Three-dimensional dislocation dynamics (DD) simulations are performed to understand the behavior of strain gradient plasticity (SGP) under inhomogeneous loading
conditions. Representative micrometer-scale loading geometries—torsion of cylindrical pillars and bending of thin films—were considered to capture intrinsic size effects arising from
plastic strain gradients. Two distinct classes of SGP theories, incremental and non-incremental formulations, were critically examined by applying non-proportional loading conditions. Non-
incremental version of strain gradient plasticity theories is characterized by certain stress quantities expressed in terms of increments of strains and their gradients, whereas incremental
version of theories employ incremental relationships between all stress quantities and the increments of strains and their gradients. From the previous work, two classes of theories
showed marked difference in the prediction of the onset of plasticity under non-proportional loading. In this work, our DD results show that plastic flow could continuously occur even
with passivation, which lead to severe hardening, without showing the elastic loading gap. Furthermore, plastically stored energy associated with geometrically necessary dislocations
(GNDs) was identified during unloading. Under torsional deformation, unlike in uniaxial tension, reverse plasticity emerged, driven by the release and motion of GNDs accumulated
during prior loading. These observations indicate that plastic strain gradients can contribute recoverably to the internal energy, underscoring the energetic role of GNDs in microscale
plasticity