نشریه علوم و فناوری کامپوزیت
سال چهارم شماره 1 (پیاپی 11، بهار 1396)

Laminated composite conical shells are used as components of aerospace, marine industries and civil engineering structures. In this research free vibration of grid stiffened composite conical shell with simply support boundary condition is studied. No study has yet been done on vibration analysis of these structures. Smeared method is employed to superimpose the stiffness contribution of the stiffeners with those of shell in order to obtain the equivalent stiffness parameters of the whole structure. The stiffeners are considered as a beam and support shear loads and bending moments in addition to the axial loads. Geodesic path is applied to the stiffeners. Equations were derived using classical shell theory of Donnell type and were solved using energy functional with the Rayleigh-Ritz method. A 3D finite element model was built using ABAQUS software. Results were compared and validated for grid stiffened structure with ABAQUS software. Comparisons and validations revealed good agreements. The effects of shell geometrical parameters and variations in the cross stiffeners angle on the natural frequencies were investigated.Results given are novel and can be used as a benchmark for further studies.

Matrix Cracking and induced delamination in symmetric cross-ply, [0m/90n]s, and angle-ply, [θ/-θ]s, laminated composites under axial and shear loading are studied by multi-scale micro-meso approach. The stress transfer model is implemented to predict the stress and displacement distribution incorporated with the ply-refinement technique in laminates. By considering crack closure condition, a series of useful inter-relationships between thermo-elastic constants for damaged and corresponding undamaged laminates are derived. By using both of stress and displacement fields and these inter-relationships the properties of damaged laminated structure are achieved. These obtained properties are used to calculate the energy release rate in load control condition for the initiation and growth of matrix cracking and induced delamination in cross-ply symmetric laminates with [0m/90n]s layup. The obtained results of mechanical properties degradation show a good agreement with the results of experiments, variational approach and finite element methods (FEM) existed in literature. By comparing the results of the energy release rate of single-layer method with variational approach and finite element methods, it was observed that the results of proposed single-layer have a considerable error due to differences in slop of stiffness versus crack density.Finally, the accuracy of single-layer micromechanical approach in predicting the mechanical properties and inaccuracies in predicting of energy release rate was confirmed

In this paper, crashworthiness analysis of a tapered sandwich column under oblique impact loading against a rigid wall is investigated by nonlinear finite element analysis.The energy absorption characteristics of honeycomb sandwich cylindrical columns in oblique crushing process depend greatly on the amount of material which participates in the plastic deformation. The interaction effects between the honeycomb and column walls greatly improve the energy absorption efficiency. The response surface method with cubic basis functions is employed to formulate specific energy absorption and peak crushing force, which reduces considerably the computational cost of crush simulations by finite element method. Based on the results of crash modeling, it is observed that the specific energy absorption has a decreasing trend by increasing the impact angle and decreasing the column thickness. On the other hand, the peek crushing force reduces when the impact angle and the column thickness are increased. Therefore, multiobjective optimization is done to maximize the specific energy absorption and minimize the peek crushing force at the same time. Furthermore, maximizing the specific energy absorption and maximizing impact load angle is performed. Finally, both local and global sensitivity analyses are employed to assess the effect of impact angle and thickness on the specific energy absorption and peak crushing force. The global sensitivity of the specific energy absorption with respect to the impact angle is observed to be more than the column thickness, while the peak crushing force has more global sensitivity to the column thickness compared to the impact angle.

In this research, mechanical properties and fracture mechanism of polymer nanocomposite reinforced by carbon nanotubes (CNTs) has been evaluated employing multiscale modeling method. Effect of CNTs’ structural defects and covalent bonds created during functionalization process are investigated in nanoscale analysis and the effect of CNTs’ dispersion, curvature and volume fraction are studied in microscale analysis. In microscale modeling both analytical and finite element methods are employed to investigate mechanical properties and their results are compared. It has been investigated that, according to mentioned parameters such as CNTs’ dispersion, volume fraction, functionalization and curvature in polymer matrix, both increase and decrease in ultimate strength of polymer nanocomposite are possible with respect to pure polymer. Moreover, polymer nanocomposite’s ultimate strength is increased and fracture brittleness is decreased significantly using functionalized CNTs. On the other hand, the CNT’s structural defects caused during functionalization process decrease polymer nanocomposite Young’s modulus. It also has been demonstrated that by increasing curvature, the improving effects of functionalized CNTs on mechanical properties of polymer nanocomposite, decrease obviously.

In this paper the kinetic theory of fracture is employed to predict fatigue life of composite cross-ply laminates. To this end, carrying stress in each layer is determined based on classical plate theory. Then using the rate dependent kinetic theory of fracture, damage index for constituents of each layer, matrix and fiber, are determined separately. According to these values, mechanical degradation model are applied separately for fiber and matrix material properties and the overall property of each layer will be updated. In this way carrying stress in each layer and consequently in fiber and matrix will be changed and fatigue failure is take place when value of fiber or matrix damage index in all layers of cross-ply laminated composite rises to the critical value. In this approach material parameters in fiber and matrix kinetic theory of fracture equations will be determined using experimental fatigue data for the longitudinal (θ= 0°) and transverse (θ= 90°) unidirectional, respectively. The huge advantage of this micromechanical model is reduction in amount of experimental test. Results from presented model show good agreement with experimental result presented by other researchers on cross-ply laminated composite plates.

In this paper, an analytical solution is presented to study acoustic transmission through a laminated composite cylindrical shell. Therefore, a new and exact model is employed to solve the three-dimensional sound transmission loss in a composite cylindrical shell based on three-dimensional linear theory of elasticity. The model presented in this paper is a composite cylindrical shell with arbitrary thickness and infinite length and immersed in a fluid. Also an oblique plane wave impinges on the external sidewall of the shell. The state space method is used to investigate laminate approximated model along with transfer matrix approach to analysis the sound transmission loss though the composite cylindrical shells. The results obtained from presents study have been compared with those of other researchers. This comparison shows an excellent agreement between the results. Finally, the parameters affecting on the sound transmission loss have been studied. The results indicate that the laminated composite cylindrical shells are more advantageous rather than other materials as a result of enhancing the sound transmission loss with change the stacking sequence of laminated composite. This characteristic can be used to increase the amount of sound transmission loss in composite shells.

Glass fiber reinforced composites pose numerous industrial applications that are because of suitable mechanical and physical properties. Drilling is a common method to connect fiber reinforced material structures. Rotary ultrasonic machining is one of the new methods in machining of fiber reinforced composites that is highly attractive in recent years. Fiber reinforced composite laminates in machining operations, especially in drilling operations which are subjected to stress concentration, tend to be delaminated. Delamination damage is strongly influenced by factors such as tool material and geometry and also machining parameters. Therefor in recent years, new tools such as diamond core drills are used in drilling of fiber reinforced composites due to their lower force creation and simultaneous drilling operation and hole internal grinding. In this research, delamination of glass fiber reinforced composites (GFRP) with a percentage of 65% fiber in rotary ultrasonic machining process using diamond core drill regarding machining parameters have been discussed. According to conducted experiments, it was observed that cutting speed increment and feed rate reduction, decreases the delamination damage and improves hole quality. Also based on achieved results, a statistical relationship between machining parameters and delamination in Minitab software offered. According to the resulting model, cutting speed has a greater impact on the amount of delamination

In this research, the production mechanism of VB-Co in situ nanocomposite powder from oxidized raw materials via mechanochemical method was studied. Regarding the adiabatic temperature of the chemical reaction, this reaction was occurred through self-progressive high-temperature synthesis or MSR. The mixed powder of raw materials (Co3O4, V2O5, B2O3 and Mg) were ground according to the stoichiometry reaction with the ratio (1:1:1:12) using a high-energy planetary ball mill in an argon atmosphere in different times where the weight ratio of powder to the bullet was 1:20. The produced composite powder was evaluated by the X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). After grinding for 15 minutes, a burning occurred in the mill and regarding the X-ray diffraction, the phases Co-VB-MgO-VO2-Mg3­(BO3)2 were generated. Increasing the time of re-grinding and after burning for 30 minutes, the unwanted phase of Mg3­(BO3)2 was decomposed, the remaining vanadium oxide restored and the final reaction fully occurred. Regarding the results of XRD and the X-ray map analyses, MgO, which was the by-product of this reaction, was removed completely from the produced powder by hydrochloric acid at a concentration of 9. As the TEM image shows the composite powder after the pickling process, the size of particles is around 20- 30 nm.

In this study, delamination growth in mode I loading of unidirectional laminated composites with R-curve effect was investigated. To this end, a finite element simulation of delamination growth in double cantilever beam (DCB) performed using the cohesive zone model (CZM) based on traction-separation laws. By increasing the delamination length in DCB specimens, the strain energy release rate increases and for this reason the simple traction-separation law such as bilinear are not capable to predict the load-displacement curve of this specimens accurately. To solve this shortcoming, cohesive zone models with multi-linear traction-separation laws were proposed to predict delamination behavior of the DCB specimens numerically. Afterwards, by minimizing the difference between experimental and numerical load-displacement curves using optimization method based on genetic algorithm, cohesive zone parameters are characterized. A comparison of the results obtained by cohesive laws with experimental data show that four-linear cohesive law can predict the maximum bridging stress as well as the experimental load-displacement curve accurately.

Nowadayas, manufacturing of carbon nanotube reinforced aluminium matrix nanocomposites have been studied by many researchers. Different techniques have been used for possessing of Al-CNT nanocomposites. But, the powder metallurgy methods have been more attractive because of the lower temperature and better control of process. In the current research, the flake powder metallurgy route was used as a slurry based method to produce Al2024-CNT nanocomposite with 1.5wt.% CNT as reinforcement. Then, the initial compacted billets of Al2024 alloy and Al2024-CNT composites were produced by cold pressing of powders. They were sintered at 550 oC and the billets were hot extruded to produce rods with 10 mm in diameter. The produced samples were investigated by means of tension, compression, hardness, density measurement, XRD, and Raman tests. The uniform dispersion of CNTs within Al2024 powder was observed using FE-SEM. It was related to the merits of wet synthesis of composite powder. In Al2024-CNT sample, the yield and ultimate strength was increased about 28% in comparison with Al2024-O; It was also increased about 20% in yield strength and 15% in tensile strength compared with Al2024-T6 sample. After optimum sintering process, the relative density, hardness, and compressive strength of Al2024-CNT nanocomposite have been increased through hot extrussion to 95.6%, 90 HB, and 547 MPa, respectively. In addition to Al2Cu and Al2CuMg intermetallics in alloyed samples, Al4C3 carbide phase was proved to be formed after sintering in composite specimen.

Grid composite structures (GCSs) owing to their unique shape which is a network of ribs, have some interesting properties such as low strength to weight ratio, low stiffness to weight ratio, high energy absorption capability and good corrosion resistance. In this study, the effect of the multi-walled carbon nanotubes (MWCNTs) addition at various weight percentages with respect to the matrix (0, 0.1, 0.25 and 0.4) on the flexural behavior of GCSs was experimentally examined. For fabricating of the composites, hand lay-up method was used, where plain E-glass and unidirectional carbon fibers impregnated to the resin mixture were used in the skin and rib parts. Afterwards, 3-point bending test was performed on these specimens and the parameters such as maximum flexural load, flexural stiffness and energy absorption were studied. Experimental results showed that, the best flexural behavior was obtained with the addition of 0.4 wt. % of MWCNTs. In this case, the maximum flexural load, flexural stiffness and energy absorption of the GCSs increased by 24%, 35% and 25%, respectively compared to the specimen without MWCNTs addition. The microstructural investigations of the fracture surfaces using electron microscopy clearly indicated the improvement in the interfacial adhesion between the fibers and epoxy matrix in the case of the nanocomposite specimen. This case plays an important role for improvement in the mechanical properties of the GCSs.

In the present study the free vibration analysis of functionally graded composite rectangular nanoplates in thermal environment is investigated. The nonlocal elasticity theory based on the exponential shear deformation theory has been used to obtain the natural frequencies of the nanoplate. In exponential shear deformation theory an exponential functions are used in terms of thickness coordinate to include the effect of transverse shear deformation and rotary inertia. Nonlocal elasticity theory is employed to investigate effect of small scale on natural frequency of the functionally graded rectangular nanoplate. The temperature is assumed to be constant in the plane of the plate and to vary in the thickness direction only. Material properties are assumed to be temperature dependent, and vary continuously through the thickness according to a power law distribution in terms of the volume fraction of the constituents. The govering equations are derived by implementing Hamilton’s principle. To show the accuracy of the formulations, present result’s in specific cases are compared with available results in literature and good agreement are seen. Finally, the effect of various parameters such as nonlocal parameter, power law indexes, width to length ratio, the thickness to length ratio, and temperature fields on the natural frequencies of rectangular FG nanoplates are presented and discussed in detail.