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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 60MPa, 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 0.93–1.33MPa√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. [Copyright &y& Elsevier]

Abstract: In order to evaluate the mechanical properties, such as Young’s modulus and strength, of Dy123 bulk superconductors and those with 10wt.% Ag2O, we performed 3-point bending tests at room (RT) and liquid nitrogen temperatures (LNT) using specimens cut from the bulks. The Young’s modulus and the bending strength increased with decrease in temperature. In the tests loading in the direction of c-axis and ones perpendicular to it, Young’s moduli were almost comparable at both RT and LNT. Although the strengths for both orientations were also comparable at LNT, those at RT were different. Young’s moduli loaded in the direction of c-axis for Ag2O added bulk specimens, 127GPa in average at RT, were almost comparable to those without Ag2O, and 134GPa at LNT, were slightly lower than those without Ag2O. On the other hand, the strengths at both RT and LNT were enhanced by 20% by the Ag addition. The mechanical properties of Dy123 bulks without Ag2O were compared with those of Y123 bulks obtained previously. The Young’s modulus for loading in the direction of c-axis was slightly lower, and the strength was comparable to those in Y123 bulks, respectively. [Copyright &y& Elsevier]

Abstract: In order to investigate the evaluation method of the Young’s modulus of RE123 single-grain bulk superconductors, 3-point bending tests have been carried out at room and liquid nitrogen temperatures. An attempt to evaluate it from the measurements of deflection of the specimen with 3mm thickness was made. The consistent stiffness of the test machine including the jig was obtained by using a dummy specimen. For the test of the bulk, increasing repeated load within a level less than the fracture load was applied to stabilize the loading condition. The Young’s modulus calculated by using the deflection of the bulk specimen obtained subtracting the machine stiffness coincided with that obtained by the conventional method, where the strain gage was adhered to the specimen. The effects of specimen thickness and the oxygen annealing sequence with respect to specimen cutting were also investigated. [Copyright &y& Elsevier]

Abstract: In order to investigate the mechanical properties of a single-grain Y123 bulk with 15wt.% Ag at liquid nitrogen temperature (LNT), three-point bending tests of specimens cut from the bulk were carried out. The dimensions of the bending test specimens were 4×3×36mm3. The loading was carried out in the c-axis or perpendicular to it such that the normal stress induced by loading was perpendicular to the c-axis. Young’s modulus at LNT by loading in the c-axis and perpendicular to it were 143 and 134GPa and the bending strengths were 115 and 128MPa, respectively. These bending strength values were higher than those at room temperature (RT) for this bulk. From the comparison with the other rare earth element based bulks, the smaller the ionic radius of Y or rare earth elements, the higher Young’s modulus and the bending strength of the bulks. This implies that the strength is basically controlled by the ideal crystal strength of bulks, irrespective of the crystal orientation and, further, defect induced during fabrication process, so long as the defect size is comparable. The step-wise undulation in the crack propagation direction on the fracture surfaces by loading in the c-axis was significant as compared with that by loading in perpendicular to it. [Copyright &y& Elsevier]

Abstract: The fracture toughness of Y123 and Gd123(Ag) single-grain bulk samples with 25 mol% RE 211 phase (and 10 wt.% Ag for the latter) prepared by the modified quench and melt-growth technique, was evaluated by the 3-point bending test at liquid nitrogen temperature (LNT) and room temperature (RT). The sharp V-notch was introduced into the specimens such that both of the loading and the crack extension directions are in the c-axis. The data for Y123 scattered from 1.2 to 2.0 MPam1/2 at LNT and 1.4 to 1.7 MPam1/2 at RT. The average value at LNT, 1.6 MPam1/2, was slightly higher than that for RT, 1.5 MPam1/2. For Gd123(Ag) the average fracture toughness at LNT, 1.8 MPam1/2 was higher than that at RT. The temperature dependence of fracture toughness for two kinds of bulks was consistent with that of the bending strength. Further, the order of the magnitude of the fracture toughness almost corresponded with that of bending strength for both bulks. [Copyright &y& Elsevier]

Abstract: In order to investigate the compressive mechanical properties of a single-grain Sm123 bulk superconductor at cryogenic temperature, compressive tests at liquid nitrogen temperature on small specimens with the dimensions of 3×3×8mm3 cut from the bulk with 45mm in diameter and 15mm in thickness were carried out. Due to the closure of pre-existing micro-cracks perpendicular to the c-axis, the average Young’s modulus in the c-axis, 93GPa, was lower than that in the direction perpendicular to the c-axis, 150GPa. While the former was higher than the value at room temperature (RT), 75GPa, the latter was slightly lower than that, 165GPa. The compressive strength in the c-axis, 466MPa, was higher than that in the direction perpendicular to the c-axis, 368MPa. This anisotropy is also ascribed to the pre-existing micro-cracks associated with the fracture mode. Both the former and the latter were higher than the values at RT, 350 and 335MPa, respectively. Most of the specimens loaded in perpendicular to the c-axis fractured along the plane perpendicular to the c-axis into a few pieces. The specimen loaded in the c-axis fractured into many small fragments, although the specimen with extraordinarily low strength fractured in a similar mode as mentioned above. [Copyright &y& Elsevier]

Abstract: Bending tests have been commonly carried out to investigate the mechanical properties of melt-processed rare earth based bulk superconductors. Tensile tests by using small specimen, however, are preferable to evaluate the detailed distribution of the mechanical properties and the intrinsic elastic modulus because no stress distributions exist in the cross-section. In this study, the tensile test method at low temperature by using specimens with the dimensions of 3×3×4mm from Y123 and Gd123 bulks was examined. They were glued to Al alloy rods at 400K by using epoxy resin. Tests were carried out at liquid nitrogen temperature (LNT) by using the immersion type jig. Although the bending strength in the direction perpendicular to the c-axis of the bulks at LNT is higher than that at room temperature (RT), the tensile strength at LNT was lower than that at RT. Many of specimens fractured near the interface between the specimen and the Al alloy rod at LNT. According to the finite element method analysis, it was shown that there was a peak thermal stress in the loading direction near the interface and it was significantly higher at LNT than that at RT. It was also shown that the replacement of the Al alloy rod to Ti rod of which the coefficient of thermal expansion is close to that of bulks significantly increased the tensile strength. [Copyright &y& Elsevier]

The monotonic and the repeated bending tests on the Y123 single grain bulk superconductor sample were carried out. The average Young''s modulus at liquid nitrogen temperature (LNT), 148 GPa, was higher than that at room temperature (RT), 132 GPa. The average bending strength at LNT, 90 MPa, was also higher than that at RT, 74 MPa. The Weibull coefficient for the bending strength at RT, 17.6, was significantly larger than that at LNT, 6.1. The specimen cut from the bottom region of the bulk superconductor showed slightly higher value of the Young''s modulus and lower value of the bending strength. The fatigue endurance limit for the repeated pulsating loading up to 5 × 104 cycles at LNT, 70 MPa, was about 80% of the bending strength. [Copyright &y& Elsevier]

Tensile and 3-point bending tests of specimens cut from an Y123 and a Gd123 bulk superconductors fabricated by using the modified QMG (Quench and Melt Growth) process were carried out at room temperature. The mechanical properties obtained by two testing method were compared. The average values of the Young''s modulus obtained by tensile tests, 122 GPa for Y123 and 109 GPa for Gd123, were lower than those obtained by bending tests, 132 and 117 GPa, respectively. The average values of the tensile strength, 39 MPa for Y123 and 42 MPa for Gd123, were also lower than those of the bending strength, 74 MPa and 71 MPa, respectively. There was no appreciable difference between the Weibull coefficient for the tensile strength and that for the bending strength, 6.9, in Gd123. On the other hand, the coefficient for the bending strength, 17.6, was significantly higher than that for the tensile strength, 6.8, in Y123. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

In order to investigate the effects of Ag content on the mechanical properties of a high performance melt-processed (Nd,Eu,Gd)Ba2Cu3Oy single grain bulk superconductors with 30 mol% Gd2BaCuO5, tensile tests of specimens cut from the bulks have been carried out at room temperature. The tensile strength in the direction perpendicular to the c-axis of the sample with 20 wt.% Ag was higher than that of the sample with 10 wt.% Ag. This is ascribed to a decrease in the void fraction and suppression of cracks by an increased Ag particles revealed by the microstructure image analysis. The almost identical value of Young’s modulus obtained for both of the samples are presumably due to the balance of the effects of the voids and soft Ag particles mentioned above. Comparing these results with that in the c-axis direction and that obtained by the compressive test, the anisotropy of the elastic behavior associated with the effect of pre-existing micro-cracks perpendicular to the c-axis was discussed. [Copyright &y& Elsevier]

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 MPa m1/2, was less as compared with that by the tensile tests, 1.1–2.5 MPa m1/2. However, the average value in tensile tests 1.9 MPa m1/2 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. [Copyright &y& Elsevier]

Abstract: Mechanical properties of low porosity Dy123 (DyBa2Cu3O x ) 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. [Copyright &y& Elsevier]

Abstract: Since the high T c 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. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

Abstract: Three-point bending tests on specimens cut from Dy123 bulks with X wt.% 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. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

Abstract: Tensile tests of bulk high T c superconductors at room temperature have been generally performed by gluing the bulk specimens to Al-alloy rods. Because of the difference in the coefficient of thermal expansion, thermal stresses were induced at cryogenic temperatures especially near the interface between the specimen and the rods. In this study, tensile testing methods with minimized effect of the thermal stress were tried by using specimens cut from Dy–Ba–Cu–O superconductors. These were: (1) The rod material of Al-alloy was replaced with Ti-alloy, which has the coefficient close to the bulk. (2) The interlayer made of the identical bulk superconductor was inserted between the specimen and the Ti-alloy rod. The nominal tensile strength at the liquid nitrogen temperature (LNT) of the specimen glued to the Ti-alloy rods was significantly higher than that glued to the Al-alloy rods. The application of the interlayers increased the strength significantly. The FEM analysis showed that the thermal tensile stress component in the direction of loading axis within the specimen at LNT is markedly reduced by the method (1) and substantially eliminated in the method (2). [Copyright &y& Elsevier]

Abstract: In order to investigate the mechanical properties of RE–Ba–Cu–O bulk superconductors, Indentation Fracture (IF) method using Vickers hardness number has been used to evaluate the fracture toughness at 40–293K. However, there are some differences between IF values and fracture toughness obtained from other mechanical test. This paper considers the influence of the elastic properties of Y–Ba–Cu–O, Gd–Ba–Cu–O and two types of Sm–Ba–Cu–O on IF values by measuring the indentation depth. As a result, the elastic recovery rate R of Y–Ba–Cu–O increased from 27% to 68% with decreasing temperature. R of Gd–Ba–Cu–O and Sm–Ba–Cu–O also increased from 10–20% to 40–50% with decreasing temperature. In this study, we calculated Vickers hardness numbers using the indentation depths at 40–293K, and evaluated IF values of bulk superconductors at 40–293K. [Copyright &y& Elsevier]