1. a Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153, Japan
2. b Research Institute for Applied Mechanics, Kyushu University, Kasuga-City, Fukuoka 816, Japan
3. c Research Institute for Applied Mechanics, Kyushu University, Kasuga-City, Fukuoka 816, Japan