Excitation of Guided Waves in Generally Anisotropic Layers Using Finite Sources

The excitation of guided wave modes in generally anisotropic layers by finite sized strip sources placed on the surfaces of the layer is examined. The general problem of arbitrarily applied harmonic surface tractions is first solved using the normal mode expansion technique in conjunction with the complex reciprocity relation of elastodynamics. This general solution is then specialized to loading situations modelling those commonly used to excite guided waves in layers for use in nondestructive evaluation. The amplitudes of the generated modes are written as the product of an “excitation function” which depends only on the distribution of the applied tractions and an “excitability function” which depends only on the properties of the specific mode(s) being excited and which determines how receptive the modes are to the applied tractions. Expressions are obtained for the −9 dB wave number and phase velocity bandwidths (σβ and σν respectively) which determine the widths of the wavenumber or phase velocity excitation spectra at the −9 dB generation point. Finally, the problem of transient loading is addressed by superimposing time harmonic solutions via an integration over the dispersion curves of the layer.