镍涂层对碳纳米管/镁复合材料界面结合强度影响的分子动力学模拟

使用分子动力学模拟方法研究了镍涂覆单壁碳纳米管(SWCNTs)增强镁基复合材料的力学行为.结果表明,镍涂覆SWCNT/Mg复合材料的杨氏模量显著大于未涂覆SWCNT/Mg复合材料的杨氏模量,在碳纳米管表面修饰的Ni涂层可有效传递碳纳米管和Mg基体之间的载荷.此外,还研究了Ni涂层数对SWCNT/Mg复合材料界面结合强度的影响.对于不同Ni涂层数,即无Ni涂层、1层Ni涂层和2层Ni涂层,涂覆1层Ni和2层Ni的SWCNT从Mg基体中完全拔出后的界面结合强度分别约为无Ni涂层SWCNT/Mg复合材料界面结合强度的3.9和11.9倍.     

多壁碳纳米管-高聚物纳米复合材料的抗蠕变性能

通过双螺杆挤出机和模压成型设备制备了两种不同长径比的多壁碳纳米管(MWNT)增强的聚丙烯(PP)纳米复合材料.实验表明,通过添加1%体积含量的MWNT,聚丙烯的抗蠕变性能得到很大提高,即长时间加载后,基体的蠕变变形量和蠕变率均显著降低.同时,在特定载荷下,纳米复合材料的蠕变寿命比纯基体提高了10倍.几种载荷传递机理导致了材料抗蠕变性能的增强:(1)碳纳米管和基体之间较好的界面性能,(2)碳纳米管限制了基体内无定型分子链的活动性,以及(3)碳纳米管的较高的长径比.差分热扫描(DSC)的结果显示了材料蠕变前后结晶的变化和载荷传递机理分析是一致的.这些实验结果显示,在不增加成本的基础上可以大大提高抗蠕变的聚合物纳米复合材料的工程应用.     

多壁碳纳米管-高聚物纳米复合材料的抗蠕变性能

通过双螺杆挤出机和模压成型设备制备了两种不同长径比的多壁碳纳米管（MWNT）增强的聚丙烯（PP）纳米复合材料。实验表明，通过添加1％体积含量的MWNT，聚丙烯的抗蠕变性能得到很大提高，即长时间加栽后，基体的蠕变变形量和蠕变率均显著降低。同时，在特定载荷下，纳米复合材料的蠕变寿命比纯基体提高了10倍。几种栽荷传递机理导致了材料抗蠕变性能的增强：（1）碳纳米管和基体之间较好的界面性能，（2）碳纳米管限制了基体内无定型分子链的活动性，以及（3）碳纳米管的较高的长径比。差分热扫描（DSC）的结果显示了材料蠕变前后结晶的变化和栽荷传递机理分析是一致的。这些实验结果显示，在不增加成本的基础上可以大大提高抗蠕变的聚合物纳米复合材料的工程应用。     

纤维增强橡胶复合材料界面力学性能有限元分析

基于剪滞理论,引入双线性内聚力模型研究了纤维与基体界面应力传递机理.采用ABAQUS模拟了非理想界面在单纤维拔出过程中的脱粘失效,分析了不同脱粘阶段界面剪应力分布情况,以及界面刚度和纤维长径比对界面应力传递和拔出载荷的影响规律.结果 表明,在纤维受载失效过程中,纤维的拔出过程可分为4个阶段,即界面的完全粘结、损伤演化、...     

增强相形态对复合材料微区力学状态影响的有限元分析

采用三维有限元方法模拟了非连续增强金属基复合材料的应力场，得到了不同长径比的椭球形增强体周围的最大主应力场和应力球张量场的分布，分析了增强体长径比对非连续增强金属基复合材料的应力场、应力集中、界面应力过渡及材料内部最危险位置的影响。与仅适用于稀疏夹杂的Eshelby单夹杂模型相比，本文模型(体积分数约为20％—60％)与工程实际更加接近，所得的椭球状增强体内部应力分布并不均匀的计算结果与Eshelby的经典解析解有所不同。     

短切碳纤维C/SiC陶瓷基复合材料的动态劈裂拉伸实验

为了探究C/SiC陶瓷基复合材料的动态断裂力学行为和破坏形态,利用分离式霍普金森压杆(split Hopkinson pressure bar,SHPB)装置对3种不同短切碳纤维体积分数的C/SiC陶瓷基复合材料进行了动态劈裂实验,并利用扫描电子显微镜扫描了C/SiC复合材料试件的破坏界面,分析了C/SiC陶瓷基复合材料的失效特征和增韧机理。实验结果表明:C/SiC复合材料在冲击劈裂实验过程中,同一短切碳纤维体积分数下试件的动态抗拉强度随着冲击气压的增大而增大; 短切碳纤维体积分数为16.0%时, 材料的抗拉强度最低; 冲击后,试件的整体破坏情况与冲击气压、短切碳纤维体积分数有关。     

共晶基陶瓷复合材料的断裂韧性

应用细观力学方法研究了由具有随机尺寸和方位的棒体共晶体构成的共晶基陶瓷复合材料的断裂韧性.首先根据棒状共晶体的细观结构特性,考虑共晶体边界处的微观滑移确定共晶陶瓷复合材料的开裂应力,当外载荷达到开裂应力时,裂纹开始扩展.然后分析裂纹表面处的棒状共晶体桥联力使裂纹产生闭合效应,减小裂纹尖端的应力集中,建立棒状共晶体桥联增韧机制;再依据棒状共晶体拔出过程中摩擦力做功,建立棒状共晶体拔出增韧机制.最后在棒状共晶体的桥联与拔出增韧机制的基础上,得到了共晶基陶瓷复合材料断裂韧性的理论表达式.结果表明共晶基陶瓷复合材料的断裂韧性与棒状共晶体的长径比密切相关.     

横观各向同性轴对称界面端奇异性研究

套筒模型是复合材料中常用的进行纤维、基体间应力传递分析的轴对称模型.在套筒模型中,中心为纤维,纤维外包裹的"套筒"有假设为各向同性基体材料的,也有假设为横观各向同性复合材料的.不失一般性,本文将纤维和基体均视作横观各向同性材料,建立了任意楔形角的横观各向同性复合材料基体包裹横观各向同性纤维的轴对称模型,采用两次坐标变换、逐次渐近等求解方法,得到了求解该模型界面端应力奇异性指数的特征方程.考虑常见的碳纤维/环氧树脂复合材料制成的压入和拔出试件,根据得到的特征方程计算了两种试件的界面端奇异性指数随碳纤维体积百分含量的变化情况,结果发现,随纤维体积百分含量的增加,两种试件界端的奇异性均呈减弱趋势.     

颗粒增强橡胶细观力学性能二维数值模拟

在细观层次上建立了具有随机分布形态的代表性体积单元,通过细观力学有限元方法对炭黑颗粒填充橡胶复合材料的宏观力学性能进行了研究。采用二维平面应变模型进行单轴压缩模拟仿真,通过施加周期边界条件保证了代表性体积单元变形场的协调性。重点研究讨论了颗粒的随机分布形态和粒径大小、刚度、体积分数对复合材料宏观应力-应变关系曲线和有效弹性模量的影响。结果表明:炭黑颗粒的填充显著提升了橡胶材料的刚度,在炭黑含量Vf=0.2513时,复合材料的有效弹性模量值高于橡胶初始模量值的2倍;复合材料的有效弹性模量随颗粒所占体积分数的增加而增大。     

碳纳米管增强硫化天然橡胶拉伸性能的分子动力学模拟

基于分子动力学(MD)模拟方法,建立了碳纳米管/硫化天然橡胶复合材料体系模型,采用ReaxFF势函数,模拟了不同碳纳米管(CNT)含量的复合材料的拉伸过程.通过计算复合材料体系的自由体积分数、均方位移及回转半径,分析了材料基本微观性质和碳纳米管团聚的机制,计算结果与实验相符.通过碳纳米管与硫化天然橡胶界面能的计算,发现在加载过程中系统总势能的变化主要由硫化天然橡胶基体引起,其中非键能起主导作用;碳纳米管由于其自身力学性能较好,且与天然橡胶分子链相互作用产生界面能,导致材料力学性能提升,材料的屈服应力随碳纳米管含量的增加而显著升高.     

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.     

Analysis of the vibrational behavior of the composite cylinders reinforced with non-uniform distributed carbon nanotubes using micro-mechanical approach

Nanocomposites are a promising new class of structural materials for the aerospace structural components. This paper presents a detailed theoretical investigation of dynamic characteristics of cylinders made of carbon nanotube-reinforced composites. The cylinders are modeled as a cylindrical shell consisting of an isotropic matrix reinforced with transversely isotropic carbon nanotubes. Two different types of carbon nanotube reinforcements are considered: single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs). The effects of carbon nanotube aspect ratio, dispersion, alignment and volume fraction on the elastic modulus are analyzed using the Eshelby–Mori–Tanaka theory. Mass and stiffness matrices are obtained via Ritz method and natural frequencies of the structure are derived through solving the eigenvalue problem. Finally, the effects of the CNT distribution on mode shapes and natural frequencies are discussed.     

范德华力对多壁纳米碳管力学性质的影响

用分子动力学方法模拟了多壁纳米碳管在压缩、弯曲变形下力与变形的关系．通过与组成多壁碳管的各单壁碳管的比较分析,揭示了多壁纳米碳管层间范德华力对碳管力学性质的影响．采用Tersoff-Brenner势描述每一单壁纳米碳管内原子间作用,采用Lennard-Jones势描述碳管壁间范德华力．计算结果表明：多壁纳米碳管的比强度明显高于单壁纳米碳管．纳米碳管的半径虽然对杨氏模量影响不大,但对纳米碳管的曲屈行为影响却相当显著。     

Nonlinear analysis of a SWCNT over a bundle of nanotubes

The deformation of a single wall carbon nanotube (SWCNT) interacting with a curved bundle of nanotubes is analyzed. The SWCNT is modeled as a straight elastic inextensible beam based on small deformation. The bundle of nanotubes is assumed rigid and the interaction is due to the van der Waals forces. An analytical solution is obtained using a bilinear approximation to the van der Waals forces. The analytical results are in good agreement with the results of two numerical methods. The results indicate that the SWCNT remains near the curved bundle provided that its curvature is below a critical value. For curvatures above this critical value the SWCNT breaks contact with the curved bundle and nearly returns to its straight position. A parameter study shows that the critical curvature depends on the stiffness of the SWCNT and the absolute minimum energy associated with the van der Waals forces but it is independent of the SWCNT's length in general. An analytical estimate of the critical curvature is developed. The results of this study may be applicable to composites of nanotubes where separation phenomena are suspected to occur.     

A cohesive law for carbon nanotube/polymer interfaces based on the van der Waals force

We have established the cohesive law for interfaces between a carbon nanotube (CNT) and polymer that are not well bonded and are characterized by the van der Waals force. The tensile cohesive strength and cohesive energy are given in terms of the area density of carbon nanotube and volume density of polymer, as well as the parameters in the van der Waals force. For a CNT in an infinite polymer, the shear cohesive stress vanishes, and the tensile cohesive stress depends only on the opening displacement. For a CNT in a finite polymer matrix, the tensile cohesive stress remains the same, but the shear cohesive stress depends on both opening and sliding displacements, i.e., the tension/shear coupling. The simple, analytical expressions of the cohesive law are useful to study the interaction between CNT and polymer, such as in CNT-reinforced composites. The effect of polymer surface roughness on the cohesive law is also studied.     

Effects of dimensional parameters and various boundary conditions on axisymmetric vibrations of multi-walled carbon nanotubes using a continuum model

Axisymmetric vibrations of multi-walled carbon nanotubes (MWCNTs) with finite length are investigated in this paper. A multi-walled carbon nanotube is modeled with multiple elastic isotropic shells. Based on a continuum model and considering van der Waals forces between tubes, a two-dimensional finite element model is developed to obtain axisymmetric natural frequencies and mode shapes of MWCNTs. First, the axisymmetric vibrational characteristics of single-walled carbon nanotubes are investigated, and then, they are compared with those of MWCNTs. The effects of van der Waals forces on radial vibration of MWCNTs are also explained. Moreover, influences of end conditions on radial and axial natural frequencies of carbon nanotubes (CNTs) with wide range of length-to-thickness ratio are studied by considering the free-free (F-F) and simply supported (S-S) end conditions. Besides, it is focused on dependence of axisymmetric mode frequencies on dimensional parameters such as length, diameter, as well as number of layers of MWCNTs. Through this, explicit expressions are found for calculating the radial breathing mode and longitudinal mode frequencies of MWCNTs.     

纤维压出实验双向脱粘现象的数值研究

本文利用轴对称边界元程序，对影响纤维同界面剪面应力分布的材料和几何参数；纤维与基体的弹性模量比，纤维体积分数，试件厚度和支撑孔尺寸进行了数值分析工作，找出了纤维压出实验中导致界面双向脱粘的主要影响参数，给出了关于性暮一经和纤维体积分数的两界面端初始剪切脱粘的发生区域。     

界面及热残余应力对超磁致伸缩复合材料有效性能的影响

本文基于Mori-Tanaka理论，考虑了界面相对超磁致伸缩复合材料的有效性能的影响，得到了具有界面相的超磁致伸缩复合材料的有效性能的一般解析表达。考虑到固化过程中热残余应力对超磁致伸缩复合材料有效性能的影响，通过数值计算，给出超磁致伸缩复合材料有效弹性模量、有效磁致伸缩应变及有效热膨胀系数随夹杂物长径比、体分比、界面参数和固化热残余应力的变化特征曲线，数值结果表明：界面和固化热残余应力对于超磁致复合材料有效性能的影响是显著的。     

混杂短纤维增强复合材料弹性模量预测

在现代工程结构中,纤维增强复合材料具有较高的刚度重量比、优异的耐久性和设计灵活性等优点,因此得到了广泛应用．本文结合细观力学中的Mori-Tanaka方法和Halpin-Tsai方法推导了混杂碳纤维和玻璃纤维增强复合材料有效弹性模量的解析表达式．通过引入参数λ,提出了计算随机方向混合纤维增强复合材料弹性模量的新模型,分析了纤维长径比和体积分数对复合材料弹性模量的影响．结果表明,复合材料的弹性性能对纤维长径比和体积分数非常敏感．根据提出的理论,混杂纤维增强复合材料的弹性模量处于单一纤维（纯碳纤维或纯玻璃纤维）增强复合材料弹性模量之间．对于单一纤维增强复合材料,采用Halpin-Tsai方法计算的复合材料弹性模量高于Mori-Tanaka方法计算结果．     

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。