In solid materials, acoustic nonlinearity arises from three sources of diverse scales. They are lattice anharmonicity, energy absorption, and imperfect interfaces. In the atomic scale, the interatomic potential contains non-parabolic terms of interatomic distance, the magnitude and characteristics being specific to the crystal. Those terms are the origin of non-Hookean behaviors of solids and also account for the thermal expansion and specific heat. If the solid has one or more mechanisms of acoustic damping, the dynamic stress-strain curve shows a hysteresis loop, whose area determines the rate of the energy dissipation per cycle. This implies that the nonlinear phenomena accompany energy absorption; but, not necessarily vice versa. In a scale comparable or larger than grain size, solids sometimes contain imperfect interfaces like small cracks caused by aging and the separation at inter-phase boundaries in natural rocks, for instance. The opposite faces are in contact with each other but not well tightly. They open, close, or grind responding to the applied stress and acoustic fields. This rectifying effect can distort the waveform causing the nonlinearity.
KeywordsInteratomic Potential Imperfect Interface Acoustic Nonlinearity Lattice Anharmonicity Tensile Plastic Deformation
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