Air induced defect formation during powder compaction

Publication Reference: 
Author Last Name: 
William Zunker and Ken Kamrin
Report Type: 
ARR - Annual Report
Research Area: 
Particle Formation
Publication Year: 
United States

The focus of the report is on development of a contact model for usage in CFD-DEM simulations. Great effort is placed in this step since it provides a basis for all future results upon which the continuum model will be built.

We present a contact model able to capture the response of interacting adhesive elastic-perfectly plastic particles under a variety of loadings. The model is built upon the Method of Dimensionality Reduction which allows the problem of a 3D axisymmetric contact to be mapped to a semi-equivalent 1D problem of a rigid indenter penetrating a bed of independent Hookean springs. Plasticity is accounted for by continuously varying the 1D indenter profile subject to a constraint on the contact pressure. Unloading falls out naturally, and simply requires lifting the plane indenter out of the springs and tracking the force. By considering the incompressible nature of this plastic deformation, the contact model is also able to account for the nonlocal effects of neighboring contacts, including formation of secondary contacts from outward displacement of the free surface. JKR type adhesion is recovered easily by simply allowing the springs to ‘stick’ to the 1D indenter’s surface. Additionally, we account for the rapid stiffening in the force-displacement curve under high confinement (e.g. during powder compaction) by triggering a superimposed bulk elastic response based on a simple criterion related to contact area. Given that the model arises from rigorous contact mechanics formulations and simple geometric arguments only a few physical inputs are needed to run the model. Namely, the average radius of the particles Ro, Young’s modulus E, Poisson ratio ν, yield stress Y , and effective surface energy Δγ. An outline of the numerical implementation of the model is included. Every aspect of the contact model is validated, either through comparison to finite element simulations or in the case of adhesion directly to the JKR theory of adhesion. These comparisons show that the proposed contact model is able to accurately capture plastic displacement at the contact, average contact stress, contact area, and force as a function of displacement under a variety of complex loadings. This gives us confidence in the predictive capability of the contact model and its ability to reflect reality when used in the upcoming CFD-DEM simulations.