Sharif Digital Repository

The development of damage detection techniques for offshore jacket structures is vital to prevent catastrophic events. This paper applies a frequency response based method for the purpose of structural healthCgreater detection efficiency. In addition, the performance of the proposed method was evaluated in relation to multiple damages. The aforementioned points are illustrated using the numerical study of a two dimensional jacket platform, and the results proved to be satisfactory utilizing the proposed methodology  

The elastic behavior of an edge dislocation placed in the shell of a free-standing core-shell nanowire is considered within the theory of surface/interface elasticity. Using the method of complex potential functions the expressions for the stress field of the dislocation, image forces on the dislocation, and the dislocation strain energy are derived and studied in detail. A special attention is paid to non-classical effects revealed within the surface/interface elasticity approach where a characteristic length parameter referred to as surface/interface modulus is introduced. These effects are (i) the stress oscillations along the shell surface and core-shell interface for negative values of... 

The elastic behavior of an edge dislocation located inside the core of a core-shell nanowire which is embedded in an infinite matrix is studied within the surface/interface elasticity theory. The corresponding boundary value problem is solved exactly by using complex potential functions. An important parameter so-called interface characteristic parameter which has the dimension of length and is a combination of the interface moduli enters the formulations. The stress field of the dislocation, image force acting on the dislocation, and the dislocation strain energy is calculated by considering the interface effect. The introduced characteristic parameter allows the examination of the... 

Hyperelastic behavior of isotropic incompressible rubbers is studied to develop a strain energy function which satisfies all the necessary characteristic properties of an efficient hyperelastic model. The proposed strain energy function includes only three material parameters which are somehow related to the physical quantities of the material molecular network. Moreover, the model benefits from mathematical simplicity, well suitting in all ranges of stretch and possessing the property of deformation-mode-independency. This reduces the required number of experimental tests for parameter calibration of the model. Results of the proposed model are compared with results of some available models... 

In this paper, employing the logarithmic (or Hencky) strain as a more physical measure of strain, the time-dependent response of compressible viscoelastic materials is investigated. In this regard, we present a phenomenological finite strain viscoelastic constitutive model, developed within the framework of irreversible thermodynamics with internal variables. The formulation is based on the multiplicative decomposition of the deformation gradient into elastic and viscoelastic parts, together with the use of the isotropic property of the Helmholtz strain energy function. Making use of a logarithmic mapping, we present an appropriate form of the proposed constitutive equations in the... 

In this paper, a new multi-scale approach is presented based on the modified boundary Cauchy-Born (MBCB) technique to model the surface effects of nano-structures. The salient point of the MBCB model is the definition of radial quadrature used in the surface elements which is an indicator of material behavior. The characteristics of quadrature are derived by interpolating data from atoms laid in a circular support around the quadrature, in a least-square scene. The total-Lagrangian formulation is derived for the equivalent continua by employing the Cauchy-Born hypothesis for calculating the strain energy density function of the continua. The numerical results of the proposed method are... 

This paper proposes a new visco-hyperelastic constitutive law for modeling the finite-deformation strain rate-dependent behavior of foams as compressible elastomers. The proposed model is based on a phenomenological Zener model, which consists of a hyperelastic equilibrium spring and a Maxwell element parallel to it. The hyperelastic equilibrium spring describes the steady state response. The Maxwell element, which captures the rate-dependency behavior, consists of a nonlinear viscous damper connected in series to a hyperelastic intermediate spring. The nonlinear damper controls the rate-dependency of the Maxwell element. Some strain energy potential functions are proposed for the two... 

In this paper, tensile strain energy absorption of two different hybrid modified epoxies has been systematically investigated. In one system, epoxy has been modified by amine-terminated butadiene acrylonitrile (ATBN) and hollow glass spheres as fine and coarse modifiers, respectively. The other hybrid epoxy has been modified by the combination of ATBN and recycled Tire particles. The results of fracture toughness measurement of blends revealed synergistic toughening for both hybrid systems in some formulations. However, no evidence of synergism is observed in tensile test of hybrid samples. Scanning electron microscope (SEM), transmission optical microscope (TOM) and finite element (FEM)... 

In this paper, a novel multi-scale approach is developed for modeling of the surface effect in crystalline nano-structures. The technique is based on the Cauchy-Born hypothesis in which the strain energy density of the equivalent continua is calculated by means of inter-atomic potentials. The notion of introducing the surface effect in the finite element method is based on the intrinsic function of quadratures, called as an indicator of material behavior. The information of quadratures is derived by interpolating the data from probable representative atoms in their proximity. The technique is implemented by the definition of reference boundary CB elements, which enable to capture not only... 

Dual phase steels (DPS)s have a microstructure consisting of a hard second phase called Martensite in the soft Ferrite matrix. In recent years, there has been interest in dual-phase steels, because the application of these materials has made significant usage; particularly in the automotive sector Composite microstructure of (DPS)s exhibit interesting characteristic mechanical properties such as continuous yielding, low yield stress to tensile strength ratios(YS/UTS), and relatively high formability; which offer advantages compared with conventional high strength low alloy steels(HSLAS). The research dealt with the characterization of damage in (DPS)s. In this study by review the mechanisms... 

Various experimental and theoretical investigations have been carried out to determine the elastic properties of nanotubes in the axial direction. Their behavior in transverse directions, however, has not been well studied. In this paper, a combination of molecular dynamics (MD) and continuum-based elasticity model is used to predict the transverse-isotropic elastic properties of single-walled carbon nanotubes (SWCNTs). From this modeling study, five independent elastic constants of an SWCNT in transverse directions are obtained by analyzing its deformations under four different loading conditions, namely, axial tension, torsion, uniform and non-uniform radial pressure. To find the elastic... 

Hyperelastic materials have high deformability and nonlinearity in load-deformation behavior. Based on a phenomenological approach, these materials are treated as a continuum, and a strain energy density is considered to describe their hyperelastic behavior. In this paper, the mechanical behavior characterization of these materials is studied from the continuum viewpoint. For this purpose, the strain energy density is expressed as sum of independent functions of the mutual multiple of principal stretches. These functions are determined by applying the governing postulates on the form of the strain energy density. It is observed that a consistent strain energy density is expressible in terms... 

The growth of slant cracks by fretting fatigue of a half plane in contact with a flat rounded pad was studied. The mode I and mode II stress intensity factors for cracks of various lengths and directions were calculated using the semi-analytical method of the distribution of dislocations, and their cumulative effect on the crack growth was investigated using the strain energy density criterion. The results showed dominance of mode I fracture on crack growth, and based on the observation of strain energy density factor versus crack orientation, the results also suggest that depending on the crack length, the most critical orientations of cracks are at 0° to 20° outward the contact zone. Good... 

The macroscopic constitutive law for a heterogeneous solid containing two dissimilar nonlinear elastic phases undergoing finite deformation is obtained. Attention is restricted to the case of spherical symmetry such that only the materials consisting of an irregular suspension of perfectly spherical particles experiencing all-round uniform loading are considered which leads to a one-dimensional modeling. For the homogenization procedure, a strain-energy based scheme which utilizes Hashin's composite sphere is employed to obtain the macroscopic stress-deformation relation added by the initial volume fraction of the particles. As applications of the procedure, the closed-form macroscopic... 

In this paper, a new visco-hyperelastic constitutive law for describing the rate dependent behavior of foams is proposed. The proposed model was based on a phenomenological Zener model: a hyperelastic equilibrium spring, which describes the steady-state, long-term response, parallel to a Maxwell element, which captures the rate-dependency. A nonlinear viscous damper connected in series to a hyperelastic intermediate spring, controls the rate-dependency of the Maxwell element. Therefore, the stress is the sum of equilibrium stress on the equilibrium spring and overstress on the intermediate spring. In hyperelastic theory stress is not calculated directly as in the case of small-strain, linear... 

The behavior of foams is typically rate-dependent and viscoelastic. In this paper, multiplicative decomposition of the deformation gradient and the second law of thermodynamics are employed to develop the differential constitutive equations for isotropic viscoelastic foams experiencing finite deformations, from a phenomenological point of view, i.e. without referring to micro-structural viewpoint. A model containing an equilibrium hyperelastic spring which is parallel to a Maxwell model has been utilized for introducing constitutive formulation. The deformation gradient tensor is decomposed into two parts: elastic deformation gradient tensor and viscoelastic deformation gradient tensor. A... 

In this paper, the concept of energy pairs in the micropolar continuum is introduced. A brief review of the micropolar continuum theory is presented for using in the subsequent derivations. A mathematical Lagrangian strain and a wryness tensor for the micropolar continuum are introduced. Using the first law of thermodynamics and for isothermal processes, the power of deformation is obtained and the energy pairs in the Eulerian and Lagrangian descriptions are defined. Also, the micropolar stress and couple stress tensors which are energy pairs to the micropolar Lagrangian strain and wryness measures are determined. © 2006 Elsevier Ltd. All rights reserved  

Delaminated composite beam under general edge loading conditions is studied. Based on a technical engineering theory an analytical procedure for calculation of strain energy release rate and its separation into modes I and II of delamination is presented. By choosing a suitable displacement field based on second-order shear-thickness deformation theory and using the principle of minimum total potential energy, the equations of equilibrium are obtained along with the appropriate boundary conditions. The J integral and its definition for different modes of fracture is used for calculation of strain energy release rate and its separation into different modes. Double cantilever beam (DCB)...