نشریه روش های عددی در مهندسی
سال بیست و نهم شماره 1 (شهریور 1389)

The Continuum Damage Mechanics is a branch of applied mechanics that predicts the initiation of cracks in metal forming processes. In this paper, a modified ductile damage model is applied to predict initiation of the micro-void as a central burst along the bar axis during the forward extrusion. The implicit integration scheme of the coupled constitutive equations is presented. The effects of various process parameters including die semi-angle, reduction in area, friction coefficient and material ductility on central bursts are discussed. To verify the quality of simulations, numerical predictions were compared with published experimental results, and good agreement was observed.

This paper is intended to analyse the post buckling behavior of annular plate under the effects of symmetric uniform loading using Rayleigh-Ritz method. This is done first by formulating the problem in large defection using Von-Karman nonlinear theory. The geometry in natural coordinate system using the correct and useful mapping and the development of displacement fields in natural coordinate system using Hierarchic, Hermitian and Lagrange shape functions respectively in interpolation of out-of-plane and in-plane displacement field of plates are also considered. Finally, we deal with thepresentation of a proper Hookian displacement field for controlling post buckling behavior of circular plates. To solve the problems, this methid enjoys a number of advantages including continuity in stress and strain, few numbers of degree of freedom and application of bounder easy conditions.,

A major part of the flow field has no complicated turbulent behavior in many turbulent flows. In order to reduce memory and CPU time, the flow field was decomposed to several blocks in which different turbulence models were employed. A two dimensional backward facing step was considered in this research. Four combinations of the Prandtl mixing length and standard k-ε models were implemented. In addition to the numerical convergence and accuracy of the results obtained, computer memory usage and CPU time consumption were investigated. Comparisons showed that employing a suitable combination of the turbulence models for individual blocks led to the results which were as accurate as those resulting from the application of high order turbulence model for all of them. The results revealed that the memory usage and CPU time were considerably decreased.

In this investigation, the thermal and residual stresess distributions developed during the circumferential butt gas tungsten arc welding (GTAW) of Incoloy 800H pipes were simulated using the finite element method. A coupled thermostructuralmodel was developed in three dimensions using ANSYS software. A subroutine based on the Goldak model was addedto the model to simulate the distribution of arc heat source. The plastic behaviour of the material was described by Von-Mises yield function and the bilinear kinematics hardening in the mechanical part of the model, which is sequentially coupled with the thermal one. In order to validate the coupled model, temperature distributions during welding and residual stresses distributions in the welded pipes were both measured using thermocouples and hole drilling method, respectively. Using this model, a process parameter study was performed for the process variables such as welding efficiency and the amount of heat input in each pass.The results showed a tension-compression distribution of the axial stresses through thickness from inside to outside surface of the pipe and a tensile distribution for the hoop stresses. Increasing the heat input by 36% increased the axial residual stresses alongthe weld centerline on the inner surface of pipe to 11%. The present simulation model is able to predict distributions of temperature and residual stresses during the GTAW process with a reasonable precision.

This paper is focused on the results of the experimental investigation and numerical analysis for changing the precast RC beam-column connections with corbels to flexural connections using fiber reinforced polymers (FRPs). The experimental study has a special focus on the flexural strength and debonding effect of boundary layer of FRP sheets attached onto the corbels and other parts of precast connections. In the current study, four half-scale external beam-to-column connections including one conventional fixed connection, and three simple precast connections with corbels strengthened withFRP sheets were casted and tested. The FRP sheets were attached in different layouts with different thicknesses. Static concentrated loads were applied on the tip of the beam simultaneously with a constant axial load on the column, and were continued up to ultimate failure. To fulfill the numerical phase of this study, non-linear static analysis on the 3-D model of the connections were performed using Ansys 8.1. A comprehensive comparison was then performed between different strengthening methods with FRP sheets using different FRP configurations, different thicknesses and dimensions, also using full mechanical anchorages. The results showed that simple precast connections could be changed to flexural connections using FRPs, whereas the sufficient development length and thickness, configuration of FRP sheets, and suitable mechanical anchorage are quiteeffective to prevent the FRP debonding. The test results of this study showed that the suitable use of FRP sheets increased the flexural capacity of a simple precast joint more than 80% of that of a full flexural RC joint.

Hammers or presses commonly used in modern manufacturing facilities produce remarkable vibrations during their operation. Foundations supporting these machines experience powerful dynamic effects which may extend to the surrounding areas and affect workers, sensitive machines within the same facility, or the neighboring residential areas. To control free-field impact-induced vibrations, wave barriers may be constructed in the vicinity of the vibration source. This paper examines the efficiency of soft barriers (open trenches) on screening pulse-induced waves for foundations resting on a deep elastic layer or a layer of limited thickness underlain by rigid bedrock. The effectiveness of open trenches as wave barriers is examined for different cases of soil layer depth, arrangement for double trenches, and trench location.A 2D finite element model is constructed and some parametric analyses are performed in the time domain. Different arrangements of wave barriers in vibration isolation for shock-producing equipment are assessed for their efficiency. The results show that using the second trench could remarkably reduce the vibration amplitude. Finally, some guidelines are defined for their use.