The use of the finite-element method to design an optimized tool for the plain-strain punch stretching test
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I. INTRODUCTION
THERE are several types of experimental and simulative tests that duplicate well each deformation mode occurring in the stamping process, as discussed in Reference 1. Among these simulative tests, the Erichsen cup test, Swift cup-drawing test, and hemispherical punch-stretching test[2] are frequently used to evaluate material formability. However, these test methods sometimes produce poor data, reproducibility, and unrealistic evaluations, especially for zinc-coated steel sheets, which are widely used in car bodies for their excellent anticorrosion properties. In order to successfully obtain the desired shape of a panel, it is necessary to avoid forming failures such as local necking and shape inaccuracy while assuring the stamping formability of sheet materials. Recently, two simple and effective test methods have been developed to evaluate the stamping formability of sheet materials. One is the new Ohio State University (OSU) formability test,[3,5] and the other is the Kim’s plane-strain stretching (PSS) test.[6–9] In the OSU formability test, a long, rectangular specimen is uniformly stretched under the action of a long, cylindrical punch of small radius until the failure of the specimen occurs in the plane-strain deformation mode. Some finite-element simulations were performed to investigate the effect of the tool geometry and the specimen size on the deformation characteristic.[4] Thereby, the optimum condition of these parameters was ascertained. However, concerning the PSS test, the adoption of a specially designed, semicylindrical punch is used to assure a plane-strain stretching deformation at the overall area of the rectangular specimen with a proper width.[6] In the original PSS test, a specimen width of 126 mm and a punch geometry of 70 mm in width, 70 mm in length, and corner radius of 5 mm yields a plane-strain deformation of the specimen for various automotive sheet materials from high-strength steel sheet to aluminum sheet. The limiting-punch-height (LPH) YOUNGSUK KIM and YOUNGDOO KWON, Professors, are with the Department of Mechanical Engineering, Kyungpook National University, Taegu 702-701, Korea. JAEBOK NAM, Senior Researcher, is with the Sheet Products and Process Research Team, Pohang Iron and Steel Company (POSCO), Pohang 790-330, Korea. MANSEOK CHU, Professor, is with the Division of Electronic and Mechanical Engineering, Donseo University, Pusan 617-716, Korea. Manuscript submitted April 1, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
value measured at plane-strain fracture is used as a measurement of material formability. The prototype tool geometry of the original PSS test was determined by repeated experimentation. In the conventional PSS test, with a high punch speed of 300 mm/min, a fracture occurs across the specimen concurrently both at the center of symmetry and near the punch-corner area. However, in a special testing condition with a relatively low punch speed, the fracture seems to occur near the punch corner first and successively propagates to the center o
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