Index of content:
Volume 113, Issue 6, June 2003
- NONLINEAR ACOUSTICS 
Local interaction simulation approach to modelling nonclassical, nonlinear elastic behavior in solids113(2003); http://dx.doi.org/10.1121/1.1570440View Description Hide Description
Recent studies show that a broad category of materials share “nonclassical” nonlinear elastic behavior much different from “classical” (Landau-type) nonlinearity. Manifestations of “nonclassical” nonlinearity include stress–strain hysteresis and discrete memory in quasistatic experiments, and specific dependencies of the harmonic amplitudes with respect to the drive amplitude in dynamic wave experiments, which are remarkably different from those predicted by the classical theory. These materials have in common soft “bond” elements, where the elastic nonlinearity originates, contained in hard matter (e.g., a rock sample). The bond system normally comprises a small fraction of the total material volume, and can be localized (e.g., a crack in a solid) or distributed, as in a rock. In this paper a model is presented in which the soft elements are treated as hysteretic or reversible elastic units connected in a one-dimensional lattice to elastic elements (grains), which make up the hard matrix. Calculations are performed in the framework of the local interaction simulation approach (LISA). Experimental observations are well predicted by the model, which is now ready both for basic investigations about the physical origins of nonlinear elasticity and for applications to material damage diagnostics.
113(2003); http://dx.doi.org/10.1121/1.1566974View Description Hide Description
In crystals the speed of the surface acoustic wave mode may approach that of the lowest-speed quasitransverse bulk mode in some directions of propagation. Under these circumstances, it is possible for energy to be transferred from the surface mode to the bulk mode by nonlinear coupling. In the present paper we investigate the possibilities for mode coupling in the (001), (110), and (111) planes of cubic crystals. A condition is given for determining the range of propagation directions with significant coupling, and numerical results are provided for eight different crystals with a range of anisotropy ratios. It is shown that even for significant excitation amplitudes the coupling is negligible for most propagation directions in the aforementioned surface cuts.
113(2003); http://dx.doi.org/10.1121/1.1570437View Description Hide Description
A theoretical investigation of the nonlinear interaction between an acoustic plane wave and an interface formed by two rough, nonconforming surfaces in partial contact is presented. The macroscopic elastic properties of such a nonlinear interface are derived from micromechanical models accounting for the elastic interaction that is characteristic of spherical bodies in contact. These results are used to formulate set of boundary conditions for the acoustic field, which are to be enforced at the imperfect interface. The scattering problem is solved for plane wave incidence by using a simple perturbation approach and the harmonic balance method. Sample results are presented for arbitrary wave polarization and angle of incidence. The relative magnitude of the nonlinear signals and their potential use toward the nondestructive evaluation of imperfect interfaces are assessed. In particular, attention is drawn to the enhanced nonlinear response of an interface insonified by a shear vertical wave in the neighborhood of the longitudinal critical angle. The motivation for this investigation is provided by the need to develop nondestructive methods to detect and localize small, partially closed cracks in metals with coarse microstructures.
Optimization of acoustic scattering from dual-frequency driven microbubbles at the difference frequency113(2003); http://dx.doi.org/10.1121/1.1570442View Description Hide Description
The second harmonic radiation of acoustically driven bubbles is a useful discriminant for their presence in clinical ultrasound applications. It is useful because the scatter from a bubble at a frequency different from the driving can have a contrast-to-tissue ratio better than at the drive frequency. In this work a technique is developed to optimize the scattering from a microbubble at a frequency different from the driving. This is accomplished by adjusting the relative phase and amplitudes of the components of a dual-frequency incident ultrasound wave form. The investigation is focused primarily on the example of dual-mode driving at frequencies of 1 MHz and 3 MHz, with the scattering optimized at 2 MHz. Bubble radii of primary interest are 0.5 to 2 μm and driving amplitudes to 0.5 atm. Bubbles in this size range are sensitive to modulation of driving. It is shown that an optimal forcing scheme can increase the target response eightfold or more. This suggests new applications in imaging and in bubble detection.