PSI - Issue 61

Koji Uenishi et al. / Procedia Structural Integrity 61 (2024) 115–121 Uenishi and Kato / Structural Integrity Procedia 00 (2024) 000 – 000

120

6

free surface, and relatively large particle movement is expected, as observed in Fig. 1(a). Similarly, next in Fig. 3, the particle movement and photoelastic isochromatic fringe patterns (a) and the pictures indicating difference of colors between the snapshots before and after the first dynamic impact (b) are shown, again from top to bottom, at time 300, 600, 900, 5,400, 5,700 and 6,000  s after the first impact. In the figure, as in Fig. 2(b), the first impact gives two lines of unidirectional stress transfer. Then, due to the delayed second impact, two more lines of unidirectional stress transfer seem to emerge. The kinetic energy scattered from the stress transfer lines is superimposed in the area surrounded by the two neighboring lines and the free surface in the middle and hence particles in this possibly trapezoidal area can move largely and buckling-like jump may occur, again as found in Fig. 1(b). It is immediately noticed that the idea of delayed double impact loads (and the highly unstable trapezoidal area due to the multiple impact loads) may be used for more efficient impact-induced fracturing and sampling of surface material of extraterrestrial asteroids, etc., in sample-return spacecraft missions. Of course, the newly identified interaction of multiple unidirectional stress transfers and confinement of kinetic energy to specific areas may assist in understanding more deeply the dynamic stability, in particular, the initiation phase of failure and flow associated with granular slopes, including rock falls and debris flows. Although great efforts have been made and valuable research results have been obtained (e.g. Staron et al., 2005; Takagi et al., 2011; Farin et al., 2014; Darbois Texier et al., 2018; Hassan and El Shamy, 2019; Tordesillas et al., 2021; Wang et al., 2022; Zhang et al., 2022) in the analyses of slope stability, external loads are often given uniformly to the models, either by (unrealistic sudden action of) gravity or so-called seismic disturbances from bottom, and the essential mechanisms of the beginning of granular failure and flow at the very local level has not been completely understood yet. Moreover, as mentioned above, the influence of dynamic disturbances like Rayleigh waves propagating along the top surface of a slope cannot be neglected. Therefore, local observations of the effect of dynamic impact loads from top such as conducted here may well enhance our comprehension of the fundamental mechanisms of the mechanical stability and initial unstable behavior of a granular slope. 3. Conclusions In order to observe the dynamic fracture phenomena caused by multiple impact loads in a two-dimensional dry granular medium, further laboratory photoelastic experiments have been conducted. It is indicated that transient particle motion, stress transfer and failure patterns inside a granular medium can vary in accordance with the change, single or delayed double, in the dynamic loading conditions. The experimentally taken photographs (and the color difference) indicate that basically the pattern of stress transfer, in this study unidirectional, is the same for both cases of single and double impact loads and areas of mechanically stable and unstable particles have been recognized. In the case of double impact loads, superposition of the effect of the neighboring lines of unidirectional stress transfer generates an area of highly unstable particles where large movement and buckling-like jump of particles as well as fragmentation (and collapse) of the granular medium can be found, however, the superposition is not quantitatively evaluated. Discrete modeling, such as found in Uenishi and Sakurai (2015), Debski and Klejment (2021) and Uenishi and Xi (2022), may be needed for the evaluation and better understanding of the stress transfer and fracture development in the granular medium under multiple dynamic impact loads. 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 23K04021. References Darbois Texier, B., Ibarra, A., Vivanco, F., Bico, J., Melo, F., 2018. Friction of a Sphere Rolling down a Granular Slope. EPL 123, 54005. Debski, W., Klejment, P., 2021. Earthquake Physics beyond the Linear Fracture Mechanics: A Discrete Approach. Philosophical Transactions of the Royal Society A 379, 20200132.