Zhen Chen

Generation of fast propagating combustion and shock waves with copper oxide/aluminum nanothermite composites

Nanothermite composites containing metallic fuel and inorganic oxidizer are gaining importance due to their outstanding combustion characteristics. In this paper, the combustion behaviors of copper oxide/aluminum nanothermites are discussed. CuO nanorods were synthesized using the surfactant-templating method, then mixed or self-assembled with Al nanoparticles. This nanoscale mixing resulted in a large interfacial contact area between fuel and oxidizer. As a result, the reaction of the low density nanothermite composite leads to a fast propagating combustion, generating shock waves with Mach numbers up to 3.

Atomistic study of the mechanical response of copper nanowires under torsion

The length, loading rate and thermal effects on the torsional response of hollow copper nanowires are investigated with molecular dynamics simulation. Evolution of atomic configuration is studied, which shows that partial dislocations nucleated from the surfaces accommodate the plastic deformation of the nanowires under torsion. With the increase in torsional angle, necking appears and the corresponding cross-section transforms from a hollow square to a solid circle. Meanwhile, atomic rearrangement from being amorphous to fcc occurs, which becomes more obvious at higher loading rates. To understand the relation between material and geometrical instabilities, the torsional buckling mode is also investigated and found to strongly depend on both wire length and temperature.

An evaluation of the MPM for simulating dynamic failure with damage diffusion

Abstract For dynamic brittle failure, conventional mesh-based methods, such as the finite element method and finite difference method, are handicapped when localized large deformations and subsequent transitions from continuous to discontinuous failure modes occur. To evaluate the potential of the material point method (MPM) in simulating dynamic brittle failure involving different failure modes, the essential features of the MPM are explored for wave and impact problems, and combined wave and diffusion problems are then solved by using the MPM. Through the comparison with the experimental, analytical and numerical data available, it appears that the MPM is a robust tool to simulate multi-physics problems such as dynamic failure under impact.

The Sandia Fracture Challenge: blind round robin predictions of ductile tearing

Existing and emerging methods in computational mechanics are rarely validated against problems with an unknown outcome. For this reason, Sandia National Laboratories, in partnership with US National Science Foundation and Naval Surface Warfare Center Carderock Division, launched a computational challenge in mid-summer, 2012. Researchers and engineers were invited to predict crack initiation and propagation in a simple but novel geometry fabricated from a common off-the-shelf commercial engineering alloy. The goal of this international Sandia Fracture Challenge was to benchmark the capabilities for the prediction of deformation and damage evolution associated with ductile tearing in structural metals, including physics models, computational methods, and numerical implementations currently available in the computational fracture community. Thirteen teams participated, reporting blind predictions for the outcome of the Challenge. The simulations and experiments were performed independently and kept confidential. The methods for fracture prediction taken by the thirteen teams ranged from very simple engineering calculations to complicated multiscale simulations. The wide variation in modeling results showed a striking lack of consistency across research groups in addressing problems of ductile fracture. While some methods were more successful than others, it is clear that the problem of ductile fracture prediction continues to be challenging. Specific areas of deficiency have been identified through this effort. Also, the effort has underscored the need for additional blind prediction-based assessments.

An investigation of the effect of interfacial atomic potential on the stress transition in thin films

In order to better understand the mechanisms of tungsten (W) film delamination from the silicon (Si) substrate, a three-dimensional molecular dynamics (MD) simulation is being conducted to investigate the formation of residual stress during the film deposition process. For the purpose of simplicity, a Morse pair potential is proposed in this paper to simulate the interactions between W and Si atoms during the film deposition process. It appears from numerical solutions that the residual stress field in the W film is very sensitive to the W–Si interfacial potential model proposed for the MD simulation. By calibrating the controlling parameters in the interfacial potential model using the comparison between the simulated stresses and experimental data, the film stress transition from tension to compression during the film deposition process could be qualitatively simulated via the proposed simulation procedure. The numerical results presented in this paper provide a better insight into the effect of interfacial atomic potential on the stress transition in thin films. In addition, it can be seen from the MD simulation that there might exist a phase transition from the crystalline Si to amorphous W–Si structure to crystalline W around the interface area. Well-designed experiments are required to verify the simulation results.

An analytical and numerical study of failure waves

Based on recent observations in shock experiments on glasses, a new failure process has been suggested for a certain type of brittle solids, in which a failure wave propagates through a solid at some distance behind the compressive stress wave near but below the Hugoniot elastic limit. Since the failure wave phenomenon is different from the usual inelastic shock waves, a combined analytical and numerical effort is made in this paper to explore the impact failure mechanisms associated with the failure wave. Based on the experimental data available, it appears that the physical picture of failure wave is related to local dilatation due to shear-induced microcracking. A mathematical argument then leads to the conclusion that the failure wave should be described by a diffusion equation instead of a wave equation, which is in line with the bifurcation analysis for localization problems. However, the occurrence of different governing equations in a single computational domain imposes both an analytical and a numerical challenge on the design of an efficient solution scheme. With the use of a partitioned-modeling approach, a simple solution procedure is proposed for failure wave problems, which is verified by the comparison with data.

Monte Carlo simulation of grain growth in two-phase nanocrystalline materials

The normal grain growth in volume-conserved two-phase nanocrystalline materials is studied using a modified Potts model, in which the grain boundary migration is driven by the interfacial energy between two phases and the grain boundary energy inside each phase. Monte Carlo simulation results show that the grain growth of one phase is constrained by the presence of the other phase. The power-law grain growth kinetics with an almost temperature-independent exponent of 0.16±0.01 (0.5 in a pure single-phase system) is predicted for two immiscible phases, which is in agreement with experimental observations.

A Numerical Study of the Size and Rate Effects on the Mechanical Response of Single Crystal Diamond and UNCD Films

To better understand the mechanical response of ultrananocrystalline diamond (UNCD) and its grain boundary mechanism, a numerical study is performed of the specimen size and rate effects on the mechanical properties of single crystal diamond and UNCD films under uniaxial and shear loading paths, respectively. To compare with the UNCD films, single crystal diamond blocks of various sizes under tensile loading in the 100 hi direction and shear loading with the {100} 110 hi slip at different rates are first investigated via the molecular dynamics (MD) simulation. A combined kinetic Monte Carlo (KMC) and MD procedure is then developed for large-scale atomistic simulation of the mechanical response of UNCD films. In this numerical procedure, two single crystal diamond films, that are formed by the KMC method based on the mechanisms of UNCD growth from carbon dimers on the hydrogen-free (001) surface, are compressed along the [001] direction with two growth surfaces contacting each other at an elevated temperature in the MD simulation to create a polycrystalline UNCD film with certain grain boundary. The mechanical response of the resulting UNCD film is investigated by applying displacement-controlled shear loading in the MD simulation, and is compared with that of single crystal diamond. The preliminary results presented in this article provide a better understanding of the size and rate effects on the material properties of diamond and the role played by the grain boundary on influencing the mechanical response of UNCD films.

Loading path effect on the mechanical behaviour and fivefold twinning of copper nanowires

The effect of loading path on the mechanical behaviour of single crystalline copper nanowires is investigated with molecular dynamics simulations. Different loading conditions including pre-tensile torsion and pre-torsional tension at different temperatures are taken into consideration. It is found that elastic pre-loading conditions can induce a distinct weakening on the resistance against plastic deformation under later applied loads. Meanwhile, coupled thermal and pre-loading effect can also facilitate the transformation from elasticity to plasticity. Formations of fivefold twins are observed in copper nanowires subjected to the loading path with tension after pre-torsion. These fivefold twins all form at the necking stage before fracture, and are found to be pre-torsion- and size-dependent but insensitive to the change in temperature and cross-sectional shape. The results reported here indicate that the loading path effect on the mechanical behaviour plays an important role in the formation of some special microstructures such as multiple twins in metallic nanowires.

Size effects on the impact response of copper nanobeams

Molecular dynamics simulations are performed to study size effects on the impact response of copper nanobeam targets subjected to impacts by copper nanobeam flyers with different impact velocities. It is found that the Hugoniot response is size-dependent, while the aspect ratio – that is, the ratio of flyer and target nanobeam heights – has a small effect. It is also observed that the propagation speed of a disordering front generated at the impact surface is close to the shock wave speed initially, but decreases as dislocations form. The thermal gradient in the target is mainly due to the quasi-temperature difference (transient spatial localization of kinetic energy) between hexagonal-close-packed atoms and face-centered-cubic atoms. The findings for the impact stress, defect evolution, and quasi-temperature could be useful for better understanding the responses of nanosystems to extreme loading conditions.