单轴压缩作用下岩石脆性破裂机制的声发射识别

Characterization of brittle failure modes of rocks under uniaxial compression using acoustic emission

监测和识别岩石变形过程的微破裂发育,有助于理解其脆性破裂机制以及预防工程灾害。基于声发射监测技术,对比分析了砂岩和花岗岩的单轴压缩破裂过程。基于声发射RA-AF裂纹分类准则,提出了裂纹分类参数累计曲线法,依据累计曲线斜率k值变化(-1~1)可推断出岩石破裂过程中张拉裂纹(k=1)和剪切裂纹(k=-1)的发育阶段,进而识别脆性破裂机制。宏观破裂特征显示,砂岩发生单斜面剪切破坏,花岗岩发生劈裂破坏。声发射撞击数、b值等特征参数分析表明,两种岩石脆性破裂过程中微破裂释放的弹性能的频数与能级特征相似,无法区别二者破裂机制。然而,累计曲线法结果显示,砂岩存在裂纹分类参数累计和线性增长(k=0.66)、非线性增长(k降低至0)和非线性降低(k降低至-0.37)等3个阶段,而花岗岩仅存在线性增长(k=0.83)和非线性增长(k降低至约0.5)两个阶段。微观结构显示,破坏砂岩的剪切带由翼型张拉裂纹组成,暗示张拉裂纹发育先于剪切裂纹发育。综合分析表明,张拉裂纹发育首先主控砂岩的变形,随后剪切裂纹发育增强,直至第3阶段剪切裂纹发育主控并导致剪切破坏;而花岗岩在变形第2阶段受张拉裂纹发育主控并发生劈裂破坏。基于声发射监测技术,裂纹分类参数累计曲线法有效识别了单轴压缩条件下的两种基本破裂模式,为识别复杂应力条件下岩石的脆性破裂过程和机制提供了简单途径。

Monitoring and distinguishing the development of microcracks in deformed rocks are helpful to understand the likely brittle failure mechanisms,which is essential for preventing rock engineering disasters.In this study,uniaxial compression experiments were performed on sandstone and granite samples to study the failure processes inversed by acoustic emission(AE)monitoring.Macroscopically,the sandstone samples exhibited a shear faulting mode while the granite samples an axial splitting mode.These two modes,however,cannot be clearly distinguished using the traditional AE methods via hits and the b value,as these parameters can reflect the frequency and energy level characteristics of elastic energy released during the cracking process only.Therefore,we propose a crack cumulative summation curve method,following the RA-AF crack classification method,which can successfully inverse and distinguish the possible different microcracking processes in deformed rock from the pure tensile microcrack development(k=1)to the pure shear microcrack development(k=-1)according to the variance of curve slope k value(i.e.,-1~1).Inversed results show that during deformation to brittle failure,the tensile microcracks development first dominates the deformation of sandstone(i.e.,k=0.66),followed by a transition between the development of the tensile and shear microcracks(i.e.,k decreased to 0),and finally by strongly developed shear microcracks(i.e.,k decreased to-0.37 from 0)dominating shear faulting.In contrast,the granite sample showed a macroscopic axial splitting mode,as no strong development of shear microcracks was observed,i.e.,k remains greater than 0,decreasing to 0.5 from 0.83.The combined effects suggest these two failure modes can be clearly distinguished by strongly developed shear microcracks in rock near failure.Microscopic observation performed on the failure sandstone clearly illustrated wing tensile cracks in the shear fracture zone.This supports our inversed results that macro shear fracturing of sandstone was formed upon the initiation of the tensile microcracks,followed by the development of the shear microcracks.This suggests AE monitoring using our proposed crack cumulative summation curve method can properly distinguish the development of microcracks and accordingly failure modes in the deformed rocks,and may be used for further study on understanding the failure process of rocks upon complex loading conditions.