围压卸载速率对砂岩力学性质影响数值试验研究

Numerical Experimental Investigations of the Effect of Confining Pressure Unloading Rate on Sandstone Mechanical Properties

【目的】为了排除天然岩石试件离散性与试验过程多种应力路径相互转换对试验结果的叠加影响,进一步探讨围压卸载速率对岩石破坏失稳的影响机理。【方法】利用离散元法开展均质数值试件在恒定轴压与两种固定围压卸载速率条件的数值试验研究,对比与讨论了两种围压卸载速率下的试件宏观与细观力学特性与规律。【结果】结果表明:两种条件下,宏观和细观破裂分布具有一定相似性和规律性,慢速围压卸荷条件较快速条件的试件承载强度更高,快速卸荷条件局部破裂分布密度相对更大。两种卸载速率试验过程出现轴压和围压的“跌落”和“上升”,反映试件经历了多阶段突发式渐进破坏过程,因局部破坏不断累积导致试件向整体失稳方向发展。两种卸载速率条件下的张拉破裂数量显著大于剪切破裂数量,张拉破裂增长速率大于剪切破裂,试件表现为张拉破坏为主,快速卸载条件破裂总数量相对更大。慢速卸载条件相对充分地调动了试件局部承载能力,使试件极限承载能力有所提高。【结论】两种条件下,20次以上声发射事件构成的裂纹分布程度最不均一,随着组成每条裂纹的声发射事件数增加,震级整体逐渐增大,快速卸荷形成裂纹过程总体释放能量相对更高,而慢速卸荷以相对较慢的速率传递应力,使裂纹形成过程释放能量相对更低、平稳且均一。

【Purposes】This work is conducted to exclude the superimposed effect of natural rock specimen discreteness and multiple stress path conversion in test process on results,and to further investigate the influence mechanism of the confining pressure unloading rate on rock failure and instability.【Methods】The discrete element method was used to carry out homogeneous numerical specimens under constant axial pressure and two fixed confining pressure unloading rates.The macroscopic and mesoscopic mechanical properties and laws of the specimens under two confining pressure unloading rates are compared and discussed.【Findings】The results show that the macroscopic and mesoscopic fracture distributions have certain similarity and regularity under the two conditions,the load-bearing strength of the specimen in slow confining pressure unloading rate is higher than that in fast condition,and the local fracture distribution density at the fast unloading condition is relatively larger.The axial and confining pressure“falls”and“rises”during the two unloading rate tests,which reflects that the specimens have undergone a multi-stage sudden and progressive failure process.The continuous accumulation of local damage leads to the overall instability of the specimens.The number of tensile fractures under the two unloading rates is significantly larger than that of shear fractures,and the growth rate of tensile fractures is greater than that of shear fractures.The slow unloading condition relatively fully mobilized the local bearing capacity of the specimen and improved the ultimate bearing capacity of the specimen.【Conclusions】Under the two conditions,the distribution of fractures composed of more than 20 AE events is the most uneven.With the increase of AE event numbers creating each fracture,the overall seismic magnitude increases gradually,and the overall energy release during the fast unloading process to form fractures becomes relatively higher,while slow unloading transfers stress at a relatively slow rate,resulting in a relatively lower,smooths,and uniform energy release during fracture formation.