Longitudinal-flexural wave mode conversion via periodically undulated waveguides with constant and graded profiles

Mode conversion between longitudinal and flexural elastic waves allows the energy in hard-to-detect bulk waves to produce surface-readable bending motion, which is valuable for structural health monitoring, ultrasonic sensing, and nondestructive testing. Here most devices rely on dense resonator arrays or intricate architectures that operate in narrow frequency ranges and are hard to fabricate. This work offers a simpler route: monolithic beams of undulating profile forming a phononic crystal that is tuned to couple longitudinal and flexural behavior. Mode locking with inverted group velocities converts incident longitudinal waves into flexural waves via reflection.