Abstract
The focusing of ultrasound using topographic lenses, typically made of plates with step changes that cause an interaction between forward- and backward-propagating guided waves, has been widely studied in recent years. However, such ‘step-change’ lenses require precise machining and moreover, the thick-thin structure can be unstable during deployment in practical inspection applications. The work reported here follows from the insight that perhaps any approach to induce a mismatch in acoustical impedance as achieved by the step-change can also lead to focusing of ultrasonic guided waves. By carefully choosing the impedance pairing, a novel material contrast lens stacking Aluminium and Molybdenum plates in series is shown to achieve focusing of ultrasound through negative refraction. The interface between the two metals causes the interaction of the forward-propagating second symmetric Lamb mode S2 into the backward- propagating first symmetric S2b. The focusing of Lamb waves is demonstrated using numerical simulations validated by experiments. Comparison with a simple Aluminium-Aluminium plate combination brings out the underlying physics of focusing using the proposed material contrast lens. Simulation results showing super-resolution imaging using the proposed material contrast lens are also presented, demonstrating the power of the proposed approach. This report opens up the possibilities of developing new lensing devices for use in medical imaging and nondestructive evaluation, among other possible applications.
Highlights
Techniques to improve the resolution of imaging systems beyond the diffraction limit[1,2,3,4] been of much interest in the recent years
Multiple investigations have been reported in the domain of acoustics[25,26,27] in order to characterize and reduce such focal spot aberrations
Due to the presence of coupled modes of wave propagation in elastic solids[28], concepts developed for acoustics are not entirely portable to ultrasonics
Summary
Techniques to improve the resolution of imaging systems beyond the diffraction limit[1,2,3,4] been of much interest in the recent years. Focusing of acoustic and elastic waves have been demonstrated through lenses made of phononic crystals[11,12,13,14,15,16] and holey metamaterials[17,18,19,20,21,22]. Ultrasonic guided wave based lenses making use of the phenomenon of negative refraction (NR) have been proposed[23,24] to show how focusing can be achieved due to thickness changes in waveguides. This paper explores a different route to achieve super-resolution using waveguide-based topographic lenses for ultrasonics. Lamb waves are guided elastic waves propagating in plate -type waveguides. An experimental demonstration of the focusing of Lamb waves using a plate waveguide with a thickness change was discussed in[23]. Researchers[33,34,35] have reported that such focusing can be achieved by mode conversion occurring at plate edges using negative reflection of Lamb waves
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