1. Department of Finemechanics, Graduate School of Engineering, Tohoku University, 6-6-11, Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan
2. Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University, 6-6-11, Aoba, Aramaki, Aoba-kuSendai 980-8578, Japan