Stress–strain effects on powder-in-tube MgB₂tapes and wires

Abstract

The effects of stress-strain on the critical current, I c , of ex situ powder-in-tube (PIT)-processed Ni-sheathed MgB 2 tapes and round wires as well as in situ PIT-processed Cu-sheathed wires at 4.2 K in a magnetic field up to 5 T have been studied. The effect of In powder addition on the Ni-sheathed MgB 2 wire was not so clear compared with that in the tape, in which the irreversible strain, ε irr , for the I c degradation onset increases significantly by the addition. This is attributed to the difference in the microstructure of the core associated with cold workings. A peak and gradual degradation behaviour of I c with strain beyond ε irr was found in the wire, whereas no evident peak and a steep degradation behaviour was found in the tape. As a possible reason, the difference in the triaxial residual stress state at 4.2 K due to the difference in geometry of the cross-section is suspected. The transverse compression tests revealed that I c of the wire did not degrade up to 270 MPa. Again, the effect of In addition was minimal. The Young's modulus of MgB 2 , 31-41 GPa, at room temperature was estimated by a tensile test of Cu sheath wire using a high-accuracy extensometer and the law of mixtures. The tensile strain dependence of I c in the Cu sheath wire was similar to that in the Ni-sheathed wire, ε irr being 0.4%. However, the stress corresponding to ε irr , 50 MPa, was about 1/10 of that for the Ni-sheath wire and the irreversible transverse compressive stress, 150 MPa, was also lower. The effect of bending strain on the I c in Cu-sheathed wire was compared with that of the tensile strain.

Influence of twisting and bending on the J and n-value of multifilamentary MgB2 strands

“…The bending-strain independent J c (B) of PIT-36 can be an average of the effect of tension and compression. Katagiri [23] reported a field-independent critical bending strain of a single-core PIT strand to be 0.45%. Although our PIT strand is 36-core, our results are quite consistent with Katagiri’s observation.…”

“…The strand was then allowed to relax mechanically, mounted onto the J c test probe (without straightening even if deformation remained), cooled to temperature, and measured. In contrast to previous bend test measurements in which application of bending in-situ after cool down or the application of bending at room temperature and its retention during cool down [20, 21, 23], the present tests were designed to evaluate the tolerance of the strand to bending during react-and-wind coil fabrication or cable winding followed by operation at cryogenic temperatures.…”

“…This type of strain is distinct from that experienced by the strand during magnet excitation in which case well-defined tensile, compressive, or transverse pressure induced strains will be applied at the operating temperature (4–30 K for MgB 2 ). Such effects, resulting for example from thermal cycling stresses and Lorentz forces generated in MgB 2 strands during magnet operation have been investigated [19–23]. In this study we focus on the effect of bending strain applied to reacted strands at room temperature, the relevant parameter for developing magnet winding procedures, cabling, and the spooling and re-spooling of reacted strands.…”

Sample-length dependence of the critical current of slightly and significantly bent-damaged Bi2223 superconducting composite tape

“…figure 1 [1]. As has been reported, the critical current value and n value (referring to the sharpness of the transition from superto normal conducting state, as shown later) of superconducting composite tape with a given length is, more or less, different from sample to sample, being distributed [1][2][3][4][5][6][7][8][9]. This means that the critical current and n value of local elements (figure 1) are distributed along the sample length, and therefore the overall critical current and n value of a sample is determined by such a distributed critical current and n value of the local elements constituting the sample.…”

Evaluation of residual stress and texture in isotope based Mg11B2superconductor using neutron diffraction

“…the critical current (I c ), which have been experimentally observed multiple times in MgB 2 superconducting systems. [29][30][31][32] The residual stresses were measured successfully on several occasions for Nb 3 Sn using neutron diffraction, 33,34 and this technique proved to be the most suitable for the powder-in-tube system due to its ability to penetrate through the sheath material. There are no published results on measurements of the residual stress in MgB 2 wires, however, presumably due to the fact that manufacturing 11 B isotope based Mg 11 B 2 wires is a prerequisite for such neutron measurements.…”