Sharif Digital Repository

The laser forming process is one of the last technologies on forming of sheet metals with laser beam heat distribution. In this process laser beam moves across the top surface of the sheet metal and the heated zone expands and causes a great moment that deforms the sheet metal. Subsequently, the heated zone gets cooled and provides a reverse strain and moment. The final bending angle is a combination of two phases. Due to the complexity of the process, it is studied with different approaches; FEM analysis and analytical as well as empirical methods. The laser forming is a sensible process regarding the material properties. Also, because of the temperature change during the process, it is... 

Governing equilibrium equations of thick-walled spherical vessels made of material following linear strain hardening and subjected to a steady-state radial temperature gradient using elasto-plastic analysis are derived. By considering a maximum plastic radius and using the concept of thermal autofrettage for the strengthening mechanism, the optimum wall thickness of the vessel for a given temperature gradient across the wall thickness is obtained. Finally, in the case of thermal loading on a vessel, the effect of convective heat transfer on the optimum thickness is studied and a general formula for the optimum wall thickness and design graphs for several different cases are presented. © 2008... 

In this paper, the extended finite element method is presented for large elasto-plastic deformation in 3D solid mechanics problems. The X-FEM computational algorithm is presented in the framework of Lagrangian description in order to model the arbitrary interfaces in large deformations. In X-FEM, the material interface is represented independently of element boundaries and the process is accomplished by partitioning the domain with several tetrahedral sub-elements whose Gauss points are used for integration of the domain of elements. The interface between two bodies is modeled by using the X-FEM technique and applying a modified level set enrichment function. In order to simulate the... 

A study of thick-walled spherical vessels under steady-state radial temperature gradients using elasto-plastic analysis is reported. By considering a maximum plastic radius and using the thermal autofrettage method for the strengthening mechanism, the optimum wall thickness of the vessel for a given temperature gradient across the vessel is obtained. Finally, in the case of thermal loading on a vessel, the effect of convective heat transfer on the optimum thickness is considered, and a general formula for the optimum thickness and design graphs for several different cases are presented. © 2004 Elsevier Ltd. All rights reserved  

In this paper, static response of a sandwich cylindrical shell under elasto-plastic deformation is investigated. The faces are made of some isotropic materials and the core is made of an orthotropic material both with linear work hardening behavior. The faces are modeled as thin cylindrical shells obeying the Kirchhoff-Love assumptions. The core material is modeled as a special orthotropic solid in which its in-plane stresses are assumed to be negligible. The Prandtl-Reuss plastic flow theory and von Mises yield criterion are used in the analysis. The governing equations are derived using the principle of virtual displacements. Using Ritz method, the equations are solved for deformation... 

This paper presents new achievements in the extended finite element modeling of large elasto-plastic deformation in solid problems. The computational technique is presented based on the extended finite element method (X-FEM) coupled with the Lagrangian formulation in order to model arbitrary interfaces in large deformations. In X-FEM, the material interfaces are represented independently of element boundaries, 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 elements. The large elasto-plastic deformation formulation is employed within the X-FEM framework to simulate the non-linear... 

In recent years, growing interest in underbalanced drilling has resulted in the rapid development of its associated equipment technology, practices, and procedures. Underbalanced drilling is used to avoid lost circulation, formation damage, and decreasing weight on bit. However, the risk of wellbore collapse due to lake of hydrostatic mud pressure is high; therefore, using good geo-mechanical model may avoid wellbore instability problems. In order to evaluate the potential for wellbore instability, it is necessary to use an elastoplastic model to compute the stresses and strains around the bore hole. Furthermore, it should be mentioned that the laser drilling process depends extremely on the... 

In this paper, decomposition of the total strain into elastic and plastic parts is investigated for extension of elastic-type constitutive models to finite deformation elastoplasticity. In order to model the elastic behavior, a Hookean-type constitutive equation based on the logarithmic strain is considered. Based on this constitutive equation and assuming the deformation theory of Hencky as well as the yield criteria of von Mises, the elastic-plastic behavior of materials at finite deformation is modeled in the case of the proportional loading. Moreover, this elastoplastic model is applied in order to determine the stress distribution in thick-walled cylindrical pressure vessels at finite... 

An Eulerian rate-independent constitutive model for isotropic materials undergoing finite elastoplastic deformation is formulated. Entirely fulfilling the multiplicative decomposition of the deformation gradient, a constitutive equation and the coupled elastoplastic spin of the objective corotational rate therein are explicitly derived. For the purely elastic deformation, the model degenerates into a hypoelastic-type equation with the Green-Naghdi rate. For the small elastic- and rigid-plastic deformations, the model converges to the widely-used additive model where the Jaumann rate is used. Finally, as an illustration, using a combined exponential isotropic-nonlinear kinematic hardening... 

The concept of equivalent linearization, in which the actual nonlinear structure is replaced by an equivalent linear single-degree-of-freedom (SDOF) system, is extended for soil-structure systems in order to consider the simultaneous effects of soil-structure interaction (SSI) and inelastic behavior of the structure on equivalent linear parameters (ELP). This is carried out by searching over a two-dimensional equivalent period-equivalent damping space for the best pair, which can predict the earthquake response of the inelastic soil-structure system with sufficient accuracy. The super-structure is modeled as an elasto-plastic SDOF system whereas the soil beneath the structure is considered... 

Models for chatter prediction in machining often use a mechanistic force model that calculate the force as the product of a material dependent cutting constant and chip area. However, in reality, the forces are the result of complex interaction between the tool and the chip, and are affected by many factors. The effects of these complex, and often nonlinear, factors on the machining dynamics may only be included in chatter prediction if the chip formation process is simulated concurrently with simulation of the machining dynamics. In this paper, finite element simulation of the chip formation process is combined with simulation of chatter dynamics and the inter-relationship between the chip... 

In this paper, a constitutive model with a temperature and strain rate dependent flow stress (Bergstrom hardening rule) and modified Armstrong-Frederick kinematic evolution equation for elastoplastic hardening materials is introduced. Based on the multiplicative decomposition of the deformation gradient,new kinematic relations for the elastic and plastic left stretch tensors as well as the plastic deformation-dependent spin tensor are proposed. Also, a closed-form solution has been obtained for the elastic and plastic left stretch tensors for the simple shear problem.To evaluate model validity, results are compared with known experimental data for SUS 304 stainless steel, which shows a good... 

In this study the effect of stress field on delamination and fiber/ matrix debonding in laminated composite panels is investigated from the micro-mechanical point of view by means of 3-D Finite Element Models. Specifically, the behavior of two-layer cross-ply symmetric laminates made up of SCS-6/Ti-24Al-11Nb Intermetallic Matrix Composite (IMC) during cooling from the processing temperature is studied. The results show that large plastic strains occur at the fiber/ matrix interface at the fiber end on the laminate free edge which may eventually extend to the interface of the layers of the laminate inflicting delamination damage. This phenomenon is more serious at the corner areas of the... 

In the present paper, the nonlinear seismic behavior of a concrete gravity damreservoir- massed foundation system is studied focusing on the foundation material nonlinearity. For this purpose, a finite element program utilizing the elasto-plastic formulation for the foundation medium is provided considering the tensile and shear failure modes. Hoek-Brown and Mohr Coulomb criteria are used as the yield and plastic potential function of the shear failure mode, respectively; and in the tensile failure mode, various levels of the tensile strength are applied to the foundation. Bearing capacity is studied considering the localized plastic elements and forming the sliding path by plastic elements... 

By considering the Bauschinger effect and the yield criterion of Tresca, an exact elasto-plastic analytical solution for a thick-walled cylindrical vessel made of elastic linear-hardening material is derived. Having the working pressure and geometric dimensions of the vessel, the distribution of the hoop and equivalent stresses are optimized in the way that the distribution of stresses becomes smooth in the vessel wall. Based on two optimizing methods of the hoop and equivalent stresses, the best autofrettage pressure is determined. It is shown that this pressure is more than the working pressure and depends on the three following variables: Bauschinger effect, working pressure and geometric...