1. Columbia Nanomechanics Research Center, Dept. of Earth and Environmental Engineering, Columbia Univ., New York, NY 10027.
2. State Key Laboratory of Automotive Safety and Energy, Dept. of Automotive Engineering, Tsinghua Univ., Beijing 100084, P.R. China.
3. Columbia Nanomechanics Research Center, Dept. of Earth and Environmental Engineering, Columbia Univ., New York, NY 10027; Dept. of Civil and Environmental Engineering, Hanyang Univ., Seoul 133-791, Korea; Int. Center for Applied Mechanics, SV Lab, Xi’an Jiaotong Univ., Xi’an 710049, China (corresponding author).