Dr. Shaofan Li is currently a full professor of applied and computational mechanics at the University of California-Berkeley. Dr. Li graduated from the Department of Mechanical Engineering at the East China University of Science and Technology (Shanghai, China) with a Bachelor Degree of Science in 1982; he also holds Master Degrees of Science from both the Huazhong University of Science and Technology (Wuhan, China) and the University of Florida (Gainesville, FL, USA) in Applied Mechanics and Ae

Abstract：Dislocations in a deformed crystal tend to aggregate into various dense formations separated by relatively dislocation free regions. These dense formations are called dislocation patterns that underlie most important crystal plasticity features such as work hardening and strain localization. In this work, we show that the previously developed multiscale crystal defect dynamics (MCDD) method (Li et al. (2015) Philosophical Magazine, 94,1414-1450. and Lyu and Li, (2017) Journal of Mechanics and Physics of Solids, 107, 379-410.) is actually a discrete dislocation pattern dynamics, in which we link the early pattern of the dislocation distribution, or the microstructure of crystal defect in general, with that of original crystal lattices as part of material genome.

The main developments of present work are: (1) Using the dislocation dynamics invariants and scaling laws, we demonstrated that the multiscale crystal defect dynamics is in fact a multiscale dislocation pattern dynamics; (2) The multiscale dislocation pattern dynamics may lead to an atomistically-informed crystal plasticity theory, and (3) It is shown that cyclic plastic responses of FCC crystals may be captured by MCDD simulations. The work is highlighted by the use of the hierarchical higher order Cauchy-Born rule approach, which models different dislocation patterns with different order of Cauchy-Born rules, so that it can capture the overall inelastic response of crystalline materials. In doing so, we have developed a MCDD based crystal plasticity finite element method (CPFEM) to simulate crystal slip and shear band formation in single crystals at sub micron scale.

In this approach, coarse-grained models are adopted for both bulk media and material interphase or process zone. In bulk elements, the first order Cauchy-Born rule is adopted, so we can formulate an atomistic enriched continuum constitutive relation to describe the material behaviors. All the nonlinear deformations are assumed to be confined inside the process zone, and the process zone between the bulk elements is remodeled as a finite-width strip whose lattice constants and atomistic potential may be the same or different from those of the bulk medium. Inside the interphase zone, the higher order Cauchy-Born rules are adopted in process zones, and a higher order strain gradient-like coarse grain constitutive model is derived, which can capture the size-effect at the small scales. All interphase or process zones are constructed such that they are part (a subset) of slip planes in a lattice space. The multiscale crystal defect dynamics has been applied to simulate both dislocation motion and crack propagations in both single crystals and polycrystalline solids. Crack branching and void formation have been found possible for different element mesh stacking fault energies, which are dictated, by the effective lattice structure or microstructure in the process zone elements.