基于超二次曲面的颗粒材料缓冲性能离散元分析

自然界或工业中普遍是由非球形颗粒组成的复杂体系,与球形颗粒相比,非球形颗粒间的高离散和咬合互锁可使冲击载荷引起的能量有效衰减实现缓冲作用.基于连续函数包络的超二次曲面单元能准确地描述非球形颗粒的几何形态,并可精确地计算单元间的接触碰撞作用.本文采用离散元方法对冲击载荷作用下非球形颗粒物质的缓冲性能进行数值分析,并与圆柱体冲击的理论结果和球体冲击的实验结果进行对比验证.在此基础之上,进一步研究了筒底作用力在不同颗粒层厚度和形状等因素影响下的变化规律.计算结果表明:不同颗粒形状都存在一个临界厚度H_c.当HH_c时,缓冲率随H的增加而增加;当HH_c时,缓冲率的变化不再显著并趋于稳定值.此外,减小颗粒表面尖锐度和增加或减小圆柱形和长方形颗粒的长宽比都会提高颗粒材料的缓冲效果.

Discrete element analysis of buffering capacity of non-spherical granular materials based on super-quadric method

Granular system commonly encountered in industry or nature is comprised of non-spherical grains. Comparing with spherical particles, high discretization and interlocking among non-spherical particles can effectively dissipate the system energy and improve the buffer capacity. The superquadric element based on continuous function envelop can form the geometric shape of irregular particles accurately, and then contact collision action between particles can be calculated easily. In this paper, we provide a comprehensive introduction to particle-particle and particle-boundary contact collision. In addition, considering different shapes and surface curvatures under various contact patterns between super-quadric particles, the linear contact force model cannot be applied to the accurate calculation of the contact force, and a corresponding non-linear viscoelastic force model is developed. In this model, the equivalent radius of curvature at a local contact point is adopted to calculate the normal contact force, and the tangential contact force is simplified based on the contact model of spherical elements. To examine the validity of the algorithm and this model, we compare the discrete element analytical results with the analytical results for a single cylinder impacting a flat wall and the previous experimental results for spherical granular material under impact load, and this method is verified by good agreement between the simulated results and the previous experimental results. According to the aforementioned method, we study the buffer capacity of non-spherical particles under impact load by the discrete element method, and the influences of granular thickness and particle shapes on the buffer capacity are discussed. The results show that a critical thickness H_c is obtained for different particle shapes. The buffer capacity is improved with increasing the granular thickness when H < H_c, but is independent of the granular thickness and particle shapes when H > H_c. Moreover, the impact peak and initial packing fraction increase significantly with increasing the blockiness. Rectangular particles account for the highest packing fraction, and the packing fraction of cylindrical particles is higher than the packing fraction of spherical particles. Therefore, Rectangular particles are more likely to form dense face-face contacts and ordered packing structures with high packing fraction. These denser packings prevent the particles from their relatively moving, and thus reducing the buffering capacity of the particles. Furthermore, the impact peak and initial packing fraction decrease with increasing or reducing the aspect ratio of cylindrical particles and the aspect ratio of rectangular particles. The aspect ratio of particle can be used to adjust the dense packing structure and reduce the stability of the system. It means that the particles have more effective buffer capacity for the non-spherical particle system.