Hybrid Ray-Mode and Wavefront-Resonance Techniques for Acoustic Emission and Scattering in Multiwave Layered Media (original) (raw)

1987

Abstract

Analytical modeling of acoustic detection of flaws in composite laminates poses a difficult problem in wave propagation and scattering. Wave excitation and propagation in the laminate itself is complicated by the layering and by the generally multiwave character (due to anisotropy, elasticity, etc.) of each layer. Scattering from a flaw, even if it is understood in an unbounded material, is complicated by the presence of the layered environment. Dealing with the overall problem requires a self-consistent combination of phenomena due to layering and scattering. In this context, attention is here being called to self-consistent hybrid ray-mode and wavefront-resonance techniques — originally introduced in electromagnetics and underwater acoustics, and more recently in seismology — which incorporate a new approach to the multiwave, multilayer problem and the scattering problem, respectively. For the layered environment, a hybrid ray-mode Green’s function seeks to remove the disadvantages of separate ray propagation and mode propagation models. Ray fields alone may be inconvenient due to the multiwave character (caused by elasticity, anisotropy, etc.) of the layers, which results in wave species coupling at boundaries, and consequent ray proliferation after a few multiple reflections. Mode fields alone may be inconvenient at high frequencies because many normal modes are then required. Thus, for general conditions encompassing a broad range of observables, neither rays nor modes alone provide an efficient algorithm for numerical implementation. For scattering of wide-band signals by targets modeling flaws or inclusions in the unlayered environment, a self-contained wavefront-resonance description seeks to reconcile the spiked wavefront-like responses observed at early times with the more smoothly oscillatory resonance-like responses observed at later times. These hybrid formulations are reviewed and illustrated by examples which clearly reveal the phenomenology noted above. Emphasis is placed on the implications of these concepts for NDE.

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