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.
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We have studied the tensile behaviour of Bi2223 superconducting composite tapes at room temperature, and the influence of the tensile damages introduced at room temperature on the critical current Ic and the n values at 77 K. In the measurement of the Ic and n values, the overall composite with a gauge length 60 mm was divided into six elements with a gauge length of 10 mm in order to find the correlation of the Ic and n values of the overall composite to those of the local elements which constitute the composite. From the measured stress–strain curve of the composite and the calculated residual strain of the Bi2223 filaments, the intrinsic fracture strain of Bi2223 filaments was estimated to be 0.09–0.12%. When the applied strain was lower than the onset strain of the filament damage, the original Ic and n values were retained both in the overall composite and the elements. In this situation, while the overall voltage at the transition from superconductivity to normal conductivity of the composite was the sum of the voltages of the constituent elements, among all elements the overall voltage was affected more by the element with the lower Ic (higher voltage). The damage of the filaments arose first locally, resulting in a reduction of the Ic and n values in the corresponding local element, even though the other elements retained the original Ic and n values. In this situation, the voltage of the overall composite stemmed dominantly from that of the firstly damaged weakest element, and the overall Ic and n values were almost determined by the values of such an element. After the local element was fully damaged, the damage arose also in other elements, resulting in segmentation of the filaments. Thus, the Ic and n values were reduced in all elements. The correlation of Ic between the overall composite and the elements could be described comprehensively for non-damaged and damaged states from the voltage–current relation.
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.
Transport current and n-value of DyBCO−coated conductor pulled in tension were measured experimentally and their relation to crack-induced current shunting was analyzed with the partial crack-current shunting model. The following features were revealed. The shunting current increases with increasing transport current and with increasing crack size. At low voltage where shunting current is low, the transport current of cracked sample normalized with respect to the transport current in non-cracked state is described with the modified ratio of non-cracked area to overall cross-sectional area of superconducting layer. At high voltage where the shunting current is high, the normalized transport current becomes higher than the modified ratio of non-cracked area. The increase in shunting current with transport current (and voltage) leads to a decrease in n-value at high current (voltage). This phenomenon is enhanced by crack extension.
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