The discovery of the disorder effect in traditional metamaterials has opened up possibilities in the search for disordered metasurfaces. Photonic, dielectric, and elastic metamaterials that exhibit added value of the disorder effect on wave-propagation physics are reported. Despite this extensive attention and progress in disordered metamaterials, elastic metasurfaces, however, involving disorder have not yet been reported. Here, we introduce the concept of disordered elastic metasurface composed of identical pillared resonators with a random arrangement in the subwavelength range. Based on theoretical formalism and direct acoustic measurement, we observe anomalous deflection and focusing effects of flexural waves in a plate and elucidate, in detail, the related physics. This research extends the disorder effect to metasurfaces and may lead to innovative acoustoelastic devices.

Based on the new conception of natural frequency vector (NFV) and natural frequency vector assurance criterion (NFVAC), a new damage detection method using the natural frequencies as damage feature is described in detail. For a specific structure, a series of damage states can be simulated based on the finite element model of the intact structure. Then, the NFVs of these damaged structures can be calculated and used to build the damage feature database of the structure. At last, the NFVAC, which is defined by the relationship of NFVs in the damage feature database and the NFV measured on the actual structure are used as damage index. An 8-story shear frame model is adopted as an example to verify the feasibility and validity of the proposed method. Both numerical simulations and model experiments for damage detection are performed, the results of damage detection for several damage cases demonstrate that using NFVAC as damage index, the damage location can be identified correctly and the damage extents can be estimated as well.