A. E. Tekkaya

An Improved Relationship between Vickers Hardness and Yield Stress for Cold Formed Materials and its Experimental Verification

Abstract Cold formed products are increasingly serving as high duty machine parts. Designers and users need to know their properties as accurate as possible. One such product property is the new yield stress, which can be approximated by the final flow stress of the workpiece material during forming. Vickers hardness measurements provide an easy and inexpensive method for evaluating the new local yield stress in cold formed workpieces. In this study, an improved relationship for the conversion of Vickers hardness values to yield stress is proposed. The agreement between theoretical and experimental results is better than 4% error.

Determining Stress-Strain Curves of Sheet Metal in the Plane Torsion Test

The plane torsion test has been studied for determining the flow curve k f (φ) of sheet metal. This test was first proposed by Marciniak and Kolodziejski for a determination of the n-value assuming Ludwiks law for the flow curve. In the present work, a new procedure is proposed by which k f (φ) can be determined allowing for deviations from the Ludwik equation. A new test apparatus and limits of application are described. First experimental results are reported, and it is shown that k f (φ) can be determined up to much higher strains than is possible by tensile tests.

Residual Stresses in Cold-Formed Workpieces

Every workpiece contains residual stresses which can affect its performance in service. Especially metal-forming processes leave high residual stresses in the workpiece because of the inherent nonuniform plastic deformation pattern. The knowledge of the magnitudes of these stresses in such workpieces is of prime importance for the industry in order to evaluate their influences and particularly to optimize the manufacturing process with regard to a convenient residual stress state. For this purpose, in this study several cold metal-forming processes have been analysed regarding the residual stresses left in the products, such as rod extrusion, containerless extrusion, tube extrusion and upsetting. The trend to evaluate convenient process parameters for a manufacturing process optimization amounts to numerical process simulations, because they become more and more economic compared to protracted experimental methods. Hence, this analysis has been basically conducted with the Finite-Element-Method. For the rod extrusion case also an experimental study has been performed in order to compare measurements with computations. The agreement is satisfactorily. As a result of the analysis, two ways of manipulating the residual stresses in extrusion-processes turn out to be as: Varying the cone-angle and/or varying the die-stiffness.

Service life estimation of extrusion dies by numerical simulation of fatigue-crack-growth

Abstract In this study, fatigue behavior of cold extrusion dies is investigated analytically. Experimental studies and practical experience have shown that fatigue cracks occur at the inlet radius of die shoulders. The highest stress concentration is found also at the same location. For this reason, effective stress intensity factors at different locations of the die-shoulder inlet of a typical axisymmetric extrusion die have been calculated by using the finite element method. The crack growth has been simulated by applying the Paris/Erdoĝan fatigue law to the computed data. Finally, service life of the extrusion die has been estimated from the crack-growth-rate. Agreement of theoretical estimations with available data is found to be satisfactory within the limits of experimental uncertainties. Furthermore the experimental behavior of the crack growth (stable-unstable-stable growth with final fracture) is simulated correctly. It has been shown that the unexplained behavior is caused by decrease of stress concentration with increasing crack-length.

Complete Numerical Solution of the Axisymmetrical Deep-Drawing Problem

The present study introduces a finite difference method to obtain the complete numerical solution of the axisymmetrical deep-drawing of a circular blank. The theory includes nonlinear strain-hardening, normal plastic anisotropy, prestrain, variable coefficient of friction as a function of deformation, an approximation to the thickness stress, and an instability criterion. Using a computer program, distributions of any variable such as stress or strain can be obtained at any stage. Theoretical solutions are in good agreement with experimental ones. The method of solution slightly overestimates the limiting drawing ratio, which can be improved by adding the effects of bending and unbending at the die.

Life Estimation of Extrusion Dies

Abstract The fatigue behaviour of cold extrusion dies is investigated. The effective stress intensity factor at different locations of the die inlet of a typical axisymmetric extrusion die has been calculated by the finite element code method. Applying the Paris/Erdogan fatigue law to this calculated data, the crack growth has been simulated. From the crack-growth-rate the life of the extrusion die has been estimated. The agreement with the experimental results is satisfactory within the limits of experimental uncertainties. Furthermore, the experimentally observed behaviour of the crack growth, i. e. stable-instable-stable growth with final fracture, is simulated perfectly.

Simulation of Metal Forming Processes

The term process simulation describes all methods by which one or more of the process parameters of a real physical process or process family is or are predicted approximately before its or their actual happening, [7.1]. The aim of the determination of these parameters in case of metal forming processes is usually one or more of the following: Checking the feasibility of the process design for producing a workpiece, Evaluating the product properties for service use, Increasing the insight about the real process in order to optimize the production sequence. The simulation in production processes aims to manufacture products economically. Therefore, the application of process simulation must be always more economical than the application of the real process.

Testing The Plastic Behaviour of Bars and thin Sheet by Torsion Tests

The (cold and hot) torsion test on cylindrical bars and the recently developed (cold) plane torsion test on thin sheet are discussed with respect to their communi ties and differences. In torsion of bars the test results are conventionally used for calculating stress and strain at the specimen surface. However, since the material properties are distorted at the surface it is recommended to calculate stress and strain at a “critical radius”, where shear stress at a given torque is almost 1ndependent of the shape of flow curve: so it can be calculated with good accuracy by a new series expansion method. The plane torsion test 1S evaluated in similar way. Sources of error as well as the uncertainty of the yield criterion are discussed.

Non-conventional extrusion of less-common materials

Abstract The theoretical background of metal forming is improving rapidly, finite-element-based methods, especially, giving an important impact in this area. Despite these developments, the production of difficult — and hence industrially interesting — parts still requires a lot of experience and ingenuity. Particularly, cold and warm extrusion is a domain of metal forming in which theoretical methods cannot aid the production of extrudates that are either of complex shape or made of less-common materials or even both. The present paper aims to give mostly unknown technological details of the production of such workpieces. The described know-how is the outcome of approximately 300 man-years experience gathered at the Institut fur Umformtechnik of the Universitat Stuttgart. The examples given represent the state-of-the-art of extrusion technology.