Issue 47

T. Kawabata et alii, Frattura ed Integrità Strutturale, 47 (2019) 416-424; DOI: 10.3221/IGF-ESIS.47.32 423 C ONCLUSION o understand the elementary process of cleavage fracture in steel in more detail, the history of crack propagation velocity in a single grain of a 3%Si - Fe alloy was measured using a small-sized multiple-strain gauge. Conclusions were drawn as follows: 1) In the three-point bending test, the brittle fracture originated from a trigger point located slightly away from the tip of the notch like an ordinary polycrystalline steel material. The trigger point was at the intersection of a twinning deformation, and apparently the void-like microcrack generated at the intersection due to strong deformation acted as the initial crack. It was estimated that the twin deformation was orthogonal through imaging of the fracture surface and it was assumed to be generated by the {110} plane as concluded by Matsuda et al. [26] 2) Crack propagation speed accelerated from 200 m/s to 500 m/s within a single grain. The location of the acceleration was near the centre of the bending test sample and the stress intensity factor was already decreasing. It can be presumed from the past research [15] and the results of other steel materials [27] that crack propagation speed is positively correlated with stress intensity factor. However, in this study acceleration was observed despite a decrease in the stress intensity factor. This cannot be easily understood. 3) If a delay or stopping of the crack tip at the grain boundary occurred, the speed of the crack propagation was accelerating—even as the stress intensity factor was decreasing—to regain the original speed after the deceleration due to delay or stopping. In the future, a more detailed experimental measurement should be performed using ideal materials for a more detailed discussion. A CKNOWLEDGEMENT he authors of this work would like to express their gratitude to the ISIJ Innovative Program for Advanced Technology at the Iron and Steel Institute of Japan for their financial support and dedicated discussions of this study. This work was also supported by JSPS KAKENHI Grant Numbers 18H05337, 17H01354. R EFERENCES [1] Yamaguchi, K., Kitada, H., Yajima, H., Hirota, K., Shirakihara, H. (2005). Development of Extremely Large Container Ships, KANRIN, 3, pp. 70–76. [2] Kawabata, T., Hirose, H. (2017). A Transition of Philosophies of Fracture Safety in Liquefied Natural Gas Storage Tanks, Twenty-seventh International Offshore and Polar Engineering Conference, San Francisco, California, USA, ISOPE-I-17-623 [3] Horikawa K. and Watanabe, N. (2005). Application of Extra-High Tensile Strength Steel for Hydropower Plants in Japan, Conference on High Strength Steels for Hydropower Plants in Takasaki, 3-1~3-8. [4] Sakai, Y. Yagawa, G. and Ando, Y., (1986). A Cooperative Study for the Evaluation of Crack Arrest Toughness of RPV Materials in Japan, Fracture Mechanics: Nineteenth Symposium, ASTM STP 969, 547-568. [5] Kobayashi, A.S., Emery A.F. and Mall, F. (1999). Dynamic Finite Element and Dynamic Photoelastic Analysis of Crack Arrest in Homalite-100, in Fast Fracture and Crack Arrest, ASTM STP627, eds. Hahn, G.T. and Kanninen, M.F., American Society for Testing and Materials, Philadelphia, pp.95-108. [6] Ravi-Chandar, K. and Knauss, W.G. (1984). An Experimental Investigation into Dynamic Fracture –III. On Steady State Crack Propagation and Branching, International Journal of Fracture, 26, pp.141-154 [7] Freund, L.B. and Clifton, R.J. (1974). On the uniqueness of elastodynamic solutions for running cracks, Journal of Elasticity 4, pp.293-299. [8] Achenbach, J., Kanninen, M.F. and Popelar, C.H. (1980). Crack Tip Fields for Fast Fracture of Elastic-Plastic Material, Journal of the Mechanics and Physics of Solids, 29(3), pp.211-225. [9] Kanazawa, T., Machida, S. and Teramoto, T. (1977). Study on Fast Fracture and Crack Arrest: The 1st Report: Numerical analysis of dynamic crack with the use of finite difference method, Journal of the Japan Society of Naval Architects (141), pp. 290-296. T T