Influence of Columnar Microstructure on Ultrasonic Backscattering
01 Jan 1995-pp 1617-1624
TL;DR: In this paper, the authors quantify the backscattered signals from microstructures with favored grain orientation and grain elongation and show that the amount of sound energy removed from the main beam depends on the size, shape, and orientation distributions of the grains.
Abstract: 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
Citations
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TL;DR: In this article, the authors considered the propagation of elastic waves through heterogeneous anisotropic media and derived simple expressions for the attenuations of the shear horizontal, quasicompressional, and quasishear waves in terms of integrations on the unit circle.
Abstract: 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.
95 citations
TL;DR: In this article, a novel approach for the detection of root flaws is proposed using an immersion ultrasonic testing method in oblique incidence and backscatter mode, where 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.
Abstract: 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.
51 citations
TL;DR: An ultrasonic backscattering model is developed for textured polycrystalline materials with orthotropic or trigonal grains of ellipsoidal shape to simulate realistic microstructures and orthotropic macroscopic material textures resulting from thermomechanical processing for a broad variety of material symmetries.
47 citations
Cites background from "Influence of Columnar Microstructur..."
...The Rose model was interpreted by Han and Thompson [21] and applied to polycrystals with elongated grains of hexagonal symmetry....
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...Ultrasonic backscattering in materials with highly oriented, columnar cubic grains was discussed by Ahmed and Thompson [15]....
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...[23] J.H. Rose, Theory of ultrasonic backscatter from multiphase polycrystalline solids, in: D.O. Thompson, D.E. Chimenti (Eds.)...
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...[5] R.B. Thompson, F.J. Margetan, P. Haldipur, L. Yu, A. Li, P. Panetta, H. Wasan, Scattering of elastic waves in simple and complex polycrystals, Wave Motion 45 (2008) 655–674....
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...1098 Please cite this article in press as: J. Li et al., Effect of texture and grain s http://dx.doi.org/10.1016/j.ultras.2014.02.020 [12] S. Ahmed, R.B. Thompson, Propagation of elastic waves in equiaxed stainless steel polycrystals with aligned [001] axes, J. Acoust....
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01 Jan 2002
TL;DR: In this paper, the fundamental principles that govern the propagation of elastic waves in metal polycrystals are discussed in the context of their influence on nondestructive evaluation, and the implications of these effects on the imaging of flaws in complex media are discussed.
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
44 citations
TL;DR: In this article, a general ultrasonic scattering model for a polycrystal with arbitrary macroscopic texture and triclinic ellipsoidal grains is developed allowing the texture and grain orientation frames to be independent and the wave propagation direction arbitrary in those frames.
26 citations
References
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TL;DR: In this article, general electromechanical reciprocity relations are applied to the calculation of elastic wave scattering coefficients observed at the electrical terminals of the transducer used in performing an experiment.
369 citations
TL;DR: In this paper, a unified approach to solve for the attenuation and phase velocity variations of elastic waves in single phase, polycrystalline media due to scattering is presented. But the approach is not applicable for any material whose singlecrystal anisotropy is not large, regardless of texture, grain elongation, or multiple scattering.
Abstract: We have developed a unified approach to solve for the attenuation and phase velocity variations of elastic waves in single‐phase, polycrystalline media due to scattering. Our approach is a perturbation method applicable for any material whose single‐crystal anisotropy is not large, regardless of texture, grain elongation, or multiple scattering. It accurately accounts for the anisotropy of the individual grains. It is valid for time‐harmonic waves with all ratios of grain size to wavelength. It uses an autocorrelation function to characterize the geometry of the grains, and thereby avoids coherent artifacts that occur if the grains are assumed to have symmetrical shapes and suggests new methods for characterizing distributions of grains that are irregularly shaped. We have carried out numerical calculations for materials that are untextured and equiaxed, and have cubic‐symmetry grains and an inverse exponential spatial autocorrelation function. These calculations agree with the previous calculations which are valid in the Rayleigh, stochastic, and geometric regions, and show the transitions between these regions. The complex transition between the Rayleigh and stochastic regions for longitudinal waves, and the severe limitations of the stochastic region for grains with fairly large anisotropy are of particular interest.
354 citations
TL;DR: In this paper, an ultrasonic scattering theory is presented which allows one to calculate the scattering coefficients and velocities of plane longitudinal and transverse waves in polycrystals as a function of the wavenumber k times grain radius a without limitation to the Rayleigh region.
Abstract: An ultrasonic scattering theory is presented which allows one to calculate the scattering coefficients and velocities of plane longitudinal and transverse waves in polycrystals as a function of the wavenumber k times grain radius a without limitation to the Rayleigh region. The theory includes mode conversion and multiple scattering and can be used to describe ultrasonic propagation in polycrystals with randomly orientated grains as well as in those with preferred grain orientation. The calculation was done for compressional waves in polycrystals of cubic symmetry with randomly orientated grains in second‐order perturbation theory using the assumption that the changes in the elastic constants and in the density of the materials from grain to grain are small. The asymptotic values at low ka (Rayleigh scattering) are exactly the same as the well‐known results from Bhatia and Moore. Numerical calculations are carried out for some examples.
149 citations
TL;DR: Hirsekorn et al. as discussed by the authors generalized the theory of ultrasonic wave propagation in textured polycrystals to calculate the directional-dependent scattering coefficients and phase and group velocities of plane waves with arbitrary propagation and polarization direction as a function of frequency.
Abstract: The theory of ultrasonic wave propagation in textured polycrystals presented previously [S. Hirsekorn, J. Acoust. Soc. Am. 77, 832–843 (1985)] for propagation and polarization parallel to symmetry axes of texture is generalized to calculate the directional‐dependent scattering coefficients and phase and group velocities of plane waves with arbitrary propagation and polarization direction as a function of frequency. The analytical calculations are carried out for cubic polycrystals with orthorhombic texture symmetry. Numerical evaluation is done for an austenitic weld metal. The results are compared to those of the frequency‐independent description of ultrasonic propagation.
68 citations
TL;DR: In this paper, the scattering coefficients and the phase and group velocities of plane compressional and shear waves in textured polycrystals were derived for cubic symmetry with rolling texture in second-order perturbation theory.
Abstract: The theory of ultrasonic propagation in polycrystals with independent and uniformly distributed orientations of the grains presented in previous papers [J. Acoust. Soc. Am. 72, 1021–1031 (1982); 73, 1160–1163 (1983)] is generalized to calculate the scattering coefficients and the phase and group velocities of plane compressional and shear waves in textured polycrystals. The calculation was done for plane waves in polycrystals of cubic symmetry with rolling texture in second‐order perturbation theory using the assumption that the changes in the material constants from grain to grain are small. In the limit texture equal to zero the analytical results are exactly the same as those for untextured polycrystals previously presented. Numerical calculations are carried out for some examples.
61 citations