فصلنامه مکانیک هوافضا
سال چهاردهم شماره 1 (پیاپی 51، بهار 1397)

Carbon / epoxy composite is one of the most useful polymer matrix composites that has special properties such as high strength-to-weight ratio, high hardness, high corrosion resistance, resistance to nuclear radiation and etc. has high consumption in different industries such as aerospace industry. Therefor loading monitoring of this type of composite is important. In order to determine various failure mechanisms, acoustic emission method has more performance than other non-destructive methods. In this research acoustic emission method was used to study carbon/epoxy composite and evaluate frequency range of flexural loading. For this purpose bending behavior of composite and relation between acoustic signals had studied. Using both fast Fourier transform and wavelet transform analysis methods in this research, led to the same result with margin of 5%. By using FFT analysis, maximum frequency of 135 KHZ was determined while using wavelet transform, this amount led to 142 KHz. Time limits that events was occurred on the specimen, monitored by online diagrams that obtained from acoustical system. Energy distribution at failure mechanisms was obtained as 17%, 29% and 48% related to matrix fracture, debonding and fiber breakage respectively. Finally failure mechanisms of composite were confirmed by SEM images. Energy distribution amounts, time limits and ascending progress of diagrams validate bending diagram.

The eccentrically stiffened cylindrical shells are one of the most important structures in aerospace industries. In this paper, semi-analytical method for eccentrically stiffened functionally graded (FG) cylindrical shells under external pressure and surrounded by an elastic medium is presented. The proposed model is based on Winkler and Pasternak elastic foundation parameters. According to the Von Karman nonlinear equations and the classical plate theory (CPT) of shells, strain displacement relations are obtained. The smeared stiffeners technique and Galerkin method, used for solving nonlinear dynamic problem. With considering three terms approximation for the deflection shape, the frequency-amplitude relation for non-linear vibrations obtained. The nonlinear dynamic response is obtained from fourth order Runge-Kutta method. The nonlinear dynamic buckling behavior of stiffened FGM shells is investigated based on the Budiansky-Roth criterion. The effect of parameters such as eccentrically stiffened, elastic foundation and excitation force on the frequency-amplitude curve of the nonlinear vibrations and parameters such as damping and loading speed on the nonlinear dynamic response of the FGM cylindrical shells have been investigated. The natural frequency, static and dynamic buckling load are analyzed, too.

Composite conical shells have extensive industrial application and stability analysis is necessary for them. In this article, stability of composite conical shells subjected to dynamic external pressure is investigated by numerical and experimental methods. In experimental tests, cross-ply glass woven fabrics were selected for manufacturing of specimens. Hand-layup method was employed for fabricating the glass-epoxy composite shells. A test-setup that includes pressure vessel and data acquisition system was designed. For detecting the buckling load, pressure history of vessel during loading was used. Because of suddenly changing in volume of pressure vessel in instability moment, a disruption in pressure history can be observable. Also, numerical analyses are performed in Abaqus software. For doing this at a distinct loading time, we load composite cone with dynamic external loading and then draw displacement of a specific node versus time. And then with continuous changing of loads and drawing it versus time, the load on which variation of displacement become significant has been considered as dynamic buckling load.
In both of experiments and numerical studies, increasing of shell’s stability threshold in dynamic loading is recognizable. Finally, results are compared together while a good correlation is observed.

Polymeric composite materials are widely used in aerospace, marine, automobile and other industries. These materials are often subjected to different defects and damages from in-service and manufacturing conditions. Interlaminar fracture or delamination is the most important of these defects. In this paper numerical and experimental study of the interlaminar mixed-mode fracture behavior of woven glass epoxy composite is performed using recently modified Arcan fixture. Mixed-mode fracture tests from pure mode I to pure mode II were performed by varying the loading angle, α from 0o to 90o. Finite-element analyses were done by ABAQUS and mode-I and mode-II non-dimensional stress intensity factors, fI (a/w) and fII (a/w) respectively, were obtained for various a/w ratios and different loading angles (mixed-mode). As the result, it can be seen that the shearing mode interlaminar fracture toughness is larger than the opening mode. This means that interlaminar cracked specimen is tougher in shear loading condition and weaker in tensile.

Carbon nanotube-reinforced polymer nanocomposites have attracted great attention from research centers, due to their enhanced mechanical properties, and many studies have been performed for development of such materials. Many experimental and theoretical studies have investigated the effect of different parameters on the properties of these materials. However, there are some limitations associated with experimental methods, such as fabrication problems and high levels of costs. Hence, molecular simulations are growingly applied for study of properties and behavior of polymer/CNT nanocomposites. In this study, the molecular dynamics method was used to calculate the mechanical properties of CNT-reinforced epoxy nanocomposites. Since epoxy is a two-component thermoset polymer, the molecular dynamics method was utilized to create cross links between the monomers. Thereafter, cross-linked epoxy polymer and carbon nanotubes were used to construct the nanocomposite with containing 1-5 wt.% of CNTs. Finally, elastic constants of nanocomposite including Young’s and shear moduli were calculated using the constant-strain method. The results of simulations revealed that the mechanical properties of CNT-reinforced epoxy polymer were improved in comparison to those of pure polymer.

In this study, Nonlinear low-velocity impact response of carbon fiber reinforced polymer(CFRP) composite plates enhanced with carbon nanotubes resting on elastic foundations in thermal environments is investigated. The effective material properties of the multi phase nanocomposite are calculated using Halpin–Tsai equations and fiber micromechanics in hierarchy. The carbon nanotubes are assumed to be uniformly distributed and randomly oriented through the epoxy resin matrix. Contact force between the impactor and the plate is obtained with the aid of the modified nonlinear Hertzian contact law models. The governing equations are derived based on principle of virtual work and solved by the finite element method with Newmark’s numerical integration method. Numerical results reveal that a small amount of CNT (1–2 percent) can increases the peak contact and decreases the peak indentation. Also the contact time duration and central deflection have decreased with increasing the CNT percentage.

In the present paper, responses of a multilayer viscoelastic composite plate against a low-velocity impact by a rigid spherical indenter is investigated. In this regard, a novel energy formulation that is suitable for impact analysis and accounts for the potential energy due to indentation is proposed and employed, for the first time. First, Hertz contact law is refined to include effect of the lower layers on the stiffness of the contact region. Voltra hierarchical integral is employed for modeling the viscoelastic material and a layerwise theory capable of considering the transverse flexibility of the layers is used to accurately model the plate behavior. To solve the governing integro-differential equations, the finite element method, trapezoidal integration method, and Newmark numerical time integration method are used. In the results section, effects of the various viscoelasticity parameters and the indenter velocity on the time histories of the contact force, indentation, and lateral deflection of the plate are investigated. Results show that due to the damping nature of the viscoelastic materials, the plate rigidity and contact force increase whereas the maximum lateral deflection and the indentation decrease. Furthermore, higher contact forces do not necessarily indicates higher indentations.

In this study, nonlinear bending analysis of functionally graded carbon nanotube reinforced composite (FG-CNTRC) cylindrical panels subjected to a uniform transverse mechanical load and thermal gradient along the radial direction is investigated. The equilibrium equations are derived based on first-order shear deformation shell theory (FSDT) and nonlinear von karman strains. Four types of uniform and functionally graded distributions of the reinforcement along the thickness direction of panels are considered. The nonlinear coupled equations of motion are solved by combination of dynamic relaxation (DR) and finite difference methods for different combinations of simply supported and clamped boundary conditions. In order to verify the current work, some obtained results are compared with the solutions reported in the literature and also ABAQUS finite element packages. In the presented parametric study, the effects of distribution of carbon nanotubes (CNTs), thickness-to-radius and length-to-radius ratios, boundary conditions, volume fraction of CNTs and panel angel is considered on the deflection and stress resultants in detail. The results show that FG-O and FG-X distributions of CNTs have the maximum and minimum values of deflection, respectively, for both simply supported and clamped boundary conditions.