Using two types of tensile testing device, we have measured the strain dependence of Ic in AgMgNi-sheathed Bi-2223 57 core tapes at 77 K. The Ic degradation onset strains for the specimens with a short gauge length of 17.6 mm varied from 0.40 to 0.78%. In the relatively long samples with seven voltage terminals 5 mm apart along the gauge length of 60 mm, the smallest strain for the initiation of Ic degradation for the sections was 0.26%. The Ic in some other sections, however, began to degrade at larger strains and no degradation was found in another section even at 0.36%, which by far exceeds the strain corresponding to the yield stress of this tape. The local elongation in the first damaged section occupies most of the total elongation in the sample, while the net strain in the latter section is thought to be significantly less than average and no damage occurred there. The bending tests using several mandrels showed considerably consistent Ic degradation behaviour with increasing strain in all the sections, indicating that the strain distribution is uniform in the longitudinal direction.

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.

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