University of Minnesota
Aerospace Engineering and Mechanics
Spring 1999 Seminar Series



Computer Simulation of Particle Breakage


Professor Charlie Campbell

Department of Mechanical Engineering, University of Southern California


Abstract


The first part of this talk will describe a computer simulation technique that is used to model shattering (involving many cracks propagating simultaneously) of solid materials. The technique was developed primary to study problems in comminution (the intentional grinding of large solid particle down to fine sizes) and attrition (the loss of useable materials by unintentional breakage) although it may be useful in studying other breakage situations. In most such situations, the breakage comes about in some sort of flowing environment. This model of developed from techniques used to study the flow of unbreakable solid particles and thus naturally lends itself to studying these types of problems. At its heart, this technique creates a simulated solid on which the breakage occurs. The solid is simulated by "gluing" together unbreakable polygonal (in 2-D) and polyhedral (in 3-D) elements. The "glue" may be thought of as a compliant connection that can only withstand a specified tensile stress. When that tensile stress is exceeded the glue breaks and a crack propagates along the interelement connection. Afterwards the broken edge will still act in a compliant manner if contacted by another solid surface (such as the contact with another fragment or the by the closing of a partially opened crack) but will no longer be able to withstand tensile forces. If the interelement compliance is linearly related to the displacement, the composite can be shown to behave as a linearly elastic material with predictable elastic constants. The model has been shown to accurately predict experimental fragment size distributions, at least for fragments that are somewhat larger than an element. This model was first used to study the impact breakage of round particles and those studies will be described in some detail. It will be shown that the generally observed breakage patterns can be understood as the results of two types of breakage. In Type I breakage, cracks propagate along lines perpendicular to the maximum tensile stresses that would be generated in an unbroken solid. However, late in the breakage process, cracks are observed to propagate in a direction perpendicular to those maximum stress lines. Such Type II cracks will be shown to result from tensile stresses generated by the buckling of the fragments produced by the Type I breakage. Time permitting, some simulations involving combined flow and breakage will be described. The most recent of this work is series of simulations that may be thought of as simplified ball milling. In this case a large unbreakable grinding ball falls onto the surface of a bed of breakable round particles. The impact causes both the breakage and scattering of the particles within the bed. The studies show that the "efficiency" of this process is strongly dependent on the interparticle friction due to the manner in which the friction controls the flow of particles around the grinding ball as it descends.

Friday, May 28, 1999
209 Akerman Hall
2:30-3:30 p.m.


Refreshments served after the seminar in 227 Akerman Hall.
Disability accomodations provided upon request.
Contact Kristal Belisle, Senior Secretary, 625-8000.