Negative refraction and focusing of ultrasonic lamb waves in a plate - How far are we from practical NDE applications?

Recently a flat Veselago lens for elastic Lamb waves displaying negative refraction and focusing has been demonstrated experimentally for the first time [1]. In the present paper we discuss this phenomenon analytically and numerically by using the 3-D Elastodynamic Finite Integration Technique (EFIT). We study refraction of Lamb waves at a plane interface consisting of an abrupt thickness change between two different plate regions. While one region supports a backward-propagating (negative group velocity) mode the other plate region supports a forward-propagating (positive group velocity) mode at exactly the same frequency and wave number. We demonstrate negative refraction and focusing numerically and compare it with the experimental results from [1]. We further discuss if and how a flat Lamb wave lens could be used for novel imaging applications in non-destructive evaluation. The experimental results from [1] and our numerical results confirm that guided ultrasonic waves provide a powerful test bed for negative-index physics. In contrast to complex metamaterials the described system is mechanically simple and thus, seems to be well suited for practical applications.