Discrete element method analysis of non‐coaxial flow under rotational shear

Abstract

SUMMARYThis study focuses on non-coaxial flow behavior of cohesionless soil undergoing cyclic rotational shear, with a special interest in the effects of particle-scale characteristics. To this end, we perform a series of 2D discrete element simulations with various particle shapes, inter-particle coefficient of friction, initial density, and stress ratios. The validity and efficacy of the numerical model is established by systematically comparing numerical simulation results with existing laboratory testing results. Such comparison shows that the numerical simulations are capable of capturing mechanical behavior observed in laboratory testing under rotational shear. We further demonstrate and quantify a strong yet simple relationship between the deviatoric part of the normalized strain increment and the non-coaxial angle, denoted by Δε ð Þ R q and ψ, respectively. This quantitative correlation between ψ and Δε ð Þ R q is independent of applied stress ratio, initial and current void ratio, and the number of cycles applied, but dependent on the principal stress orientation and particle-scale characteristics. At the same Δε ð Þ R q , specimens with higher inter-particle friction angle or smaller particle aspect ratio show greater non-coaxial angles. A simple model ψ ¼ 45 ∘ m Δε ð Þ R q is able to fit this ψ-Δε ð Þ R q relationship well, which provides a useful relationship that can be exploited in developing constitutive models for rotational shearing.

Macro deformation and micro structure of 3D granular assemblies subjected to rotation of principal stress axes

“…And the principal direction of strain increment aligns more in the stress increment direction, as shown in Fig. 20, consistent with experimental study [15,28].…”

“…21b. Sample DEB00Y09 is less contractive than sample DEB00Y09, consistent with the experimental observation [15,28]. It is interesting to note that the sample sheared at low b value (b = 0.0) experience a dilative strain in the out-of the plain with ε yy being negative while sheared at a higher b value (b = 0.5), ε yy is highly contractive.…”

“…It is known that the intermediate principal stress has a significant effect on the material deformation to stress rotation [15,28]. To investigate this effect, the specimen has been sheared at different b values (b = 0.0, b = 0.5 and b = 1.0), and then subjected to principal stress rotation with η = 0.9.…”

“…Numerically, discrete element method (DEM) [21] has been used in numerous studies on material responses to biaxial/triaxial shearing and direct/simple tests [22][23][24][25][26]. Responses of two dimensional granular materials to rotation of principal stress axes have been investigated [27,28]. The trend of noncoaxial deformation and contractive volume changes have been qualitatively reproduced and explained based on structural evolution.…”

“…External loading is applied on the boundary consisting of a string of boundary particles [26,[28][29][30][31] or a set of rigid mass-less surfaces [21,27,32]. For particle-based boundaries, the boundary particles are often chosen according to their positions as the out-most particles.…”