Bending of silver-sheathed (Bi,Pb)-2223 tapes investigated by magneto-optical flux visualization

The effect of bending on silver-sheathed Bi-2223 tape is investigated by means of magneto-optical (MO) visualization of flux distributions. One single piece of rolled tape is used throughout all experiments starting from as-prepared, and subsequently the tape is bent to 4 mm diameter and then down to 1 mm diameter in 1 mm steps. Flux patterns are visualized using an intact tape, i.e. the visualization is done through the silver sheath after removing the bending strain. It is shown that bending of the tape causes a series of cracks running perpendicular to the tape axis (rolling direction). This crack pattern remains practically unchanged down to 1 mm bending diameter. The flux penetration changes from nearly homogeneous (as-prepared) to entirely crack-governed. Vortices are found to penetrate the tape preferentially along the cracks and from there into the remaining bulk. In bent samples, a very homogeneous flux distribution is observed when the external field is decreased after having reached the full-penetration field. A further decrease of the field forces vortices to leave the sample along the cracks, and trapped vortices remain within the Bi-2223 grains (grain clusters). With each step of bending, the values for the first flux penetration and full-penetration field are decreased. The observations of flux patterns at 1 mm bending diameter clearly demonstrate that intergranular currents are still flowing in the sample. Our observations demonstrate the capability of the MO technique for non-destructive testing of superconducting tapes.