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.     

碳纳米管增强Nb-Si基复合材料界面应力传递的研究

基于均匀化理论,建立了碳纳米管增强Nb-Si基复合材料的代表体积元模型,并采用剪切滞后模型对碳纳米管增强Nb-Si基复合材料界面上的应力分布和传递机制进行了研究,探讨了分子间作用力、杨氏模量比β、长径比α、体积分数?等对其应力分布和传递机制的影响。结果表明,复合材料界面应力分布的变化主要集中在碳纳米管的两端,最大的应力都是分布在加载端或拔出端,然后向另一端传递;分子间作用力、杨氏模量比、长径比、体积分数等参数对界面应力的传递均有一定的影响,其中长径比和体积分数的影响最明显,体积分数为0.02时拔出端的界面剪切应力值相对于体积分数为0.0025时增大幅度达到近7倍,而长径比从200减小到50时,其应力传递距离增大了近8倍。     

固化温度对碳纤维/环氧基体界面行为影响的分析

对界面粘结性能及热残余应力影响下的单纤维复合材料的界面行为进行了分析。采用界面的弹性-软化内聚力模型,用解析法对单纤维复合材料由固化引起的热残余应力、以及单纤维碎断过程纤维的轴向应力分布进行了模拟,得到了碳纤维/环氧树脂在常温和高温固化两种情况的界面粘结性能。结果表明：与常温固化相比,高温固化后,界面的剪切强度增幅不大,界面的断裂韧性显著增加;高温固化后形成的界面,使界面的软化提前、界面的脱粘延迟;高温固化产生的纤维轴向和界面径向热残余应力对界面的软化均有延迟作用;界面径向热残余应力还对界面的脱粘有延迟作用。     

颗粒增强复合材料中微观热应力和残余应力分析

运用空间配位体密堆模型和球对称分析单元，分析计算了颗粒增强复合材料经历温度变化后产生的微观热应力和残余应力。分析结果表明，温度升高时界面产生径向张应力，降温时产生压应力。存在一个基体开始发生塑性变形的临界温差ｔ＿ｐ，其值随增强体体积分数Ｖ＿ｐ增加而降低。除组元间热膨胀系数差和弹性常数外，基体材料本构关系和屈服强度对热应力和残余应力均有很大影响。随Ｖ＿ｐ增加，微观应力和水静宏观应力幅值上升。但粒子周围塑性区尺寸近似与Ｖ＿ｐ无关。给出了不同变温条件下残余应力的确定方法。     

Thermal vibration and apparent thermal contraction of single-walled carbon nanotubes

It is of fundamental value to understand the thermo-mechanical properties of carbon nanotubes. In this paper, by using molecular dynamics simulation, a systematic numerical investigation is carried out to explore the natural thermal vibration behaviors of single-walled carbon nanotubes and their quantitative contributions to the apparent thermal contraction behaviors. It is found that the thermo-mechanical behavior of single-walled carbon nanotubes is exhibited through the competition between quasi-static thermal expansion and dynamic thermal vibration, while the vibration effect is more prominent and induces apparent contraction in both radial and axial directions. With increasing temperature, the anharmonic interatomic potential helps to increase the bond length, which leads to thermally induced expansion. On the other hand, the higher structural entropy and vibrational entropy of the system cause the carbon nanotube to vibrate, and the apparent length of nanotube decreases due to various vibration modes. Parallel analytical and finite element analyses are used to validate the vibration frequencies and provide helpful insights. The unified multi-scale study has successfully decoupled and systematically analyzed both thermal expansion and contraction behaviors of single-walled carbon nanotube from 100 to 800 K, and obtained detailed information on various vibration modes as well as their quantitative contributions to the coefficient of thermal expansion in axial and radial directions. The results of this paper may provide useful information on the thermo-mechanical integrity of single-walled carbon nanotubes, and become important in practical applications involving finite temperature.     

Numerical model for composite material with polymer matrix reinforced by carbon nanotubes

Due to the high stiffness and strength, as well as their ability to act as conductors, carbon nanotubes are under intense investigation as fillers in polymeric materials. The nature of the carbon nanotube/polymer bonding and the curvature of the carbon nanotubes within the polymer have arisen as particular factors in the efficacy of the carbon nanotubes to actually provide any enhanced stiffness or strength to the nanocomposite. Here the effects of carbon nanotube curvature and interface interaction with the matrix on the nanocomposite stiffness are investigated using nanomechanical analysis. In particular, the effects of poor bonding and thus poor shear lag load transfer to the carbon nanotubes are studied. In the case of poor bonding, carbon nanotubes waviness is shown to enhance the composite stiffness.     

导电混凝土传感器的热变形与机敏性研究

论文设计了具有不同灵敏度的水泥基传感器.测试了传感器的热变形特征与机敏性规律.验证了传感器埋入混凝土进行结构变形检测的可行性.热膨胀系数测定实验发现:与传统认为的受热膨胀不同,添加了碳纳米管的传感器具有热胀-热缩特性.通过对比传感器单独加载与埋入混凝土中加载,发现了大掺量的碳纳米管传感器的压阻效应更易受到混凝土干缩应力...     

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。     

微拉曼光谱研究M55JB碳纤维/微滴的拉伸变形行为

使用评价纤维/基体界面力学性能的新方法纤维微滴拉伸测试,来研究M55JB碳纤维/环氧树脂基体之间的界面应力传递性能。使用自制的微加载装置对碳纤维/环氧树脂微滴试样进行对称式拉伸测试,用微拉曼光谱仪记录下不同应变下的嵌入微滴内纤维上的拉曼频移信号,经过应力/频移关系转换成纤维轴向应力。实验结果显示,微滴内纤维轴向应力随载荷而明显增加。根据界面力平衡模型得到相应的界面剪切应力呈反对称式分布,在纤维嵌入端存在剪应力集中。新测试方法能保证嵌入微滴内纤维上的应力呈对称式分布,而且能降低纤维嵌入端附近的应力奇异性。     

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.     

INSTANTANEOUS THERMAL EXPANSION BEHAVIOR OF HETEROGENEOUS MATERIALS WITH A WORK-HARDENING ELASTOPLASTIC MATRIX AND ELASTIC SPHEROIDAL INHOMOGENEITIES

The instantaneous thermal expansion behavior of two-phase hetero-geneous materials subjected to a uniform temperature change is explored in the presentstudy.The matrix phase is assumed to be a work-hardening ductile metal and thedispersive phase is assumed to consist of either aligned or randomly-oriented,elastic,spheroidal inhomogeneities.The plastic flow and decreasing stiffness of the matrixduring Eshelby's transformation strain of the equivalent inclusions are accounted for byusing the deformation theory of plasticity.The explicit results of the instantaneousoverall thermal expansion coefficients and the critical inelastic temperature changes arepresented for aligned disc-and fiber-inclusions.For the spherical and randomly-oriented spheroidal inclusion,the present study demonstrates that when the yielding ofthe composites is governed by the average matrix stress,the overall response is alwayselastic in spite of the temperature change.     

Investigation of stress-strain behavior of single walled carbon nanotube/rubber composites by a multi-scale finite element method

The excellent properties of carbon nanotubes have generated technological interests in the development of nanotube/rubber composites. This paper describes a finite element formulation that is appropriate for the numerical prediction of the mechanical behavior of rubber-like materials which are reinforced with single walled carbon nanotubes. The considered composite material consists of continuous aligned single walled carbon nanotubes which are uniformly distributed within the rubber material. It is assumed that the carbon nanotubes are imperfectly bonded with the matrix. Based on the micromechanical theory, the mechanical behavior of the composite may be predicted by utilizing a representative volume element. Within the representative volume element, the reinforcement is modeled according to its atomistic microstructure. Therefore, non-linear spring-based line elements are employed to simulate the discrete geometrical structure and behavior of the single-walled carbon nanotube. On the other hand, the matrix is modeled as a continuum medium by utilizing solid elements. In order to describe its behavior an appropriate constitutive material model is adopted. Finally, the interfacial region is simulated via the use of special joint elements of variable stiffness which interconnect the two materials in a discrete manner. Using the proposed multi-scale model, the stress-strain behavior for various values of reinforcement volume fraction and interfacial stiffness is extracted. The influence of the single walled carbon nanotube addition within the rubber is clearly illustrated and discussed.     

Local displacements and load transfer in shape memory alloy composites

Although there has been a significant amount of research dedicated to characterizing and modeling the response of shape memory alloys (SMAs) alone, little experimental work has been done to understand the behavior of SMAs embedded in a host material. The interaction between SMA wires and a host polymer matrix was investigated by correlating local displacements and stress fields induced by the embedded wires with SMA/polymer adhesion. Most SMA composite applications require transfer of strain from the wire to the matrix. In these applications, maximum interfacial adhesion between the SMA wire and the polymer matrix is most desirable. The adhesion was varied by considering four different surface treatments: untreated, acid etched, hand sanded and sandblasted. The average interfacial bond strength of the SMA wires embedded in an epoxy matrix was measured by standard pull out tests. Sandblasting significantly increased the bond strength, whereas hand sanding and acid cleaning actually reduced interface strength. In situ displacements of embedded, surface-treated SMA wires were measured using heterodyne interferometry, whereas the resulting stresses induced in the polymer matrix were investigated using photoelasticity. Increased wire adhesion resulted in lower axial wire displacement and higher interfacial stresses due to the restraining effect of the matrix on the actuated wire. A simplified theoretical analysis was carried out to estimate the shear stress induced in the matrix due to wire actuation. The maximum shear stress predicted for the case of a perfect interfacial bond was about 7 percent larger than the value measured experimentally for the sand-blasted wire.     

A time dependent micro-mechanical fiber composite model for inelastic zone growth in viscoelastic matrices

In this work, a fiber composite model is developed to predict the time dependent stress transfer behavior due to fiber fractures, as driven by the viscoelastic behavior of the polymer matrix, and the initiation and propagation of inelastic zones. We validate this model using in situ, room temperature, micro-Raman spectroscopy fiber strain measurements. Multifiber composites were placed under constant load creep tests and the fiber strains were evaluated with time after one fiber break occurred. These composite specimens ranged in fiber volume fraction and strain level. Comparison between prediction and MRS measurements allows us to characterize key in situ material parameters, the critical matrix shear strain for inelastic zones and interfacial frictional slip shear stress. We find that the inelastic zone is predominately either shear yielding or interfacial slipping, and the type depends on the local fiber spacing.     

Creep and recovery of polypropylene/carbon nanotube composites

The creep and recovery of polypropylene/multi-walled carbon nanotube composites were studied. It was found for thermoplastics in general that the creep strain reduces with decreased temperature, and with enhanced content of carbon nanotubes. The incorporation of nanotubes improved the recovery property remarkably, especially at high temperature. The unrecovered creep strain of nanocomposites with content of 1 and 2.8 vol.% carbon nanotubes decreased by 53% and 73% compared to that of polymer matrix. To understand the mechanisms, the Burger’s model and Weibull distribution function were employed since the variations in the simulating parameters illustrated the influence of nano-fillers on the creep and recovery performance of the bulk matrix. To further study the recovery properties, the particular contribution of each Burger’s element to the total deformation was obtained and the recovery percentage was calculated. The time-temperature-superposition-principle was applied to predict the long-term creep behavior.     

The analysis of thermoplastic characteristics of special polymer sulfur composite

Specific chemical environments step out in the industry objects. Portland cement composites (concrete and mortar) were impregnated by using the special polymerized sulfur and technical soot as a filler (polymer sulfur composite). Sulfur and technical soot was applied as the industrial waste. Portland cement composites were made of the same aggregate, cement and water. The process of special polymer sulfur composite applied as the industrial waste is a thermal treatment process in the temperature of about 150–155 \(^{\circ }\hbox {C}\). The result of such treatment is special polymer sulfur composite in a liquid state. This paper presents the plastic constants and coefficients of thermal expansion of special polymer sulfur composites, with isotropic porous matrix, reinforced by disoriented ellipsoidal inclusions with orthotropic symmetry of the thermoplastic properties. The investigations are based on the stochastic differential equations of solid mechanics. A model and algorithm for calculating the effective characteristics of special polymer sulfur composites are suggested. The effective thermoplastic characteristics of special polymer sulfur composites, with disoriented ellipsoidal inclusions, are calculated in two stages: First, the properties of materials with oriented inclusions are determined, and then effective constants of a composite with disoriented inclusions are determined on the basis of the Voigt or Rice scheme. A brief summary of new products related to special polymer sulfur composites is given as follows: Impregnation, repair, overlays and precast polymer concrete will be presented. Special polymer sulfur as polymer coating impregnation, which has received little attention in recent years, currently has some very interesting applications.     

Meshless method for nonlinear heat conduction analysis of nano-composites

Carbon nanotubes (CNTs) may become ideal reinforcing materials for high-performance nano-composites due their exceptional properties. Still, much work is needed to be done before the potentials of CNT based composites can be fully realized. The evaluation of effective material properties of nano-composites is one of many difficult tasks. Simulations using continuum mechanics approach can play a significant role in the analysis of these composites. In the present work, nonlinear heat conduction analysis of CNT based composites has been carried out using continuum mechanics approach. Element free Galerkin method has been applied as a numerical tool. Thermal conductivities of nanotube and polymer matrix are assumed to vary quadratically with temperature. Picard and quasi-linearization schemes have been utilized to obtain the solution of a system of nonlinear equations. Cylindrical representative volume element has been used to evaluate the thermal properties of nano-composites. Present simulations show that the temperature dependent matrix thermal conductivity has a significant effect on the equivalent thermal conductivity of the composite, whereas temperature dependent nanotube thermal conductivity has a small effect on the equivalent thermal conductivity of the composite. The results obtained by Picard method have been found almost similar with those obtained by quasi-linearization approach.     

Thermal expansion of copper-carbon fiber composites

The thermal expansion data are obtained for carbon fiber copper matrix composites prepared from variously arranged crossed ply unidirectional monolayers and copper foils. Four types of specimens are made with different thickness of monolayers and copper foils and constant carbon fiber content. The initial thermal treatment of all specimens was the same. After being cooled from the bonding temperature, they were cycled up to 673 K three times. Due to this treatment the cracks were created in the monolayers. The specimens were additionally cycled between the temperature of 293 K to 623 K and coefficient of thermal expansion calculated from the linear slope for lower temperatures was about 10⁻⁵ m/m/K and decreased to zero at 600 K due to the very low coefficient of thermal expansion of the carbon fibers. Thermal expansion led to large hysteresis by plastic flow of the copper matrix. The hysteresis loops after additional cycling did not change. No other cracks were observed by optical microscopy.     

SMA复合材料在热载条件下的残余应力分析

本文基于三相复合圆柱模型发展了增量型的分析方法,讨论在ＳＭＡ复合材料中由于ＳＭＡ材料相变以及各相材料热特性随温度变化引起的残余应力。研究基体与过渡恸介面和纤维与过渡界面间的残余应力,同时讨论由于基体相的变化对残余应力的影响。特别研究了涂层和复合材料基体间界面处的残余应力受纤维体积比、涂层厚度、纤维最大相变应以及基体中纤维取向等影响,而且讨论了计及应力对相就运动方程的影响时对ＳＭＡ复合材料相变温度和     

The effect of carbon nanotubes in enhancing the thermal transport properties of PCM during solidification

This study is aimed to prepare a novel class of nanofluid phase change material (NFPCM) by dispersing a small amount of multi-walled carbon nanotubes (MWCNT) in liquid paraffin, to enhance the heat transfer properties and examine the characteristics of the NFPCM during the solidification process. The stable NFPCMs are prepared by dispersing the MWCNT in liquid paraffin at 30°C with volume fractions of 0.15, 0.3, 0.45 and 0.6% without any dispersing agents. The rheology measurement illustrates the Newtonian fluid behavior in the shear stress range of 1–10?Pa. The differential scanning calorimetric results showed that there is no observable variation in the freezing/melting temperature of the NFPCM, and only a small observable change in the latent heat values. The thermal conductivity of various NFPCM is measured. The enhancement in thermal conductivity increases with the increased volume fraction of the MWCNT, and shows a weak dependence on the temperature. Further, for the NFPCM with a volume fraction of 0.6%, there is an appreciable increase in heat transfer with a reduction in the solidification time of 33.64%. The enhancement in the heat transfer performance would alleviate the major problems that have been encountered in the conventional phase change materials since several years.