Abstract
The effect of surface passivation on the torsional behavior of thin metallic wires is studied within two frameworks of higher-order strain gradient plasticity, i.e. the simplified version of Gudmundson theory and the Polizzotto theory. The elastoplastic solution to the wire torsion problem is obtained. The flexibility of the higher-order theories is emphasized by solving the torsion problem for two boundary conditions, i.e. with and without surface passivation. The two types of models predict similar trends related to the size effect and the passivation effect in the wire torsion. By accounting for a free energy involving one energetic length scale, a formula relating the normalized torque to the wire radius is derived. The formula confirms a standardized increase in strain hardening with diminishing wire diameter. The distributions of plastic shear strain and its gradient along the radial coordinate for the unpassivated and passivated wires are given. It is found that the plastic flow would be strongly suppressed at the passivated wire surface. Numerical results also show that the energetic length scale plays an important role in modelling the elevation of strain hardening due to passivation.
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