1. Department of Civil Engineering and Soft Matter Research Center, Zhejiang University, Hangzhou 310058, China
2. Department of Civil and Environmental Engineering and Mechanical Engineering, Northwestern University, Evanston, IL 60208
3. State Key Laboratroy of Structural Analysis for Industrial Equipment, Dalian University of Technology, Dalian 116024, China
4. Department of Mechanical Engineering, University of Colorado, Bouldera, CO 80309 e-mail:
5. Department of Mechanical Engineering, Kyung Hee University, Seocheon-dong, Giheung-gu, Yongin-si, Gyoenggi-do 446-701, Korea
6. Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China
7. Center for Mechanics and Materials, Tsinghua University, Beijing 100084, China
8. Department of Materials Science and Engineering, Beckman Institute, and Materials Research Laboratory, University of Illinois, Urbana, IL 61801