Measurement of friction under sheet forming conditions
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INTRODUCTION
EXPERIMENTAL die tryout has been traditionally used to ensure formability of a given sheet metal part. However, because of high costs and long times required, this practice has been minimized by the use of finite element modeling (FEM). The accuracy of such modeling is limited by the knowledge of boundary conditions, material properties, and process parameters. The use of inaccurate friction coefficients, for example, can have a more profound effect than most material properties in the FEM simulation.[1,2,3] Comparisons among simulation and experimental results show that the lack of quantitative friction values under realistic forming conditions is a serious deficiency in the current stage of forming simulation.[4–8] The friction between the workpiece and tooling has an important effect on the material flow, the strain distribution, and the forming forces in sheet forming operations.[9–16] It also takes part in determining the forming failures (e.g., tearing and wrinkling) and their locations.[17] Studies of asperity deformation under various contact pressures[18–24] have been conducted to determine the microscopic friction models in forming processes. The results indicate that the applicable friction model depends on contact pressure. In sheet metal forming, the contact pressure between the workpiece and tooling is relatively low, and the frictional interaction is carried out through asperities.[25] A comparison between experiment and calculation in the drawbead simulation test[13] showed that a constant friction coefficient model could satisfactorily represent the frictional behavior W. WANG, formerly Graduate Student, Department of Materials Science and Engineering, The Ohio State University, is Project Engineer, Research and Development Division, Inland Steel, East Chicago, IN 46212. R.H. WAGONER, Professor, is with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210-1179. X.-J. WANG, Professor and Chair, is with the Department of Metal Forming, University of Science and Technology Beijing, Beijing 100 083, People’s Republic of China. Manuscript submitted September 30, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
at low and intermediate pressures but not necessarily at high pressures. Because frictional force is known to be a complicated function of material properties and process parameters, it is unrealistic to expect a single test to represent friction for a range of forming processes. In order to understand friction and assess frictional values in FEM simulation, tests should be carried out under conditions close to real production processes. Friction testing techniques for sheet forming can be classified into two groups. The first involves significant sliding and concurrent thickness reduction, usually by through-thickness compressive loading with little or no bending.[26–29] These tests are usually used to simulate plane-strain drawing, ironing, and extrusion. The second group involves a workpiece that is stretched and bent during the e
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