Volume 119, Issue 6, June 2006
Index of content:
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
119(2006); http://dx.doi.org/10.1121/1.2197807View Description Hide Description
This paper describes the implementation of equations of dynamic equilibrium in a finite element (FE) code for modeling, in axisymmetry, the propagation of torsional wave modes along metallic pipes coupled to solid elements. Materials constituting either pipes and/or surrounding elements can be absorbing media, the absorption being caused by either their viscoelasticity or scattering on their internal structure (or both). Complex moduli are used as input data to equations of dynamic equilibrium, which are solved in the frequency domain rather than in the temporal domain. Their real and imaginary parts represent material elasticity and damping, respectively. A new definition of efficient and easy-to-implement absorbing regions that suppress undesired reflections from boundaries is proposed. The resolution of equations in the frequency domain, together with the use of absorbing regions, lead to significant reductions in the number of mesh elements and also in the number of iterations required for describing problems of propagation and scattering. Through two examples, the model is validated by successful comparisons of numerical predictions with experimental data. Then, a third example is presented to illustrate the importance of properly modeling waves damping when using FE models for setting-up or optimizing NDT techniques.
Bulk conical and surface helical acoustic waves in transversely isotropic cylinders; application to the stiffness tensor measurement119(2006); http://dx.doi.org/10.1121/1.2198182View Description Hide Description
A point-source-point-receiver technique, based on laser generation and laser detection of acoustic waves, allows determination of mechanical properties of anisotropic cylinders. The anisotropic nature of the material and the geometry of the samples make the acoustic signature difficult to interpret. In addition to multiple surface waves, quasi-longitudinal and quasi-shear bulk waves are diffracted and acoustic rays are reflected with or without mode conversion at the cylinder surface. Moreover both bulk and surface diffracted waves have a dispersive behavior. To bypass the intricacies, wave fronts are synthesized with signals provided by scanning a straight line on the cylinder with the laser point source. Conical waves propagating in the volume and helical waves propagating along the surface are then numerically produced. The recovery of the stiffness-tensor components is based on the inversion of the bulk waves, phase velocities. The method is presented and applied to signals simulated or experimentally recorded for a composite material. The five independent stiffness coefficients of the hexagonal symmetry are thus measured with waveforms provided by a single scan along the cylinder surface. The method provides a unique mean for the noncontact measurement of elastic properties of cylindrical parts.
119(2006); http://dx.doi.org/10.1121/1.2195292View Description Hide Description
Dynamic viscoelasticpolymer data is traditionally time-temperature shifted to obtain a temperature shift function (TSF) and then fitted to various analytical models. The process of obtaining the TSF can introduce considerable procedural or operator bias. Nevertheless the Havriliak and Negami (HN) model using the TSF methodology can generally describe polymers that are rheologically simple. In this paper the “wicket” plot is utilized as an important tool in analyzing data, as it is completely independent of time-temperature shifting (TTS). Using the wicket plot the data is fit to the HN equation to determine the four material HN constants independent of TTS. Having obtained the complete spectra of dynamic properties the specific relaxation time (frequency) at each temperature is obtained by matching the HN curve to the experimental data at that temperature thus determining the TSF. The procedure is illustrated by analyzing computer-generated data with random error in modulus and loss and finally real data on a standard material.
119(2006); http://dx.doi.org/10.1121/1.2201251View Description Hide Description
The efficacy of ultrasound-assisted thrombolysis as an adjunct treatment of ischemic stroke is being widely investigated. To determine the role of ultrasound hyperthermia in the process of blood clot disruption, the acousto-mechanical and thermal properties of clotted blood were measured in vitro, namely, density, speed of sound, frequency-dependent attenuation, specific heat, and thermal conductivity. The amplitude coefficient of attenuation of the clots was determined for , , and ultrasound at room temperature . The attenuation coefficient ranged from in porcine clots and from in human clots. The experimentally determined values of specific heat and thermal conductivity for porcine clotted blood are and , respectively, and for human clotted blood are and , respectively. Measurements of the acousto-mechanical and thermal properties of clotted blood can be helpful in theoretical modeling of ultrasound hyperthermia in ultrasound-assisted thrombolysis and other high-intensity focused ultrasound applications.
Ultrasonic evaluation of residual stresses in flat glass tempering: Comparing experimental investigation and numerical modeling119(2006); http://dx.doi.org/10.1121/1.2197806View Description Hide Description
In order to control residual stress distribution in glass, techniques based on the phenomenon of photoelasticity are efficient, though subject to the inherent limitations of all optical techniques. To mitigate these limitations, we exploit the phenomenon of acoustoelasticity to estimate residual stress distribution, using surface acoustic waves. Experimental results are obtained for a thick soda-lime silicate flat glass plate that had been subjected to nonuniform thermal tempering and whose stress distribution is calculated using finite element modeling. The estimated stress distributions provided by our ultrasonic method compare quite well with the results from the modeling, from both the qualitative and quantitative points of view.