Yielding of metal powder bonded by isolated contacts

A macroscopic constitutive law is developed for the plastic yielding of a random aggregate of perfectly plastic spherical metal particles. The particles are bonded perfectly by isolated contacts and deformation occurs by plastic yielding of material at and near these contacts. The configuration is treated as isotropic and homogeneous as far as particle size and properties are concerned. The results are considered valid for aggregates with densities ranging from about 60% to around 90% of the theoretical fully dense level. The yield surface is obtained from the plastic dissipation at necks between particles given an imposed macroscopically uniform strain rate. The contact yield surface resulting from this analysis is sensitive to pressure as well as to deviatoric stress. The plastic strain rate direction is outwardly normal to the yield surface. Densification takes place when pressure is present, but a notable feature is a vertex on the yield surface at the points of pure positive and negative pressure. Consequently, plastic flow in the presence of pure pressure is nonunique, and deviatoric components may be superposed on densification.