  Although continuum theory has been widely used to describe the long-range
elastic behavior of dislocations, it is limited in its ability to describe
mechanical behaviors that occur near dislocation cores. This limit of the
continuum theory mainly stems from the discrete nature of the core region,
which induces a drag force on the dislocation core during glide. Depending on
external conditions, different drag mechanisms are activated that govern the
dynamics of dislocations in their own way. This is revealed by the resultant
speed of the dislocation. In this work, we develop a theoretical framework that
generally describes the dynamic drag on dislocations and, as a result, derive a
phenomenological constitutive equation. Furthermore, given that a low-angle
grain boundary (LAGB) can be regarded as an array of dislocations, we extend
the model to describe the mobility law of LAGBs as a function of misorientation
angle. As a result, we prove that both dislocations and LAGBs follow the
developed constitutive equation with the same mathematical form despite their
different governing drag sources. The suggested model is also supported by
molecular dynamics simulations. Therefore, this work has significance for a
fundamental understanding of the dynamic drag acting on defects and facilitates
a general description of various drag mechanisms.
