DETERMINATION OF FLOW CURVES BY MEANS OF A COMPRESSION TEST UNDER STICKING FRICTION CONDITIONS USING AN ITERATIVE FINITE-ELEMENT PROCEDURE

A common method for the determination of flow curves is the applicatio n of a compression test. Using this method, friction in the interface between the die and the specimen leads to a bulging of the sample and thereby to an inhomogeneous stress and strain state. The calculation o f the flow stress from experimentally determined force-displacement cu rves implies a uniaxial stress state, but this will produce an error b ecause of the above-mentioned bulging, when friction occurs. One metho d of avoiding these sources of error is to minimize friction, e.g. by the use of lubricants together with useful geometries of the samples. Another strategy, described in this paper, applies sticking friction c onditions during the testing, the calculation of the flow curve being done by the use of an iterative procedure, applying a corrective funct ion. This corrective function can be calculated by a finite element (F E) analysis of the upsetting test. It will be shown that the first ite ration gives adequate results, that the corrective function itself dep ends on the shape of the specimen and that the corrective function is not dependent on the hardening behaviour of the material, which means that if one sample geometry is used, the corrective function itself ne ed not be calculated for every test. (C) 1998 Elsevier Science S.A.