(205g) Anisotropic Mechanical Properties of Compacted Powders with Cohesive Contacts

In this work the multi-particle finite element method (MPFEM) is used to simulate the mechanical properties of compacted powders with cohesive contacts. The mechanical properties under consideration are the elastic properties (Young’s modulus, shear modulus and Poison ratio) and the yield properties (uniaxial yield strength, yield surface). The MPFEM model consists of a representative volume element (RVE) of monodisperse, spherical, deformable particles which are compacted during simulation. A simple contact model is used to describe the interaction of particles. The contact model includes repulsive forces, friction forces and cohesion forces and is therefore suitable to describe tensile strength of powders in addition to the compression strength.

The whole MPFEM simulation is divided in the compaction step and the testing step. In the compaction step, a RVE of randomly closed-packed spheres is compacted to a defined packing density in the range of 0.65 to 0.95. Different compaction modes are used to evaluate the influence of the strain path during compaction. In addition to the two most common compaction modes isostatic and closed die, five extra modes with different strain paths are considered. The strain path during compaction is described by means of the ratios of the three main principal strains and the average strain (, , ). The strength of the contact cohesion is varied to investigate the whole range from non-cohesive powder to completely bonded powders. After compaction the powder is unloaded to obtain the reference state for the testing step.

During the testing step the RVE of compacted powder is reloaded to determine its mechanical properties. First of all, the RVE is loaded in particular normal and tangential directions while the deformation is recorded to determine the elastic properties. Then the uniaxial yield strength for compression and tension is determined in x-, y- and z-direction. Finally, the RVE is stressed in many different arbitrary directions to yield, in order to generate the whole yield surface of the compacted powder.

The results of the simulations show the mechanical properties of compacted powder as function of the packing density after compaction, the cohesion strength of the contacts and the strain path. Algebraic equations are derived to condense the simulation results of the parameter study. A continuous mathematical description of the mechanical properties is given as function of the relative density, the contact cohesion strength and the three strain ratios during compaction. The equations can be used to calculate the mechanical properties of compacted powder for arbitrary packing densities, contact cohesion and strain paths.

The MPFEM model in this work mechanistically describes the anisotropic properties of compacted powder. The derived equations for the mechanical properties can be used in a future work as input data in a conventional FEM model of powder compaction. In this way the spatial distribution of the powder properties inside a whole part of compacted powder can be investigated. This way, the stability of parts can be investigated for different powder properties and different compaction conditions.