Determination of flow curves by means of a compression test under sticking friction conditions using an iterative finite-element procedure

摘要

A common method for the determination of flow curves is the application of a compression test.Using this method, friction in the interface between the die and the specimen leads to a bulging of thesample and thereby to an inhomogeneous stress and strain state. The calculation of the flow stressfrom experimentally determined force-displacement curves implies a uniaxial stress state, but this willproduce an error because of the above-mentioned bulging, when friction occurs. One method ofavoiding these sources of error is to minimize friction, e.g. by the use of lubricants together withuseful geometries of the samples. Another strategy, described in this paper, applies sticking frictionconditions during the testing, the calculation of the flow curve being done by the use of an iterativeprocedure, applying a corrective function. This corrective function can be calculated by a finiteelement (FE) analysis of the upsetting test. It will be shown that the first iteration gives adequateresults, that the corrective function itself depends on the shape of the specimen and that thecorrective function is not dependent on the hardening behaviour of the material, which means that ifone sample geometry is used, the corrective function itself need not be calculated for every test.