Volume 121, Issue 1, January 2007
Index of content:
- GENERAL LINEAR ACOUSTICS 
121(2007); http://dx.doi.org/10.1121/1.2382749View Description Hide Description
Many cold-working processes for polycrystalline metals cause alignment of the grains with a single symmetry axis. This type of microstructure is called fiber texture. The existence of a preferred orientation of the grains has a significant influence on the propagation and scattering of ultrasonic waves, which are often used for material inspection. Knowledge of the wave attenuation of such textured materials is of both theoretical and practical interest to nondestructive testing and materials characterization. In this article, the quantitative relations between fiber texture and wave attenuations of hexagonal crystals are presented. The texture is characterized by a Gaussian distribution function that contains a single parameter that governs the transition of the texture from perfectly aligned crystals to statistically isotropic. Under this assumption, the materials of interest have a varying degree of transverse isotropy representative of processing conditions. Simple expressions for the attenuations of the three modes of waves are given in a concise, generalized representation. Finally, numerical results are presented and discussed in terms of the directional, frequency, and texture dependence. The results presented are expected to improve the understanding of the microstructure evolution during thermomechanical processing.
121(2007); http://dx.doi.org/10.1121/1.2390674View Description Hide Description
The design of transducers to excite and detect guided waves is a fundamental part of a nondestructive evaluation or structural health monitoring system and requires the ability to predict the radiated guided wave field of a transmitting transducer. For most transducers, this can be performed by making the assumption that the transducer is weakly coupled and then integrating the Green’s function of the structure over the area of the transducer. The majority of guided wavemodeling is based on two-dimensional (2D) formulations where plane, straight-crested waves are modeled. Several techniques can be readily applied to obtain the solution to the forced 2D problem in terms of modal amplitudes. However, for transducermodeling it is desirable to obtain the complete three-dimensional (3D) field, which is particularly challenging in anisotropic materials. In this paper, a technique for obtaining a far-field asymptotic solution to the 3D Green’s function in terms of the modal solutions to the forced 2D problem is presented. Results are shown that illustrate the application of the technique to isotropic (aluminium) and anisotropic (cross-ply and unidirectional composite) plates. Where possible, results from the asymptotic model are compared to those from 3D time-marching finite element simulations and good agreement is demonstrated.
121(2007); http://dx.doi.org/10.1121/1.2400662View Description Hide Description
Coherent backscattering of waves by a random medium is spectacular evidence of interference effects despite disorder and multiple scattering. It manifests itself as a doubling of the wave intensity reflected exactly in the backward direction. This phenomenon has been observed experimentally in optics, acoustics, or seismology. While optical measurements are realized in far-field conditions with a plane wave illumination and a beamwidth much larger than the wavelength, ultrasonic experiments are carried out with wideband controllable arrays of (nearly) pointlike transducers that directly record the wave field, in amplitude and phase. Therefore it is possible to perform beamforming of the incoming and outgoing wave fields before computing the average backscattered intensity. In this paper, the advantages of plane wavebeamforming applied to the study of the coherent backscatteringeffect are shown. Particularly, the angular resolution, the signal-to-noise ratio, as well as the estimation of the enhancement factor can be improved by beamforming. Experimental results are presented with ultrasonic pulses, in the range, propagating in random collections of scatterers. Since the coherent backscatteringeffect can be taken advantage of to measure diffusive parameters (transport mean free path, diffusion constant), plane-wave beamforming can be applied to the characterization of highly scattering media.
121(2007); http://dx.doi.org/10.1121/1.2387127View Description Hide Description
Recent works have been performed concerning the guided elastic waves in soft porous elastic frames and the related normal displacement of the surface generated by a mechanical excitation. In this work, the mechanical excitation is replaced by a monopole acoustic field in air above the porous structure. Two cases are considered: a frame bonded on a rigid impervious layer and a free frame. The normal and the radial velocity induced by the monopole field at the surface of the frame is predicted with the Biot theory and the Sommerfeld representation of the monopole field. Measurements are performed with a laser vibrometer on a urethane foam. It is shown that the characterization of the excited modes can lead to an evaluation of the rigidity coefficients of the frames at acoustical frequencies.