نشریه علوم و فناوری کامپوزیت
سال سوم شماره 4 (پیاپی 10، زمستان 1395)

Due to the extensive use of composite materials in various industries, recognizing the failure models of these materials is very important. In this article, the behavior and failure models of composite pipes made by filament-wound with fiberglass, with internal liner and silica nanoparticles under local impact tested and examined. Filament winding is performed using a semi-automatic filament-wound device. Nano particles of silica that are used in the manufacturing the samples during the process of mixing and for better homogenization, the ultrasound is used. The filament winding angle of all 16 tubes was ±55. Impact test, using a gas gun with speeds of 118, 113, 108 and 100 meters per second is done. Add silica nanoparticles increases the elastic modulus and strength of the matrix. However, the existence of brittle liner, the composite shell behavior puts under its effect. In all tests, penetration of the projectile into the tube were occurred. The failure area due to impact, were same to the diameter of the projectile. Rupture of fibers failures in the matrix is the most important models of failure that were observed in impacted composite tubes. The experimental observation were reported, discussed and commented upon.

Extended finite element method (XFEM) is one of the strongest numerical methods that its basis is finite element method. In this method, using of enriching the nodes and increasing of their degrees of freedom (from 2 to 4 or even upto 10) virtually and without verifying the mesh and geometry of discountinuty, can model the system. In FEM crack geometry must be align with mesh edges which needs changing meshes in every steps of crack propagation and take so much time and too many analysis. One of the main objectives in this study is the expression of a novel method for modeling fatigue crack growth more easily and achieve the life of the structure by calculating the stress concentration factors. In this paper by using XFEM in ABAQUS, real 3D crack trajectory in single side repair has been simulated. Variation of fracture parameterin thickness direction of cracked panel with different patch lay-ups has been studied. In this paper, tests include mixed mode crack propagation. By examining the XFEM results of this research, FEM and experimental results of previous studies, it was found that the results of XFEM in comparison to experimental results have less error.

Grid structures most widely used in the aerospace, missile and Marine industry because have made: ideal mechanical properties, special stiffness and high strength. In this research, experimental and numerical investigations of the effect of longitudinal and horizontal ribs have been on flexural behavior of grid composite plates. For this purpose, four types of grid plates were considered with triangle, rhombus, large square, small square ribs. For the building these plates, silicone mold was designed and built and also was used for making plates from hand lay-up and hand-wound layer technique. Samples were subjected to a three-point bending test that for this purpose, the fixture was designed and built. From the numerical solution of the problem and compared with experimental results was observed that there is very little difference between experimental and numerical results. Results show that longitudinal ribs had a tremendous effect on the specific maximum load So that by adding a horizontal rib increase in plate strength to 594%. However, the horizontal ribs not only increases specific maximum load and specific stiffness, but also reduces the 14.7% strength and 9.26% specific stiffness because of increased weight. Priority to the ribs in terms of maximum strength is a large square, small square, triangular and rhombus and in terms of maximum stiffness is a large square, small square, rhombus and triangular.

The aim of this research is to investigate interface evolution during centrifugal casting of Al-Cu bimetal composite. In this work, 100 grams aluminum melt was cast into a 150°C preheated Cu cylindrical bush rotating at 700, 900, 1500, and 3000 rotation per minute (rpm) inserted in a vertical centrifugal casting (VCC) machine. Obtained samples were studied using optical microscope (OM) and scanning electron microscopy (SEM) equipped with EDS system and also microhardness test on various available phases. Centrifugal force, due to the rotational speed, leads to increase in the cooling rate. Cooling rate increment not only causes thinning the interface but also with increasing nucleation sites leads to modify the resulted microstructure. EDS results showed that the achieved interface consisted of four discrete layers from the Cu side, including Cu2Al, AlCu, Al2Cu continuous layers, Al2Cu precipitates scattering in anomalous eutectic structures and finally α-Al/Al3Cu anomalous eutectic structure near the Al side. Micro hardness measurements showed that hardness of various presented phases decreases in both Al and Cu side but have a maximum pick, more than 500 Vickers, near to Cu bush as a result of intermetallic compound formation.

Compared to thermoplastic, thermoset matrix composites and thermoset processes is more complex and less controllability. Cure kinetics, determine the network morphology in which the mechanical and physical properties of cured product may be assessed. So, knowing of thermoset curing kinetics is essential to process development, quality control and achieve desirable products. Hence, in this work, cure kinetics of an epoxy resin EPON 828/ curing agent dicyandiamide / accelerator Duran system for the production of the epoxy/glass fiber prepreg using in wind turbine blades. For this, curing epoxy system was carried out using Differential Scanning Calorimetry and the effect of temperature and the nanosilica weight percent studied. To investigate the effect of temperature, isothermal DSC test was carried out at five different temperature without the presence of nanoparticles and to assess the influences nanosilica with 0, 4 and 6 weight percentage is done. Obtained figures by MATLAB curve fitting were fitted well with autocatalytic Kamal model and model parameters achieved for each tested sample. Results showed an increase in temperature of the isothermal test, leads the increase of the rate of cure and the complete of curing process. Addition of nanosilica cause the increase of the rate of cure at low value α but delay at the beginning of cure and decrease significantly overall heat of reaction

In this paper, the vibration suppression capabilities of magnetorheological layer in smart beams is investigated. A three-layered beam including magnetorheological elastomer layer sandwiched between two elastic layers is considered. By assuming the properties of magnetorheological layer in the pre-yield region as viscoelastic materials behavior, the governing equations of motion and the corresponding boundary conditions are derived using Hamilton’s principle. Due to field-dependent shear modulus of magnetorheological layer, the stiffness and damping properties of the smart beam can be changed by application of magnetic field. This feature is utilized to suppress the unwanted vibration of the system. The appropriate magnetic field applied over the beam is chosen through a fuzzy controller for improving the transient response. The designed fuzzy controller uses the modal displacement and velocity of the beam as its inputs. The modal parameters of the sandwich beam including the natural frequencies and mode shapes are obtained and validated with existing results. Using the Galerkin method, the temporal equation governing beam’s motion is obtained and then the vibration of smart sandwich beam is investigated using numerical simulations. The results show that the magnetorheological layer along with the designed fuzzy controller can be effectively used to suppress the unwanted vibration of the system. The qualitative and quantitative knowledge resulting from this research is expected to enable the analysis, design and synthesis of smart beams for improving the dynamic performance of smart engineering structures.

In this research interface of aluminum-brass bimetal composite, fabricated by a vertical centrifugal casting machine, were investigated. At first, brass bushes were preheated at 100-300 °C temperature range and then aluminum melt with 1.5 melt-to-solid volume ratio was cast into cylindrical bush rotating at 800, 1600, and 2000 (rpm), respectively. Obtained samples were studied using optical microscope (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and Image J software. Results show that metallurgical joint in this work is probably due to particular dissolving condition, provided by multiple mechanical forces involved and also possible solid diffusion at the end of solidification process. Study of the EDS results show that layers formed at the achieved interface consisted of four discrete layers from the brass side, including Al3Cu5Zn4, Al3Cu3Zn, Al3Cu precipitates scattering in aluminum matrix and finally α-Al/Al3Cu anomalous eutectic structure near the aluminum side.

Composite sandwich structures with grid stiffened core (SSGSC) are one of the new structural configurations applied in advanced industries such as aerospace, that are made of two thin face sheet layers attached to the top and bottom of a grid stiffened core. Due to the good advantages such as high specific strength, not only in aerospace but also they are used in other engineering applications, such as military industry, ship building, rail transport, oil platform etc. In the present study three composite SSGSC samples made of different material and thiknesses are fabricated with hand lay-up method using a silicon rubber mold and epoxy resin. Also, two metallic samples of the same dimensions as the copmposite ones, including a monolithic and a SSGSC samples made of aluminum are fabricated. In order to study their behavior subjected to the quasi-static transverse loads, the samples undergo three-point bending tests. Results of the practical tests on the composite samples showed that beyond the failure of the face sheets, the grid stiffened core will tolerate the load, also there are no delamination between the face sheet layers due to good curing process. It was found that changing the fibers of the face sheet from Glass to Carbon with the same thikness, improves strength-to-weight ratio of the SSGSC samples rather than increasing the thickness of the face sheet of the same material.

Epoxy toughening is very important in industry, because high brittleness and low fracture energy. In this work different toughening techniques was used for epoxy curing system. The techniques include change of the curing temperature and simultaneous addition of silica nanoparticles on curing system. The glass transition temperature of epoxy with this curing system is approximately 118 , if the curing temperature is too closer to glass transition temperature then curing is complete, So tensile modulus and other properties such as strength is increased; In this work curing temperature was changed from 105 to 90 . While it was just pure epoxy to be tested, tensile modulus decrease and fracture energy increases, as result imperfect curing at 90 . But addition of nano-particles to the curing system increased tensile modulus in both temperatures curing system, this increased in both curing system is equal while the equal nominal surface of the nanoparticles used. On the other hand toughness and fracture energy in both curing system increased with increased nominal surface of nanoparticles when nanosilica is used, this raise in the fracture toughness at curing temperature of 90 is always greater than from the fracture toughness at curing temperature of 105 . In generally seen curing at 90 and increase in the nominal surface of nano- particles which were strongly increased fracture toughness, and modulus. The fundamental mechanisms that are effective in increasing the toughness of nano composites, they are used to analyzing the increase in fracture toughness of nanocomposites.

In this paper, dynamic response of simply-supported composite sandwich beam with viscoelastic and transverse flexible core is investigated, analytically. Three-layered sandwich panel theory is used to analyze the beam. Hamilton's principle is employed to obtain governing equations of motion. In this paper, GHM method is used to model the viscoelastic core of the beam. Advantage of GHM model in according to classical models is including the frequency dependent characteristic of viscoelastic materials. Modal superposition method is used to convert partial differential equations of motion to ordinary differential equations with time varying coefficients. Newmark method is applied to solve the ODE with a numerical approach. Results of the present model are validated by analytical results published in the literatures. Innovation of this paper is considering frequency dependency of material property in viscoelastic core with using GHM model and utilizing three-layered sandwich panel theory in dynamic analysis of composite sandwich beam. The article investigates the dynamic response of beam with viscoelastic core by using GHM model to illustrate advantages of the GHM model over the Kelvin-Voigt model. As well as, a parametric study is also included in this paper to investigate the effect of different parameters such as thickness and density of core and stiffness of composite sandwich beam face sheets on the beam frequency and damping rate of beam dynamic response. The obtained results show that GHM model by considering the frequency dependency behavior of viscoelastic material presents a more accurate description of dynamic response of the beam with viscoelastic core.

In recent decades polymeric composites have received considerable attention from different industrial sectors due to their outstanding properties. Despite the multi-purpose properties, polymers undergo creep even at room temperature which is considered as a disadvantage for their long-term applications. Numerous methods have been suggested by researchers in order to predict creep in polymeric composites. In this article, a brief review is conducted on fundaments of creep in polymers and different theoretical methods presented for creep modeling in long fiber reinforced laminated composites are categorized. Then, a new method for evaluating long-term creep in polymeric composites relying on short-term experimental data on pure resin is developed. The developed model is just in need of simple tension-creep tests on pure resin as input and creep behavior of pure resin is evaluated accordingly. Then, the results are used to estimate creep behavior a single composite laminate and finally creep behavior of laminated composites with arbitrary lay-up configurations is theoretically characterized. In parallel, the capability of micromechanical rules in estimating creep behavior of composites using its constituent’s behavior is investigated. A comparison between published experimental observations and theoretically obtained results imply on proper performance of developed modeling procedure for analyzing creep phenomenon in polymeric composites.

In this research, the sol solutions of silica nanoparticles were prepared by hydrolysis of tetra-methoxysilane (TMOS) in the presence of methanol. Then, Sol containing silica nanoparticles was added to Isophthalic polyester in 0 to 40 weight percent. The prepared nanocomposites were molded and processed at room temperature in the laboratory condition. In this study, the effect of silica nanoparticles and also the effect of secondary post curing temperature on mechanical properties of the polyester nanocomposite were evaluated. Fourier-transform infrared spectroscopy, FT-IR results showed that Si-O-Si bonds are formed in the polyester silica nanocomposites. The results of Tensile test show that the cured samples at room temperature have more strain and less tensile strength in compared with the post curing nanocomposites at 100˚C. The results of Dynamic-Mechanical Analysis (DMA) on the nanocomposite samples containing (20% TMOS) show an increase in glass transition temperature and real modulus (E') by increasing the post curing temperature. In addition, the results of Dynamic light scattering (DLS) and scanning electron microscopy (SEM) indicate that the size of the particles are 35-40 and 53 nm in the (20%TMOS) nanocomposite sample, respectively. However, the size of formed nanoparticles increases to 87 nm with post curing temperature at 100˚C.