# A Tunable Metamaterial Joint for Mechanical Shock Applications Inspired by Carbon Nanotubes
## 1 Introduction  
In recent decades, materials science has moved forward to modern areas that focus on the development of advanced nanomaterials [1] and metamaterials [2] for efficient use in devices and systems as well as novel engineering and industrial applications. With the development of nanotechnology, the mechanical behavior of a variety of organic and inorganic nanomaterials was thoroughly investigated [3] with respect to their nanosizes, forms of crystallinity, dimensionalities, chemical functionalization, atomistic periodicities, and the natures of their molecular structures using experimental [4] and theoretical methods [5]. Today, there is a lot of evidence and information that permits the efficient design of nanomaterials with controllable, tuned mechanical responses [6, 7]. Special attention has been given to the mechanical modeling and characterization of graphene-like molecules, due to their exceptional mechanical properties [8] as well as their ability to adapt to various mechanical requirements and working conditions by simply altering their size, shape, crystallinity, or functionalization [7, 9]. Their unique mechanical and physical properties are due to their unique carbon hexagonal lattice structure [10]. The hexagonal atomistic structure of graphene and carbon nanotubes (CNTs), due to its strength and deformation abilities, has attracted interest for applications not only at the nanoscale but at the microscale as well. The idea of imitating the structural characteristics of nanomaterials in larger components via the introduction of relevant metamaterials with hexagonal or honeycomb lattice patterns [11, 12] seems to produce impressive results in demanding engineering fields such as automotive and aerospace engineering. Discretedesign schemes in metamaterials may provide a variety of enhanced properties such as high strength, stiffness, and fracture toughness, and light weight [13]. Nevertheless, to realize such a capability, i.e., imitate the molecular structure of nanomaterials such as graphene and CNTs at natural scales, the adoption of special manufacturing technologies is required. The production of material components that are inspired by the structural characteristics of nanomaterials would be a very difficult task if typical manufacturing processes are utilized, considering the significantly discrete natures of their geometries. Providentially, at present, the difficulty in producing complex three-dimensional (3D) hexagonal-like geometries at a micro-level may be overcome by the utilization of modern additive manufacturing procedures such as 3D printing or laser sintering [14; 15]. In addition, it is now well understood that the complete mechanical behavior of wireframe-like metamaterials can be easily and accurately predicted through the use of simple finite element (FE) models, and even by using two-noded-line finite elements [15]. Generally, the FE method (FEM) may be seen as the ultimate method for accomplishing ideal metamaterial designs [15; 16; 17], without wasting time, material, equipment, and human resources, in order to conduct complex parametric experimental studies. The recent additive manufacturing breakthroughs and the well-established FEM simulation tools offer new opportunities for discoveries in structural engineering. One of the main advantages of metamaterials is the fact that their mechanical performance and physical properties may be tuned by modifying their microstructural geometries [18]. Small structural modifications in metamaterial-based components may allow the efficient adjustment of several targeting properties such as strength, stiffness, and plasticity. Impressively, by using clever microstructural configurations, the final performance of the 3D-printed metamaterial may be considerably uncoupled from the fundamental characteristics of the basic raw material.