Evaluation of Mechanical Properties and Fracture Mechanism of Single Crystal Bulk High Tc Superconductors

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By using small specimens of bulk high T_c oxide superconductor, the mechanical properties were evaluated and the mechanisms of fracture were clarified.1.The fracture toughness of Sm based bulk evaluated by 3-point bending and tensile tests were 1.0-1.3 MPam^<1/2>. Due to the pre-existing micro-cracks in a-b plane, values for specimens with notch in c-axis, were higher than those perpendicular to it. They increased as the temperature decrease from RT to 77K.2.The 3-point bending strength and Young's modulus of Y based bulk at 77K was higher than those at RT. These were slightly higher in the top region of bulk. The bending fatigue endurance limit for 5x10^4 cycles at 77K was 70 MPa, which is 80% of bending strength.3.Compressive stress-strain curves are linear for loading perpendicular to c-axis and non-linear for c-axis loading. The compressive elastic constants and strength showed anisotropy, due to the opening and closing behavior of the micro-cracks. They were higher at 77K and also higher than those in the tensile tests. Although the strength increased with increase in the length of specimen, it decreased when the length became 4 times of the thickness due to delamination along cleavage plane and buckling.4.The tensile strength of bulk joints, where the Y bulks were joined in {110} planes with Er or Ag added Y123, were 14 MPa or 27 MPa. They were lower than 46 MPa for Y bulk. Some solidified solution areas were observed in the middle of joint region in the former and within the Y bulk in the latter were found to be the origin of fracture. The exclusion of these regions which correspond to the degraded magnetic flux trapping areas should leads to the improvement of strength of joints.