Volume 104, Issue 2, August 1998
Index of content:
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
104(1998); http://dx.doi.org/10.1121/1.423309View Description Hide Description
This paper shows how the energy transfer between electromagnetic waves and elastic waves at the surface of fluid or solid materials could constitute a new way to generate ultrasonicwaves without contact for the purpose of nondestructive evaluation and control. When time-gated microwaves strike the surface of a material, there is a generation of ultrasonicwaves. The times-of-flight of these elastic waves inside the material decrease when the power of the incident microwaves increases. Therefore, the diminution of the propagation path indicates that the energy transformation appears in a volume close to the surface, the depth of which is increasing along with the power of the microwaves and depends on the material properties. Consequently, the elastic wave generation is attributed to the evolution of the power of the microwave during the gate width that governs the frequency content of the ultrasonicwaves. The generation of ultrasound is checked in water and polymers. If the microwaves are not too attenuated in the material, the ultrasonicwaves can be generated both at input (air–solid) and output (solid–air) interfaces. These peculiarities can generate new applications in the nondestructive evaluation and control of material.
104(1998); http://dx.doi.org/10.1121/1.423331View Description Hide Description
A new improved continuum mixture model is developed for the propagation of axisymmetric longitudinal waves in fibrous composites. The major improvement on the original model of Hegemier, Gurtman, and Nayfeh [Int. J. Solids Struct. 9, 395 (1973)] is achieved by the inclusion of the axial rate of change of the radial displacement in the shear constitutive relations which was neglected previously. This model has also been extended to treat situations in which the fiber and the matrix are anisotropic. The improved model is found superior to the original one, when compared with the recently acquired experimental data and exact solutions.
104(1998); http://dx.doi.org/10.1121/1.423332View Description Hide Description
The influence of materialattenuation on Lamb wave dispersion behavior has been studied analytically. As the attenuation is increased, keeping the ratio of the bulk longitudinal to bulk shear wave attenuation constant at the value measured in high-density polyethylene, the degree of coupling between the shear and longitudinal partial waves decreases and the phase velocity dispersion curves for different modes of the same symmetry can cross; this is not possible for an elastic plate. With increasing attenuation, some modes become asymptotic to the bulk longitudinal velocity at high frequency, rather than to the bulk shear velocity. At high values of attenuation, there is minimal coupling between the longitudinal and shear partial waves and the behavior of the longitudinal modes is analogous to that of a “fluid plate.” The predicted group velocities and attenuations of selected modes in a high-density polyethylene plate have been checked experimentally, and good agreement was obtained.
104(1998); http://dx.doi.org/10.1121/1.423333View Description Hide Description
Experimental evidence of surface waves above thin porous layers of thickness varying from 1.8 to 6 mm of plastic foams having a porosity close to one is presented. Phase velocity measurements at 40 kHz indicate the presence of waves propagating slower than homogeneous plane waves. The pole of the reflection coefficient related to these waves is detected by near-field holography at 20 kHz. The different experimental results are compared with predictions obtained by replacing the air inside the layers by an equivalent fluid, and using methods developed in the context of electromagnetism to localize the poles of the reflection coefficient.
104(1998); http://dx.doi.org/10.1121/1.423334View Description Hide Description
The acoustic wave field generated in front of a submerged fiber tip by short laser pulses is theoretically and experimentally studied by fast imaging and optical pressure measurements. It is shown that the finite size of the fiber causes strong tensile stress leading to cavitation. Depending on the absorption coefficient of the laser radiation, cavitation-induced bubble formation occurs inside (low absorption) or outside (high absorption) the volume of heat deposition. The results are used to characterize the cavitation bubble formation mechanisms and to predict possible consequences for applications of fiber-guided short-pulsed laser sources in medicine.