Gradient-index (GRIN) refers to a system where the refractive index changes spatially within a specific region. GRIN phononic crystals are capable of not only amplifying the magnitude of wave energies but also controlling the directional nature of the wave propagation, thus offering substantial benefits with regard to energy harvesting (EH) improvements. Here, we propose a systematic design method for GRIN phononic crystals which combine the two-dimensional Reissner–Mindlin plate model and a genetic algorithm for optimization. This design process allows us to design a GRIN phononic crystal with any arbitrary refractive index profile or complex shape of the unit cells. The experimentally verified focusing capability of the GRIN phononic crystals led to the realization of piezoelectric energy harvesting with a maximum areal power density value of up to 240.4 mW/m², considerably outperforming the existing non-GRIN-based EH systems without direction controllability.

This study aims to investigate elastic wave localization that leverages defect band splitting in a phononic crystal with double defects through in-depth analysis of comparison of numerical and experimental results. When more than one defect is created inside a phononic crystal, these defects can interact with each other, resulting in a distinctive physical phenomenon from a single defect case: defect band splitting. For a phononic crystal consisting of circular-hole type unit cells in a thin aluminum plate, under A₀ (the lowest antisymmetric) Lamb waves, both numerical simulations and experiments successfully confirm the defect band splitting phenomenon via frequency response functions for the out-of-plane displacement calculated/measured at the double defects within a finite distance. Furthermore, experimental visualization of in-phase and out-of-phase defect mode shapes at each frequency of the split defect bands is achieved and found to be in excellent agreement with the simulated results. Different inter-distance combinations of the double defects reveal that the degree of the defect band splitting decreases with the increasing distance due to weaker coupling between the defects. This work may shed light on engineering applications of a multiple-defect-introduced phononic crystal, including broadband energy harvesting, frequency detectors, and elastic wireless power transfer.

Defect detection during pulse-echo ultrasonic testing (UT) is challenging when defects are located in a dead zone where the echoes from the defects are overshadowed by disturbances from the initial ringing signal of the UT transducer. The time-gate method is one of the most widely used approaches in UT to filter out such unwanted components, but defects in the dead zone are virtually impossible to detect using conventional methods. This paper proposes an autoencoder-based end-to-end ultrasonic testing method to detect defects within the dead zone of a transducer. The autoencoder is designed to predict the normal behavior of ultrasonic signals including disturbances, thus enabling the identification of even subtle deviations made by defects. To advance the performance of the autoencoder further with a limited amount of training data, a two-step training procedure is presented, involving training using pure normal signals measured from a defect-free specimen and re-training using pseudo-normal samples identified by the autoencoder with a smart thresholding strategy. This two-step procedure enables us to develop an adaptive autoencoder model that can be effectively employed to process the newly measured ultrasonic signals. For a demonstration of the proposed method, UT-based B-scan inspections of aluminum blocks with near-surface defects are conducted. The results suggest that the proposed method outperforms the conventional gate-based inspection approach with regard to its ability to identify the sizes and locations of near-surface defects.

Achieving total mode conversion from longitudinal to shear waves for a broad incident angle has been a big scientific challenge in elastic fields, which was impossible to be achieved in classical elastic wave theory. In this paper, we propose and realize a refractive transmodal elastic metasurface that can convert an incident longitudinal wave to a shear wave for a broad incident angle. Here, the total mode conversion is achieved via a sufficiently large phase gradient, while the full transmission is achieved with the impedance-matched single-layered metasurface. Numerical and experimental investigations show that the proposed metasurface can provide almost total mode conversion for a broad incident angle from −20.4$°$ to 22.3$°$ . We expect that the proposed refractive transmodal metasurface can be applied in various ultrasonic systems.

In this study, we achieved energy localization and amplification of flexural vibrations by utilizing the defect mode of plate-attachable locally resonant metamaterials, thereby realizing compact and low-frequency vibration energy suppression and energy harvesting with enhanced output performance. We designed a cantilever-based metamaterial unit cell to induce local resonance inside a periodic supercell structure and form a bandgap within the targeted low-frequency range of 300–450 Hz. Subsequently, a defect area was created by removing some unit cells to break the periodicity inside the metamaterial, which led to the isolation and localization of the vibration energy. This localized vibration energy was simultaneously converted into electrical energy by a piezoelectric energy harvester coupled with a metamaterial inside the defect area. Consequently, a substantially enhanced energy harvesting output power was achieved at 360 Hz, which was 43-times higher than that of a bare plate without metamaterials. The proposed local resonant metamaterial offers a useful and multifunctional platform with the capability of vibration energy isolation and harvesting, while exhibiting easy handling via attachable designs that can be tailored in the low-frequency regime.