Ultrasonic Field Patterns and Broadband Imaging-Simulations in Transversely Isotropic Media

Thorough application of NDE — and imaging techniques in anisotropic media has to overcome those inherent problems like beam splitting, beam distortion and deviation between wave propagation direction and energy flow, known as beam skewing. A theory of elastic wave propagation in transversely isotropic media which properly accounts for these effects has been presented previously [1], an overview has been presented at last year’s conference [2]. Analytic expressions have been derived characterizing the propagation of Gaussian Wave Packets (GWPs) in these media thus making possible the simulation of real pulse propagation. In order to provide information from a practical point of view the theory has been evaluated to yield field patterns of GWPs in unidirectional graphite-epoxy as well as centrifugally cast stainless steel. The plane wave spectral decomposition of Green’s dyadic and triadic functions, also presented last year, has been used to derive the algorithm of forward-backward-propagation of elastic wavefields yielding a simple solution to the inverse problem, the so-called elastodynamic holography. With this basic imaging technique, field distributions of GWPs are propagated forth and back; for comparison, the conventional isotropic algorithm [3] is also applied, resulting in field distributions which differ both in intensity and position. Finally the simulations, being evaluated in frequency domain, are performed at multiple frequencies thus in principle making up a mode-matched FT-SAFT [4], neglecting the vector scattering amplitudes.