抗转动特性对颗粒材料分散性失稳的影响研究

Impact of rotation resistance on diffuse failure of granular materials

对于岩土类的颗粒材料,在特定的应变加载路径下会发生非局部化的失稳现象,此时应力状态处于Mohr-Coulomb屈服面内,试样整体急剧失稳。采用颗粒离散元方法,研究抗转动特性对颗粒材料在等比例应变加载路径下宏、细观力学特性的影响。模拟发现,较为松散的试样更易发生分散性失稳,此时颗粒集合体的应力-应变状态满足Hill材料失稳准则。采用考虑颗粒转动的接触模型进行离散元模拟,通过改变颗粒间接触的转动摩擦系数,从宏观和细观层面分析等比例应变加载路径中颗粒材料的稳定性。颗粒抗转动能力的增强可以降低材料发生分散性失稳的可能性,随着转动摩擦系数的增加,应力路径由应变软化逐渐转为应变硬化,原本会发生分散性失稳的松散颗粒集合体表现出与密实颗粒集合体相似的宏观力学特性;颗粒集合体的内部结构表现出相应的细观作用机制,转动摩擦系数的增加有效地抑制了颗粒转动,虽然降低了颗粒体系的配位数,但增加了颗粒之间的接触力,增强了颗粒体系力链结构的稳定性和各向异性,形成稳定的结构持续抵抗外荷载的施加,从而试样整体不会形成松散的接触状态而失去稳定性。

Under the certain strain path, granular materials may exhibit non-localized failure mode corresponding to an abrupt occurrence of failure with the stress states within the Mohr-Coulomb yielding surface. Using the discrete element method （DEM）, mechanical properties of granular material with different rolling friction coefficients are examined under proportional strain loading paths. For particle assemblies of different densities, looser assemblies tend to have diffuse failure, which qualifies Hill,s second-order work criterion. A contact model considering rolling friction is adopted in DEM analysis. By changing the rolling friction coefficient, micro-macro mechanical properties of particle assembly are studied under a specific proportional strain-loading path. The enhancement of particle rolling resistance reduces the likelihood of diffuse failure. At the macro level, stress path changes from strain softening to strain hardening as the rolling friction coefficient increases, and the looser particle assembly exhibits the similar mechanical behaviours as the denser one. The micro-structure shows the corresponding mechanism of action. As the rolling friction coefficient increases, the angular velocities of particles and their distribution will be well controlled. Although the coordination number decreases, the contact forces are enhanced, and the instabilities and anisotropies of the force chain distribution can be improved in the system. All the microscopic parameters manifest a stable structure, which can resist the increasing load, thus the specimen will not form a loose contact state and diffuse failure will not occur.