PSI - Issue 28

Koji Uenishi et al. / Procedia Structural Integrity 28 (2020) 2072–2077 Uenishi et al./ Structural Integrity Procedia 00 (2020) 000 – 000

2077

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primary fracture plane, due to the reflection of the fracture-induced body waves. At these amplified regions, the secondary fracture can be initiated. Upon a total split of the specimen by the primary fracture, Rayleigh surface (R) waves are produced and propagated upwards and downwards along the vertical free surface. The surface waves move to the edges of the specimen, reflected (RR) and then diffracted to cause further fracture, or more exactly, the resumption of propagation of the arrested secondary fracture, at a later stage. If the secondary fracture is involved in the calculations, RR wave(s) can be diffracted at the initiation point(s) of the secondary fracture on the right free surface and move along the fracture surfaces to reactivate the propagation of the arrested secondary fracture. Indeed, the time of the diffracted RR wave arrival at the tip of the arrested secondary fracture and that of resumption of fracture propagation coincide with each other ( t = 340  s). Overall, the connection between the subsidiary (secondary and reactivated secondary) fracture and the dynamic waves can be summarised as follows: (1) Generation of the rupture front wave around the tip of the primary fracture; (2) Initiation of the secondary fracture due to the waves associated with the primary fracture; and (3) Resumption of propagation of the secondary fracture by the reflected and diffracted Rayleigh surface wave. 4. Conclusions Our experimental observations of the fracture evolution in brittle solids with multiple small-scale parallel cracks have revealed that after a perfect split of the specimen into two by the propagation of the primary fracture, the secondary fracture can be generated and propagated at a distance from the primary fracture in the opposite direction, i.e. fracture propagation may jump even without further external loading. Since the specimen is completely divided by the primary fracture, the external uniaxial load applied quasi-statically by the tensile testing machine to the specimen until the total split becomes zero when the secondary fracture starts. Apparently, the secondary and further (reactivation of the arrested secondary) fractures, or a cluster of fractures, are induced by dynamic waves owing to the development of the primary fracture and not caused by additional external energy supply. The extension of the secondary and further fractures may be hidden in the global stress-strain diagram with a sharp stress drop. The impact of such subsequent fractures may be detected globally in dynamic wave radiation patterns such as doublet or a cluster of ruptures in the solid Earth, i.e. earthquakes. Now we are investigating possible effects of every crack length and distribution pattern on e.g. the fracture propagation speed and trying to capture more rigorously the relation between the speed / wavelength of the Rayleigh waves and the secondary fracture evolution. We are also examining the influence of the initial inclination angle of the set of parallel cracks for the seismological study of normal faulting with some dip angle. Acknowledgements The research has been financially supported by the Japan Society for the Promotion of Science (JSPS) through the “KAKENHI: Grant -in- Aid for Scientific Research (C)” Program under grant number 20K04680. References Anderson, T. L., 2017. “Fracture Mechanics: Fundamentals and Applications, Fourth Edition”. CRC Press, Boca Raton, pp. 706. Gomez, Q., Uenishi, K., Ionescu, I. R., 2020. Quasi-Static versus Dynamic Stability Associated with Local Damage Models. Engineering Failure Analysis 111, 104476. Uenishi, K., Fukuda, Y., Yoshida, T., Sakaguchi, S., Ionescu, I. R., 2018. Collective Mechanical Behavior and Stability of a Group of Cracks in Brittle Solids. Structural Integrity 5, 244–245. Uenishi, K., Fukuda, Y., Kame, N. 2019. Individual mechanical interaction of multiple cracks and its relation with the collective behavior: experimental observations under quasi-static loading conditions, Third International Conference on Structural Integrity and Durability. Dubrovnik, Croatia, 2 pages. Viktorov, I. A., 1962. Rayleigh Waves in the Ultrasonic Range. Soviet Physics − Acoustics 8, 118–129.