Volume 104, Issue 3, September 1998
Index of content:
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
104(1998); http://dx.doi.org/10.1121/1.424346View Description Hide Description
A new method for ultrasonic tomography based on genetic algorithms is proposed for the prediction of the geometry of an inclusion of known physical properties in a given specimen. New inversion operators are introduced in order to take full advantage of the physical properties of the system investigated. The efficiency of the proposed method is tested through a comparison with other existing techniques for the solution of the inverse problem. The accuracy of the procedure is verified by using a variety of sets of synthetic data. Good and fast convergence is obtained even in the case of complex geometries if parallel processing is adopted.
Heterogeneous structure of modes and Kramers–Kronig relationship in anisotropic viscoelastic materials104(1998); http://dx.doi.org/10.1121/1.424347View Description Hide Description
Composite materials made of fibers and a viscoelastic matrix, exhibit an orthotropic viscoelastic behavior that is described by a tensor with nine independent complex viscoelastic moduli. This tensor makes it possible to compute the velocity and attenuation of heterogeneous or homogeneous modes in any direction. In experiments, the immersion method of a plate shaped sample insonified by plane ultrasonic waves is used to measure this complex tensor. The liquid/solid interface generates heterogeneous quasilongitudinal and quasishear bulk modes that propagate through the plate with velocities and attenuations that depend on the frequency. In a viscoelastic material,velocity and attenuation are linked by the Kramers–Kronig relations. For heterogeneous modes, the attenuation that needs to be used is the projection of the damping vector on the wave vector. This paper shows that these relations limited to a “local” frequency band can be experimentally verified and permit one to link anisotropicvelocity and attenuation dispersion of quasilongitudinal and quasishear modes only if their heterogeneous structure is taken into account. The extrapolation of the material properties determined by ultrasonic measurements, towards low frequencies, relies on this feature in combination with a model of the attenuation evolution versus frequency.
104(1998); http://dx.doi.org/10.1121/1.424348View Description Hide Description
The forced oscillations of a system consisting of two finite liquid columns in a duct separated by a gas bubble are studied in the linear approximation. It is found that thermal processes in the gas induce a very significant damping in the system, which can exceed viscous damping even in capillaries with a submillimeter diameter. The study is motivated by the possibility of using gas bubbles as actuators in microdevices.
104(1998); http://dx.doi.org/10.1121/1.424349View Description Hide Description
Among the various techniques available, ultrasonicLamb waves offer a convenient method of examining composite materials. Since the Lamb wavevelocity depends on the elastic properties of a material, an effective tool exists to evaluate composites by measuring the velocity of these waves.Lamb waves can propagate over long distances and are sensitive to the desired in-plane elastic properties of the material. This paper discusses a study in which Lamb waves were used to examine fiber volume fraction variations of approximately 0.40–0.70 in composites. The Lamb wavemeasurements were compared to fiber volume fractions obtained from acid digestion tests. Additionally, a model to predict the fiber volume fraction from Lamb wavevelocity values was evaluated.