Effect Of Transverse Compressive Stress Research Articles

Study of Transverse Compressive Stress Effects on the Critical Current of Bi-2223/Ag HTS Tapes

Study of Transverse Compressive Stress Effects on the Critical Current of Bi-2223/Ag HTS Tapes

Effect of transverse compressive stress applied at room temperature on Nb3Sn Rutherford cables

The accelerator magnets for the high luminosity upgrade of the large Hadron collider use Nb3Sn conductor to achieve the required in-field performance. To sustain the Lorentz forces during operation, a pre-compression is applied to the coils during the fabrication of the magnet. This can lead to an irreversible degradation of the Nb3Sn conductor due to its mechanical sensitivity. In this study, the impact of the pre-compression applied to the conductor at room temperature is investigated using a reacted double-stack specimen made of two Nb3Sn Rutherford cables. The cables have a keystone angle and are stack with a non-inverted configuration, without compensating the angle. The rectangular specimen is submitted to increasing transverse compressive stresses at room temperature applied perpendicularly to its width. The non-inverted configuration and the rectangular shape of the total specimen can thus lead to stress concentration. The pressure applied covers the range from 130 MPa to 190 MPa with a 10 MPa step increase. After each cumulated stress level, transport current measurements are performed in liquid helium and in background fields of up to 9.6 T in the FReSCa test station at CERN Metallographic analyses of several samples are made at selected stress levels. Monotonic and cumulated stresses are applied and the impact of mechanical cycling is analyzed. Procedures are specifically developed to minimize surface damage during samples’ preparation. The observations are compared with the electrical measurements in order to correlate the irreversible effect of the transverse pressure with the A15 damage state in the cross section. The transport current measurements of the double-stack specimen show degradation of critical current and n-value starting at respectively 170 MPa and 160 MPa cumulated loadings. However, cracks in the A15 phase are already observed in a metallographic sample subjected to 140 MPa.

Transport Properties up to 1000 A of ${\rm Nb}_{3}{\rm Sn}$ Wires Under Transverse Compressive Stress

The effect of transverse compressive stress on the superconducting transport currents of multifilamentary Nb3Sn wires have been measured at 19 T using a newly developed probe allowing gauge lengths as long as 125 mm. Two different wire types have been studied: an experimental Bronze Route wire (from our laboratory) with 14'641 filaments of 4.5 mum diameter and a RRP internal Sn wire (from OST), with 60 filament bundles, each having a diameter of 70 mum. It was found that the RRP wire exhibits higher stress sensitivity. For the Bronze Route wire our data also comprise a comparison of the Ic behavior for rectangular and round cross sections. The problem of current transfer has been solved using a new type of Walters spring (one turn). The spring has been cut parallel to the wire sample into two parts so the measured sample length covers a whole turn. One part can be moved axially and forces up to 40 kN are applicable. The new device will be used for the investigation of various types of superconducting wires, comprising Nb3Sn and Nb3Al, in view of their application in systems with high transverse compressive forces, in particular high field dipoles (e.g. NED).

Transverse Compressive Stress Effects on the Critical Current of Bi-2223 Tapes

Bi-2223 tapes are usually subjected to tensile strain, bending strain and compressive stress in the process of fabrication and applications. In this study, we have measured the critical current of Bi-2223 tapes as a function of transverse compressive stress. The effects of the cross section size and processing defects on the stress characteristics of AgMn-alloy sheathed Bi-2223 tapes are compared. The experimental results showed that the onset transverse compressive stress for degradation of Ic was lower in the thinner tape with a thin Ag sheath due to the stress concentration on the irregular interface. The bubbled tapes were more sensitive to the transverse stress compared to the nonbubbled tapes.

Effect of transverse compressive stress on internal reinforced Nb 3Sn superconducting wires and coils

Lorentz force is applied to coil windings when the magnet is charged. The coil windings are subjected to huge hoop tensile stress and transverse compressive stress in both the radial and axial directions due to the Lorentz force. The higher the magnetic field and the larger the bore, the larger the Lorentz force applied to the coil windings. Thus, not only the hoop stress, but also transverse compressive stress should be taken into account when designing the magnet. In this paper, we focus on transverse compressive stress, and the influence of this stress on critical current is explored for a standard Nb 3Sn wire and a Cu–NbTi-reinforced Nb 3Sn wire. It was confirmed that Cu–NbTi sufficiently reinforced the superconducting wire, not only for tensile stress, but also for transverse compressive stress. The effect of reinforcement arrangement in the cross section was also investigated. In the case that Cu–NbTi reinforcement is arranged at the center of the wire, there was less deterioration in superconducting property than in the case of the reinforcement being arranged in the outer part of the wire.

Transverse compressive stress effect in Y-Ba-Cu-O coatings on biaxially textured Ni and Ni-W substrates

Electromechanical properties of yttrium-barium-copper-oxide (YBCO) coatings on both pure Ni and Ni-5at.%W alloy rolling-assisted, biaxially-textured substrates (RABiTS) were investigated. The effect of transverse compressive stress on transport critical-current densities (J/sub c/) was measured on samples at 76 K and self magnetic field. Transverse compressive stress can significantly degrade J/sub c/ in YBCO deposited on pure Ni RABiTS unless sufficient frictional support is provided to the sample or the substrate is given a work-hardening treatment. On the other hand, results obtained for YBCO on Ni-5at.%W alloy RABiTS show that the tolerance to transverse stress of these conductors is significantly improved. These electromechanical properties are interpreted with scanning-electron micrographs of the microstructure of the samples after electromechanical testing, as well as stress-strain characteristics measured on RABiTS substrates at 76 K. The tensile yield strength, Young's modulus, and proportional limit of elasticity of candidate RABiTS substrate materials are tabulated and compared.

Transverse compressive stress effects on the critical current of Bi-2223/Ag tapes reinforced with pure Ag and oxide-dispersion-strengthened Ag

We have measured the effect of transverse compressive stress on the critical current of two similar sets of Bi2Sr2Ca2Cu3Ox (Bi-2223) tapes in a direct comparison between tapes reinforced with pure Ag and oxide-dispersion-strengthened (ODS) Ag. The critical-current degradation at a transverse stress of 100 MPa was about 10% for the Bi-2223/pure Ag tapes and only about 3% for the ODS Ag tapes. In one case for each sample type the degradation was significantly greater, and there was an apparent correlation with porosity in the superconductor filaments. The degradation was the same for critical-current criteria from 5 μV/cm down to 0.1 μV/cm.

Highly-strengthened alumina-copper alloy matrix (Nb,Ti)/sub 3/Sn conductor fabricated by using the tube process

In order to fabricate a large-bore, high-field magnet which achieves a low foil weight and volume, a high strength compound superconducting wire is required. The alumina-copper strengthened (Nb,Ti)/sub 3/Sn wire which contained 30 vol.% alumina-Cu alloy fabricated by using the tube process has been developed and tested for critical current density, mechanical properties, transverse compressive stress effects and related properties. As a result, it was found that the reinforced (Nb,Ti)/sub 3/Sn wire had high transverse compressive stress tolerance, for example only 3% decrease which was 1/3 of the Cu matrix wire in I/sub c//I/sub c0/ at 60 MPa and reversibility in I/sub c/ between 0 and 200 MPa. Using the newly developed reinforced-(Nb,Ti)/sub 3/Sn wire, it will be possible to fabricate a lightweight, large-bore, high-field and compact superconducting magnet in the near future.

Development of (Nb, Ti) 3 Sn and Nb-Ti conductors for 15 T superconducting magnet of 40 T class hybrid magnet

Tube processed (Nb, Ti) 3Sn and NbTi conductors have been developed for a 15 T class superconducting magnet with a 400 mm clear bore of a 40 T class hybrid magnet. Prior to testing the actual conductor, the transverse compressive stress effects on the critical current ( I c) of the (Nb. Ti) 3Sn conductors were studied using a 1 7 scale reduced conductor. The I c of the samples with clearance of gap length of over 0.3 mm between Cu-housings and (Nb,Ti) 3Sn monolith reached over 90% of unloaded I c( I c0) at 14 T and 60 MPa of the transverse compressive stress. Based on the above-mentioned results, a new structure for full-scale Cu-housing (Nb,Ti) 3Sn conductors has been designed and fabricated. Fabricated (Nb, Ti) 3Sn conductors flowed more than 3870 A at 15 T and had 300 M Pa of 0.2% proof stress. All these values exceeded the values in the specification with a good margin.

Transverse stress effects in Nb/sub 3/Sn cables

The effect of transverse compressive stress on the critical current of solder-filled and unfilled Nb/sub 3/Sn cables is reported. The conductor used in this study is a Nb/sub 3/Sn Rutherford cable manufactured with a bronze-process wire of 0.92 mm diameter. Like epoxy-impregnated cables, solder-filled cables exhibit much less degradation than wire samples when subjected to the same stresses. On the other hand, unfilled specimens are irreversibly damaged at the thin edge when loaded to 160 MPa, and show significantly higher degradation than similar specimens of the solder-filled cable. A finite-element calculation of the stress state inside a particular composite superconductor indicates that more compressive stress is developed in the virgin wire than in a straight wire segment in a real cable environment.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Critical-current degradation in multifilamentary Nb/sub 3/Al wires from transverse compressive and axial tensile stress

The effect of transverse compressive stress and axial tensile stress on the critical current of multifilamentary Nb/sub 3/Al superconducting wires has been measured. Compared with commercial Nb/sub 3/Sn, Nb/sub 3/Al exhibits a relatively small sensitivity to both axial and transverse stress. For a given degradation of critical current at 9 T, Nb/sub 3/Al will tolerate about twice as much axial stress as Nb/sub 3/Sn and about five times the transverse stress. The elastic modulus of Nb/sub 3/Al, 169+or-20 GPa, was measured for the first time at cryogenic temperatures and found comparable to that of Nb/sub 3/Sn. A comparison between the effect of axial and transverse stress showed that Nb/sub 3/Al, like Nb/sub 3/Sn, is more sensitive to transverse stress than axial stress. For a given level of critical-current degradation, the transverse stress tolerance of Nb/sub 3/Al is about half the axial stress tolerance. The favorable electromechanical characteristics of Nb/sub 3/Al, compared with Nb/sub 3/Sn, may allow increased operating limits for the next generation of large high-field superconducting magnets.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The transverse stress effect on the critical current of jelly-roll multifilamentary Nb/sub 3/Al wires

Experiments were conducted to determine the effect of transverse compressive stress (TCS) on the critical current of jelly-roll multifilamentary Nb/sub 3/Al wire (0.8-mm dia.) for magnetic flux densities up to 12 T. For comparison, identical experiments were performed for bronze-process Ti-alloyed multifilamentary Nb/sub 3/Sn wire (1.0-mm dia.). Although the unstressed critical current density of Nb/sub 3/Al was inferior to that of (NbTi)/sub 3/Sn at high fields, under applied TCS Nb/sub 3/Al exhibited less critical current degradation than (NbTi)/sub 3/Sn. For example, at 12 T and 150 MPa. TCS-induced critical current degradation was approximately 20% for Nb/sub 3/Al, whereas it was approximately 65% for (NbTi)/sub 3/Sn.

Influence of transverse compressive and axial tensile stress on the superconductivity of PbMo 6S 8 and SnMo 6S 8 wires

The behaviour of the critical current density, J c, of PbMo 6S 8 and SnMo 6S 8 monofilamentary wires with external transverse compressive stress, σ t, and axial tensile stress, σ a, was investigated. Varying the reinforcing steel content in the wire matrix, different axial prestress states, e.g. the compressive, tensile and unstressed state were realized. The observed variation of J c, versus σ a was significantly smaller compared with that of Nb 3Sn wires, which is explained by the higher upper critical magnetic field, B c2, of the Chevrel phase. The elastic strain range of 0.6–0.8% (with 0.2% compressive prestrain of the filament) fulfils the requirements for future technical applications of this type of wire material. The effect of transverse applied compressive stress, σ t, on I c and B c2 ∗ of the Chevrel phase wires was investigated in magnetic fields up to B = 22 T and T = 4.2 K. Analogous to Nb 3Sn wires, a strong degradation of the critical current density with σ t was observed. As for the axial tensile stress, the influence of σ t on J c is found to increase with the applied magnetic field. At 20 T the transverse compressive stress effects reduce J c about twice as much as the axial tensile stress. At present externally steel reinforced PbMo 6S 8 wires can be compared with Nb 3Sn wires, the effect of transverse stress appears to be considerably smaller in the Chevrel phase wire, possibly as a consequence of the higher upper critical field with respect to Nb 3Sn.

Effect of transverse compressive stress on the critical current of cabled Nb/sub 3/Sn conductor

A test apparatus has been designed and built for the investigation of cabled conductors subjected to transverse load. The apparatus permits measurement of compressive stress on a loaded multistage flat cable when a transport current is applied to one of its constituent subcables. With the apparatus, the critical-current behavior of a solder-filled Nb/sub 3/Sn cable made by the external diffusion method exposed to transverse loads up to 165 MPa, was investigated. A field-dependent degradation of the critical current was found. The effect is completely reversible up to about 100 MPa. Solder-filled cables as used in large-current applications are less sensitive to transverse compressive stress than single wires. A 150 MPa compressive load on the SULTAN cable, for example, degrades the critical current by 31% at 11 T. This is still a high enough value in view of the large transverse compressive stress acting on conductors required for fusion magnets. >

Effect of transverse compressive stress on the critical current and upper critical field of Nb3Sn

A large reversible degradation of the critical current of multifilamentary Nb3Sn superconductors has been observed when uniaxial compressive stress is applied transverse to the conductor axis at 4 K. In bronze-process multifilamentary Nb3Sn, the onset of significant degradation occurs at about 50 MPa. In an applied field of 10 T, the magnitude of the effect is about seven times larger for transverse stress than for stress applied along the conductor axis. The transverse stress effect increases with magnetic field and is associated with a reversible degradation of the upper critical field. The intrinsic effect of transverse stress on the upper critical field is about ten times greater than for axial stress. Although axial stresses on the Nb3Sn filaments are greater than transverse stresses in most applications, the transverse stress effect will need to be considered in the internal design of large magnets because of the greater sensitivity of Nb3Sn to transverse stress. It is shown that the transverse stress from the Lorentz force on the conductor is proportional to conductor thickness. This will place limits on conductor dimensions and the spacing between distributed reinforcement in large magnets. The effect may be particularly significant in cabled conductors where large transverse stress concentrations can occur at strand crossover points.

Relationships between critical current and stress IN NbTi

The effects of various types of stress on the critical current of a multifilamentary NbTi superconductor are reported. Degradation of critical current due to axial tension applied at 4 K, transverse compression applied at 4 K, and hairpin bending strain applied at room temperature has been measured. The degradation from axial tension is much greater than from transverse compression in many practical cases because the soft copper matrix limits the build-up of transverse compression. The degradation from typical levels of transverse compression is only about 4% at 8 T, for example. For axial tension, on the other hand, higher stresses can occur that will degrade the critical current by 24%, for example, at 7 T and 2.7% strain. Both the axial-tensile and the transverse-compressive stress effects are about 98% reversible; thus the degradation will be seen only when the conductor is under operational stress. The results indicate that a primary origin of the critical current degradation in NbTi is a stress-induced reversible decrease in the upper critical field.