Showing papers on "Micromechanics published in 1999"

New Micromechanics Design Theory for Pseudostrain Hardening Cementitious Composite

Abstract: The micromechanics design theory has realized random short fiber-reinforced cement composites showing pseudostrain hardening (PSH) behavior with over 5% of strain capacity under tension. Nevertheless, this existing theory currently is limited to specific constituent properties, which does not account for chemical bond and fiber rupture. This article presents a new design theory that eliminates this restriction, achieving fiber rupture type PSH-random short fiber-reinforced cement composites with high-performance hydrophilic fibers like polyvinyl alcohol fibers. Uniaxial tensile tests are conducted employing polyvinyl alcohol fiber composites, the results of which support the validity of the proposed theory. Furthermore, parametric study employing the proposed theory quantitatively evaluates the effects of composite's micromechanics parameters, such as bond strength and fiber strength, on composite performance. This parametric study reveals that continuously increasing the degree of fiber rupture (fiber rupture intensity) enhances the strength performance of composites but not energy performance. However, an optimum rupture intensity exists for maximizing energy performance, which is critical for PSH behavior. The consistency between theoretical predictions and experimental results consequently demonstrates that the proposed theory can be utilized practically as a powerful and comprehensive tool for PSH composite design.

Mechanical properties of porous materials

Abstract: Porous materials are commonly found in nature and as industrial materials such as wood, carbon, foams, ceramics and bricks. In order to use them effectively, their mechanical properties must be understood in relation to their micro-structures. This paper studies the mechanical properties of a few common porous materials: carbon rods, ceramics, polymeric foams and bricks. The characterisation of pore structures was performed using a Mercury Porosimeter. Detailed information was obtained on the density, porosity, surface area and pore size distribution. A large number of experiments on either bending or compression were conducted in order to obtain their macro-mechanical properties such as Young's modulus, hardness and strength. Based on the experimental observations, theoretical models were employed to predict the macro-properties from the micromechanics viewpoint. By studying the deformation of pores the global behaviour was calculated. Two simple formulae for the elastic modulus, E, were proposed: for low values of porosity, φ, E = E0(1 − 2φ) (1 + 4φ2) where E0 is the elastic modulus when the porosity is zero; for high value of porosity such as for foams E = E0 (1 − φ)2. The theoretical results agreed well with the experimental ones. The study has provided insights into the mechanical properties of porous materials over a wide range of porosity values.

An efficient implementation of the generalized method of cells for unidirectional, multi-phased composites with complex microstructures

Abstract: An efficient implementation of the generalized method of cells micromechanics model is presented that allows analysis of periodic unidirectional composites characterized by repeated unit cells containing thousands of subcells The original formulation, given in terms of Hill's strain concentration matrices that relate average subcell strains to the macroscopic stains, is reformulated in terms of the interfacial subcell tractions as the basic unknowns This is accomplished by expressing the displacement continuity equations in terms of the stresses and then imposing the traction continuity conditions directly The result is a mixed formulation wherein the unknown interfacial subcell traction components are related to the macroscopic strain components Because the stress field throughout the repeating unit cell is piece-wise uniform, the imposition of traction continuity conditions directly in the displacement continuity equations, expressed in terms of stresses, substantially reduces the number of unknown subcell traction (and stress) components, and thus the size of the system of equations that must be solved Further reduction in the size of the system of continuity equations is obtained by separating the normal and shear traction equations in those instances where the individual subcells are, at most, orthotropic Comparison of execution times obtained with the original and reformulated versions of the generalized method of cells demonstrates the new version's efficiency As demonstrated through examples, the reformulated version facilitates previously unattainable detailed analysis of the impact of fiber cross-section geometry and arrangement on the response of multi-phased unidirectional composites

Crazing in semicrystalline thermoplastics

Abstract: The deformation processes in crystalline polymers have been studie ever since the discovery of chain folding in 1957. Since then, scientists have been intrigued by the different steps of the transformation of the folded-chain lamellar structure of single crystals or of macroscopically isotropic, often spherulitic, polymers into fibrous morphologies (see Refs. 1 and 2 for early reviews). The importance of molecular tilt, of inter- and intralamellar slip, and of micronecking were rapidly recognized [1–4]. In this paper, we discuss the analogies and differences with respect to crazing of glassy amorphous polymers. Obviously, there is an extensive body of literature on the micromechanics of crazing (see the reviews in Refs. 5–9). On the basis of these studies, it has been established that crazes in amorphous polymers are well-defined regions with approximately planar boundaries that extend perpendicular to the direction of maximum principal tensile stress and that contain highly stretched and voided ...

Micromechanics models for mechanical and thermomechanical properties of 3D through-the-thickness angle interlock woven composites

Abstract: In this paper, a laminate block modeling approach for three-dimensional (3D) through-the-thickness angle interlock woven composites is used to develop one finite element analysis (FEA) model and two analytical models, namely the “ZXY model” and the “ZYX model”. These models can be used to determine the mechanical properties and the coefficients of thermal expansion for 3D through-the-thickness angle interlock woven composites. A parametric study shows that there is good agreement between these FEA and analytical models. The parametric study also demonstrates the effects of the fiber volume fraction of the warp weaver (i.e., z yarn) and the space between two adjacent filler yarns on the mechanical properties and the coefficients of thermal expansion. Finally, the present models are found to correlate reasonably well with the predicted and measured results available in the literature.

Boundary element method homogenization of the periodic linear elastic fiber composites

Abstract: The paper presented is devoted to the Boundary Element Method based homogenization of the periodic transversely isotropic linear elastic fiber-reinforced composites. The composite material under consideration has deterministically defined elastic properties while its components are perfectly bonded. To have a good comparison with the FEM-based computational techniques used previously, the additional Finite Element discretization is presented and compared numerically against BEM homogenization implementation on the example of engineering glass–epoxy composite. The homogenization method proposed has rather general characteristics and, as it is shown, can be easily extended on n -component composites. On the contrary, we can consider and homogenize the heterogeneous media with randomly defined material properties using Monte-Carlo simulation technique or second order perturbation second probabilistic moment approach.

Micromechanical modeling for behavior of cementitious granular materials

Abstract: Crack damage is commonly observed in cementitious granular materials. Previous analytical models based on continuum mechanics have limitations in analyzing localized damages at a microscale level. In this paper, a micromechanics approach is adopted that considers a contract law for the interparticle behavior of two particles connected by a binder. The model is based on the premises that the interparticle binder initially contains microcracks. As a result of external loading, these microcracks propagate and grow. Thus, binders are weakened and fail. Theory of fracture mechanics is employed to model the propagation and growth of the microcracks. The contact law is then incorporated in the analysis for the overall damage behavior of material using a discrete element method. Using this model, the stress-strain behaviors under uniaxial and biaxial conditions were simulated. A reasonable agreement is found between the predictions and experimental results.

Progressive Failure Modeling in Notched Cross-Ply Fibrous Composites

Abstract: This study models and simulates progressive failure initiating from a notch tip of a laminated fibrous composite specimen subjected to tensile in-plane loading. The micro/macro-level approach is used. The micro-level analysis uses the 3-D unit-cell model while macro-level analysis uses the finite element analysis technique. A cross-ply laminate with double-edge notches was studied to investigate delamination, fiber splitting, and transverse matrix cracking in the specimen. Numerical results are compared to previous experimental work.

The effect of strain path on material behaviour during hot rolling of FCC metals

Abstract: Models representing material behaviour are now an essential component of the development process for rolled products. Although models based on physical parameters are being proposed, most current models employ empirical equations, which assume that the deformation can be characterized by the strain rate, temperature and the equivalent plastic strain. However, deformation in a flat product rolling pass involves a partial reversal of shear strain, and in long product and section rolling there are more complex changes in strain path in sequential passes. This paper briefly reviews the mapping of strain paths and their effects on the micromechanics of deformation and the resulting flow stress. The influence of in–grain heterogeneity of strain is discussed in relation to the development of dislocation structures and their effects on texture evolution and subsequent recrystallization behaviour. The effects on recrystallization kinetics and resulting grain size are sufficiently large to lead to significant errors in modelling the local behaviour in multipass rolling, if strain–path effects are not considered.

Micromechanics Analysis Code With Generalized Method of Cells (MAC/GMC): User Guide

Abstract: The ability to accurately predict the thermomechanical deformation response of advanced composite materials continues to play an important role in the development of these strategic materials. Analytical models that predict the effective behavior of composites are used not only by engineers performing structural analysis of large-scale composite components but also by material scientists in developing new material systems. For an analytical model to fulfill these two distinct functions it must be based on a micromechanics approach which utilizes physically based deformation and life constitutive models and allows one to generate the average (macro) response of a composite material given the properties of the individual constituents and their geometric arrangement. Here the user guide for the recently developed, computationally efficient and comprehensive micromechanics analysis code, MAC, who's predictive capability rests entirely upon the fully analytical generalized method of cells, GMC, micromechanics model is described. MAC/ GMC is a versatile form of research software that "drives" the double or triply periodic micromechanics constitutive models based upon GMC. MAC/GMC enhances the basic capabilities of GMC by providing a modular framework wherein 1) various thermal, mechanical (stress or strain control) and thermomechanical load histories can be imposed, 2) different integration algorithms may be selected, 3) a variety of material constitutive models (both deformation and life) may be utilized and/or implemented, and 4) a variety of fiber architectures (both unidirectional, laminate and woven) may be easily accessed through their corresponding representative volume elements contained within the supplied library of RVEs or input directly by the user, and 5) graphical post processing of the macro and/or micro field quantities is made available.

Micromechanics modeling of smart composites

Abstract: This paper discusses a summary of analytical modeling as applied to selected smart composites which include piezoelectric composites, shape memory alloy (SMA) fiber composites, and piezoresistive composites. First we discuss the definition of ‘smart materials’ which exhibit coupling among mechanical, thermal and electromagnetic behavior, then the Eshelby’s formulations based on a simple algebraic method for predictions of two types of smart composites properties are stated; piezoelectric and SMA composites, followed by the percolation model which is applied to obtain the strain‐electric conductivity relations of elastomer composites. The predictions based on these models are shown to be in good agreement with limited experimental results. q 1999 Elsevier Science Ltd. All rights reserved.

Role of microstructure in the thermomechanical behavior of SMA composites

Abstract: A numerical study on the role of microstructure in the thermomechanical behavior of shape memory alloy (SMA) composites under uniaxial tension is performed. The simulation is based on the micromechanics model established recently by the authors. The influence of the shape and volume fraction of SMA on the overall behavior of the composite as well as on the internal stress and strain evolution is investigated. The strengthening effect of SMA on ductile matrix is illustrated. The obtained results demonstrate several interesting features of the new composite and may serve as a quantitative basis for the microstructure design of this composite in the future.

A Micromechanics-Based Finite Element Model for Compressive Failure of Notched Uniply Composite Laminates Under Remote Biaxial Loads

Abstract: A micromechanics based failure initiation predictive capability for analyzing notched composite laminates loaded remotely in multiaxial compression is reported. The model relies on the results from a previous experimental study that investigated compression failure mechanisms in special uniply composite laminates. The finite element method (FEM) was used in the solution process. The experimental results showed that the dominant mode of failure initiation was kink banding near the hole edge. The kink band was confined in extent to a distance within one half of the hole radius. The fibers within the kind band were rotated both in plane and out of the plane of the laminate. The position of the kink band with respect to the center of the notch depended on the remote biaxial load ratio. In the FEM, the region in which kink banding takes place is contained within a finite size rectangular area, and is meshed as an alternatingly stacked region of fiber and matrix layers. The values of boundary loads on this rectangular area which correspond to kink banding is related to the remotely applied loads via an available form analysis for orthotropic laminates. Good agreement is found between experiment and analysis for a wide range ofmore » notch sizes.« less

Theory of Reinforcement Damage in Discontinuously-Reinforced Composites and Its Application

Abstract: In particle or short-fiber reinforced composites, cracking or debonding of the reinforcements is a significant damage mode because the damaged reinforcements lose load carrying capacity. This paper deals with a theory of the reinforcement damage in discontinuously-reinforced composites and its application. The composite with progressive cracking damage contains intact and cracked reinforcements in a matrix. To describe the load carrying capacity of the cracked reinforcement, the average stress of a broken ellipsoidal inhomogeneity in an infinite body which was proposed in the previous paper is introduced. An incremental constitutive relation of the composites with progressive cracking damage of the reinforcements has been developed based on Eshelby's equivalent inclusion method and Mori and Tanaka's mean field concept. This damage theory can describe not only cracking damage but also debonding damage of the reinforcements by modifying teh load carrying capacity of damaged reinforcements. Influence of the reinforcement damage on the stress-strain response and elastic stiffness of the composites is discussed. It is noted that tha full-debonding damage gives the lower limit of the stress-strain relation of the composite with progressive reinforcement damage.

Simulation of dynamic compaction of metal powders

Abstract: This article presents numerical studies on the deformation of particles during dynamic compaction of metal powders. The analysis of the process is based on a micromechanics approach using multiple particle configurations. The material considered is elastoviscoplastic with interparticle friction. Two-dimensional studies on particles in close packed arrangement were carried out using plane strain conditions for deformation and thermal response. The finite element method using an explicit dynamic analysis procedure was used for the simulations. The influence of speed of compaction, strain hardening, strain rate dependency, interparticle friction and size of the powder particles on the final shape and temperature variations within the particles were analyzed. The studies offer useful information on the shape and temperature variations within the particles. The results provide a better understanding of the dynamic compaction process at the micromechanics level.

A Micromechanics-Based Approach to the Failure of Saturated Porous Media

Abstract: This contribution is devoted to the implementation of a homogenization method for deriving the strength or failure properties of a fluid-saturated porous medium, from those exhibited by its individual constituents at the microscopic level. Within this context, a specific attention is paid to the possibility of adopting an effective stress formulation. While the case of a purely cohesive solid matrix provides the first illustrative example where the ‘effective stress principle’ as originally stated by Terzaghi is fully applicable, the analysis is then particularly focused on porous sandstones, modelled as periodic packings of cemented rigid grains. A closed-form analytical expression is thus obtained for the strength criterion of such rock materials, which proves to be a function of a generalized effective stress formed as a linear combination of the total stress and the pore pressure, as in the case of poroelasticity. It is shown in particular that the key microstructural parameter involved in this formulation is the ratio between the intergranular contact area and the grain cross-section area. A possible extension of the homogenization procedure in order to account for a still more realistic description of the sandstone microstructure is finally outlined.

Micromechanics Based Failure Analysis of Composite Structural Laminates

Abstract: : A micromechanics based multicontinuum theory and associated numerical algorithm was used to extract, virtually without a time penalty, the stress and strain fields for a composites' constituents during routine structural finite element analysis. Using this constituent information, a stress-based failure criterion was developed and used to construct a progressive failure algorithm for investigating the material failure strengths of composite structural laminates. The criterion is fully three-dimensional and requires a minimum number of experimentally derived constants. The failure prediction methodology attempts to maintain high computational efficiency, ease of input, and numerical accuracy. A finite element based analysis tool was developed and used to predict failure of a variety of laminates under uniaxial and biaxial loads. Two-dimensional failure surfaces for laminates under biaxial loads are generated and compared against those developed from using the Tsai-Wu failure criterion and from experimental data. The proposed failure criterion was shown to be superior to Tsai-Wu and in good agreement with experimentally determined failure loads.