Acoustic emission behavior under bending deformation of YBCO bulk superconductor

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 (sigma(B)) versus total AE energy (Sigma E-AE), a linear relationship was found between Sigma E-AE and sigma(n)(B). 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. (c) 2005 Elsevier B.V. All rights reserved.