A Coupled Crystal Plasticity and Anisotropic Yield Function Model to Identify the Anisotropic Plastic Properties and Fri
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UCTION
THE frictional behavior between a workpiece and forming tool is considered a critical issue regarding the determination of dimensional accuracy of the final product and the defects that occur during the bulk-metal forming process. In addition, friction is closely associated with the load capacity of forming machines, and thus influences the design and lifespan of tools. Numerous test methods have been proposed to determine such frictional behavior. Since initial studies by Kudo[1] and Hawkyard and Johnson,[2] the ring compression test has been the most commonly used due to its simplicity.[3–5] However, the simple deformation that occurs during the ring compression test does not practically represent bulk metal forming. Moreover, the
HYUK JONG BONG is with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210. DOHYUN LEEM is with the Department of Mechanical Engineering, Northwestern University, 2145 Sheridan road, Evanston, IL 60208. JINWOO LEE is with the Materials Deformation Department, Korean Institute of Materials Science (KIMS), Changwon, Gyeongnam 51508, South Korea. JINJIN HA is with the Department of Mechanical Engineering, University of New Hampshire, 33 Academic Way, Durham, NH 03824. MYOUNG-GYU LEE is with the Department of Materials Science and Engineering, Korea University, Seoul 02841, South Korea. Contact e-mail: [email protected] Manuscript submitted January 2, 2017. Article published online November 20, 2017 282—VOLUME 49A, JANUARY 2018
amount of free surface generated during the test is insufficient compared with the actual bulk metal forming. Thus, the friction coefficient determined from the ring compression test is often incompatible with that of actual forming applications. To overcome the disadvantages associated with the ring compression test, several alternative test methods have been devised, which include spike forging,[6] backward extrusion tests,[7,8] combined forward and backward extrusion methods,[7,9] and injection upsetting methods.[10] These tests are considered more suitable for determining the friction coefficient under harsh deformation conditions with a large amount of free surface generation. However, these methods require a non-linear calibration curve to determine the friction coefficient and complex test apparatuses. Recently, a new method, known as the ‘‘tip test,’’ was devised to determine the friction coefficient on the basis of the backward extrusion process.[11] During the tip test, a radial tip is formed on the extruded end of a circular billet. Intensive experimental and finite element (FE) analyses revealed that the radial tip distance is first-order dependent on the friction coefficient of the interface between the tool and workpiece.[11–18] In the tip test, the friction coefficients are determined using an inverse FE approach. Specifically, the best-fitting friction coefficients are iteratively determined via a trial-and-error approach, which reproduce the experimentally measured radial tip distance. Since the tip test
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