Tensile and Bending Fracture Behaviors and Their Influence on Critical Current of Multifilamentary Bi2223/Ag/Ag-Mg Composite Tape

Abstract

The process of damage extension under bending and tensile strains at room temperature and its influence on critical current at 77 K under zero magnetic field were studied for a mutifilamentary Bi2223/Ag/Ag_ Mg alloy superconducting composite tape. Under the tensile strain, the damages such as transverse cracking of the Bi2223 filaments, longitudinal cracking along the fila ment axis and interfacial debonding between Bi2223 and Ag took place. Under the bending strain, in addition to these damages, protrusion of the compressive side due to buckling, accompanied by compressive fracture of the filaments, was observed. From the analysis of the residual stress, tensile stress_ strain curve and tensile damage_ induced reduction in critical current, the Young's modulus, residual strain and intrinsic tensile fracture strain of the Bi2223 filaments at room temperature were estimated to be 98 GPa, |0.16÷ and 0.09`0.13÷, respectively. The change of critical current with bending strain was calculated by sub stituting these estimated values and compressive strength of the filaments as a variable into the proposed model in which the in fluence of the damages both in tensile and compressive sides on critical current was incorporated. From the comparison of the cal culation and experimental results, the compressive strength of Bi2223 filaments was estimated to be several times higher than the tensile one.

Estimation of Critical Current of Bent Bi2223 Composite Tape by Application of Superconducting Core Shape-Incorporated Model

“…．引張の場合の不 可逆ひずみ e T, irr の値は，Bi2223 フィラメント固有の引張破 壊ひずみ(e f )と熱処理温度からの冷却過程でフィラメントに 導入される圧縮熱残留ひずみ(e r )の差(＝e f -e r ，以下損傷誘 起パラメータと呼ぶ)である 2 4,7,8,13,14) ．曲げの場合の不可逆 ひずみ e B, irr は，中立軸より上方(引張側)の領域において， 銀合金シース直下の Bi2223 フィラメントの試料長手方向の 引張ひずみが，損傷誘起パラメータに到達するときの曲げひ ずみである 7,8,13) ． 曲げひずみと臨界電流の関係は超伝導コア(Bi2223 が Ag に埋め込まれた領域)の形態によって変わってくる．Fig.…”