Friday, February 11, 2005
1:00pm – 2:00pm
Professor Alan
Needleman
Department of
Mechanical Engineering
Brown University
ÒSize Effects and
Scaling in Discrete Dislocation PlasticityÓ
Abstract:
Plastic flow in
crystalline materials is size dependent over length scales of the order of tens
of microns and smaller. This size dependence arises in a variety of contexts;
e.g. the grain size dependence of the flow strength, the indentation size effect
and the size dependence of the thermo-mechanical response of thin films. One
well-appreciated origin of size effects is associated with imposed plastic
strain gradients and geometrically necessary dislocations. In addition, strain
gradients and boundary layers leading to size dependent response can occur in
circumstances where, at least in principle, a more or less homogeneous response
is possible but where the physics of dislocation motion prevents it. Discrete
dislocation plasticity analyses of various plastic flow processes will be used
to illustrate a range of size effects. Particular attention will be given to
the scaling, both with size and with material properties, that is predicted by
the calculations. Implications for various deformation and fracture processes
will be discussed.