连续分布界面相对SiCp/Al复合材料热导率影响的数值模拟

采用有限元方法对SiCp/Al复合材料的导热性能进行了数值模拟, 建立了含界面相颗粒增强铝基复合材料测试模型, 研究了不同界面相种类、厚度对复合材料热导率的影响。结果表明: 当界面相与SiC/Al结合理想时, 且界面相在颗粒表面呈连续分布时, 复合材料热导率随着界面层热导率的增加而增大, 但增加的幅度由快变慢; 复合材料热导率随界面层厚度的变化取决于界面层厚度t与颗粒粒径a的比值, 当t/a很小或t/a较大时, 热导率随界面层厚度的变化很小, 当t/a较小时, 热导率随界面层厚度的变化则与界面层热导率有关。     

考虑夹杂间相互影响的弱界面颗粒复合材料等效模量预测的一般细观力学方法

基于Ju和Chen提出的颗粒增强复合材料的一般细观力学方法,采用Qu给出的一阶近似修正的Eshelby张量,本文建立了考虑夹杂之间相互影响的含任意分布弱界面颗粒增强多相复合材料等效模量预测的一般细观力学方法,通过体积均匀化方法得到了一组细观力学本构方程。分别推导了忽略夹杂之间相互影响的弱界面多相复合材料宏观模量表达式以及考虑夹杂之间相互影响的弱界面两相复合材料的宏观模量表达式。此外,还给出了几种特殊复合材料体系在考虑界面弱化下的宏观等效性能参数的解析表达式,在界面粘结完好的假设下,给出的表达式均可以退化成经典的细观力学预测结果。在考虑夹杂之间相互影响的情况下,本文预测结果与实验数据吻合很好,验证了模型的有效性。     

颗粒增强复合材料非理想界面刚度和有效模量的理论估计

采用内聚力模型(CZM)描述颗粒增强复合材料(PRCs)颗粒/基体非理想界面的力学行为,采用细观力学的Mori-Tanaka(M-T)方法和稀释解方法研究非理想界面刚度对该复合材料有效模量的影响。结果表明:复合材料某一体积含量下的有效模量和其颗粒/基体非理想界面刚度之间存在单调递增的关系曲线。同一复合材料不同体积含量的有效模量和非理想界面刚度关系曲线存在一个临界交点(CP),该点对应的临界界面刚度控制着颗粒体积分数对有效模量的影响。研究了基体和增强相的力学性能以及增强颗粒的尺寸对CP点临界界面刚度的影响。采用有效模量-界面刚度关系曲线,结合实验测得的有效模量,提出了估算PRCs的非理想界面刚度的方法,进而估计了复合材料的宏观有效模量。      

随机植物短纤维复合材料界面性能对有效模量和拉伸行为的影响

研究了界面性能对随机短云杉纤维增强聚丙烯(PP)复合材料宏观拉伸性能的影响。采用双线性内聚力模型(CZM)描述随机短云杉纤维和PP基体间非理想界面的力学行为,建立了含非理想界面的随机短纤维增强复合材料代表性单元(RVE)的二维有限元模型,考虑了纤维含量、长细比、随机分布和随机各向异性弹性以及PP基体弹塑性的影响;模拟了不同纤维含量复合材料的实验拉伸应力应变曲线。结果表明,短云杉纤维/PP基体间非理想界面刚度与复合材料有效弹性模量之间有单调递增的曲线关系,即E-K曲线;同一复合材料不同纤维含量的E-K曲线簇有一个临界交点。在交点右侧强界面刚度区复合材料有效模量随着纤维含量的增加而提高,在交点左侧弱界面刚度区有效模量随着纤维含量的增加而减少。三种不同体积含量10%、20%和49%的云杉/PP复合材料的非理想界面刚度可用E-K曲线和实验测得的宏观有效弹性模量确定,云杉/PP界面初始破坏位移和界面完全破坏位移也可根据模拟拉伸应力应变曲线确定。数值分析结果能用非理想界面刚度来解释和理解随机短植物纤维体积含量对复合材料宏观有效模量的影响。     

短纤维增强橡胶密封复合材料界面疲劳损伤行为

依据广义自洽方法,建立了包含芳纶纤维、界面相、橡胶基体和等效介质的代表性体积单元(RVE)模型。采用自定义材料子程序对内聚力疲劳累积损伤模型进行编译,分别在基体/界面相的界面和纤维/界面相的界面设置内聚力单元,研究界面相性能参数对纤维增强橡胶密封复合材料(SFRC)界面疲劳损伤行为的影响。探讨了界面相厚度和模量的确定方法,获得了不同界面相厚度和模量下SFRC界面脱粘起始位置以及脱粘起始疲劳次数。结果表明,较低的界面相模量能够抑制界面脱粘的产生;随着界面相厚度的增加,界面脱粘的起始疲劳次数增加,SFRC抗疲劳损伤能力得到提高。     

石墨鳞片-碳纤维协同增强铜基复合材料的制备与热物理性能

采用真空热压技术制备了石墨鳞片-碳纤维协同增强铜基复合材料,研究了碳纤维含量对复合材料的组织结构、抗弯强度与热导率的影响.结果表明,石墨鳞片-碳纤维/铜基复合材料界面结合良好;当碳纤维体积分数为0.5vol％～1.5vol％时,碳纤维能够均匀分散在基体中,并有效提升复合材料的抗弯强度.当碳纤维体积分数为1.5vol％时...     

不同界面SiC纤维束复合材料的拉伸力学行为

利用国产三代SiC纤维通过化学气相渗透工艺(CVI)制备不同界面厚度和基体体积分数的SiC纤维束复合材料,并对其拉伸力学行为进行研究;同时,通过有限元方法研究界面厚度和基体体积分数对SiC纤维束复合材料热残余应力的影响。有限元分析结果表明:该纤维束复合材料的界面存在较为明显的径向和环向热残余应力,而且这两种应力均随着界面厚度增加而减小,随着基体体积分数的增加而增加。拉伸实验结果表明:随着界面厚度增加SiC纤维束复合材料的拉伸强度有增大趋势,且纤维拔出长度也相应增加;但在界面厚度相同的情况下,过高的基体体积分数将导致复合材料拉伸强度和韧性下降。     

一种新的β-锂霞石增强铜基复合材料热性能

采用热压烧结工艺成功制备了一种新的β-锂霞石增强铜基复合材料.利用扫描电镜和透射电镜对复合材料的微观组织进行了分析,并对不同体积分数复合材料的致密性,热膨胀性能和热传导性能进行了测试.结果表明:β-锂霞石颗粒在铜基体中分布均匀,界面清晰,不发生界面反应;体积分数对复合材料致密性、热膨胀系数和热导率有明显影响,当β-锂霞石颗粒体积分数超过40%时,复合材料的致密性有明显下降,热膨胀系数在(9～15.4)×10-6/K,同时热导率在50～170W/m·K.     

纤维增强聚合物复合材料的三相动态粘弹模型及其界面结构参数

本文以复合材料界面作为中间相(界面相),借助Takayanagi的两相共混模型及Ziege的修正公式,导出了单向纤维增强聚合物复合材料的三相动态粘弹共混模型,得到了界面层的几何结构混合参数(r_i、V_i、λ_L及中间参数B)及界面相的各个动态粘弹性参数.这些参数可以根据复合材料的动态粘弹性实验结果求得,利用上述导出的模型,对玻璃纤维单向增强不饱和聚酯的动态粘弹性进行了试验,结果表明,随着温度的变化,界面相的厚度r_i、体积分数V_i及动态粘弹参数也会发生一定的变化,存在着界面相与基体相之间的相互转变.      

AlN/橡胶复合材料等效热导率数值计算

利用计算机生成不同的AlN/橡胶复合材料等效结构单元,基于三维格子玻尔兹曼模型计算了复合材料的等效热导率。实验制备了AlN/橡胶复合材料,并测定了不同填充量下复合材料的热导率,用以验证模型的有效性。将LBM计算结果与实验结果及Maxwell、Bruggeman、Nielsen等模型进行了比较,发现本文数值计算结果与Maxwell模型吻合较好,相比较于Bruggeman模型与Nielsen模型更加接近实验值。研究了AlN颗粒尺寸及分布方式对复合材料导热性能的影响。结果表明,一定体积分数范围内,粒径较小的AlN颗粒填充橡胶复合材料的等效热导率较大,当体积分数增大到20%,粒径较大的复合材料内先开始形成导热网络,大大提高了热导率;随机分布比均匀分布方式下的复合材料的等效热导率大,不同的粒子空间分布结构是影响复合材料热导率的关键因素。     

Collective effects in a dense system of large bubbles

The rising velocity and interphase transport of a dense bubble swarm in a homogeneous liquid are investigated, along with the motion and external mass transfer in an inhomogeneous porous medium.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 41, No. 6, pp. 1057–1066, December, 1981.     

Fabrication of Lotus‐type Porous Metals and their Physical Properties

Lotus‐type porous metals whose long cylindrical pores are aligned in one direction were fabricated by unidirectional solidification in a pressurized gas atmosphere. The pores are formed as a result of precipitation of supersaturated gas when liquid metal is solidified. The lotus‐type porous metals with homogeneous size and porosity of the evolved pores produced by a mould casting technique are limited to the metals with high thermal conductivity. On the other hand, the pores with inhomogeneous pore size and porosity are evolved for metals and alloys with low thermal conductivity such as stainless steel. In order to obtain uniform pore size and porosity, a new “continuous zone melting technique” was developed to fabricate long rod‐ and plate‐shape porous metals and alloys even with low thermal conductivity. Mechanical properties of tensile and compressive strength of lotus‐type porous metals and alloys are described together with internal friction, elasticity, thermal conductivity and sound absorption characteristics. All the physical properties exhibit significant anisotropy. Lotus‐type porous iron fabricated using a pressurized nitrogen gas instead of hydrogen exhibits superior strength.     

Thermal behavior of an elliptic inhomogeneity surrounded by a compliant interphase layer

Summary This paper examines the thermal behavior of a plane elastic compliant interphase layer surrounding an elliptic inhomogeneity which is embedded within an infinite matrix. To allow continuity of traction but discontinuity of displacements, the compliant interphase is modeled as a spring layer with a vanishing thickness. Furthermore, to obtain the resulting thermal stresses, the complex variable method was used, together with a series solution. A commerical finite element package was used to validate the theoretical predictions. The results reveal that thermal stresses vary with the aspect ratio of the inhomogeneity and the parameterh describing the spring constant of the interphase layer for four different types of inhomogeneous materials: aluminum, copper, gold and silver. In all these cases, the matrix was assumed to be made of silicon and the thermal stresses were assumed to result from a uniform change in temperature.     

Thermal conductivity of inhomogeneous materials

The effective thermal conductivity is calculated from the rate of entropy production per unit volume. Thermal conductivity and the temperature field are expressed in terms of Fourier components and these are related. The rate of entropy production is then obtained in terms of the volume-averaged thermal conductivity and the Fourier components of thermal conductivity. A simple expression for the effective thermal conductivity is found. In the case of striations it leads to well-known results. The formalism is applied to solids with inhomogeneously distributed solutes. It is shown that the thermal conductivity is less than the volume-averaged thermal conductivity and that homogenization by diffusion increases the thermal conductivity. Similar results would apply to the electrical conductivity of inhomogeneous alloys.     

Modeling the thermal conductivity of diamond reinforced aluminium matrix composites with inhomogeneous interfacial conductance

A reasonable model for describing the thermal conductivity of diamond reinforced aluminium matrix composites behaving a distinctive character of inhomogeneous distribution of interfacial thermal conductance on diamond surfaces is proposed in terms of an equivalent diameter approach combined with a double effective-medium approximation scheme. Theoretical analyses for the thermal conductivity of diamond reinforced Al (Si) composites prepared by different infiltration techniques (squeeze casting (SQ), gas pressure infiltration (GPI)) are given for rediscovering the existing experimental results considering inhomogeneous conductance behavior. Numerical results using present model agree reasonably well with the experimental observations and explore new findings, i.e. the diffusion bonding also occurs at Al–diamond {1 1 1} interfaces of GPIed composites; the interconnected particles is possibly existed in GPIed Al/diamond composites.     

Scattering of Thermal Waves and Non-steady Effective Thermal Conductivity of Unidirectional Fibrous Composites with Functionally Graded Interface

In this paper, a thermal wave method is applied to investigate the non-steady effective thermal conductivity of unidirectional fibrous composites with a functionally graded interface, and the analytical solution of the problem is obtained. The Fourier heat conduction law is applied to analyze the propagation of thermal waves in the fibrous composite. The scattering and refraction of thermal waves by a cylindrical fiber with an inhomogeneous interface layer in the matrix are analyzed, and the results of the single scattering problem are applied to the composite medium. The wave fields in different material layers are expressed by using the wave function expansion method, and the expanded mode coefficients are determined by satisfying the boundary conditions of the layers. The theory of Waterman and Truell is employed to obtain the effective propagating wave number and the non-steady effective thermal conductivity of composites. As an example, the effects of a graded interface on the effective thermal conductivity of composites are graphically illustrated and analyzed. Analysis shows that the non-steady effective thermal conductivity under higher frequencies is quite different from the steady thermal conductivity. In the region of intermediate and high frequencies, the effect of the properties of the interface on the effective thermal conductivity is greater. Comparisons with the steady thermal conductivity obtained from other methods are also presented.     

Three-dimensional Green’s functions for a steady point heat source in a functionally graded half-space and some related problems

Three-dimensional Green’s functions are derived for a steady point heat source in a functionally graded half-space where the thermal conductivity varies exponentially along an arbitrary direction. We first introduce an auxiliary function which satisfies an inhomogeneous Helmholtz equation. Then by virtue of the image method which was first proposed by Sommerfeld for the homogeneous half-space Green’s function of a steady point heat source, we arrive at an explicit expression for this function. Finally with this auxiliary function, we derive the three-dimensional Green’s functions due to a steady point heat source in a functionally graded half-space. Also investigated in this paper are the temperature field induced by a point heat source moving at a constant speed in a functionally graded full-space; the electric potential due to a static point electric charge in a dielectric full-space with electric field gradient effects; and the two-dimensional time-harmonic dynamic Green’s function for homogeneous and functionally graded materials with strain gradient effects.     

Approximation of inhomogeneous thermal lines via series connection of homogeneous thermal lines

A method is given for approximating an arbitrary inhomogeneous thermal line (ITL) as a series connection of homogeneous thermal lines (HTL). The method is compared with other approximation methods.     

Dependence of the Effective Thermal Pressure Coefficient of a Particulate Composite on Particles Size

Model for overall thermal pressure coefficient (TPC) of a composite reinforced with layered particles is developed. It is based on the incremental homogenization scheme that replaces layered inclusions by homogeneous ones producing the same effect. Various laws of property variation across the interphase are discussed. It is shown that the most important parameters controlling effective TPC are the relative thickness of the interphase layer and the mean value of TPC across this layer.     

Thermal conductivity of composites

A general expression for the effective thermal conductivity of inhomogeneous media in terms of the Fourier components of the spatial variation of the conductivity is applied to composites consisting of inclusions in a continuous matrix. It is reformulated in terms of the mean square fluctuations of the conductivity. Specific cases treated are spherical inclusions and long cylinders, both random and with preferred directions. The results hold provided the difference in thermal conductivities is small or provided the concentration of inclusions is not too large. The theory fails if the thermal conductivity of the matrix is much smaller than that of the inclusions. The same considerations also apply to electrical conductivity.