Orateur
Description
Particle shape is known to significantly influence packing behavior and mechanical response of granular media [1, 2]. Non-convex particles, in particular, exhibit complex interlocking mechanisms that affect both the packing state and shear strength [3]. However, a systematic understanding of how particle shape governs the transition from static packing to frictional failure remains open, particularly for highly non-convex particles. In this work, we investigate this relationship using hexapod-shaped particles with aspect ratios (⍺), which control the degree of non-convexity, ranging from 1 to 15. Nine samples of 10,648 mono-sized hexapods are prepared by means of isotropic compaction under periodic boundary conditions. The dense isostatic packings then served as initial states for triaxial shear simulations under quasi-static conditions. All the simulations were carried out using an in-house code (rockable) [4], which implements the classical Discrete Element Method (DEM) for arbitrary particle shape [5]. We observe a non-monotonic variation of the packing fraction of these packings with increasing α, while interlocking increases monotonically. The coordination number Z increases steadily with α, then rapidly beyond a critical value as highly non-convex hexapods establish contacts with second neighbors. The constraint number Zc (average number of geometrical constraints per particle) reaches a nearly isostatic value of 12, due to the absence of friction in the preparation process, for all non-spherical hexapods (compared to 6 for spheres), indicating a highly connected initial fabric that resists contact loss at the onset of shearing. The shear response with frictional particles reveals a clear connection between particle shape and shear strength. As α increases, the normalized deviatoric stress increases more rapidly, with peak strength rising by nearly 80% from spheres (α = 1) to hexapods with α = 9. This amplification arises from the ability of longer-armed hexapods to resist sliding and rotation through geometric interlocking, playing a more dominant role than friction alone. Despite this dramatic increase in shear strength, yet, the Mohr-Coulomb failure envelope passes through the origin at peak and critical states, ruling out geometric cohesion as a true material property. Future work will examine whether introducing true adhesive forces (e.g., through capillary bridges or cementation) would interact with particle shape to produce a genuine cohesion intercept.
Keywords: Discrete Element Method, Non-convex particles, Granular flow, Shear strength, Granular material
References
1. Trieu-Duy Tran, Saeid Nezamabadi, Jean-Philippe Bayle, Lhassan Amarsid, Farhang Radjai, Effect of interlocking on the compressive strength of agglomerates composed of cohesive nonconvex particles, Advanced Powder Technology 36, (2) 2025, 104780, https://doi.org/10.1016/j.apt.2025.104780
2. Trieu-Duy Tran, Saeid Nezamabadi, Jean-Philippe Bayle, Lhassan Amarsid, Farhang Radjai, Contact networks and force transmission in aggregates of hexapod-shaped particles Soft Matter, 20, 3411-3424, 2024, https://doi.org/10.1039/D3SM01762A
3. E Az´ema, F Radjaı, Stress-strain behavior and geometrical properties of packings of elongated particles. Phys. Rev. E 81, 051304 (2010).
4. Vincent Richefeu, Gaël Combe, Pascal Villard, Jean-Yves Delenne, Lhassan Amarsid, et al. Rockable. 2025 ⟨hal-04933604⟩
5. Cundall, P.A., Strack, O.D.L.: A discrete numerical model for granular assemblies. Géotechnique 29(1), 47–65 (1979)