majid eskandari shahraki

In this paper, free vibration of laminated composite cylindrical shells reinforced with circumferential rings, are investigated with experimental, analytical and finite element methods and natural frequencies are obtained. The analysis is carried out for clamp-free and clamp-clamp boundary conditions and the results are compared with each other. To solve the problem, the equilibrium Equations of motions are written according to the classical shells theory and after simplification, the structural stiffness and mass matrices and the frequency Equation are derived using Galerkin method. The results obtained in this paper, are compared with the results available in the literatures, and the results of experimental and finite element methods and good agreement is observed.

This paper presents the buckling and vibration characteristics of a Mindlin rectangular nanoplate with simply supported boundary conditions and Navier approach. In order to consider the small scale effects, the modified couple stress theory, with one length scale parameter, is used. In modified couple stress theory, strain energy density is a function of strain tensor, curvature tensor, stress tensor and symmetric part of couple stress tensor. The critical buckling load values and vibration frequencies of different modes are separately solved. The governing equations are numerically solved and results are verified with literature. The effect of material length scale, length, width and thickness of the nanoplate on the buckling loads and vibration frequencies are investigated and the results are presented and discussed in details.

This paper attempts to study the vibration characteristics of a simply supported nth order rectangular nanoplate using the modified couple stress theory. The modified couple stress theory, which has only one length scale parameter, has been used to consider the small-scale effects. The basic and auxiliary equations of the nanoplate are obtained after determining the strain energy, kinetic energy, and external work and substituting them into Hamilton’s equation. Then, the vibrations of the simply supported nth order rectangular nanoplate with a thickness of h are investigated by substituting the boundary and force conditions into the governing equations. Navier’s method is used for the solution. The results indicate that the frequencies of the different modes of the nth order nanoplate decrease with an increase in the length-to-thickness ratio of the nanoplate. Furthermore, the frequency is the smallest when the effect of the size parameter is not considered (classical theory), and it increases with an increase in the size effect. In addition, the frequency is smallest for the first mode and increases for the subsequent modes. Also, the vibrational frequency increases with an increase in the order of the nanoplate.

In this research the free vibration analysis of simply supported grid stiffened doubly curved shells by using a refined higher order theory is presented. The advantage of the present theory in comparison with other higher order theories is investigation of the effects of trapezoidal shape factor in the stress resultants in order to obtain more accurate frequency results. The governing equations of motion and boundary conditions are obtained using Hamilton’s principle and solved by using the Galerkin method. In the case of grid stiffened shells, a distribution function is introduced for describing the physical discontinuity between the ribs and the bays. The results are validated by making comparison to those existed in the literature or those obtained using the present numerical simulation in ABAQUS/Standard solver. In most cases, validations illustrated excellent agreement between the results . Finally, the effects of geometrical properties, material property and layup on the frequency responses of the shell are discussed.

Microgravity and cosmic radiation are the space environmental stresses which can cause DNA damage in living organisms. Radiations injurie the cell DNA directly through the interaction of charged particles with DNA molecules or indirectly by the production of free radicals. In addition, radiation can alter cell wall composition, activate free radical scavenging enzymes, and accumulate antioxidant compounds. Although plants have evolved some mechanisms to deal with the damages, space conditions, especially microgravity can play a role in repairing DNA damage. More DNA damages can induce double strands breaks of DNA, chromosome abnormality, micro-nuclei formation, and increase the risk of cell death. In this study, effect of space environmental stresses on DNA damage and response mechanisms will be investigated in space flight or simulated conditions.

In this paper bending and buckling characteristics of third-order shear, and deformation nanoplates were investigated using the modified couple stress theory and Navier type solution. It can be useful for designing and manufacturing micro-electromechanical and nano-electromechanical systems. The modified couple stress theory was applied to provide the possibility of considering the effects of small scales that have only one material length scale parameter. In this theory, the strain energy density is a function of the strain tensor components, curvature tensor, stress tensor, and the symmetric part of the couple stress tensor. After obtaining the strain energy, external work, and buckling equations, the Hamilton principle is employed to derive the governing equations. Furthermore, by applying boundary and loading conditions in the governing equations, the bending and buckling of a third-order shear deformation nanoplate with simply-supported bearings are obtained and the Navier’s solution is used to solve the equations. The results indicate that the third-order nanoplate subjected to sinusoidal loading yields smaller values of dimensionless bending than it does while subjected to uniform surface traction. It was also found that by increasing the length to thickness ratio, the value of the dimensionless bending of nanoplate decreases but by increasing the aspect ratio of the plate, this value increases. Furthermore, it was shown that the critical buckling load of the third-order nanoplate under uniaxial loading increases by increasing the ratio of the length scale parameter to the thickness of the nanoplate but it decreases by increasing the length to thickness ratio of the nanoplate.

In this paper a third-order rectangular nanoplate model is developed for the bending and vibration analysis of a graphene nanoplate based on a modified couple stress theory. The bending rates and dimensionless bending values under uniform surface traction and sinusoidal load, and the frequencies of the nanoplate are all obtained for various plate's dimensional ratios and material length scale to thickness ratios. The governing equations are numerically solved. The effect of material length scale, length, width and thickness of the nanoplate on the bending and vibration ratios are investigated and the results are presented and discussed in details.

In aerospace structures, due to the fluctuations of excitation sources such as the engine propulsion, there is a possibility of dynamic instability, which is a destructive phenomenon. In this paper, the dynamic stability of composite grid-stiffened cylindrical shells under a combinational loading of static and fluctuating forces has been investigated using the Dunnelly theory for thin-walled shells. Using the equivalent stiffness method, the stiffness of composite grid structures has also been calculated by the method of reinforcements impregnation. The development of a normal mode for motion equations leads to the system of Matthew-Hill equations. The Boltin method is used to determine the instability regions to solve the Matthew-Hill equations. The effect of iso-grid mesh reinforcement parameters such as the rib angle, circumferential and annular rib spacing and the rib cross section as well as the effect of cylindrical shell length to radius and thickness to radius ratios have been tested and compared. The validation of the natural frequency and dynamic stability results has been done by comparison with Abaqus software and other researchers' articles. The results show that by decreasing the angle of the helical ribs, in the shells of composite grid cylinders, the main instability frequency increases and the amplitude of the instability region decreases.

One of the leading challenges in the fabrication of natural fiber composites is the adhesion of the fibers to the matrix. Since improving the adhesion between the fibers and the matrix changes the mechanical properties of natural composites, fiber surface modification has been considered. In this study, the effect of adhesion and surface treatment of date fibers on improving mechanical properties (tension, bending, impact) was investigated. Surface treatment was performed on date palm fibers using two chemicals, sodium chloride, and sodium hydroxide. These fibers were modified in four groups with sodium hydroxide, sodium chloride, sodium chloride, then sodium hydroxide, and finally sodium hydroxide, then sodium chloride. By making different samples and performing standard mechanical tests (tensile, impact bending) and electron microscope (SEM) tests, thermal analysis (TGA), fiber tension, fiber extrusion from the matrix (Pull out) the effects of different groups on improving the adhesion and mechanical properties of composites was examined and verified. The results showed that the surface treatment fiber with sodium chloride showed more resistance against stretching, extrusion of the fiber from the matrix, and degradation due to temperature increase compared to the fiber without surface treatment and with surface treatment. Also, mechanical tests on composites showed an increase and improvement in tensile strength of samples reinforced with sodium hydroxide-modified fibers, and the highest flexural strength with sodium-chloride-modified fibers and a decrease in impact strength with surface treatment fibers in all four groups. ‌‌

In this paper, a Mindlin rectangular nanoplate model is developed for the bending and vibration analysis of a graphene nanoplate based on a modified couple stress theory. In order to consider the small scale effects, the modified couple stress theory, with one length scale parameter, is used. In modified couple stress theory, strain energy density is a function of strain tensor, curvature tensor, stress tensor and symmetric part of couple stress tensor. After obtaining the strain and kinetic energy, external work and substituting them in the Hamilton’s principle, the main and auxiliary equations of the nanoplate are obtained. Then, by manipulating the boundary conditions the governing equations are solved using Navier approach for bending and vibration of the nanoplate. The bending rates and dimensionless bending values under uniform surface traction and sinusoidal load and different mode frequencies are all obtained for various plate's dimensional ratios and material length scale to thickness ratios. The effect of material length scale, length, width and thickness of the nanoplate on the bending and vibration ratios are investigated and the results are presented and discussed in details.

In this paper a Nth order nanoplate model is developed for the bending and buckling analysis of a graphene nanoplate based on a modified couple stress theory. The strain energy, external work and buckling equations are solved. Also using Hamilton’ principle, main and auxiliary equations of nano plate are obtained. The bending rates and dimensionless bending values under uniform surface traction and sinusoidal load, the dimensionless critical force under a uniaxial surface force in x direction are all obtained for various plate's dimensional ratios and material length scale to thickness ratios. The governing equations are numerically solved. The effect of material length scale, length, width and thickness of the nanoplate on the bending and buckling ratios are investigated and the results are presented and discussed in details.

In this study, the buckling analysis of multilayer composite closed cylindrical shells, as well as composite grid cylindrical shells, was investigated using a high-order theory modified from Reddy’s third-order shear theory under simply supported conditions. The advantage of the present theory compared to other high-order theories is the calculation of the effect of the term of the shell section trapezoidal coefficient on relations related to displacement and strain fields, which improves the accuracy of the results. In grid shells, the discontinuous distribution of stiffness and mass of the shell between the reinforcing ribs and the distance between them is expressed by a suitable distribution function. In the case of integrated and grid cylindrical shells, the validation of the results was performed in comparison with other studies, as well as with the results of the numerical solution obtained using Abaqus software. It is shown that for the first buckling mode, the critical load first increases and then reaches a constant value and for the second buckling mode, the critical load first decreases and then reaches a constant value. Also, in the case of grid shells, by increasing the ratio of the cavity dimension to the dimensions of the whole shell, whether in single-cavity or multi-cavity mode, the present theory and finite element solution find more difference, indicating the higher accuracy of the present theory for integrated shells.

In this paper, a solution procedure is presented for free vibration of combined cylindrical-conical composite shells including the shear deformation effect of the shell. The solution presented in this study is obtained directly from the governing equations for five displacement components according to Hamilton’s principle. This solution is in the form of a power series in terms of a particularly convenient coordinate system. In this study, the effects of geometry and material parameters on the natural frequencies are investigated. Also, to illustrate the validity of the present solution procedure, analytical results are verified with many studies and compared with those of the present numerical ABAQUS analysis. The outcomes showed a good agreement between the obtained results. The novelty of the present study is incorporating the transverse shear deformation in calculating the natural frequencies of the joined cylindrical-conical shells. In previous literature, this topic has not been studied in such a wide scope.

This study was designed to investigate the ballistic behavior of ceramic-reinforced aluminum composite plates numerically and experimentally and to present an optimal sample design. The parameters studied were ceramic reinforcement percentage and type of matrix alloy. This study used the matrix alloys 6061, 7075, and 5083. The percentage of ceramics used in this study is 15, 30, and 45% by weight. The samples are in three thicknesses of 20, 25, and 30 mm. 27 simulated samples were numerically analyzed with Abaqus finite element software in this study based on existing ballistic protection criteria, one then determines the optimal numerical sample. Using the squeeze casting method, a laboratory sample has been made and experimentally tested to evaluate the numerical results. Lastly, the numerical analysis and the experimental test were compared and the optimal sample was determined.

The paper studied the analysis of vibrations of rectangular carbon nanotube-reinforced composite plates. To this end, a three-layer nanocomposite plate - two layers with the targeted distribution of carbon nanotubes as FG-X at the top and bottom and a layer without an amplifier in the middle of the plate - were analyzed. The governing equations for this problem are based on First-order Shear Deformation Theory (FSDT). The distribution of nanotubes on these plates is as targeted FG-X. The effect of various types of SWCNTs distributions in the direction of thickness on the vibrational behavior of nanocomposite plates was examined. The effective properties of nanocomposite materials Functionally Graded Carbon Nanotube-Reinforced Composite (FG-CNTRC) were estimated using the rule of mixtures. Detailed parametric studies were performed to determine the effects of the volume fraction of carbon nanotubes and the thickness-to-length ratio of the plate on the natural frequency responses and the shape of the plate mode. The equations obtained in this problem were coded in MATLAB software, the nanocomposite plate was modelled in ABAQUS software, and the comparison of the results obtained from the numerical solution with ABAQUS software showed relatively right consistency with the results obtained from the analytical solution.

In this paper, the quasi-static test and the damage of the thin-walled composite cylinder were numerically simulated using ABAQUS. Then, a comparison was made between the results of this simulation and those obtained from experimental studies followed by their validation. In the next step, several parameters affecting the energy absorption rate including outer diameter-to-cylinder height ratio, thickness-to-outer diameter ratio, and angle of damage initiation mechanism were selected. They were optimized by modelling different states in ABAQUS. The number of tests is reduced by the design of experiments using response surface methodology and the optimal specimen is extracted by this software. Finally, optimum adsorbent is fabricated and tested. Considering enhanced energy absorption, increased mean reaction force, and reduced initial maximum force, the optimal design parameters include the inner diameter-to-cylinder height ratio of 0.2, thickness-to-inner diameter ratio of 0.1, and angle of damage initiation mechanism of 45°.

The main part of the eye is the retina covering the entire back section of the eye. Eye disease is one of the most important cause of disability and even death in developed countries as well as in developing countries. Disorders created in the retina that occur due to special diseases can be detected by specific retinal images. Studying the variations in retinal photos in a special time could help physicians to diagnose the associated diseases. In this paper, the detection of blood veins in retina photos was investigated. For this purpose, first a new method is proposed to promote the quality of retina photos by combining the histogram adjustment and gray level grouping. We use the feature vector to classify the pixels. Next, a method for classifying the images based on the feature extraction vector is required. The use of neural networks is one of the best and most widely used methods of machine learning for classification. We used a 3-layer Perceptron to classify pixels.