1. Department of Nanomechanics, Graduate School of Engineering, Tohoku University, 6-6-11-716, Aoba Aramaki, Aobaku, Sendai, Miyagi 980-8579, Japan e-mail:
2. Department of Nanomechanics, Graduate School of Engineering, Tohoku University, 6-6-11-716, Aoba Aramaki, Aobaku, Sendai, Miyagi 980-8579, Japan
3. Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University, 6-6-11-716, Aoba Aramaki, Aobaku, Sendai, Miyagi 980-8579, Japan