Hyung-Seop Shin

Critical current degradation behaviour in Bi-2223 superconducting tapes under bending and torsion strains

In this paper, we have investigated the Ic degradation behaviours in Ag alloy sheathed Bi-2223 superconducting tapes under bending and torsion strains. In particular, we have examined the homogeneity of Ic degradation along the longitudinal direction of tapes by adopting multiple voltage terminals. The bending modes influenced the Ic degradation behaviour in Bi-2223 tapes. A gradual and consistent decrease of Ic could be observed at all sections, which was a different behaviour from that under tensile loading. Up to eirr, Ic showed a nearly similar degradation at each section, due to a simultaneous initiation of cracks from defects indicating that uniform deformation occurred in tapes with bending. Over eirr, some variations in the Ic degradation occurred due to the difference of crack growth rates at each section. In particular, the behaviour appeared significantly with hard bending. The degradation of Ic in Bi-2223 tapes by torsion occurred gradually when the torsion angle exceeded 150°. However, the Ic degradation by torsion strain was small compared with other types of loading. Also, the Ic degradation behaviour at each section along the longitudinal direction of tapes was gradual and consistent, representing a uniform torsional deformation in the tapes.

The strain effect on critical current in YBCO coated conductors with different stabilizing layers

The tensile strain dependences of the critical current (Ic) in YBa2Cu3O7−δ (YBCO) coated conductors fabricated by using the rolling-assisted biaxially textured Ni–W substrates (RABiTS)–pulsed laser deposition (PLD) method were examined at 77 K and in self magnetic field. Cu and stainless steel layers were used as stabilizers to the YBCO coated conductor, and the effects of stabilizing layers on the strain tolerance of Ic were investigated, compared with the case without a stabilizing layer. The lamination of stabilizer produced an increase in the yield strength and strain tolerance of Ic in coated conductors. All YBCO coated conductors tested showed a reversible strain effect and a peak in the relation between Ic and applied strain. The peak strain of Ic and the irreversible strains for Ic degradation were enhanced when the YBCO coated conductor was laminated with a stabilizing layer. For the case laminated with a stainless steel layer, Ic recovered reversibly until the applied strain reached to about 0.5% and showed its peak at a strain of 0.42%, comparing to the case without a stabilizing layer, which were 0.21% and 0.18%, respectively. It can be predicted that the lamination of a stabilizing layer produced a significant residual compressive strain to the YBCO film during cooling to 77 K, which influenced the axial strain tolerance of YBCO coated conductors. Therefore, the Ic–tensile strain relation in YBCO coated conductors could be explained by a two-stage deformation; stage I is the region where YBCO film behaves elastically and Ic recovers when the stress is released. Stage II is the region where Ic decreases irreversibly attributable to the cracking induced in the YBCO film due to the significant plastic deformation of the substrate or the stabilizing layer.

The sliding wear resistance behavior of NiAl and SiC particles reinforced aluminium alloy matrix composites

Abstract The sliding wear resistance behavior of NiAl and SiC particles reinforced aluminum alloy matrix composites against S46C steel was studied. Experiments were performed within a load range of 3.5 N to 82.7 N at a constant sliding velocity of 0.15 m s 1 . The sliding distance was 1000 m. Two types of composites, NiAlp/Al and SiCp/Al with different volume fractions (5 vol.% and 10 vol.%), were used. At low loads, where particles acted as load bearing constituents and prevented the aluminum matrix being directly involved in the wear process, the wear resistance of the SiCp/Al and the NiAlp/Al composites was superior to that unreinforced aluminium alloy. The wear rates of SiCp/Al and 10 pct NiAlp/Al composites at 3.5 N were about one factor of 10 lower than that of aluminium alloy. With increasing applied load, the wear rates of the composites increased to levels comparable to those of unreinforced matrix alloys. At 9.4 N, the wear rates of the composites and aluminum alloy were almost the same. The wear rates of NiAlp/Al and SiCp/Al composites above 13.5 N were much lower than those of aluminum alloy, since the severe wear of aluminum alloy at higher loads was hampered by incorporating the SiC or NiAl particles into the matrix. The wear rates of the counterface material, S45C steel, worn against aluminum alloy, were lower than those worn against the SiCp/Al composites at the entire applied load range. The wear rates were increased with the volume fraction of SiC particles. The NiAlp/Al composites wore the steel at the maximum wear rate at lower loads near 5 N. The NiAl particle was easily fractured when the applied load increased; as a result, the wear rates of steel against NiAlp/Al became smaller and were almost the same as those worn against aluminum.

Test for measuring cut resistance of yarns

A test procedure for evaluating the cut resistance of yarns under tension-shear loading conditions is described and demonstrated. A knife blade is pressed transversely at a constant rate against a yarn gripped at its ends, the load-deflection relation is measured, and the energy required to cut through the yarn is computed. Results for Zylon (polybenzobisoxazole or PBO) are presented. The cut energy and strain to initiate cutting depend on the sharpness of the blade, the slicing angle, and the pre-tension in the yarn. The dependencies are explained by changes in failure mode of the fibers within the yarn. The test provides information needed to extend a computational model of ballistic response of fabrics to sharp fragments and to design a cutting tool.

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.

Impact tensile behaviors of 9% nickel steel at low temperature

In order to prevent catastrophic failure of engineering structures in an aircraft crash, explosive or impulsive loading, fracture properties under dynamic loading conditions should be investigated. One major application is the integrity of liquefied natural gas (LNG) storage tanks. The investigation of effects of strain rate as well as tested temperature on the deformation behavior for it will be necessary. In this study, in order to clarify the strain rate effect on the deformation behavior of 9% nickel steel, tensile tests were performed in a strain rate range of 10 -3 to 5 × 10 2 s -1 and at temperatures of 77 and 293 K. A drop weight testing apparatus was slightly modified to accommodate the application of the dynamic tensile load to specimens and instrumented to detect the applied dynamic load. The corresponding displacement was simultaneously measured by a laser displacement meter. Small-size specimens were used. The obtained results made clear a strain rate hardening behavior, especially distinct hardening trend over I s -1 order of strain rate in 9% Ni steel. The strain rate increase up to 5 × 10 2 s -1 , produced an increase of about 20 and 33% in the yield strength at 77 and 293 K, respectively, as compared with ones at static conditions. However, the elongation of 9% Ni steel held to the static value even at the condition of 250 s -1 , 77 K. At 77 K, a low-temperature hardening became significant, therefore the strain rate hardening effect appeared less as compared with the case at 293 K.

Critical Current Degradation Behavior in YBCO Coated Conductors Under Torsional Strain

The Ic degradation behaviors of a YBCO coated conductor (CC) tape (RABiTS/MOD) was investigated using a sample holder which gives torsional angles to HTS tapes. The Ic degradation in YBCO CC tape under torsional strains occurred gradually which is a characteristic feature under torsion. Uniform torsional deformation was induced in the YBCO CC tape evident from the consistent Ic degradation behavior at each subsection along the longitudinal direction of the tape. Similar with the tension case, the reversible behavior of Ic under torsional loading was found. The irreversible strain, epsivirr.t., was ~0.6%. The critical strain defined by the 95% Ic retention criterion was 1.4% which was located within the irreversible limit. The n-value-thetas behavior in the YBCO CC tape was similar to the Ic/Ic0-thetas behavior.

Characterization of transverse tensile stress response of critical current and delamination behaviour in GdBCO coated conductor tapes by anvil test

Recent problems in superconducting devices operating at the high magnetic field regime include the possibility of substantial critical current, Ic, degradation due to the delamination of the coated conductor (CC) tapes. The delamination may originate from excessive tensile radial stress and cleavage stress developed due to epoxy curing, cool-down and Lorentz force generated. Therefore, it is necessary to investigate the Ic response under transverse loading and the delamination behaviour in REBCO CC tapes to predict its performance in practical device applications. In this study, using the anvil test method, the delamination strength and the transverse tensile stress response of Ic in GdBCO CC tapes were investigated. Under transverse tensile stress, the Ic degradation in GdBCO CC tapes showed both abrupt and gradual behaviours. A reversible Ic degradation behaviour was also observed which was similar to the case under uniaxial tension. The electro-mechanical and mechanical delamination strengths were determined, wherein both increased when a smaller-sized anvil was used during the delamination test. The electro-mechanical delamination strength was approximately half of the mechanical delamination strength in the GdBCO CC tapes tested. Four delamination modes were observed and well correlated with electro-mechanical delamination strength and Ic degradation behaviour. Lastly, delamination sites within the CC tapes were also correlated with the Ic degradation behaviour that had been observed.

Intrinsic strain effect on critical current in Cu-stabilized GdBCO coated conductor tapes with different substrates

The intrinsic strain effect on critical current, Ic in Cu-stabilized GdBCO coated conductor (CC) tapes under bending and uniaxial tension has been investigated. The bending deformation tolerance of Ic in GdBCO CC tapes, fabricated by reactive co-evaporation by deposition and reaction (RCE-DR) with substrate materials of Hastelloy and stainless steel and fabricated by the metal organic chemical vapour deposition (MOCVD) process was evaluated. The intrinsic strain response of Ic under bending was found to be independent of the fabrication process, the substrate material and the geometry of the sample. For samples with a Hastelloy substrate, the intrinsic strain response of Ic/Icmax under bending was well correlated with those under uniaxial tension. However, for samples with a stainless steel substrate, these had a large strain sensitivity for Ic under uniaxial tension even though this showed a much higher irreversible strain limit up to 1.05%.

Bending Strain Characteristics of the Transport Property in Lap-Jointed Coated Conductor Tapes

The electro-mechanical properties of lap-jointed ReBCO coated conductor (CC) tapes have been investigated. Different contact configurations considering its geometry were adopted in this study. The configuration where the YBCO layer is in close proximity showed much less joint resistance. In all joint configurations, the electrical resistance decreased as the joint length increased. For HTS tapes, the joint resistance-joint length behavior could be explained by parallel circuit analysis. In easy and hard bending modes, the Ic degradation behaviors in lap-jointed HTS tapes were examined. The critical radius for 95% Ic retention in each bending mode were determined.