Strain Gauge Method for Evaluating a Three-Dimensional Residual Strain State in ${\rm Nb}_{3}{\rm Sn}$ Wires

Abstract-Concerning the strain effect of superconducting properties for Nb
₃
Sn wires, it is necessary to investigate a three-dimensional strain state that Nb
₃
Sn superconductors truly experience in the composite wire. However, it is very difficult for Nb
₃
Sn wires to obtain the three-dimensional residual strain components experimentally. We adopted the strain gauge that is directly glued onto the 1 mm outer diameter Nb
₃
Sn wire, in order to quantitatively measure the three-dimensional distortions. To evaluate axial and lateral distortions of the wire, strain gauges were set in both axial and lateral directions. This measurement system can obtain the distortion detail in fields up to 27 T at temperatures ranging from 4.2 to 20 K. We measured the upper critical field B
_c2
% in a three-dimensional strain state for Nb
₃
Sn wires. It was found that the wire architecture changes each residual strain in axial and lateral directions of the wire. Moreover, the B
_c2
strain sensitivity that is related to the B
_c2
variation is also affected by its architecture. We found that the axial tensile strain variation 0.3% roughly corresponded to the lateral compressive strain variation 0.1% for Nb
₃
Sn wires. This means that the ratio of the lateral strain and the axial one for Nb
₃
Sn wires is 0.3. The ratio of both residual strains in the axial and lateral directions is very important to examine the strain effect of Nb
₃
Sn wires in detail.