Adaptive multiscale method for two-dimensional nanoscale adhesive contacts

There are two separate traditional approaches to model contact problems: continuum and atomistic theory. Continuum theory is successfully used in many domains, but when the scale of the model comes to nanometer, continuum approximation meets challenges. Atomistic theory can catch the detailed behaviors of an individual atom by using molecular dynamics (MD) or quantum mechanics, although accurately, it is usually time-consuming. A multiscale method coupled MD and finite element (FE) is presented. To mesh the FE region automatically, an adaptive method based on the strain energy gradient is introduced to the multiscale method to constitute an adaptive multiscale method. Utilizing the proposed method, adhesive contacts between a rigid cylinder and an elastic substrate are studied, and the results are compared with full MD simulations. The process of FE meshes refinement shows that adaptive multiscale method can make FE mesh generation more flexible. Comparison of the displacements of boundary atoms in the overlap region with the results from full MD simulations indicates that adaptive multiscale method can transfer displacements effectively. Displacements of atoms and FE nodes on the center line of the multiscale model agree well with that of atoms in full MD simulations, which shows the continuity in the overlap region. Furthermore, the Von Mises stress contours and contact force distributions in the contact region are almost same as full MD simulations. The method presented combines multiscale method and adaptive technique, and can provide a more effective way to multiscale method and to the investigation on nanoscale contact problems.     

A Combined Molecular Dynamics and Finite Element Analysis of Contact and Adhesion of a Rough Sphere and a Flat Surface

A combined molecular dynamics and finite element model and simulation of contact and adhesion between a rough sphere and a flat surface has been developed. This model uses the results of molecular dynamics (MD) simulations, obtained using an embedded atom potential, of a nanoscale Ru-Ru asperity contact. A continuum finite element model of an elastic–plastic microscale Ru-Ru contact bump is then created. In this model, the surface roughness is represented by a system of nanoscale asperities, each of which is represented by a nonlinear hysteretic force vs. distance relationship. The nonlinear hysteretic character of these relations is determined from curve-fits of the MD results. Load vs. interference and contact area vs. interference are determined using this two-scale model for loading and unloading. Comparisons with a single-scale continuum model show that the effect of the nanoscale asperities is to reduce both the adhesion and the real area of contact. The choice of Ru as the material for this work is due to its relevance in microswitches.     

A Local Region Molecular Dynamics Simulation Method for Nanoscale Sliding Contacts

Computational efficiency and accuracy always conflict with each other in molecular dynamics (MD) simulations. How to enhance the computational efficiency and keep accuracy at the same time is concerned by each corresponding researcher. However, most of the current studies focus on MD algorithms, and if the scale of MD model could be reduced, the algorithms would be more meaningful. A local region molecular dynamics (LRMD) simulation method which can meet these two factors concurrently in nanoscale sliding contacts is developed in this paper. Full MD simulation is used to simulate indentation process before sliding. A criterion called contribution of displacement is presented, which is used to determine the effective local region in the MD model after indentation. By using the local region, nanoscale sliding contact between a rigid cylindrical tip and an elastic substrate is investigated. Two two-dimensional MD models are presented, and the friction forces from LRMD simulations agree well with that from full MD simulations, which testifies the effectiveness of the LRMD simulation method for two-dimensional cases. A three-dimensional MD model for sliding contacts is developed then to show the validity of the LRMD simulation method further. Finally, a discussion is carried out by the principles of tribology. In the discussion, two two-dimensional full MD models are used to simulate the nanoscale sliding contact problems. The results indicate that original smaller model will induce higher equivalent scratching depth, and then results in higher friction forces, which will help to explain the mechanism how the LRMD simulation method works. This method can be used to reduce the scale of MD model in large scale simulations, and it will enhance the computational efficiency without losing accuracy during the simulation of nanoscale sliding contacts.     

A Local Region Molecular Dynamics Simulation Method for Nanoscale Sliding Contacts

Computational e ciency and accuracy always conflict with each other in molecular dynamics(MD) simulations. How to enhance the computational e ciency and keep accuracy at the same time is concerned by each corresponding researcher. However, most of the current studies focus on MD algorithms, and if the scale of MD model could be reduced, the algorithms would be more meaningful. A local region molecular dynamics(LRMD) simulation method which can meet these two factors concurrently in nanoscale sliding contacts is developed in this paper. Full MD simulation is used to simulate indentation process before sliding. A criterion called contribution of displacement is presented, which is used to determine the e ective local region in the MD model after indentation. By using the local region, nanoscale sliding contact between a rigid cylindrical tip and an elastic substrate is investigated. Two two?dimensional MD models are presented, and the friction forces from LRMD simulations agree well with that from full MD simulations, which testifies the e ectiveness of the LRMD simulation method for two?dimensional cases. A three?dimensional MD model for sliding contacts is developed then to show the validity of the LRMD simulation method further. Finally, a discussion is carried out by the principles of tribology. In the discussion, two two?dimensional full MD models are used to simulate the nanoscale sliding contact problems. The results indicate that original smaller model will induce higher equivalent scratching depth, and then results in higher friction forces, which will help to explain the mechanism how the LRMD simulation method works. This method can be used to reduce the scale of MD model in large scale simulations, and it will enhance the computational e ciency without losing accuracy during the simula?tion of nanoscale sliding contacts.     

Molecular Dynamics Study on Friction Due to Ploughing and Adhesion in Nanometric Scratching Process

In this study, three-dimensional MD simulations are carried out to study the nanometric scratching process. The ploughing friction coefficient and the adhesion friction coefficient are distinguished for the first time using MD simulations. The contribution of chip to friction coefficient is also evaluated. The simulation results show that the macroscale theory can qualitatively evaluate the ploughing friction coefficient, but it slightly overestimates the ploughing friction coefficient on the nanoscale for the scratching depths studied. It is found that the adhesion friction coefficient is independent of the scratching depth as predicted by macroscale theory. It is also found that the contribution of chip to friction coefficient is independent of the scratching depth and cannot be neglected on the nanoscale.     

某超临界锅炉吹灰器罩壳内水冷壁管表面裂纹产生原因

某超临界锅炉运行1.2万h后,在其吹灰器罩壳内水冷壁管背火面发现密集横向裂纹.通过资料调研、材料成分检验以及开裂部位显微组织、断口形貌和拉伸性能分析,研究了水冷壁管表面裂纹产生的原因.结果表明:水冷壁管材料正常,其背火面管外壁裂纹为热疲劳裂纹;吹灰器套管外径偏大顶在吹灰孔周边水冷壁管上,温度偏低导致吹灰蒸汽带水,水沿着...     

Evaporation and disjoining pressure of ultrathin film on substrate: a molecular dynamics study

The equilibrium evaporation of the thin liquid film on a solid substrate is studied by performing molecular dynamics simulation (MD). The evaporation properties are obtained for a simple atomic system in three-phase coexistence and compared with those of the modeling methods based on the disjoining pressure. The thickness of the film is varied and goes down to as small as a few molecular diameters. The MD results show that the evaporation rate of the ultrathin film on a high-energy substrate is smaller than that of a macroscopic film because of the solid molecular potential effective over the film. The comparison with the mean field theory based on the kinetic theory of gases and the classical Hamaker theory of disjoining pressure shows a sizable discrepancy when the film thickness is comparable to the size of the interfacial region. The discrepancy mostly disappears when the disjoining pressure is derived by using the static properties from MD. This indicates that the evaporation modeling on ultrathin film of which the disjoining pressure is an integral part critically requires an accurate representation of the property.     

Molecular Dynamic Studies on Deformation of Polymer Resist During Thermal Nano Imprint Lithographic Process

Molecular dynamic simulation of nano imprint lithography (NIL) in which nanoscale patterned stamp is pressed onto amorphous polyethylene (PE) surface is performed to study the deformation behavior of polymer resist. Force fields including bond, angle, torsion, and Lennard Jones potential are used to describe the inter-molecular and intra-molecular forces of PE molecules and stamp. Periodic boundary condition is used in horizontal direction and canonical NVT ensemble is employed to control the system temperature. Using the simulation results, the behavior of polymer resist is investigated during the imprinting process. The mechanism of resist deformation is analyzed by considering various parameters including the surface geometry, atom distribution, and density. Especially, the density in the bottom (emboss) region is found to be larger than the top (cavity) region due to compression of polymer molecules. The result indicates that small scale patterning of polymer resist largely depends on compression rather than the flow of molecules. The numerical results are compared with the local density measurement data using the atomic force microscope (AFM). They exhibit a similar behavior at least in a qualitative sense. Additional simulations, where the stamp geometry and molecular mobility are varied and are performed to investigate the deformation characteristics of polymer resist. From these simulations, important elements for understanding of NIL process are obtained.     

Calibration of the analytical nonlocal shell model for vibrations of double-walled carbon nanotubes with arbitrary boundary conditions using molecular dynamics

In the present study, the free vibration response of double-walled carbon nanotubes (DWCNTs) is investigated. Eringen's nonlocal elasticity equations are incorporated into the classical Donnell shell theory accounting for small scale effects. The Rayleigh-Ritz technique is applied to consider different sets of boundary conditions. The displacements are represented as functions of polynomial series to implement the Rayleigh-Ritz method to the governing differential equations of nonlocal shell model and obtain the natural frequencies of DWCNTs relevant to different values of nonlocal parameter and aspect ratio. To extract the proper values of nonlocal parameter, molecular dynamics (MD) simulations are employed for various armchair and zigzag DWCNTs, the results of which are matched with those of nonlocal continuum model through a nonlinear least square fitting procedure. It is found that the present nonlocal elastic shell model with its appropriate values of nonlocal parameter has the capability to predict the free vibration behavior of DWCNTs, which is comparable with the results of MD simulations.     

单晶硅磨削过程分子动力学仿真并行算法

建立单晶硅超精密磨削过程的三维分子动力学仿真模型,分析分子动力学仿真串行程序特点和并行仿真的可行性,提出基于区域二次划分的分子动力学仿真并行算法.编制并行仿真程序,进行分子动力学仿真,从瞬间原子位置图方面分析单晶硅超精密磨削过程的加工机理.将并行仿真结果与串行程序仿真结果进行对比分析,从瞬间原子位置图和系统能量方面验证并行程序结果的正确性,在仿真规模和计算时间方面并行程序有很大优势,从而说明并行仿真程序是有效的,可以应用在不同原子规模的分子动力学仿真计算中.     

高强低合金钢焊接过程多物理场耦合数值模拟

目前高强低合金钢焊接数值模拟中,采用热-力耦合分析时,忽略固态相变效应,残余应力模拟值与试验测量值误差较大。为提高焊接数值模拟精度,根据多场耦合关系,基于传热学、固态相变理论和连续介质力学,建立焊接过程多物理场耦合本构方程,并通过子程序将其嵌入到通用隐式有限元程序中。采用数值模拟与试验分析的方法研究高强低合金钢小试样的自由膨胀试验、相变塑性试验及平板焊接试验应力及各应变分量的演变。研究结果表明：固态相变体积变化引起的相变应变对残余应力有显著影响,不但改变了残余应力的大小,甚至改变了残余应力的符号,考虑相变塑性应变时会降低应力的水平。残余应力改变程度与相变程度有关系：完全相变区影响最大,部分相变区次之,未发生相变区最小。相变应变和相变塑性应变最终大小相当。研究方法为深入了解高强低合金钢焊接过程和焊接工艺优化提供了理论基础。     

Lubricant-Induced Spacing Increases at Slider–Disk Interfaces in Disk Drives

In this article, we explore the physical mechanisms for lubricant migration on recording head slider surfaces and how this migration leads to increased slider–disk spacing during disk drive operations. This is done using both a new experimental methodology, called the “droplet stress test,” and through simulation. In our simulations, we compare the air shear-induced lubricant migration modeled either as viscous flow of a continuum liquid film with zero slip or as wind driven slippage of molecules across the surface. The experimental data are best fitted using the viscous flow model to determine an effective viscosity for the sub-nanometer thick lubricant films. This effective viscosity tends to be somewhat less than the lubricant bulk viscosity due to air shear promoting the slippage of lubricant molecules across the surface. Our experimental results also indicate that the potential spacing increase from the pickup of disk lubricant on the slider is limited by the mobile fraction of the dewetting thickness of the lubricant film on the slider.     

Molecular dynamics modeling of a single diamond abrasive grain in grinding

In this paper the nano-metric simulation of grinding of copper with diamond abrasive grains, using the molecular dynamics (MD) method, is considered. An MD model of nano-scale grinding, where a single diamond abrasive grain performs cutting of a copper workpiece, is presented. The Morse potential function is used to simulate the interactions between the atoms involved in the procedure. In the proposed model, the abrasive grain follows a curved path with decreasing depth of cut within the workpiece to simulate the actual material removal process. Three different initial depths of cut, namely 4 ?, 8 ? and 12 ?, are tested, and the influence of the depth of cut on chip formation, cutting forces and workpiece temperatures are thoroughly investigated. The simulation results indicate that with the increase of the initial depth of cut, average cutting forces also increase and therefore the temperatures on the machined surface and within the workpiece increase as well. Furthermore, the effects of the different values of the simulation variables on the chip formation mechanism are studied and discussed. With the appropriate modifications, the proposed model can be used for the simulation of various nano-machining processes and operations, in which continuum mechanics cannot be applied or experimental techniques are subjected to limitations.     

半灵活主链型液晶聚合物的结构特性

采用分子动力学(MD)模拟方法研究了半灵活主链型液晶聚合物(LCPs)的结构特性,其中分子链通过最新定义的模型——Solo-LJ-SP-GB模型来描述。该模型已被证实所需计算时间不到传统GB/LJ模型的十分之一,可大大地提高计算效率。通过模拟半灵活主链型LCPs 的液晶形成过程,研究了LCP分子中不同个数的间隔体对该材料热力学特性的影响。研究发现其热力学特性呈现出与间隔体个数相关的奇偶效应,这与现有试验结果相当吻合。通过对半灵活主链型LCPs的局域空间方位时间相关函数及平移灵活性的测量,并与传统GB/LJ模型的模拟结果相比较,发现两种模型显示出相似的各向异性及灵活性。     

Fluid, solid and fluid-structure interaction simulations on patient-based abdominal aortic aneurysm models

This article describes the use of fluid, solid and fluid-structure interaction simulations on three patient-based abdominal aortic aneurysm geometries. All simulations were carried out using OpenFOAM, which uses the finite volume method to solve both fluid and solid equations. Initially a fluid-only simulation was carried out on a single patient-based geometry and results from this simulation were compared with experimental results. There was good qualitative and quantitative agreement between the experimental and numerical results, suggesting that OpenFOAM is capable of predicting the main features of unsteady flow through a complex patient-based abdominal aortic aneurysm geometry. The intraluminal thrombus and arterial wall were then included, and solid stress and fluid-structure interaction simulations were performed on this, and two other patient-based abdominal aortic aneurysm geometries. It was found that the solid stress simulations resulted in an under-estimation of the maximum stress by up to 5.9% when compared with the fluid-structure interaction simulations. In the fluid-structure interaction simulations, flow induced pressure within the aneurysm was found to be up to 4.8% higher than the value of peak systolic pressure imposed in the solid stress simulations, which is likely to be the cause of the variation in the stress results. In comparing the results from the initial fluid-only simulation with results from the fluid-structure interaction simulation on the same patient, it was found that wall shear stress values varied by up to 35% between the two simulation methods. It was concluded that solid stress simulations are adequate to predict the maximum stress in an aneurysm wall, while fluid-structure interaction simulations should be performed if accurate prediction of the fluid wall shear stress is necessary. Therefore, the decision to perform fluid-structure interaction simulations should be based on the particular variables of interest in a given study.     

Multi-scale simulation of the nano-metric cutting process

Molecular dynamics (MD) simulation and the finite element (FE) method are two popular numerical techniques for the simulation of machining processes. The two methods have their own strengths and limitations. MD simulation can cover the phenomena occurring at nano-metric scale but is limited by the computational cost and capacity, whilst the FE method is suitable for modelling meso- to macro-scale machining and for simulating macro-parameters, such as the temperature in a cutting zone, the stress/strain distribution and cutting forces, etc. With the successful application of multi-scale simulations in many research fields, the application of simulation to the machining processes is emerging, particularly in relation to machined surface generation and integrity formation, i.e. the machined surface roughness, residual stress, micro-hardness, microstructure and fatigue. Based on the quasi-continuum (QC) method, the multi-scale simulation of nano-metric cutting has been proposed. Cutting simulations are performed on single-crystal aluminium to investigate the chip formation, generation and propagation of the material dislocation during the cutting process. In addition, the effect of the tool rake angle on the cutting force and internal stress under the workpiece surface is investigated: The cutting force and internal stress in the workpiece material decrease with the increase of the rake angle. Finally, to ease multi-scale modelling and its simulation steps and to increase their speed, a computationally efficient MATLAB-based programme has been developed, which facilitates the geometrical modelling of cutting, the simulation conditions, the implementation of simulation and the analysis of results within a unified integrated virtual-simulation environment.     

微型机械力学行为的分子动力学模拟研究进展

介绍了国外分子动力学模拟在微型机械力学行为分析中的研究进展;阐述了分子动力学模拟的基本方程、分子动力学的算法以及应用于微型机械力学行为分析中的势函数等.最后介绍了几个在微型机械力学行为分析中应用分子动力学模拟的例子.     

Numerical methodologies for investigation of moderate-velocity flow using a hybrid computational fluid dynamics — molecular dynamics simulation approach

Numerical approaches are presented to minimize the statistical errors inherently present due to finite sampling and the presence of thermal fluctuations in the molecular region of a hybrid computational fluid dynamics (CFD) — molecular dynamics (MD) flow solution. Near the fluid-solid interface the hybrid CFD-MD simulation approach provides a more accurate solution, especially in the presence of significant molecular-level phenomena, than the traditional continuum-based simulation techniques. It also involves less computational cost than the pure particle-based MD. Despite these advantages the hybrid CFD-MD methodology has been applied mostly in flow studies at high velocities, mainly because of the higher statistical errors associated with low velocities. As an alternative to the costly increase of the size of the MD region to decrease statistical errors, we investigate a few numerical approaches that reduce sampling noise of the solution at moderate-velocities. These methods are based on sampling of multiple simulation replicas and linear regression of multiple spatial/temporal samples. We discuss the advantages and disadvantages of each technique in the perspective of solution accuracy and computational cost.     

Creep life assessment of aero-engine recuperator based on continuum damage mechanics approach

The creep life of an aeroengine recuperator is investigated in terms of continuum damage mechanics by using finite element simulations. The effects of the manifold wall thickness and creep properties of brazing filler metal on the operating life of the recuperator are analyzed. Results show that the crack initiates from the brazing filler metal located on the outer surface of the manifold with the wall thickness of 2 mm and propagates throughout the whole region of the brazing filler metal when the creep time reaches 34900 h. The creep life of the recuperator meets the requirement of 40000 h continuous operation when the wall thickness increases to 3.5 mm, but its total weight increases by 15%. Decreasing the minimum creep strain rate with the enhancement of the creep strength of the brazing filler metal presents an obvious effect on the creep life of the recuperator. At the same stress level, the creep rupture time of the recuperator is enhanced by 13 times if the mismatch between the minimum creep rate of the filler and base metal is reduced by 20%.