Sharif Digital Repository

The size of the rock specimen affects the stress concentrates in the vicinity of the top/bottom of the rock specimen during the evaluation of the geomechanical parameters in the laboratory, which causes unreliable results. However, the appropriate size for geomechanical evaluation is not well understood yet because of limitations in the sampling and analysis. In this study, a series of numerical simulations using a finite element package was conducted to investigate the effect of sample aspect ratio, fluid saturation, and porosity, on the mechanical behavior of the rock under elastic and poroelastic conditions. In addition, two concepts, stress/strain homogeneity index (SHI) and... 

Strain and stress concentrations are studied for elastomers at finite deformations. Effects of strain-induced crystallization, filler reinforcement and deformation rate are also investigated, and micromechanical descriptions are provided for the observed results. A simple problem is subjected to finite element simulations to show the results evidently. Material parameters are obtained from experimental tests conducted on standard tensile samples of filled and unfilled natural rubber (NR) as well as styrene–butadiene rubber (SBR) as crystallizing and non-crystallizing rubbers, respectively. In all simulations, the strain concentration factor KE is shown to decrease monotonically where the... 

Due to inadequacy of the classical continuum theories at the nano-scale when dealing with defects, stress concentrators, and relevant deformation phenomena in solids, a refined approach that can capture the discrete atomic features of solids is essential. The inability to detect the size effect, giving unrealistically high values for some components of the stress field right on the edge of the stress concentrators, and infirmity to address the complex interaction between small inhomogeneities, cracks and as such when they are only a few nanometers apart, are among some of the drawbacks of the classical approach. An atomistic study which employs atomic finite element method in conjunction... 

A modified analytical model is developed for analysis of 3D elastic stress fields in short fiber composites subjected to an applied axial load. Two sets of exact displacement solutions for the matrix and fiber are derived based on the theory of elasticity. The superposition state of these solutions are then used to obtain the analytical expressions for the 3D stress field components over the entire composite system including the fiber end region, which is modeled by the use of imaginary fiber technique. The main difference with the previous works here is that the stress field is considered to be a function of both radial and axial directions. Such an assumption made it possible to calculate... 

The efficiency of fiber/matrix interface bonding in composites has an important influence on their overall performance. This article deals with the simulation of interface imperfection effect as it grows under fatigue loading. The relations have been derived based on the constituents' properties for the primary damage mode in unidirectional composites and boundary conditions defined for elements of the micromechanics model. Conformation of model stiffness predictions with the best described residual stiffness model indicates the way to find the remaining constants in the simulated relation. Finally through this relation, cyclic corrected stress components in constituents could be determined.... 

Radial forging is an open die forging process used for reducing the diameters of shafts, tubes, stepped shafts and axels, and for creating internal profiles in tubes. Among parameters affecting process variables, the die geometry is of fundamental importance and greatly influences variables such as forging load, stress distribution on the dies, metal flow during deformation, and surface finish of the forged product. In this paper a generalized slab method analysis of radial forging process is presented which can handle this process with curved shape dies. Results for dies with various curves are presented and it is shown that the analysis reduces to that of Lahoti and Altan [2] when the die... 

In a shell and tube heat exchanger, the failure of tube-to-tubesheet welds results in high-pressure water jet which erodes the refractory in front of the tubesheet. Finite element method was employed to simulate the welding process and post weld heat treatment (PWHT) to find the factors affecting the failure in tube-to-tubesheet weldments. Residual stresses in two different geometries of tube-to-tubesheet weldment were calculated through uncoupled thermal-structural analysis. The results showed that the values of residual stresses are higher in heat exchanger of site 1 than site 2 due to more weld passes and geometry of connection. Also, the maximum stress in site 1 occurs at the shellside... 

In a shell and tube heat exchanger, the failure of tube-to-tubesheet welds results in high-pressure water jet which erodes the refractory in front of the tubesheet. Finite element method was employed to simulate the welding process and post weld heat treatment (PWHT) to find the factors affecting the failure in tube-to-tubesheet weldments. Residual stresses in two different geometries of tube-to-tubesheet weldment were calculated through uncoupled thermal-structural analysis. The results showed that the values of residual stresses are higher in heat exchanger of site 1 than site 2 due to more weld passes and geometry of connection. Also, the maximum stress in site 1 occurs at the shellside... 

In this study the energy concept along with the classical plate theory (CPT), first and third order shear deformation theories (FSDT and TSDT) are used to predict the large deflection and through the thickness stresses of a FGM plate. For defining the volume fraction of the FGM constituent materials three different functions are considered; simple power-law (PFGM), exponential (E-FGM) and sigmoid (S_FGM) functions. Power-law and exponential functions are commonly used tocontrol the variations of properties of FGMs. However, with both functions, a stress concentration appears due to abruptchange of the volume fraction of the constituents. Therefore, a sigmoid FGM is used to define a new... 

The stress distribution on open-ended Carbon Nanotubes (CNTs) embedded in a composite material is considered in this work and an analytical solution for the stress distribution has been obtained. The effects of CNT's thickness and CNT's length on the distribution of stress have been investigated. To find the governing relations, continuity equations of the axisymmetric problem in cylindrical coordinate (r,o,z) are used. Under some assumptions, the governing equations are solved and by using constitutive equations and applying the boundary conditions, an equation which relates the stress applied to the representative volume element with the stress distribution on the CNT, has been found. The... 

One of the engineers concern in designing bridges and structures under moving load is the uniformity of stress distribution. In this paper the analysis of a variable cross-section beam subjected to a moving concentrated force and mass is investigated. Finite element method with cubic Hermitian interpolation functions is used to model the structure based on Euler-Bernoulli beam and Wilson-⊖ direct integration method is implemented to solve time dependent equations. Effects of cross-section area variation, boundary conditions, and moving mass inertia on the deflection, natural frequencies and longitudinal stresses of beam are investigated. Results indicates using a beam of parabolically... 

Within the elasticity formulation the most general displacement field for hygrothermal problems of long laminated composite plates is presented. The equivalent single-layer theories are then employed to determine the global deformation parameters appearing in the displacement fields of general cross-ply, symmetric, and antisymmetric angle-ply laminates under thermal and hygroscopic loadings. Reddy's layerwise theory is subsequently used to determine the local deformation parameters of various displacement fields. An elasticity solution is also developed in order to validate the efficiency and accuracy of the layerwise theory in predicting the interlaminar normal and shear stress... 

In this research, a finite element model was developed in order to simulate the two-step residual stress distribution of a thermal barrier coating system, considered to be used in diesel engine cylinder head, with a real roughness and real porosity. Two steps including the bond coat and the top coat deposition processes were taken into account. The real geometry of coating layers, including the roughness and the porosity, was also considered based on a scanning electron microscopy image. Then, effects of the convective heat transfer coefficient and initial substrate and substrate/bond coat preheating temperatures on the residual stress were studied. Obtained results illustrate that the... 

Interaction of Carbon Nanotubes (CNTs) and nanovoids embedded in a metal matrix is studied comparatively. For this purpose, two nearby CNTs/nanovoids are modeled as two similar cylindrical inclusions/holes in an infinite matrix. The nonlocal stresses around the CNTs/nanovoids are obtained by applying the integral constitutive equation of the nonlocal elasticity to the stresses from the complex stress potential method. Also, in order to bring different nonlocality effects of dissimilar media into account, the influence function of the nonlocal elasticity is modified. Effects of the CNTs/nanovoids size and distance as well as far-field loading ratio on the stress state in the matrix are... 

In this study, a near equiatomic NiTi shape memory alloy was hot rolled at 800°C using thethickness reductions of 30 and 50%. Optical and transmission electron microscopy, together withX-ray diffraction were used to demonstrate the microstructural changes associated with the hotrolling at different thickness reductions. Repeated transformation cycling was employed toinvestigate the evolution of R phase during cycling. Microstructural observations revealed thepresence of deformation twins embedded in an elongated grain matrix in the hot rolled material.Moreover, it was found that with increasing degree of thickness reduction, the size and number ofdeformation twins increased throughout the... 

The usual continuum theories are inadequate in predicting the mechanical behavior of solids in the presence of small defects and stress concentrators; it is well known that such continuum methods are unable to detect the change of the size of the inhomogeneities and defects. For these reasons various augmented continuum theories and strain gradient theories have been proposed in the literature. The major difficulty in implication of these theories lies in the lack of information about the additional material constants which appear in such theories. For fcc metals, for the calculation of the associated characteristic lengths which arise in first strain gradient theory, an atomistic approach... 

In the present research, results from the finite element (FE) stress analysis of 108 models, validated using test data obtained from the experimental investigation, was used to propose probability distribution models for stress concentration factors (SCFs) in internally ring-stiffened tubular KT-joints under four types of in-plane bending (IPB) loads. The SCF is one of the crucial parameters in the fatigue reliability analysis of tubular joints. It exhibits considerable scatter which calls for greater emphasis in accurate derivation of its governing probability function. According to the knowledge of the authors, no comprehensive research has been carried out on the probability distribution... 

A torsion test recently implemented for solid oxide fuel cell sealant materials is analyzed as a method for measuring the shear strength of sealant for solid oxide fuel cells. The finite element method is used to simulate the stress distribution in the hourglass-shaped steel specimens with intermediate sealant layer with different specimen's dimensions and configurations. Also, it is analyzed how stress concentration changes if the sealant does not completely fill the gap or is squeezed out of gap. The reduction of seal thickness to outer radius ratio results in an increase in stress concentration at the outer edge of sealant. The developed specimens with a hollow halve steel plate as well... 

Three-dimensional thermo-mechanical stress analysis of composite joints with bi-adhesive bonding is presented using the full layerwise theory. Based on three-dimensional elasticity theory, sets of fully coupled governing differential equations are derived using the principle of minimum total potential energy and are simultaneously solved using the state space approach. Results show that bi-adhesive bonding substantially relieves the edge effects. Moreover, series of parametric studies reveal the nonlinear effects of bonding length ratio and the relative stiffness and coefficient of thermal expansion of the mid- and side-adhesives. It is also concluded that the optimum design of a bi-adhesive...