金属基纳米复合材料等效弹性模量的均匀化方法数值模拟

均匀化理论利用位移场双尺度渐近展开建立有限元列式，本文将其与有限元通用程序相结合，应用于金属基复合材料的弹性本构数值模拟。通过对不同尺度增强相金属基复合材料等效模量的数值模拟，考察了均匀化方法的适用情况。数值计算结果表明，对常规尺度增强相金属基复合材料，均匀化方法可以较准确地预测其等效弹性模量；对纳米增强相金属基复合材料，该方法仍可给出较好的预测，但存在某种程度的系统偏差。通过对纳米尺度增强机理的分析讨论，认为纳米增强相与基体材料问的界面效应可能有别于连续介质假设，指出可以考虑采用离散原子-连续介质耦合模型改进数值模拟结果。     

Theoretical analysis of electric, magnetic and magnetoelectric properties of nano-structured multiferroic composites

Electric, magnetic and magnetoelectric properties of the nano-structured multiferroic composites were studied by using an energy formulation with the consideration of the surface, interface, and size effect. Coupled thermodynamic evolution equations with respect to the spontaneous polarization and magnetization were established, in which the elastic fields in the matrix and inclusions were solved based on the Eshelby's equivalent inclusion concept and the Mori–Tanaka method. Physical properties of the composite, such as the spontaneous order parameters, piezoelectric/piezomagnetic properties, and the magnetoelectric coupling effect are highly dependent on the stress state and the microstructures of the nano-composites. Magnetoelectric coupling voltage coefficient was unstable in the vicinity of the critical size and disappeared below the critical size. The model is versatile enough for various composite structures.     

Rheological properties and percolation in suspensions of multiwalled carbon nanotubes in polycarbonate

This paper is concerned with several issues related to the rheological behavior of polycarbonate/multiwalled carbon nanotube nanocomposites. The composites were prepared by diluting a masterbatch of 15 wt.% nanotubes using melt-mixing method, and the dispersion was analyzed by SEM, TEM, and AFM techniques. To understand the percolated structure, the nanocomposites were characterized via a set of rheological, electrical, and thermal conductivity measurements. The rheological measurements revealed that the structure and properties were temperature dependent; the percolation threshold was significantly lower at higher temperature suggesting stronger nanotube interactions. The nanotube networks were also sensitive to the steady shear deformation particularly at high temperature. Following preshearing, the elastic modulus decreased markedly suggesting that the nanotubes became more rigid. These results were analyzed using simple models for suspensions of rod-like particles. Finally, the rheological, electrical, and thermal conductivity percolation thresholds were compared. As expected, the rheological threshold was smaller than the thermal and electrical threshold.     

Two-fold enhancement in tensile strength of carbon nanotube–carbon fiber hybrid epoxy composites through combination of electrophoretic deposition and alternating electric field

A new carbon nanotube (CNT)–hybridized carbon fiber (CF) was introduced in an attempt to improve interfacial strength between CF and polymeric matrix. Amine-functionalized CNTs was radially deposited on the CF surface through a combination of alternating electric field with electrophoretic deposition process. Radial deposition of CNTs on CF formed a unique porous structure around CF that could significantly increase the interfacial adhesion through interlocking of polymeric matrix. Tensile properties and fatigue life of the reinforced composites were investigated in order to study the effect of interfacial adhesion on mechanical properties of reinforced composites. Results indicate that the radial deposition of CNT on CF can remarkably enhance the compatibility of polymeric matrix with CF. This improvement in compatibility of polymeric matrix with CNT–hybridized CF resulted in considerable enhancement in mechanical properties of composites. The interfacial reinforcing mechanism was explored through fractography of reinforced composites and possible failure modes have been precisely discussed.     

The thermal effect on axially compressed buckling of a double-walled carbon nanotube

The thermal effect on axially compressed buckling of a double-walled carbon nanotube is studied in this paper. The effects of temperature change, surrounding elastic medium and van der Waals forces between the inner and outer nanotubes are taken into account. Using continuum mechanics, an elastic double-shell model with thermal effect is presented for axially compressed buckling of a double-walled carbon nanotube embedded in an elastic matrix under thermal environment. Based on the model, an explicit formula for the critical axial stress is derived in terms of the buckling modes of the shell and the parameters that indicate the effects of temperature change, surrounding elastic medium and the van der Waals forces. Based on that, some simplified analysis is carried out to estimate the critical axial stress for axially compressed buckling of the double-walled carbon nanotube. Numerical results for the general case are obtained for the thermal effect on axially compressed buckling of a double-walled carbon nanotube. It is shown that the axial buckling load of double-walled carbon nanotube under thermal loads is dependent on the wave number of axially buckling modes. And a conclusion is drawn that at low and room temperature the critical axial stress for infinitesimal buckling of a double-walled carbon nanotube increase as the value of temperature change increases, while at high temperature the critical axial stress for infinitesimal buckling of a double-walled carbon nanotube decrease as the value of temperature change increases.     

材料高温力学性能理论表征方法研究进展

随着科学技术的迅猛发展,材料在高温领域的应用越来越广泛。然而高温下材料的力学性能和常温相比有很大差异,材料的高温力学性能研究和表征已成为当前的研究热点。论文文对材料在高温下力学行为理论表征方法研究的最新进展进行了总结和回顾。着重介绍了近年来高温陶瓷材料的断裂强度、金属材料的屈服强度、弹性模量与本构关系的温度相关性理论表征方法的研究进展。最后,总结已有研究工作的特点和不足之处,对材料高温力学性能理论表征方法的后续研究进行了展望,就进一步研究提供建议。     

Multiscale micromechanical modeling of the constitutive response of carbon nanotube-reinforced structural adhesives

Appropriate formulations are developed to allow for the atomistic-based continuum modeling of nano-reinforced structural adhesives on the basis of a nanoscale representative volume element that accounts for the nonlinear behavior of its constituents; namely, the reinforcing carbon nanotube, the surrounding adhesive and their interface. The newly developed representative volume element is then used with analytical and computational micromechanical modeling techniques to investigate the homogeneous dispersion of the reinforcing element into the adhesive upon both the linear and nonlinear properties. Unlike our earlier work where the focus was on developing linear micromechanical models for the effective elastic properties of nanocomposites, the present approach extends these models by describing the development of a nonlinear hybrid Monte Carlo Finite Element model that allows for the prediction of the full constitutive response of the bulk composite under large deformations. The results indicate a substantial improvement in both the Young’s modulus and tensile strength of the nano-reinforced adhesives for the range of CNT concentrations considered.     

Influence of applied in-plane strain on transverse thermal conductivity of 0°/90° and plain weave ceramic matrix composites

A computationally economic finite-element-based stress analysis model, developed previously by the authors, has been extended to predict the thermal behaviour of ceramic matrix composites with strain-induced damage. The finite element analysis utilises a solid element to represent a homogenised orthotropic medium of a heterogeneous uni-directional tow. The non-linear multi-axial strain dependent thermal behaviour has been discretised by multi-linear curves, which have been implemented by a user defined subroutine, USDFLD, in the commercial finite element package, ABAQUS. The model has been used to study the performance of two CMC composites: a SiC (Nicalon) fibre-calcium aluminosilicate (CAS) matrix, 0°/90° cross-ply laminate Nicalon-CAS; and, carbon fibre-dual carbon-SiC matrix (C/C-SiC), plain weave laminate DLR-XT. The global through-thickness thermal conductivity degradation with composite uni-axial strain has been predicted. Comparisons have been made between the predictions and experimental data for both materials, and good agreement has been achieved. For the Nicalon-CAS 0°/90° cross-ply the dominant mechanism of thermal conductivity degradation is combined fibre failure and associated wake debonding; and, for the DLR-XT plain weave the same mechanisms act in combination with out-of-plane shear failure.     

Continuum models of multi-walled carbon nanotubes

Molecular mechanics (MM) simulations have been carried out to determine energetically favorable double-walled carbon nanotube (DWNT) structures, and analyze their infinitesimal extensional, torsional, radial expansion/contraction, and bending deformations. Loads are applied either to one wall or simultaneously to both walls of an open-ended DWNT. These results are compared against single-walled carbon nanotube (SWNT) results to determine differences and similarities between responses of SWNTs and DWNTs, and the validity of using SWNT results to predict the response of a DWNT. It is found that for small deformations such as simple tension and torsion, results for a DWNT can be derived from those for its constituent SWNTs within 3% error. Results of radial expansion/contraction of a SWNT are used to deduce an expression for the van der Waals force. Based on these results, a continuum model is proposed for a MWNT whose response to mechanical deformations computed using engineering theories is the same as that of the MWNT obtained via MM simulations. The continuum structure is comprised of concentric cylindrical tubes interconnected by truss elements. Young’s modulus, Poisson’s ratio, the thickness of each concentric tube, and the stiffness of the truss elements are given. The proposed continuum model is validated by studying bending and the onset of global buckling deformations of a DWNT and its proposed equivalent continuum structure. Carbon nanotubes can be replaced by their equivalent continuum structures when deriving mechanical properties of nanotube reinforced polymeric composites.     

具有正温度系数特性的导电低密度聚乙烯基复合材料的摩擦磨损性能

采用共混法制备了以低湿度聚乙烯为基体的含碳黑的导电复合材料,考察了导电复合材料的摩擦磨损性能及正温度系数特性,同时探讨了导电复合材料的摩擦熔融现象对其摩擦磨损性能的影响。结果表明：当低密度聚乙烯基导电复合材料与GCr15钢配副时,在较高pv值下,摩擦系数呈现周期性降低现象;温度对复合材料的导电性能具有重要影响,而聚四氟乙烯及聚苯酯等辅助导电剂对其PTC特性无影响。     

单壁碳纳米管屈曲的原子/连续介质混合模型

用数学和力学研究所，上海 200072)//力学学报.--2004,36(6).--744～748 提供了一种运用原子/连续介质混合(hybrid atomic/continuum,HAC)方法解决纳米力学问题的思路. 通过在连续介质力学模型中引入利用分子力学方法获得物性参数，建立了预测单壁碳纳米管临界屈曲参数的HAC模型. 结果表明, HAC模型具有与连续介质力学模型可比拟的简洁性, 同时可表征纳米管微观结构特征对屈曲参数的影响. 计算结果表明，Zigzag纳米管的抗屈曲性能优于Armchair纳米管. 基于Tersoff-Brenner作用势的分子动力学结果证实了这一结论.     

考虑碳纳米管非均匀分布的复合材料电导率计算

碳纳米管作为导电相在机敏复合材料中广泛应用,但碳纳米管为团簇材料,在基体中很难均匀分散。本文考虑碳纳米管的非均匀分布特性,提出了计算碳纳米管复合材料电导率的数值方法。通过引入随机谐和函数,建立了碳纳米管体积分数的三维随机场模型。基于细观力学的有效介质理论、Mori-Tanaka方法和H-S界限理论,考虑碳纳米管之间的隧穿效应,发展了复合材料微小体积单元的电导率计算方法。在此基础上,构建了考虑碳纳米管非均匀分布的复合材料等效电导率三维有限元计算模型。数值分析结果与试验值能够很好吻合,表明这一方法可以准确计算碳纳米管复合材料的电导率。本文进一步分析了碳纳米管非均匀分布对复合材料电导率的影响。     

Thermal effects on interfacial stress transfer characteristics of carbon nanotubes/polymer composites

The paper presents an analytical method to investigate thermal effects on interfacial stress transfer characteristics of single/multi-walled carbon nanotubes/polymer composites system under thermal loading by means of thermoelastic theory and conventional fiber pullout models. In example calculations, the mechanical properties and the thermal expansion coefficients of carbon nanotubes and polymer matrix are, respectively, treated as the functions of temperature change. Numerical examples show that the interfacial shear stress transfer behavior can be described and affected by several parameters such as the temperature field, volume fraction of CNT, and numbers of wall layer and the outermost radius of carbon nanotubes. From the results carried out it is found that mismatch of thermal expansion coefficients between the carbon nanotubes and polymer matrix may be more important in governing interfacial stress transfer characteristics of carbon nanotubes/polymer composite system.     

Relative orientation constraints in the nonlinear large displacement analysis: application to soft materials

This study dealt with the dynamic stability and geometrical nonlinear problems of carbon nanotube/fiber/polymer composite (CNTFPC) cylindrical panels without or with delamination around a central cutout. A multiscale analysis using the Hewitt and Malherbe model was performed to determine the carbon nanotube (CNT) weight ratios, thickness–radius ratios, thickness–length ratios, and delamination area ratios around a cutout. A delamination around a central cutout was modeled in two dimensions by introducing continuity conditions of displacements at the delamination boundaries. The proposed approach in this study has been verified by previous studies. Parametric results showed the significance of a proper CNT ratio and curvature for better structural performance on the dynamic instability and nonlinearity of delaminated CNTFPC cylindrical panels.

     

A new method for characterizing the interphase regions of carbon nanotube composites

The elastic properties of a carbon nanotube (CNT) reinforced composite are affected by many factors such as the CNT–matrix interphase. As such, mechanical analysis without sufficient consideration of these factors can give rise to incorrect predictions. Using single-walled carbon nanotube (SWCNT) reinforced Polyvinylchloride (PVC) as an example, this paper presents a new technique to characterize interphase regions. The representative volume element (RVE) of the SWCNT–PVC system is modeled as an assemblage of three phases, the equivalent solid fiber (ESF) mimicking the SWCNT under the van der Waals (vdW) forces, the dense interphase PVC of appropriate thickness and density, and the bulk PVC matrix. Two methods are proposed to extract the elastic properties of the ESF from the atomistic RVE and the CNT-cluster. Using atomistic simulations, the thickness and the average density of interphase matrix are determined and the elastic properties of amorphous interphase matrix are characterized as a function of density. The method is examined in a continuum-based three-phase model developed with the aid of molecular mechanics (MM) and the finite element (FE) method. The predictions of the continuum-based model show a good agreement with the atomistic results verifies that the interphase properties of amorphous matrix in CNT-composites could be approximated as a function of density. The results show that ignoring either the vdW interaction region or the interphase matrix layer can bring about misleading results, and that the effect of internal walls of multi-walled carbon nanotubes (MWCNTs) on the density and thickness of the dense interphase is negligible.     

HT-7U超导托卡马克装置极向场线圈的等效弹性常数及其残余刚度

HT－7装置的超导极向场线圈由导管，超导材料和绝缘体组合而成，具有复杂的正交各向异性的性质。本文从微观力学的观点出发，将极向场线圈整体作为一种复合材料来考虑，采用微观力学的复合律和损伤力学的细观损伤模型，求出了极向场线圈的等效弹性常数和残余刚度；并且，残余刚度的计算结果已得到了实验验证。这些材料特性已被用于有限元分析的输入数据。     

Multi-scale modeling of tensile behavior of carbon nanotube-reinforced composites

A multi-scale representative volume element (RVE) for modeling the tensile behavior of carbon nanotube-reinforced composites is proposed. The RVE integrates nanomechanics and continuum mechanics, thus bridging the length scales from the nano- through the mesoscale. A progressive fracture model based on the modified Morse interatomic potential is used for simulating the behavior of the isolated carbon nanotubes and the FE method for modeling the matrix and building the RVE. Between the nanotube and the matrix a perfect bonding is assumed until the interfacial shear stress exceeds the corresponding strength. Then, nanotube/matrix debonding is simulated by prohibiting load transfer in the debonded region. Using the RVE, a unidirectional nanotube/polymer composite was modeled and the results were compared with corresponding rule-of-mixtures predictions. A significant enhancement in the stiffness of the polymer owing to the adding of the nanotubes is predicted. The effect of interfacial shear strength on the tensile behavior of the nanocomposite was also studied. Stiffness is found to be unaffected while tensile strength to significantly decrease with decreasing the interfacial shear strength.     

Efficiency and optimisation of fin with temperature-dependent thermal conductivity: a simplified solution

An analytical simplified solution is proposed for temperature distribution and fin efficiency, when thermal conductivity is temperature dependent. An optimal linearization technique is used to solve the nonlinear equation. Based on classical solution, some accurate results are obtained and presented with thermal conductivity parameter and fin parameter. Arithmetic mean temperature is less precise than an equivalent thermal conductivity. Optimal thickness for rectangular fin is derived.     

Nonlinear low-velocity impact on damped and matrix-cracked hybrid laminated beams containing carbon nanotube reinforced composite layers

This paper investigates the low-velocity impact response of a shear deformable laminated beam which contains both carbon nanotube reinforced composite (CNTRC) layers and carbon fiber reinforced composite (CFRC) layers. The effect of matrix cracks is considered, and a refined self-consistent model is selected to describe the degraded stiffness caused by the damage. The beam including damping effects rests on a two-parameter elastic foundation in thermal environments. Based on a higher-order shear deformation theory and von Kármán nonlinear strain–displacement relationships, the motion equations of the beam and impactor are established and solved by means of a two-step perturbation approach. The material properties of both CFRC layers and CNTRC layers are assumed to be temperature-dependent. To assess engineering application of this hybrid structure, two conditions for outer CNTRC layers and outer CFRC layers are compared. Besides, the effects of the crack density, volume fraction of carbon nanotube, temperature variation, the foundation stiffness and damping on the nonlinear low-velocity impact behavior of hybrid laminated beams are also discussed in detail.