Enhanced plastic distortion based micromorphic model for flexible control of scaling effects

A novel plastic distortion based micromorphic model is proposed, incorporating higher-order multi-kinematic decomposition and generalized power-law defect energy to accurately capture size-dependent effects. For the first time in micromorphic modeling, higher-order dissipation effects are introduced, providing enhanced description of strengthening and hardening phenomena beyond existing micromorphic approaches. Analytical investigation of the proposed model reveals the crucial role of the micromorphic relaxation, governed by the penalty parameter, in reproducing realistic scaling behaviors. The model predicts two distinct asymptotic regimes for size effects at very large and very small geometrical sizes. At intermediate sizes, it yields realistic scaling relationships with adjustable scaling exponent between 0 and 2, allowing for a more precise description of size-dependent strengthening and hardening. Numerical simulations using the proposed model demonstrate its capability to reproduce complex cyclic responses with nonlinear hardening effects, capturing key microstructural mechanisms. Comparison with discrete dislocation dynamics (DDD) simulations under cyclic loading conditions shows excellent agreement for intermediate geometrical sizes, accurately reflecting size-dependent strengthening and hardening evolution. However, challenges remain in predicting cyclic responses at very small scales, indicating a restricted range of applicability in extreme miniaturized conditions. Despite this limitation, the proposed micromorphic framework provides a versatile and robust alternative to classical gradient plasticity theories, offering greater flexibility in modeling small-scale plasticity mechanisms. This work establishes the foundation for future refinements through systematic parameter identification, experimental validation, and broader comparisons with existing and emerging size-dependent plasticity models.