Modelling the cyclic torsion of polycrystalline micron-sized copper wires by distortion gradient plasticity

In this contribution we show that the distortion gradient plasticity recently proposed by our group, characterised by a higher-order plastic potential leading to reliable predictions under non-proportional loading, can predict experimental data of literature on the cyclic torsion of copper wires of diameter ranging from 18 to 42 micrometres. To reach our goal, we plug our recent constitutive proposal in a framework that we have previously developed for the torsion problem, which is based on the pivotal theory established in 2004 by Gurtin, relying on Nye's dislocation density tensor to describe size effects in micron-scale metal plasticity. We implement the new model in a finite element code and identify its parameters by resorting to the Coliny evolutionary algorithm within the software Dakota.