Numerical study of inter‐particle bond failure by 3D discrete element method

Abstract

The cohesive-frictional nature of cementitious geomaterials raises great interest in the discrete element method (DEM) simulation of their mechanical behavior, where a proper bond failure criterion is usually required. In this paper, the failure of bond material between two spheres was investigated numerically using DEM that can easily reproduce the failure process of brittle material. In the DEM simulations, a bondedgrain system (composed of two particles and bond material in between) was discretized as a cylindrical assembly of very fine particles connecting two large end spheres. Then, the bonded-grain system was subjected to compression/tension, shear, rolling and torsion loadings and their combinations until overall failure (peak state) was reached. Bonded-grain systems with various sizes were employed to investigate bond geometry effects. The numerical results show that the compression strength is highly affected by bond geometry, with the tensile strength being dependent to a lesser degree. The shear, rolling and torsion strengths are all normal force dependent; i.e., with an increase in the normal force, these strengths first increase at a declining rate and then start to decrease upon the normal force exceeding a critical value. The combined actions of shear force, rolling moment and torque lead to a spherical failure envelope in a normalized loading space. The fitted bond geometry factors and bond failure envelopes obtained numerically in this three-dimensional study are qualitatively consistent with those in previous two-dimensional experiments. The obtained bond failure criterion can be incorporated into a future bond contact model.where the three fitting parameters are C 1 = 1.444, C 2 = À0.702 and C 3 = 0.168. Figure 7 presents the 2D experimental results adapted from [44]. In Figure 7(a), the end particle size (D s ) in the experiments was fixed at 12 mm, whereas the 528 Z. SHEN, M. JIANG AND R. WAN compressive displacements as marked in Figure 8(a). The black lines represent compressive forces, and the red lines represent tensile forces (in the online version of this paper). Figure 8(b) shows that the bond material initially crushes at the periphery, and the crushed zone extends inward progressively. No force is transmitted through the crushed zone. Figure 8(c) presents similar observations in 2D experiments. Comparison with experiments.The outermost peripheral zone of the bond material is mechanically the weakest. The longer (in the normal direction) this zone is, the less overall compressive force it can bear because of the increased susceptibility to lateral buckling. The buckling itself is a result of the complex stress distribution within the bond material. See more details on this issue in [36] where FEM was used to visualize the stress fields at a bond contact. This buckling mechanism can explain the end shape and slenderness effects to be discussed below. 3.1.3.2. End shape. The nondimensional quantity D s /D b characterizes the end shape of the bond material. A smaller D s /D b means that...

Discrete Element Method Analyses of Bond Degradation Evolutions for Cemented Soils

“…Upon reaching the failure criterion, the bond undergoes breakage, resulting in abrupt reductions in force and moment in the contact to their residual values. A bond failure criterion proposed in Shen et al [34] is simplified [33] and applied in the simulations. This bond failure criterion considers the bond size effect and the coupled effect of the normal force, tangential force, rolling moment, and twisting torque on the bond failure.…”

“…The bond compressive and tensile strengths, denoted by R b nc and R b nt , respectively, are expressed as follows [34]:…”

“…of the bond depend on the compressive strength R b nc , the tensile strength R b nt , and the bond size [34], where S b i (I = s, r, and t, denoting shearing, rolling, and twisting, respectively) captures the impact of the normal force on the shearing, rolling, and twisting strengths. A simplified form of S b i can be expressed as follows [33,34]:…”

“…) is a dimensionless normal force, and the strength envelope shape parameters m s = 0.5, m r = 0.3, and m t = 0.36 are adopted according to the data obtained by Shen et al [34].…”

“…In general, a bond may be subjected to the normal and shear forces, rolling moment, and torque. A unified failure criterion can be characterized as follows [34]:…”