Sharif Digital Repository

In this paper, an atomistic–continuum homogenization multiscale method is developed to study the nonlinear behavior of heterogeneous nanomaterials. The atomistic representative volume element (RVE) with vacancy and/or void defects are analyzed by employing the fully atomistic method, in which the nucleation, migration, and elimination of dislocation, as well as the dislocation-vacancy interaction, are captured. The coarse-scale material domain is modeled within the framework of the nonlinear finite element method, and the impression of nanoscale material defects is investigated by upscaling the stress tensor and tangent modulus from the atomistic RVE based on the Hill-Mandel principle. The... 

In this paper, a new single cone-cap plasticity with an isotropic hardening rule is presented for powder materials. A general form is developed for the cap plasticity, which can be compared with some common double-surface plasticity models proposed for powders in literature. The constitutive elasto-plastic matrix and its components are derived based on the definition of yield surface, hardening parameter and nonlinear elastic behavior, as a function of relative density of powder. Different aspects of the model are illustrated and the procedure for determination of powder parameters is described. Finally, the applicability of the proposed model is demonstrated in numerical simulation of... 

In this paper, a new computational technique is presented based on the eXtended Finite Element Method (X-FEM) in pressure-sensitive plasticity of powder compaction considering frictional contact. In X-FEM, the need for mesh adaption to discontinuity interface is neglected and the process is accomplished by partitioning the domain with some triangular sub-elements whose Gauss points are used for integration of the domain of the elements. The technique is applied by employing additional functions, which are added to approximate the displacement field of the elements located on the interface. The double-surface cap plasticity model is employed within the X-FEM framework in numerical simulation... 

As computer simulation increasingly supports engineering design, the requirement for a computer software environment providing an integration platform for computational engineering software increases. A key component of an integrated environment is the use of computational engineering to assist and support solutions for complex design. In the present paper, an integrated software environment is demonstrated for multi-disciplinary computational modeling of structural and geotechnical problems. The SUT-DAM is designed in both popularity and functionality with the development of user-friendly pre- and post-processing software. Pre-processing software is used to create the model, generate an... 

In this paper, a new computational technique based on the extended finite element method (X-FEM) is presented in elasto-plastic behavior of pressure-sensitive material. The cap plasticity model is employed within the X-FEM framework in numerical simulation of powder die pressing. The double-surface plasticity model includes a failure surface and an elliptical yield cap, which closes the open space between the failure surface and hydrostatic axis. The yield cap expands in the stress space according to a specified hardening rule. Application of the X-FEM in simulation of two phase plasticity continuums is presented in an incremental manner and the role of sub-elements in the partition of unity... 

In this paper, a multiscale finite element framework is developed based on the first-order homogenization method for fully coupled saturated porous media using an extension of the Hill-Mandel theory in the presence of microdynamic effects. The multiscale method is employed for the consolidation problem of a 2-dimensional saturated soil medium generated from the periodic arrangement of circular particles embedded in a square matrix, which is compared with the direct numerical simulation method. The effects of various issues, including the boundary conditions, size effects, particle arrangements, and the integral domain constraints for the microscale boundary value problem, are numerically... 

Standard finite element models, i.e. finite element methods that use standard constitutive models, suffer from excessive mesh dependence when strain-softening models are used in numerical analyses and cannot reproduce the size effect commonly observed in quasi-brittle failure. In this paper, an h-adaptive analysis for the mixed finite element solution of solid mechanics problems is presented with special reference to metal powder forming involving localization due to material instability. A remeshing strategy is employed to compute the distribution of required element size using the estimated error distribution. The numerical results are obtained for a Von-Mises yield criterion applied to a... 

The main objective of this paper is to present a numerical simulation of mechanical interaction (bond mechanism) of lugged FRP rods with concrete. A finite element analysis is performed and the major causes of bond in lugged rod's interface with concrete are summarized. The concept of ascending and descending behavior of a rib under pull-out load is discussed. A simple procedure is employed to model the crack formation and propagation in the concrete block below the rib. Details and general aspects of model are described and numerical results are compared with experiments. Finally, it has been concluded that the proposed model can be effectively used for the simulation of bond behavior of... 

In this paper, an h-adaptive mesh refinement is presented based on the gradient of deformation in the modeling of localization due to material instability. As the classical continuum models suffer from pathological mesh dependence in the strain-softening models, the governing equations are regularized by adding rotational degrees-of-freedom to the conventional degrees-of-freedom. Adaptive strategy using element elongation is applied to compute the distribution of required element size using the estimated error distribution. Once the new element size and its alignment have been indicated, an automated procedure is used to construct the mesh according to a predetermined size and elongation... 

Purpose - The purpose of this paper is to present a shape optimization technique for powder forming processes based on the genetic algorithm approach. The genetic algorithm is employed to optimize the geometry of component based on a fixed-length vector of design variables representing the changes in nodal coordinates. The technique is used to obtain the desired optimal compacted component by changing the boundaries of component and verifying the prescribed constraints. Design/methodology/approach - The numerical modeling of powder compaction simulation is applied based on a large deformation formulation, powder plasticity behavior, and frictional contact algorithm. A Lagrangian finite... 

In this paper, the uniaxial cold compaction process of metal nano-powders is numerically analyzed through the Molecular Dynamics (MD) method. The nano-powders consist of nickel and aluminum nano-particles in the pure and mixed forms with distinctive contributions. The numerical simulation is performed using the different number of nano-particles, mixing ratios of Ni and Al nano-particles, compaction velocities, and ambient temperatures in the canonical ensemble until the full-dense condition is achieved. In the MD analysis, the inter-atomic interaction between metal nano-particles is modeled by the many-body EAM potential, and the interaction between frictionless rigid die-walls and metal... 

In this paper, a three-invariant cap plasticity with a kinematic hardening rule is presented for powder materials. A general form is developed for the cap plasticity which can be compared with some common double-surface plasticity models proposed for powders in literature. The constitutive elasto-plastic matrix and its components are derived based on the definition of yield surface, hardening parameter and non-linear elastic behavior, as function of relative density of powder. The procedure for determination of powder parameters is described. Finally, the applicability of the proposed model is demonstrated in numerical simulation of triaxial and confining pressure tests. © 2006 Elsevier B.V.... 

In this paper, the computational aspects of large deformation frictional contact are presented in powder forming processes. The influence of powder-tool friction on the mechanical properties of the final product is investigated in pressing metal powders. A general formulation of continuum model is developed for frictional contact and the computational algorithm is presented for analyzing the phenomena. It is particularly concerned with the numerical modeling of frictional contact between a rigid tool and a deformable material. The finite element approach adopted is characterized by the use of penalty approach in which a plasticity theory of friction is incorporated to simulate sliding... 

In this paper, an application of the reproducing kernel particle method (RKPM) is presented in plasticity behavior of pressure-sensitive material. The RKPM technique is implemented in large deformation analysis of powder compaction process. The RKPM shape function and its derivatives are constructed by imposing the consistency conditions. The essential boundary conditions are enforced by the use of the penalty approach. The support of the RKPM shape function covers the same set of particles during powder compaction, hence no instability is encountered in the large deformation computation. A double-surface plasticity model is developed in numerical simulation of pressure-sensitive material.... 

In this paper, a three-invariant cap plasticity model with an isotropic hardening rule is presented for numerical simulation of powder compaction processes. A general form is developed for single-cap plasticity which can be compared with some common double-surface plasticity models proposed for powders in literature. The constitutive elasto-plastic matrix and its components are derived based on the definition of yield surface, hardening parameter and non-linear elastic behaviour, as function of relative density of powder. Different aspects of the new single plasticity are illustrated by generating the classical plasticity models as special cases of the proposed model. The procedure for... 

Gold (Au) nanoparticles are widely used in diagnosing cancer, imaging, and identification of therapeutic methods due to their particular quantum characteristics. This research presents different types of aqueous models and potentials used in TIP3P, to study the effect of the particle size and density of Au clusters in aquatic environments; so it can be useful to facilitate future investigation of the interaction of proteins with Au nanoparticles. The EAM potential is used to model the structure of gold clusters. It is observed that in the systems with identical gold/water density and different cluster radii, gold particles are distributed in aqueous environment almost identically. Thus, Au... 

Purpose - In this paper, the polygonal-FEM technique is presented in modeling large deformation - large sliding contact on non-conformal meshes. The purpose of this paper is to present a new technique in modeling arbitrary interfaces and discontinuities for non-linear contact problems by capturing discontinuous deformations in elements cut by the contact surface in uniform non-conformal meshes. Design/methodology/approach - The geometry of contact surface is used to produce various polygonal elements at the intersection of the interface with the regular FE mesh, in which the extra degrees-of-freedom are defined along the interface. The contact constraints are imposed between polygonal... 

In this paper, a polygonal finite element method is presented for crack growth simulation with minimum remeshing. A local polygonal mesh strategy is performed employing polygonal finite element method to model the crack propagation. In order to model the singular crack tip fields, the convex and concave polygonal elements are modified based on the singular quarter point isoparametric concept that improves the accuracy of the stress intensity factors. Numerical simulations are performed to demonstrate the efficiency of various polygonal shape functions, including the Wachspress, metric, Laplace and mean value shape functions, in modeling the crack tip fields. Eventually, analogy of the... 

In this paper, two new approximation functions are introduced. These new techniques, which are referred herein as the multi-triangles method (MTM) and weighted multi-triangles method (WMTM) are applied for the approximation of unknowns and their derivatives at the points of interest. The approximations are performed in terms of the unknowns corresponding to the field nodes which are the vertices of the region surrounding the desired point and determined by Delaunay triangulations. The capability and accuracy of the proposed approximation functions are compared with the other approximating techniques in the meshless finite volume (MFV) frame work for some benchmark problems. Numerical... 

The local buckling behavior of perfect/defective and single/multi-walled carbon nanotubes (CNTs) under axial compressive forces has been investigated by the molecular dynamics approach. Effects of different types of defects including vacancy and Stone-Wales (SW) defects and their configurations on CNTs with different chiralities at room temperature are studied. Results show that defects largely reduce the buckling stress and the ratio of immediate reduction in buckling compressive stress of the defective CNT to the perfect one, but have little influence on their compressive elastic modulus. SW defects usually reduce the mechanical properties more than vacancy defects, and zigzag CNTs are...