(A) study on wave focusing characteristics of elastic wave refraction patch

Abstract

Wave manipulation has been widely employed in various industries and engineering fields. It encompasses techniques such as frequency filtering to conceal energy within specific frequency bands, wave guiding to induce energy along specific paths, and wave focusing for achieving energy harvesting and shock mitigation. Notably, wave focusing has found applications in ultrasound imaging, optical lens systems, acoustic navigation systems, and more, achieving concentration in sound, light, radio, and ultrasonic waves. From a mechanical engineering perspective, the concentration of stress waves has been utilized for energy harvesting and shock mitigation. In the fields of mechanical engineering and aerospace, the primary focus has been on nondestructive evaluation and energy harvesting using elastic waves. Research efforts have explored practical applications through methods such as wave filtering and guiding using metamaterials' Bragg effect, as well as refraction and concentration using phononic crystals. However, these methods often face challenges in terms of difficult fabrication, design complexity, and limited frequency ranges. Efforts have been made to simplify wave manipulation, one of which involves utilizing thickness variations in a flat plate. In thin plates, thickness variations induce variations in the refractive index within the region, enabling wave focusing and guiding. Previous studies have created wave focusing lenses using the global highest refractive index point (thinnest thickness) in the plate. However, this approach weakens the structural integrity of the thin plate and requires precise fabrication due to the extreme thinness. In this study, we aim to overcome these limitations by proposing a patch-form elastic wave focusing lens that can be attached to an extremely thin plate structure. By designing the patch area to have a local highest point, wave focusing is achieved even with a thicker thickness compared to the base plate. This design has been validated through analysis and experimentation. For the analysis, we have employed the numerical simulation methods, such as ray-tracing and finite element methods, and established 2-D auto stage scanning system for experimental analysis. As comparing the results, the wave focusing characteristics of the elastic wave refraction patch was investigated and validated. The proposed approach is advantageous as it can be applied to most plate structures (sandwich structures), offers a wide applicable frequency range, and does not introduce structural flaws.