Theory of Acoustic Emission

ABSTRACT A theory of acoustic emission is presented based on a Green's function type of formalism, rather than on the conventional count rate concept. Sources are represented by stress drop rate tensors and conditions are derived from which the source can be considered small in terms of wavelength and distance to the transducer. These “pseudopoint” sources are examined over a restricted frequency bandwidth, called the “informative bandwidth.” Such a bandlimited system may be described by a tensor transfer function type of formalism, facilitating the analysis and reducing the inverse problem—where the source is not known a priori –to a deconvolution operation. Due to the tensor nature of the source, multiple transducer measurements are necessary to reconstruct the source stress drop. The difficulty of using spectral techniques for data analysis in the presence of multiple sources is discussed. The forward problem for acoustic emission to be expected from plasticity, phase transformations, and cracks is solved within the pseudopoint approximation and the concept of retarded dipole density is introduced to deal with the problem of time delays from propagating inhomogeneity sources. In addition, the strong directionality of the signal with respect to source type and orientation is illustrated by calculating the acoustic emission signals generated by loop expansion of slip and climb (prismatic collapse) type dislocations. This directionality is shown to be the same for isotropic materials as that associated with Mode II/III or Mode I cracks and a comparison is provided between emission from expanding penny-shaped cracks and that from dislocation loops. Finally, the concepts of stress-controlled and materials-controlled acoustic emission are introduced and their significance is discussed in terms of relating AE source descriptions and material mechanisms.

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