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Workshop on Strain and Gradient Plasticity: Applications and Theory

January 24, 2008
Time: 3:00 - 3:45pm
Location: IMMS Conference Room – TA3, Building 4200, Suite 101

John W. Hutchinson
School of Engineering and Applied Sciences, Harvard University

(Lectures will be held over the Access Grid from UCSB)

Plastic deformation at the micron scale is assuming increasing
importance in areas of technical application, including MEMS, thin
films and coatings, metal-matrix composites with micron sized
reinforcing particles, and in attempts to understand fundamentals of
material fracture. Recent experiments have revealed aspects of
micron scale deformation that are not present in bulk, or
conventional, plasticity. Of major importance is a strong size
dependence whereby the effective yield strength increases with
decreasing specimen size when non-uniform straining occurs. The
phenomenon is revealed by indentation hardness tests, by bending and
torsion tests, by straining of thin films bonded to a substrate, and
by the well-known dependence of polycrystalline yield stress on grain
size. In effect, smaller is stronger when deformation involves
gradients in the plastic strain.

The lectures will begin with an overview of a broad selection of
recent experimental data that motivates the need for plasticity
theories applicable at the micron scale. The underlying physical
basis of the theory will also be addressed. The review of
experimental data will be followed by the introduction of a
phenomenological strain gradient plasticity theory which reduces to
the most commonly used conventional plasticity theory (so-called J2
flow theory) when the scale of the deformations is not small.
Illustrative examples based on the theory will be presented. Some
discussion of numerical approaches will be given because numerical
methods are even more essential for generating solutions based on
strain gradient theories than for conventional plasticity. The
lectures will conclude with a discussion of open issues and prospects
for the future.



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