Yield and plastic flow of soft metals in small volumes loaded in tension and flexure

Theories of small-scale plasticity often invoke effects of strain gradient, and this is best tested by comparison of experimental stress–strain data obtained with and without well defined strain gradients. We provide new results to add to the body of data for 25–150-µm Cu wires in tension, 10–125-µm Cu and Ni foils in flexure and 10–125-µm Ni foils in tension, and test whether the data can adequately discriminate between the theories. What the collected data shows is that there are size effects in yield strain, as well as in the strain-hardening behaviour in the low-strain and high-strain regimes. Within the experimental scatter, the data is largely consistent with theories that invoke, and those that do not invoke, effects of strain gradients. The tension data in particular are too scattered, and the differences in the theoretical predictions are not sufficiently stark, to discriminate between the theories. However, we find that the flexure data for Cu and Ni agree within experimental error, indicating that material-specific properties such as elastic moduli and stacking fault energies are not involved in the size effect.