抗转动对颗粒材料组构特性的影响研究

Impact of rotation resistance on fabric of granular materials

颗粒材料大多由不规则形状的颗粒组成,如砂土、谷物等,抵抗转动是不规则形状颗粒的固有特性。已有研究表明,颗粒抗转动特性对其宏观力学特性有显著影响。因此,在颗粒材料的细观数值模拟中或采用非圆颗粒,或在圆颗粒离散元模拟中采用考虑抗转动的接触模型。采用有限元-离散元耦合方法(FDEM)和离散元方法(DEM)分别对椭球形状颗粒和具有抗转动能力的圆球颗粒进行三轴剪切数值模拟,指出了采用抗转动接触模型考虑颗粒形状影响的局限性,并基于颗粒的局部排布结构揭示了形状影响的细观来源。峰值内摩擦角和剪胀均随着转动摩擦系数和形状偏离圆球程度而单调增加,但颗粒形状对它们的影响呈现出明显的收敛趋势。细观组构分析也表明,虽然颗粒形状和转动摩擦都能显著增强组构各向异性,但是组构各向异性的演化模式有明显的区别。造成以上结果的差异在于其抵抗转动的影响机制不同。转动摩擦是通过限制颗粒转动,增强了颗粒间的稳定承载能力,而非圆颗粒是通过咬合作用形成稳定的局部排列结构。由于椭球颗粒腹部比端部能够传递更大的接触力,颗粒受剪切后发生转动,颗粒长轴倾向于正交大主应力方向,呈现交错排列,颗粒间相互锁定。

Granular materials are mostly composed of irregularly shaped particles,such as sand,grain and so on.Resistance to the rotation is an inherent characteristic of irregularly shaped particles.Previous studies have shown that the anti-rotation characteristics of particles have a significant effect on their macro-mechanical responses.Therefore,the non-spherical particles or the spherical particles with a contact model considering rolling resistance are widely employed in the mesoscopic numerical simulations of granular materials.In this paper,the combined finite and discrete element method(FDEM)and discrete element method(DEM)are used to simulate the triaxial tests of ellipsoidal particles and spherical particles with rolling resistance,respectively.The limitations of the rolling resistance model capturing particle shape effects are pointed out,and the meso sources of shape influence are revealed from the perspective of particle configuration structure.Peak deviatoric stress and dilatancy both change monotonically with rolling friction coefficient and the degree of deviation from a spherical shape,but the influence of particle shape on them shows obvious convergence.The mesoscopic fabric analysis also shows that although both particle shape and rolling resistance can significantly enhance the fabric anisotropy,but there are significant differences in the fabric anisotropy evolution mode between the two particle systems.The difference of the above results lies in the different influence mechanism of rotation resistance.Rolling friction enhances the stable bearing capacity of particles by limiting the rotation of particles,while non-spherical particles form a stable local arrangement structure through interlocking.Since the middle of ellipsoid particles can transfer greater contact force than the end,the particles rotate during shearing,and the long axis of the particles tends to be orthogonal to the direction of the largest principal stress,presenting a staggered arrangement,which means that particles lock together.