Graded Elastic Waveguide Metamaterial Rod for Up-Conversion of Longitudinal Axisymmetric Guided Ultrasonic Wave Modes

Cylindrical or circular rod-type waveguides are of much interest in applications such as measurement of flow, temperature, material properties in harsh environments, and also in medical diagnostics. However, multiple waveguide modes exist in such systems, out of which only some are of interest to certain applications. For example, L(0,3) longitudinal mode excitation can optimally transmit elastic waves into the test specimen and help in better sensing and measurement when compared to other modes within the family of longitudinal guided waves. This paper demonstrates the up-conversion of longitudinal modes within the family of guided ultrasonic rod waves (from L(0,2) to L(0,3)), which is of interest to certain waveguide transducer applications. The mode up-conversion is demonstrated using numerical simulations and experiments. An analysis is used to bring more insights and guide the design of the metamaterial in this process.