Two-phase Composite Material Research Articles

Magnetocaloric effect in zero-applied magnetic fields

The substantial change in a magnetic Curie temperature of 294 K and degree of magnetic order were investigated in a 10-nm La0.7Sr0.3MnO3/Pb(Mg1/3Nb2/3)O3-PbTiO3(001) two-phase composite material through pure voltage control in the absence of external magnetic fields. The mass-specific isothermal entropy change calculated was greater than 2 J/(kg × K), obtained using an electric field of 7 kV/cm, which serves as proof of the concept of using the magnetocaloric effect in zero-applied magnetic fields for sizeable entropy change. Fabrication strategies of the two-phase multiferroics ensembles are also discussed.

Direct dynamic tensile study of concrete materials based on mesoscale model

Dynamic tensile tests of concrete materials at high strain rates are primarily splitting experiments and spalling experiments at present, however, certain limitations exist in these methods to indirectly study dynamic tensile mechanical behaviour of concrete materials. Thus it is necessary to perform direct dynamic tensile study of concrete materials with a large-diameter split Hopkinson tensile bar (SHTB). Concrete is considered as a two-phase composite material which composed of coarse aggregates and cement matrix in this paper, then cylindrical and dumbbell-shaped three-dimensional mesoscale concrete models are established and applied to SHTB simulation research. Three contact modes between concrete specimen and member bars are discussed based on existing experiments and our numerical simulation analysis. The results show that established finite element models can effectively simulate SHTB tests and mesoscale concrete models also reflect proper tensile behaviour at high strain rates, which also provides convenience to mesoscopic numerical simulation research of many other heterogeneous materials. At last, a proper contact mode is chosen to perform mesomechanics analysis of concrete materials subjected to four types of tensile impact wave, then five mesoscale concrete models with different aggregates content are established and corresponding numerical simulation analysis are conducted.

Effective Medium Theory for the Elastic Properties of Composite Materials with Various Percolation Thresholds.

It is discussed that the classical effective medium theory for the elastic properties of random heterogeneous materials is not congruous with the effective medium theory for the electrical conductivity. In particular, when describing the elastic and electro-conductive properties of a strongly inhomogeneous two-phase composite material, the steep rise of effective parameters occurs at different concentrations. To achieve the logical concordance between the cross-property relations, a modification of the effective medium theory of the elastic properties is introduced. It is shown that the qualitative conclusions of the theory do not change, while a possibility of describing a broader class of composite materials with various percolation thresholds arises. It is determined under what conditions there is an elasticity theory analogue of the Dykhne formula for the effective conductivity. The theoretical results are supported by known experiments and show improvement over the existing approach. The introduction of the theory with the variable percolation threshold paves the way for describing the magnetorheological properties of magnetoactive elastomers. A similar approach has been recently used for the description of magneto-dielectric and magnetic properties.

Experimental Investigation and Analytical Modeling of Chloride Diffusivity of Fly Ash Concrete.

Owing to its importance in the assessment of reinforced concrete structures, it is essential to determine the chloride diffusivity of fly ash concrete. This paper presents an investigation into the diffusion characteristics of chloride ions in fly ash concrete. Through experiment, the relationship between chloride diffusivity and curing age up to 1800 days is measured and the effects of curing age, water/binder ratio, aggregate volume fraction, and fly ash content (i.e., percentage of total cementitious material by mass) on chloride diffusivity are evaluated. It is found that the chloride diffusivity decreases with the increase of curing age, aggregate volume fraction, and fly ash content, but increases with the increase of water/binder ratio. In analytical modeling, an equivalent aggregate model is constructed and the equivalent interfacial transition zone (ITZ) thickness is derived analytically. With the equivalent aggregate model, three-phase fly ash concrete reduces to a two-phase composite material. By extending the Maxwell method, the chloride diffusivity of fly ash concrete is formulated. Finally, the validity of the analytical method is verified by experimental results.

A Green operator-based elastic modeling for two-phase pantographic-inspired bi-continuous materials

We propose an estimate for the effective elastic properties of a new imagined two-phase bi-continuous composite material type with a so-called ``pantographic-inspired” (P-I) architecture in the sense of a matrix reinforcement which is a 3D fiber network capable of large, pantographic-like, deformations, owing to particular properties carried by the fiber interconnections. Such fiber networks co-continuous with the matrix are described from an assemblage of inter-penetrated planar alignments of parallel infinite, identical and equally distant, rods or beams that we call fiber layers or FPAs for ``fiber planar alignments”. Piece-wise linear deformation of such a structure is constrained, as 2D pantographs are, by the specific relations that link the evolutions of the FPA misorientations, of the fiber inter distance and concentration within each FPA, and of the given characteristics to the through-layer interconnections. We derive effective properties within a well defined first gradient elastic homogenization scheme for macro-homogeneous composite structures, based on explicating the involved mean Green operator (mGO) for the representative pattern of such a P-I networked reinforcement in an isotropic matrix. This mGO for P-I composites is built in making use of previously derived and presented mGOs for rod and beam FPAs. Comparisons with Young and shear modulus variations during numerical homogeneous extension simulations show that the proposed modeling provides relevant effective property evolutions with the advantage of being analytical. Comparing so estimated force-displacement curves with numerical ones for 2D pantographs enables to identify ways to further account in the modeling for the specific strengthening effects of the FPA interconnections in pantographs. This tends to prove such P-I composite structures, still to be manufactured, to possibly also behave similarly to pantograph ones.

Thermoelastic Stress and Deformation Analyses of Functionally Graded Doubly Curved Shells

In this paper, the authors develop Reissner’s mixed variational theorem (RMVT)-based finite layer methods for the three-dimensional (3D) coupled thermoelastic analysis of simply supported, functionally graded, doubly curved (DC) shells with temperature-independent material properties. A two-phase composite material is considered to form the shell, and its material properties are assumed to obey a power–law distribution of the volume fractions of the constituents through the thickness direction of the shell. The effective material properties are estimated using the Mori–Tanaka scheme. The accuracy and convergence rate of these RMVT-based finite layer methods are validated by comparing their solutions with the quasi 3D and accurate two-dimensional solutions available in the literature.

Influence of service time of recycled coarse aggregate on the mechanical properties of recycled aggregate concrete

The mechanical properties of recycled aggregate concrete (RAC) may be affected by the service time of parent concrete used to produce the recycled coarse aggregate (RCA), which has not been experimentally investigated. Current models primarily relate RAC elastic modulus to its compressive strength. These models cannot describe the difference in the mechanisms of the influence of the incorporation of RCA on the concrete elastic modulus and that on the concrete strength. This paper first investigated how the compressive strength, static modulus of elasticity and shrinkage deformation of the RAC are influenced by the service time of the RCA (1, 18 and 40 years). The RCA was adopted as 0%, 30%, 50% and 100% replacement of the natural coarse aggregate. Based on two-phase composite material theory, the elastic modulus model was then developed for RAC which is related to the elastic modulus of the companion natural aggregate concrete by accounting for the influence of residual mortar content (CRM) to achieve a better accuracy in wider parameter ranges. The model to predict the development of the compressive strength over time was also proposed for the RAC by modifying the EC2 model. Results obtained indicated that RAC compressive strength was affected by the service time of the parent concrete, and the newly proposed models showed good accuracy in predicting RAC mechanical properties.

Temperature Effects during Impact Testing of a Two-Phase Metal-Ceramic Composite Material.

Metal-ceramic composite (MCC) materials can be used for manufacturing high-responsibility structures such as jet engines or cutting tools. One example of these materials is a two-phase wolfram carbide (WC) and cobalt (Co) composite. This MCC is a combination of hard WC grains with a Co metallic ductile binder. The resulting microstructure is a combination of two phases with significantly different mechanical behaviors. In this study, we investigate impact conditions, starting with an illustrative example of the Taylor impact bar where—although the process is very rapid—the equivalent plastic strain and temperature are higher in the adiabatic solution than those in the coupled solution. On exposing the WC/Co composite with a metallic binder to impact loading, heat is generated by plastic deformation. If the process is fast enough, the problem can be treated as adiabatic. However, a more common situation is that the process is slower, and the heat is generated in the ductile metallic binders. As a result, the associated grains are heated due to the conduction effect. Consequently, the process should be treated as coupled. When the impact is applied over a short time period, maximum temperatures are significantly lower if the process is analyzed as coupled rather than as adiabatic. The grains are immediately affected by temperature increase in the binders. Therefore, the heat conduction effect should not be omitted.

A KPCA-based parameterization model for composite materials representation

It is supposed that there exist means to generate material microstructure representations by varying processing parameters numerically [1] or experimentally [2], the goal of this work is to establish a parameterized geometrical description model from learning the set of given snapshot instances. In this work, we propose a parameterization model for the representation of the Representative Volume Elements (RVE) of composite materials based on Kernel Principal Component Analysis (KPCA) method. The set of synthetic RVE snapshots governed by a series of control parameters is firstly mapped into a high-dimensional feature space. Then, Principal Component Analysis (PCA) is performed, together with the establishment of approximated response surfaces of the retained PCA projection coefficients. We showcase the performance of KPCA-based surrogate by applying it for an optimal design of the effective mechanical properties of a two-phase composite material.

Effective Medium Method for Chloride Diffusion Coefficient of Mature Fly Ash Cement Paste.

The chloride diffusion coefficient of concrete plays an essential role in the durability assessment and design of concrete structures built in chloride-laden environments. The purpose of this paper is to present an effective medium method (EMM) for evaluating the chloride diffusion coefficient of mature fly ash cement paste. In this method, a numerical method is used to estimate the degrees of hydration of cement and fly ash. Fly ash cement paste is then modeled as a two-phase composite material, composed of a solid phase and a pore space. By introducing the percolation theory, the EMM is modified to derive the chloride diffusion coefficient of fly ash cement paste in an analytical manner. To verify the EMM, a chloride diffusion test of fly ash cement paste at a curing age of up to 540 days is conducted. It is shown that, within a reasonable fly ash content, a larger fly ash content and/or curing age results in a smaller chloride diffusion coefficient. The chloride diffusion coefficient decreases with a decreasing water/binder ratio. Finally, the validity of the EMM is verified with experimental results.

A new model for response of laterally loaded piles in soil-rock mixtures

The S-RM is treated as an equivalent homogenous c-φ geomaterial. A hyperbolic p–y curve, which is governed by initial stiffness and ultimate resistance, is used to describe the nonlinear interaction between the S-RM and the pile. The initial stiffness and ultimate resistance are derived based on two-phase composite material theory and empirical equations for friction angle and cohesion of S-RMs, respectively. The predicted responses of laterally loaded piles in S-RMs with different volumetric block proportions are validated with 3D numerical simulations. The influencing factors on the p–y curves are also investigated in this paper.

A parametric model for the changes in the complex valued conductivity of a lung during tidal breathing

Classical homogenization theory based on the Hashin–Shtrikman coated ellipsoids is used to model the changes in the complex valued conductivity (or admittivity) of a lung during tidal breathing. Here, the lung is modeled as a two-phase composite material where the alveolar air-filling corresponds to the inclusion phase. The theory predicts a linear relationship between the real and the imaginary parts of the change in the complex valued conductivity of a lung during tidal breathing, and where the loss cotangent of the change is approximately the same as of the effective background conductivity and hence easy to estimate. The theory is illustrated with numerical examples based on realistic parameter values and frequency ranges used with electrical impedance tomography (EIT). The theory may be potentially useful for imaging and clinical evaluations in connection with lung EIT for respiratory management and control.

Tunable negative permittivity and permeability of yttrium iron garnet/polyaniline composites in radio frequency region

The present work demonstrated the tunable negative permittivity and permeability properties of a two-phase composite material consisting of conductive matrix [polyaniline (PANI)] and magnetic filler [yttrium iron garnet (YIG)]. The negative permittivity observed in the YIG/PANI composites is accompanied by the formation of conductive PANI network, which induced the plasma oscillation of carriers in PANI obeying the Drude model. Through theoretical calculation, negative permeability can be realized and further adjusted by external magnetic field via the domain wall and spin resonances of YIG.

Computational Modelling of Structures with Non-Intuitive Behaviour.

This paper presents a finite-element analysis of honeycomb and re-entrant honeycomb structures made of a two-phase composite material which is optimized with respect to selected parameters. It is shown that some distributions of each phase in the composite material result in the counter-intuitive mechanical behaviour of the structures. In particular, negative values of effective Poisson’s ratio, i.e., effective auxeticity, can be obtained for a hexagonal honeycomb, whereas re-entrant geometry can be characterized by positive values. Topology optimization by means of the method of moving asymptotes (MMA) and solid isotropic material with penalization (SIMP) was used to determine the materials’ distributions.

Numerical analysis of the rheological behaviors of basalt fiber reinforced asphalt mortar using ABAQUS

This study aims to investigate the rheological behaviors of basalt fiber (BF) reinforced in asphalt mortar. The samples of basalt fiber asphalt mortar (BFAM) at various BF contents were prepared and were then submitted for the compressive creep test (CCT) and bending creep test (BCT) at high and low temperatures, respectively. In addition, the numerical simulation in finite element ABAQUS software was performed to model CCT and BCT, where the internal structure of the fiber-reinforced materials was considered to build a two-phase composite material model (fiber and mortar matrix) and further analyze the influence of BF on the rheological behaviors. Testing results indicate that BFAM has an excellent reinforcement performance compared with the control sample (0% fiber content). Moreover, numerical analyses show that the addition of fiber into asphalt mortar causes stress redistribution in BFAM and decreases the average stress value of the mortar matrix. The creep results of the three-dimensional numerical simulation using Burgers constitutive model for BFAM exhibit excellent agreement with those of the testing.

THREE-DIMENSIONAL ANALYSIS OF PERMEABILITY OF CONCRETE WITH CRACKS

Concrete with cracks is considered to be a two-phase composite material with a porous matrix and cracks. The interaction direct derivative (IDD) expression was used to estimate the effective permeability of periodic crack model. The conventional seepage numerical analysis method associated with continuous solid elements has some difficulties in mesh coordination, so a new method which combines embedded element technique with elastic analogy was developed to solve this problem. Based on the assumption of penny cracks, a model with three dimensional periodic microstructures was carried out to represent the geometric arrangement of cracks, and the effect of different geometric factors, namely crack diameter, crack density, structure periodicity, crack inclination, and space interval, were studied. The numerical result derived from the new method, was compared with the theoretical solution from IDD. The results reveal that the IDD-derived solution is suitable for low crack density, but it starts to underestimate the finite element results at higher crack density, especially when cracks are nearly touching. During their approach to each other, the effective permeability can be described by an asymptotic logarithmic expression.

Disorder-induced stiffness degradation of highly disordered porous materials

The effective mechanical behavior of multiphase solid materials is generally modeled by means of homogenization techniques that account for phase volume fractions and elastic moduli without considering the spatial distribution of the different phases. By means of extensive numerical simulations of randomly generated porous materials using the lattice element method, the role of local textural properties on the effective elastic properties of disordered porous materials is investigated and compared with different continuum micromechanics-based models. It is found that the pronounced disorder-induced stiffness degradation originates from stress concentrations around pore clusters in highly disordered porous materials. We identify a single disorder parameter, φsa, which combines a measure of the spatial disorder of pores (the clustering index, sa) with the pore volume fraction (the porosity, φ) to scale the disorder-induced stiffness degradation. Thus, we conclude that the classical continuum micromechanics models with one spherical pore phase, due to their underlying homogeneity assumption fall short of addressing the clustering effect, unless additional texture information is introduced, e.g. in form of the shift of the percolation threshold with disorder, or other functional relations between volume fractions and spatial disorder; as illustrated herein for a differential scheme model representative of a two-phase (solid–pore) composite model material.

A potential-of-mean-force approach for fracture mechanics of heterogeneous materials using the lattice element method

Fracture of heterogeneous materials has emerged as a critical issue in many engineering applications, ranging from subsurface energy to biomedical applications, and requires a rational framework that allows linking local fracture processes with global fracture descriptors such as the energy release rate, fracture energy and fracture toughness. This is achieved here by means of a local and a global potential-of-mean-force (PMF) inspired Lattice Element Method (LEM) approach. In the local approach, fracture-strength criteria derived from the effective interaction potentials between mass points are shown to exhibit a scaling commensurable with the energy dissipation of fracture processes. In the global PMF-approach, fracture is considered as a sequence of equilibrium states associated with minimum potential energy states analogous to Griffith’s approach. It is found that this global approach has much in common with a Grand Canonical Monte Carlo (GCMC) approach, in which mass points are randomly removed following a maximum dissipation criterion until the energy release rate reaches the fracture energy. The duality of the two approaches is illustrated through the application of the PMF-inspired LEM for fracture propagation in a homogeneous linear elastic solid using different means of evaluating the energy release rate. Finally, by application of the method to a textbook example of fracture propagation in a heterogeneous material, it is shown that the proposed PMF-inspired LEM approach captures some well-known toughening mechanisms related to fracture energy contrast, elasticity contrast and crack deflection in the considered two-phase layered composite material.

A Brownian motion simulation for the chloride diffusivity of concrete

The purpose of this paper is to present a Brownian motion simulation for the chloride diffusivity of concrete. According to stereological theory, the probability density function and cumulative distribution function for the circular aggregates in terms of the number of aggregates are derived. By coating each aggregate with an interfacial transition zone (ITZ) layer of equal thickness, concrete is then reduced to a two-phase composite material, composed of a bulk cement paste and equivalent aggregates. The equivalent ITZ thickness and the chloride diffusivity of each equivalent aggregate are formulated in an analytical manner. The Brownian motion simulation is used to compute the chloride diffusivity of concrete. Finally, the validity of the numerical simulation is verified with two sets of experimental results and the effects of the aggregate area fraction, the chloride diffusivity of ITZ, and the ITZ thickness on the chloride diffusivity of concrete are evaluated in a quantitative manner. The paper concludes that the numerical simulation can predict the chloride diffusivity of concrete with an average relative error smaller than 6% for the two selected verification examples.

An interphase model for effective elastic properties of concrete composites

In this work, the Eshelby tensor of a finite spherical domain is employed to construct an interphase model to evaluate elastic properties of concrete composites. Explicit formulations of the interphase model via multi-inclusion method for a class of concrete composites are derived. The theoretical estimate based on an improved Hashin–Shtrikman bounds for the Young’s modulus of two-phase concrete composite material are used as a comparison result in the analysis, and the influence of the interfacial transition zone (ITZ) on elastic properties of three-phase concrete composite is studied. Moreover, the homogenization results predicted by the proposed interphase model are compared with the published experimental data. Results obtained in this work show that the proposed interphase model can provide a very accurate estimate of the effective elastic properties of complex concrete composite materials.