Mechanics of Advanced Composite Structures
Volume:7 Issue: 2, Summer Autumn 2020

In this study, the frequency response of rectangular sandwich plates with multi-layer face sheets and electrorheological (ER) fluid cores is investigated. The assumed electro-rheological fluid as a core is capable of changing the stiffness and damping of structures. In modelling the sandwich panel implemented for the first time, first-order shear deformation theory and the second Frostig's model are applied for the face sheets and thick cores, respectively. The sandwich panel under study is supposed to simply support boundary in all edges, and the Galerkin approach is implemented for discretizing the problem. In the result section, impacts of various parameters such as electric field, aspect ratio, the thickness of the ER layer, and thickness ratio on vibrational characteristics of the structure are discussed in detail.  The obtained results highlight the notable effects of the electric field on natural frequencies, which can make the structure flexible within the desired range. It is also pointed out that the dynamic behavior and stability of the system can be controlled by changing the magnitude of the ER fluid layer.

This paper presents a numerical solution and optimization for a functionally graded material cylinder with an elliptic hole subjected to mechanical pressure. To obtain the governing equations, an elliptic cylindrical coordinate was used. The material properties were considered in a way in order to vary with power-law function along the elliptic cylindrical direction. The differential quadrature method was used for solving the equations. In addition, by using von-Misses stress along the thickness, the optimal values for various material inhomogeneity and the geometry of the cylinder investigated. The results showed that the inconsistency in shape of the hole in the cylindrical vessel can affect the expected results and the stresses in thickness of cylinder were changed. Furthermore, it was shown that with low values of the functionally graded material index, the geometry of the cylinder had a more significant effect on von Mises stress. Additionally, with high values for the material index, the values for von Misses stress converged together and the material inhomogeneity had a less noticeable effect on stress. The results also showed that for various geometries of the cylinder and holes, the best value for material homogeneity to reach the optimum value for von Misses stress was changed. The presented results were consistent with those reported in previous publications.

In this study, an efficient finite element model with two degrees of freedom per node is developed for buckling analysis of axially functionally graded (AFG) tapered Timoshenko beams resting on Winkler elastic foundation. For this, the shape functions are exactly acquired through solving the system of equilibrium equations of the Timoshenko beam employing the power series expansions of displacement components. The element stiffness matrix is then formulated by applying the developed shape functions to the total potential energy along the element axis. It is demonstrated that the resulting shape functions, in comparison with Hermitian cubic interpolation functions, are proportional to the mechanical features of the beam element, including the geometrical properties, material characteristics, as well as the critical axial load. An exhaustive numerical example is implemented to clarify the efficiency and simplicity of the proposed mathematical methodology. Furthermore, the effects of end conditions, material gradient, Winkler parameter, tapering ratio, and aspect ratio on the critical buckling load of AFG tapered Timoshenko beam are studied in detail. The numerical outcomes reveal that the elastic foundation enhances the stability characteristics of axially non-homogeneous and homogeneous beams with constant or variable cross-section. Moreover, the results show that the influence of non-uniformity in the cross-section and axially inhomogeneity in material characteristics play significant roles in the linear stability behavior of Timoshenko beams subjected to different boundary conditions.

This work investigates the sensitivity analysis of vibrating laminated composite rectangular plates in interaction with inviscid fluid using the modified higher-order shear deformation plate theory. The EFAST method which is based on variance and is independent of any assumption of linearity and uniformity between inputs and outputs is utilized for sensitivity analysis of laminated composite rectangular plates. Theoretical formulations, both for the laminated rectangular plates in interaction with inviscid, incompressible and irrotational fluid and the sensitivity analysis technique are summarized here. A Cartesian coordinate system is used to describe governing equations of fluid-structure interaction. Hamilton's variational principle is used to derive the Eigen problem of the complex system. A numerical investigation is carried out by using the Galerkin method and the boundary conditions of the plate are simply supported. A set of admissible displacement functions which satisfy identically the geometric boundary conditions are used to calculate the wet natural frequencies of the plate. In the numerical examples, the effect of the aspect ratio, thickness ratio and material orthotropy orientation of the plate, depth ratio and width of the fluid on the fundamental natural frequency of the vibrating laminated composite rectangular plates are examined and discussed.

In this paper, the effects of filler type, filler content, functionalization, and the use of hybrid nanofillers on nanocomposite mechanical properties are investigated. For this purpose, several nanocomposite types were modeled and analyzed using Molecular Dynamics method. In the molecular dynamic’s simulations, crosslinking and nanofiller/matrix interface effects were considered. First thermoset epoxy resin with 75% crosslinking ratio between DGEBA resin and DETA hardener were simulated to determine pure resin properties. Then nanocomposites consisting of single walled carbon nanotubes (SWCNT), nanographene (NG), carbon nanoparticle (CNP), functional single walled carbon nanotubes (SWCNT-COOH), and functional nanographene (nanographene oxide) in thermoset epoxy were modeled and analyzed using Materials Studio software. In addition, filler weight fraction was increased from 2.5 to 10 percent in order to investigate the effects of filler content on nanocomposite mechanical properties. The results indicated that increasing nanofiller weight fraction from 0 to 7.5% resulted in an increase in nanocomposite elastic modulus for three non-functional nanofiller types. Moreover, functionalization improving nanocomposite properties as the highest increase in resin elastic modulus were obtained for the SWCNT-COOH reinforced epoxy for filler contents up to 7.5 weight percent. Also, agglomeration occurred at filler contents higher than 7.5 weight percent in the NG/epoxy, SWCNT/epoxy nanocomposites. Finally, the use of hybrid nanofillers reduced/prevented agglomeration for filler contents even up to 10 weight percent.

Damage detection in Concrete-Filled Tubes (CFSTs) and the study of their application in special structures such as high-rise buildings, towers and bridges are important issues. CFST columns are widely studied by researchers and engineers due to the simultaneous utilization of both their steel and concrete properties. Hence, any damage to this structural element may result in extremely severe and irreversible injury. In this study, we researched the factors that may be involved in such damage. In addition, identification of a particular type of damage that may be due to the buckling of steel tube plates was performed in this study as well. Since, the buckled part in the column is eliminated from the system or decreases its bearing capacity significantly, in this study, in order to simulate the damage, a part of the CFST column steel wall was cut off and eliminated. And, since buckling is more likely to occur in the middle of the column, damage has been located in the middle of the column. After preparing the specimen and performing the tests, the modal data was extracted and was entered in the MATLAB software to be analyzed with the aid of the wavelet transform tool. The results showed that the frequency was reduced and the mode shape of the specimens did not match entirely before and after the damage with the Modal Assurance Criteria (MAC), which indicated damage in a specimen. To identify the location of the damage, the mode shape obtained from the experimental modal was given to the wavelet transform as the input signal and Daubechies (Db) wavelet was applied to correctly identify the location of the damage.

Bones are natural composites, which are consisted of mineral fibers, which strengthen the organic matrix.  Such composites are exposed to both monotonic and cyclic loadings, which could predispose the structure to failure. One such failure mechanism could be the fatigue phenomenon. In this article, the scatter-band and the reliability response of bovine tibia bones were predicted in the load-controlled fatigue condition. The one-point rotary-bending fatigue machine was utilized to carry out standard tests at two different loading levels, 0.4 and 0.6 kg for three various loading frequencies, 10, 20 and 30 Hz for tibia bones. For scatter-band predictions, three confidence levels (85, 90, and 95%) were selected. Addiotionally, lower/upper bands were drawn for a selected target function, including the ratio of logarithmic fatigue lifetimes to stress levels. For the reliability prediction, three different distribution functions were considered. Results showed that, by decreasing the confidence level, the scatter-band would be narrower. Besides, the Probability of failure generally increased at 0.6 kg of the loading level, when the loading frequency increased.

Generally, in-served cylindrical shells buckling usually takes place not merely under one of the basic loads, i.e., axial compression, lateral pressure, and torsion, but under a combination of them. The buckling behavior of fiber-metal laminate (FML) cylindrical shells under combined axial and torsional loading is studied in this paper. The Kirchhoff Love-type assumption is employed to study the axial buckling load. Then, an extended finite element (FE) model is presented and results are compared. A number of consequential parameters such as lay-up arrangement, metal type and metal volume fraction are employed and enhancement of buckling behavior of the shell is also studied. Finally, the interaction of axial /torsional loading is analyzed and discussed. The results show that as the metal volume fraction rises to 15%, the endurable buckling load increases almost 43% more than the state in which there is no metal layer. The numerical results show that increasing the metal volume percentage leads to a decrease in buckling performance of the structure under axial loading.

This paper presents a finite element method (FEM) for linear and geometrically nonlinear behaviours of cross ply square laminated composite plates (LCPs) subjected to a uniform distributed load (UDL) with simply supported boundary conditions (SS-BCs). The original MATLAB codes were written to achieve a finite element (FE) solution for bending of the plate. In geometrically nonlinear analysis, changes in geometry take place when large deflection exists to consequently provide nonlinear changes in the material stiffness and affect the constitutive and equilibrium equations. The Von Karman form nonlinear strain displacement relations and a new inverse trigonometric shear deformation hypothesis were used for deriving the FE model. Here, in-plane displacements made  use of an inverse trigonometric shape function to account for the effect of transverse shear deformation. This hypothesis fulfilled the traction free BCs and disrupted the necessity of the shear correction factor (SCF). Overall the plate was discretized using the eight-node isoparametric serendipity element. The equilibriums governing equations associated boundary conditions were obtained by using the principle of virtual work (PVW). The numerical results were obtained for central deflections, in-plane stresses and transverse shear stresses for different stacking sequences of cross ply laminates. The results were also computed by the FE software ANSYS for limited cases. The results obtained showed an acceptable agreement with the results previously published. The findings suggested the future use of a new FE model for linear and nonlinear laminated composite plate deformation.

Recently, strengthening of steel sections using carbon fiber reinforced polymer (CFRP) has come to the attention of many researchers. For various reasons, such structures may be placed under combined loads. The deficiency in steel members may be due to errors caused by construction, fatigue cracking, and other reasons. This study investigated the behavior of deficient square hollow section (SHS) steel members strengthened by CFRP sheets under two types of the combined loads. To study the effect of CFRP strengthening on the structural behavior of the deficient steel members, 17 specimens, 12 of which were strengthened using CFRP sheets, were analyzed. To analyze the steel members, three dimensional (3D) modeling and nonlinear static analysis methods were applied, using ANSYS software. The results showed that CFRP strengthening had an impact on raising the ultimate capacity of deficient steel members and could recover the strength lost due to deficiency, and the impact of CFRP strengthening on rising and recovering the ultimate capacity of the steel members under loading scenario 2 was more than the steel members under scenario 1.

In this study, using the dynamic relaxation method, nonlinear mechanical and thermal buckling behaviors of functionally graded cylindrical shells were studied based on first-order shear deformation theory (FSDT). It was assumed that material properties of the constituent components of the FG shell vary continuously along the thickness direction based on simple power-law and Mori-Tanaka distribution methods separately. An axial compressive load and thermal gradient were applied to the shell incrementally so that in each load step the incremental form of governing equations were solved by the DR method combined with the finite difference (FD) discretization method to obtain the critical buckling load. After convergence of the code in the first increment, the latter load step was added to the former so that the program could be repeated again. Afterwards, the critical buckling load was achieved from the mechanical/ thermal load-displacement curves. In order to validate the present method, the results were compared with other papers and the Abaqus finite element results. Finally, the effects of different boundary conditions, grading index, rule of mixture, radius-to-thickness ratio and length-to-radius ratio were investigated on the mechanical and thermal buckling loads.

In the present research, an experimental study was carried out to assess the vibrational behavior of Acrylonitrile-Butadiene-Styrene (ABS) based Nano composites reinforced by Nano-silica particles. Therefore, the twin extruder methodology was used to fabricate the Nano composite samples.   The silica content and extrusion temperature were considered as variable parameters. The samples were prepared based on bending test standards and then subjected to dynamic mechanical and thermal analysis machines. To identify the effect of SiO2 content and presence of defects in the fabricated samples, 12 experiments were carried out and the obtained results analyzed based on scanning electron microscopy (SEM) images of the samples’ cross section and the graphs, which were obtained from the aforementioned tests. As a result, it was found from the results that by increasing the silica content up to 2%, the static and dynamic strength of the fabricated Nano-composite were significantly enhanced. However, by a further increase of silica content, it was found that the fabricated samples showed brittle behavior causing reduction of strength properties. On the other hand, for defected samples, the static and dynamic forces of the fabricated composite reached a maximum at 3% and 4% of Nano-silica content, respectively. It was also found from the results that the increase of silica content caused a reduction in the damping behavior of fabricated composites for both the perfect and defected samples. This trend could be attributed to the fact that an increase of silica content increased the storage modulus in common surfaces between polymeric layers and the reinforcement material.

In this research, the nonlinear dynamics of an electrostatically actuated non-uniform microbeam equipped with a damping film and a piezoelectric layer have been studied. The nonlinear behaviour of the system was modelled using the von Karman geometrical strain terms. In addition, the strain gradient theory was utilized and the Hamilton principle was applied to obtain equations of motion and boundary conditions, respectively. The obtained equations were reduced using the Galerkin method, and the reduced equations were solved with the multiple scale method. The size-dependent responses were then investigated for primary, super-harmonic, and sub-harmonic resonances. The influence of beam width, beam thickness, and distance between electrodes on the resonant frequency response was studied along with nonlinearity of the system. The results showed that the static and forced vibration behaviours of microbeams strongly depended on the size of the electrodes.

These days, different types of carbon nano-fillers are used widely as a reinforcement agent in polymer composites like fullerenes, carbon nanotubes, graphene nano-platelets, and graphite platelets. Moreover, graphene-based materials and their composites have shown promising characteristics for a wide variety of applications in nano-science and nano technology.  Adding graphene as a reinforcing agent in a polymer matrix has improved the overall performance and properties of these substances. In this review, the general properties of the nano-particle in polymers have been studied. Also, the effect of these nano particles on the mechanical, thermal, and electrical prope rties of polymer composites has been investigated. It was demonstrated that filling graphene platelets in polymer materials improves their mechanical, thermal, and electrical properties.

In this paper, the dynamic response of a simply - supported relatively thick composite sandwich curved beam under a moving mass is investigated. In contrast to previous works, the geometry of beam is considered to be in a curved form. Moreover, the rotary inertia and the transverse shear deformation are also considered in the present analysis. The governing equations of the problem are derived using Hamilton's principle. Then, the obtained partial differential equations are transformed to the ordinary differential equations with time varying coefficients, using the modal analysis method. Fourth-order Runge-Kutta method is applied to solve the ordinary differential equations in an analytical – numerical form. The obtained results are validated by the results existed in the literature. Performing a thorough parametric study, the effects of some important parameters such as the mass and the velocity of moving mass, the radius of curvature of the beam, the core thickness to the total thickness ratio and the stacking sequences of the face sheets on the dynamic response are investigated. It is observed that increasing the mass and the velocity of moving mass and the radius of curvature of beam, result in an increase, decrease and increase of the dynamic deflection of curved beam, respectively.

In this study, active control of free and forced vibration of rotating thin laminated composite cylindrical shells embedded with two magnetostrictive layers is investigated by means of classical shell theory. The shell is subjected to harmonic load which is exerted to inner surface of the shell in thickness direction.  The velocity feedback control method is used in order to obtain the control law. The vibration equations of the rotating cylindrical shell are extracted by means of Hamilton principle while the effects of initial hoop tension, centrifugal and Coriolis accelerations are considered in the vibration equations. The differential equations of the rotating cylindrical shell are converted to ordinary differential equations by means of modified Galerkin method.  The displacement of the shell is obtained using modal analysis. The free vibration results of this study are validated by comparison with the results of open literature. Also, the validity of the forced vibration results is proved by comparison with the fourth order Runge-Kutta method's result. Finally, the effects of several parameters including circumferential wave number, rotational velocity, the whole orthotropic layers thickness, magnetostrictive layers thickness, length, the amplitude and exciting frequency of the load on the vibration characteristics of the rotating cylindrical shell are investigated.