岩石巴西劈裂强度与裂纹扩展颗粒尺寸效应研究

Study on particle size effects on strength and crack coalescence behavior of rock during Brazilian splitting test

基于一组经室内巴西试验结果标定的类岩石材料细观参数,通过颗粒流程序(PFC)建立圆盘试样离散元模型,对含不同颗粒粒度中心直切槽圆盘试样进行巴西劈裂模拟,分析颗粒尺寸对荷载-位移曲线、拉伸强度和破裂模式的影响,揭示裂纹扩展过程中细观力场、微裂纹以及能量演化规律。研究结果表明:切槽圆盘试样荷载-位移曲线可分为单峰值(type I)、峰值之后软化(type II)以及峰值之后强化(type III)3种;切槽圆盘试样拉伸强度显著比完整圆盘的低,降幅与切槽倾角和颗粒半径相关;当切槽倾角不变时,拉伸强度总体上随着颗粒半径的增大而增大;而当颗粒半径不变时,拉伸强度随着切槽倾角的增大而减小;当切槽倾角相同时,不同颗粒半径中心直切槽圆盘试样破裂模式显著不同,颗粒尺寸主要影响中心直切槽圆盘试样次生裂纹的萌生和扩展;边界对试样作的功首先用于克服颗粒间黏结以产生裂纹,裂纹在应变能的作用不断扩展,在裂纹产生之后,摩擦能才开始起作用;颗粒的运动程度很低,因此动能很小;边界能与抗拉强度总体上呈正比关系,即边界能越大,拉伸强度越大。

A discrete element disc specimen model was constructed using a set of microscopic parameters which were calibrated by the experimental results of intact rock-like material disc specimen during Brazilian splitting test in particle flow code（PFC）.Then PFC was adopted to simulate Brazilian test for central straight notched Brazilian disc（CSNBD） specimens. The effects of particle size on the load-displacement curves, tensile strength and failure mode of CSNBD specimens were analyzed, and the evolution laws of meso-force, micro-crack and energy during the process of macrocrack initiation, propagation and coalescence of CSNBD specimen were revealed. The results show that complete axial load-displacement curves can be divided into three types, i.e., single peak（type I）, softening after first-peak（type II） and hardening after first-peak（type III）.Compared with the intact disc specimen, the tensile strength of CSNBD specimen all decreases, and the reducing extent is related to the notch angle and particle size. By keeping the notch angle constant, the tensile strength shows a downwards trend with the increase of particle size, while keeping the particle size constant, the tensile strength decreases with the increase of the notch angle. The failure modes of CSNBD specimens are dependent on particle size. The particle size mainly affects the initiation and propagation of secondly cracks. The power produced by boundary wall is firstly used to overcome the bond among particles. After the crack initiation, friction energy begins to work, and the crack is propagated by strain energy. Since the particle moves slowly, the kinetic energy is small. The boundary energy is positively correlated with the tensile strength, i.e., the bigger boundary, the higher tensile strength.