1. Department
of Mechanical Engineering, Northwestern University, 2145 Sheridan
Road, Evanston, Illinois 60208-3111, United States
2. Department
of Mechanics and Engineering Science, College of Engineering, Peking University, Beijing 100871, China
3. Department
of Civil and Environmental Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3109, United States
4. Department
of Mechanical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea
5. Department
of Mechanical Engineering, Columbia University, New York, New York 10027, United States
6. Theoretical
and Applied Mechanics, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3111, United States