1. Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL 60208; AML, Department of Engineering Mechanics, Center for Mechanics and Materials, Tsinghua University, Beijing 100084, China e-mail:
2. Department of Micro/Nano Electronics, Shanghai Jiao Tong University, Shanghai 200240, China; Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL 60208 e-mail:
3. Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208 e-mail: