Bending fatigue characteristics of DyBaCuO bulk superconductor

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.