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This research aims to develop a new constitutive model using classic micromechanics methods to predict the stress -strain behavior of elastomeric nanocomposites. For this purpose, using the concept of eigen strain in polymer nanocomposites, an improved self - consistent implicit model was applied to develop a new constitutive model while it was used only to predict the elastic properties of these materials before. Additionally, using this model, a constitutive model relying on a modified self-consistent model and incremental theory is presented to predict the stress -strain behavior of elastomeric nanocomposites. The model was verified by comparing results obtained from the theoretical and experimental models. Examining the results has shown that the newly developed constitutive model based on the improved self -consistent micromechanics method can predict the stress -strain behavior of elastomer nanocomposites with better accuracy compared to other theoretical models, including Mori -Tanaka's model. [ABSTRACT FROM AUTHOR]

Once a composite laminate is subjected to quasi -static tensile or fatigue loading, some damage modes initiate and propagate in the laminate. The first damage mode is the matrix crack that forms in the layers with an angle to the loading direction. Although not leading to breakage, these cracks reduce the equivalent mechanical properties of the composite laminate. In this paper, a new nonlinear analytical model is presented and used to predict the stiffness degradation of the cross -ply composite laminates. For this purpose, a new third -order polynomial function is proposed as the Helmholtz free energy of the composite, and the appropriate equations are derived. A microscopic experimental test is designed and accompanied by the analytical model to investigate the damage progression in a glass/epoxy cross -ply laminate. Also, finite -element micromechanical models with periodic boundary conditions (PBC) are proposed and used to determine the damage constants. The model is validated against the 3D micromechanical models and the quasi -static uniaxial loading - unloading experimental tests. The validation shows a very good agreement between the model and the experiments. [ABSTRACT FROM AUTHOR]

In this research, an analytical method is presented for predicting the viscoelastic and dynamic behavior of polymer nanocomposite. The analytical model is achieved by coupling the SUC micromechanical model with standard linear solid model. Boltzmann superposition principle is used to develop the constitutive equations. First, the strain associated with a relaxation experiment is considered, and then by using the idea of linearity as embodied in the Boltzmann superposition principle, the resulting stress history is predicted. Eventually, the creep function corresponding to the relaxation modulus is obtained and the hysteresis loop for nanocomposite material is represented. Creep response is sinusoidal in time and a function of stress history. Loss and storage modulus and material behavior in Laplace domain are obtained using standard linear solid model and SUC micromechanical model, respectively. Standard linear solid model is achieved by paralleling the Kelvin model with Maxwell model. The model is validated with experimental results. Effects of different interphase thickness, CNT volume fraction and phase angle on hysteresis loop is studied. Obtained results reveal that increasing the CNT volume fraction and phase angle leads to decreasing and increasing the nanocomposite hysteresis loop area, respectively. Also, Interphase thickness contains considerable effects on the nanocomposite dynamic behavior. [ABSTRACT FROM AUTHOR]

In this work, an elastoplastic constitutive model is planned to analyze the effects of adding silica nanoparticles on the overall elastic-plastic stress-strain curves of the polymer matrix nanocomposites. The elastic modulus of the nanocomposites is evaluated by combination of the Mori-Tanaka and Eshelby micromechanical models considering interphase region formed due to the interaction between silica nanoparticles and the polymer matrix. Then, the elastic-plastic stress-strain curves of nanocomposites are extracted by employing a micromechanics-based ensemble-volume averaged homogenization procedure. To prove the validity of the developed method, the predictions are compared to the experimental data existing in the literature. The effects of volume fraction and diameter of silica nanoparticles, thickness and adhesion exponent of the interphase on the polymeric nanocomposite elastic-plastic stress-strain curves are extensively examined. Stiffer elastoplastic behavior is found in the presence of interphase region. The results clearly indicate that the strengthening of the silica nanoparticle-reinforced polymer nanocomposites is improved with (1) increasing nanoparticle volume fraction, (2), decreasing the nanoparticle diameter, (3) increasing the interphase thickness and (4) decreasing the interphase adhesion exponent. Finally, the elastic-plastic stress-strain curves of silica nanoparticle/polymer nanocomposites under biaxial loading are achieved. [ABSTRACT FROM AUTHOR]

The effects of physical, thermal and mechanical parameters on curing microthermal residual stresses are studied, based on the energy method, to present a theoretical solution for prediction of residual stress felds. A fnite element analysis is developed to compare the theoretical and numerical results together. There is found to be a good agreement between the results of the two methods. However, due to the edge effect, the fnite element method is not able to satisfy the boundary conditions at the composite ends. An increase in the fber length leads to a signifcant increase in axial and shear stresses. In addition, for long fber composites the axial and shear residual stresses distribution become more uniform along the fber length and suddenly decrease to zero at the composite edge. The results of the two methods demonstrate that besides the physical characteristics of composites, the order of mismatch in thermal and mechanical properties of the fber and matrix has a considerable infuence on the axial and shear residual stress felds. It must be noted that the radial stress distribution is approximately independent of these parameters and only its maximum value is changed at fber end. The presented analytical solution not only satisfes all governing boundary conditions, but also yields the residual stress felds according to longitudinal and radial coordinates. A shear stress concentration occurs at vicinity of composite edge that signifcantly intensifes due to increasing the mismatch in thermal and elastic properties of the fber and matrix. High mismatch in coeffcient of the thermal expansion and Young’s modulus of the fber and matrix causes a substantial increase in the axial and shear stresses, while the difference between the Poisson’s ratios of the composite constituents has not any signifcant effect on the residual stress feld.

The aim of this paper is investigation of progressive damage in a metal matrix composite lamina using coupling of micromechanical method and continuum damage mechanics viewpoint. The micromechanical method is a representative volume element based method known simplified unit cell method which possesses the capability of investigating of progressive damage and plastic behavior in the representative volume elements. The studied damage is isotropic and anisotropic based on continuum damage mechanics viewpoint. Under investigation composite system is Carbon/Aluminum composite. The matrix behavior is considered as isotropic and elastoplastic and the fiber behavior is transversely isotropic and elastic. The fiber arrangement within the matrix is regular. The matrix elastoplastic behavior model is included as bi-linear behavior and solution method is successive approximation method. According to available previous studies, Siliconcarbide/Titinium composite system is noticed for validation and comparison with experimental data. Also the effect of fiber volume fraction on the damage progression routine is studied. The results show that by increasing the longitudinal and transverse loadings, the damage variable grows in the fiber direction and perpendicular to the fiber direction and the axial and transverse Young's modulus decrease subsequently. Also the results prove that in longitudinal loading, considering anisotropic damage, damage progression in the fiber direction is more than its growth in perpendicular to the fiber direction. Whereas, under transverse loading, damage growth in perpendicular to the fiber direction is faster. [ABSTRACT FROM AUTHOR]

The application of woven fabrics in composites manufacturing has been increased because of their special mechanical behavior. Due to the complexity of modeling and simulation of these composites, in this research a micromechanics based analytical model has been developed to predict the elastic properties of woven fabric composites. The present model is simple to use and has a high accuracy in predicting the elastic properties of woven fabric composites. One of the most important effective factors on the modeling accuracy is utilizing a proper homogenization method. Therefore, a new homogenization method has been developed by using a laminate analogy based method for the woven fabric composites. The proposed homogenization method is a multi-scale homogenization procedure. This model divides the representative volume element to several sub-elements, in a way that the combination of the sub-elements can be considered as a laminated composite. To determine the mechanical properties of laminates, instead of using an iso-strain assumption, the assumptions of constant in-plane strains and constant out of plane stress have been considered. Then, the proposed homogenization model has been combined with a micromechanical model to propose the new micromechanical model. The applied assumptions improve the prediction of mechanical properties of woven fabrics composites, especially the out of plane elastic properties. The proposed model has been evaluated by comparing the predicted results with four available experimental results available in the literature, and the accuracy of the present model has been shown. [ABSTRACT FROM AUTHOR]

Mechanical properties of polymeric materials are significantly sensitive to the loading rate. Therefore, it is necessary to develop a dynamic constitutive model to investigate their strain rate dependent mechanical behavior. In this study, first by conducting torsion experiments the shear behavior of neat and reinforced epoxy with carbon nano-fibers (CNFs) was studied experimentally. Then, the Johnson-Cook (J-C) model was modified to be able to model the shear behavior of neat polymers. The strain rate effects on elastic behavior of polymers were considered by introducing a material equation. Then, by combining the modified Johnson-Cook (MJ-C) model with a micromechanical model (Halpin-Tsai model) and using pure polymer experimental tests and mechanical properties of carbon nano fiber, the strain rate dependent mechanical behavior of polymers reinforced with CNFs at arbitrary strain rates and volume fraction of carbon nanofiber has been predicted. The new model presented in this research is called the dynamic-micromechanical constitutive model. The predicted results for the neat and nano-phased polymers were compared with conducted and available experimental results. It has been shown that the present dynamic constitutive model can predict the strain rate dependent mechanical behavior of polymeric materials with good accuracy. [ABSTRACT FROM AUTHOR]

In this paper, a 3D time dependent micromechanical model is presented to study the elastoviscoplastic behavior of aligned fiber reinforced composites in the presence of interfacial damage subjected to multi-axial loading. The Representative Volume Element (RVE) of the composite consists of three phases including fiber, matrix and fiber/matrix interphase. The interphase is considered as adistinct phasewith a definite thickness that covers the outer surface of fibers. The difference between manufacturing process temperature to room temperature introducing thermal residual stresses is included in the analysis. The fiber and interphase are assumed to be elastic, while the matrix exhibits elastic-viscoplastic behavior with isotropic hardening. The Bodner-Partom viscoplatic theory is used to model the time dependent inelastic behavior of the matrix. The Needleman model is employed to analyze interphase damage. For metal matrix composites, it is shown that while predictions based on undamaged interphase are far from the reality, the predicted stress-strain behavior including interphase damage and thermal residual stresses demonstrate very good agreement with experimental data. Furthermore, the elastic properties of the composites with various aspect ratios are extracted by the micromechanical model. The elastic behavior predictions of the composite are also very close to experimental data and the other available models. [ABSTRACT FROM AUTHOR]

Grains in polycrystalline texture have anisotropic deformation nature. This cause material to show completely different behavior at meso and micro scale than they do at macro scale. To be specific, deformation at these scales is heterogeneous and cannot be modeled using constitutive equation in continuum plasticity. In this paper, in order to investigate deformation behavior of 316L stainless steel at micro scale a crystal plasticity finite element (CPFE) modeling system has been developed. The crystal plasticity equations were implemented in the ABAQUS/Implicit FE code through a user-defined material subroutine, UMAT. Verification was done through comparing the CPFE result against those obtained through implementing crystal plasticity formulation in MATLAB software. Comparison show good agreement between the analytical and CFFE result. Afterward, three dimensional simulation of tensile test on Stainless Steel type 316L is carried out using CPFE method and continuum macro mechanic FE. Deformation characteristic and localization behavior of single grain specimen during tensile test has been captured and predicted using CPFE method; on the other hand, macro mechanic finite element is unable of predicting localization and evolution of lattice at micro and meso scale. At the last part, a set of CPFE analysis are conducted on representative volume elements with 10 Grain and 5 set of different grain orientations. Results show a scattering in plastic part of stress-strain response of material with switching from one set of grain orientation to another set. It has been found that the material behavior at these scales is highly direction dependent. [ABSTRACT FROM AUTHOR]

In this study, a three-dimensional micromechanics-based analytical model is developed to study the effects of regular and random distribution of silica nanoparticles on the thermo-elastic and viscoelastic properties of polymer nanocomposites. The Representative Volume Element (RVE) of nanocomposites consists of three phases including silica nanoparticles, polyimide matrix and interphase. Since the polymer in the vicinity of the nanoparticles shows distinct properties from those of the bulk matrix, due to nanoparticle--polymer matrix interactions this region as interphase is considered in micromechanical modeling with specific thickness and properties. In order to simulate random distribution of silica nanoparticles into polyimide matrix, the RVE is extended to cxrxh cubic nano-cells in three dimensions. Perfect bonding conditions are applied between the constituents of RVE. It is assumed that all three phases of the RVE are homogeneous and isotropic to obtain the thermo-elastic response of nanocomposite. The extracted thermo-elastic properties by the micromechanical model with random distribution of silica nanoparticles are closer to the experimental data. To predict the effective viscoelastic properties of the nanocomposites, silica nanoparticles are modeled as a linear elastic material, while polyimide matrix and interphase are assumed to be as a linear viscoelastic material. The model is also used to examine the influence of varying interphase properties and silica nanoparticle size on the effective nanocomposite behavior. The overall creep behavior of the nanocomposite for several stress levels is also presented. [ABSTRACT FROM AUTHOR]

In recent years many investigations have been performed on design and fabrication of micro mechanical manipulators. These manipulators have a wide application in industry, specifically medical applications. A practical usage of this manipulator is endoscopy system. In the endoscopy system, a small manipulator with high maneuverability and flexible body is needed to make the probe's movement into the colon easier than classic manipulators. In this paper a basic flexible module is developed for use in this structure. Connecting three wires of shape memory alloy uniformly distributed in circumference of a compressive spring with angle of 120 degrees, it is possible to make an sufficiently large displacement in the end planes with small strain of wires. In this paper, a model is developed to define the spring deformation which in the following is coupled with the model of SMA wire presented by Brinson to describe the module behavior definitely. Dynamic modeling and simulation is implemented in MATLAB-Simulink and module performance in addition to proper geometry for the considered application is extracted. Through the results of simulation verified by experiment, proper parameters of module for providing more deflection and rotation when activated by SMA are extracted. [ABSTRACT FROM AUTHOR]

In comparison with conventional concrete, the polymer concrete has considerably better mechanical properties. Meanwhile, polymer concrete is more expensive than traditional cement concretes. Considering the good mechanical properties of polymer concrete, its applications can be developed by reducing manufacturing costs. In this study, in order to improve the mechanical properties and reduce the manufacturing costs of polymer concrete, a new formulation is presented. In this formulation, the more common mixture of large and small aggregates was replaced by silica sand. In addition, in order to reduce the manufacturing costs, polyester resin was used instead of epoxy resin as the base material. The compressive and three-point bending tests showed that the polyester-based concrete had a higher compressive strength (20%) in comparison with the epoxy-based concrete, while their bending strength was approximately the same. As a result, the manufacturing cost for the polymer concrete prepared using this new formulation can be reduced without decrements in mechanical properties. Moreover, a two-phase micromechanical model was applied to estimate the compressive strength of the polymer concrete. Using the two-phase micromechanical model, the mechanical properties of the polymer concrete were predicted. The results obtained from the modeling and experiments were in good agreement with each other (6.8% error). The excellent mechanical properties of polyester-based concrete developed in the present research and its low cost in comparison with the epoxy-based concrete are key factors that can help its wider applications.