Strain gauge method for evaluating a three-dimensional residual strain state in Nb₃Sn wires

Concerning the strain effect of superconducting properties for Nb ₃Sn wires, it is necessary to investigate a three-dimensional strain state that Nb₃Sn superconductors truly experience in the composite wire. However, it is very difficult for Nb₃Sn 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 ₃Sn 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 Nb₃Sn 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 Nb₃Sn wires. This means that the ratio of the lateral strain and the axial one for Nb₃Sn 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 Nb₃Sn wires in detail.