1. The George W. Woodruff School of Mechanical Engineering Georgia Institute of Technology Atlanta GA 30332 USA
2. Applied Mechanics and Structure Safety Key Laboratory of Sichuan Province, School of Mechanics and Engineering Southwest Jiaotong University Chengdu Sichuan 610031 P. R. China
3. School of Materials Science and Engineering Georgia Institute of Technology Atlanta GA 30332 USA
4. Key Laboratory of Advanced Technologies of Materials (Ministry of Education) School of Materials Science and Engineering Southwest Jiaotong University Chengdu Sichuan 610031 P. R. China