1. Department of Aerospace Engineering, Iowa State University, Ames, IA 50010, USA
2. Department of Materials Science and Engineering, Iowa State University, Ames, IA 50010, USA
3. Department of Engineering Mechanics, Tsinghua University, Beijing 100084, P. R. China
4. Department of Chemistry and Engineering Science, Wartburg College, Waverly, IA 50677, USA