3556 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005 Fatigue Behavior and Its Influence on the Critical Current of Externally Reinforced Bi-2223 Superconducting Tape Hyung-Seop Shin, John Ryan C. Dizon, Ki-Hyun Kim, Sang-Soo Oh, and Dong-Woo Ha Abstract—The endurance evaluation of HTS conductors for practical applications is necessary. The mechanical properties and the critical current, , of multifilamentary Bi-2223 super- conducting tapes, externally reinforced with stainless steel foils, subjected to high cycle fatigue loading in the longitudinal direc- tion were investigated at 77 K. The relations between the applied stress amplitude and the fatigue life (S-N curves) were obtained using the externally reinforced Bi-2223 tapes, and its transport property was evaluated with the increase of repeated cycles at different stress amplitudes. The effect of the stress ratio, R, on the degradation behavior under fatigue loading was also examined considering the practical application situation of HTS tapes. Mi- crostructure observation was performed in order to understand the degradation mechanism in fatigued Bi-2223 tapes. Index Terms—Bi-2223 tapes, critical current, electrical fatigue limit, fatigue, stress ratio. I. INTRODUCTION I N APPLICATIONS such as coils, motors, power cables and magnets, HTS tapes are subjected to various kinds of stresses. The effects of stress/strain on the superconducting properties, the strain tolerance of the critical current, , in the Ag sheathed BSSCO tapes, have been studied because of the importance in practical aspects [1]–[4]. It also is well established that the degradation mechanism in HTS tapes is due to crack formation in the superconducting filaments which irreversibly lowers the transport critical current. In BSSCO tapes, usually, the onset of degradation is caused by the initiation of cracks in superconducting filaments and the degradation characteristic of the with strain is influenced by the subsequent growth of cracks. The improvement of the mechanical property, critical current density and critical strain tolerance of BSSCO tapes have been achieved by adopting multi-filaments [5], and by alloying the sheath or reinforcing BSSCO tapes externally with metallic foils such as stainless steel [6], [7]. Manuscript received October 5, 2004. This work was supported by a grant from the Center for Applied Superconductivity Technology under the 21st Cen- tury Frontier R&D Program funded by the Ministry of Science and Technology, Korea. H.-S. Shin is with the School of Mechanical Engineering, Andong National University, 760-749 Kyungbuk, Korea (e-mail: hsshin@andong.ac.kr). J. R. C. Dizon and K.-H. Kim are with the Department of Mechanical En- gineering, Graduate School, Andong National University, 760-749 Kyungbuk, Korea (e-mail: johnryancdizon@hotmail.net). S.-S. Oh and D.-W. Ha are with the Applied Superconductivity Research Group, Korea Electrotechnology Research Institute, 641-120 Kyungnam Korea (e-mail: ssoh@keri.re.kr). Digital Object Identifier 10.1109/TASC.2005.849358 Fig. 1. Cross-sectional views of the sample supplied. During the service life of electric devices using supercon- ducting tapes, the components will be subjected to cyclic loading which are caused by various reasons such as thermal cycling, variation in Lorentz forces due to changes in coil current, and alternating centrifugal forces during motor op- eration. Therefore, in order to apply HTS tapes to practical superconducting devices such as magnets, power transmission cables and motors, the endurance evaluation of the tapes which includes the mechanical and electrical reliability are necessary. Although mechanical tests of Bi-2223 tapes have already been performed in many laboratories, available information or data on the mechanical and electrical properties of these HTS tapes under fatigue loading are still insufficient [8]–[11]. Therefore, the investigation of fatigue behaviors of BSSCO tapes will be of great significance in the design of superconducting devices, especially in the aspect of their reliability [12]. In this study, the mechanical properties and the critical current, , of multifilamentary Bi-2223 superconducting tapes, exter- nally reinforced with stainless steel foils, subjected to high cycle fatigue loading were investigated at 77 K. Especially, the influ- ence of the stress ratio on the electrical property of Bi-2223 tape under fatigue loading was examined. In addition, microstructure examination was performed in order to understand the damage mechanism which occurred under fatigue loading. II. EXPERIMENTAL PROCEDURE A commercially available Ag-sheathed Bi-2223 multifila- mentary superconducting tape was supplied for the experiments (manufacturer: AMSC; high strength reinforced wire). The tape was fabricated by the powder-in-tube (PIT) method and reinforced with stainless steel foils soldered on both surfaces. Fig. 1 shows the cross-sectional view of the sample. Tape width and thickness are 4.2 mm and 0.3 mm, respectively. The at 77 K is about 150 A. Tensile and high cycle fatigue tests of the Bi-2223 tape were conducted at RT and 77 K using a hydraulic-servo material testing machine (Instron type 8516, loadcell capacity: 5 kN). The total length of the specimen and the gauge length between 1051-8223/$20.00 © 2005 IEEE