1. Professor, Dept. of Aeronautical Structural Engineering, Northwestern Polytechnical Univ., P.O. Box 118, Xi’an, Shaanxi 710072, P.R. China; Adjunct Professor, State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi’an Jiaotong Univ., Xi’an, Shaanxi 710049, P.R. China