Journal of Aerospace Mechanics
Year:2016 | Volume:12 | Issue:2 (44) (STRUCTURES AND MATHERIALS MECHANICAL BEHAVIOR)|Papers Count:10

This paper describes time-dependent stress and deformation redistribution response of thick–walled spherical vessels made of functionally graded materials (FGM) under radial temperature fields and an internal pressure. The material properties except poisson's ratio are power functions of radius. Total strains are the sum of mechanical, thermal and creep strains. By using equations of equilibrium, stress-strain and strain-displacement as well as using Norton’s law for creep constitutive model and considering the Prandtl - Reuss relations the stress distributions at loading moment and their rates with respect to time are obtained. Time-dependent redistribution of stresses is calculated as the sum of stress distribution at loading moment and integral of stress rates. Radial temperature distribution, history of stresses, displacements, stress rates and the effect of temperature rise on creep evolution are reported in this paper. It is concluded that the influence of tangential stress variations is more notable than radial stress`s and the creep behavior is more significant at high temperature applications. Also by means of FGM materials one can build vessels so that after passing specified time from loading, the stresses and deformations stand at anticipated rates.

In this paper thermoelastic behavior of a functionally graded material (FGM) conical shell with clamped edges condition is investigated. Material properties vary according to power law along the thickness direction. Thermo elastic properties of the shell are assumed to vary along the thickness by a power law distribution with Poisson ratio is held to be constant. By using first order shear deformation theory, strain-displacement relations and constitutive equations, governing differential equations can be derived in term of mid radius displacements and rotation. By using the differential quadrature method (DQM) the obtained governing equations can be solved. To validate the accuracy of the present procedure, numerical results were compared with those available in published literature. Finally the effect of gradient index, geometry dimension and thermo mechanical load on the bending behavior of conical shell is considered.

The objective of this study was quantified of uncertainty design parameters and included their effects explicitly in structural analysis for evaluated reliability indexes. On the other hand, structural reliability methods are of little use if they cannot be used in conjunction with the best available software for current structural analysis. Hence, it becomes very important to couple structural reliability algorithms with finite element analysis of structural problems. So in this research such coupling can be performed. It is used an algorithm for evaluation of first and second order approximation of reliability indexes based on the finite element programs. In this algorithm the limit state function and relative’s gradient of uncertainty parameters computed using finite element model. So structural model directly analyzed without any approximation such as response surface methodology. Based on the probability of fracture mechanics, strain energy release rate is evaluated for isotropic and composite material to demonstrate the application of the algorithm. In the paper also, the Young’s modulus probability distribution is determined for 26 steel samples with over the 35 service life years.

This paper presents an experimental investigation of the shear behaviour of UD fibre glass epoxy under cyclic loading. Experimental data of such materials under loading cycles, involving loading, unloading, reversed loading, unloading from reversed loading and reloading, are essential. Cyclic rail shear fixture (CRSF) as a modified version of ASTM D 4255/D 4255 M fixture has been used in this study. Some specimens tested under low cycle fatigue loading. The results of nonlinearity and level of damage in these specimens are considerable. In part II, a model applies to any phase of the cyclic loading up to failure. The model has been compared with test results. Excellent agreements between the prediction and experimental results have been observed in all respects.

The in-plane shear stress-strain relationship of UD fibre-reinforced composites is a well-known example in which nonlinearity behaviour plays a dominant role while the linear-elastic regime is relatively small. This paper describes a new phenomenological damage model for prediction of the shear behaviour. The observed nonlinear behaviour may well be the result of combined effects of nonlinear elasticity, viscosity, plasticity and microscopic damage and some of these processes in the material are irreversible in nature. The model applies to any phase of the cyclic loading, such as loading, unloading, reversed loading, unloading from reversed loading and reloading up to failure. The model has been compared with test results. Excellent agreements between the prediction and experimental results have been observed in all respects. In order to incorporate this nonlinear shear model in a comprehensive material model so that it can be applied in general structural analysis. A material user subroutine UMAT has been written for this model and all results obtained by this user subroutine via commercial finite element code ABAQUS.

In this article, according to the thin shell theory of Flugge as well as using a new mathematical procedure, the linear buckling of waffle cylindrical shell under axial load is investigated. Skin and grids are composed of the same isotropic material and the classical boundary conditions of clamped and simply supported are considered. Firstly, using a new mathematical modeling, the structural stiffness matrix of skin and grids is obtained. Then, the equilibrium equations of the shell are derived based on the new stiffness matrix. Finally, these equations are solved using differential quadrature method. The results are compared by the finite element analyses. Comparative results reveal that the present procedure is very stable and accurate. Also, the effects of various geometrical parameters on the linear buckling loads are presented.

The main purpose of this study is to investigate nonlinear buckling analysis of Functionally Graded (FG) cylindrical shells under uniform axial compressive loads by dynamic relaxation (DR) method. The mechanical properties of shell vary continuously throughout the thickness direction according to the power-law, exponential function and the Mori-Tanaka distribution. The poisson’s ratio of the FG cylindrical shell is constant for power-law and exponential function. But in the Mori-Tanaka distribution variations of poisson's ratio is determined as a function of the thickness direction. The incremental form of nonlinear formulations are based on first order shear deformation theory (FSDT) and large deflection von Karman equations. The DR method combined with the finite difference discretization technique is employed to solve the equilibrium equations. Some comparison study is carried out to compare the current solution with the results reported in the literature and the ones obtained by the Abaqus finite element software for the isotropic cylindrical shells. Finally, numerical results are presented for critical buckling load with various boundary conditions, grading indices, radius -to- thickness ratio, length -to- radius ratio and variation of poisson’s ratio.

In this article, bending-stretching analysis of thick rectangular plate made of functionally graded material has been investigated based on higher order shear and normal deformable theory which was introduced by Batra and Vilodi. In this theory, both shear and normal deformation effects in the thickness direction are considered. Also, the deflection of plate is not considered as a constant in the thickness direction. A power law function is used to describe the mechanical properties of the functionally graded plate. The numerical results have been presented for first to fifth order shear and normal deformable theory in tables and diagrams. The effects of functionally graded material properties on deflection and stresses of plate have been studied for various thickness and dimensions of the plate. It is shown that this theory gives accurate results not only for thin and moderately thick plates, but also for functionally graded thick plates.

In this article the effect of circular cutout size, fiber orientation, and reinforcer edge size on buckling behavior of composite plates is investigated by experimental and numerical. Samples have clamped boundary conditions on L-shaped edges and free on other two sides. In the experimental method, the E-glass/epoxy L-shaped samples with and without circular cutout is applied in three different lay-up under uni-axial compression. Then the buckling and failure loads will be recognized. In the second method, according to numerical method (Ansys 10), the effect of reinforcer edge size changing and size of circular cutout on composite plate buckling in four different lay-ups with the same experimental materials properties is investigated. The results show that there is the optimum height for reinforcer edge during different conditions that increasing this size just causes the extra weight and cost and does not have a good effect on buckling load.