CONTINUUM MODELS WITH RUPTURED/RESTORED MICROSTRUCTURE
A. I. Leonov
Department of Polymer Engineering, The University of Akron, OH
Abstract
Many multi-phase systems with small attractive colloidal particles
display a pec uliar rheological/mechanical behavior when the particle
concentration is above a certain gelation (or a percolation)
threshold. A simplest class of such systems is dispersions where solid
colloidal particles are dispersed in a low molecular weight
fluid. Because of the inter-particle attractive interactions,
dispersion s can create a particulate network, which is usually
ruptured in flow with forma tion of flocs, and restored again at
rest. The filled polymers are another examp le of such a system. They
include a broad variety of cured and uncured rubber co mpounds
employed in rubber and tire industries. Here, depending on the type of
p olymer and filler, a dominant physical bonding can be established
either between small particles of filler or between filler particles
and polymer matrix. Again , this secondary network existing at rest
can be destroyed by stresses, with a l ong restoration after
unloading. The third class of the phenomena to which the g eneral
approach is applied is the elasto-viscoplasticity in metals. Here,
beyond a critical (yielding) level of stresses, a sharp and time
dependent transition from elastic behavior to plastic flow occurs in
active loading, caused by the sl iding of metallic grains along
multiple dislocation lines. Also, a long time str ess-strain
relaxation with restoration of structure, hardening and stress
localization, and frozen memory effects happen in metals. All these
systems demonstrat e such interactive phenomena as yielding,
thixotropy, nonlinear viscoelasticity, frozen memory, and stress
localization. A continuum approach based on qualitative
micro-structural physics and t hermodynamic arguments has been
elaborated for modeling these phenomena. It oper ates without any
yield criteria but predicts yielding as a bifurcation type tran sition
from solid- to liquid-like behavior.
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