1. State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu, Sichuan 610031, People's Republic of China
2. Applied Mechanics and Structure Safety Key Laboratory of Sichuan Province, School of Mechanics and Engineering, Southwest Jiaotong University, Chengdu 610031, People's Republic of China
3. Institute of Applied Mechanics, University of Kaiserslautern, P.O. Box 3049, Kaiserslautern D-67653, Germany