Introduction

The problem of elastic wave propagation in anisotropic layers has received a fair amount of attention in the literature during the past several decades, and recent interest in this subject has increased even more. This is undoubtedly due, at least in part, to the increased use of composite materials in many new facets of structure design. Composite materials that are mechanically anisotropic offer many benefits over more conventional material – a higher stiffness-to-weight ratio, for example. This advantage of composites is in turn due to the fact that their mechanical properties, such as elastic moduli, can be tailored to be high in the directions that are expected to see high loads while remaining considerably lower in other directions. This directional dependence of the mechanical properties of composites classifies them as anisotropic media.

The benefits of using composites come at the cost of a more complicated mechanical response to applied loads, static or dynamic. The anisotropic nature of the solid introduces many interesting wave phenomena not observed in isotropic bodies: a directional dependence of wave speed, a difference between phase and group velocity of the waves, wave skewing, three wave velocities instead of two, and many somewhat more subtle differences. An understanding of the nature of waves in plates made of anisotropic materials is certainly required if one wants to use these materials effectively in structure design or if one wants to inspect them using ultrasonic methods.