BENDING OF SILVER-SHEATHED (BI,PB)-2223 TAPES INVESTIGATED BY MAGNETOOPTICAL 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 sta rting from as-prepared, and subsequently the tape is bent to 4 mm diam eter and then down to 1 mm diameter in 1 mm steps. Flux patterns are v isualized 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 t o the tape axis (rolling direction). This crack pattern remains practi cally unchanged down to 1 mm bending diameter. The flux penetration ch anges from nearly homogeneous (as-prepared) to entirely crack-governed . Vortices are found to penetrate the tape preferentially along the cr acks and from there into the remaining bulk. In bent samples, a very h omogeneous flux distribution is observed when the external field is de creased after having reached the full-penetration field. A further dec rease of the field forces vortices to leave the sample along the crack s, and trapped vortices remain within the Bi-2223 grains (grain cluste rs). With each step of bending, the values for the first flux penetrat ion and full-penetration field are decreased. The observations of flux patterns at 1 mm bending diameter clearly demonstrate that intergranu lar currents are still flowing in the sample. Our observations demonst rate the capability of the MO technique for non-destructive testing of superconducting tapes.