| Size effects at micro-nano scales have been observed by various experiments in both polymers and metals. Similar to metals, our nano-indentation tests with different tip geometries show that such size effects in polymers are associated with an increase in higher order displacement gradients with decreasing size. However, experimental findings discussed in this study reveal that the nature of such higher order displacement gradients, and subsequently the nature of size effects, in polymers are quite different from metals. In contrast to metals where size effect is associated with non-uniform plastic deformation, in polymers size effect is, at least in parts, of elastic nature. Corresponding to these findings, a size dependent elasto-plastic couple stress theory is developed where the higher order displacement gradients have reversible components. The theory is implemented into a nonlinear finite element framework to numerically capture the size effects observed in the experiments. |
