Acoustic emission behavior under bending deformation of YBCO bulk superconductor

Abstract

Bending tests were conducted on U-notched specimens cut from a YBCO bulk superconductor. Acoustic emission (AE) signals obtained under loading parallel or perpendicular to the c-axis were analyzed to investigate the correlation between crack growth behavior and the AE signals. As a result of analyzing log–log plots of strength ( σ B) versus total AE energy ( ΣE AE), a linear relationship was found between ΣE AE and σ B n . Cracks could be broadly divided into two types based on the value of n as an index of crack growth behavior. One type consisted of microcracks originating from cleavage planes and gas holes; these crack propagated parallel to the c-axis and had an n index value of approximately 0.7. The other type was a main crack that originated from the U-notch and had an n index value of approximately 6.5. A sample (A) loaded parallel to the c-axis showed mean bending strength of 74.8 MPa. Cracks displaying two different growth patterns of n = 0.7 and 6.5 were presented in this sample. Microcracks parallel to the c-axis occurred in the vicinity of 5–10 MPa. This sample was characterized by mixed crack growth of a main crack and microcracks. A sample (B) loaded perpendicular to the c-axis displayed mean bending strength of 43 MPa. A main crack occurred in the vicinity of 20 MPa and displayed a single growth pattern of n = 6.5. By analyzing AE signals in this way in the process of conducting a strength evaluation, it was possible to evaluate the failure process of the bulk superconductor in relation to the strength level induced by the applied load.