The propagation of elastic waves through heterogeneous anisotropic media is considered. Appropriate ensemble averaging of the elastic wave equation leads to the Dyson equation which governs the mean response of the field. The Dyson equation is given here in terms of anisotropic elastic Green’s dyads for the medium with and without heterogeneities. The solution of the Dyson equation for the mean response is given for heterogeneities that are weak. The formalism is further specified for the case of equiaxed cubic polycrystalline metals with a single aligned axis. The Green’s dyads in this case are those for a transversely isotropic medium. Simple expressions for the attenuations of the shear horizontal, quasicompressional, and quasishear waves are given in terms of integrations on the unit circle. The derived expressions are limited to frequencies below the geometric optics limit, but give the attenuations in a direct manner. Comparisons with previous results are also discussed. It is anticipated that a similar approach is necessary for the study of wave propagation in complex anisotropic materials such as fiber‐reinforced composites. In addition, the results are applicable to diffuse ultrasonic inspection of textured polycrystalline media.

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Elastic wave propagation and scattering in heterogeneous, anisotropic media: Textured polycrystalline materials

As a solid-state welding process, Friction Stir Welding (FSW) offers a variety of advantages over traditional welding processes. Problems that are typically occurring due to the cooling of the liquid phase, such as solidification cracking and formation of porosity, generally do not occur in FSW. Nevertheless, as a result of suboptimal settings of the welding process parameters and certain uncontrollable conditions, FSWs are still associated with a number of specific flaws, e.g. root flaws and wormholes. Ultrasonic non-destructive testing and evaluation techniques (NDT&E) can be used for quality assessment of friction stir welded joints. In this paper, a novel approach for the detection of root flaws is proposed using an immersion ultrasonic testing method in oblique incidence and backscatter mode. The backscattered energy C-scan images obtained after an empirical positioning and proper time gating can be straightforwardly interpreted by direct comparison with typical ‘flaw’ patterns, allowing for identification and localization of the root flaws in the weld. The method is illustrated for FSW butt joints of the AlZnMgCu (7XXX series) alloy.

Non-destructive ultrasonic examination of root defects in friction stir welded butt-joints

Effect of texture and grain shape on ultrasonic backscattering in polycrystals.

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

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

Elastic wave scattering in random anisotropic solids

Most structural materials are polycrystalline, that is, they are composed of numerous discrete grains, each having a regular, crystalline atomic structure The elastic properties of the grains are anisotropic and their crystallographic axes are differently oriented When an ultrasonic wave propagates through such a polycrystalline aggregate, it is scattered at the grain boundaries The fraction of sound energy thus removed from the main beam is responsible for important phenomenons like attenuation and dispersion of the main beam, and background “noise” associated with a given ultrasonic inspection system The amount of sound energy removed from the main beam depends on the size, shape, and orientation distributions of the grains If the grains are equiaxed and randomly oriented, propagation direction of the ultrasonic wave has no effect upon the magnitude of the scattered energy Such is not the case when an acoustic wave travels through materials like centrifugally cast stainless steel and austenitic stainless steel welds, which are used extensively in nuclear power plants The microstructures of these stainless steels vary from randomly oriented, equiaxed grains to highly oriented, columnar grains1,2 Since the backscattered signals tend to mask the signals from small and subtle defects, the estimation of probability of detection of such defects requires quantitative description of these signals Consequently, an effort has been undertaken in this research to quantify the backscattered signals from microstructures with favored grain orientation and grain elongation

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=2568&context=qnde

S. Ahmed

Papers

Influence of Columnar Microstructure on Ultrasonic Backscattering