Reversible Strain Limit Research Articles

Mechanical and critical current characteristics of high-strength BiSrCaCuO multi-filamentary tapes reinforced with thicker Ni-alloy laminations with various pre-tensions

We investigated the stress–strain relation and the applied strain dependence of the critical current of high-strength Bi2Sr2Ca2Cu3O (Bi2223) R&D tapes reinforced by thick Ni-based alloy sheets with various pre-tensions applied during the lamination process. We confirmed that the thickening of Ni-alloy laminations is useful for enhancing Young’s modulus compared with commercial tapes ( thick laminations). decreases gradually and reversibly with applied strains and then degrades irreversibly with larger tensile and compressive strains. The reversible strain regime tends to shift toward the tensile side with larger , indicating that application of large is effective in improving the reversible tensile strain limit. The observed can be reproduced well by model considering -linear critical current density, cross-sectional strain distribution, and Weibull-type distribution of filament damage. We clarified that different irreversible degradations with large tensile and compressive strains can be understood by differences in the dimensions and number of cracks and ruptures (tensile strains) and bucklings (compressive strains).

Critical current degradation behavior of 7-filamentary Ba1−x K x Fe2As2 tapes under uniaxial strain

122-type iron-based superconductors (IBS) have attracted extensive attention due to their excellent performance. The 100 m class Ba1−x K x Fe2As2 (Ba-122) 7-filamentary IBS tapes were successfully fabricated by the Institute of Electrical Engineering, Chinese Academy of Sciences. In this study, we have performed tests of the 7-filamentary Ba-122 short tape samples (Bas samples) and the 100 m class 7-filamentary Ba-122 tape samples (Ba100 samples) under various kinds of strain using U-shaped bending spring devices. Both types of samples were still reversible even when the applied compressive strain reached −0.65%. In addition, the reversible tensile strain limit of the Bas and Ba100 samples were 0.25% and 0.28%, which increased to 0.38% and 0.41% after considering the effect of the cooling process.

Mechanical—electromagnetic property of stainless sheet laminated BSCCO–2223 wires

In order to elucidate the effect of applied tensile stress/strain on the critical current of DI-BSCCO-2223 wires, the critical current measurement and the tensile test were carried out at 77 K. Neutron diffraction measurements were also performed at 77 K to get direct information about the local strain exerted on the BSCCO superconducting filaments. It was reported that the critical current of BSCCO wires decreases linearly in a small range of tensile stress/strain and that the critical current returns reversibly on reducing the stress/strain. The gradual decrease of critical current was observed within the reversible region. When the tensile stress /strain increased beyond a characteristic value, the critical current decreased rapidly due to the brittle fracture of BSCCO filaments. It is proposed that the reversible stress/strain limit of critical current (Rrec and Arec) can be defined experimentally as the 99% Ic recovery stress (R99%rec) and strain (A99%rec). The reversible strain limit is proposed to consist mainly of three factors of pre-strain, thermal strain and tensile fracture strain of SC filaments themselves. The calculated relation between the Rrec and Arec could well explain quantitatively the observed relation between 99% Ic recovery stress and strain.

Effects of differently hardened brass foil laminate on the electromechanical property of externally laminated CC tapes

The mechanical properties of REBCO coated conductor (CC) wires under uniaxial tension are largely determined by the thick component layers in the architecture, namely, the substrate and the stabilizer or even the reinforcement layer. Depending on device applications of the CC tapes, it is necessary to reinforce thin metallic foils externally to one-side or both sides of the CC tapes. Due to the external reinforcement of brass foils, it was found that this could increase the reversible strain limit from the Cu-stabilized CC tapes. In this study, the effects of differently hardened brass foil laminate on the electromechanical property of CC tapes were investigated under uniaxial tension loading. The tensile strain dependence of the critical current (Ic) was measured at 77 K and self-field. Depending on whether the Ic of CC tapes were measured during loading or after unloading, a reversible strain (or stress) limit could be determined, respectively. The both-sides of the Cu-stabilized CC tapes were laminated with brass foils with different hardness, namely 1/4H, 1H and EH. From the obtained results, it showed that the yield strength of the brass laminated CC tapes with EH brass foil laminate was comparable to the one of the Cu-stabilized CC tape due to its large yield strength even though its large volume fraction. It was found that the brass foil with different hardness was mainly sensitive on the stress dependence of Ic, but not on the strain sensitivity due to the residual strain induced in the laminated CC tapes during unloading.

Hoop Stress Test of $\hbox{GdBa}_{2}\hbox{Cu}_{3}\hbox{O}_{\rm y}$ Coated Conductor

Mechanical property of GdBa2Cu3Oy (GdBCO) coated conductor was investigated at 4.2 K in a magnetic field by two ways, which are the tensile test and the hoop stress test. The tensile stress/strain dependence of critical current (Ic) of a 2 mm width conductor was explored at 4.2 K, 18 T. The result provided that Ic -reversible strain limit existed in between 0.43% and 0.46%, corresponding to 907 MPa and 960 MPa in stress, and the elastic constant was 203 GPa. The hoop stress test has been performed at 4.2 K, 11 T. A test coil was fabricated by winding a 5 mm width conductor on a 270 mm diameter GFRP bobbin by 1.5 turns. The maximum value of applied hoop stress, which was deduced from a product of magnetic field, current density and coil radius, was 1322 MPa. Five strain gauges glued on the conductor surface showed almost the same values, which were in a range of 0.64% to 0.67%, indicating the uniform longitudinal deformation. Furthermore, the hoop stress-strain characteristics were linear, suggesting an elastic deformation. The deduced elastic constants were in a range of 196-204 GPa. It was confirmed that the GdBCO coated conductor performance was deteriorated irreversibly by 1322 MPa hoop stress, whereas not by 1302 MPa hoop stress.

Measurement system for strain effect on Ic in coated conductor tapes at 77 K under magnetic field using a pair of permanent magnets

In this study, in order to investigate the strain effect in the critical current, Ic of coated conductor (CC) tapes under magnetic field a newly devised measurement system using a pair of permanent magnets was constructed. The level of magnetic field applied could be controlled by the distance between magnets located in parallel. The distribution of magnetic field between the permanent magnets was checked using a hall probe sensor. External magnetic field was applied perpendicular to the CC tapes surface. Uniaxial tensile strain was applied to the tape by using devised loading frame mounted on a universal testing machine and the strain induced was measured by using a Nyilas type extensometer. Ic/Ic0–εt relation of the CC tape was first measured at self field. External magnetic field was then varied to investigate the dependency of critical current, Ic, and its effect on the reversible strain limit under uniaxial tension. Finally, the transport property characteristic of SmBCO CC tapes under magnetic field with uniaxial tension using permanent magnet is compared to the case using superconducting magnet.

Improvement of Reversible Strain Limit for Critical Current of DI-BSCCO Due to Lamination Technique

The DI (dynamically innovative)-BSCCO-Bi2223 tapes achieved high critical current as well as high modulus of elasticity. Further the reversible strain limit and the corresponding stress for critical current have been remarkably increased by means of lamination technique. During the course of development, their optimized architecture has been designed based on the principle of the rule of mixture for maximizing the force free strain exerted on the superconducting component. The reversible strain/stress limit ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rev</sub> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rev</sub> ) was defined as a strain, at which the critical current recovers to the level of 99% <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">co</sub> . Selecting several kinds of laminating materials and changing condition of the fabrication, the excellent Cu alloy-3ply tape with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">co</sub> of 311 A/cm was realized of which <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rev</sub> and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rev</sub> reached 0.42% and 300 MPa, respectively. Further during the theoretical analysis, the increase of reversible strain limit were made clear to be attributed to the increase of thermally induced residual strain as well as the compensation effect of laminated layers against a local fracture mode.

Reversible strain limit of critical currents and universality of intrinsic strain effect forREBCO-coated conductors

Intensive research work has been carried out in order to develop industrially available HTSREBCO-coated conductors under the NEDO project in Japan. Recently, several groups inthe project succeeded in the development of high performance coated conductors. Theircharacteristic features have been evaluated in terms of mechanical propertiesand their influence on critical currents. The mechanical properties at RT and77 K were analyzed on the basis of the rule of mixtures. The force-free strain(Aff) was analytically deduced, which indicates the strain at which the residual stress exertedon the superconducting layer becomes zero. Tensile strain dependence on criticalcurrents could be divided into elastic and brittle regions. The reversible strain limit(Arev) was defined as a strain at which the critical current recovers elastically to the level of 99%Ico. Within the elastic region, the critical current showed a convex strain dependence,which is explained as Ekin’s intrinsic strain effect. The degradation beyond thereversible strain limit was attributed to a fracture of the superconducting layer. As awhole, the present study made clear quantitatively the tensile strain behavior ofcritical currents and proposed a reasonable definition for the reversible strain limit.

Strain characteristics of critical current in ISD/Dy–Ba–Cu–O coated conductors

The strain dependence of critical currentIc of ISD (inclined substrate deposition) Dy123 coated conductor tape has beenstudied by two kinds of bending and tensile testing apparatus at room (RT) andliquid nitrogen temperature (77 K). In the continuous bending-tension tests byGoldacker type apparatus, in which a Dy123 layer was subjected to tensile strain,Ic began to decrease rapidly beyond the strain of 0.4% after initialmoderate degradation. Bending back in the test deterioratedIc intermittently and the reversible strain limit was reduced toless than 0.2%. In the bending-compression tests, however, theIc was slightly increased up to the strain of 0.35 to 0.4% and then it decreased. In thediscontinuous bending-tension tests by using FRP holders at RT, the reversible strain wassmaller compared with that of the continuous bending tests at 77 K due to thedifference in thermal compressive residual strains. The strain characteristics ofIc obtained by tensile tests are almost the same as that by the bending tests.

Reversible Strain Dependence of Critical Current in 100 A Class Coated Conductors

The strain dependence of the critical current was studied for YBCO and DyBCO coated conductors with different buffer layers on Hastelloy substrates. A maximum of I/sub c/ was observed for both the YBCO and DyBCO tapes, however the sign of the strain at the I/sub c/ peak was opposite for the two superconductors. A reversible variation of I/sub c/ with applied strain was found and the reversible strain limit was observed to depend on the buffer layer. For the IBAD-CeO/sub 2//YSZ buffered YBCO tapes, I/sub c/ recovers reversibly when the applied strain is reduced starting from 0.30%. For those with an ISD-MgO buffer layer the irreversible degradation starts at a strain less than 0.22%. The reason for this difference is discussed based on microscopic observations. Quenching occurred during V-I measurements after the applied strain exceeded 0.30%, which is close to the yield strain of the composite tape.

Intrinsic strain effect on critical current and its reversibility for YBCO coated conductorswith different buffer layers

The uniaxial strain dependence of the critical current was examined for YBCO coated conductors withIBAD-CeO2/YSZ or ISD-MgO buffer layers on Hastelloy substrates.Ic increased with increasing applied strain, reached a maximum anddecreased for higher strain values. The reversible strain region ofIc variation was observed. The reversible strain limit depends on the buffer layers. For theCeO2/YSZ bufferedtape, Ic recovered the initial value even after the applied strain reached0.30%. On the other hand, an irreversible degradation ofIc wasobserved at the strain less than 0.22% for the MgO buffered tape. The overall relationship betweenthe Ic normalized by the peak value and the intrinsic strain in YBCO obeyed a unified scalingfunction. Quenching occurred at the strain close to the yield strain of the Hastelloysubstrate.

Analysis of stress–strain behavior in Bi2223 composite tapes

Tensile test was carried out for Bi2223/Ag/Ag alloy composite tapes at RT, 77 and 7 K. Two yielding points are observed in the stress–strain curves. From the stress–strain behavior of the components and critical current ( I c) as a function of tensile strain, it was found that the microscopic reason for these yieldings is attributed to yielding of Ag alloy and fracture of Bi2223, respectively. The strain at the second yielding has temperature dependence and it becomes larger with decreasing measured temperature. From the thermo-mechanical analysis, it can be explained by temperature dependence of compressive residual strain of Bi2223. Reversible recovery of I c was found during loading–unloading test. The relationship between the reversible strain limit and the intrinsic strain of Bi2223 was discussed.

強化ブロンズ法Ｎｂ３Ｓｎ線材の機械的性質と臨界電流の応力／ひずみ特性

This paper reviews the stress/strain characteristics of some typical highly strengthened bronze-processed practical Nb3Sn superconducting wires examined in the HFLSM, IMR of Tohoku University up to the magnetic field of 14.5 T at a temperature of 4.2 K. Internal reinforcement of the superconducting composite wires using the materials with high strength and high electrical conductivity such as in situ processed or jelly rolled Cu-Nb microcomposite, Ta, and recently developed Cu-Nb-Ti compound was proved to be promising, although the stability of the wire is decreased to some extent. Although the intrinsic axial tensile strain sensitivity of Ic was not markedly changed, the reinforcements changed not only mechanical properties such as the 0.2% proof stress but also the parameters relevant to the tensile strain characteristics of Ic, the strain corresponding to the Ic peak, em, and reversible strain limit, eirr, where permanent damage such as cracking starts to take place in the Nb3Sn filaments. Furthermore, the transverse compressive stress sensitivity of Ic decreased and the reversible stress limit increased. The effect of parameters on the Cu-Nb composite reinforced bronze-processed Nb3Sn wires and the strain characteristics are intensively discussed.

Dependence of the critical current in ex situ multi- and mono-filamentary MgB2/Fe wires on axial tension and compression

The transport critical current Ic of single- and four-core MgB2/Fe wires under tensile and compressive axial strain ε was measured with a U-shaped spring set up. In all wires, Ic increases linearly and reversibly with applied tensile strain, up to a sample-dependent reversible strain limit εirr. The same reversible and linear Ic(ε) relation is found when applying compressive strain. This shows how the Ic(ε) increase with tension can be understood as a gradual release of the thermal pre-compression strain induced in the MgB2 filaments of the composite. The value of the reversible strain limit εirr depends mainly on the degree of pre-compression, but also on the shape of the composite wire. It is highest in a square wire and lowest in the flattest tape. Tensile strain levels above εirr cause an irreversible degradation of Ic. This Ic(ε) degradation, due to filament cracking, is significantly steeper in heat-treated wires than in as-deformed ones. The different Ic(ε) regimes are analysed and the mechanical behaviour of the MgB2/Fe composite is compared with that of typical Nb3Sn wires and Bi,Pb(2223) tapes.

Stress/strain dependence of critical current in Nb 3Sn superconducting wires stabilized with Cu–Nb microcomposites – effect of Nb content

The effects of niobium content in the Cu–Nb microcomposite on the mechanical property and the axial tensile or transverse compressive stress/strain characteristics of the critical current I c in a bronze processed Nb 3Sn multifilament superconducting wire with the Cu–Nb microcomposite were evaluated at a temperature of 4.2 K in a magnetic field of 14 T. An increase in niobium content increases the 0.2% proof stress as well as both tensile and transverse compressive stress tolerance of I c of the wire. High yield stress value of the Cu–Nb microcomposite increases the strain at peak I c, ε m, as well as the reversible strain limit of I c, ε irr in the wire as compared to those in the Cu stabilized wire. An increase in niobium content in Cu–Nb microcomposite, however, decreases them due to decrease in the coefficient of thermal expansion.

Properties of `z-stack' elongate (Bi 2− xPb x)Sr 2Ca 2Cu 3O 10− δ/Ag high temperature superconductors manufactured by the `tape-in-tube' technique

Multifilamentary z-stack samples of tape-in-tube wires and tapes have been prepared using a flat tape re-stacking procedure. Self-field I C measurements at 77 K show values of 24±2 A and 22±1 A for 2×11 multifilamentary z-stack tapes and round wires, respectively. These I C measurements correspond to J C values=1.62×10 8 A m −2 and 7.7×10 7 A m −2, respectively, for tape and wire configurations. While most of the fabrication factors (such as the superconducting packing fraction) were found to be similar to those found in conventional powder-in-tube (Bi 2− x Pb x )Sr 2Ca 2Cu 3O 10− δ /Ag composites, some of the tape properties such as the 5% ( ε=0.46) reversible bend strain limit, showed an improvement from conventional powder-in-tube tape values. Measurements in magnetic fields show that twisting z-stack filaments in a round wire format can reduce the magnetic field anisotropy of tape-in-tube tapes compared to conventional powder-in-tube tapes.

Tensile strain/transverse compressive stress effects in bronze processed Nb-matrix Nb/sub 3/Sn wires

Mechanical properties and the strain/stress dependence of the critical current I/sub c/ of Nb-matrix Nb/sub 3/Sn multifilamentary wires fabricated through bronze route were evaluated up to a magnetic field of 14 T at 4.2 K. The wire showed a 0.2% proof stress of 550MPa, which is 3.4 times higher than that in the conventional bronze processed Nb/sub 3/Sn wires. The strain sensitivity of I/sub c/ and the reversible strain limit, 0.8%, were almost the same, but the axial tensile strain for the peak I/sub c/, 0.1%, was small compared with those in the conventional wires. On the other hand, the transverse compressive stress sensitivity of I/sub c/ was remarkably low. The irreversible stress where I/sub c/ on unloading no longer recovers to the initial value was larger than 300MPa, which is several times higher than those in conventional wires. Thus, these results show that the wire is highly tolerant to the external stress/strain. >

実用Ｎｂ３Ｓｎ超電導線材の構成と臨界電流のひずみ依存性

An investigation was made on the strain dependence of critical cuurent Ic for three kinds of practical Nb3Sn superconducting wires which are in the course of development, with an emphasis on the correlation with wire constitution.In the internal tin diffusion processed wires with a single tin core at the center, the reversible strain limit, εirrev was small and the strain sensitivity of Ic was high. This is ascribed to the strain concentration induced by Kirkendall voids observed in the center of fracture surface.Internal reinforcement using SUS 405 steel increased both εm, the strain corresponding to peak Ic, and the strain sensitivity, and decreased the εirrev in addition to a marked increase of flow stress. In the light of both the serration in the stress-strain relation and fracture surface morphology, the degradation of strain characteristics are attributable to the strain concentration in Nb3Sn layer associated with localized deformation of the steel at low temperatures.In the external tin diffusion in situ processed wires, both Nb3Sn layer formed in the periphery and the unreacted core in the center of the wire controlled the strain characteristics. The smaller the core size was, the larger the critical current density and the smaller the εm or εirrev. Inhomogeneity of tin diffusion in the cross section as well as in the longitudinal direction resulted in deviation of the core and tin balling, respectively. They deteriorated the strain characteristics of the wire significantly.Improvements of the strain characteristics in these wires through proper constitution and heat treatment were briefly discussed.

Ｎｂ３Ｓｎ導体構成の歪み特性への影響

To study characteristics of stress distribution in the conductor is necessary for the design of a superconducting magnet working stably at high magnetic fields. Strain effects of conductors depend on the layer thickness of Nb3Sn filaments and the structure of the conductor. The dependence of reversible strain and pre-strain effects on the layer thickness were investigated using superconductor fabricated by liquidus-solidus process that could vary the layer thickness of Nb3Sn in the wide range. The reversible strain limit decreased with increasing Nb3Sn layer thickness. It was found that the pre-strain of the conductors depended on how Nb3Sn filaments were grouped and distributed in the stabilizer like Cu.

Analysis Method for Both Internal Stresses and Microstructural Effect Under Pure Rolling Fatigue Conditions

For pure rolling fatigue conditions, the effect of microstructural changes and internal stresses due to grinding and fatigue processes itself was studied. Using Sachs’ method, the field of internal stresses was determined and it has been shown that its evolution is correlated with microstructural changes. These microstructural changes lead to changes in mechanical properties, especially to the reversible strain limit stress (R.S.L.S.) which has been determined in each point of the subsurface contact. Assuming a given relation between R.S.L.S. and fatigue limit, and using fatigue criterion which includes R.S.L.S., the effect of internal stresses, microstructural evolution, and Hertz stresses was analyzed. It has been found that the grinding internal stresses have no significant effect. On the contrary, the R.S.L.S. value was strongly reduced by grinding thermal effect in subsurface region, and was also strongly changed by the fatigue process. Finally, this analysis method was used to determine the best field of internal stresses necessary to minimize the fatigue damage.