On the effective thermal conductivity of carbon nanotube reinforced polymer composites

In this paper, a theoretical model has been developed for predicting the effective thermal conductivity of an aligned multi-walled nanotube polymer composite. This model is based on an effective medium theory that has been developed for composites containing aligned spheroidal inclusions with imperfect interfaces. To incorporate the nanotube structure into this theory, a continuum model of the nanotube geometry is developed by considering its structure and the mechanism of heat conduction through it. Results show that the overall conductivity will be much lower than expected due to the fact that in the composite, the outer nanotube layer carries the bulk of the heat flowing through the nanotube. It is also seen that the high nanotube–matrix boundary resistance does not significantly affect the overall conductivity. The effective conductivity was also found to be highly sensitive to the nanotube diameter.     

A numerical study on carbon nanotube pullout to understand its bridging effect in carbon nanotube reinforced composites

Carbon nanotube (CNT) reinforced polymeric composites provide a promising future in structural engineering. To understand the bridging effect of CNT in the events of the fracture of CNT reinforced composites, the finite element method was applied to simulate a single CNT pullout from a polymeric matrix using cohesive zone modelling. The numerical results indicate that the debonding force during the CNT pullout increases almost linearly with the interfacial crack initiation shear stress. Specific pullout energy increases with the CNT embedded length, while it is independent of the CNT radius. In addition, a saturated debonding force exists corresponding to a critical CNT embedded length. A parametric study shows that a higher saturated debonding force can be achieved if the CNT has a larger radius or if the CNT/matrix has a stronger interfacial bonding. The critical CNT embedded length decreases with the increase of the interfacial crack initiation shear stress.     

Joining of natural fiber reinforced composites using microwave energy: Experimental and finite element study

The development of natural fiber reinforced polymer composites has received widespread attention due to their environment friendly characteristics over the synthetic fiber based polymer composites. Although, different categories of natural fiber reinforced composites have been developed, their joining has not been explored extensively. In the current article, natural fibers (nettle and grewia optiva) reinforced polylactic acid green composites and polypropylene based partially biodegradable composites have been developed. These composites have been joined with an innovative microwave heating process in the presence of suitable susceptor. Samples have also been joined with the well known adhesive bonding technique for comparison purposes. Joint strength has been evaluated in each case as per standard procedures and results showed that microwave joining provides higher joint strength as compared to adhesive bonding. Microwave heating process has also been simulated with standard multiphysics finite element (FE) software to analyze the microwave heating mechanism. The results of the experimental study are in close agreement with the finite element investigation.     

Thermal conductivity of carbon nanotube cross-bar structures

We use non-equilibrium molecular dynamics (NEMD) to compute the thermal conductivity (κ) of orthogonally ordered cross-bar structures of single-walled carbon nanotubes. Such structures exhibit extremely low thermal conductivity in the range of 0.02-0.07 W m(-1) K(-1). These values are five orders of magnitude smaller than the axial thermal conductivity of individual carbon nanotubes, and are comparable to the thermal conductivity of still air.     

Nanoscratch behavior of carbon nanotube reinforced aluminum coatings

Nanoscratch experiments have been carried out on plasma sprayed aluminum alloy coatings reinforced with 0, 5 and 10 wt.% carbon nanotubes (CNTs). Scratches have been performed at loads of 1000, 2000 and 3000 μN load using Berkovich indenter. The contact and true wear volumes of the scratches have been calculated. The nano-scale wear resistance is shown to increase by 4 times by addition of 10 wt.% CNTs. Improvement in the wear resistance is discussed with respect to strengthening effect and increased elastic recovery by addition of CNTs. Direct evidence of increased recovery and small decrease in the coefficient of friction with CNT content is provided using the true and instantaneous depth plots and the corresponding scanning probe microscope and scanning electron microscope images of the scratches. Friction coefficient was found to be load independent and was found to vary slightly with the CNT content.     

Damping characteristics of carbon nanotube reinforced aluminum composite

A multi-walled carbon nanotube (MWNTs) reinforced 2024Al composite was successfully fabricated by a procedure of mixing 2024Al powders and CNTs, cold isostatic press and hot extrusion. The damping behaviors of the composite were investigated with frequency of 0.5, 1.0, 5.0, 10, 30 Hz, at a temperature of 25–400 °C. The experimental results show that the frequency significantly affects the damping capacity of the composite when the temperature is above 230 °C; meanwhile, the damping capacity of the composite with a frequency of 0.5 Hz reaches 975 × 10^− 3, and the storage modulus is 82.3 GPa when the temperature is 400 °C, which shows that CNTs are a promising reinforcement for metal matrix composites to obtain high damping capabilities at an elevated temperature without sacrificing the mechanical strength and stiffness of a metal matrix.     

Modeling of interfacial debonding crack in particle reinforced composites using Voronoi cell finite element method

In this paper, Voronoi cell finite element method (VCFEM), introduced by Ghosh and coworkers (1993), is applied to describe the matrix-inclusion interfacial debonding for particulate reinforced composites. In proposed VCFEM, the damage initiation is simulated by partly debonding of the interface under the assumption of the critical normal stress law, and gradual matrix-inclusion separations are simulated with an interface remeshing method that a critical interfacial node at the crack tip is replaced by a node pairs along the debonded matrix-inclusion interface and a more pair of nodes are needed to be added on the crack interface near the crack tip in order to better facilitate the free-traction boundary condition and the jumps of solution. The comparison of the results of proposed VCFEM and commercial finite element packages MARC and ABAQUS. Examples have been given for a single inclusion of gradually interfacial debonding and for a complex structure with 20 inclusions to describe the interfacial damage under plane stress conditions. Good agreements are obtained between the VCFEM and the general finite element method. It appears that this method is a more efficient way to deal with the interfacial damage of composite materials. The financial support by the Special Funds for the National Major Fundamental Research Projects G19990650 and the National Natural Science Foundation of China No. 59871022 are gratefully acknowledged.     

Multiscale modeling of carbon nanotube reinforced polymer composites

This article examines the effect of interfacial load transfer on the stress distribution in carbon nanotube/polymer composites through a stress analysis of the nanotube/matrix system. Both isostrain and isostress loading conditions are investigated. The nanotube is modeled by the molecular structural mechanics method at the atomistic level. The matrix is modeled by the finite element method, and the nanotube/matrix interface is assumed to be bonded either perfectly or by van der Waals interactions. The fundamental issues examined include the interfacial shear stress distribution, stress concentration in the matrix in the vicinity of nanotube ends, axial stress profile in the nanotube, and the effect of nanotube aspect ratio on load transfer.     

Interface toughness of carbon nanotube reinforced epoxy composites

Traditional single-fiber pull-out type experiments were conducted on individual multiwalled carbon nanotubes (MWNT) embedded in an epoxy matrix using a novel technique. Remarkably, the results are qualitatively consistent with the predictions of continuum fracture mechanics models. Unstable interface crack propagation occurred at short MWNT embedments, which essentially exhibited a linear load-displacement response prior to peak load. Deep embedments, however, enabled stable crack extension and produced a nonlinear load-displacement response prior to peak load. The maximum pull-out forces corresponding to a wide range of embedments were used to compute the nominal interfacial shear strength and the interfacial fracture energy of the pristine MWNT-epoxy interface.     

Calorimetric study on mechanically milled aluminum and multiwall carbon nanotube composites

Pure aluminum reinforced with carbon nanotube (CNT) composites have been prepared by high energy attritor milling up to 48 hrs. Differential Scanning Calorimetry (DSC) has been carried out to investigate apparent activation energy and order of the reaction between carbon nanotubes and aluminum by Kissinger equation and Crane equation under non-isothermal conditions. The DSC results clearly reveal that an exothermic reaction occurs before the melting of aluminum. The effect of milling time on the initiation of this exothermic reaction has been studied. The peak temperature of the reaction of carbon nanotubes and aluminum is found to depend on the heating rate during the continuous heating. Apparent activation energy was found to get doubled after milling for 36 hrs compared to 24 hrs milled samples. The mechanism of the reaction kinetics which depends on reaction order is instantaneous nucleation and one dimensional growth for both samples. Formation of Al₄C₃ was confirmed by X-ray diffraction (XRD) of as-milled powders and after performing DSC of the milled powders.     

Thermal conductivity of boron nitride reinforced polyethylene composites

The thermal conductivity of boron nitride (BN) particulates reinforced high density polyethylene (HDPE) composites was investigated under a special dispersion state of BN particles in HDPE, i.e., BN particles surrounding HDPE particles. The effects of BN content, particle size of HDPE and temperature on the thermal conductivity of the composites were discussed. The results indicate that the special dispersion of BN in matrix provides the composites with high thermal conductivity; moreover, the thermal conductivity of composites is higher for the larger size HDPE than for the smaller size one. The thermal conductivity increases with increasing filler content, and significantly deviates the predictions from the theoretic models. It is found also that the combined use of BN particles and alumina short fiber obtains higher thermal conductivity of composites compared to the BN particles used alone.     

Investigation of carbon nanotube reinforced aluminum matrix composite materials

We have increased the tensile strength without compromising the elongation of aluminum (Al)–carbon nanotube (CNT) composite by a combination of spark plasma sintering followed by hot-extrusion processes. From the microstructural viewpoint, the average thickness of the boundary layer with relatively low CNT incorporation has been observed by optical, field-emission scanning electron, and high-resolution transmission electron microscopies. Significantly, the Al–CNT composite showed no decrease in elongation despite highly enhanced tensile strength compared to that of pure Al. We believe that the presence of CNTs in the boundary layer affects the mechanical properties, which leads to well-aligned CNTs in the extrusion direction as well as effective stress transfer between the Al matrix and the CNTs due to the generation of aluminum carbide.     

平纹编织碳纤维增强树脂复合材料离散电导率建模方法

编织碳纤维增强树脂复合材料(CFRP)的电阻抗分布具有各向异性、异质性、几何结构复杂等特点。建立电阻抗分布模型是利用电磁涡流无损检测技术获取编织CFRP缺陷及疲劳损伤信息的关键关节。基于电阻抗张量建模理论,采用多层编织结构CFRP二维平面的分块均化电学特性表征方法,建立编织结构CFRP的简化电阻抗分布模型,从而实现编织结构CFRP电磁特性的精确、快速有限元分析。在有限元仿真基础上,通过设计双空气旋转线圈电磁传感器对平纹编织CFRP进行电磁无损检测,选用阻抗的极坐标图描述被测材料沿不同方向的阻抗变化趋势,通过实验验证有限元建模的正确性。最后利用所提出的建模方法模拟了双空气旋转线圈传感器对平纹编织CFRP的结构缺陷及循环载荷疲劳的检测效果。      

Interface characteristics of carbon nanotube reinforced polymer composites using an advanced pull-out model

An advanced pull-out model is presented to obtain the interface characteristics of carbon nanotube (CNT) in polymer composite. Since, a part of the CNT/matrix interface near the crack tip is considered to be debonded, there must present adhesive van der Waals (vdW) interaction which is generally presented in the form of Lennard-Jones potential. A separate analytical model is also proposed to account normal cohesive stress caused by the vdW interaction along the debonded CNT/polymer interface. Analytical solutions for axial and interfacial shear stress components are derived in closed form. The analytical result shows that contribution of vdW interaction is very significant and also enhances stress transfer potential of CNT in polymer composite. Parametric studies are also conducted to obtain the influence of key composite factors on bonded and debonded interface. The result reveals that the parameter dependency of interfacial stress transfer is significantly higher in the perfectly bonded interface than that of the debonded interface.     

Thermal conductivity and thermal expansions of aluminum/carbon composites based on wood templates

Wood with its rational and aesthetic inner structures was used as a template to fabricate aluminum/carbon composites in this research. Porous carbon was first pyrolyzed from the wood templates. The final composites were then obtained by injecting aluminum alloy into porous carbon. The microstructures, thermal conductivity, and thermal expansions of these composites were then analyzed. The results indicate that the structures of the aluminum/carbon composites are controlled by the natural structure of wood. Moreover, the composites exhibit a lower coefficient of thermal expansion than aluminum and a higher thermal conductivity than porous carbon.     

Microstructural characteristics and mechanical properties of carbon nanotube reinforced aluminum alloy composites produced by ball milling

The influence of milling time on the structure, morphology and thermal stability of multi-walled carbon nanotubes (MWCNTs) reinforced EN AW6082 aluminum alloy powders has been studied. After structural and microstructural characterization of the mechanically milled powders micro- and nano-hardness of the composite powder particles were evaluated. The morphological and X-ray diffraction studies on the milled powders revealed that the carbon nanotubes (CNTs) were uniformly distributed and embedded within the aluminum matrix. No reaction products were detected even after long milling up to 50 h. Nanotubes became shorter in length as they fractured under the impact and shearing action during the milling process. A high hardness of about 436 ± 52 HV is achieved for the milled powders, due to the addition of MWCNTs, after milling for 50 h. The increased elastic modulus and nanohardness can be attributed to the finer grain size evolved during high energy ball milling and to the uniform distribution of hard CNTs in the Al-alloy matrix. The hardness values of the composite as well as the matrix alloy compares well with that predicted by the Hall–Petch relationship.     

A study of continuous filament reinforced aluminum matrix composites

Some principal results of the research work on metal matrix composites at Beijing Institute of Aeronautical Materials, concerning CVD-produced continuous B and SiC filaments reinforced aluminum and its alloys, are summarized. The processing, fiber degradation, interface, mechanical properties and fracture behavior of the composites are discussed.Abbreviations CVD chemical vapor deposition - MMCs metal matrix composites - ROM rule of mixtures     

An object oriented implementation of a front tracking finite element method for directional solidification processes

This paper focuses on the numerical simulation of phase‐change processes using a moving finite element technique. In particular, directional solidification and melting processes for pure materials and binary alloys are studied. The melt is modelled as a Boussinesq fluid and the transient Navier–Stokes equations are solved simultaneously with the transient heat and mass transport equations as well as the Stefan condition. The various streamline‐upwind/Petrov–Galerkin‐based FEM simulators developed for the heat, flow and mass transport subproblems are reviewed. The use of classes, virtual functions and smart pointers to represent and link the particular simulators in order to model a phase change process is discussed. The freezing front is modelled using a spline interpolation, while the mesh motion is defined from the freezing front motion using a transfinite mapping technique. Various two‐ and three‐dimensional numerical tests are analysed and discussed to demonstrate the efficiency of the proposed techniques. Copyright © 1999 John Wiley & Sons, Ltd.     

Investigating the mechanical properties of single walled carbon nanotube reinforced epoxy composite through finite element modelling

Varying experimental results on the mechanical properties of carbon nanotube reinforced polymer composites (CNTRPs) have been reported due to the complexities associated with the characterization of material properties in nano-scale. Insight into the issues associated with CNTRPs may be brought through computational techniques time- and cost-effectively. In this study, finite element models are generated in which single walled carbon nanotube models are embedded into the epoxy resin. For modelling interface regions, two approaches named as non-bonded interactions and perfect bonding model are utilized and compared against each other. Representative volume finite element (RVE) models are built for a range of CNTRPs and employed for the evaluation of effects of diameter and chirality on the Young’s modulus and Poisson’s ratio of CNTRPs, for which there is a paucity in the literature. The outcomes of this study are in good agreement with those reported available in the literature earlier. The proposed modelling approach presents a valuable tool for determining other material properties of CNTRPs.