Asymptotic descriptions of the acoustic signatures of surface-breaking cracks.

Abstract

This describes work on modeling asymptotically the acoustic signature, measured with a scanning acoustic microscope having a line-focus or point-focus lens, of a surface-breaking crack. This is a continuation of the work presented at the Spring 1991 ASA Meeting [J. Acoust. Soc. Am. 89, 1923 (A) (1991)]. The crack is modeled as a vertical discontinuity that can be characterized by reflection and transmission coefficients. Asymptotic expressions describing the incident and scattered wave fields, including the leaky Rayleigh waves scattered from the discontinuity, are given. The acoustic signatures are calculated by using an electromechanical reciprocity identity to relate the wave fields at the specimen’s surface to the voltage at the terminals of the microscope’s transducer. For the line-focus microscope, explicit expressions for the reflection coefficient of a normally incident Rayleigh wave reflecting from a surface-breaking crack are given in terms of the acoustic signatures for a perfect and cracked surface. For the point-focus microscope, explicit expressions for the same reflection coefficient, even though the Rayleigh wave now propagates in two dimensions rather than one, are also given in terms of the signatures for a perfect and a cracked surface. Initial work to assess the influence of the condition of the specimen’s surface upon the acoustic signature will be reported. [Work partially supported by MRC at UI.]