Degenerate weakly nonlinear elastic plane waves

TL;DR

This paper derives evolution equations for weakly nonlinear elastic plane waves in anisotropic hyperelastic crystals, revealing how wave behavior depends on crystal symmetry and propagation direction, with explicit examples provided.

Contribution

It introduces a unified framework for deriving amplitude evolution equations for quasi-longitudinal and quasi-transverse waves in anisotropic media, highlighting the influence of crystal symmetry.

Findings

Single evolution equation for quasi-longitudinal waves across all directions.

Coupled equations for quasi-transverse waves along degeneracy directions.

Decoupling of transverse wave equations in higher symmetry axes.

Abstract

Weakly nonlinear plane waves are considered in hyperelastic crystals. Evolution equations are derived at a quadratically nonlinear level for the amplitudes of quasi-longitudinal and quasi-transverse waves propagating in arbitrary anisotropic media. The form of the equations obtained depends upon the direction of propagation relative to the crystal axes. A single equation is found for all propagation directions for quasi-longitudinal waves, but a pair of coupled equations occurs for quasi-transverse waves propagating along directions of degeneracy, or acoustic axes. The coupled equations involve four material parameters but they simplify if the wave propagates along an axis of material symmetry. Thus, only two parameters arise for propagation along an axis of two-fold symmetry, and one for a three-fold axis. The transverse wave equations decouple if the axis is four-fold or higher. In the absence of a symmetry axis it is possible that the evolution equations of the quasi-transverse waves decouple if the third order elastic moduli satisfy a certain identity. The theoretical results are illustrated with explicit examples.