Interpreting the internal length scale in strain gradient plasticity

Gradient plasticity frameworks arise from the consideration of the gradient of the plastic strain as an independent variable. In doing so a new material parameter, the internal length (or gradient coefficient) is introduced, which governs the gradient effects. Despite the efficiency of these theories, there does not exist a unified interpretation of the internal length, and its relationship with the material microstructure has not been clearly understood. The present article provides a review on the various interpretations that recent studies have provided for the internal length that go beyond fitting it to experimental data.