On the nucleation and growth of voids at high strain-rates

The nucleation and growth of voids at high strain-rate is studied in copper as a model face centered cubic (fcc) material using large scale molecular dynamics (MD) methods. After a brief introduction to dynamic fracture, results are presented for the homogeneous nucleation of voids in single crystal copper and the heterogeneous nucleation in nanoscale polycrystalline copper. The simulations suggest void growth occurs through anisotropic dislocation nucleation and emission in agreement with experiment and the observed anisotropy of the tensile flow stress in fcc crystals. A phenomenological model for the transition from intergranular to transgranular fracture at high strain-rate is presented. This revised version was published online in July 2006 with corrections to the Cover Date.     

Molecular dynamics simulations of spallation in metals and alloys

We report on the results of large-scale molecular dynamics simulations of the mechanical behavior of two-dimensional metallic systems. The specific impact phenomenon studied is that in which a flyer of mass M moving with x-velocity v impacts a target of mass 2M moving with x-velocity −v/2. Simulations of such a spallation experiment have been performed for a generic metal, modelled with an embedded atom potential and also for a Cu-Ni alloy system, modelled with truncated Lennard-Jones potentials.

Our simulations indicate cold-welding upon impact, and shock wave generation, followed by rebound from the boundaries. The alloy was less ductile than the generic metal and consequently the system came apart due to the cooperative effect of the reflected shock waves.     

Spallation caused by the diffusion and agglomeration of vacancies in ductile metals

In this paper, the spallation process for the ductile metals under plane shock loading is discussed in theory. By employing the phase transition theory and non-equilibrium theory, the spallation process may be understood as a result of the diffusion and agglomeration of the generated vacancies. Through the detailed theoretical analysis, the following important points are concluded: (1) the spalling temperature, a new concept, is proposed first and the appearance of spallation critical behavior is proved; (2) the quantitative grain size, tensile strain rate and temperature dependence of both the damage evolution rate and the void growth velocity is obtained; (3) the existence of a characteristic size for the voids and a characteristic stress at the void boundary is discovered first, and their magnitude depend on the vacancy excitation energy and the average volume of one vacancy; (4) the temperature of metal near the growing void is found to be high, possibly causing the metal to melt, and it decreases quickly with the distance away from the void; (5) the area of the plastic zone, surrounding one formed spherical void, is clarified; (6) the viewpoint is put forward that the void growth may arise from the agglomeration of vacancies rather than the emission of dislocations when the shocking temperature approaches spalling temperature. Most of the above theoretical results are novel and obtained first.     

Molecular dynamics study of thermal properties of noble metals

Molecular dynamics simulations have been applied to investigate thermal properties of Ag and Au. Semi-empirical potentials, based on the embedded atom method (EAM) have been employed to calculate lattice parameter, energy per atom, mean square displacements and radial distribution function for the two metals. Thermal properties like specific heat, thermal coefficient of linear expansion and melting temperature are deduced from the calculated parameters. Results are found to compare well with the experimental results.     

Multi-scale applications to high strain-rate dynamic fracture

Though the bulk of the Bodega Bay Multi-Scale Modeling workshop was devoted to understanding flow stress within the multi-scale paradigm, a dedicated session was devoted to Dynamic Failure and Fracture. Here, we review recent developments with emphasis on work presented at the workshop in the area of ductile dynamic fracture. The paper begins with a discussion of the relevant experimental observations, followed by an overview of the mechanisms of void nucleation and growth at high strain-rate, including dislocation processes (see the companion review by Bulatov in this issue). While the connection to the continuum is at its infancy, we present some directions that hold promise. Shear localization from the continuum perspective is presented in the companion review by Becker et al. This section finishes with a brief summary of issues that need to be resolved to apply the full apparatus of multi-scale modeling to dynamic fracture. This revised version was published online in June 2006 with corrections to the Cover Date.     

A spectral scheme for the simulation of dynamic mode 3 delamination of thin films

Motivated by recent developments in laser-induced spallation testing of thin film structures, we develop a spectral scheme for the simulation of dynamic failure of thin films. In this first study, we focus on the anti-plane shear (mode 3) loading case. The scheme relies on an exact spectral representation of the elastodynamic solutions in the substrate and in the film, and their combination through interface conditions that involve general cohesive failure and/or frictional contact models. A detailed modal analysis of the response of a single spectral mode is performed to assess the stability and precision of the resulting numerical scheme. A set of dynamic fracture problems involving non-propagating and propagating cracks are simulated to show the ability of the numerical scheme to capture the effect of wave reflection on the near-tip stress and displacement fields, and on the dynamic motion of a crack along the film/substrate interface.     

Computer simulation of point defects in fcc metals using EAM potentials

The activation energies for the motion of interstitials and vacancies, vacancy clusters have been calculated using embedded atom potentials. The most exciting results obtained are that the stable interstitial configuration is a [1 0 0] split type and the saddle point was a body-centered interstitial. This is quite different from widely accepted computational results. The activation energies for the motion of vacancies, di-vacancies and tri-vacancies have been calculated. The atom above the center of a 60° tri-vacancy relaxes to the center of the tetrahedron in copper but does not relax to the center of the tetrahedron in silver. In the case of gold the energy relaxed to the center is very small and the atoms move around in the tetrahedron. The details depend upon the potentials.     

Fabrication and characterization of poly-d-l-lactide/nano-hydroxyapatite composite scaffolds with poly (ethylene glycol) coating and dexamethasone releasing

The control of pore size and structure, drug release capacity, and biodegradation of scaffolds is of importance for bone tissue engineering. In this study, a technique combining polymer coagulation, cold compression molding, salt particulate leaching and drug coating method was developed to fabricate poly (ethylene glycol)/dexamethasone coated porous poly-d-l-lactide/nano-hydroxyapatite (PDLLA/nano-HAp) scaffolds. These scaffolds possess homogenous pore networks with high porosity (66-82%) and controllable pore size (200-300 μm). The compressive moduli and strength of the scaffolds after incorporation of nano-HAp were improved by 50% and 20%, respectively. The surface hydrophilicity of the scaffold was significantly improved by poly (ethylene glycol)/dexamethasone coating and nano-HAp addition, leading to a higher initial drug loading amount. The results showed that the drug release behavior of the scaffolds after 35-day immersion in water could be adjusted by varying the porosity level and by incorporation of 20 wt.% of nano-HAp.     

修正Tersoff势在熔态硅分子动力学模拟中的应用

采用Ｔｅｒｓｏｆｆ 势,通过粒子间相互作用距离Ｒ、Ｓ的不同取值加以修正,并进行了液态硅的分子动力学模拟。模拟的结果表明其径向分布函数能与Ｘ 射线衍射、中子散射实验相一致。模拟得到液态硅配位数６－９ ,键长０－２５４ｎｍ 。键角几率分布出现两个峰值,～５７°和～１０２°,表明液态硅中存在复杂的结构。模拟结果还表明,液态硅中硅原子间联接成一种网络状结构,但大多数硅原子与其近邻硅原子仍保持近似于正四面体的局部结构。     

Lattice distortion and thermal stability of nano-crystalline copper

Molecular dynamics simulations of high temperature annealing of copper bicrystals with varying grain sizes in nano-meter range have been carried out. Planar 1 1 1-tilt CSL grain boundaries are set. An EAM potential of FS type is used for calculating inter-atomic forces in copper. For comparison, similar simulations for aluminum and tungsten have been conducted. The results show that in the copper bicrystals of present grain boundary geometry, mismatch between the {1 1 1} planes of the neighboring grains occurs at the grain boundary, resulting in a general shear lattice distortion within the grains. The shear strain is inversely proportional to the grain size. The energy of such mismatched grain boundary is found lower than that of the mismatch-free grain boundary. For aluminum such kind of mismatch is much smaller, and for tungsten no such mismatch appears. The nano-sized copper bicrystals with grains smaller than a critical size are found instable at high temperature, where grain boundary motion and atomistic reconstruction lead to annihilation of the grain boundaries after an incubation time.     

分子结构对高分子材料动态力学性能影响的研究进展

从材料的阻尼性能的表征出发,系统总结了高分子材料阻尼机理研究状况。主要以LA和TA分析法为例,从分子设计角度,总结阐述了近年来有关分子结构对高分子材料动态力学性能影响的研究成果和存在的问题,并对未来此方面的研究进行了简单的预测。     

Buckling of defective single-walled and double-walled carbon nanotubes under axial compression by molecular dynamics simulation

Buckling of defective single-walled and double-walled carbon nanotubes (SWCNTs and DWCNTs, respectively) due to axial compressive loads has been studied by molecular dynamics simulations, and results compared with those of the perfect structures. It is found that single vacancy defect greatly weakens the carrying capacity of SWCNTs and DWCNTs, though it does slight harm to the effective elastic modulus of the tubes. The influence of defects on the buckling properties of nanotubes is related to the density of the defects, and the relative position of defects also plays an important role in buckling of DWCNTs. The van der Waals force among atoms in the inner and the outer tubes of short defective DWCNTs makes the critical buckling strain of DWCNTs greater than that of the inner tube.     

Multiresolution atomistic simulations of dynamic fracture in nanostructured ceramics and glasses

Multimillion atom molecular-dynamics (MD) simulations are performed to investigate dynamic fracture in glasses and nanostructured ceramics. Using multiresolution algorithms, simulations are carried out for up to 70 ps on massively parallel computers. MD results in amorphous silica (a-SiO₂) reveal the formation of nanoscale cavities ahead of the crack tip. With an increase in applied strain, these cavities grow and coalesce and their coalescence with the advancing crack causes fracture in the system. Recent AFM studies of glasses confirm this behavior. The MD value for the critical stress intensity factor of a-SiO₂ is in good agreement with experiments. Molecular dynamics simulations are also performed for nanostructured silicon nitride (n-Si₃N₄). Structural correlations in n-Si₃N₄ reveal that interfacial regions between nanoparticles are amorphous. Under an external strain, nanoscale cavities nucleate and grow in interfacial regions while the crack meanders through these regions. The fracture toughness of n-Si₃N₄ is found to be six times larger than that of crystalline -Si₃N₄. We also investigate the morphology of fracture surfaces. MD results reveal that fracture surfaces of n-Si₃N₄ are characterized by roughness exponents 0.58 below and 0.84 above a certain crossover length, which is of the order of the size of Si₃N₄ nanoparticles. Experiments on a variety of materials reveal this behavior. The final set of simulations deals with the interaction of water with a crack in strained silicon. These simulations couple MD with a quantum-mechanical (QM) method based on the density functional theory (DFT) so that chemical processes are included. For stress intensity factor K=0.4 MPa m^1/2, we find that a decomposed water molecule becomes attached to dangling bonds at the crack or forms a Si-O-Si structure. At K=0.5 MPa m^1/2, water molecules decompose to oxidize Si or break Si-Si bonds.     

Indentation of a laminated composite plate with an interlayer rectangular void

We employ the Eshelby–Stroh formalism to study generalized plane strain infinitesimal deformations caused due to the indentation by a rigid circular cylinder of an elastic laminated plate with a through-the-width rectangular void between two adjoining layers. Assuming that the void does not close during the indentation process, we find the indentation modulus (i.e., the slope of the indentation load vs. the indentation depth curve) as a function of the void size, the void position, elastic moduli of the layers, and boundary conditions at the edges. The change in the indentation modulus caused by an interlayer void parallel to the major surfaces of an anisotropic plate can potentially be used to estimate the void size and location.