3552 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005 Effect of Mechanical Loading on the Degradation Behavior of Bi-2223 Tapes Carlos A. Baldan, Carlos Y. Shigue, Ernesto Ruppert Filho, and Ulisses R. Oliveira Abstract—During the winding process of HTS coils the tapes of Bi-2223 are subjected to the inﬂuence of bending strain, axial strain, compressive force and torsional deformation resulting in degradation. In the literature the effects of the individual strain components are separately analyzed in spite of during coil winding and energizing the strain-stress effects are combined. In this work using commercial tapes of Bi-2223 Ag/AgMg with and without stainless steel reinforcement several samples were wound on cylindrical FRP G-10 holder in which different combined strains are applied. Measurements of characteristic curves are done to determine the degree of critical current degradation and the operational limits. The results are compared with the values of short samples and other specimens subjected to deformation generated by loading types such as tensile and bending strain. Index Terms—Bi-2223 tapes, critical current, HTS coils, degradation, stress-strain. I. INTRODUCTION T HE HIGH temperature superconductor tapes are now available in long lengths for electrical applications [1], such as magnets, cables, current limiters, generators and mo- tors with different winding geometries where the tapes are subjected to various kinds of stress and strain. During the winding process the HTS tapes are subjected to torsion and bending strains, associated with winding tension applied on the conductor [2]–[6]. In operation, the tapes are subjected to tensile strain due to hoop stress, as well compressive strain due to different thermal expansion properties for each constituent material which is present. The mechanical deformation will be present for any geometry and the inﬂuence of the tensile strain and torsion strain on degradation should be investigated as the combined effects with the bending strain. In order to evaluate each of these deformation contributions, several measurements for separated effects and combined effects were carried out to determine the allowable deformation limits of these tapes. The relationship at 77 K for various torsion angles with combined applied tensile loads are measured to evaluate the degradation for Bi-2223/Ag/AgMg tapes with and Manuscript received October 4, 2004. This work was supported in part by the Companhia Paulista de Força e Luz—CPFL under Contract 4600000767. C. A. Baldan is with Faculdade de Engenharia Química de Lorena—FAEN- QUIL-Departamento de Engenharia de Materiais-DEMAR, Lorena, SP, Brazil. He is also with the Faculdade de Engenharia de Guaratinguetá- FEG/Unesp—Guaratinguetá, SP, Brazil (e-mail: cabaldan@demar.faenquil.br). C. Y. Shigue and U. R. Oliveira are with Faculdade de Engenharia Química de Lorena—FAENQUIL-Departamento de Engenharia de Materiais-DEMAR, Lorena, SP, Brazil (e-mail: cyshigue@demar.faenquil.br). E. Ruppert Filho is with Universidade Estadual de Campinas—UNICAMP, Faculdade de Engenharia Elétrica e da Computação, Campinas, SP-Brazil (e-mail: ruppert@fee.unicamp.br). Digital Object Identiﬁer 10.1109/TASC.2005.849357 without stainless steel reinforcement [7], [8]. The evaluation of HTS performance will be done in terms of the whole cur- rent-voltage curve. These results can be applied to the evaluation of power dissipation and the effect of short time over-critical current periods in a coil [9]. The degradation results for the Bi-2223 tapes due to ten- sile, torsional and combined strain is analyzed comparing the results obtained for two types of tape, with and without external reinforcement. II. EXPERIMENTAL A sample holder for the torsion, tensile and combined test of HTS tapes was designed to ensure a uniform torsional deforma- tion in 77 K with adjustable gauge length from 3 to 8 cm. For the torsional strain test after soldering the tape, the bolts located at the both ends can be rotated to achieve the desirable angle thus producing the correspondent deformation. The torsional deformations with the angles between 30 to 360 degrees were applied at room temperature (RT) and soon after the measurements were carried out at 77 K to obtain the curves using the standard four-point probe method with 1 criterion to determine the value. The same procedure was used for tensile test to evaluate the degradation with tensile strain applied under self-ﬁeld at 77 K. The combined strain test was carried out with torsion strain and tensile strain applied to measure the curve and to evaluate the degradation. After applying torsional stress in the sample at RT the system was immersed in liquid nitrogen bath and for the combined tensile test different loads were applied using known masses added to the bottom of the sample holder. For each mass, the curve was measured to determine the and -index. The -index is calculated by the ﬁtting of following equation to experimental data: (1) where is the electric ﬁeld, is a proportional constant, is the current, and is the critical current. The equation used to evaluate the torsional strain applied with the angle rotation is: (2) where is the thickness of the tape, is the torsion angle ex- pressed in radians and is the gauge length of the sample where the torsion is applied. Two types of commercial Bi-2223/Ag/AgMg multiﬁlamen- tary superconducting tapes were used. The tape speciﬁcations 1051-8223/$20.00 © 2005 IEEE