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

摘要

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 I-c 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.