Abstract

Micron scale size-effects in metals have been confirmed in numerous experiments, exhibiting the general trend that smaller is stronger. For torsion of micron scale wires and bending of thin films, it has been found that normalized twisting and bending resistance in the plastic deformation range increase with decreasing specimen size. In micro- and nano-indentation testing increased hardness for smaller indentations are measured. It is generally accepted these size-effects in metals are due to so-called geometrically necessary dislocations that arise in the presence of very large plastic strain gradients. Such gradients may be imposed due to inhomogeneous overall deformation as in the examples above, but they can also arise under homogeneous deformation when boundaries are passivated, such that plastic deformation is hindered due to blockage of dislocations.

The first part of this presentation presents a basis for numerical modeling of strain gradient plasticity, both in the context visco-plastic materials and time-independent plasticity.

The second part treats size-effects in void growth, with emphasis on extensions of conventional porous metal yield surfaces to include micron scale size-effects.