Topographic metamaterials for ultrasonic non-destructive evaluation

Ultrasound is mainly used in Non-Destructive Evaluation of industrial structures and non-invasive structures and non-invasive diagnostics in medicine, owing to attractive features such as absence of radiation and affordability of associated electronics. Recently there has been much interest in improving the resolution of ultrasonic evaluation using various metamaterials concepts. Phononic crystals and resonant structures have demonstrated the negative refraction and focusing, at different scales. The drawbacks of these are complex design and manufacturing constraints. This paper studies the potential of Topographical Waveguides (TW) for improving the resolution of Ultrasonic inspection, by manipulating the wave field using backward propagating waves. The existence of backward waves in elastic wave guides such as plates, is well known from literature. Due to topographical change, the guided elastic waves in plates, undergo refraction and mode conversion; the forward propagating wave gets converted to backward wave. The phase and group velocity of backward propagating waves are antiparallel. Recent research has studied the phenomenon of anomalous refraction and mode focusing of Zero Group Velocity (ZGV) modes in topographies. The ZGV in thin plates occurs at a specific point, where the group velocity goes to zero while the phase velocity remains finite. At the same wave number the forward and backward propagating waves will interfere, and at that point group velocity becomes zero. This work studies the feasibility of TW lenses for improved resolution of Ultrasonic NDE by using ZGV mode. An aluminium plate, with a step thickness change is considered to show the concept of ZGV. Finite Element (FE) simulation results are presented. Experiments are carried over to validate.