[Show abstract][Hide abstract] ABSTRACT: International RRT was carried out in order to establish the test method for mechanical properties of commercial BSCCO superconductive tapes under cooperation of seven research laboratories. From the stress versus strain curve, the following quantities were evaluated; modulus of elasticity, 0.2% proof strength, fracture strength and stresses at fixed strains. The scatter of measured values was analyzed by evaluating the relative standard uncertainty (RSU), which is the standard uncertainty divided by the average. The expected value of RSU for N=3 was derived for each mechanical quantities. In order to make clear the major contribution to the scatterings, the F test was applied. The major source of RSU’s was attributed mostly to the influence of inter-laboratory scattering.
[Show abstract][Hide abstract] ABSTRACT: Mechanical properties of low porosity Dy123 (DyBa2Cu3Ox) bulks with different Dy211 (Dy2BaCuO5) content were evaluated through bending tests and Vickers indentation tests of specimens cut from the bulks. The bending strength and the Young’s modulus of the low porosity bulks were higher than those of porous Dy123 bulks evaluated in the previous study. The bending strength of the low porosity bulks, which was decreased by the oxygen annealing, increased with increase of the Dy211 content. The fracture toughness of the area with uniform dispersion of the Dy211 particles was superior to that with low density of the Dy211 particles.
Physica C Superconductivity 10/2009; 469(15):1207-1210. DOI:10.1016/j.physc.2009.05.018 · 1.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In order to improve the mechanical properties of the Dy123 bulk material, a precursor was melted in 100% O2 atmosphere to obtain bulk with low porosity. Evaluations of the mechanical properties in the Dy123 bulk were carried out through bending tests and Vickers indentation tests. The mechanical properties such as Young's modulus and bending strength were improved by eliminating pores. The mechanical properties of Dy123 bulks melt-processed in 50 and 75% O2 atmosphere were also evaluated and the relationship between the mechanical properties and the porosity associated with the oxygen annealing was discussed.
[Show abstract][Hide abstract] ABSTRACT: Important operating parameters, temperature characteristics and radical production in an air plasma flow generated by a low-power pulsed arc discharge were experimentally clarified for lean combustion enhancement and surface treatment. Furthermore, the time-dependent thermofluid field downstream from the torch was also clarified numerically and the downstream temperature well agreed with experimental data. Finally, the time evolution of production and decay of the chemical species in air plasma were clarified numerically under a high electric field.
International Journal of Heat and Mass Transfer 03/2009; 52(7):1778-1785. DOI:10.1016/j.ijheatmasstransfer.2008.10.009 · 2.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have developed a novel critical current and stability measurement experimental setup, which utilizes a closed electric circuit with a multi-strand superconducting cable. The feature of this setup is mechanical loading applied to the multi-strand cable in the transverse direction. It was reported that Lorentz forces caused degradation in the critical current of the ITER-TFMC conductor. Furthermore, these phenomena were mainly observed in the ITER full-size conductors with large Lorentz forces under high magnetic fields. The advantage of our setup is critical current measurement under mechanical stresses comparable to those in the full-size conductor under high magnetic fields. By employing an inductive critical current measurement technique, we conducted an experiment with a transport current of about 10 kA without any power supply or current leads. In our experiments, we observed significant degradation in critical currents due to a compressive stress of about 30 MPa. We applied an innovative technique to mitigate the critical current degradation in mechanically loaded Nb3Sn superconducting multi-strand cables. We molded one such cable with ice and tested it. No degradation occurred in the icemolded cable. In addition, stability was also ensured due to the large thermal conductivity of ice. Thus, we have successfully mitigated the degradation in the critical current of the Nb3Sn conductor by ice molding.
Plasma and Fusion Research 01/2009; DOI:10.1585/pfr.3.042
[Show abstract][Hide abstract] ABSTRACT: Since the high Tc rare-earth based bulk superconductor is subjected to the tensile load in radial and circumferential direction by the Lorentz force generated in the magnetization process, the evaluation of the strength by the tensile test is indispensable. Ultimate tensile strength of the bulk superconductor depends on the defects in each sample. Many artificial specimens containing voids were generated for numerical stress calculations. The distribution of the voids diameter in each artificial specimen was based on the observations of Dy123 containing 25wt% Dy211 (abbreviated as Dy25). Furthermore, the effect of a center crack superposed to the field of the voids on the strength was analytically evaluated. The strength depends on both the size and the location of the voids. The maximum crack length which has eventually no effect on the strength was evaluated. By the evaluation method proposed in this study, it was found that if there had been no void in the Dy25 bulk sample, the tensile strength could have been estimated to be 63MPa. The voids increase the stress intensity factor at the crack tip. If there is a crack with 0.16mm or more in the Dy25 superconductor bulk with the porosity 10%, the fracture may not be originated around a void but at a crack tip.
Physica C Superconductivity 09/2008; 468(15):1415-1418. DOI:10.1016/j.physc.2008.05.209 · 1.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The strain dependence of critical current Ic of a commercial RABiTS (Rolling-Assisted Biaxially Textured Substrates) YBCO coated conductor tape with Cu stabilizing layers on both surfaces has been studied by two kinds of bending apparatus and a tensile apparatus at 77K and 293K. According to the voltage measurements using multi-taps in the longitudinal direction, local Ic degradation was also observed. In the continuous bending-tension test at 77K, in which YBCO layer was subjected to the tensile strain, Ic began to decrease rapidly beyond the strain of 0.6% after initial moderate degradation. In the continuous bending-compression tests, the Ic began to decrease rapidly beyond the strain of 0.4%. The strain for the beginning of degradation of Ic obtained by tensile tests at 77K was almost the same as that by the bending-tension tests. Bending test at RT using FRP holders showed Ic degradation not so different from that by the continuous bending tests at 77K. In all tests, the Ic initially degraded locally irrespective of the position in the specimens. Furthermore, Ic degradation by repeated bending was also studied.
Physica C Superconductivity 09/2008; 468(15):1697-1701. DOI:10.1016/j.physc.2008.05.244 · 1.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The tests of mechanical properties of the rare earth single grain bulk superconductors have been conducted. In this study, three-point bending high cycle fatigue tests loaded in the a/b-axis direction and the bending fatigue crack propagation tests loaded in the c-axis direction of Dy123 bulk superconductor with 25 vol.% 211 particles have been carried out at room temperature. No specimen has broken up to the number of cycles 106 at the maximum stresses less than 60 MPa, the fatigue limit. This value equals approximately to 88% of the average bending strength of the bulk superconductor obtained by the three point bending tests. All the specimens subjected to cyclic stress above the fatigue limit were broken within the number of cycles of about 2 × 104. The threshold stress intensity factor for the crack propagation of Dy123 bulk was in the range of 0.93–1.33 MPa√m. The crack propagation rate obtained during the propagation test significantly scattered. The optical microscopy showed that the fatigue crack propagated with significant deflection to a/b direction making use of pre-existing a/c or b/c cracks. This is in marked contrast with the monotonic bending test of notched specimen, in which the crack propagates in the c-axis direction with small amount of deflection.
Physica C Superconductivity 09/2008; 468:1424-1427. DOI:10.1016/j.physc.2008.05.245 · 1.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Bending tests of the as-grown and the annealed specimens cut from Dy123 bulk samples were carried out. The Young’s modulus of the annealed specimen was lower than that of the as-grown specimen. On the other hand, the fracture strength of the annealed specimen was slightly higher than that of the as-grown specimen. The fracture toughness of the annealed specimen was also higher. The smaller bulk showed excellent mechanical properties in comparison with the larger bulk. The mechanical properties of these bulks are discussed in terms of the porosity and the mechanical properties of the matrix evaluated by Vickers indentation tests.
Physica C Superconductivity 09/2008; 468(15):1395-1398. DOI:10.1016/j.physc.2008.05.119 · 1.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The tensile strain dependence of the upper critical field B<sub>c2</sub> for Nb<sub>3</sub>Sn wires was measured using a new apparatus for applying tension at low temperature. The critical current I<sub>c</sub> calculated from the strain-dependent B<sub>c2</sub> measurements is the almost consistent with the measured I<sub>c</sub>. In the measurements by a new apparatus, the axial and the lateral applied strains were examined by the strain gauges attached onto the wire. The deviatoric strain epsiv<sub>dev</sub> and the hydrostatic strain epsiv<sub>hyd</sub> were estimated by the axial and the lateral strain. We found the deviatoric strain dependence of B<sub>c2</sub> is different between the tensile and compressive strain states. This means that the hydrostatic strain also should be taken into account in order to understand the strain effects on the superconducting properties of Nb<sub>3</sub>Sn.
[Show abstract][Hide abstract] ABSTRACT: It was reported that Lorentz force caused degradation of critical current in the ITER-TFMC conductor. We have used our novel experimental setup, which utilizes the closed electric circuit concept for critical current and stability measurements of multi-stand superconducting cables. The feature of this setup is mechanical loading applied to the multi-strand cable in the transverse direction. Significant degradation in the critical current of the cable was observed when the average compressive stress was about 20 MPa. This degradation was found irreversible after unloading. We tested the cable with epoxy or ice molds as well. No degradation was observed in the molded cables. We also tested the cable with smaller void fraction. In this case, significant degradation in critical current was observed.
[Show abstract][Hide abstract] ABSTRACT: An international RRT has been carried out in order to establish the test method for mechanical properties of commercial Nb3Sn superconductive wires under the cooperation of eleven worldwide research groups. From the stress-strain curve, the following quantities were evaluated; modulus of elasticity, transition of elastic to plastic deformation, proof strength, tensile strength and elongation to fracture. The scatter of measured values was analyzed to evaluate the COV, which is the standard deviation divided by the average. The results made clear how the experimental conditions influence the determination of physical quantities. The most important point is that large COVs for modulus of elasticity and proof strength from the initial slope are caused by the narrow elastic limit. Methods have been discussed to improve the statistics of experimental results obtained from the international RRT.
[Show abstract][Hide abstract] ABSTRACT: Results of three test methods were compared among different laboratories to determine a standard measurement method of critical current (I-c) as a function of bending strain for Ag alloy-sheathed Bi-2223 superconductor tapes. The die-press, the weight-wind, and the bending-rig methods were used. Specimens were bent at room temperature and the I-c values were measured at 77 K and in the self-field. After I-c measurement of an unbent specimen, bending of the specimen and subsequent IT, measurements were done from 0.2% to 1.0% in 0.2% steps of strain on single specimens. Both I-c and I-c/I-c0 values start to decrease from 0.4%' strain and continue to decrease up to 1.0% strain. Moreover, data scattering starts to increase stepwise at 0.4% strain, being a function of the magnitude of I-c and I-c/I-c0 values with strain for all the test methods. Regarding the statistical analysis on the data, results showed that the data obtained using the die-press method were reasonable and no serious defects were associated with this method. On the other hand, results of the bending-rig method showed a lower I-c/I-c0 degradation rate with bending strain than those of other two methods, which exhibited nearly the same degradation rate. The use of solder for the current contact induced extra thermal strain in the specimens. Consequently, the data for the irreversible strain scatter. Examining the difference in the resistance of the current contact part between the method using solder and the way a specimen is mechanically clamped, the mechanical method is useful and does not affect the test results. Results show that the mechanical method for the current contact increases the test method's effectiveness and reliability.
Journal of the Japan Institute of Metals 04/2008; 72(4):268-277. DOI:10.2320/jinstmet.72.268 · 0.33 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Fracture toughness evaluations of specimens cut from low porosity Dy123 single-grain bulks melt-processed in pure oxygen were carried out at room temperature. While the porosity of a conventional Dy123 single-grain bulk melt-processed in air measured in the previous study is about 20%, those of the Dy123 bulks melt-processed in pure oxygen are below 4%. The minimum of the fracture toughness values of the low porosity bulk is 15% higher than that of the conventional bulk, and the maximum of the former and that of the latter are close to each other. The difference in the fracture toughness among these bulks is discussed in association with the porosity and the mechanical properties of the matrix evaluated by Vickers indentation tests.
Journal of Physics Conference Series 03/2008; 97(1):012138. DOI:10.1088/1742-6596/97/1/012138
[Show abstract][Hide abstract] ABSTRACT: Fracture toughness evaluations on Dy123 single-grain bulks with X wt.% Ag (X=5, 7.5 and 10) melt-processed in air were carried out at room temperature, and the scattering of the fracture toughness values is discussed in terms of the distributions of pore and Ag particle in these bulks. The fracture toughness is improved by the Ag addition; the average values are 1.34, 1.44 and 1.54 MPa m1/2 for X=5, 7.5 and 10, respectively. The porosity near the top surface of these bulks is distinctly low. In X=5, an increase of the fracture toughness value with decrease of the porosity is clearly observed. According to the extrapolation of data, it is deduced that the fracture toughness value at 0 % porosity will be 20 % higher than the average value.
Journal of Physics Conference Series 03/2008; 97(1):012136. DOI:10.1088/1742-6596/97/1/012136
[Show abstract][Hide abstract] ABSTRACT: High-temperature superconducting bulks with highly oriented crystallographic structures are expected to be applied for high field quasi-permanent magnets, current leads and so on. However, the bulks sometimes fracture due to the thermal stress on the cooling process or the electromagnetic force during the magnetization. Thus, it has been recognized that improvement and understanding of the mechanical properties of bulks are indispensable for practical application. In this review, we summarize the present status of evaluation process of the mechanical properties for various bulks. The Young’s modulus, Poisson’s ratio, fracture strength, fracture toughness and hardness are evaluated by tensile, bending, compression and hardness tests. The mechanical properties are anisotropic, mainly due to the crystallographic structure and pre-existing micro-cracks associated with it. Data obtained are summarized and the influential parameters associated with the microstructure and testing condition for the mechanical properties are explained.
[Show abstract][Hide abstract] ABSTRACT: Abstract— The effects of bluing, associated with drawing strain, on the fatigue strength of eutectoid steel wires have been investigated. The fatigue limit increases by bluing and the increase is more significant with higher drawing strain. The peak in the fatigue limit with regard to the drawing strain in the wires, at a strain of 2.5, disappears after bluing. On the other hand, in the ferritic steel wires investigated for comparison, the fatigue limit gradually increases with the drawing strain up to 7.7. Furthermore, no appreciable change in the fatigue limit due to bluing is found. Based on the results of hardness tests on fatigue specimens with- and without-bluing, it is deduced that the decrease of the fatigue limit beyond the peak drawing strain in the eutectoid steel wire can partly be attributed to insufficient locking of the high-density dislocations by solute atoms. The effect of relaxation of residual stress during bluing is also briefly discussed.
[Show abstract][Hide abstract] ABSTRACT: To evaluate the fracture toughness of Dy123 with X mol% Dy211 (X=15, 20 and 25) bulks, bending tests on V-notched specimens were carried out at room temperature. The average value of the fracture toughness for X=25, 1.47MPam1/2, was higher than those for X=15 and 20, 1.24 and 1.17MPam1/2, respectively. The porosity of specimens cut from the top region for X=25 was extraordinarily low and the fracture toughness was high. According to the relation between the fracture toughness and the porosity, the fracture toughness of the material of no pore was estimated to be 1.75MPam1/2.
Physica C Superconductivity 10/2007; 463:357-361. DOI:10.1016/j.physc.2007.03.437 · 1.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Three-point bending tests on specimens cut from Dy123 bulks with Xwt.% Ag (X=5, 7.5 and 10) were carried out at room temperature. Bending test specimens with 2.8×2.1×24mm3 were cut from the bulk samples and then annealed in O2 atmosphere at 723K for 100h. The Young’s modulus decreased with increase of the Ag content, 136, 128 and 124GPa for X=5, 7.5 and 10, respectively. On the other hand, the fracture strength increased, 86, 89 and 91MPa. The Ag content dependences of the Young’s modulus and the fracture strength are analyzed through the area fraction of the pores and Ag particles in each bending test specimen.
Physica C Superconductivity 10/2007; 463:362-366. DOI:10.1016/j.physc.2007.03.438 · 1.11 Impact Factor