Sharif Digital Repository

In this study, the tractive rolling contact problem between a rigid cylinder and a graded coating is investigated. The main objective of this study is to investigate the effect of the stiffness ratio, the coefficient of friction and the coating thickness on the surface contact tractions, the surface in-plane stress, the stick zone length and the creep ratio parameter that may have a bearing on the fatigue life of the component. Assuming that the shear modulus varies exponentially through the thickness of the coating, the governing integral equations associated with the rolling contact problem are constructed. Furthermore, it is supposed that the contact patch is controlled by a central stick... 

A theoretical approach is presented to derive an explicit formula for buckling load and postbuckling path of advanced grid stiffened conical shells (stiffeners with laminated composite skins). Different types of fiber paths of grids including stringer, ring, and helical are considered. The simply supported truncated conical shell with imperfection is subjected to axial compression. Basic formulations are constructed using the classical theory of shells and von Karman type of nonlinear strain-displacement relationships. The equilibrium and compatibility equations are solved by Galerkin procedure, and an explicit relation is obtained to predict the equilibrium paths. Results for different... 

An analytical method for reconstructing residual stresses within axisymmetric cylinders is presented. The method uses the stress equilibrium and boundary conditions together with an Airy stress function that satisfies these conditions. The method allows the reconstruction of residual stresses from limited measurements. The analysis is also coupled to a least squares approximation and a regularization analysis to provide stability of the inverse problem. The application of the method is demonstrated for four cases; two correspond to analytical solutions for residual stresses created during autofrettage and two sets of experimental results for through wall measurements of residual stresses. In... 

The most common use of FG materials is as barrier coating against large thermal gradients. Thermal stresses in FG materials, if not released, may cause structural discontinuities in outer surfaces or even inside the material such as cracks, debonding, etc. In this research work, using Finite element method and micromechanical modeling of FG thermal barrier coatings, stresses under thermal and mechanical loadings of the same and different phases have been investigated. Also, the effect of some parameters such as refinement and offsetting of particles on stresses are studied. As for the loading, thermal cycle and in-phase and out-of-phase thermo-mechanical cyclic loadings are considered. The... 

An exact thermoelasticity solution for a two-dimensional thick composite consisting of homogeneous and functionally graded layers is presented. The thermomechanical properties of functionally graded layers are assumed to vary exponentially through the thickness while the Poisson's ratio is taken to be constant. The heat transfer problem is solved under steady state condition accounting for the heat convection. Utilizing the stress function the governing equation reduces to a fourth order inhomogeneous partial differential equation which is solved exactly using Fourier series method. A comparative study is done between two sandwich structures with homogeneous and functionally graded coatings,... 

Welding process is widely used in manufacturing of many important engineering components. For such structures, the most important problem is the development of residual stresses and distortion due to welding. Welding tensile residual stresses have a detrimental effect and play an important role in an industrial environment. Crack initiation and propagation in static or fatigue loading, or in stress corrosion can be greatly accelerated by welding tensile stresses. Practically, however, it is often very difficult to characterize the residual stress state completely, while the knowledge of the complete residual stress distribution in structures is essential for assessing their safety and... 

Residual stresses, created in a steel beam by elastic-plastic bending, are predicted using an approximate analysis and the finite element method. The predictions are compared to experimental measurements obtained from the application of incremental centre hole drilling, deep hole drilling and neutron diffraction methods. Finite element simulations of the incremental centre hole drilling and deep hole drilling methods applied to the predicted residual stresses permitted an assessment of their ability to reconstruct the stresses. An analytical reconstruction analysis using an Airy stress function together with boundary and equilibrium conditions is developed and applied to the predictions and... 

Free vibration analysis of a transversely stiffened circular thin hollow cylinder made of functionally graded materials (FGMs) is analytically evaluated. Functionally graded materials are inhomogeneous composites which are usually made from a mixture of metal and ceramic. The gradient compositional variation of the constituents from one surface to the other provides an elegant solution to the problem of high transverse shear stresses induced when two dissimilar materials with large differences in material properties are bonded. In this paper, application of an FGM made of two different materials is investigated by applying Ritz method. While cylinder is assumed to be thin, strain energy... 

This paper presents a stability analysis for fractal cracks. First, the Westergaard stress functions are proposed for semi-infinite and finite smooth cracks embedded in the stress fields associated with the corresponding self-affine fractal cracks. These new stress functions satisfy all the required boundary conditions and according to Wnuk and Yavari's (2003 Eng. Fract. Mech. 70 1659-74) embedded crack model they are used to derive the stress and displacement fields generated around a fractal crack. These results are then used in conjunction with the final stretch criterion to study the quasi-static stable crack extension, which in ductile materials precedes the global failure. The material... 

In this paper, a size-dependent formulation is presented for Timoshenko beams made of a functionally graded material (FGM). The formulation is developed on the basis of the modified couple stress theory. The modified couple stress theory is a non-classic continuum theory capable to capture the small-scale size effects in the mechanical behavior of structures. The beam properties are assumed to vary through the thickness of the beam. The governing differential equations of motion are derived for the proposed modified couple-stress FG Timoshenko beam. The generally valid closed-form analytic expressions are obtained for the static response parameters. As case studies, the static and free... 

Based on the first-order shear deformation plate theory with von Karman non-linearity, the non-linear axisymmetric and asymmetric behavior of functionally graded circular plates under transverse mechanical loading are investigated. Introducing a stress function and a potential function, the governing equations are uncoupled to form equations describing the interior and edge-zone problems of FG plates. This uncoupling is then used to conveniently present an analytical solution for the non-linear asymmetric deformation of an FG circular plate. A perturbation technique, in conjunction with Fourier series method to model the problem asymmetries, is used to obtain the solution for various clamped...