Unloading of powder compacts and their resulting tensile strength

We examine the unloading of an agglomerate of perfectly plastic spherical particles that have been compacted beyond their elastic limit and that exhibit some adhesion. The behaviour at the contact scale is derived from the model of Mesarovic and Johnson for the decohesion of two spherical particles [1]. The resulting springback that the powder compact experiences is calculated using the Discrete Element Method. An analytical equation for the extent of springback is proposed and coincides well with simulation results. The alteration of the compact during unloading is investigated through contact loss and the decrease of the contact area. The tensile strength of the resulting compact is calculated in isostatic conditions. While isostatic conditions represent the basis of this work, the effect of closed die conditions on the anisotropy of springback is also investigated. Finally, the consequences of adding hard elastic particles to form a composite are analysed in terms of the springback extent and the resulting loss of tensile strength of the agglomerate. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.