题目：Multiscale Crystal Defect Dynamics: A dual-lattice process zone model to nanoscale plasticity

报告人：Shaofan Li (Department of Civil and Environmental Engineering University of California-Berkeley ）

主持人：段慧玲特聘研究员

时间：2014年6月3日（周二）12:10—13:10

地点：工学院1号楼 学术报告厅（210室）

报告简介：
In this work, we present the theoretical and computational formulations of a multiscale crystal defect dynamics (MCDD) for the simulation of crystal defects at small scales. The main novelties of the proposed MCDD are: (1) We use the dual-lattice tessellation to construct a dual-lattice process zone model that can represent different types of crystal defects in a single crystal; (2)We adopt a fourth-order (four scales) hierarchical strain gradient theory to model constitutive behaviors of various defect process zones, in which the atomistic-informed higher order Cauchy?Born rule is employed, and (3)We employ the Barycentric finite element technique to construct finite element shape functions for polygonal and polyhedral process zone elements. The proposed MCDD method provides an efficient and viable alternative for both molecular dynamics and dislocation dynamics in simulations of defect evolutions such as void growth, dislocation nucleation, and fracture. In particular, MCDD offers a mesoscale description for dynamic lattice microstructure, defect microstructure, and their interactions. The method offers a possible solution for studying nanoscale and mesoscale crystalline plasticity. 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 deformation is 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.

报告人介绍：
Dr. Shaofan Li is currently a 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 Aerospace Engineering in 1989 and 1993 respectively. In 1997, Dr. Li received a PhD degree in Mechanical Engineering from the Northwestern University (Evanston, IL, USA), and he was also a post-doctoral researcher at the Northwestern University during 1997-2000. In 2000, Dr. Li joined the faculty of the Department of Civil and Environmental Engineering at the University of California-Berkeley. Dr. Shaofan Li has also been a visiting Changjiang Professor in the Huazhong University of Science and Technology, Wuhan, China (2007-2013). Dr. Shaofan Li is the recipient of ICACM Computational Mechanics Award (2013), USACM Fellow Award (2013), A. Richard Newton Research Breakthrough Award (2008), and NSF Career Award (2003). Dr. Li has published more than100 articles in peer-reviewed scientific journals with h-index 36 (Google Scholar), and he is also the author of two research monographs/graduate textbooks.