K. Kasaba

Local variations in the critical current degradation of Ag/Bi2223 tape by tensile and bending strains

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.

Tensile mechanical properties of (Nd,Eu,Gd)–Ba–Cu–O bulk superconductors at room and liquid nitrogen temperatures

Abstract In order to investigate the mechanical properties of a recently developed high performance melt-processed (Nd,Eu,Gd)Ba 2 Cu 3 O y (NEG-123) single grain bulk superconductor, tensile tests of specimens cut from the bulk have been carried out at room temperature, 293 K, and at liquid nitrogen temperature, 77 K. Tensile stress was applied in a - or b -direction. The maximum value of tensile strength and Youngs modulus at 77 K, 41 MPa and 158 GPa were higher than those at 293 K, 33 MPa and 134 GPa in the NEG-123 sample with 40 mol% (Nd,Eu,Gd) 2 BaCuO 5 (NEG-211) secondary phase particles, 0.5 wt% Pt and 10 wt% Ag 2 O. Tensile strength of the sample added 30 mol% NEG-211 and the sample added 30 mol% Gd 2 BaCuO y instead of 40 mol% NEG-211 that was added to the sample described above have also been investigated.

Mechanical properties of Gd123 bulk superconductors at room temperature

Abstract In order to investigate the mechanical properties of Gd123 single-grain bulk superconductors fabricated using a modified quench and melt growth method, tensile tests in the direction parallel and perpendicular to the c -axis have been carried out at 293 K by using the small specimens cut from bulk superconductors. As for the mechanical properties perpendicular to the c -axis, there was no significant difference between those in the crystal growth direction and those perpendicular to it. While the average value of the Young’s modulus of the bulk sample with 33.0 mol%-Gd211 secondary phase particles, 118 GPa, was higher than that of the bulk sample with 28.7 mol%-Gd211, 111 GPa, the average value of the tensile strength of the former, 36 MPa, was lower than that of the latter, 40 MPa. The tensile strength and the Young’s modulus in the c -axis, 10 MPa and 37 GPa, were quite low compared with those mentioned above. Poisson’s ratio based on the transverse strain in the c -axis, 0.15, was significantly smaller than that perpendicular to it, 0.30. In the specimens with higher length, however, the difference was decreased to some extent. With regard to the anisotropy of the Poisson’s ratio, the effect of a pre-existing micro-crack opening in the c -axis direction was discussed coupled with the constraints at the interfaces between the specimen and the sample holder.

Fracture toughness evaluation of YBCO bulk superconductor

Abstract Evaluation of the fracture toughness of a Y–Ba–Cu–O bulk superconductor with 28.6 mol% Y-211 and 15 wt.% Ag has been investigated by tensile tests at room temperature using specimens with a sharp V-notch. The notch was introduced into the specimens such that its direction of depth was parallel or perpendicular to the c -axis and the loading direction was in the a ( b )-axis. Fracture toughness evaluation by three-point bending tests was also carried out. In addition, usual tensile tests and three-point bending tests were also carried out in the specimens without V-notch. The scatter of measured fracture toughness values by the bending tests, 1.6–2.1 MPaxa0m 1/2 , was less as compared with that by the tensile tests, 1.1–2.5 MPaxa0m 1/2 . However, the average value in tensile tests 1.9 MPaxa0m 1/2 coincided with that in bending tests. The maximum and the average value of bending strength, 109 and 104 MPa, were higher than those of tensile strength, 72 and 56 MPa, respectively.

Mechanical properties of Sm-Ba-Cu-O bulk superconductors at room temperature

Abstract In order to investigate the mechanical properties of Sm–Ba–Cu–O bulk superconductors fabricated using TSMG method such that Sm123:Sm211 in molar ratio was 100− Z : Z ( Z =16.7, 28.6), tensile tests in the direction perpendicular to the c -axis have been carried out at room temperature by using the small specimens cut from the bulk samples. Although, non-linear behavior was seen in a few specimens, stress–strain curves were almost linear until the fracture. The non-linear behavior is presumably ascribed to the opening of pre-existing micro-cracks parallel to the c -axis and the slow propagation of them. The Young’s modulus for both samples was almost equal. Poisson’s ratio based on the transverse strain parallel to the c -axis was smaller than that based on the strain perpendicular to it. One of the possible reasons for it will be the reduced transverse shrinkage by the constraint at the interface between the specimen and the aluminum alloy tensile rod coupled with the opening of the pre-existing micro-cracks perpendicular to the c -axis. Tensile strength for the sample with 28.6 mol% of Sm211 was not higher than that with 16.7 mol%. It is presumably ascribed to the low cohesive force between the Sm211 particles and the matrix. The effect of loading condition with respect to that of the crystal growth on the strength was not clear. No significant difference in the strength was found between the specimens including a growth sector boundary and those without it.

Fractographic observations on Sm-Ba-Cu-O bulk superconductors fractured by tensile tests

The fracture surface morphology of top-seeded melt-grown SmBa 2 Cu 3 O x bulk superconductors after tensile tests in (100) direction at room temperature was observed using SEM. There were voids with and without Ag particles of diameters ranging from 80 to <10 μm on the fracture surface of high density sample, while the size was 300 μm at the largest on the low density sample. Ag particles of 10 to 20 μm had the matching mate on the other side of the fracture surface. In the high density samples, the propagation of the crack was easily traced through the flow markings with steps on the fracture surface, while it was difficult to trace in low density samples.

Fatigue crack growth under compressive loading

S/C magnets have many components which were designed to be compressively stressed. This paper describes a possible fatigue crack growth even under fully compressive stresses. In general crack is thought to close under compressive stress condition. But, if there is a tensile residual stress field in the specimen, the crack can open and grow to a considerable length. Compressive cyclic loading tests were made on the compact tension specimen with a hole at the center of ligament region where a tensile residual stress field was introduced by an excessive compressive pre-stress. Elastoplastic Finite Element Method (FEM) analysis indicated that the crack growth relaxed and re-distributed the residual tensile stress to some extent. In consequence it is experimentally verified that the crack grew to the length nearly to the fracture.

Tensile strain/transverse compressive stress dependence of critical current in Bi(2212) superconducting tapes with Zr-reinforced Ag sheath

Abstract The tensile mechanical property and the axial tensile strain/transverse compressive stress dependence of the critical current I c in mono- and 19-core Bi(2212) superconducting tapes with Zr-reinforced Ag sheaths were evaluated at a temperature of 4.2xa0K and a magnetic field of 14xa0T. Replacement of Ag sheath by Ag-Zr alloy sheath increased the 0.2% proof stress and the onset tensile strain for I c degradation although I c decreased markedly. Double layer sheath with Ag inner layer and Ag-Zr alloy outer layer improved both the stress-strain relation and the strain characteristics of I c without remarkable degradation in I c . The increase in the number of core from 1–19 degraded the tensile strain vs . I c characteristics of the tape, although no marked change in critical current density is observed. The alloying of sheath also improved the transverse compressive stress vs . I c characteristics. The effects of Zr alloying of Ag sheath on the mechanical behavior and the stress/strain dependence of I c of the superconducting composites are briefly discussed with emphasis on texturing of the oxide superconductor.

Mechanical properties of Gd123 bulk superconductor at liquid nitrogen temperature

In order to investigate the mechanical properties of the Gd123 single grain bulk superconductor at low temperature, bending tests were carried out. The bending fracture strength at LNT (liquid nitrogen temperature) scattered from 83 to 120 MPa was higher than that at RT, from 62 to 86 MPa. However, the Youngs modulus at LNT scattered from 109 to 123 GPa was comparable to that at RT, from 110 to 129 GPa. The Weibull coefficient for the bending strength at LNT, 6.5, was almost identical with that at RT, 6.9. The endurance limit for the repeated pulsating bending up to the5 /spl times/ 10/sup 4/ cycles at LNT, 70 MPa, was about 70% of the average value of the monotonic bending strength.

Stress–strain effects on powder-in-tube MgB2 tapes and wires

The effects of stress–strain on the critical current, Ic, of ex situ powder-in-tube (PIT)-processed Ni-sheathed MgB2 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 MgB2 wire was not so clear compared with that in the tape, in which the irreversible strain, eirr, for the Ic 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 Ic with strain beyond eirr 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 Ic of the wire did not degrade up to 270 MPa. Again, the effect of In addition was minimal. The Youngs modulus of MgB2, 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 Ic in the Cu sheath wire was similar to that in the Ni-sheathed wire, eirr being 0.4%. However, the stress corresponding to eirr, 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 Ic in Cu-sheathed wire was compared with that of the tensile strain.