Cross-property relations for two-phase planar composites

The overall property of a composite material is dictated by parameters that characterize its microstructure. Theoretically, cross-links between different physical properties of the same material have been established by eliminating all or partially these microstructural parameters. Practically, such a correlation may be used to determine one property from another once the latter is measured or calculated: the success of this approach depends on whether the correlation is insensitive to the detailed material microstructure. In the present paper, cross-property relations for planar two-phase composites are examined using both analytical approaches and the digital-based finite element method. Both isotropic and transversely isotropic two-phase planar composites are studied. Focus is placed on studying how the microstructure (e.g., shape, size, distribution and volume fraction of inclusions) affects the correlation between two different overall properties of the composite. At a fixed volume fraction, questions on whether the correlation is one-to-one and whether it is sensitive to large material contrast (e.g., voids or rigid inclusions) or how the inclusions are distributed in the matrix will be answered.     

Stress–strain model for concrete confined by FRP composites

In this paper, a stress–strain model for concrete confined by fiber reinforced polymer (FRP) composites is developed. The model is based on the results of a comprehensive experimental program including large-scale circular, square and rectangular short columns confined by carbon/epoxy and E-glass/epoxy jackets providing a wide range of confinement ratios. Ultimate stress, rupture strain, jacket parameters, and cross-sectional geometry were found to be significant factors affecting the stress–strain behavior of FRP-confined concrete. Such parameters were analyzed statistically based on the experimental data, and equations to theoretically predict these parameters are presented. Experimental results from this study were compared to the proposed semi-empirical model as well as others from the literature.     

A three-dimensional steel/concrete interface model including corrosion effects

This paper presents a new constitutive law for modelling the steel/concrete interface by including main corrosion effects. A three-dimensional formulation has been proposed based on continuum damage mechanics. The theoretical framework of the thermodynamics of irreversible processes has been applied in order to guarantee the respect of both conservation and evolution principles. Results are presented in order to point out the main features of the proposed model. The identification of material parameters, one of the major points when invoking this model, is detailed. In order to show its efficiency, pull-out tests with and without the presence of corrosion have been simulated. The effects of corrosion on failure loads are clearly in accordance with experimental results. A qualitative comparison between the damage pattern and the experimental crack path due to corrosion is also included in the paper.     

A large deformation breakage model of granular materials including porosity and inelastic distortional deformation rate

A general constitutive model of crushable granular materials is developed within the context of large deformations. The time evolution equations for breakage, inelastic porous compaction and dilation, and distortional deformations are coupled by a yield surface and restrictions are imposed to ensure that these inelastic processes are dissipative. Some of the most salient mechanisms of such materials are described, including: (1) stiffness dependent on the breakage (a variable index of grading), porosity, and pressure; (2) critical comminution pressure and isotropic hardening, also dependent on the breakage and porosity; (3) jamming transition between solid and gaseous states; (4) a dilation law that embodies competition between porous compaction (due to the rate of breakage) and bulking (porous dilation at positive pressure due to the rate of inelastic distortional deformation); and finally, (5) the non-unique critical state relation between stress and porosity, in terms of the loading history and grading changes.     

两相复合材料等效介电常数数值计算

为精确计算复合材料等效介电常数,采用随机分布模型,在对材料电性能本构方程研究的基础上,设计了两相复合材料等效介电常数计算的体积加权平均算法(VWAM)。研究结果表明,所设计的算法计算结果稳定性较高。在复合材料组成相介电常数比值小于10时,数值计算结果与理论和实验结果吻合度非常高。在其比值大于10时,所设计的算法适用于高介电常数组成相体积分数较高情况。研究方法有望应用于多相复合材料的相关等效电磁参数计算。研究结果为复合材料设计提供参考。     

On the calculation of deformation curves for two-phase cermets

Analytical algorithms for the construction of tensile and compression stress-strain diagrams of two-phase cermets are proposed, which are based on the concept of mean stresses in the phase volume and physical equations of the theory of small elastoplastic strains. The deformation properties and strength of cermet are supposed to depend on such parameters of its microstructure as mean size and coefficient of variation of solid phase grain-size distribution, contiguity coefficient of solid phase grains, and mean thickness of the metal phase inter-layers. A numerical analysis of the characteristic parameters of deformation curves for WC-Co hard metals has been carried out over a wide range of cobalt concentration and carbide grain size. A good agreement between the theoretical values of ultimate tensile and compression strength and known experimental values has been established. The constructed model deformation curves for hard metals may be regarded as alternatives to the corresponding experimental curves of stress against relative change in specimen length. __________ Translated from Problemy Prochnosti, No. 3, pp. 99–111, May–June, 2006.     

Prediction of creep deformation in ductile two-phase alloys by a continuum mechanics model

The creep deformation of the ductile two-phase alloys was analysed on the basis of the continuum mechanics model which incorporated the projection concept proposed by Evans and Wilshire. The calculated creep curves were compared with the experimental ones in ferrjte-pearlite steels. It was found that the continuum mechanics model was able to predict the whole creep deformation process of the ductile two-phase alloys from the onset of creep loading to the final rupture, if the creep-deformation and creep-rupture data of the individual phases which constituted the two-phase alloys were known. A steady-state creep in the ductile two-phase alloys was predicted by the continuum mechanics model to occur, even if the constituent phases did not have the inherent steady-state creep. This was caused by the internal stresses arising from the creep-strength difference between second-phase and matrix in the two-phase alloys. This steady-state creep was observed in ferrite-pearlite steels during creep at 873 K. The predicted rupture life on the basis of the continuum mechanics model was correlated well with the experimental results in ferrite-pearlite steels, although the former was somewhat shorter than the latter under higher creep stresses. The continuum mechanics model was able to apply to the life prediction and the creep-strength design of the ductile two-phase alloys.     

Stress-strain rate relations for high-temperature deformation of two-phase Al-Cu alloys

Al-Cu alloys containing 6, 11, 17, 24 and 33 wt% Cu, annealed for 0.5–100 h, were deformed by the differential strain-rate test technique over a strain-rate range of 4×10^–6 to 3×10^–2s^–1 at temperatures ranging from 460–540°C. Superplastic behaviour, with strain-rate sensitivity, m0.5, and activation energy, Q=171.5 kJ mol^–1, is shown by the Al-24Cu and Al-33Cu alloys at lower strain rates and higher temperatures. All the alloys show m0.20 at higher strain rates, but the average activation energy for deformation of the Al-6Cu, Al-11Cu, and Al-17Cu alloys is evaluated to be 480.7 kJ mol^–1, in contrast to a lower value of 211 kJ mol^–1 for the Al-24Cu and Al-33Cu alloys. Instead of grain size, the mean free path between particles is suggested to be a more appropriate microstructural parameter for the constitutive relationship for deformation of the Al-Cu alloys.     

A model to similate the strength and deformation of concrete in compression

Summary A simple model is developed to represent the strength and deformational characteristics of concrete when subjected to a rate of strain or rate of stress or creep or relaxation testing under uniaxial compression.

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Résumé On représente par un modèle simple les caractéristiques de résistance et de déformation du béton soumis à une compression axiale, le fluage et la relaxation étant ainsi étudiés.

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The deformation of hybrid-particulate composites

The deformation behaviour of a hybrid-particulate epoxy composite has been examined. The Young's modulusE and yield stress have been determined as a function of temperature and volume fraction of rigid glass spheres, for both a simple epoxy matrix and a two-phase epoxy toughened by the presence of rubber particles. In addition, the effect of improving the particle-matrix interface with a silane bonding agent has been studied. It is found that there is a steady increase in Young's modulus with the volume fraction of spheres for all systems. In contrast, the yield stress is only found to increase with the volume fraction of rigid particles when the epoxy matrix is not toughened with rubber. It is found that the yield stress is virtually independent of particle volume fraction when a rubber-modified epoxy matrix is employed. Finally, it is found that for all compositions tested the Young's modulus and yield stress increase with increasing temperature. The overall behaviour has been discussed in terms of existing theories concerning the deformation behaviour of amorphous polymers.     

Computational results of a model for chloride ingress in concrete including convection, drying-wetting cycles and carbonation

Over the past decades a considerable research effort has been attributed to the modelling of chloride ingress into reinforced concrete in order to predict its durability. Traditional models are based on the error-function. In the present paper computational results of an advanced model are presented, which takes environmental temperature and humidity fluctuations, chloride binding, diffusion and convection, as well as carbonation effects into account. These qualifications make the model particularly suitable for simulating drying-wetting cycles, an example of which is included. The good performance of the moisture sub-model is demonstrated in an example concerning drying cement paste. Another example deals with a simple submersion case. The examples are based on laboratory and real-life experiments with documented concrete or paste compositions and environmental conditions, as well as the eventual measured chloride profiles. In general, the computational results are in good agreement with the measurements. The paper concludes with a comparison between simulation results of the present model and the error-function model for the case of cyclic drying-wetting exposure. It appeared that the implicit negligence of the moisture fluctuations by the error-function model, forced that model to deploy strongly deviating material characteristics to reach agreement with the measured chloride profile.     

热固性树脂基复合材料固化变形影响因素分析

采用整体-子模块化方法建立了描述复合材料固化全过程的三维有限元模型。以L 形层合板为例,分析了固化工艺、结构设计和模具等因素对固化变形的影响方式和程度。数值模拟结果表明：升温速率和对流换热系数通过改变峰值温度影响回弹角,固化压力通过改变树脂分布和含量影响回弹角;铺层方向引起的结构力学性能的变化是回弹角差异较大的主要原因,厚度对固化变形的影响需考虑其对峰值温度和结构刚度变化两方面因素的综合影响,拐角半径的变化对固化变形的影响较小;模具形式通过改变树脂分布梯度和模具对结构的作用力位置影响回弹角,模具材料和形式的选择对于固化变形控制具有重要意义。     

Limiting state of two-phase composites under uniaxial tension

We propose a model based on the statistical analysis of fracture processes and a concept of effective medium when a system of structural elements of different kinds is regarded as a quasihomogeneous medium with certain effective properties. This approach enables one to compute parameters of the stable stage of fracture of composite materials. The suggested model is used for the prediction of critical values of the parameters corresponding to the transition to unstable fracture under uniaxial tension. We also compare two-phase composites with brittle and plastic phases with strong and weak interfaces between the structural elements, respectively.Frantsevich Institute for Problems in Materials Science, Ukrainian Academy of Sciences, Kiev. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 6, pp. 51–58, November–December, 1995.     

A two-phase model for dry density-varying granular flows

A granular flow is normally comprised of a mixture of grain-particles (such as sand, gravel or rocks) of different sizes. In this study, dry granular flows are modeled utilizing a set of equations akin to a two-phase mixture system, in which the interstitial fluid is air. The resultant system of equations for a two-dimensional configuration includes two continuity and two momentum balance equations for the two respective constituents. The density variation is described considering the phenomenon of air entrainment/extrusion at the flow surface, where the entrainment rate is assumed to be dependent on the divergent or convergent behavior of the solid constituent. The density difference between the two constituents is extremely large, so, as a consequence scaling analysis reveals that the flow behavior is dominated by the solid species, yielding small relative velocities between the two constituents. A non-oscillatory central (NOC) scheme with total variation diminishing (TVD) limiters is implemented. Three numerical examples are investigated: the first being related to the flow behaviors on a horizontal plane with an unstable initial condition; the second example is devoted to simulating a dam-break problem with respect to different initial conditions; and in the third one investigates the behavior of a finite mass of granular material flowing down an inclined plane. The key features and the capability of the equations to model the behavior are illustrated in these numerical examples.     

Quantitative spectral analysis for flaw detection in concrete

The resonance method of transient stress wave propagation is employed for the detection of flaws in concrete. Quantitative analysis of the spectra identifies the resonance mode due to reflection from the flaw, and enables the exclusion of the flexural modes of the concrete plate above the flaw. Two-dimensional scanning of a test point provides the information about the depth of internal flaws. The test results of a large concrete block containing several types of artificial flaws are presented.     

Flow and deformation of fresh concrete

The results of studies made from the standpoint of rheology on methods of predicting flows and deformations of fresh concrete and mortar as basic research for rationalization of concrete construction are described. This paper contains as follows:

1. method of measuring viscosity of fresh concrete and mortar;
2. method of estimating deformation of fresh concrete due to its own weight;
3. proposal of the “Inclined Pipe Test Method” as a new method of testing consistency of grout mortar.

     

Flow rule for the plastic deformation of particiulate metal-matrix composites

Finite element calculations are performed on models of particulate metal-matrix composites to study the applicability of a quadratic yield function and an associated flow rule. The matrix, here taken to be Al, is described by a J₂ flow rule for an isotropic material and the reinforcing particles, either SiC or TiC, are taken to be elastic. Two different types of three-dimensional models of the composite are considered: (1) a simple cubic lattice of spherical particles and (2) random digital models that are approximately isotropic. Only in the second type of model can the existence of a flow rule be established. The validity of a flow rule in random models is associated with the absence of shear planes that extend throughout the solid without intercepting any particles. Such planes can be drawn in the simple cubic lattice for some shear deformations and particle volume fractions. Localized shear bands occurring on these planes results in a shear response essentially identical to that of the unreinforced matrix material, which precludes the determination of the shear response from the uniaxial deformation of the composite.     

A physical model for plastic deformation

A general theoretical model for plastic deformation is presented, which is based on considerations of the variation of internal stored energy during deformation. It is proposed that the deformation rate will always be such that the rate of energy dissipation in the deforming material is minimal. The physical justification of this principle is discussed. The model is applied to dislocation deformation in metals and the result is then compared with experimental observations in aluminium.