In situ strain measurements by neutron diffraction and residual stress estimation in Ag-sheathed Bi2223 superconducting composite tapes

Heretofore, an accurate residual stress in Ag-sheathed superconducting composite tape could not be obtained from a measurement using conventional low energy x-rays due to the large x-ray absorption in Ag phase. Therefore, we developed a method for evaluating the residual strain in the filaments using neutron diffraction and finite element method (FEM) analysis, measuring the mechanical curvature. As for the residual strain of the Ag phase and Bi phase in a high strength type specimen, the Bi phase has a compressive longitudinal strain, and the Ag phase has a tensile strain; these are thermally balanced residual stresses at room temperature that depend on the mismatch of the coefficient of thermal expansion (CTE). On the other hand, there was very little thermally induced stress at the room temperature in the high Ic specimen. The numerical residual strain obtained by the FEM analysis agrees with experimental neutron method results. In a high critical current (Ic) type specimen, the existence of a stress relaxation mechanism during the heat treatment is suggested.