J. Han Department of Mathematics, Tianjin University 300072 Tianjin, China e-mail: hanjian @tju.edu.cn J. Han. N. Jiang Y. Tian Department of Mechanics, Tianjin University 300072 Tianjin, China e-mail: nanj@tju.edu.cn N. Jiang Y. Tian Tianjin Key Laboratory of Modern Engineering Mechanics, 300072 Tianjin, China