Transport critical currents of Bi(2223) tapes at 77K under mechanical stress

The transport critical current, j(c), depends on the mechanical stress al,plied during or before the j(c)-determination. Two experiments with different aim were performed : a) bending of the tape tests the resistance against winding (coils, power cables) b) axial strain tests give information on the effect of forces acting on the conductor during operation. Bending of Bi(2223)/Ag tapes from a radius r = infinity to r = 1 cm as well as an axial strain of about 0.2% decreases j(c) by some 80%. For bended as well as for elongated tapes we find an almost unaffected field dependence of j(c), which is only on a lower j(c) level due to crack formation, i.e, a reduction of the percolation path. The strong j(c) reduction can be avoided by a reinforcement of the sheath material by dispersion hardening with 2% Mg. The j(c) decrease after bending in these improved tapes is only some 20%, the maximum axial strain tolerance about 0.8%. The yield of the reinforced tapes is 300-350MPa, which is more than 5 times the value for Ag sheathed tapes.