不同围压卸荷速率下砂岩力学特性数值试验研究

Numerical study on mechanical properties of sandstone under varied unloading rates of confining pressure

现有关于岩石试件在卸围压条件下力学特性的研究成果,无法完全排除试件离散性及受载过程中应力路径变化对试验结果的影响。利用离散元法开展均质试件在恒定轴压与4种围压卸荷速率下的数值试验研究,对比、分析其宏观与细观力学特性及能量演化规律。结果表明:随着围压卸荷速率的降低,试件极限承载强度不断提高,试验过程中逐步出现轴压和围压的跌落与反弹,反映了试件经历多阶段突发式渐进破坏过程,由局部破裂不断积累导致试件向失稳方向发展。卸荷过程中,试件张拉破裂增长速率大于剪切破裂,以张拉破坏为主。4种条件下,随着形成每条裂纹的声发射事件数增加,总体震级波动范围逐渐减小,整体震级逐渐增大。随着卸荷速率的减小,能量密度出现明显的台阶式上升特征,卸荷过程中试件突发性破裂程度逐渐提高。相对高速卸荷以近似线性方式使试件短时间内能量密度迅速变化,相对低速卸荷条件下的试件则有相对充足的时间以阶梯状增加方式改变能量密度。本研究可为卸围压条件下岩石破坏失稳机制分析提供理论和工程参考。

Existing researches on the mechanical properties of rock specimens under unloading confining pressure suffers the inability of completely remove the impacts on testing results due to specimen differences and stress paths changes during loading process.This study carried out numerical tests with discrete element method by deploying homogeneous specimens suffering constant axial pressure and four unloading rates of confining pressure,and the macro⁃micro mechanic and energy evolution laws are compared and analyzed.The results showed that the ultimate bearing capacity of the specimens increased continuously with the decrease of the unloading rate,and the unloading processes of the confining pressure expressed obvious falls and bounces of the axial pressure and confining pressure.It indicated a multi⁃stage sudden and progressive failure process,consecutive accumulation of localized cracking contributed to the instability of the specimen.For the unloading process,the growth rate of tensile cracking was greater than that of shear cracking,and tension⁃induced failure predominated.Under the four unloading rate conditions,with the increase of acoustic emission events accompanying each cracking,overall fluctuation range of the seismic magnitude gradually decreased,but overall seismic magnitude increased step by step.With the decrease of unloading rate,energy density expressed apparent staged arising characteristic,and the extent of sudden cracking of the specimen increased gradually at the unloading process.Compared with the high⁃speed unloading that facilitated rapid changes of energy density of the specimen with an approximate linear manner,relative low⁃speed unloading allowed for enough time for the specimen to change the energy density with a staged increasing manner.This research could provide theoretical and engineering reference for the analysis of rock failure instability and mechanism under unloading confining pressure environment.