PSI - Issue 61

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

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1. Introduction Ranging from landslides and debris flows to the formation of the geological flame structure (Yasuda and Sumita, 2014) and impact-induced controlled fracturing of asteroid surfaces in sample-return spacecraft missions (Sawada et al., 2017), granular materials and their fracture and collapse can be found in wide areas of science and engineering applications. Motivated by an earthquake-generated unique tensile failure in a fill slope in Sendai, Japan, where both theoretical continuum (Uenishi, 2010) and numerical discrete (Uenishi and Sakurai, 2015) investigations suggest the important role of Rayleigh surface waves that are caused by dynamic impact (earthquake) and traveling along the top free surface, in our previous work, transfer of stresses and evolution of waves and fractures in a dry granular slope that is subjected to dynamic impact on its top free surface have been investigated by means of dynamic photoelasticity. Every two-dimensional granular slope consists of penny-shaped birefringent polycarbonate particles that are set on top of a rigid horizontal plane with some inclination (slope) angle (e.g. 60 degrees). On its top free surface, one single dynamic impact load is given, and through the experimental observations of time-dependent particle motion and dynamic stresses, two different failure patterns have been identified: (1) complete collapse or mass flow caused by unidirectional (force-chain-like) stress transfer; and (2) toppling-type separation of the slope face of the medium due to widely spreading multi-dimensional waves (Uenishi and Goji, 2018). Then, some mechanical confinement and material heterogeneities are added to the granular slopes and their influence on granular dynamics, including buckling phenomena inside granular media, has been studied experimentally as well as numerically via discrete element simulations (Uenishi and Xi, 2022). However, as stated in both Uenishi and Goji (2018) and Uenishi and Xi (2022), the mechanical characteristics of granular materials have not been thoroughly comprehended so far, and especially those in light of fracture dynamics still remain unexplored. Here, for a deeper understanding of the granular fracture dynamics, multiple impact loads are given to a dry granular medium, and mechanical interaction of particles as well as fracture and stability of the granular medium are examined experimentally. 2. Granular media under single and delayed double dynamic impact loads Like the earlier study (Uenishi and Goji, 2018; Uenishi and Xi, 2022), 1403 penny-shaped birefringent particles (diameter 8 mm, thickness 3 mm) are cut out from polycarbonate plates by a digitally controlled laser cutter and piled up on a rigid horizontal plane to form a two-dimensional rectangular granular medium with 33 rows of particles. Single or double dynamic impact loads are given to the top free surface of the granular medium by one or two steel balls (sphere of diameter 30 mm, mass 110.3 grams) that are free-falling from a height of about 100 mm above the top surface. The time-dependent stress variations and fracture evolution have been recorded by a high speed video camera (Photoron FASTCAM SA5) at a frame rate of 20,000 frames per second (fps). From the experimental observations, the impact velocity of the steel ball is found to be 1.43 m/s, roughly equal to the theoretical value, some 1.4 m/s. In Fig. 1, the experimentally recorded photographs indicate the particle movement and isochromatic fringe patterns inside each particle for a granular medium subjected to (a) single and (b) delayed double impact loads. Photographs are taken at 0, 6, 12, 18 and 24 milliseconds (ms) after the first impact given by the steel ball. In the first experiment with a single impact load, Fig. 1(a), it is immediately noticed that particles just below the point of impact remain at rest while others move relatively largely. In Fig. 1(b) where the second impact is imparted 5.4 ms after the first one with a horizontal distance of 96 mm, relatively large particle movement as well as buckling-like jump of the particles on the top row and fragmentation of the granular medium can be found in a trapezoidal area. In order to see the dynamics behind the abovementioned particle movement, first, more detailed information for the single impact case is given in Fig. 2 where, in addition to the photoelastic isochromatic fringe patterns (a), the pictures showing difference of colors between the snapshots before and after the dynamic impact (b) are listed at a shorter temporal interval, from top to bottom, at time 300, 600, 900, 1,200 and 1,800 microseconds (  s) after the impact. In the figure, two lines of unidirectional (force-chain-like) stress transfer emerge after the impact of the steel ball. The particles below these two lines can transmit the kinetic energy rather easily, widely into the surroundings and thus into the far field in a scattered way, and they are mechanically at a stable state. On the other hand, above the two lines, the number of particles that can bear kinetic energy is relatively small due to the existence of the top