An Improved Experimental Method for Determining the Workability Diagram
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An Improved Experimental Method for Determining the Workability Diagram S. Alexandrov & D. Vilotic & Z. Konjovic & M. Vilotic
Received: 26 February 2012 / Accepted: 5 September 2012 / Published online: 21 September 2012 # Society for Experimental Mechanics 2012
Abstract The workability diagram is often used for ductile fracture predictions in metal forming processes. Its determination requires the measurement of the strain to fracture in several tests in which the triaxiality ratio is supposed to be known and fixed throughout the process of deformation. One of such tests is the uniaxial tension test. In practice, however, it is very difficult (or even impossible) to keep the triaxiality ratio fixed in this test because of necking. It is shown in the present paper that the uniaxial tension test can be replaced with the collar test for obtaining a point of the workability diagram when the fracture criterion based on an average value of the triaxiality ratio is adopted. It is also shown that the collar test provides a more accurate prediction of the strain to fracture. The test carries out at quasistatic strain rates. Keywords Collar test . Tensile test . Necking . Workability diagram . Ductile fracture metal forming
Introduction Empirical ductile fracture criteria are often adopted to predict the fracture initiation in bulk metal forming processes [1]. The approach based on the forming limit curve usually
S. Alexandrov (*) A.Yu. Ishlinsky Institute for Problems in Mechanics, Russian Academy of Sciences, 101-1 Prtospect Vernadskogo, 119526 Moscow, Russia e-mail: [email protected] D. Vilotic : Z. Konjovic : M. Vilotic Faculty of Technical Sciences, University of Novi Sad, Novi Sad, Serbia
used in sheet forming (see, for example, [2]) is not considered in the present paper. Some of empirical ductile fracture criteria are based on the workability diagram [3, 4] which is a mathematical relation between the strain to fracture and the triaxiality ratio assuming that the latter is fixed throughout the process of deformation. Tests to obtain a measure of workability are stopped once the initiation of fracture has occurred at any location and the subsequent process of fracture is not investigated. Conventional tests for determining the workability diagram are the uniaxial tension test, the uniaxial compression test, and the torsion test [3]. The theoretical values of the triaxiality ratio in these tests based on the assumption of uniform states of stress and strain are 1, −1 and 0, respectively. In practice, however, it is very difficult (or even impossible) to satisfy the theoretical requirements in the case of tension and compression tests because of necking and barreling, respectively. The Rastegaev method is often used to significantly reduce friction and, therefore, barreling in the compression test [5, 6]. When the fracture criterion based on an average value of the triaxiality ratio is valid [7, 8] and fracture initiates at a traction free surface, a much simpler method has been proposed in [8]. According t
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