Yielding Behaviour in Strain Controlled Compactions of Metal Powder Assembles.

Abstract

Metal powder assembles show some volumetric change under irreversible deformation, and their yield function (yield curve) is represented in the space of equivalent deviatoric stress q and hydrostatic pressure p. In the mechanics of metal powder assembles yield curves are assumed to be contours of constant density. However, there is something questionable in this concept. We already examined the consistency of the concept referring to the experimental data of tri-axial compression tests carried out in the past by other investigators, who compressed cylindrical specimens subjected to constant lateral pressure. The results showed that the concept of constant density was inconsistent with the actual constitutive behaviour under the assumption that associated flow rule is applicable and yield curve expands in a similar shape with increasing of compressive deformation. Here, we newly referred to the data obtained from compaction tests by Brown and A-Chedid who collected them using uni- and tri-axial strain control systems. In these compaction tests the direction of stress vector (p, q) do not change largely, and so whole view of yield curve is not obtainable. But the correspondence between directions of stress and strain rate vectors can be determined in the compactions. The comparison of the results with those in compression tests does not show good agreement. That suggests the necessity of inspection on the suitability of constitutive model used in the mechanics of metal powder assembles. Hardening strain rate ey is defined here which is composed of volumetric strain rate ep and equivalent deviatoric one eq accompanying with constant parameter. Finally, it is found that this definition is valid from the view point of inherent characteristic of hardening.