Elastic twisting metamaterial for perfect longitudinal-torsional wave mode conversion

In this work, we design a twisting metamaterial for longitudinal-torsional (L-T) mode conversion in pipes through exploring the theory of perfect transmodal Fabry-Perot interference (TFPI). Assuming that the axial and radial motions in pipes can be decoupled, we find that the metamaterial can be designed in a rectangular coordinate system, which is much more convenient than that in a cylindrical system. Numerical calculation with detailed microstructures shows that an efficient L-T mode conversion can be obtained in pipes with different radii. In addition, we fabricate mode-converting microstructures on an aluminum pipe and conduct ultrasonic experiments, and the results are in good agreement with the numerical calculations. We expect that the proposed L-T mode-converting metamaterial and its design methodology can be applied in various ultrasonic devices.