نشریه علوم و فناوری کامپوزیت
سال سوم شماره 1 (پیاپی 7، بهار 1395)

In this paper, the effect of carbon nanotubes on mode I strain energy release rate at crack growth initiation of epoxy-based nanocomposites was studied analytically. In this theoretical model, effect of carbon nanotubes debonding from its surrounding resin at the crack tip was postulated as one of the causes of increasing of the strain energy release rate of nanocomposites in comparison with the pure resin. Furthermore, a representative volume element was considered at the nanoscale. The assumed representative volume element contains carbon nanotubes, surrounding resin and the interphase. The available mechanical properties and the thickness of the interphase in the literature were used. Finally, a model for increasing the strain energy release rate of nanocomposites due to presence of carbon nanotubes was introduced based on mechanical properties and geometric parameters of carbon nanotubes and resin. It must be noted that enhancement of strain energy release rate in comparison with the pure polymer was investigated by correlation between nano, micro and macro-scales. To validate the proposed analytical model, results were compared with other experimental results available in the literature. The results show that the present model has a reasonable error and is able to model the effect of single-wall and multi-wall carbon nanotubes on nanocomposites strain energy release rate.

Grid-stiffened composite (GSC) structures have been maturely developed in aerospace, aircraft and automobile industries due to their attractive properties such as high specific strength and stiffness, superior load bearing capacity, and excellent energy absorption capability. These structures undergo various loading conditions in service. In the present study, iso-GSC structures reinforced with silica nanoparticles (SiO2) have been investigated in terms of their capability to improve the mechanical properties during transverse loading. At first, a silane coupling agent (3-glycidoxypropyltrimethoxysilane/3-GPTS) was introduced onto the silica nanoparticle surface and the effects of silica content (0, 1, 3, and 5 wt.% with respect to the matrix material) on the three-point flexural response of isogrid E-glass/epoxy composites were assessed. Based on the Fourier transform infrared (FT-IR) spectra, it was inferred that the 3-GPTS coupling agent was successfully grafted onto the surface of silica nanoparticles after modification. The results showed that nano-SiO2 particles incorporation affected the flexural properties of the isogrid fibrous composites. Maximum improvements in the flexural load and energy absorption were obtained after adding 3 wt.% nano-SiO2 particles. In this condition, up to 14% and 25% increase in the maximum flexural load and energy absorption, respectively were observed, compared to the sample without silica addition. In these structures, a considerable amount of energy absorption occurred beyond primary failure at the peak load point. Furthermore, the flexural stiffness was increased by increasing the silica loading. In conclusion, this study suggests that the addition of modified silica nanoparticles is a promising method to improve the flexural properties of the grid-stiffened fibrous composite structures.

One of the most important issues about the composites behavior in different loading conditions is the initiation and propagation of various damage modes that have significant effects on the application of these materials. Fiber/matrix debonding is one of the first damage modes that appears in different composites and causes the formation of other damage modes like matrix cracking. In the present study, by using the cohesive zone model (CZM) as well as an extended finite element method (XFEM) and by applying a transverse loading on different representative volume elements (RVE’s) in micromechanical scale, the effects of initiation and propagation of different damage modes like fiber/matrix debonding and matrix cracking will be studied. To this aim, the authors start by studying the behavior of cohesive zone model and validating the applied method by simulating the previous researchs. Then, the effects of cohesive zone on different volume elements will be studied and the results will compare with each other. Finally by entering the effects of matrix cracking initiation and propagation using the extended finite element method, effects of cohesive zone damage and matrix cracking will be studied simultaneously based on finite element method and using Abaqus software

The impregnation prediction of melted thermoplastic into reinforced fiber is the one of the challenges in pultrusion of thermoplastic composites. In this paper, tow impregnation models has been presented to analyze the impregnation in the pultrusion process of thermoplastic composites. At first, a simple model based on Darcy’s law is provided. In the first model, without considering microscopic impregnating flow, velocity of macroscopic flow and pressure of the molten thermoplastic calculated along the axis of the pultrusion mold. The calculated pressure in the macroscopic flow in the axial direction is combined with Darcy equations in microscopic and macroscopic flow, then the simple equation is obtained to calculate the radial velocity of melted thermoplastic and the radius dimension of the dry region of the fiber agglomeration. This dry region radius shows an estimate of the degree of impregnation. In the second part, a developed model has been proposed. In this model, microscopic flow and the radius changes of agglomeration are considered. Darcy equation is written in two radial microscopic and macroscopic axial direction flows. The perfect equation considering the macroscopic and microscopic flow is proposed. This equation represents the relationship between pressure drops in macroscopic axial and microscopic radial direction and the radius of the dry region and agglomeration radius with other constants. An iterative solution algorithm for solving the developed model is used. Both models for a series of inputs have been calculated. The degree of impregnation and radius of dried fiber region in agglomerations has been calculated.

Nanocomposites based on PA6/ABS (60/40) containing 3 phr of POE-gr-MA and 2, 5 and 8 wt.% of CaCO3 nanoparticles (10-15 nm) were prepared by melt compounding, using a co-rotating twin-screw extruder, followed by injection molding process. The morphology and impact properties were characterized. The inclusion of coated CaCO3 nanoparticles into PA6/ABS altered the morphology, and as a consequence, ABS particle size in PA6 matrix was increased. This result was attributed to the nonpolar natures of ABS and coated CaCO3 as well as very small nanoparticles’ size. Incorporation of CaCO3 nanoparticles noticeably affected the impact properties. By adding 2 wt.% of nanoparticles, the impact strength in notched samples was increased as high as 160% when compared to net PA6/ABS. Due to their high flexibility, no break was observed in unnotched samples containing 2 and 5 wt.% of nanoparticles.

In this paper, structure and morphology of powder coatings created by electrostatic spraying method and also the corrosion protection properties of these coatings, were investigated. For this purpose, 5%Wt nanoclay was embedded in polyester matrix using different and continuous mixing methods. Fabricated nanocomposite powder and pure polyester powder were applied on the carbon steel by electrostatic spraying method. Morphology and structure of nanoclay was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM and TEM micrograph showed lamellar structure of nanoclay. Also the morphology and structure of the created coatings were analyzed by SEM. The results of SEM for coatings showed that the nanocomposite coatings have less pores and are more compact than pure coatings. The corrosion properties of coatings were investigated by immersion test and electrochemical impedance spectroscopy (EIS). Corrosion test results showed that the corrosion resistance of the nanocomposite coatings have increased about 30 times than pure coatings

In this paper, electromechanical response analysis of a rotating piezoelectric cylinder with functionally graded material under thermomagnetic fields is presented. In this study all mechanical, magnetic, electrical and thermal piezoelectric material properties, were considered to follow an identical power law in the radial direction. Using heat transfer equation in one dimension (1D) with symmetric boundary conditions, the temperature changes in the steady-state can be achieved. According to Maxwell electrodynamics equations, Lorentz magnetic force is obtained due to the presence of an externally applied axial magnetic field. Using the equation of temperature distribution in the cylinder wall thickness under considered boundary conditions and the use of electromechanical relations by acquiring the magnetic force, inhomogeneous differential equation is derived and then solved by analytical method. Also, the ANSYS finite-element software is employed for thermo-piezo-mechanical analysis of a rotating functionally graded piezoelectric cylinder. By providing a numerical example, the effect of various parameters such as the intensity of the magnetic field and temperature and coefficient of heterogeneous material on the stress and strain behavior, electric potential distributions and radial displacement of cylinder is investigated. To validate the results, comparisons are made with the solutions for FGM cylinder available in the literature.

In the last decades, the production of smart materials have led to modern structures with superior properties. Among these materials one may points to the Shape Memory Alloys (SMAs) which show the capability of retaining the large plastic strains when exposed to outer temperature or traction loadings. The developement of SMA acuators in the forms of wire and stent have attracted many attentions in the fields of engineering and smart structures. In this regard, an analytical model is represented for the composite beam with shape memory alloy wires resting on the Pasternak elastic foundation. The composite beam is simply-supported in both sides and pre-strained SMA wires are embedded in the middle of the cross section. The governing equations of Euler-Bernoulli, Rayleigh, Shear and Timoshenko beams are extracted using the Hamilton's principle. By heating the beam, strain recovery operation will produce a tensile force along the beam. This tensile force in turn will produce a compressive force against the beam supports. The resulted force is modeled by martensite transformation. By normalizing the governing equations, analytical relations are provided to evaluate the exact solution of natural frequency. The validity of results are established by comparing the typical solution with similar solution in the literature. Based on the analyses, the effects of Pasternak foundation coefficients, number of SMA wires, thickness to span ratio, recoverable strain limit and span to width ratio on the natural frequency in temperature above the austenite finish temperature are found and represented by using the engineering beam theories

Production of Mg-Al bimetal composite, for weight reduction of industrial components and decrease in fuel consumption, has taken attentions in the transport industry during recent years. In this research Mg melt was poured at 700 Celsius into Al hollow cylinder, with 1.5 melt to solid volume ratio (Vm/Vs), preheated at various temperature including; 320, 400 and 450 Celsius, respectively, while they were rotating at 1600 rpm in a vertical centrifugal casting machine. Effect of the cooling process after pouring, and also contraction behavior of magnesium-aluminum bimetal during solidification, under centrifugal force, was studied. Preheating temperature from 350 to 450 Celsius led to the increasing of reaction layer and phase changes. Study of microstructure using scanning electron microscope (SEM) equipped with x-ray spectroscopy showed Al3Mg2 and Al12Mg17 intermetallic compounds, eutectic structure and Mg solid solution are formed in the interface. Keep the casting in the casting machine, while rotating and cooling to the range of 150 Celsius, prevented creation of contraction cracks and separation of two layers composite alloy.

A micromechanical model is presented to analyze the fracture response of unidirectional composite materials considering the nonlinear behavior of matrix material under loading more than the yield strength as well as the fiber-matrix debonding and matrix cracking. The composite microstructure is characterized with repeating unit cell with regular or random fiber-packing patterns. The micromechanical model is employed for composite material with aluminum matrix and carbon fibers. The high rigidity fibers are modeled as linear isotropic elastic material, while matrix material is characterized with elastic-plastic model. The damage initiation stage in matrix material is described by principal strain criterion accompanied with damage evolution considering stiffness degradation up to crack formation. The bonding between fiber and matrix is modeled using cohesive model, in which damage initiation criterion depends on the normal and shear strength of the cohesive zone. The micromechanical model is employed to study the effects of fiber distribution, fiber volume fraction, fiber-matrix bonding strength on the crack propagation through the microstructure as well as the stress-stain graph up to the fracture of microstructures.