Elastic-Wave Propagation in Random Polycrystals: Fundamentals and Application to Nondestructive Evaluation

Abstract

The fundamental principles that govern the propagation of elastic waves in metal polycrystals are discussed in the context of their influence on nondestructive evaluation. The major influence of the polycrystalline microstructure is to determine the velocity, attenuation and backscattering of the elastic waves. For randomly oriented, equi-axed polycrystals, these effects are reasonably well understood. Waves travel at the same velocity in all directions and are exponentially attenuated at a rate controlled by the frequency and grain size. Signals backscattered from the grains, also controlled by the wavelength and grain size, produce a background noise that competes with flaw signals. The same basic phenomena exist in more complex materials. However, the understanding of these phenomena is not as well understood. Recent progress towards the development of such an understanding is discussed within this chapter. Examples include cases in which the grains have preferred crystallographic orientation, elongation in one or more dimension, or correlations in orientation from crystallite to crystallite. The latter case is particularly rich, in that the two dimensions scales of the media, associated with the grain size and the correlation length, can lead to a number of unusual phenomena such as highly anisotropic backscattering and phase modulations of an elastic beam. These modulations make the measurement, and even definition, of attenuation problematic. The current status of experimental observation and theoretical description of these phenomena is discussed. The chapter concludes with a discussion of the implications of these effects on the imaging of flaws in complex media.