Scale Effects on the Ballistic Penetration of Graphene Sheets

AbstractCarbon nanostructures are promising ballistic protection materials, due to their low density and excellent mechanical properties. Recent experimental and computational investigations on the behavior of graphene under impact conditions revealed exceptional energy absorption properties as well. However, the reported numerical and experimental values differ by an order of magnitude. In this work, we combined numerical and analytical modeling to address this issue. In the numerical part, we employed reactive molecular dynamics to carry out ballistic tests on single, double, and triple-layered graphene sheets. We used velocity values within the range tested in experiments. Our numerical and the experimental results were used to determine parameters for a scaling law. We find that the specific penetration energy decreases as the number of layers (N) increases, from ∼15 MJ/kg for N = 1 to ∼0.9 MJ/kg for N = 350, for an impact velocity of 900 m/s. These values are in good agreement with simulations and experiments, within the entire range of N values for which data is presently available. Scale effects explain the apparent discrepancy between simulations and experiments.</jats:p

Elastic properties of graphyne-based nanotubes

Graphyne nanotubes (GNTs) are nanostructures obtained from rolled up graphyne sheets, in the same way carbon nanotubes (CNTs) are obtained from graphene ones. Graphynes are 2D carbon-allotropes composed of atoms in sp and sp2 hybridized states. Similarly to conventional CNTs, GNTs can present different chiralities and electronic properties. Because of the acetylenic groups (triple bonds), GNTs exhibit large sidewall pores that influence their mechanical properties. In this work, we studied the mechanical response of GNTs under tensile stress using fully atomistic molecular dynamics simulations and density functional theory (DFT) calculations. Our results show that GNTs mechanical failure (fracture) occurs at larger strain values in comparison to corresponding CNTs, but paradoxically with smaller ultimate strength and Young's modulus values. This is a consequence of the combined effects of the existence of triple bonds and increased porosity/flexibility due to the presence of acetylenic groups

Friction and Adhesion of Different Structural Defects of Graphene

Graphene structural defects, namely edges, step-edges and wrinkles are susceptible to severe mechanical deformation and stresses under frictional operations. Applied forces cause deformation by folding, buckling, bending and tearing the defective sites of graphene, which lead to a remarkable decline in normal load and friction bearing capacity. In this work, we experimentally quantified the maximal normal and friction forces corresponding to the damage thresholds of the different investigated defects as well as their pull-out (adhesion) forces. Horizontal wrinkles (with respect to the basal plane, i.e. folded) sustained the highest normal load, up to 317 nN, during sliding, whereas for vertical (i.e. standing collapsed) wrinkles, step-edges and edges, the load bearing capacities are up to 113 nN, 74 nN and 63±5 nN, respectively. The related deformation mechanisms were also experimentally investigated by varying the normal load up to the initiation of the damage from the investigated defects and extended with the numerical results from Molecular Dynamics and Finite Element Method simulations

High temperature quasistatic and dynamic mechanical behavior of interconnected 3D carbon nanotube structures

CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL E NÍVEL SUPERIORFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCarbon nanotubes (CNTs) are one of the most appealing materials in recent history for both research and commercial interest because of their outstanding physical, chemical, and electrical properties. This is particularly true for 3D arrangements of CNTs which enable their use in larger scale devices and structures. In this paper, the effect of temperature on the quasistatic and dynamic deformation behavior of 3D CNT structures is presented for the first time. An in situ high-temperature nanomechanical instrument was used inside an SEM at high vacuum to investigate mechanical properties of covalently interconnected CNT porous structures in a wide range of temperature. An irreversible bucking at the base of pillar samples was found as a major mode of deformation at room and elevated temperatures. It has been observed that elastic modulus and critical load to first buckle formation decrease progressively with increasing temperature from 25 degrees C to 750 degrees C. To understand fatigue resistance, pillars made from this unique structure were compressed to 100 cycles at room temperature and 750 degrees C. While the structure showed remarkable resistance to fatigue at room temperature, high temperature significantly lowers fatigue resistance. Molecular dynamics (MD) simulation of compression highlights the critical role played by covalent interconnections which prevent localized bending and improve mechanical properties.142291299CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL E NÍVEL SUPERIORFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL E NÍVEL SUPERIORFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOSem informaçãoSem informação2013/08293-7S.O. acknowledges financial support from a LANL Director's Postdoctoral Fellowship. LDM, and DSG acknowledge the Brazilian Research Agencies CNPq, CAPES, and FAPESP for financial support. DSG also acknowledges the Center for Computational Engineering and Sciences at Unicamp through the FAPESP/CEPID Grant No. 2013/08293-7, for computational and financial support. N.M.P. is supported by the European Commission H2020 under the Graphene Flagship Core 2 grant no. 785219 (WP14, Composites) and under the FET Proactive ("Neurofibres. no. 732344), as well as by the Italian Ministry of Education, University and Research (MIUR) under the "Departments of Excellence" grant no. L.232/2016 and by Fondazione Caritro under "Self-Cleaning Glasses" no. 2016.0278. R.A.B. acknowledges financial support from Fondazione Caritro