Cooperative deformation of carboxyl groups in functionalized carbon nanotubes

Functionalized carbon nanotubes have tremendous potential for nanotechnology applications such as in the fabrication of polymeric carbon fibers. However, approaches to design carbon nanotube structures by using functional groups as glue and carbon nanotubes as stiff building blocks to reach superior mechanical strength and toughness at the fiber level with limited amount of materials remains poorly understood. Inspired by the outstanding mechanical properties of spider silk, here we present a bio-inspired structural model of carbon nanotube based fibers connected by weak hydrogen bonds (H-bonds) formed between functional carboxyl groups as the molecular interface. By applying shear loading, we study how the deformation of H-bonds in functional groups is affected by the structural organization of the carboxyl groups, as well as by the geometry of constituting carbon nanotubes. The analysis of H-bond deformation fields is used to compute the extent of significant deformation of inter-CNT bonds, defining a region of cooperativity. We utilize an exponential function (exp (?x/ξ)) to fit the deformation of H-bonds, with the cooperative region defined by the parameter ξ, and where a higher value of ξ represents a weaker exponential decay of displacements of carboxyl groups from the point where the load is applied. Hence, the parameter ξ characterizes the number of carboxyl groups that participate in the deformation of CNTs under shear loading. The cooperativity of deformation is used as a measure for the utilization of the chemical bonds facilitated by the functional groups. We find that for ultra-small diameter CNTs below 1 nm the external force deforms H-bonds significantly only within a relatively small region on the order of a few nanometers. We find that the mechanical properties of carbon nanotube fibers are affected by the organization of H-bonds in functional carboxyl groups. Both, the grouping of functional groups into clusters, and a specific variation of the clustering of functional groups along the CNT axis are shown to be potential strategies to improve the cooperativity of deformation. This allows for a more effective utilization of functional groups and hence, larger overlap lengths between CNTs in fibers. The effect of structural organization of functional groups is not only significant in very small diameter CNTs, but also in larger diameter CNTs as they are most commonly used for engineering applications. Notably larger-diameter CNTs naturally show a larger cooperative deformation range. Our model can be applied to other functional groups attached to CNTs, and could in principle also include strong bonds such as covalent or ionic bonds, or other weak bonds such van der Waals forces or dipole–dipole interactions.     

A DAMAGE MECHANICS MODEL FOR TWISTED CARBON NANOTUBE FIBERS

Carbon nanotube fibers can be fabricated by the chemical vapor deposition spinning process. They are promising for a wide range of applications such as the building blocks of high-performance composite materials and micro-electrochemical sensors. Mechanical twisting is an effective means of enhancing the mechanical properties of carbon nanotube fibers during fabrication or by post processing. However, the effects of twisting on the mechanical properties remain an unsolved issue. In this paper, we present a two-scale damage mechanics model to quantitatively investigate the effects of twisting on the mechanical properties of carbon nanotube fibers. The numerical results demonstrate that the developed damage mechanics model can effectively describe the elastic and the plastic-like behaviors of carbon nanotube fibers during the tension process. A definite range of twisting which can effectively enhance the mechanical properties of carbon nanotube fiber is given. The results can be used to guide the mechanical twisting of carbon nanotube fibers to improve their properties and help optimize the mechanical performance of carbon nanotube-based materials.     

植物纤维增强复合材料力学高性能化与多功能化研究

论文从植物纤维的微观结构、化学组成以及力学性能入手,针对植物纤维增强复合材料的界面性能,综述了国内外采用植物纤维表面处理方法来提升复合材料力学性能的研究进展,分析了所遇到的瓶颈,并进一步从复合材料结构设计的角度出发,充分利用植物纤维独特的多层次、多尺度的微观结构特点,通过揭示植物纤维增强复合材料多层次、多尺度的界面力学损伤破坏机制,实现了植物纤维增强复合材料的界面调控和力学高性能化。在此基础上,提出了植物纤维增强复合材料兼顾阻燃和声学性能的结构设计原则和特有的界面力学研究方法。此外,也介绍了相关基础研究成果在航空、轨道交通等领域的示范应用,并针对实现绿色复合材料的结构功能一体化的应用提出了未来研究方向。     

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.     

碳纳米管纤维及其传感器力电性能实验研究

基于UTM T150微纳米力学测试系统搭建了纤维力电耦合测试平台,并对碳纳米管纤维进行了多组力电耦合作用下的拉伸性能实验研究。通过观察分析碳纳米管纤维的电阻-应变曲线,发现其电阻变化与应变大小密切相关,随着应变的不断变大,碳纳米管纤维的电阻也在逐渐增加。基于此,设计制作了碳纳米管纤维柔性应变传感器,并将其贴在标准试件上分别进行了弯曲变形测试、单次/循环加载性能测试以及标定实验。结果表明:当对试件进行加载时,碳纳米管纤维柔性应变传感器的电阻会随着载荷的增大而增大;当载荷逐渐消失时,其电阻会逐渐减小直至回归初始电阻值附近。这表明碳纳米管纤维具备优异的电学与力学特性,并且利用它所制作的柔性应变传感器也有着较好的灵敏度与稳定性。     

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.     

Electro-Thermal Effects and Deformation Response of Carbon Fiber

A carbon fiber mat is a sheet composed of intercrossing short carbon fibers, which has more stable and lower electrical resistivity compared with dispersed short carbon fiber mixed in cement. Thereby carbon fiber mat cement could exhibit obvious electro-thermal effect. When electrified, the temperature of composite structures made up of cement mortar and carbon fiber mat will rise rapidly. If the temperature field is not uniform, temperature difference will cause structures to deform, which can be used to adjust the deformation of structures. The temperature field and deformation response driven by the electro-thermal effects of a type of carbon fiber mat cement beams are studied. Firstly, the temperature and deformation responses are studied using theories of thermal conduction and elasticity. Secondly, experimental results are given to verify the theoretical solution. These two parts lay the foundation for temperature and deformation adjustment.     

ELECTRO-THERMAL EFFECTS AND DEFORMATION RESPONSE OF CARBON FIBER MAT CEMENT BEAMS

A carbon fiber mat is a sheet composed of intercrossing short carbon fibers, which has more stable and lower electrical resistivity compared with dispersed short carbon fiber mixed in cement. Thereby carbon fiber mat cement could exhibit obvious electro-thermal effect. When electrified, the temperature of composite structures made up of cement mortar and carbon fiber mat will rise rapidly. If the temperature field is not uniform, temperature difference will cause structures to deform, which can be used to adjust the deformation of structures. The temperature field and deformation response driven by the electro-thermal effects of a type of carbon fiber mat cement beams are studied. Firstly, the temperature and deformation responses are studied using theories of thermal conduction and elasticity. Secondly, experimental results are given to verify the theoretical solution. These two parts lay the foundation for temperature and deformation adjustment.     

原位微拉曼测试技术在碳纳米管纤维和薄膜材料力学性能研究中的应用

利用宏观应力联合原位微拉曼测试技术对双壁碳纳米管(Carbon Nanotube,CNT)纤维和薄膜材料的力学性能进行了实验分析,探讨了拉伸加载期间纤维和薄膜内CNT的载荷响应及其与宏观力学性能的关联,揭示了两种材料力学性能差异性的微观机理。实验分析表明,CNT纤维和薄膜的拉伸变形呈现弹性、强化和损伤断裂三个阶段,但其内的CNT只发生弹性变形,没有塑性形变,且没有明显的损伤或键的断裂,纤维和薄膜呈现阶段性拉伸变形的原因可归结为滑移。纤维的弹性模量显著高于薄膜,是薄膜的4.7倍,原因是弹性阶段纤维中CNT的轴向伸长对宏观应变的贡献较大。纤维和薄膜的拉伸强度相差较小,原因是强化阶段薄膜内不断有大量CNT进入承载队伍,这也使得薄膜具有比纤维更高的韧性。     

EFFECTS OF DIFFERENT FUNCTIONALIZATION SCHEMES ON THE INTERFACIAL STRENGTH OF CARBON NANOTUBE POLYETHYLENE COMPOSITE

The mechanical performance of carbon nanotube(CNT) reinforced polymer composites is primarily controlled by the dispersive capacity and interfacial shear strength of CNTs in polymer matrices. CNT functionalizations will improve dispersion and strengthen interfacial bonding of CNTs in matrices. To understand the effects of different functionalization schemes on the interfacial strength of CNT-polymer composites, pullout of the covalent, noncovalent, and mixed functionalized single-walled carbon nanotube(SWCNT) from polyethylene(PE) matrix was simulated by using molecular dynamics, respectively. The results show that the SWCNT-PE interfacial shear strength is significantly improved by SWCNT functionalizations, particularly by mixed functionalization.     

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.     

天然纤维增强复合材料力学性能及其应用

本文介绍了天然纤维的化学成分、结构以及力学性能;综述了天然纤维的表面处理方式,分析了其作用机理,并讨论了表面处理对其复合材料力学性能的影响;从增强体形式出发,介绍了短纤维、纤维毡、纤维织物以及单向纤维增强复合材料,并研究了成型工艺、纤维含量和表面处理等对其拉伸、弯曲、界面性能和冲击强度以及断裂韧性的影响;最后总结了天然纤维增强复合材料在汽车、建筑土木等领域的应用现状,并展望了其发展和应用前景。     

非对称铺层复合材料层合板质量优化设计

复合材料广泛应用于航空航天等领域,追求轻量化设计已经成为研究重点.对复合材料层合板质量优化设计,可以减少层合板的纤维用量,减小层合板的质量,降低成本.首先研究复合材料层合板在承受轴向载荷时,产生的形变量、应力示意图,分析容易发生失效部位;以层合板铺层厚度为设计变量,最大应变、铺层比例等为约束条件,最小化层合板质量为优化...     

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.     

平纹机织碳纤维复合材料的多尺度随机力学性能预测研究

平纹机织碳纤维复合材料在结构上具有多尺度特性和空间随机性. 同时, 组分材料会因存储条件和组成相成分、批次的不同导致力学性能有所差异. 当考虑各尺度结构和组分性能参数不确定性进行随机力学性能预测时, 存在以下两个难点: 一是随机变量众多, 使得对不确定性传递方法的精度和效率提出了要求; 二是由于随机参数之间存在高维相关性, 需要建立高精度的相关性模型. 针对以上问题, 本文提出了基于混沌多项式展开和Vine Copula的平纹机织复合材料多尺度随机力学性能预测方法, 综合考虑了平纹机织碳纤维复合材料微观及介观尺度的材料、结构随机参数, 基于自下而上层级传递的策略逐尺度地研究力学性能不确定性. 该方法采用Vine Copula理论构造相关随机变量的高维联合概率分布, 并运用非嵌入式混沌多项式展开法实现不确定性传递. 结果显示, 本方法构造的相关性模型几乎与原模型一致, 且能够高效准确地实现各尺度力学性能的随机预测.      

A periodic unit-cell simulation of fiber arrangement dependence on the transverse tensile failure in unidirectional carbon fiber reinforced composites

The effect of fiber arrangement on transverse tensile failure in unidirectional carbon fiber reinforced composites with a strong fiber-matrix interface was studied using a unit-cell model that includes a continuum damage mechanics model. The simulated results indicated that tensile strength is lower when neighboring fibers are arrayed parallel to the loading direction than with other fiber arrangements. A shear band occurs between neighboring fibers, and the damage in the matrix propagates around the shear band when the interfacial normal stress (INS) is sufficiently high. Moreover, based on the observation of Hobbiebrunken et al., we reproduced the damage process in actual composites with a nonuniform fiber arrangement. The simulated results clarified that the region where neighboring fibers are arrayed parallel to the loading direction becomes the origin of the transverse failure in the composites. The cracking sites observed in the simulation are consistent with experimental results. Therefore, the matrix damage in the region where the fiber is arrayed parallel to the loading direction is a key factor in understanding transverse failure in unidirectional carbon fiber reinforced composites with a strong fiber/matrix interface.     

多尺度复合材料力学研究进展

多尺度复合材料力学是运用多尺度分析思想研究空间分布非均匀材料力学性能的学科, 近年来,多 组分、多层级先进材料的蓬勃发展和微纳米实验观测手段的不断进步,有力地推动了该学科的研究,论文围绕非均 匀材料力学性能的多尺度分析,首先从微纳米尺度到宏观尺度综述了常用的理论分析方法;接着分别针对非均匀 连续介质和离散体系介绍了常用的多尺度计算模拟方法;然后结合本课题组在纳米复合材料、抗冲击吸能材料、随 机网络材料和多层级自相似材料等方面的研究工作,举例说明了如何综合运用多种方法对各种复杂材料系统进行 多尺度分析;最后,展望了该领域还需进一步发展和完善的若干方向。     

轻质高强点阵材料及其力学性能研究进展

点阵材料是一种新型轻质高强材料, 同时具备形状控制、致动、能量吸收和传热等多种功能. 文章综述了点阵材料的拉伸主导型设计原则、点阵构型和制备工艺. 拉伸主导型点阵材料的比强度和比刚度明显强于一般胞元材料, 在低密度时质量效率更加突出. 根据材料的基本构型特征主要介绍了三维八角点阵以及夹层点阵材料, 比较分析了熔模铸造法和冲压折叠成型工艺的特点. 总结了研究点阵材料力学性能的理论方法和试验研究成果, 研究表明缺陷对点阵材料力学性能的影响明显小于一般胞元材料. 对点阵材料在形状控制与致动、传热和数值计算方面的应用研究成果进行了介绍. 文中归纳了作者近期在炭纤维点阵复合材料方面的工作, 给出了制备炭纤维隐身点阵格栅的探索性工作. 主要包括炭纤维点阵复合材料的三维编织工艺和二维点阵格栅的嵌锁工艺以及隐身点阵格栅反射率试验测试结果.      

碳纤维增强摩阻材料的摩擦磨损特性研究

利用Ｄ－ＭＳ摩擦磨损试验机研究了自制的碳纤维增强摩阻材料的碳纤维含量、表面状态、强度及长度对其摩擦磨损性能的影响．结果表明：碳纤维含量对摩阻材料的摩擦磨损性能有显著影响,低含量时主要起减摩作用,高含量时主要起抗犁削作用;经过表面改性的碳纤维与粘结剂结合强度较高,能改善摩阻材料的摩擦磨损性能,高强度碳纤维增强摩阻材料具有较好的摩擦磨损性能;碳纤维长度对摩阻材料的摩擦磨损性能和加工性能具有一定的影响．     

An Experimental Technique for the Dynamic Characterization of Soft Complex Materials

We describe an experimental technique to study the dynamic behavior of complex soft materials, based on high-speed microscopic imaging and direct measurements of dynamic forces and deformations. The setup includes high sensitivity dynamic displacement measurements based on geometric moiré interferometry and high-speed imaging for in-situ, full-field visualization of the complex micro-scale dynamic deformations. The method allows extracting dynamic stress-strain profiles both from the moiré interferometry and from the high-speed microscopic imaging. We discuss the advantages of using these two complementing components concurrently. We use this technique to study the dynamic response of vertically aligned carbon nanotube foams subjected to impact loadings at variable deformation rates. The same technique can be used to study other micro-structured materials and complex hierarchical structures.