Toward a further understanding of size effects in the torsion of thin metal wires: An experimental and theoretical assessment

Both torsion and tensile tests are performed on polycrystalline copper wires with diameters in the range 20–50 μm. A significant size effect in both the initial yielding and the plastic flow is observed in torsion. In contrast, only a minor effect is seen in tension. The physical basis of the size effects in wire torsion is elucidated in the light of the geometrically necessary dislocation argument and of the critical thickness effect. Three phenomenological theories of strain gradient plasticity, due to Fleck and Hutchinson, to Chen and Wang and to Aifantis and co-workers, are assessed within the context of wire torsion, and the corresponding rigid-plastic solutions are derived. Distinctions between the theories are highlighted through comparison with experiment, emphasizing the difference in predicted trends in the size dependence of initial yielding and of hardening rate. Additionally, the key aspects of a new torsion balance technique for wire torsion are presented. An in-situ torsional vibration method for calibrating the torque meter with precision is addressed. The systematic experimental and theoretical assessment suggests that the size effect in the initial yielding is mainly due to the constraints that the external geometrical size put on a finite strained volume, while the size dependence in the plastic flow is principally owing to the geometrically necessary dislocations associated with the plastic strain gradients.