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Size-Dependent Mechanical Properties
The design of macroscopic components relies on the fact that the response of almost all engineering materials is independent of size at length scales above, roughly, a millimeter.
At size scales around a micrometer and smaller, this does no longer hold. These size effects are particularly important for irreversible processes like plasticity and fracture. In plasticity, there is a growing body of experimental evidence that suggests that ``smaller is harder''. During deformation processes that involve strain gradients, such as torsion, bending and indentation, the material is relatively stronger when specimens become smaller but the material's microstructure remains the same. On the other hand, in tension this gradient effect is not active but opposite size effects have been observed due to statistical effects. Fracture is often size dependent because the physical mechanism is directly related to the microstructure, whose characteristic size becomes of the same order of magnitude as the specimen when dimensions become smaller. A related issue in miniature engineering is that surfaces become relatively more important. Surface properties such as contact and friction, however, are also size dependent and we are only beginning to understand them.
These size effects are becoming essential ingredients of design in the era of miniaturization. Their implementation requires at least two main steps: (i) identification and understanding of the origin of size effects; (ii) incorporation in continuum models that can be used in the design stage. The second step implies that these models have to be nonlocal in nature, contrary to the local material models that underly today's procedures. While we are building up an understanding of the origin of size effects, it has also become evident that the basic structure of such a nonlocal theory is essentially unknown. There are few, if any, universal guidelines for the development of nonlocal theories, which explains the abundance of models proposed in the mechanics of materials community during the last decade. A recent, promising new line of thinking for microscale plasticity is aiming at a field theory of dislocations based on statistical treatments.
On top of these fundamental issues, engineering in MEMS and microsystems is one of the main technological applications that is facing the intriguing mechanical problems that play a role at smaller scales.
In view of the above developments, this workshop aims at bringing together physicists and engineers who are dealing with size-dependent mechanical properties. Nonlocal plastic deformation phenomena will be at the core of the meeting, including plasticity related to fracture, indentation, contact and friction.
While experimental characterization and development of nonlocal theories at the (sub)micron scale is the nucleus of the programme, the workshop explicitly aims at improving the interaction with microsystems engineers, on the one hand, and with physicists working on atomic-scale problems of defects and surfaces, on the other hand. Discussions with the former will help to focus the models to the current design problems, while atomic-scale information is needed to further nurture the theories with physical content.
This workshop is part of the EU Marie-Curie Research Training network SizeDepEn (http://sizedepen.mrginfo.com).