Journal of Science and Technology of Composites
Year:2015 | Volume:2 | Issue:3|Papers Count:8

In this paper, based on a systematic procedure in the field of theory of elasticity, it is tried to establish an exact closed form solution for the problem of a long-fiber composite representative volume element (RVE) under uniform tension loading condition. To this end, the RVE of long fiber composites is considered. The RVE is composed of two concentric cylinders where the inner and outer ones are of fiber and matrix materials, respectively. Consequently, this study deals with an axially symmetric problem of the theory of elasticity. Using the Love function for axisymmetric problems, the Navier equilibrium equations for displacement components are converted to a single biharmonic equation in terms of the Love function. Then, a general expression for the separable solutions of biharmonic equation in cylindrical coordinates is derived. Next, according to the boundary conditions and physical interpretations, appropriate terms from separable solutions are chosen and a solution is suggested. In the final step, the unknown constants in the suggested solution are computed by using the boundary conditions. The obtained exact solution satisfies the entire field equations of theory of elasticity. Using this solution, the exact distribution of stress and displacement field components of the RVE is determined and related numerical results are presented. Based on the exact solution, an expression for the equivalent longitudinal modulus of composite is obtained and compared with the well-known rule of mixture. This comparison reveals that the rule of mixture can be a good engineering approximation for the equivalent longitudinal modulus.

In this article two dimensional sliding contact of a rigid cylindrical punch on a functionally graded (FG) semi-infinite medium is studied. The modulus of elasticity in the graded layer is calculated based on TTO model approximation. This model defines a parameter q which considers the microstructural interactions between the constituting phases. The governing equations are solved by finite difference (FD) method by means of MATLAB software. The effects of different parameters such as non-homogeneity, q, the dimensions of the punch, the thickness of the graded layer and the coefficient of friction are investigated on the force-displacement diagram of the punch, stress distribution and modes I and II stress intensify factors (KI and KII). The results show that the contact force and the stress distribution under the punch are affected by the thickness of the graded layer, radius of the punch and the non-homogeneity coefficient. But the variation of q has no effects on the contact force and the stress distribution under the punch. In the absence of the friction, KI is always non-positive and the crack has no tendency to grow under this mode. Increasing the friction coefficient, increases KI but decreases KII.

In recent decades, continuum damage mechanics based models, has been of interest to researchers in the area of damage in composites. These models with considering the effects of damage on the mechanical properties of composites and without considering detailed phenomenon of the damage, can analyze wide range of failure modes, loadings and layups. The aim of this paper is to analyze holed composite laminates using continuum damage mechanics. The general damage theory that serves as the foundation for the model of this paper is the model proposed by Ladeveze, which involves damage and inelastic strains to estimate residual stiffness. The Ladeveze model is implemented in a user material defined subroutine of the ABAQUS software, to analyze four different laminate sequences of [02/902]s, [0/90/0/90]s, [452/-452]s, and [0/90/45/-45]s. The analysis includes initiation and growth of damage parameters, subsequently a parametric study for the influence of diameter of hole and the inelastic strain material constant in Ladeveze model. Hence, it was shown that increasing the hole diameter, reduces the laminate strength, while the inelastic strain material constant is proportional to transverse inelastic strains.

In this paper, a method based on Chebyshev polynomials is developed for examination of the post-buckling behaviour of thin rectangular anti-symmetric cross-ply composite laminated plates with different boundary conditions under end shortening in their plane. Classical laminated plate theory is used for developing equilibrium equations that it produces acceptable results for thin plates. In this method, the equilibrium equations are solved directly by substituting the displacement fields with equivalent finite Chebyshev polynomials. Using this method allows developing the mathematical model of composite laminated plates with different boundary conditions on all edges. Equations system is introduced by discretizing equilibrium equations and boundary conditions with finite Chebyshev polynomials. Nonlinear terms caused by the product of variables are linearized by using quadratic extrapolation technique to solve the system of equations. Since number of equations is always more than the number of unknown parameters, the least squares technique is used to solve the system of equations. Some results for anti-symmetric cross-ply composite plates under end-shortening in their plane with different boundary conditions are computed and compared with those available in the literature, wherever possible.

In this study, the stress field and specially the interlaminar stresses in a composite cylinder with finite length which is subjected to radial load are studied. The displacement based layer-wise theory (LWT) of Reddy is used for formulation and solution of a composite cylinder which is subjected to internal and external pressure. The principle of minimum total potential energy is used to derive the governing equations and the appropriate boundary conditions (BC) for the problem. The governing equations of the problem include a set of coupled ordinary differential equations in the terms of the unknown displacement functions of the mathematical surfaces in the LWT. A set of new variables are defined and the governing equations of the problem are solved analytically. The free edge boundary conditions are considered in the analysis. In the numerical results, the distribution of the interlaminar stresses and in-plane stresses in the symmetric and un-symmetric laminated composite cylinders which are subjected to internal or external pressure are presented. It is seen that the layer stacking has important effect on the distribution and magnitude of the interlaminar stresses in the cylinder. It is observed that the maximum value of the interlaminar normal stresses in the free edge of the cylinder is bigger than the applied radial pressure and the interlaminar shear stress is in the order of the applied pressure.

In this paper, free vibration of an embedded double-layered nano-plate reinforced by functionally graded carbon nanotubes (FG-CNT) is analytically investigated. Carbon nanotubes are distributed through the thickness in two ways: uniform distribution and symmetrically linear distribution (or decreasing-increasing layup). To accurately model this nanocomposite behavior, the elastic medium around the nano-plate is modeled by Pasternak elastic foundation and the Van der waals forces between two nano-plates are taken into account. Governing equations of motions are obtained using energy method in association with Eringen nonlocal theory and solved by Navier method for a simply-supported rectangular plate. Finally, the effect of elastic foundation parameters, different distributions of CNT and nonlocal parameters are investigated on the vibration behavior of orthotropic double-layer nano-plate. Results show that natural frequencies of a double-layer nano-plate increase by increasing the Winkler elastic constants while Pasternak elastic constant has less effect on the results. Also, increasing the nonlocal parameter at a constant length decreases the natural frequencies. By increasing the length to thickness ratio (L/h) of nano-plate, the nonlocal frequencies reduce and natural frequency of symmetrically linear distribution is more than those of uniform distribution for constant value of L/h.

In the present study, the tensile strength and hardness of polymeric nanocomposites containing multi-walled carbon nanotubes (MWCNT) are investigated. For this purpose, polyamide-6 and MWCNTs melt compounded using a twin-screw extruder in various weight percentages. For investigate the influence of MWCNTs and injection molding processing parameters on tensile strength and hardness, the samples in various MWCNT contents including 0, 0.5, 1 and 1.5 weight percentages and under different processing parameters including holding pressure and injection temperature, injected molding according to design of experiments using Taguchi method. The specimens were injected and have been tested. According to the results of signal to noise ratio, the weight percentage of MWCNT is the most effective parameter on the tensile strength and hardness of samples. Also, the results demonstrated that by adding 1 wt% of MWCNT, the tensile strength of samples increased almost 31% and by addition of 1.5 wt% MWCNT, the hardness increased about 15%. The results of analysis of variance (ANOVA) indicated that wt% of MWCNT with 68% and 76% influence on tensile strength and hardness of samples, respectively, is more effective than processing parameters.

Al/Mg composite have new applications in various industries such as car manufacturing and aerospace. In this work characteristic of metallurgical interface and the behavior of melt-solid-gas during production of Al/Mg composite via vertical centrifugal casting process was investigated Aluminium melt, at 700oC and volume ratio of nearly one, was poured into a solid Mg rings preheated up to 30, 100, 150 and 200oC, respectively while rotating at 1200 rpm, in a vertical centrifugal casting machine. After casting samples were cut and interfacial microstructure was studied by optical and scanning electron microscopes. Results show that the heat content of Al melt lead to dissolving preference prominent locations and creating local Mg rich regions. Study of the Al/Mg interface also showed that intermetallic compounds, Al3Mg2, Al12Mg17 are formed. Furthermore, gas pores may form, on notches present on the rough surface of the solid Mg ring, possibly as a result of melt/gas reaction. Possible states and conditions of the formation of bubble and gas porosity are discussed.