ABSTRACT

Lightweight mechanical metamaterials with exceptional specific stiffness and strength are useful in many applications, such as transportation, aerospace, architectures and etc. These materials show great potential in mechanical tasks where the weight of the material is restrained due to economical or energetic reasons. To achieve both the lightweight and the high specific mechanical properties, the metamaterials are often made of periodic cellular structures with well-designed unit cells. The strategies in designing and improving such cellular structures become the key in the studies of such mechanical metamaterials.

In this work, we designed three types of mechanical metamaterials and used both experimental and numerical approaches while probing their mechanical responses. They were: i) composite bending dominated hollow lattices (HLs); ii) triply periodic minimal surfaces (TPMSs) and extended TPMSs (eTPMSs); iii) corrugated TPMSs.

We have demonstrated a few strategies in designing and improving the specific stiffness or strength via these examples of mechanical metamaterials. While fabricating carbon/ceramic composite bending dominated HLs with advanced 3D printing and deposition techniques, we proved that using the composite layered material against the single layer of ceramic was effective in improving the specific mechanical performances of the metamaterials. In the meantime, recovery and cyclic stability of the structures were enabled as well, thanks to the carbon nanolayers suppressing crack propagations. Next, while studying the mechanical nature of TPMSs, whose local mean curvatures are zero at every spot, we discovered that under isotropic deformation TPMSs were stretch dominated with no stress concentrations within the shell structure. They also exhibited the optimal specific bulk moduli approaching the Hashin-Shtrikman upper bound. Furthermore, we established a strategy to smoothly connect the zero-mean-curvature surfaces in TPMSs with the extension of zero-Gaussion-curvature surfaces, forming new ‘eTPMSs”. These new shellular structures traded off their isotropy and showed improved specific Young’s modulus along their stiffest orientation compared to their TPMS base structures. This discovery revealed that despite that the TPMSs had the optimal bulk moduli when the thickness approached zero, they did not necessarily have the optimal Young’s moduli. Lastly, we introduced corrugated sub-structures to existing TPMSs to improve their mechanical properties, such as Young’s modulus, yield strength and failure strength in compression. It was found that the corrugated sub-structure could effectively suppress the local bending behavior and could redirect crack propagations while such structures were under uniaxial compression. Therefore, the specific mechanical properties of the corrugated TPMSs were significantly boosted compared to smooth TPMSs.