Mechanical Energy Absorption Properties of Nanoscale Hierarchical Schwarzite-Based Structures Applied to Additive Manufacturing

We carried out classical molecular dynamics simulations to study the mechanical behavior of 4 Schwarzites [1,2] from the primitive family. Schwarzites are crystalline nanostructures with negative Gaussian curvature that are created by covering a TPMS (Triply Periodic Minimal Surface) with carbon rings containing 6 up to 8 atoms. Through the combinations of these 4 structures, we also generated 6 new energetically stable ones, which will be referred to as hierarchical structures, with the goal that they could have enhanced mechanical properties. Using the open-source software LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) we performed ballistic impact and compressive deformation tests on all structures. Our ballistic tests showed that the hierarchical structures are indeed much more resistant than any individual Schwarzite 'motif' that was used to create them, as the ballistic penetration depth for the Schwarzites were found to be at least 3 times as large as the distances for the hierarchical ones. Our compressive simulations have also shown that all structures can withstand uniaxial compressive stress up to the order of GPa and can be compressed past 50 percent strain without structural collapse. Our most resistant hierarchical structure has an estimated compressive strength of 260 GPa and a Specific Energy Absorption (EPA) of 45.95 MJ/kg while possessing a mass density of only 685 kg/m³. Therefore, our results show that these structures could be excellent lightweight candidates for applications that require significant mechanical energy absorption. Our tests for the Schwarzites also indicate that their structural’ mechanical behavior is strongly dependent on their unique topology, as the Schwarzites with lower density values were found to better absorb impact. This idea is further validated by compressive tests performed on 3D printed versions of all our new structures [3]. Interestingly, the general trends of the 3D printed structures' under compressive strain (characterized by its stress-strain curve) are the same observed from the molecular dynamics simulations for the atomic models. This unexpected correlation between microscale and macroscale suggests that the elastic behavior is dominated by the topology features and that molecular dynamics simulations can be an efficient tool to design new 3D printed structures with macroscale functionalities.<br/>[1] A. L. Mackay, H. Terrones, <i>Nature </i>1991, <i>352</i>, 762.<br/>[2] Levi C. Felix, Cristiano F. Woellner and Douglas S. Galvao. (2019). Mechanical and energy-absorption properties of schwarzites. Carbon. 157. 10.1016/j.carbon.2019.10.066.<br/>[3] Leonardo V. Bastos, Chandra S. Tiwary, Douglas S. Galvão, Cristiano F. Woellner. Mechanical energy absorption properties of nanoscale hierarchical Schwarzite-based structures applied to additive manufacturing (submitted).