Some thoughts on dynamic effective properties -- a working document

TL;DR

This paper critically examines the use of effective constitutive relations in dynamic wave problems within laminated elastic media, highlighting challenges at higher frequencies and proposing alternative analytical approaches.

Contribution

It investigates the limitations of effective medium theories for dynamic waves and introduces methods based on Green's functions to better understand wave transmission.

Findings

Effective properties are problematic beyond the first pass band.

Causality and passivity constraints affect effective medium models.

Green's function approach offers an alternative perspective.

Abstract

The main purpose of this work is to address the question of the utility of "effective constitutive relations" for problems in dynamics. This is done in the context of longitudinal shear waves in an elastic medium that is periodically laminated, with attention restricted to plane waves propagating in the direction normal to the interfaces. The properties of such waves can be found by employing Floquet theory, implemented via a "transfer matrix" formulation. Problems occur at frequencies beyond those that define the first pass band, associated in part with the difficulty of assigning a unique wavenumber to the wave. This problem is examined, paying careful attention to the requirements of causality and passivity. The transmission of waves into a half-space is discussed by studying the impedance of the half-space, both directly and in the "effective medium" approximation, and an alternative way of looking at this problem, based on construction of the Green's function, is developed.