stress/strain on HTS tape when designing HTS applications. The transport properties of HTS tapes under stress/strain have been widely studied [2]–[18], especially the effects of stress/strain on the critical current. But the theoretical and experimental research concerning the influence of stress/strain Manuscript received October 3, 2004. This work was supported by the Office of Naval Research through the Center for Advanced Power Systems. G. M. Zhang is on leave from the Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100080, China. He is now with the Center of Advanced Power systems, Florida State University, Tallahassee, FL 32310 USA (e-mail: gzhang@caps.fsu.edu). D. N. Nguyen, A. Mbaruku, and P. V. P. S. S. Sastry are with the Center for Advanced Power Systems, Tallahassee, FL 32319 USA (e-mail: pamidi@caps.fsu.edu). J. Schwartz is with the FAMU-FSU College for Engineering, National High Magnetic Field Laboratory and Center for Advanced Power Systems, Talla- hassee, FL 32310 USA (e-mail: Schwartz@magnet.fsu.edu). Digital Object Identifier 10.1109/TASC.2005.848238 on AC losses are relatively few. Hence, investigation of AC losses of HTS tapes under stress/strain is of importance. In our previous work [2], [3], we measured and analyzed the stress/strain (tensile and bending) effects on the critical cur- rent and AC loss of the same sample in liquid nitrogen. In the practical applications, such as for the winding of magnet using Bi2223/Ag tape, it is needed to know the effects of stress/strain at room temperature (RT) on the critical current and AC loss. Here, the Bi2223/Ag samples were pulled to different stress/strain at room temperature, while the critical currents and AC losses of the samples were measured at 77 K. The measured results were presented and compared with the values calculated from the formula previously developed. It shows that the measured data fits well with the calculated values. II. EXPERIMENTAL A. Samples Choice The short samples were cut from the same 55-filamentary Bi2223/Ag tape manufactured by American Superconductor Corporation. Its cross-section is 3.98 mm 0.22 mm. All short samples chosen had roughly the same critical currents and N values (the critical currents are about 117 A and N values are about 20). B. Tensile Test The tensile tests were carried out at room temperature by a stress/strain machine that gives highly accurate measurement of applied load and displacement, from which the strain is cal- culated [4]. In the test, each sample was pulled to a certain strain. To estimate the irreversible strain, we first measured the stress/strain curve up to breaking strain. Then the strain values for the measurement of critical currents and AC losses can be properly decided accordingly. C. Ic-Strain Test The critical currents were measured with the four-contact method using 1 criterion. The length of the short samples is 7 cm and the distance between the potential taps is 4.5 cm. The critical currents were measured at 77 K. D. AC Loss Measurements AC losses of the samples were measured using the electrical method [2]. Before straining, the samples’ critical currents were measured at 77 K. After the samples were strained to different stress/strains at room temperature, the critical currents and AC 1051-8223/$20.00 © 2005 IEEE