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.

中文翻译：

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关于应变梯度塑性高阶条件在细金属线循环扭转中的作用：实验和建模

对经过和不经过表面钝化的微米级铜线进行循环扭转测试，以研究高阶条件在非比例载荷下细线塑性行为中的作用。在对称循环中观察到典型的强化尺寸效应。钝化铜线的扭转响应强度增强更为明显。在加载-卸载循环期间发现了不寻常的包辛格效应，这在钝化线中更为明显。基于 Gudmundson 应变梯度塑性理论的有限元实现是针对线材扭转而开发的，以表征观察到的与尺寸相关的现象。引入高阶边界条件来模拟钝化表面。给出并比较了钝化和未钝化线的塑性应变、应力分量和几何必要位错密度的预测径向分布。这项工作为理解应变梯度塑性的高阶条件的作用提供了合理的基础。