Recent advances in experimental techniques have enabled impact tests of ultrathin films. For example, microprojectile impact tests of ultrathin polymer films have revealed that their specific penetration energy is about ten times more than that of the conventional armor materials. On the other hand, metallic nanostructures have demonstrated extraordinary mechanical properties. These observations suggest that multilayer arrangements of nanoscale polymer and metal films could possess superior ballistic impact resistance. In order to test this hypothesis, we simulated the impact tests of multilayer aluminum-polyurea nanostructures using molecular dynamics (MD). Our simulations demonstrate that the ballistic limit velocity (V 50 ) and the specific penetration energy of the multilayers and aluminum nanofilms are significantly higher than the experimentally measured values for any material. In order to further investigate the mechanisms associated with the observed superior ballistic performance of multilayers, we computed their V 50 using an existing membrane model and another analytical model reflecting a two-stage penetration process. Our results demonstrate a potential bottom-up design pathway for developing flexible barrier materials with superior dynamic penetration resistance.

Polytetrafluoroethylene (PTFE) has been studied as a low friction surface coating since its discovery. The high wear-rate of PTFE reduces the usefulness of the polymer for mechanical purposes; however, combining PTFE with polydopamine (PDA) has been shown to greatly reduce the film wear-rate. During rubbing tests involving PDA/PTFE thin films, a tenacious layer of PTFE remains intact after substantial testing even though pure PTFE film layers are destroyed quickly. Understanding the interface mechanics that allow PTFE and PDA to adhere so well during experimental rubbing tests is necessary to improve the wear-rate of PDA/PTFE thin films. In this study, we use density functional theory (DFT) and molecular dynamics (MD) simulations to investigate the adhesive properties and interface deformation mechanisms between PDA and PTFE molecules. Steered molecular dynamics (SMD) is then performed on isolated pairs of PDA and PTFE molecules to investigate different modes of deformation from equilibrium. PDA trimer oligomers were identified as the most adhesive to PTFE and selected to use in a PDA/PTFE thin film, where nano-indentation and scratch tests are performed. Our results indicate that a combination of the unique deformation mechanisms of PDA molecules and the penetration of PTFE molecules into the PDA substrate provide the PTFE/PDA interface with its wear resistance.

Massively parallel molecular dynamics (MD) simulations have been performed to understand the plastic deformation of metals. However, the intricate interplay between the deformation mechanisms and the various material properties is largely unknown in alloy systems for the limited available interatomic potentials. We adopt the meta-atom method proposed by Wang et al., which unifies MD simulations of both pure metals and alloys in the framework of the embedded atom method (EAM). Owing to the universality of EAM for metallic systems, meta-atom potentials can fit properties of different classes of alloys. Meta-atom potentials for both aluminum bronzes and hypothetic face-centered-cubic (FCC) metals have been formulated to study the parametric dependence of deformation mechanisms, which captures the essence of competitions between dislocation motion and twinning or cleavage. Moreover, the solid-solution strengthening effect can be simply accounted by introducing a scaling factor in the meta-atom method. As the computational power enlarges, this method can extend the capability of massively parallel MD simulations in understanding the mechanical behaviors of alloys. The calculation of macroscopic measurable quantities for engineering oriented alloys is expected to be possible in this way, shedding light on constructing materials with specific mechanical properties.

In this paper, a hybrid quasi-static atomistic simulation method at finite temperature is developed, which combines the advantages of MD for thermal equilibrium and atomic-scale finite element method (AFEM) for efficient equilibration. Some temperature effects are embedded in static AFEM simulation by applying the virtual and equivalent thermal disturbance forces extracted from MD. Alternatively performing MD and AFEM can quickly obtain a series of thermodynamic equilibrium configurations such that a quasi-static process is modeled. Moreover, a stirring-accelerated MD/AFEM fast relaxation approach is proposed in which the atomic forces and velocities are randomly exchanged to artificially accelerate the “slow processes” such as mechanical wave propagation and thermal diffusion. The efficiency of the proposed methods is demonstrated by numerical examples on single wall carbon nanotubes.

We postulate that an equivalent continuum structure (ECS) of a single-walled carbon nanotube (SWCNT) is a hollow cylinder with mean radius and length equal to that of the SWCNT, and find the thickness of the ECS so that its mechanical response in free vibrations is the same as that of the SWCNT. That is, for mechanical deformations, the ECS is energetically equivalent to the SWCNT. We use MM3 potential to study axial, torsional, radial breathing and bending vibrations of several traction free–traction free SWCNTs of different helicities and diameters and compare them with the corresponding vibrational modes and frequencies of traction free–traction free ECSs obtained by using the three-dimensional linear elasticity theory and the finite element analysis (3D-FEA). The consideration of free ends eliminates the effects of boundary conditions and avoids resolving equivalence between boundary conditions in the analyses of SWCNTs and their ECSs. It is found that the wall thickness of the ECS (and hence of a SWCNT) is ∼ 1 Å and Young’s modulus of the material of the ECS (and hence of the SWCNT) is ∼ 3.3 TPa . Both quantities are independent of the helicity and the diameter of the SWCNT. We also study radial breathing mode (RBM) vibrations with the molecular dynamics and the 3D-FEA simulations, and compare them with experimental findings. Accuracy in the assignment of spectral lines for RBMs in the Raman spectroscopy is discussed.

The large number of degrees-of-freedom of finite difference, finite element, or molecular dynamics models for complex systems is often a significant barrier to both efficient computation and increased understanding of the relevant phenomena. Thus there is a benefit to constructing reduced-order models with many fewer degrees-of-freedom that retain the same accuracy as the original model. Constructing reduced-order models for linear dynamical systems relies substantially on the existence of global modes such as eigenmodes where a relatively small number of these modes may be sufficient to describe the response of the total system. For systems with very many degrees-of-freedom that arise from spatial discretization of partial differential equation models, computing the eigenmodes themselves may be the major challenge. In such cases the use of alternative modal models based upon proper orthogonal decomposition or singular value decomposition have proven very useful. In the present paper another facet of reduced-order modeling is examined, i.e., the effects of “local” nonlinearity at the nanoscale. The focus is on nanoscale devices where it will be shown that a combination of global modal and local discrete coordinates may be most effective in constructing reduced-order models from both a conceptual and computational perspective. Such reduced-order models offer the possibility of reducing computational model size and cost by several orders of magnitude.

The isothermal compression, up to nearly half the liquid densities, of argon, nitrogen, and methane at three different temperatures ranging from 0–150 deg C are investigated by using the cell method originally developed by Lennard-Jones and Devonshire. A two-potential model is adopted in the evaluation of the classical partition function to partially take into account the contributions arising from the correlations of molecular motions. The Lennard-Jones 6–12 potential and the Kihara hard core model are used, respectively, for the calculation of the energy of the geometrically symmetric lattice and the energy of augmentation arising from molecular motions. The potential parameters used in the calculations are those derived from fitting the second virial coefficient of dilute gases. The resulting calculated isotherms are compared with experimental measurements; agreement obtained is good for the range of density considered. The Kihara hard-core model with parameters determined for dilute gases is found to be ineffective at very high densities. This agrees with the conclusion derived from the Monte Carlo and molecular dynamics calculations that the Lennard-Jones 12–6 potential is tolerably good in representing the effective pair potential at liquid densities.