Sharif Digital Repository

The purpose of this work is to introduce an inverse finite element tool for prediction of the blank geometry and forming severity in stamping of 3D sheet metal parts. To achieve this goal total deformation theory of plasticity has been employed. In this method the final 3D part is unfolded on the flat surface and therefore the problem is reduced to two dimensional computations. Some tooling effects including blank holder force, bending and unbending and effect of draw bead are also considered. To investigate the capability of this work, predictions of the blank shape and strain distribution and forming severity have been performed on two circular and square parts. The results have been... 

Cold roll forming is a process for manufacturing of long sheet metal products with constant cross sections. The base of this process is bending and formation of plastic deformation in sheet. In this process, bending operation occurs in several forming stations. The purpose of this project is estimation of the distance between stations in roll forming. This work is based on determination of deformation length. In this study, first, the researches done in the roll forming is expressed. Then, a relation for estimation of deformation length has been derived that could be used in different stages of roll forming. Compared with other works, the work hardening effect has been considered in the... 

The inverse finite element method was derived from ideal forming to predict the development blank of sheet forming process and the strain distribution according to the geometry of final product. In this project we present an inverse finite element method to obtain strain and stress distribution in final shape and design initial blank of deep drawing process. The procedure is as follows: First, strain distribution in deep drawn part is estimated by kinematics. Then by using Hill’s anisotropic plasticity and according to the associated plastic flow rule, stress distribution is obtained in deep drawn orthotropic part.Finally, initial blank is designed by considering external forces between... 

In designing of sheet metal forming processes, knowledge of the maximum permissible strains in the sheet is of prime importance for the design engineer. Thus, a large amount of experimental and theoretical researches have been carried out to estimate these limiting strains. A renowned and prevalent theoretical method for prediction of forming limits is the Marciniak and Kuczynski model. In this model, it is assumed that a small groove exists in the sheet even before applying the loads; and due to the weakness of this region, necking starts from there. The major problem of the M-K model is requiring of an experimental point to “calibrate” the results. Furthermore, while experiments indicate... 

In the classical theories of plasticity, plastic deformation of material is modeled with a macro- scopic viewpoint and without considering the existing physical mechanisms at the crystal scale. However, plastic deformation stems from slip phenomena and evolution of crystalline structure. Furthermore the effects of initial texture of material on its mechanical properties, like formability, and also texture evolution of material can only be calculated by considering these physical mecha- nisms.The main purpose of this thesis is to predict forming limit diagrams of sheet metals based on crystal plasticity approach by considering the effect of initial material texture and presentation of... 

This study presents an image processing technique for automatic strain and stress measurement in sheet metals. In order to measure the strain automatically, the sheet blank is electrochemically etched to create circular and rectangular grids. After press forming of the sheet, circular grids are deformed into elliptical grids and rectangular grids are deformed into quadrilateral grids. Different photos are captured for using in image processing technique. These photos are taken in different positions. Finally strain can be measured by processing these photos and the stress is obtained by using Levy-Mises flow rule. In this study, three methods of image processing technique are simultaneously... 

Sheet metals are widely used in different industries such as ship building and pressure vessels. In these industries, One important problem is Line Heating Forming. This method needs simple initial equipment and is economically advantageous.In this research heat transfer between gas torch and plate, cooling effects and edge effects are studied more precise. Impingement jet model is used to simulate effect of heat source (flame) and cooling air on plate. The temperature distribution in sheet metal due to the rates of cooling air and heated gas (flame) are calculated by using commercial engineering software, Fluent. Then, the computed temperature field by FLUENT is fed into the ANSYS FEM... 

In this dissertation, using experimental and numerical methods, the effects of grain morphology on the formability of sheet metals are investigated. In this regard, in the first stage, by conducting heat treatment on two different sheet metals with bcc and fcc crystal structures, different grain sizes are produced. Using metallographic observations, these grain structures are examined and the grain sizes are measured. Tensile test and Nakazima formability test have been implemented on these specimen. The grains are also shaped in the rolling process and different grain morphologies are created. In the second stage, a software has been developed to generate various grain structures and... 

Cyclic Contraction Expansion Extrusion (CCEE) is a new Severe Plastic Deformation (SPD) method used to refine the microstructure and improve mechanical properties of Magnesium AZ91 alloy. In this study, different CCEE process zones are investigated via optical microscopy to understand this process better. The average grain size in the contraction zone decreased to 8.5 microns from an initial value of 106 microns. In the expansion zone, the average grain size was equal to 4.8 microns. Microhardness tests were conducted to study the improvements in mechanical properties. Microhardness of the sample increased from the initial value of 66 HV to 74.4 HV in the contraction zone and 88 HV in... 

One of the common methods of removing blood clots is angioplasty with stents. Due to the fact that the mechanical properties of the stent are directly related to the microtube it is made of. Therefore, investigating the mechanical properties of microtubes is very important. One of the ways used to evaluate the plasticity of microtubes; Drawing a graph is the limit of shaping. The purpose of this research is to extract the forming limit diagram of magnesium microtube by simulating the hydroforming process in Abaqus software. First, in order to confirm the modeling, the hydroforming process for aluminum has been simulated and compared with the existing experimental results. The comparison... 

Properties of stents are significantly dependent on the mechanical properties of the microtubes used in their construction. Therefore, investigating the influence of various factors on the mechanical behavior of magnesium microtubes employed in the fabrication of biodegradable stents can play an indispensable role in the development of the emerging industry of biodegradable stent manufacturing. In this dissertation, the effects of different parameters on the mechanical behavior of magnesium microtubes used in the construction of biodegradable stents have been examined. To achieve this, the objectives of this dissertation have been divided into three main sections. In the first section, a... 

The purpose of this work is to introduce an inverse finite element tool for prediction of the blank geometry and forming severity in stamping of 3D sheet metal parts. To achieve this goal total deformation theory of plasticity has been employed. In this method the final 3D part is unfolded on the flat surface in one step and on predefined surface in multistage analysis. The problem for one step analysis is reduced to two dimensional computations. The effect of curvature of initial blank and the number of stages in forming process are also considered. To investigate the capability of this work, predictions of the blank shape and strain distribution and forming severity have been performed on... 

In this study, the forming limit stress and strain diagrams have been determined based on the strain gradient plasticity theory. Forming limit stress diagram (FLSD) has been reported as being much less path-dependent and much more favorable than the forming limit diagram (FLD) in representing forming limits in the numerical simulation of sheet metal forming processes. Therefore, the purpose of this study is to develop a methodology for prediction of the forming limits both in strain and stress spaces. All simulations are based on strain gradient theory of plasticity in conjunction with the so-called Marciniak-Kuczynski (M-K) approach. This method introduces an internal length scale into... 

It is the goal of this thesis to develop an analytical model for the hydro-mechanical deep drawing (HDD) process of an axisymmetric sheet metal with the fixed gap method to evaluate the effects of some assumptions such as: proportional loading, plane stress, and constant thickness conditions. The effect of parameters on the HDD process is also studied. The main model is developed with considering the normal stress and part thickness change, non-proportional loading, bending and unbending effects at the top of the cup wall. The interrelationships between geometrical and mechanical variables are obtained in the finite difference form based on the incremental strain theory, thereby being solved... 

The inverse Finite Element method (IFEM) has been used for estimation of the initial length of tube, axial feeding and fluid pressure in tube hydroforming. The already developed IFEM algorithm, which has been used in this work, is based on the total deformation theory of plasticity. Although the nature of tube hydroforming is three-dimensional deformation, a technique has been used to perform the computations in two-dimensional space. Therefore, compared with conventional forward finite element methods, the present computations are quite fast with no try and error process. The solution provides all the components of strain. Using the forming limit diagram (FLD), the components of strain can... 

Knowing the magnitude of the maximum strain before necking is of utmost importance in the processes of designing sheet metals. Thus, numerous experimental and theoretical efforts have been done to answer these questions. In order to predict allowed strains in biaxial stretching, an experimental diagram was introduced called “Forming Limit Diagram” in which major strain is plotted versus minor strain and if the corresponding point to the strain on the sheet is higher than this diagram, necking will occur. One of the most popular methods of obtaining forming limit diagrams is using a hemispherical rod for shaping sheet, which is named as “Nakazima Test” and then the critical strains are... 

In recent years, severe plastic deformation processes have been widely used to correct the grain size of metals. One of these methods is equal channel angular pressing. In this research, the improvement of mechanical property and structural changes of pure magnesium and AZ91 alloy after the ECAP process are investigated. The mold is made of H13 steel with a 90-degree channel crossing angle and a zero-degree external curve angle. The purpose of this study is to find the smallest average grain size at the optimum temperature on these materials. First, pure magnesium and AZ91 are subjected to the ECAP process at temperatures of 250, 300, 350, and 400 °C to calculate the best operating... 

This research presents inverse isogeometric models based on the deformation theory of plasticity to predict the blank and strain distribution in sheet metal forming processes. In this study, the NURBS functions are utilized for both modeling of a final part and analysis of the forming process. Therefore, the developed models require only one modeling and analysis representation, in contrast to finite element models which deal with two separate modeling and analysis representations. In this research, a membrane one-step inverse isogeometric model is initially presented to analyze sheet metal forming processes. This method has various advantages such as solving the governing equations in... 

A broad range of plain carbon steels have ferrite-pearlite microstructures, which are used in many industrial applications due to the good strength and formability provided by the typical combination of ferrite grains and pearlite colonies. The mechanical behavior of ferrite-pearlite steels inherently originates from the configurations of ferrite grains and pearlite colonies, which comprised alternating lamellae of cementite with randomly distributed orientations in a ferrite matrix. In this thesis, three different experimental-numerical approaches were employed to investigate the deformation behavior, damage mechanisms, and formability of the ferrite-pearlite steels. At the first step, two... 

In this research, using the crystal plasticity finite element method, the effects of various microstructural features on behavior of spherical steel sheets have been investigated. The cementite phase ratio, the grain sizes of ferrite and cementite, and the percentage of residual pearlite in the steel structure due to incomplete annealing are the major microstructural parameters studied in this work. A grain generator software has been developed to generate ferrite grains as well as cementite particles distributed in the ferrite matrix. A hard coating with special properties as an intermediate layer around the cementite grains has been considered to simulate the contact between ferrite grains...