On the role of higher-order condition of strain gradient plasticity in the cyclic torsion of thin metallic wires: Experiments and modeling

Abstract

Cyclic torsion tests are performed on micron-scale copper wires with and without surface passivation to study the role of the higher-order condition in the plastic behavior of thin wires under non-proportional loading. A typical strengthening size effect is observed in the symmetric cycles. More obvious strength enhancement exists in the torsional response of passivated copper wires. An unusual Bauschinger effect is found during the loading-unloading cycles, which is more pronounced in passivated wires. The finite element implementation based on Gudmundson's strain gradient plasticity theory is developed for wire torsion to characterize the observed size-dependent phenomena. The higher-order boundary conditions are introduced to simulate the passivated surface. The predicted radial distributions of plastic strain, stress components, and geometrically necessary dislocation density for the passivated and unpassivated wires are given and compared. This work provides a reasonable basis for understanding the role of higher-order conditions of strain gradient plasticity.