J.K. Shin

Residual and fracture strains of Bi2223 filaments and their relation to critical current under applied bending and tensile strains in Bi2223/Ag/Ag alloy composite superconductor

Mechanical and electromagnetic stresses are exerted on Bi2223∕Ag∕Ag alloy superconducting composite tapes during fabrication∕winding and operation, which cause reduction in critical current when the Bi2223 filaments are damaged. In the damage process, the thermally induced residual strain and fracture strain of the Bi2223 filaments play a dominant role. The aim of the present work was to propose a comprehensive method for estimation of these strain values and a quantitative description method of the relation of critical current to the applied bending∕tensile strain, and to examine the accuracy of the method in comparison with the experimental results. The residual strain of Bi2223 filaments in the composite tape was measured by the x-ray diffraction method. The measured residual strain value was used for analysis of the load-strain curve, from which the intrinsic fracture strain of filaments was estimated. The relation of critical current to applied bending∕tensile strain was predicted by the proposed cal...

Modeling analysis of the critical current of bent Bi2223 composite tape based on the damage strain parameter and the shape of the core

We present a new modeling approach to describe the relation between the critical current and the applied bending strain of a multifilamentary Bi2223/Ag/Ag alloy superconducting composite tape. In the model, the shape of the superconducting core, in which the Bi2223 filaments that transport the superconducting current are embedded in Ag, and the damage strain parameter, defined as the difference between the intrinsic tensile fracture strain and the residual strain of the filaments in the sample length direction (= current transport direction), are combined with the exerted strain distribution in the bent sample. The extent of the damage to the core and the corresponding critical current are expressed as a function of bending strain. The present approach describes well the measured critical current–bending strain relation for the three different fabrication-route samples.

Thermally induced residual strain accumulation in Bi2223/Ag/Ag alloy composite superconductor

A method to estimate the thermally induced residual strain accumulation under varying temperature in a Bi2223/Ag/Ag alloy composite superconductor was presented, in which the mechanical property values measured from the stress?strain curves of the samples with different residual strain states, the residual strain value of Bi2223 filaments in the composite tape at room temperature measured by x-ray diffraction and the reported coefficients of thermal expansion of the constituents (Bi2223, Ag and Ag alloy) in the relevant temperature range were incorporated. This method was applied to estimate the change of the residual strain of all constituents of the high critical current type composite tape fabricated by American Superconductor Corporation as a function of temperature. The residual strain value at 77?K estimated by this method and the reported fracture strain of Bi2223 filaments accounted well for the measured strain tolerance of the critical current at 77?K.

Distribution of normalized critical current of bent multifilamentary Bi2223 composite tape

The distribution of the normalized critical transport current (critical current normalized with respect to the original value) of Bi2223/Ag/Ag alloy composite tape under bending strain of 0%–0.833% was studied experimentally and analytically. The experimental results were analyzed by a modeling approach based on the relation of the heterogeneous damage evolution to the distribution of the critical current. The main results are summarized as follows. (1) The measured distribution of the critical current values varying with bending strain was described well by the present approach. (2) When all specimens were damaged at high bending strains (0.338%–0.833% in the present work), the distribution of the critical current of the bent-damaged specimens was expressed by the three-parameter Weibull distribution function, the reason for which was revealed. (3) The distribution of the irreversible strain was estimated, with which the influence of the increase in the fraction of damaged specimens on the variation of c...

Thermally and mechanically induced residual strain and strain tolerance of critical current in stainless steel-laminated Bi2223/Ag/Ag alloy composite superconductors

It is known that the tensile strain tolerance of the critical current of Bi2223 composite superconductors is improved when the compressive residual strain of Bi2223 filaments in the current transport direction (sample length direction) is enhanced. In the present work, the thermally and mechanically induced residual strain accumulation process of a stainless steel-laminated Bi2223/Ag/Ag alloy superconducting composite tape fabricated at American Superconductor Corporation was studied. A calculation procedure is presented for a description of the strain change of the constituents (Bi2223, Ag, Ag alloy and stainless steel) during cooling and heating, and during lamination, followed by the stress relaxation. As the input values, the mechanical property values of the constituents and the residual strain of Bi2223 filaments in the laminated composite tape at room temperature, measured with x-rays, were used. Such an approach revealed the change of residual strain of each constituent in the thermal and mechanical process in the laminated composite tape. Then, from a comparison of the residual strain accumulation in the laminated tape with that in the insert tape, the effects of the stainless steel lamination on the enhancement of the compressive residual strain of Bi2223 filaments and the improvement of the tensile strain tolerance and strain tolerance window of critical current at 77 K of the composite tape are discussed.

Analysis of the residual strain change of Bi2212, Ag alloy and Ag during the heating and cooling process in Bi2212/Ag/Ag alloy composite wire

The residual strain change of Bi2212 and Ag during the cooling and heating process in the Bi2212/Ag/Ag alloy composite superconductor was studied. First, the residual strain of Bi2212 filaments at room temperature was measured by the x-ray diffraction method. Then, the Youngs moduli of the constituents (Bi2212 filaments, Ag and Ag alloy) and yield strains of Ag and Ag alloy were estimated from the analysis of the measured stress–strain curve, based on the rule of mixtures. Also, the coefficient of thermal expansion of the Bi2212 filaments was estimated from the analysis of the measured thermal expansion curve of the composite wire. From the modeling analysis using the estimated property values and the residual strain of Bi2212 filaments, the changes of residual strain of Bi2212, Ag alloy and Ag with temperature during the cooling and heating process were revealed.