Index of content:
Volume 120, Issue 2, August 2006
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
120(2006); http://dx.doi.org/10.1121/1.2206512View Description Hide Description
Ultrasound spectroscopy has many applications in characterizing dispersions, emulsions, gels, and biomolecules. Interpreting measurements of sound speed and attenuation relies on a theoretical understanding of the relationship between system properties and their effect on sound waves. At its basis is the scattering of a sound wave by a single particle in a suspending medium. The problem has a well-established solution derived by expressing incident and scattered fields in terms of Rayleigh expansions. However, the solution is badly conditioned numerically. By definition, in the long-wavelength limit, the wavelength is much larger than the particle radius, and the scattered fields can then be expressed as perturbation series in the parameter (wave number multiplied by particle radius), which is small in this limit. In addition, spherical Bessel and Hankel functions are avoided by using alternative series expansions. In a previous development of this perturbation method, thermal effects had been considered but viscous effects were excluded for simplicity. Here, viscous effects, giving rise to scattered shear waves, are included in the formulation. Accurate numerical correspondence is demonstrated with the established Rayleigh series method for an emulsion. This solution offers a practical computational approach to scattering which can be embodied in acoustic instrumentation.
120(2006); http://dx.doi.org/10.1121/1.2211587View Description Hide Description
Rail breaks caused by rolling contact fatigue defects are of growing concern to the railway industry. Very often critical defects cannot be detected reliably by conventional inspection methods. Low-frequency surface waves with a high penetration depth have the potential to overcome such difficulties. In this paper, the properties of surface wave modes in both new and worn rails are investigated and the implications for rail inspection are discussed. The dispersion curves of the dominant surface wave modes were determined up to a frequency of using a finite element model and show excellent agreement with experimental data. One surface wave mode was identified to be nondispersive and not significantly affected by cross-section changes due to wear at frequencies above . It exhibits a relatively homogeneous energy distribution in the upper half of the rail head and is therefore suitable for inspection purposes. The problem is that there exist several other surface modes with very similar propagation properties. The interference of these multiple modes, which may be generated either directly by the exciter or by mode conversion at defects, means that the received amplitude is position dependent. For this reason, accurate defect sizing will be difficult.