Abstract in English:Finite element mesh of multi-directional 4D carbon/carbon (C/C) composite was reconstructed from 2D images obtained by X-ray tomography. Thus, imperfections in the composite such as voids, misalignment and cross-section distortion of the fibre bundles were directly incorporated in the finite element mesh. 2D images of the composite were also used for the characterization of the porosity in the composite. The effect of these micro structural imperfections was studies by assuming perfect bonding at the bundle/matrix interface. The initial mechanical properties of the composite were obtained from unit cell analysis using asymptotic homogenization and moduli in x, y and z directions were 39, 25 and 44 GPa. However, matrix and bundle/matrix interfacial cracks were also clearly visible in the X-ray tomographic images. Later, the effects of debonding was incorporated by using frictional cohesive interaction at bundle/matrix interfaces and matrix cracking was modeled by degrading the elastic properties of matrix. Final, the response of the composite was studied under six individual load cases.
Abstract in English:Using the general purpose finite element package Abaqus, an investigation has been carried out to examine the dynamic response of steel stiffened plates subjected to uniform blast loading. The main objective of this study is to determine the dynamic response of the stiffened plates considering the effect of stiffener configurations. Several parameters, such as boundary conditions, mesh dependency and strain rate, have been considered in this study. Special emphasis is focused on the evaluation of midpoint displacements and energy of models. The modeling techniques were described in details. The numerical results provide better insight into the effect of stiffener configurations on the nonlinear dynamic response of the stiffened plates subjected to uniform blast loading.
Abstract in English:Structural foams are a relatively new class of materials with peculiar characteristics that make them very attractive in some energy absorption applications. They are currently used for packaging to protect goods from damage during transportation in the case of accidental impacts. Structural foams, in fact, have sufficient mechanical strength even with reduced weight: the balance between the two antagonist requirements demonstrates that these materials are profitable. Structural foams are generally made of microcellular materials, obtained by polymers where voids at the microscopic level are created. Although the processing technologies and some of the material properties, including mechanical, are well known, very little is established for what concerns dynamic impact properties, for the design of energy absorbing components made of microcellular foams. The paper reports a number of experimental results, in different loading conditions and loading speed, which will be a basis for the structural modeling.
Abstract in English:This work presents a new metamodel for reinforced panels under compressive loads, typically used in light-weight aircraft structures. The metamodel represents a replicable cell structure of integrally machined panels. The presented formulation for conception is based on the synthesis of four stability criteria: section crippling, web buckling, flange buckling and column collapse. The aluminum alloy, a typical choice in modern aircraft industry, is selected and the structure is expected to work in the linear elastic domain. In order to evaluate the accuracy and to validate the analytical tool, the procedure is applied in the pre-sizing of the fuselage basic structural components of a 9-passenger executive aircraft. The pull-up maneuver, one of the critical load conditions in most of aircrafts, causes the maximum compressive stresses in lower fuselage panels. Finite element models are presented to the resulting fuselage configuration. The optimal configuration achieved through the application of the analytical tool yields to an innovative structure from those usually adopted in the aeronautical industry. This structural configuration is presented and discussed. The developed metamodel proved to be effective, presenting satisfactory results with adequate accuracy for the initial stages of light-weight aircraft structure.
Abstract in English:In this work, tensile and shear modulus, as well as Poisson's ratio of a woven orthotropic nanocomposite plate were determined from vibration data. Plates used in the experiment were 16 layers S2-glass/epoxy composite manufactured by vacuum assisted wet layup. The nanocomposite plate was obtained by adding 0%, 1%, 2%, 5% and 10% nanoclays in weight to the epoxy matrix. A Finite Element model of the composite plate combined with a gradient method was applied to obtain an approximate numerical solution to experimental data in order to estimate the mechanical properties. Two different modal procedures were employed: the Laser Anemometry and Hammer test. A modal analysis was made in both cases to determine structural mode shape and associated frequencies, modal and mechanical properties for different nanoclay composite plates, as well as assessing its sensitivity to Modal Analysis.
Abstract in English:The paper presents an analysis of the possibility of increasing the accuracy and stability of machining of low-rigidity shafts while ensuring high efficiency and economy of their machining. An effective way of improving the accuracy of machining of shafts is increasing their rigidity as a result of oriented change of the elastic-deformable state through the application of a tensile force which, combined with the machining force, forms longitudinal-lateral strains. The paper also presents mathematical models describing the changes of the elastic-deformable state resulting from the application of the tensile force. It presents the results of experimental studies on the deformation of elastic low-rigidity shafts, performed on a special test stand developed on the basis of a lathe. An estimation was made of the effectiveness of the method of control of the elastic-deformable state with the use, as the regulating effects, the tensile force and eccentricity. It was demonstrated that controlling the two parameters: tensile force and eccentricity, one can improve the accuracy of machining, and thus achieve a theoretically assumed level of accuracy.
Abstract in English:In this paper, free vibration response of a hybrid composite plate was studied. Effects of some geometrical, physical and material parameters on response of the composite plates embedded with shape memory alloy (SMA) wires were investigated, which have not been reported in the literature thus far. Some of these parameters included important factors affecting free vibration response of the smart hybrid composite plates. The SMA wires were embedded within the layers of the composite laminate. First-order shear deformation theory (FSDT) was utilized to obtain the governing equations of hybrid composite plates. Transverse shear and rotary inertia effects of the plate were taken into consideration. For simply-supported boundary conditions, systematic closed form solutions were obtained by Navier's technique. It was established that dynamic behavior of the smart hybrid composite plate depended on various parameters such as volume fraction, temperature dependent recovery stress and tensile pre-strain of SMA wires and aspect ratio of the laminated hybrid plate.
Abstract in English:This paper is devoted to the study of propagation of Rayleigh waves in a homogeneous isotropic microstretch generalized thermoelastic diffusion solid half-space. Secular equations in mathematical conditions for Rayleigh wave propagation are derived for stress free, insulated/impermeable and isothermal/isoconcentrated boundaries. The phase velocity, attenuation coefficient, the components of normal stress, tangential stress, tangential couple stress, microstress, temperature change and mass concentration are computed numerically. The path of surface particles is also obtained for the propagation of Rayleigh waves. The computationally stimulated results for the resulting quantities are represented to show the effect of thermally insulated, impermeable boundaries and isothermal, isoconcentrated boundaries alongwith the relaxation times. Some particular cases have also been deduced from the present investigation.
Abstract in English:In this paper, a mathematical model of large amplitude vibration of a uniform cantilever beam arising in the structural engineering is proposed. Two efficient and easy mathematical techniques called variational iteration method and He's variational approach are used to solve the governing differential equation of motion. To assess the accuracy of solutions, we compare the results with the Runge-Kutta 4th order. An excellent agreement of the approximate frequencies and periodic solutions with the numerical results and published results has been demonstrated. The results show that both methods can be easily extended to other nonlinear oscillations and it can be predicted that both methods can be found widely applicable in engineering and physics.
Abstract in English:The purpose of this work is to investigate experimentally the effects of mixed-mode I/II loading on the fracture behavior of polymer mortars. Three point bend beam specimens with offcenter notch were used in plane-strain fracture toughness tests. Two approaches have been simultaneously used to determine the crack opening displacements. The crack mouth opening displacement was measured by means of a clip gauge, while the crack tip opening displacement was estimated through the Digital Image Correlation method. Fracture parameters, such as critical values of crack opening displacement, energy release rate and mixed-mode stress intensity factor were estimated. In addition, the values of crack mouth opening displacement and crack tip opening displacement were used to evaluate the rotational factor. Finally, the effect of sliding mode-II in fracture was not easily observed using clip gage, however this effect was noticed employing Digital Image Correlation method.