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This study evaluated two fundamental assumptions about the geometry of the wood cell wall to develop a mechanistic characterization of wood behavior at the scale of wood structural components. This paper used a three-step homogenization scheme implemented through homogenization techniques and finite-element analyses to evaluate a multilayered cell wall and a simplified single-layer method. Both earlywood and latewood geometries were considered. A three-part sensitivity analysis assessed the influence of wood constituent proportions, microfibril angle, and cellulose crystallinity on cell wall stiffness. Sensitivity to constituent ratios was evaluated through a Monte Carlo simulation of 1,000 sampled points and correlation analysis. Sensitivity to microfibril angle and cellulose crystallinity was evaluated through parametric characterization. The investigation found that both methods provided an accurate assessment of wood stiffness when compared with experimentally determined values. Care should be taken when modeling higher densities of wood, and use of either method in higher order models should be determined by the resolution of input data and computational needs. [ABSTRACT FROM AUTHOR]

The load-carrying capacity of rock-socketed piles depends on the shaft resistance at the pile-rock interface which is governed by the interactions between the pile, rock, and soft interface materials (smear or infill). Despite recent advancements in 3D experimental pile testing of smeared interfaces, the knowledge in this field still remains quite limited primarily due to the experimental limitations, and high costs and fabrication challenges associated with extensive laboratory tests. This study presents a comprehensive numerical investigation to simulate 3D rock-socketed piles with smear using the discrete element method (DEM). By correlating the observations with the experimental x-ray CT images, the interactions between the pile, rock and smear are investigated for various smear area proportions in terms of the micromechanics at the asperity level. The movements of smear and rock debris in the socket and their influence on the various asperity failure modes in relation to the shaft resistance development are evaluated by monitoring the force and damage evolution in DEM. Finally, the calibrated DEM model is utilized to examine the influence of the various smear fabric parameters on the shaft response. Based on the observations, it was inferred that the shear failure mode primarily dictates the interface damage evolution, with a significant contribution from mixed-mode failure involving both tensile and shear damages. Insights from the results indicated that the effect of smear placement on the load-bearing capacity of the pile is found to be minimal compared to the other smear fabric parameters (thickness and area proportion). Moreover, the critical smear thickness to asperity height ratio was determined to be 1.75 beyond which the smear controls the load-bearing capacity of the pile. The discussions presented in this study provide a novel understanding into the smear fabric effect and act as a foundation for further research aimed at improving the reliability and efficiency of rock-socketed pile construction in soft rocks. [ABSTRACT FROM AUTHOR]

The effect of particle shape on the suffusion of gap-graded soils is an essential although poorly understood subject in geotechnical engineering that requires further investigation. This work presents a macroscale and microscale numerical investigation into the effect of particle shape on the suffusion of gap-graded granular materials. Rounded, elliptical, and convex particles with the same volume-equivalent diameter and varying shape coefficients were generated and used to produce samples. Next, a series of resolved coupled computational fluid dynamics (CFD) and discrete-element method (DEM) simulations were performed to provide evidence of the effect of particle shape on the suffusion susceptibility of gap-graded soils. The evolution of particle orientation, moment, and drag force coefficient were analyzed to determine the mechanisms by which particle shape exerts influence. The fine angular particles under seepage flow were found to adjust their orientation, reducing the projected area of the particle perpendicular to the fluid flow direction. Fine particles in high-flow-velocity regions had a smaller projected area and drag force coefficient. The continuous rotation of the irregularly shaped particles during suffusion implies that their migration should counteract the moments exerted by the surrounding particles. In the sample containing various irregularly shaped particles, the initial position of the most irregularly shaped particle was closer to the outlet, implying that irregularly shaped particles are less susceptible to suffusion. [ABSTRACT FROM AUTHOR]

Asphalt mastic is a binary composite in which the bitumen matrix is embedded with filler particles. By taking interparticle interactions and the effects of bitumen origins and filler types into account, this research seeks to achieve a higher predictive precision of the modulus of asphalt mastics using micromechanical models. To achieve this goal, five classical micromechanical models and the Ju-Chen model (JCM) that effectively considers interparticle interactions were adopted to predict the moduli of 25 kinds of asphalt mastics composed of five bitumen origins and five filler types. The results show that the prediction accuracy of the general self-consistent model (GSCM) is the highest among the classical models, which is close to the accuracy of JCM (uniform) with the uniform distribution assumption for the radial distribution function. The JCM (P-Y) with the Percus-Yevick (P-Y) distribution assumption has the highest accuracy of all models, with average goodness of fit of 0.984 for all asphalt mastics. The dilute model (DM) exhibits higher accuracy in two bitumen with smaller low-temperature moduli, whereas the other five models in this study demonstrate higher accuracy in the other three bitumen with bigger low-temperature moduli. For the two models with the highest accuracy, the GSCM and JCM, the filler types have basically no effect on the prediction accuracy of these models. For other models except for GSCM and JCM, the smaller volume fractions of fillers lead to higher prediction accuracy, which can be attributed to the lower filler concentrations in asphalt mastics exhibiting weaker interparticle interactions. This study provides a certain reference for the performance prediction of asphalt mastic composites. [ABSTRACT FROM AUTHOR]

The design of rock-socketed pile foundations requires careful consideration of critical construction aspects such as the socket roughness and smear for accurately estimating the shaft load capacity. Despite the effect of smear being investigated extensively over the years, previous studies are limited in accurately characterizing the effect of smear on the shaft response. This paper presents a comprehensive investigation on the load-bearing mechanisms of model smeared rock-socketed piles with different smear fabrics (configurations) at the pile-rock interface. These model piles were loaded into synthetic soft rocks while being intermittently subjected to three-dimensional (3D) X-ray computed tomography (CT) imaging. Interpretations from the X-ray CT images were correlated with the pile-head load-displacement behavior to gain insights into the load transfer attributes between the pile, smear, and rock. The various interactions between these components were deduced by monitoring the micromechanics at the pile-rock interface through shaft energy utilization, interface void volume, smear volume, and sand density mapping. Based on the observations, it was inferred that the load transfer between the smear and the rock is primarily dependent on the percentage and position of smear occupancy at the interface. Results indicated that for the partially smeared interfaces, the interface mechanics are a combination of smeared and clean shafts with the residual resistances dependent upon the combined compression of smear and rock debris at the interface. Moreover, it was observed that the continuous presence of smear for more than half area of the leading faces of the pile asperities results in the smeared regions of the pile taking precedence over the unsmeared regions in governing the load-carrying capacity of the shaft. The discussions presented in this study can provide a strong base to further study the smear effect, subsequently aiding in enhancing the existing design guidelines to economically construct piles in soft rock. [ABSTRACT FROM AUTHOR]

A novel micromechanics-based model in brittle rocks is presented during the confined tensile fracture. This model is formulated by using the stress intensity factor of crack under distinct loading modes in the fracture mechanics theory. The confined tensile fracture mechanisms with and without initial crack friction are established. The development process from the confined tensile fracture without initial crack friction to that with initial crack friction is studied. The stress–strain constitutive relationship describing the transformation mechanism from the unloading of axial stress at a compressive state to the loading of axial stress at a tensile state is also studied during the confined tensile fracture. The tension–compression transformation mechanism codetermined by the friction of the initial crack, the angle of initial crack inclination, and the confining pressure is proven. With the decrease of the angle of initial crack inclination or the increase of the confining pressure, the confined tensile fracture constitutive model tends to be dominated by the tensile fracture mechanism with initial crack friction. The confined tensile fracture of brittle rocks tends to happen at the larger angle of initial crack inclination. The reasonableness of the presented model is checked by contrasting the experimental data. Effects of the friction, density, inclination angle and size of the initial crack, and the confining pressure on the tensile relation curve of strain and stress, the tensile relation curve of crack length and stress, the tensile elastic modulus, the tensile stress of crack initiation, and the tensile strength are discussed. [ABSTRACT FROM AUTHOR]

The cumulative damage to rock and soil mass caused by blasting in tunnel, highway, hydropower, and other projects is very complex, but the process of damage evolution is still unclear. This study took Zhujiapo mining area limestone as the research object and analyzed the change in its strength and mechanical parameters with periodic blasting loads. Combined with damage mechanics theory and microcrack propagation zone theory, a micromechanics constitutive model considering cumulative damage evolution was developed. The results show that (1) periodic blasting loads have a continuous cumulative effect on microcrystalline limestone, the peak strength and elastic modulus of which decrease exponentially with increasing blasting cycles; (2) periodic blasting loads cause crystal fractures, microcrack initiation, and propagation; and (3) the micromechanics damage constitutive model considering confining pressure and periodic blasting loads well describes the micromechanics behavior of microcrystalline limestone in the Zhujiapo mining area. The research results provide a theoretical reference to the study of rock damage mechanical properties under periodic blasting loads. [ABSTRACT FROM AUTHOR]

Fillers are known as dust fractions in asphalt mixes having a predominant particle size less than 75 micrometer (μm). Typical filler concentration in an asphalt mix varies from 4% to 12% (by aggregate weight). The effects of filler intrinsic properties on mastic behavior can get magnified with the concentration of filler. Thus, in filler-rich mixes like stone matrix asphalt (SMA), the filler characteristics and concentration can be sensitive factors, playing significant roles in the performance of asphalt mastic and mix. The study considered six different types of fillers to capture the effect of filler characteristics on mastic. The mastics were prepared with a constant filler concentration by volume (F/Bvol=0.49), which was determined by a typical SMA mix design. The fillers were characterized based on their surface area, size distribution, shape, Rigden voids (RV), clay content, and chemical composition. The filler effects on mastic were evaluated based on the stiffness (complex modulus) and stiffness ratio. The weak correlation observed between the mastic stiffness and the fixed and free binder reveals that the RV and filler fixing factor (FFF) can mislead regarding the stiffening potential of filler material. The surface areas by Brunauer–Emmett–Teller (BET) and Blaine's methods were insufficient to represent the stiffening potential while the stiffness showed a better correlation with the concentration of finer gradations in the filler evinced by a higher coefficient of P10 (percentage passing 10 μ sieve size) and fineness modulus (FM) parameters. Further, the micromechanics based understanding of mastic behavior explains the mastic stiffening phenomenon as the combined effects of adsorbed asphalt layer thickness and interparticle interaction. The linear relation observed between stiffness ratio and newly derived micromechanism stiffening factor (MMSF) indicates that in filler-rich mastics, the stiffness may be improved with a finer sized filler (lower FM) and higher volume concentration. However, the adsorbed layer thickness didn't show correlation with any of the filler properties considered. The study recommends a limiting value for mastic stiffness in terms of stiffness ratio which is established based on MMSF factor. A limiting value for stiffness of mastic could be significant to avoid dry mixes and subsequent poor fatigue performance. [ABSTRACT FROM AUTHOR]

To simulate the behavior of fiber-reinforced concrete, an accurate modeling of the concrete and fiber phases as well as the fiber–concrete interaction is required. This study proposes a new model to account for fiber-matrix interactions based on micromechanics. The concrete matrix is simulated using the lattice discrete particle model (LDPM), a mesoscale model for heterogeneous materials. The fiber is explicitly represented using elastoplastic beam elements. The fiber–concrete interaction algorithm features a slideline model and a constitutive model to describe the bond-slip relation. The slideline model includes a tie algorithm for interactions between concrete and slideline, and a penalty constraint between the slideline and the fiber. The proposed model is examined by simulating different types of fiber pullout tests. These simulations examine the model validity in capturing the fiber–concrete interaction both in the direction orthogonal to the fiber and in the direction of pullout. Also, the proposed model is calibrated and validated by comparing numerical simulations with experimental data from the literature, including fiber pullout under confinement, pullout of hooked fiber, and pullout of inclined fiber that leads to matrix spalling. The good agreement between the simulation results and the experimental data in terms of force versus displacement curve demonstrates the effectiveness of the proposed approach in simulating fiber–concrete interaction. In addition, using the proposed model, the contributions from different toughening mechanisms can be quantitatively compared. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study of the mechanical properties of a binary mixture of similar prolate ellipsoids of various particle shapes and fines' contents using the discrete element method. The ratio of the major axis of a small ellipsoid to that of a large ellipsoid is 0.2. Because all particles have the same shape, the particle shape of the binary mixture is defined unambiguously. The effect of fines content is examined with samples of the same particle shape. The effect of particle shape is examined by comparing the samples of different particle shapes. Very dense samples are generated and sheared under drained and constant volume conditions. The macroscopic and microscopic behaviors of these samples are studied at both peak and critical states. The mechanical behavior can be grouped into two or three regions for our binary mixture depending on the fines content. Although the critical state line varies with fines content, a unified critical state line is identified using the intergranular void ratio, which is a function of fines content and a parameter. The parameter is a constant for the samples of the same particle shape. This constant is different for the samples of different particle shapes. The finding explains why some studies defined the parameter as a function of fines. The intergranular void ratio does not relate well to the peak friction angle. The microscopic behavior is examined through contact normal, particle orientation, and stress partition. Although some microscopic parameters can identify the region where the sample belongs, we do not find any dominant microscopic parameter that relates to the macroscopic behavior of a binary mixture of various fines contents. [ABSTRACT FROM AUTHOR]

We analyze in two dimensions the mechanical behavior of materials with granular microstructures modeled by means of a variationally formulated strain-gradient continuum approach based on micromechanics and show that it can capture microstructural-size-dependent effects. Tension-compression asymmetry of grain-assembly interactions, as well as microscale damage, is taken into account and the continuum scale is linked to the grain-scale mechanisms. Numerical results are provided for finite deformations and substantiate previous research. As expected, results show interesting size-dependent effects that are typical of strain-gradient modeling. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to study the microleveled rejuvenating effects of different rejuvenating agents on aged asphalt binders. In this study, an original asphalt binder (Shuanglong) with a grade of Pen70 was aged through an accelerated aging process specified by a strategic highway research program (SHRP). Two types of rejuvenating agents (one virgin asphalt binder and another petroleum product named HNCCY-1) were used to rejuvenate the aged asphalt binder at different concentrations. An atomic force microscopy (AFM) was used to study the microleveled properties of the rejuvenated aged asphalt binders, whereas the dynamic shear rheometer (DSR) was for rheological properties. The results showed that: (1) the number of bee structures increased, but their size decreased in the topographies of height or adhesion of the rejuvenated asphalt binders as the concentration of both the rejuvenating agents increased; (2) the roughness indexes gradually decreased and approached those of the original asphalt binder; (3) the diameter and threshold height decreased; and (4) the Young's modulus and reduced modulus became smaller. [ABSTRACT FROM AUTHOR]

Discrete-element method (DEM) simulations of three-dimensional (3D) assemblies of ellipsoid particles were used to evaluate the critical state (CS) for both drained and undrained (constant volume) conditions. A series of conventional triaxial cyclic liquefaction tests with symmetrical cyclic deviatoric stress (σd) with initial q=0 kPa were simulated to develop a relationship between the cyclic stress ratio (CSR=σd/2σ0′) and the number of cycles required for initial liquefaction (NL), where σ0′ is the mean effective normal stress at the end of consolidation. Both cyclic mobility and instability type behaviors were observed depending on the initial void ratio (e0) and σ0′. The micromechanics quantities, i.e., the coordination number (CN), von Mises fabric (FvM), fabric anisotropy intensity (αc), and stress-strain behavior, suggested that cyclic mobility and instability may depend on the phase transformation and instability state, respectively. The cyclic resistance ratio (CRR15), i.e., CSR at NL=15 , showed a unique relation with the initial state parameter (ψ0), irrespective of e0 and σ0′. Two series of postliquefaction monotonic simulations with and without reconsolidation exhibited a unique CS, which perfectly matched with the original CS line. The FvM also reached a unique, narrow range at the CS. The postliquefaction settlement during reconsolidation also showed a linear relation with ψ0. [ABSTRACT FROM AUTHOR]

In this paper, an optimization-based computational framework for the spatial tailoring of a metal-ceramic composite panel subjected to high-speed flow is discussed. The framework includes the modeling, evaluation, and optimization of the spatial material grading and thermostructural response of the metal-ceramic composites over a wide range of temperatures. The framework relies on micromechanics and a finite-element analysis (FEA) of representative volume elements (RVEs) to obtain the overall elastic, thermoelastic, and thermal properties of the graded microstructure as functions of temperature and spatial position. The effective thermostructural response of the airframe is analyzed using the FEA. The time-dependent thermal and structural loads are representative of a characteristic high-speed trajectory. Optimal multivariable material distribution is determined numerically using a constrained sequential quadratic programming (SQP) method of surrogate models to evaluate the response at multiple design locations efficiently. Three example cases are presented to showcase the developed framework. In all three example cases, optimal material variation and panel thickness are found such that they reduce the section mass when compared to a benchmark titanium (Ti-6Al-4V) structural skin and Acusill II thermal protection system (TPS) solution. Furthermore, these studies demonstrate that the use of metal-ceramic spatially tailored materials makes excellent material choices for operation in the high-speed environment. [ABSTRACT FROM AUTHOR]

A micromechanics-based model is developed to study the fatigue response of cementitious composites. Microcrack growth, which is the predominant mechanism responsible for fatigue damage, is explicitly modeled at the mesoscale. The damaged state at the macroscopic scale is determined by using energetic criterion. The dissipated energy associated with each stage of microcrack propagation is computed numerically based on the elastic solutions of the stress and displacement quantities at the mesoscale. The model is used to predict the fatigue life of plain concrete beams under fatigue load cycles. The influence of the various properties of the constituent phases on the fatigue life of the composite is investigated through a parametric study. [ABSTRACT FROM AUTHOR]

This paper presents the results of a comprehensive micromechanical study to improve the understanding of the coupled hydro-thermo-mechanical (HTM) processes during injection of pressurized cold fluid in permeable hot rock. At the microscopic level, it is expected that fluid flow–induced convective temperature changes in the voids will dominate the conductive heat flow from grain to grain. However, the coupled interactions between fluid flow, thermal convection, and conduction, and the resulting changes in local permeability due to thermo-poroelastic stresses at the grain scale remain poorly understood. Moreover, almost all previous studies at a large scale have focused only on thermal conduction. This study used the discrete-element method (DEM) modified to account for convective heat transport to model the particulate interactions between rock grains and between rock deformation, fluid pressure, and temperature. The results indicate that wellbore pressure, provided it is less than the fracturing threshold, has a less significant role in rock cooling around the wellbore, which is governed predominately by rock permeability, as previously thought. Different applied flow rates into the wellbore resulted in low and high wellbore pressures at short times in the absence of rock fracturing. As expected, convective heat transport dominated over conduction, resulting in a cooled ring around the wellbore without a significant thermal gradient zone due to conductive heat flow. As the fluid infiltrates and cools the rock, preferential fluid flow paths occur as fingering instabilities that are oriented toward the minimum principal horizontal stress even in the absence of initial local anisotropic porosity variations. [ABSTRACT FROM AUTHOR]

This paper presents the effects of nano-SiO2 and nano-CaCO3 on the microstructure of high-volume fly ash (HVFA) cement paste. The microstructures of HVFA cement pastes containing 40 and 60% Class F fly ash were evaluated at 28 days using nanoindentation, X-ray diffraction (XRD), thermogravimetric (DTA/TGA), and mercury intrusion porosimetry (MIP) analyses. A reduction of calcium hydroxide (CH) was seen in XRD analysis of HVFA pastes containing nanoparticles. This observation was also confirmed in the DTA/TGA analysis. The nanoindentation results also showed the evidence of pozzolanic reaction in the HVFA pastes, where the addition of 2% nano-SiO2 and 1% nano-CaCO3 increased the volume fractions of high-density and low-density calcium silicate hydrate (C-S-H) gels and confirmed the ability of nanoparticles to reduce the porosity of HVFA pastes, which was consistent with the MIP analysis. The improved nanostructure and microstructure of HVFA pastes due to the addition of nano-SiO2 and nano-CaCO3 in this study show that high-strength and highly durable sustainable concrete can be produced with lower repair and maintenance requirements for the concrete structures. [ABSTRACT FROM AUTHOR]

This paper presents the effects of nano-SiO2 and nano-CaCO3 on the microstructure of high-volume fly ash (HVFA) cement paste. The microstructures of HVFA cement pastes containing 40 and 60% Class F fly ash were evaluated at 28 days using nanoindentation, X-ray diffraction (XRD), thermogravimetric (DTA/TGA), and mercury intrusion porosimetry (MIP) analyses. A reduction of calcium hydroxide (CH) was seen in XRD analysis of HVFA pastes containing nanoparticles. This observation was also confirmed in the DTA/TGA analysis. The nanoindentation results also showed the evidence of pozzolanic reaction in the HVFA pastes, where the addition of 2% nano-SiO2 and 1% nano-CaCO3 increased the volume fractions of high-density and low-density calcium silicate hydrate (C-S-H) gels and confirmed the ability of nanoparticles to reduce the porosity of HVFA pastes, which was consistent with the MIP analysis. The improved nanostructure and microstructure of HVFA pastes due to the addition of nano-SiO2 and nano-CaCO3 in this study show that high-strength and highly durable sustainable concrete can be produced with lower repair and maintenance requirements for the concrete structures. [ABSTRACT FROM AUTHOR]

Formation of ice plays a key role in the behavior of concrete materials at low temperatures in cold and wet regions. The internal stresses generated during the freeze-thaw process could cause serious damage and other durability problems to concrete structures. However, just concerning the stage while the temperature is below 0°C, ice could reduce the stress concentration in the porous matrix by filling the capillary pores and result in a significant increase of elastic modulus and strength, which is usually beneficial for concrete under mechanical loads (either static or fatigue). Meanwhile, pore pressures at low temperatures will also play an important role in either accelerating or delaying the microcracking. In order to establish a mesoscale approach [which usually treats concrete as a composition of coarse aggregate, mortar, and interfacial transition zone (ITZ) between them] based on the rigid-body spring method (RBSM) for the aforementioned issues, in the first stage, the ice-strengthened elastic properties of the mortar are estimated based on multiscale continuous micromechanics. Then a mesoscale model based on RBSM is developed to simulate the nonlinear mechanical behavior under uniaxial compression, tension, and splitting for concrete, in which the ice-strengthened elastic properties in mortar and ITZ, as well as the pore pressures caused by ice formation, are taken into consideration. The range and tendency of the simulated strength values agree well with experimental data. [ABSTRACT FROM AUTHOR]

Granular materials are the most recurrent form of solid-state matter on Earth. They challenge researchers and engineers in various fields not only because they occur with a broad variety of grain sizes, shapes and interactions in nature and industry, but also because they show a rich panoply of mechanical states. Despite this polymorphism, all these different types of soils, powders, granules, ores, pharmaceutical products, etc., are instances of the granular matter with the same least common denominator of being sandlike (psammoid in Greek), i.e., solid grains interacting via frictional contacts. This review describes milestone contributions to the field of granular materials since the early elastic-plastic models developed for soils in the 1950s. The research on granular materials has grown into a vast multidisciplinary field in the 1980s with increasing focus on the microstructure and owing to new experimental tools and discrete simulation methods. It turns out that the granular texture, particle-scale kinematics, and force transmission are far more complex than presumed in early micromechanical models of granular materials. Hence, constitutive relations cannot easily be derived from the particle-scale behavior although advanced continuum models have been developed to account for anisotropy, intermediate stress, and complex loading paths. The subtle elastic properties and origins of bulk friction will be discussed, as well as the effects of particle shape and size distributions. The review covers also recent developments in macroscopic modeling such as the thermomechanical approach, anisotropic critical state theory, nonlocal modeling approach, inertial flows, and material instabilities. Finally, a brief account is given of open issues and some new frontiers and challenges in the field. [ABSTRACT FROM AUTHOR]

Homogenization of random media is a widely used practical and efficient tool to estimate the effective mechanical behavior of composite materials. However, when the microstructure evolves with respect to time, due to phase transformations, care should be taken when upscaling behaviors that themselves involve time, such as viscoelasticity. A natural assumption is to consider that the influence of microstructure evolution is negligible once the material is macroscopically loaded: it allows one to take advantage of the correspondence principle when the phases are nonaging linear viscoelastic. A new approach is proposed to overcome this limitation, building an equivalent composite replacing transforming phases by fictitious aging viscoelastic phases. This equivalent composite has a constant microstructure but is made up of aging linear viscoelastic phases. Its effective behavior can then be estimated taking advantage of recent approaches to homogenize such behaviors. Applications to cement paste are proposed, referring to a simplified morphological model. In particular, the approximation introduced when neglecting microstructure evolution is investigated. Qualitatively, compliance functions from evolving microstructure are found to be closer to the experimental ones. [ABSTRACT FROM AUTHOR]

Composites of piezoelectric materials are widely used in practical applications such as medical devices, nondestructive testing devices, and intelligent or smart structures. The study of mechanics and effective properties of piezocomposites is crucial to the design and development of this class of materials. In this paper, a micromechanics model is developed to determine the effective properties of piezoelectric fiber-reinforced composite materials with imperfect fiber-matrix interface bonding conditions. The micromechanics analysis is based on the periodic microfield micromechanics theory and the boundary element method (BEM). The imperfect bonding between the piezoelectric fibers and matrix is taken into account and represented by a spring-factor parameter. Selected numerical results are presented to show the influence of fiber-volume fraction, material properties of fiber and matrix, and interface bonding conditions on the effective properties of piezoelectric fiber-reinforced composites. [ABSTRACT FROM AUTHOR]

Railway ballast comprises unbounded discrete grains that are often used to form a load-bearing platform for tracks. Ballast degradation as trains pass over the tracks and infiltration of external fines including slurried (pumped) fine subgrade soils are two of the main reasons for ballast fouling. Fouling causes tracks to settle and also reduces the load-bearing capacity, which is associated with a reduction in internal friction and increased lateral spreading of the ballast layer. This paper presents a study of mobilized friction angle, volumetric behavior, and associated evolutions of contact and fabric anisotropy of fouled ballast subjected to monotonic triaxial loading using a series of large-scale triaxial tests and discrete element modeling. Monotonically loaded and drained triaxial tests were carried out on ballast with levels of clay fouling that varied from 10 to 50% void contamination index (VCI) subjected to three confining pressures of 10, 30, and 60 kPa. The results showed that an increase in the level of fouling decreased the mobilized friction angle and increased the ballast dilation. The discrete element method (DEM) was used to study the mobilized friction angle and fabric anisotropy of fresh and fouled ballast by simulating actual large-scale triaxial tests. Irregular shaped grains of ballast were simulated by clumping bonded circular balls with appropriate sizes and positions together. Ballast fouling was approximately simulated in DEM by adding 1-mm particles into the pore spaces of the fresh ballast. The predicted mobilized friction angles and volumetric changes obtained from the DEM simulations agreed well with those measured in the laboratory, indicating that the peak friction angle of fouled ballast and dilation decreased as the degree of fouling increased. The DEM simulations provided an insight into the distribution of contact force chains, contact orientations, and evolution of fabric anisotropy of fresh and fouled ballast that could not be captured in the laboratory. These observations are important for a better understanding of the loaddeformation behavior of fouled ballast from the perspective of micromechanics. [ABSTRACT FROM AUTHOR]

Dilatancy behavior plays a key role in asphalt mixture rutting. Therefore, it is important to understand the mechanism of dilation for asphalt pavement design. In this study, material dilation was approached by considering different stress-dilatancy models. The uniaxial compressive test was employed to measure the dilation of asphalt mixtures under various test temperatures. Radial and volumetric deformations of asphalt mixture specimens were obtained in a multifunctional material system using special radial displacement transducers. The testing results were interpreted through dilatancy theory. The experimental results show that the softening point of asphalt played an important role in the dilation behavior of asphalt mixtures. Analysis suggests that the dilation was governed by the kinematic constraints imposed by the aggregate skeleton. The constitutive relationship of dilation of asphalt mixtures that can be used to describe both the contraction and dilation is discussed by considering material composition and test conditions. A digital specimen of an asphalt mixture was established by a discreteelement modeling (DEM) and incorporated aggregate of irregular shape, the asphalt mortar phase, and the air void phase. Results show that the virtual specimen exhibited similar dilatancy to the lab-compacted specimen in triaxial compression tests. [ABSTRACT FROM AUTHOR]

Within a micromechanical framework, a constitutive model for sand capable of predicting noncoaxiality between stress and strain increments is presented. Anisotropy of elastic and plastic material properties is considered by introducing fabric-like second-order coefficient tensors that arise from a description of the interparticle contacts of sand grains. Strength anisotropy is further accounted for by introducing an interlocking parameter that describes the relative ease of sliding depending on the orientation of potential failure planes. The model is calibrated to and validated against two sets of hollow cylinder triaxial compression experiments consisting of various orientations of major principle stress directions, including tests conducted with the principle stress axes noncoaxial with the material axes. Model predictions of noncoaxiality between stress and strain increments are examined and compared with laboratory measurements. The results show that by considering the arrangement of interparticle contacts, the model is capable of predicting such noncoaxiality without the need to specify a priori any assumptions about the noncoaxial behavior. [ABSTRACT FROM AUTHOR]

Discrete simulation and continuum modeling are two competing methods for analyzing the behavior of complex material systems and granular assemblies. In this paper, a two-dimensional (2D) granular micromechanics model for micromorphic media is presented. The granular micromechanics method is enhanced by incorporating nonclassical terms in the kinematics. These include, in addition to the usual macroscale displacement gradient, the fluctuations in displacement gradient and also the second gradient of displacement. Microscopic intergranular force vectors and macroscopic stress tensors conjugate to the kinematic measures are defined, and the macroscopic strain energy density function is set equal to the volume average of grain-pair energies. As a result, continuum stiffness tensors are formulated on the basis of intergranular stiffness coefficients and fabric parameters defining the geometry of grains and their contacts. To demonstrate the applicability of the continuum method, a random granular assembly is analyzed with both the developed model and discrete modeling method. Results of the discrete analysis then are used to identify the macroscale (or continuum) and the microscale (or grain-pair) stiffness coefficients for a randomly generated assembly of disks. Effects of the three kinematic fields, macrodisplacement gradient, fluctuations in displacement gradient, and its second gradient, are compared. It is found that the derived grain-scale stiffness coefficients are not equal to the ones used in the discrete simulation. This implies that the intergranular stiffness coefficients used for continuum modeling do not represent stiffness of an isolated grain-pair, rather they represent a collective behavior from the grain-pair and its surroundings. An advantage of the granular micro-mechanics method is that the grains locations and their contacts are not needed. Conversely, for discrete modeling, one needs exact data about grains' positions and contacts in addition to their contact properties, which can be very complicated, perhaps intractable, for real materials and complex grain assemblies. [ABSTRACT FROM AUTHOR]

The irregularity of particle motions during quasi-static deformation is investigated using discrete element (DEM) simulations of sphere and sphere-cluster assemblies. A total of three types of interparticle movements are analyzed: relative motions of particle centers, relative motions of material points of two particles at their contact, and the traversal of contacts across the surfaces of particles. Motions are a complex combination of rolling, sliding, and elastic distortion at the contacts, and all motions are highly irregular and variant, qualities that increase with increasing strain. The relative motions of particle centers diverge greatly from those of an affine displacement of the particles. The motions of the nonconvex sphere-cluster particles were more regular that those of the spheres. The paper also investigates the effect of the distance between two remote particles and their pair-wise relative displacements. Even for particle pairs separated by more than six intermediate particles, the relative motions do not conform with the mean deformation (affine) field. Force chains are shown to be transient features, which survive only briefly across elapsed strains. [ABSTRACT FROM AUTHOR]

Within the framework of mean-field homogenization methods, a lattice version of the cascade micromechanics model for the estimation of the effective permeability of microcracked materials with a physically consistent percolation threshold is proposed. Also, a link is investigated between the cascade scheme, self-similarity, and critical phenomena. The cascade lattice micromechanics model is compared with the dilute, the Mori-Tanaka and the self-consistent lattice schemes and validated bymeans of numerical experiments using a computational lattice microcrack-pore network model. Different scenarios of the microcrack probability and the permeability contrast between the pore channels and the microcracks are investigated. [ABSTRACT FROM AUTHOR]

Creep of cementitious materials results from the viscoelastic behavior of the reaction products of cement and water, called hydrates. In the present paper, a single isochoric creep function characterizing well-saturated portland cement hydrates is identified through downscaling of 500 different nonaging creep functions derived from three-minute-long tests on differently young cement pastes with three different initial water-to-cement mass ratios. A two-scale micromechanics representation of cement paste is used for downscaling. At a scale of 700 microns, spherical clinker inclusions are embedded in a continuous hydrate foam matrix. The latter is resolved, at the smaller scale of 20 microns, as a highly disordered arrangement of isotropically oriented hydrate needles, which are interacting with spherical water and air pores. Homogenization of viscoelastic properties is based on the correspondence principle, involving transformation of the time-dependent multiscale problem to the Laplace-Carson space, followed by quasi-elastic upscaling and numerical back-transformation.With water, air, and clinker behaving elastically according to well-accepted published data, the hydrates indeed show one single power-law-type creep behavior with a creep exponent being surprisingly close to those found for the different cement pastes tested. The general validity of the identified hydrate creep properties is further corroborated by using them for predicting the creep performance of a 30-year-old cement paste in a creep test lasting 30 days: the respective model predictions agree very well with results from creep experiments published in the open literature. [ABSTRACT FROM AUTHOR]

In this study, numerical simulation of rock blocks with nonpersistent open joints under uniaxial compression was undertaken using the particle flow modeling method. First, the micromechanical parameter values of intact material were calibrated through a trial-and-error process using macromechanical laboratory test results. Then, a back-analysis procedure was used to calibrate the joint gap and joint micromechanical parameter values using laboratory test results conducted on jointed rock blocks. Afterward, the effects of joint dip angle, joint persistency, and joint gap on the mechanical behavior of block models having nonpersistent open joints was investigated using the calibrated micromechanical parameter values. The joint dip angle and joint persistency were found to play significant roles in the failure mode, strength, and stress–strain relationship of jointed blocks. The joint gap played a significant to negligible role in the mechanical behavior of jointed block models gradually when the joint dip angle was increased from 0 to 90°. The contact and interaction of joint surfaces were found to have a significant influence on the mechanical behavior of jointed blocks. [ABSTRACT FROM AUTHOR]

This study investigates the effect of high temperature on microstructure and mechanical properties of high-toughness fiberreinforced cementitious composites. Pore size distribution, microstructure, and mechanical properties are tested after exposure to 20, 200, 400, 600, and 800°C. The variation of chemical compositions after elevated temperatures leads to color changes of fibers and matrix, which can be clearly verified from energy dispersive X-ray analysis (EDAX). Furthermore, pore size of the composites decreases gradually at 200 and 400°C. Compressive strength varies consistently with the microstructure, showing that fire damage does not inevitably lead to the deterioration of mechanical properties. However, residual flexural properties decrease significantly because of the melting of PVA fiber. As a covering layer of the structures, the composites exhibits excellent fire protection to reinforced concrete columns. [ABSTRACT FROM AUTHOR]

Based on the microstructure-based discrete-element method (DEM), this study aims to investigate the effect of vertical aggregate homogeneity, i.e., aggregate homogeneity in vertical cross sections, on the uniaxial penetration test of asphalt mixtures. An aggregate homogeneity index, which can be used to evaluate aggregate homogeneity in a two-dimensional (2D) cross section, was briefly introduced. The vertical aggregate distribution was evaluated by the index. Microstructure-based discrete-element modeling of a uniaxial penetration test was accomplished by a discrete-element program called particle flow code in two dimensions. The effect of vertical aggregate homogeneity on penetration strengths regarding a uniaxial penetration test was simulated by the DEM. The obtained results were verified by uniaxial penetration testing. Results show that the effect of vertical aggregate homogeneity on penetration strengths can be numerically simulated by the microstructure-based DEM. The penetration strengths in the uniaxial penetration test are anisotropic. Vertical aggregate homogeneity also dominates the variation of penetration strengths in the uniaxial penetration test. A good correlation between vertical aggregate homogeneity and the variation of penetration strengths is also observed. [ABSTRACT FROM AUTHOR]

In this paper, the interface debonding and slipping of carbon fiber-reinforced ceramic-matrix composites (CMCs) subjected to different fatigue loading sequences have been investigated using the micromechanics approach. There are two different types of fatigue loading sequences considered: (1) cyclic loading under low peak stress for N1 cycles, and then high peak stress for N2 cycles; and (2) cyclic loading under high peak stress for N1 cycles, and then low peak stress for N2 cycles. Based on the fatigue damage mechanism of fiber slipping relative to matrix upon unloading/reloading, the interface debonded and slip lengths are determined by fracture mechanics approach. The relationships between interface debonding, interface slipping, interface wear, cycle number, fatigue peak stress, and fatigue loading sequence have been determined. The effects of peak stress level, interface wear, cycle number, and loading sequence on the interface debonding and slipping of fiber-reinforced CMCs have been analyzed. With increasing cycle number and the peak stress level, the interface debonding and slipping range increase, leading to the increase of unloading residual strain and hysteresis loops area, and the decrease of hysteresis loops modulus of fiber-reinforced CMCs. The cyclic fatigue hysteresis loops of unidirectional C/SiC composite under multiple fatigue peak stress levels of σmax = 200, 220, and 240 MPa have been predicted. [ABSTRACT FROM AUTHOR]

The asymptotic homogenization method of differential equations with rapidly oscillating coefficients is used to evaluate the effective elastic stiffness of masonry structures. The equilibrium equations as well as the constitutive relationships at different scales are determined from the asymptotic expansion of the displacement field. The formulation is developed for a general three-dimensional problem. It is then particularized for a simple one-dimensional (1D) model that is applicable to available experimental results of a masonry prism. Key concepts of the method such as periodicity of the fields are highlighted in this 1D problem. A multiscale finite-element model was developed for a two-dimensional unit cell to solve the canonical cell equation that arises in homogenization, which provides numerical solution of the effective elastic moduli. In particular, the numerical study focuses on the in-plane action of the masonry panels that are used as the experimental specimens in laboratory tests. Comparing experimental and analytical (from classical computational homogenization) results, the multiscale homogenization model for linear elastic masonry structures is validated. The main advantage of this model is its straightforward extension for the study of the dynamic homogenization of masonry structures. Furthermore, the developed model is suitable for a comprehensive study of the stress localization in masonry structures for the determination of its strength-based limiting properties. [ABSTRACT FROM AUTHOR]

Classical micromechanics approaches for heterogeneous media assume perfect bonding between phases, implying that both displacement and stress vectors are continuous across the interface between the phases. When nanoinclusions are involved, a stress vector discontinuity in the local equilibrium has to be accounted for. In this framework, this paper derives an approximate solution of the Lippmann- Schwinger (L-S) equation, which accounts for these surface stresses. This approach suggests introducing the concept of an equivalent particle that combines the particle with the surrounding interface, which can be directly implemented in any standard homogenization procedure, such as the Mori-Tanaka scheme. Analytical expressions for the stiffness tensor of the equivalent particle is derived for spheroidal inclusions, accounting for a wide range of nanoinclusion shapes and dimensions. Finally, an energy-based analysis proves how the dramatic increase of the elastic properties is controlled, for a given volume fraction, by the smallest size of the nanoinclusions. [ABSTRACT FROM AUTHOR]

A contact mechanics model, based on the Hertzian elastic contact theory and cementation coating development at particulate scale, was established to predict the time-dependent small-strain stiffness of Class C fly ash-stabilized soils during curing. The cementation coating development model was developed at particulate level based on the Arrhenius law to predict the contact radius growth. A hyperbolic time-temperature relationship was proposed to capture the temperature change of fly ash-stabilized soils and links the pozzolanic reaction rate with curing time. Model-predicted small-strain stiffness was evaluated through both published and experimental test results with good success. The micromechanics modeling indicated that the small-strain stiffness of fly ash-stabilized soil depends on the contact area between fly ash and soil particles and the soil particles' shear modulus. Most of the small-strain stiffness of the stabilized soil was developed within the first 7 days of curing. In addition, a parametric study and a sensitivity analysis were carried out, which indicated that the proposed contact mechanics model was reliable and robust for predicting the time-dependent small-strain stiffness of soils stabilized with Class C fly ash (or other cementitious stabilizers). [ABSTRACT FROM AUTHOR]

Locally averaged random fields can be used to represent composite material properties at an intermediate scale. These types of fields provide the basis for stochastic simulation to generate nominally identical realizations of random composite microstructures to be used in reliability analysis. One challenge in choosing an appropriate local averaging technique and mesoscale length parameter is the difficulty of quantifying the accuracy of the locally averaged model. Although mesoscale models may offer advantages over the use of a representative volume element (RVE), particularly in regions of high stress gradients, mesoscale approximations do not converge to an effective medium, so the accuracy of the mesoscale representation cannot be quantified. In this paper, a matrix norm error is developed to test locally averaged material property approximations where loading is not specified a priori. This norm-based approach illustrates the advantage of local averaging over global averaging when small-scale variation in load may be present. This metric provides a basis for comparing locally averaged material property approximations of a benchmark composite microstructure. [ABSTRACT FROM AUTHOR]

The effects of material variability on the mechanical response and failure of composites under high strain rate and impact loading are investigated in this paper. A previously developed strain rate-dependent, sectional micromechanics model is extended to account for the variability in microstructure and constituent material properties. The model presented in this paper also includes a three-dimensional damage law based on a work potential theory and a microscale failure criterion. Microstructural characterization of the composite is performed to obtain the statistical distributions needed for the stochastic methodologies. A Latin hypercube sampling technique is used to model the uncertainties in fiber volume fraction and viscoplastic material constants. A comparison of general Monte Carlo simulation and Latin hypercube-based Monte Carlo shows that the Latin hypercube technique converges using fewer simulations. The modulus and failure strain obtained using the developed methodology show good correlation with the experimental data. This novel stochastic sectional model is shown to correlate better with the available experimental data compared with the deterministic sectional model. A laminate level, parametric study is also conducted to investigate the effect of uncertainty on the residual energy of a composite laminate during impact. [ABSTRACT FROM AUTHOR]

Characterizing dynamic failure is critically important to a number of applications, among others including armor, material fragmentation, and structural blast. In brittle materials, this failure is driven by crack growth from pre-existing flaws in the material microstructure. Structural scale models that explicitly address the cracks associated with each individual flaw are computationally infeasible; therefore, a model that accurately links flaw population to dynamic failure strength provides a much-needed connection between the microscale and macroscale. The current paper introduces a micromechanical model that addresses the effects of both air-entrained pores and slitlike flaws on the strain-rate dependent uniaxial compressive strength of the material. In particular, four variants of the model are addressed: a two-dimensional (2D) model with only pore flaws, a 2D model with both pores and slit-like flaws, a pseudo-three-dimensional (3D) model with only pore flaws, and a pseudo-3D model with both pores and slit-like flaws. To demonstrate the relative success of each of these approaches, the model is based on microstructural characterization and subsequent Kolsky bar tests on air-entrained mortar. Air-entrained mortar provides an excellent model material for this study, since the pore population introduced by air-entrainment is characterized relatively easily and the slit-like flaw population is deduced from the sand gradation. Furthermore, the sample sizes used in the Kolsky bar set-up are larger than the length scale of the microstructure of mortar, so that the samples are reasonably representative and provide a good basis of comparison with the micromechanics model. The micromechanics model is shown to provide reasonable agreement with experimentally obtained uniaxial compressive dynamic strength of air-entrained mortar. [ABSTRACT FROM AUTHOR]

In multiphase particulate composites, the deviation and mismatch of the elastic moduli of different particles may significantly affect the overall mechanical performance of the composites. This study investigates the effects of such deviations on the macroscopic properties of multiphase composites via an iterative micromechanics-based method. The elastic properties of the particles are assumed to obey certain statistical distributions. In the proposed iterative method, the composites are divided into multiple two-phase composites and their strain concentration tensors are derived by means of the inclusion matrix-reference medium model, which is a modification of the generalized self-consistent method. Iterative solutions are established that take into account the effects of the variation in the elastic properties of the particles in terms of the effective shear and bulk moduli. The findings show that the proposed iterative method converges quickly and that the results agree well with the experimental data for three-phase composites. In addition, the model indicates that the variation in the elastic properties of the particles does have a significant effect on the effective moduli of the composites. [ABSTRACT FROM AUTHOR]

A postbuckling analysis is presented for functionally graded material (FGM) cylindrical panels resting on elastic foundations subjected to axial compression under heat conduction. Two kinds of micromechanics models, namely, Voigt model and Mori-Tanaka model, are considered. The material properties of FGMs are assumed to be graded in the thickness direction according to a simple power law distribution in terms of the volume fractions of the constituents and are assumed to be temperature-dependent. The formulations are based on a higher-order shear deformation theory with a von Kármán-type of kinematic nonlinearity. The panel-foundation interaction and thermal effects are also included. The nonlinear prebuckling deformations and initial geometric imperfections of the panel are both taken into account. A singular perturbation technique along with a two-step perturbation approach is employed to determine the buckling loads and postbuckling equilibrium paths. The effects of volume fraction index, temperature variation, foundation stiffness, panel curvature ratio as well as the inplane boundary condition of straight edges on the postbuckling behavior of FGM cylindrical panels are discussed in detail. [ABSTRACT FROM AUTHOR]

This study aims to propose a new model for clay behavior by incorporating physical-chemical effects acting between clay clusters using the Chang and Hicher micromechanical approach. Local mechanisms are thus introduced through repulsive and attractive forces similar to double-layer van der Waals forces, which are obtained from the derivation of energy potentials. A specific study of local parameters and their evolution is conducted using experimental data from the mineralogy variation provoked by the variation of a remolded saturated reconstituted clayey mixture consisting of kaolinite and montmorillonite ranging from 0 to 100% montmorillonite. Using scale transition, the results of micro-macrocalculations under an isotropic loading path show a very good agreement with experimental results. [ABSTRACT FROM AUTHOR]

Using a microscopic modeling approach, a simplification of the discrete element's shape was done by using irregular convex polyhedrons to reproduce the geometry-dependent behavior of rockfills. Numerical modeling of the triaxial compression tests was conducted using a combined FEM and discrete-element method (FEM/DEM). The micromechanical properties of the numerical sample were calibrated to match the macroscopic response of the real material. The numerical results quantitatively agree with the laboratory results, which reproduce typical features of the mechanical behavior of rockfills, such as nonlinear stress-strain relationship, compaction, and shear dilatancy. The combined FEM/DEM simulation provides an opportunity for a quantitative study of the microstructure of particle assemblages, which give a better understanding of the mechanical characteristics of rockfills. [ABSTRACT FROM AUTHOR]

Architectural and decorative concrete (ADC) has become an enormously popular product for both building interiors and exteriors, combining an aesthetic finish with structural capabilities. Cracking and chipping of ADC product during handling and transportation, however, is of great concern due to the brittleness of ADC materials. This paper looks at the development of a white strain-hardening cementitious composite (SHCC) as an alternative ADC material to address the above-mentioned challenges. It was found that the replacement of ordinary portland cement and fly ash with white cement altered SHCC microstructure which is unfavorab to the tensile strain-hardening behavior. A micromechanical model was engaged for composite optimization through ingredients selection and component tailoring to reengineer white SHCCs. [ABSTRACT FROM AUTHOR]

In the reported study the viability of using computed tomography (CT) scanning for assessing flushing defects in thin sprayed seal (chipseal) surfacings was explored. The study was undertaken to investigate the micromechanical interactions that occur within chipseal layer materials in order to examine their relationship to the origination of flushing, using CT scanning techniques. In particular, the effectiveness of using image analysis techniques to analyze the changes in air voids that occur within a chipseal layer during loading was investigated. The presented study was based on laboratory testing of chipseal pavement samples (cores) from in-service pavements in the Auckland and Waikato regions of New Zealand. The cores, with diameters of 200 mm and thicknesses ranging from 32.4 mm to 44.5 mm, were subjected to varying levels of lateral cyclic loading using a wheel-tracking machine, and the deformation that occurred on the surface of the cores was measured. Two small specimens were extracted from each loaded core, one specimen from the wheel-tracked area of the core and the other from the untracked area of the core. The specimens were scanned using a CT scanner, and the resulting scan images were analyzed using image analysis techniques to determine the distribution of air voids within each specimen. The air voids within the tracked and untracked specimens of each core were compared to examine the changes that had occurred to the distribution of air voids during loading. The results of the study showed that image analysis is an effective tool to analyze air voids within a chipseal layer. [ABSTRACT FROM AUTHOR]

This paper investigates the stochastic nonlinear flexural response of laminated composite plates resting on a two-parameter Pasternak elastic foundation with Winkler cubic nonlinearity and random system properties subjected to transverse uniform lateral pressure and hygrothermal loading. System properties, such as the lamina material properties, coefficients of thermal expansion, coefficients of hygroscopic expansion, and lateral load, are modeled as basic random variables using a micromechanical model. A higher-order shear deformation theory in the von Kármán sense is used to model the system behavior of the laminated plate. A direct iterative-based nonlinear FEM, in conjunction with the first-order perturbation technique previously developed by the authors, is extended for the hygrothermal problem to obtain the second-order response statistics, i.e., the mean and variance of the nonlinear transverse deflection of the plate. Typical numerical results for the second-order statistics of the nonlinear transverse central deflection of composite plates subjected to uniform temperature and moisture distribution over the plate surface and through the plate thickness are obtained for various combinations of foundation parameters, uniform lateral pressures, staking sequences, volume fraction, aspect ratio, plate thickness ratio, and boundary conditions under environmental conditions. The approach has been compared with those available in the literature and an independent Monte Carlo simulation (MCS). [ABSTRACT FROM AUTHOR]

This paper presents the three-dimensional discrete element method (DEM) that was used to study the shear behavior of fresh and coal fouled ballast in direct shear testing. The volumetric changes and stress-strain behavior of fresh and fouled ballast were simulated and compared with the experimental results. Clump logic in particle flow code in three dimensions () incorporated in a subroutine was used to simulate irregular-shaped particles in which groups of 10-20 spherical balls were clumped together in appropriate sizes to simulate ballast particles. Fouled ballast with a various void contaminant index (VCI) ranging from 20 to 70% VCI was modeled by injecting a specified number of miniature spherical particles into the voids of fresh ballast. The DEM simulation captures the behavior of fresh and fouled ballast as observed in the laboratory, showing that the peak shear stress of the ballast assembly decreases and the dilation of fouled ballast increases with an increasing VCI. Furthermore, the DEM also provides insight to the distribution of contact force chains and particle displacement vectors, which cannot be determined experimentally. These micromechanical observations clearly justify the formation of a shear band and the evolution of volumetric changes during shearing. The reduced maximum contact force associated with increased particle contact area due to fouling explains the decreased breakage of fouled ballast. An acceptable agreement was found between the DEM model predictions and laboratory data. [ABSTRACT FROM AUTHOR]

Although stress-strain models for the cyclic behavior of clay have been studied extensively, the models have not been examined from the point of view of the intercluster contact level. In the current paper, we extend the recently developed micromechanical approach to model the cyclic behavior of clay. Undrained triaxial tests under one-way and two-way cyclic loadings on normally consolidated and overconsolidated clay are simulated to evaluate the applicability of the approach. The computed evolution of local stresses and local strains at intercluster scale due to externally applied cyclic loads are also discussed. [ABSTRACT FROM AUTHOR]

Thermal cracking of Portland cement concrete (PCC) decreases rideability and accelerates deterioration of concrete pavements. Coefficient of thermal expansion (CTE) is one of the most important parameters to evaluate the thermal sensitivity of PCC. The AASHTO mechanistic-empirical pavement design guide (MEPDG) requires CTE as a basic input for concrete pavement design, which has increased interest in studies related to concrete CTE in the United States. Several test methods have been developed and used to determine concrete CTE. Nevertheless, concrete CTE testing is time-consuming. Most of the currently available concrete CTE prediction models are empirical and do not reflect the microstructure of PCC. This paper developed a micromechanical model based on thermal mechanical analysis to predict concrete CTE. Concrete CTE data found in the literature validated the applicability of the developed model. Factors affecting concrete CTE were examined using the proposed model. The model has the potential to estimate concrete CTE for concrete pavement design and to help select appropriate raw materials for PCC to achieve low CTE. [ABSTRACT FROM AUTHOR]

This paper presents a study on the thermal postbuckling response of a shear deformable functionally graded cylindrical shell of finite length embedded in a large outer elastic medium. The surrounding elastic medium is modeled as a Pasternak foundation. Two kinds of micromechanics models, namely the Voigt model and Mori-Tanaka model, are considered. The governing equations are based on a higher-order shear deformation shell theory that includes shell-foundation interaction. The thermal effects are also included and the material properties of functionally graded materials (FGMs) are assumed to be temperature dependent. The governing equations are solved by a singular perturbation technique. The numerical results show that in some cases the FGM cylindrical shell with intermediate volume fraction index does not have intermediate buckling temperature and thermal postbuckling strength. The results reveal that Voigt model and Mori-Tanaka model have the same accuracy for predicting the thermal buckling and postbuckling behavior of FGM shells. The results confirm that for the case of heat conduction, the postbuckling equilibrium path for geometrically perfect FGM cylindrical shells with simply supported boundary conditions is no longer of the bifurcation type. [ABSTRACT FROM AUTHOR]