A large, magnetic-field-dependent, reversible reduction in critical current density with axial strain in YBa 2 Cu 3 O 7−δ coated conductors at 75.9 K has been measured. This effect may have important implications for the performance of YBa 2 Cu 3 O 7−δ coated conductors in applications where the conductor experiences large stresses in the presence of a magnetic field. Previous studies have been performed only under tensile strain and could provide only a limited understanding of the in-field strain effect. We now have constructed a device for measuring the critical current density as a function of axial compressive and tensile strain and applied magnetic field as well as magnetic field angle, in order to determine the magnitude of this effect and to create a better understanding of its origin. The reversible reduction in critical current density with strain becomes larger with increasing magnetic field at all field angles. At 76 K the critical current density is reduced by about 30% at −0.5% strain when a magnetic field of 5 T is applied parallel to the c-axis of the conductor or 8 T is applied in the ab-plane, compared to a reduction of only 13% in self-field. Differences in the strain response of the critical current density at various magnetic field angles indicate that the pinning mechanisms in YBa 2 Cu 3 O 7−δ coated conductors are uniquely affected by strain.

A reversible strain effect on transport critical current I c was found in Bi 2 Sr 2 CaCu 2 O 8+x (Bi-2212) high-temperature superconducting round wires. I c showed unambiguous reversibility at 4 K and 16 T up to an irreversible strain limit of about 0.3 % in longitudinal tension, prompting hope that the Bi-2212 conductor has the potential to sustain mechanical strains generated in high-field magnets. However, I c was not reversible under longitudinal compression and buckling of Bi-2212 grain colonies was identified as the main reason. A two-component model was proposed, which suggests the presence of mechanically weak and strong Bi-2212 components within the wire filaments. Porosity embedded in the weak component renders it structurally unsupported and, therefore, makes it prone to cracking under strain ε. I c (ε) is irreversible in tension if the weak component contributes to the transport critical current but becomes reversible once connectivity of the weak component is broken through strain increase or cycling. A modified descriptive strain model was also developed, which illustrates the effect of strain in the Bi-2212 conductor and supersedes the existing descriptive model. Unlike the latter, the new model suggests that higher pre-compressive strains should improve I c if buckling of Bi-2212 grains does not occur, and should result in a wider I c (ε) plateau in the applied tensile regime without degradation of the initial I c. The new model postulates that a reversible strain effect should exist even in the applied compressive strain regime if buckling of Bi-2212 grains could be prevented through elimination of porosity and mechanical reinforcement of the wire.

We present a new experimental technique to measure the delamination strength under transverse tensile stress of YBa 2 Cu 3 O 7−δ coated conductors for electric power applications. The delamination strength, defined as the tensile stress at which the ceramic layers delaminate from one another, is measured at 76 K for different sample configurations. The delamination strength is reduced by as much as 40% when the conductor is slit to smaller width, a standard fabrication process, and this reduction is due to damage to the ceramic layers near the edges of the conductor. We found that the delamination strength of slit coated conductors can be raised significantly by reinforcing the conductor by laminating it with copper strips and adding solder fillets at the edges. In relatively strong conductors, where the delamination strength is as high as 15 MPa, the critical current does not degrade before actual delamination. This fact greatly simplifies sample characterization of practical high-strength conductors, since only mechanical measurements need to be made. The critical current does, however, degrade significantly as a function of transverse stress before delamination in weak conductors that have relatively low delamination strength below 15 MPa. We discuss how a soft metallic layer in YBCO coated conductors may limit the transverse stress that the superconducting layer experiences in applications.

Nb 3 Sn superconducting wires made by the restacked-rod process (RRP ®) were found to have a dramatically improved resilience to axial tensile strain when alloyed with Ti as compared to Ta. Whereas Ta-alloyed Nb 3 Sn in RRP wires showed permanent damage to its current-carrying capacity (I c) when tensioned beyond an intrinsic strain as small as 0.04%, Ti-doped Nb 3 Sn in RRP strands exhibits a remarkable reversibility up to a tensile strain of about 0.25%, conceivably making Ti-doped RRP wires more suitable for the high field magnets used in particle accelerators and nuclear magnetic resonance applications where mechanical forces are intense. A strain cycling experiment at room temperature caused a significant drop of I c in Ta-alloyed wires, but induced an increase of I c in the case of Ti-doped strands. Whereas either Ti or Ta doping yield a similar enhancement of the upper critical field of Nb 3 Sn, the much improved mechanical behavior of Ti-alloyed wires possibly makes Ti a better choice over Ta, at least for the RRP wire processing technique. * Contribution of NIST, an agency of the US government, not subjected to copyright.

A benchmarking experiment was conducted to compare strain measurement facilities at the National Institute of Standards and Technology (NIST) and the University of Twente. The critical current of a bronze-route wire, which was fabricated for the International Thermonuclear Experimental Reactor (ITER), was measured as a function of axial strain and magnetic field in liquid helium at both institutes. NIST used a Walters' spring strain device and University of Twente used a bending beam ("Pacman") apparatus. The ITER bronze-route wire investigated had a very high irreversible strain limit that allowed comparing data over a wide range of applied strain between 1% and 1%. Similarities of the data obtained by use of the two apparatuses were remarkable, despite the many differences in their design and techniques.