Advances in cloaking properties of elastic wave metamaterials

From myth stories to science fiction, people have never stopped exploring “invisibility”. With the continuous development of metamaterial technology, the research of cloaking properties has gradually moved from theory to application. Metamaterials realize the wave regulation principle and characteristics from the physical scale of sub-wavelength. Some special physical properties enable metamaterials to break through the performance limitations of traditional materials and provide a new idea for the design of acoustic, structural vibration and elastic wave cloaking devices. Due to the vectorial characteristics of elastic waves, longitudinal and transverse waves often interact and couple together, which makes the wave phenomenon more complex than electromagnetic waves and acoustic waves, and the theoretical analysis is more difficult, so it brings special challenges to the research of elastic wave metamaterials. In this review, the advances in cloaking properties of elastic wave metamaterials are introduced from different mechanisms such as coordinate transformation, hyperelastic theory, scattering cancellation and metasurface phase regulation. The cloaking characteristic of elastic wave metamaterials is introduced in terms of basic concepts, working principles and performance advantages, and its application prospects and challenges are discussed.Firstly, we introduce the elastic wave cloaking metamaterials based on the coordinate transformation method. By the geometric transformation of virtual space and physical space, the coordinate transformation method can flexibly adjust the wave propagation path according to the control equation, so that the elastic wave can recover its original wave field after bypassing a specific target area, and realize the invisibility effect. This method has unique advantages in designing accurate propagation paths. However, in practical applications, this method still faces several challenges. For example, the parameters of cloaking materials tend to change with radius, and the materials are often anisotropic, so it is still a challenge to realize the ideal broadband cloaking elastic metamaterial.Secondly, the hyperelastic theory is an important method for designing elastic wave stealth metamaterials, which enables the alteration of the parameters through the application of prestress/predeformation. The key to this method is to use the applied prestress/predeformation to achieve the heterogeneity and anisotropy of the structure. In theory, metamaterials designed based on hyperelasticity theory can satisfy the cloaking effect in any mapping case, but it is difficult to construct nonlinear elastic solids with specific strain energy functions in practice. Therefore, the elastic wave cloak designed based on this theory also faces great challenges in experimental verification and practical application. Thirdly, the scattering cancellation method shows strong broadband adaptability and low-frequency invisibility performance in elastic wave-cloaking metamaterials. Different from the coordinate transformation method, scattering cancellation is designed by micro-structure to make the scattered waves of the object cancel with the original physical scattering, to realize the total scattering of the whole system to be zero and to present the invisibility state of the object. This method is relatively simple and easy to implement, but it is still a challenge to realize the cloaking of irregular objects, and how to design high-performance scatterer structures and achieve accurate coherent phase cancellation among multiple scatterers are still the difficulties for future research.Then, the mechanism of metasurface phase regulation for elastic metamaterials is introduced. The metasurface cloak has the advantages of being ultra-thin, lightweight, easy to fabricate, and suitable for cloaking three-dimensional objects of arbitrary size and shape. The metasurface cloak has potential application value in industrialized large-size cloaking and illusion, but this method is designed for a specific incidence angle and frequency, so it has some limitations such as a relatively narrow frequency band and limited incidence angle range. In conclusion, the cloaking technology of elastic wave metamaterials is developing rapidly in the direction of multi-physical field coupling, intelligent control, broadband cloaking and high integration. Although the field is still facing many challenges, with the rapid development of smart materials, manufacturing processes and artificial intelligence, elastic wave metamaterial cloaking will be applied in many fields such as national defense and security, earthquake engineering, noise control and medical imaging.