1. State Key Laboratory for Strength and Vibration of Mechanical Structures Xi'an Jiaotong University Xi'an 710049 P. R. China
2. State Key Laboratory of Porous Metal Materials Northwest Institute for Nonferrous Metal Research Xi'an 710016 P. R. China
3. State Key Laboratory of Traction Power Southwest Jiaotong University Chengdu 610031 P. R. China