基于界带模型的碳纳米管声子谱的辛分析

针对碳纳米管声子谱的数值计算方法研究,基于对偶体系和辛几何算法提出了一套全新的计算方法和相应的界带结构模型,通过将碳纳米管模拟成不同的结构力学模型,利用分析结构力学中的振动理论来计算碳纳米管的色散关系.理论框架包括:周期结构的变分原理、周期结构中波的传播分析、子结构方法、界带理论和声子色散关系的基本算法.数值算例验证了理论和算法的有效性,而且也指出了针对碳纳米管的声子谱的计算,界带模型相对于其它传统模型存在着一定的优势.     

连续系统的界带分析方法

基于分析结构力学提出的界带分析方法,将子结构间的分界面延拓为有一定宽度的分界带/分界域,从而可以用于分析计算结构的非局部效应。界带分析方法首先在离散结构的分析计算中取得了成功,从而验证了该套理论算法的准确性。离散结构按界带宽度（影响域范围）划分子结构,因而限制了子结构区段积分计算的最小步长;而连续系统则要求可以实现任意步长的积分运算。通过引入步进的计算方法,界带分析方法可以实现任意步长的积分计算,进而可以解决连续系统的积分问题。通过数值算例验证了连续系统的界带分析方法的准确性和可行性,也为进一步研究该套计算方法在分析动力学中的应用打下基础。     

Prediction of buckling characteristics of carbon nanotubes

In this paper, to investigate the buckling characteristics of carbon nanotubes, an equivalent beam model is first constructed. The molecular mechanics potentials in a C–C covalent bond are transformed into the form of equivalent strain energy stored in a three dimensional (3D) virtual beam element connecting two carbon atoms. Then, the equivalent stiffness parameters of the beam element can be estimated from the force field constants of the molecular mechanics theory. To evaluate the buckling loads of multi-walled carbon nanotubes, the effects of van-der Waals forces are further modeled using a newly proposed rod element. Then, the buckling characteristics of nanotubes can be easily obtained using a 3D beam and rod model of the traditional finite element method (FEM). The results of this numerical model are in good agreement with some previous results, such as those obtained from molecular dynamics computations. This method, designated as molecular structural mechanics approach, is thus proved to be an efficient means to predict the buckling characteristics of carbon nanotubes. Moreover, in the case of nanotubes with large length/diameter, the validity of Euler’s beam buckling theory and a shell model with the proper material properties defined from the results of present 3D FEM beam model is investigated to reduce the computational cost. The results of these simple theoretical models are found to agree well with the existing experimental results.     

Molecular structural mechanics approach to carbon nanotubes on graphics processing units

A molecular structural mechanics approach to carbon nanotubes on graphics processing units (GPUs) is reported. As a powerful parallel and relatively low cost processor, the GPU is used to accelerate the computations of the molecular structural mechanics approach. The data structures, matrix-vector multiplication algorithm, texture reduction algorithm, and ICCG method on the GPU are presented. The computations for Young's moduli of carbon nanotubes by the molecular structural mechanics approach on the GPU show its accuracy. The running times of large degree of freedom (DOF) carbon nanotubes, whose DOF is larger than 100,000, on the GPU are compared against those on the CPU, proving the GPU can accelerate the computations of the molecular structural mechanics approach to carbon nanotubes.     

Parametric variational principle and quadratic programming method for van der Waals force simulation of parallel and cross nanotubes

A parametric variational principle for van der Waals force simulation between any two adjacent nonbonded atoms and the corresponding improved quadratic programming method for numerical simulation of mechanical behaviors of carbon nanotubes are developed. Carbon nanotubes are modeled and computed based on molecular structural mechanics model. van der Waals force is simulated by the network of bars (called bar network) with a special nonlinear mechanical constitutive law (called generalized parametric constitutive law) in the finite element analysis. Compared with conventional numerical methods, the proposed method does not depend on displacement and stress iteration, but on the base exchanges in the solution of a standard quadratic programming problem. Thus, the model and method developed present very good convergence behavior in computation and provide accurate predictions of the mechanical behaviors and displacement distributions in the nanotubes. Numerical results demonstrate the validity and the efficiency of the proposed method.     

Analytical and numerical techniques to predict carbon nanotubes properties

In this paper, two different approaches for modeling the behaviour of carbon nanotubes are presented. The first method models carbon nanotubes as an inhomogeneous cylindrical network shell using the asymptotic homogenization method. Explicit formulae are derived representing Young’s and shear moduli of single-walled nanotubes in terms of pertinent material and geometric parameters. As an example, assuming certain values for these parameters, the Young’s modulus was found to be 1.71 TPa, while the shear modulus was 0.32 TPa. The second method is based on finite element models. The inter-atomic interactions due to covalent and non-covalent bonds are replaced by beam and spring elements, respectively, in the structural model. Correlations between classical molecular mechanics and structural mechanics are used to effectively model the physics governing the nanotubes. Finite element models are developed for single-, double- and multi-walled carbon nanotubes. The deformations from the finite element simulations are subsequently used to predict the elastic and shear moduli of the nanotubes. The variation of mechanical properties with tube diameter is investigated for both zig-zag and armchair configurations. Furthermore, the dependence of mechanical properties on the number of nanotubules in multi-walled structures is also examined. Based on the finite element model, the value for the elastic modulus varied from 0.9 to 1.05 TPa for single and 1.32 to 1.58 TPa for double/multi-walled nanotubes. The shear modulus was found to vary from 0.14 to 0.47 TPa for single-walled nanotubes and 0.37 to 0.62 for double/multi-walled nanotubes.     

Nanoscale finite element models for vibrations of single-walled carbon nanotubes: atomistic versus continuum

By the atomistic and continuum finite element models, the free vibration behavior of single-walled carbon nanotubes （SWCNTs） is studied. In the atomistic finite element model, the bonds and atoms are modeled by the beam and point mass elements, respectively. The molecular mechanics is linked to structural mechanics to determine the elastic properties of the mentioned beam elements. In the continuum finite element approach, by neglecting the discrete nature of the atomic structure of the nanotubes, they are modeled with shell elements. By both models, the natural frequencies of SWCNTs are computed, and the effects of the geometrical parameters, the atomic structure, and the boundary conditions are investigated. The accuracy of the utilized methods is verified in comparison with molecular dynamic simulations. The molecular structural model leads to more reliable results, especially for lower aspect ratios. The present analysis provides valuable information about application of continuum models in the investigation of the mechanical behaviors of nanotubes.     

An analytical molecular structural mechanics model for the mechanical properties of carbon nanotubes

An analytical molecular structural mechanics model for the prediction of mechanical properties of defect-free carbon nanotubes is developed by incorporating the modified Morse potential with an analytical molecular structural model. The developed model is capable of predicting Young’s moduli, Poisson’s ratios and stress–strain relationships of carbon nanotubes under tension and torsion loading conditions. Results on the mechanical properties of single-walled carbon nanotubes show that Young’s moduli of carbon nanotubes are sensitive to the tube diameter and the helicity. Young’s moduli of both armchair and zigzag carbon nanotubes increase monotonically and approach Young’s modulus of graphite when the tube diameter is increased. The nonlinear stress–strain relationships for defect-free nanotubes have been predicted, which gives a good approximation on the ultimate strength and strain to failure of nanotubes. Armchair nanotubes exhibit higher tensile strength than zigzag nanotubes but their torsion strengths are identical based on the present study. The present theoretical investigation provides a very simple approach to predict the mechanical properties of carbon nanotubes.     

A structural mechanics approach for predicting the mechanical properties of carbon nanotubes

Based on molecular mechanics, a structural mechanics model of carbon nanotubes (CNTs) was developed with special consideration given to the bending stiffness of the graphite layer. The potentials associated with the atomic interactions within a CNT were evaluated by the strain energies of beam elements which serve as structural substitutions of covalent bonds in a CNT. In contrast to the original model developed by Li and Chou (Int. J. Solids Struct. 40(10):2487–2499, 2003), in the current model the out-of-plane deformation (inversion) of the bond was distinguished from the in-plane deformation by considering a rectangular cross-section for the beam element. Consequently, the model is able to study problems where the effect of local bending of the graphite layer in a carbon nanotube is significant. A closed-form solution of the sectional properties of the beam element was derived analytically. The model was verified through the analysis of rolling a graphite sheet into a carbon nanotube. Using the present model, the buckling behavior of nanotubes under bending is simulated. The predicted critical bending angle agrees well with molecular dynamics simulations.     

模态分析与动态子结构方法新进展

综述在模态分析与动态子结构方法研究的一些最新进展.首先回顾经典的位移展开定理和模态叠加原理.为了加速经典方法的收敛速度、提高计算效率, 进一步介绍两个新的结构位移展开定理(采用固定界面模态的位移展开新定理, 给出采用低阶固定界面模态的高精度位移展开式;采用混合模态的位移展开新定理, 给出采用低阶混合模态表示的高精度位移展开式)和相应的动力学新解法.相应上述3个位移展开定理, 介绍采用解析推导的方法构造出3类动态精确子结构方法, 各种子结构模态综合法实质上都是它们的某种近似与变化形式, 从而形成系统的动态子结构分析技术.上述介绍的模态分析与动态子结构方法新进展与经典模态分析技术一起形成结构动力学分析技术的系统理论.      

GUIDED CIRCUMFERENTIAL WAVES IN DOUBLE-WALLED CARBON NANOTUBES

A model of guided circumferential waves propagating in double-walled carbon nan- otubes is built by the theory of wave propagation in continuum mechanics,while the van der Waals force between the inner and outer nanotube has been taken into account in the model.The dispersion curves of the guided circumferential wave propagation are studied,and some dispersion characteristics are illustrated by comparing with those of single-walled carbon nanotubes.It is found that in double-walled carbon nanotubes,the guided circumferential waves will propagate in more dispersive ways.More interactions between neighboring wave modes may take place.In particular,it has been found that a couple of wave modes may disappear at a certain frequency and that,while a couple of wave modes disappear,another new couple of wave modes are excited at the same wave number.     

Estimation of material properties of nanocomposite structures

A method for the numerical modelling of mechanical behaviour of nanocomposite materials reinforced with the carbon nanotubes, based on computational homogenization as a multi-scale method, is presented. Since the carbon nanotube inside of the representative volume element (RVE) is modelled as a space frame structure, theoretical background and a proper way of modelling of carbon nanotubes is given. Novelty in this paper is an incorporation of interactions, based on the weak van der Waals forces and modelled by nonlinear rod elements, into a multiscale model as described below. An algorithm is developed for analysis of those interactions. Since the problem of modelling nanocomposite structures is a three-dimensional multi-scale problem, one part of this work is dedicated to multi-scale modelling methods, especially to the first order computational homogenization. Computational homogenization and representative volume element are the basis of the presented numerical model of the nanocomposites. Nano scale model is based on beam and non-linear rod finite elements. For the purpose of the software verification, examples, i.e. models of the nanocomposite material are presented. Obtained results are compared with the results given by the other authors.     

Continuum Mechanics Modeling and Simulation of Carbon Nanotubes

The understanding of the mechanics of atomistic systems greatly benefits from continuum mechanics. One appealing approach aims at deductively constructing continuum theories starting from models of the interatomic interactions. This viewpoint has become extremely popular with the quasicontinuum method. The application of these ideas to carbon nanotubes presents a peculiarity with respect to usual crystalline materials: their structure relies on a two-dimensional curved lattice. This renders the cornerstone of crystal elasticity, the Cauchy–Born rule, insufficient to describe the effect of curvature. We discuss the application of a theory which corrects this deficiency to the mechanics of carbon nanotubes (CNTs). We review recent developments of this theory, which include the study of the convergence characteristics of the proposed continuum models to the parent atomistic models, as well as large scale simulations based on this theory. The latter have unveiled the complex nonlinear elastic response of thick multiwalled carbon nanotubes (MWCNTs), with an anomalous elastic regime following an almost absent harmonic range.     

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综述了连续介质方法在碳纳米管研究中的最新进展. 主要叙述梁、壳模型, 膜模型,多尺度方法,分子结构力学方法,非局部连续介质方法,以及无网格法的基本原理、 基本方法,及其最新进展,指出其局限性,并预测连续介质方法在碳纳米管研究的发展趋势 和方向.     

An accurate molecular mechanics model for computation of size-dependent elastic properties of armchair and zigzag single-walled carbon nanotubes

In this paper, an analytical solution based on a molecular mechanics model is developed to evaluate the mechanical properties of armchair and zigzag single-walled carbon nanotubes (SWCNTs). Adopting the Perdew–Burke–Ernzerhof (PBE) exchange correlation, the density functional theory (DFT) calculations are performed within the generalized gradient approximation (GGA) to evaluate force constants used in the molecular mechanics model. After that, based on the principle of molecular mechanics, explicit expressions are proposed to obtain surface Young’s modulus, Poisson’s ratio and surface shear modulus of SWCNTs corresponding to both types of armchair and zigzag chiralities. Based on the DFT calculations, it is found that the flexural rigidity of graphene is independent of the type of chirality which indicates the isotropic characteristic of this material. Moreover, it is observed that for the all values of nanotube diameter, surface Young’s modulus for the armchair nanotube is more than that of zigzag nanotube. It is shown that the trend predicted by the present model is in good agreement with other models which confirms the validity as well as the accuracy of the present molecular mechanics model.     

水凝胶多场耦合计算力学

作为一种具有多场耦合特性的智能柔体材料,水凝胶的制备技术、性能表征与结构应用得到迅速发展。本文在分析水凝胶本构理论和结构设计的基础上,提出了水凝胶多场耦合计算力学的基本方法和范式,包括微观粗粒化分子动力学模拟和宏观耦合有限元方法等,计算了化学-力学耦合作用下水凝胶材料与结构的变形和应力,给出了多个数值算例与结果比较。研究指出多场耦合计算力学将成为水凝胶材料和结构分析的主要手段,并推动水凝胶等这类智柔材料的性能设计与工程应用。     

界带分析的基本理论和计算方法

基于应用力学的辛数学理论,阐述了界带分析的基本理论与计算方法,主要包括:界带分析的基本概念、界带分析的变分原理、单个与不同原子周期链的波的色散关系、子结构界带分析计算方法以及数值算例等多方面内容。研究工作在表明辛数学理论优势和潜力的同时,也为相关问题的深入研究奠定了理论与算法基础。