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The infiltration behavior of epoxy on the surface of the aluminum alloy sheets with different roughness and surface energy was studied to enhance adhesion performance. In this study, the aluminum alloy sheets were treated with the process of phosphoric acid anodizing (PAA) under different parameters. The as-formed uniform pits and nanometer-scale pores contributed to the change of apparent surface energy and surface roughness. Then the morphologies of the interface between epoxy and the aluminum alloy sheet were observed by SEM. The adhesion strength was characterized by the lap-shear tensile test. The results indicated that the modes of epoxy wetting had mainly two types. Apparent surface energy played a leading role in improving adhesive bonding at the state of fully wetting, while the value of roughness could affect adhesion strength greatly at the state of incompletely wetting. The specimen exhibited the ultimate interfacial adhesion strength of 52.45 MPa and optimal hygrothermal ageing resistance

This paper proposes an innovative and simple elastic metasurface, which is one kind of ultrathin metamaterial, to abnormally deflect normally and obliquely incident flexural waves. In particular, negative refraction of flexural wave is also realized. The proposed metasurface can support high transmission and full phase control for transmitted waves. The metasurface is composed of three-dimensional (3D) functional units which are combined with multiple slender pillars for phase shift accumulation. The slots are filled in the 3D functional units to avoid coupling between adjacent functional units. Firstly, based on the generalized Snell’s law, the metasurface in the sub-wavelength regime is theoretically designed. Secondly, the deflecting functionality of the metasurface for incident flexural waves is numerically demonstrated. Finally, the deflecting functionality is verified by the corresponding experiments. The proposed metasurface has a great application potential in many fields, such as ultrasonic detection, energy harvesting, and vibration control.

Traditional asymmetric transmission of elastic waves is mostly based on mode conversion, which presents a real challenge to get pure-mode elastic waves. In this letter, without the aid of mode conversion, we present an innovative concept of asymmetric flexural wave transmission within a wide frequency band, based on dual-layer elastic wave gradient metasurfaces. According to the generalized Snell's law, we theoretically and numerically design and experimentally demonstrate the asymmetric flexural wave transmission by tuning the supercell lengths of dual-layer metasurfaces. The experimental results confirm that the proposed design provides a wide effective frequency band feature, which agrees well with the theoretical analysis and predictions. Our concept offers the flexibility to control the wave energy flow, opening the route to pragmatic applications in many fields, such as ultrasonic detection, energy harvesting, and vibration control.

In this research, we systematically study the flexural waves diffraction. Based on the diffraction mechanism, we propose the concept of subwavelength lossy gradient elastic metasurface for flexural waves absorption. We theoretically reveal that the highefficiency absorption behavior stems from maximum multireflection-enhanced absorption of 0 th order diffraction, and experimentally show that robust flexural wave quasi-omnidirectional absorption in the frequency range extending approximately from 340 Hz to 1000 Hz (larger than 1.5 octaves). In addition, we propose a general approach which involves new physics of adjusting the arrangement sequence of subunits to suppress the 1 st diffraction mode, to further reduce the sub-wavelength thickness of the metasurface while maintaining its high-efficiency absorption. Our designs could provide new routes to broadband vibration suppression and cancellation in lowfrequency by lossy elastic metamaterials and metasurfaces.