Creep of Heat-Resistant Composites of an Oxide-Fiber/Ni-Matrix Family

A creep model of a composite with a creeping matrix and initially continuous elastic brittle fibers is developed. The model accounts for the fiber fragmentation in the stage of unsteady creep of the composite, which ends with a steady-state creep, where a minimum possible average length of the fiber is achieved. The model makes it possible to analyze the creep rate of the composite in relation to such parameters of its structure as the statistic characteristics of the fiber strength, the creep characteristics of the matrix, and the strength of the fiber-matrix interface, the latter being of fundamental importance. A comparison between the calculation results and the experimental ones obtained on composites with a Ni-matrix and monocrystalline and eutectic oxide fibers as well as on sapphire fiber/TiAl-matrix composites shows that the model is applicable to the computer simulation of the creep behavior of heat-resistant composites and to the optimization of the structure of such composites. By combining the experimental data with calculation results, it is possible to evaluate the heat resistance of composites and the potential of oxide-fiber/Ni-matrix composites. The composite specimens obtained and tested to date reveal their high creep resistance up to a temperature of 1150°C. The maximum operating temperature of the composites can be considerably raised by strengthening the fiber-matrix interface.     

基于有限元的金属基短纤维复合材料MMC的一种统计蠕变模型

在有限元分析的基础上建立了一个单向应力状态下金属基短纤维复合材料(MMC)的统计蠕变模型.首先建立细胞模型并进行有限元分析,得到了单向应力状态下材料细观尺寸及载荷方向对宏观蠕变响应的影响规律.通过在细胞模型中增加一界面层(考虑材料特性和厚度)来研究基体和纤维的界面对MMC宏观蠕变响应的影响.基于细胞模型的数值结果,提出了一适用于纤维平面随机分布的随机统计模型,该模型考虑了纤维的断裂.通过试验获得纤维的统计分布规律.分析结果表明随机统计模型可以满意地描述试验结果.进一步讨论了材料细观尺寸,纤维的断裂特性以及界面层的材料特性和厚度对MMC宏观蠕变响应的影响.     

Multifibre polymer composites: Prediction of deformation and thermophysical properties

Conclusions A theoretical and experimental investigation was carried out to examine the possibilities of a structural approach for prediction of elastic constants, creep functions and thermophysical characteristics of hybrid polymer composites reinforced with anisotropic fibres of several types. The theoretical solutions were obtained by generalizing the self-consistent method for the case of a three phase model. The effects of brittle fibre breakdown under tension in the direction of reinforcement of a unidirectional hybrid composite were studied under conditions of a short-term loading and a long-term creep. It has been shown that a creep of viscoelastic fibres plays a principal role in creep of the hybrid composite. It is just this creep that significantly increases the fibre damage during creep of the composite.A variant of the solution has been proposed for predicting the thermorheologically complex behavior of hybrid composites containing not only elastic but also viscoelastic thermorheologically simple components with different temperature-time shift factors. The peculiarities of thermal expansion of hybrid composites and the possibilities for a purposeful control of thermal expansion coefficients by hybridization were studied. The considered thermal interval included a region of transition of the polymer matrix from a glass state into a viscoelastic one.The control tests were performed for specimens of organic/glass, organic/carbon, glass/carbon and organic/boron polymer composites with different ratios of fibre volume contents. On the whole, the obtained accuracy of predicting the characteristics of the examined hybrid composites may be considered as acceptable for engineering applications.Published in Mekhanika Kompozitnykh Materialov, Vol. 30, No. 3, pp. 299–313, May–June, 1994.     

High-Temperature Creep of Fibrous Metal-Matrix Composites under Varying Stresses

The present paper is aimed at testing the hypothesis about the failure of the relatively weak fiber/matrix interface under cyclic loading, which causes an increase in the steady-state creep rate. The hypothesis is tested qualitatively by comparing the creep behavior of composite specimens with various interface strengths under the conditions mentioned (loading-unloading-loading to the original level). The hypothesis is tested semi-quantitatively by estimating the interface strength in relation to the action decreasing the strength. The latter requires the use of a microstructural calculation model. Both the approaches are used in the paper, and the results found support this hypothesis. The experimental data obtained are an additional argument for the necessity of developing metal-matrix composites with a strong interface, which can be a basis for real creep-resistant high-temperature composites.     

Numerical Prediction of the Strain State of Thermoset Matrix/Mineral Filler Composite Plates

The numerical prediction of the fields of inelastic strains (the linear invariant of the tensor of inelastic strains) in thermoset polyester/marble filler composite plates is discussed. A uniformly distributed load is applied to the plates, which lie on a steel base. The strain fields are predicted by means of the boundary element method by using creep test data for the composites and the polyester matrix itself. Identical creep tests were performed for two ages of the materials (1 month and 13 years), which allowed evaluating the aging effect. The study is carried out in two stages. At the first stage, the application of the generalized Maxwell-Gurevich equation to the thermoset matrix/mineral filler composite is demonstrated. The model parameters determined from the experimental creep data is used for the second stage, where the state of inelastic strains in the plates is predicted by applying the boundary element method. The influence of composite formulation (filler content) and physical aging of the polyester matrix on the state of inelastic strains in the plates is shown.__________Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 41, No. 2, pp. 145–156, March–April, 2005.     

A distributed friction model for energy dissipation in carbon nanotube-based composites

Being lighter and stiffer than traditional metallic materials, nanocomposites have great potential to be used as structural damping materials for a variety of applications. Studies of friction damping in the nanocomposites are largely experimental, and there has been a lack of understanding of the damping mechanism in nanocomposites. A new friction contact model is developed to study the energy dissipation of carbon nanotube (CNT)-based composites under dynamic loading. The model incorporates the spatially-distributed nature of the CNT in order to capture the stick/slip phenomenon at the interface and treats the total slip force in a statistical sense. The effects of several parameters on energy dissipation are investigated, including the excitation’s amplitude, the interaction between CNT’s ends and matrix, the orientation, concentration, and diameter distribution of the CNTs inside the matrix. The results are in good agreement with experimental observations in the literature.     

Theory of plasticity and creep taking account of microstrains

A relationship between the theories of plasticity and creep of the type /1, 2/ and theories based on the concept of slip is set up. A most logical structure is proposed for the constitutive equations of the theory which is convenient for engineering calculations.

It has been shown /3/ that the theory of slip /4/ results from the theories /1, 2/. However, it remains unclear whether a deeper connection exists between these theories. Moreover, the connection between creep theories constructed using the approach in /1, 2/ and creep theories based on the slip concept was not generally examined. A survey of the development of polycrystalline strain theory /5/ yields a complete representation of the state of matters in plasticity and creep theories.     

碾压混凝土坝施工层面变形分析模型

针对碾压混凝土坝施工层面对大坝变形产生显著影响的问题,深入研究了施工层面的变化性质及规律,提出了层面不同阶段变形的模拟方法,建立了施工层面有厚度和无厚度分析模型,提出的模型能反映层面的弹性变形、衰减蠕变、不可逆变形以及加速蠕变等变形状态．实例分析表明:所提出的碾压混凝土坝施工层面有厚度和无厚度分析模型能较客观地模拟大坝的结构变化形态,尤其是施工层面有厚度分析模型较完整地模拟了层面的渐变规律,其计算结果与原位监测成果吻合较好．同时,提出的方法和建立的分析模型可推广应用于常规混凝土坝,特别是坝基内断层和夹层等变形规律的分析．     

Creep simulation in polymer matrix — disperse filler composites

The deformation properties of composites with disperse fillers in close to maximum amounts were investigated. Two types of matrices were selected: epoxy resin with high structural stability in filling, and a crystalline polymer-trioxane-dioxolane copolymer. Materials with a high and low relative surface area-diatomite and ceramic-were used as fillers. The elastic properties were simulated with the Kerner model and creep was simulated with its viscoelastic analog. The model was extended to a matrix-filler-buffer layer model. The possibility of estimating the part of the matrix entering the pores of the filler was examined. Differences in the use of the structural models in predicting elasticity and viscoelasticity were examined. The best results in describing creep were obtained for epoxy resin-ceramic filler composites which have the least interaction between matrix and filler. The possibility of estimating the change in the degree of crystallinity of the matrix and formation of buffer layers in filling with active fillers was examined.Presented at the Ninth International Conference on the Mechanics of Composite Materials (Riga, October, 1995).Translated from Mekhanika Kompozitnykh Materialov, Vol. 31, No. 6, pp. 754–768, November–December, 1995.     

Effect of nanofiller aspect ratio on the stress relaxation and creep response of toughened pom composites

Ternary composites composed of polyoxymethylene (POM), polyurethane (PU), and sodium fluorohectorite (FH) or sodium bentonite (BN) were produced by the melt compounding masterbatch (MB) technique. The related MB was produced by mixing the PU latex with water-swellable FH or BN. The dispersion of the nanofillers in the composites was studied by X-ray diffraction techniques. The crystallization of the POM-based systems was inspected by polarized optical microscopy (PLM). The stress relaxation and creep properties of the composites were determined in short-time stress relaxation and creep tests (creep at various temperatures), respectively. The POM/PU/FH composites produced by the MB technique outperformed the POM/PU blend and the POM/PU/BN system in respect to most of the stress relaxation and creep characteristics. This fact was attributed to the higher aspect ratio of FH compared with that of BN. The master curves (creep compliance vs. time) constructed by employing the time-temperature superposition principle showed that the Findley power law was fully applicable to the experimental results obtained.     

On the acoustics of sonic composites

The primary goal of this paper is to propose an alternative method for obtaining the band structures of the 3D sonic composites without/ with point defects. The point defects are vacancies or foreign interstitial atoms which are supported by the interfaces between the hollow spheres and the matrix. The proposed method is used to simulate a sonic plate composed of an array of acoustic scatterers which are piezoceramic hollow spheres embedded in an epoxy matrix. The scatterers are made from functionally graded materials with radial polarization [1, 2]. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Unit-cell simulations of damage and failure in PRMMCs

The purpose of this work is damage and failure modeling in multiphase metallic materials via unit-cell simulations and homogenization methods. To this end, we investigate such behaviour in particle-reinforced metal matrix composites (PRMMCs). Taking into account the processes of void nucleation (due, e.g., to particle debonding and/or cracking) and growth, we examine the effect of phase composition, particle sizes and distributions, as well as the nature of the particle/matrix interface, on damage and failure in the unit cell. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

高体积百分比颗粒增强聚合物材料的有效粘弹性性质

聚合物材料通常表现为粘弹性性质．为了改进聚合物材料的力学性能,通常将某种无机材料以颗粒或纤维的形式填充到聚合物中,从而得到增强、增韧的聚合物基复合材料．提出了一个新的细观力学模型,用于预测颗粒增强聚合物复合材料的有效粘弹性性质,尤其针对高体积百分比的颗粒夹杂复合材料,该方法基于Laplace变换和双夹杂相互作用的弹性模型．计算了玻璃微珠/ED-6复合材料的有效松弛模量以及恒应变率下的应力应变关系．计算结果表明在高体积百分比下该文方法比基于Mori Tanaka方法预测的粘弹性效应明显减弱．     

Investigation of the mechanical behavior of two-component granular composites in terms of structural models

A structural model is suggested for elastomers filled with particles of two fractions — with diameters exceeding 10 μm and submicronic ones. In each fraction, the particle diameter varies randomly, but between the fractions, the average particle diameter differs by several orders of magnitude. It is assumed that the small particles, together with the matrix, behave as a homogeneous medium relative to the large ones. By using this model, the mechanical behavior of composites based on elastomers filled with different volume contents of solid particles is investigated. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 43, No. 2, pp. 191–200, March–April, 2007.     

An effective permeability model to predict field-dependent modulus of magnetorheological elastomers

Magnetorheological elastomers (MREs) are composites where magnetic particles are suspended in a non-magnetic solid or a gel-like matrix. MREs are shown to have a controllable, field-dependent shear modulus. Most of conventional MREs models are based on magnetic dipole interactions between two adjacent particles of the chain. These models can predict the field-dependent properties of MREs with simple chain-like structures. In this paper, an effective permeability model is proposed to predict the field-induced modulus of MREs. Based on the effective permeability rule and taking into account the particle’s saturation, the model is proposed to predict the mechanical performances of MREs with complex structure and components. The effectiveness of the model is justified by a designed novel MREs.     

Variational Phase Field Modeling of Laminate Deformation Microstructure in Finite Gradient Crystal Plasticity

The macroscopic mechanical behavior of many materials crucially depends on the formation and evolution of their microstructure. In this work, we consider the formation and evolution of laminate deformation microstructure in plasticity. Inspired by work on the variational modeling of phase transformation [5] and building on related work on multislip gradient crystal plasticity [9], we present a new finite strain model for the formation and evolution of laminate deformation microstructure in double slip gradient crystal plasticity. Basic ingredients of our model are a nonconvex hardening potential and two gradient terms accounting for geometrically necessary dislocations (GNDs) by use of the dislocation density tensor and regularizing the sharp interfaces between different kinematically coherent plastic slip states. The plastic evolution is described by means of a nonsmooth dissipation potential for which we propose a new regularization. We formulate a continuous gradient-extended rate-variational framework and discretize it in time to obtain an incremental-variational formulation. Discretization in space yields a finite element formulation which is used to demonstrate the capability of our model to predict the formation and evolution of laminate deformation microstructure in f.c.c. Copper with two active slip systems in the same slip plane. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of defects to the band-gaps generation

Effect of defects to the band-gaps generation in sonic composites is investigated in this paper. Acoustic scatterers are composed by piezoceramic hollow spheres of functionally graded materials. Defects are vacancies or foreign interstitial atoms which are supported by the interfaces between the hollow spheres and the matrix. The presence of defects in sonic composites is related to the generation of localized modes in the vicinity of the point defect with a significant evanescent behavior of the waves outside the defect point. As result, the full band-gap is wider than in the case of no defects. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Effects of Poisson Contraction on Matrix Cracking in Brittle-Matrix Composites

The effects of Poisson contraction on matrix cracking in unidirectional fiber-reinforced brittle-matrix composites are studied in this paper. The fibers, initially held in the matrix by a compressive pressure due to the thermal expansion mismatch, are subjected to frictional slipping over the matrix as soon as a fiber-bridged crack is formed. The friction between the fibers and the matrix is assumed to follow the Coulomb friction law. A shear-lag model, which includes the Poisson contraction and the friction due to the relative fiber/matrix slipping, is adopted to calculate the stress and strain fields in the fibers and matrix. Using the energy balance approach, a relation for the critical matrix cracking stress for propagating of a semi-infinite fiber-bridged crack is derived. The results obtained show that the Poisson contraction has a strong effect on the predicted matrix cracking stress in brittle-matrix composites, especially in composites with a stiff matrix.     

Numerical Determination of Damping in Metal Matrix Composites

Damping in metal matrix composites is mainly caused by inelastic matrix deformation induced by the great difference in the mechanical properties of the single constituents of the materials. In this study, the finite-element method in combination with a highly accurate material model is employed to examine the effects of both the fiber volume fraction and the external loading amplitude on the energy dissipation process in an Al/SiC composite under a cyclic mechanical load.     

Boundary slip phenomena in a binary gas mixture

The slip phenomena in gas mixtures are of fundamental significance in the specification of boundary conditions for flows in the slip regime. In a recent paper, new explicit results for the slip coefficients appropriate to binary gas mixtures were reported. The present work being reported extends the previous work to a higher level of accuracy by involving a higher order Chapman-Enskog expansion. In particular, new expressions for the slip coefficients are presented which are applicable for arbitrary models of the intermolecular interaction. Limiting expressions for the slip coefficients are given (for a simple gas) and the accuracy of the theory is discussed. Numerical calculations of the slip coefficients for different binary gas mixtures using the first and second order Chapman-Enskog approximations and the rigid sphere and Lennard-Jones (12-6) potential models have been carried out. The thermal creep and diffusion slip coefficients are found to be sensitive to the order of the approximation and to the potential model used. A comparison of the new higher order results with some of our previously obtained experimental data for the thermal transpiration effect has also been carried out and shows excellent agreement between the theory and the experiments which confirms the accuracy of the theory.