1. Institute of Structural Mechanics, China Academy of Engineering Physics, P.O. Box 919-414, Mianyang City, Sichuan Province, 621900, P.R. China; formerly Protective Technology Research Center, School of Civil and Environmental Engineering, Nanyang Technological Univ., Nanyang Ave., Singapore 639798.
2. Dept. of Mechanical, Aerospace, and Manufacturing Engineering, UMIST, P.O. Box 88, Manchester M60 1QD, UK (corresponding author).