单轴加载条件下花岗岩声发射及波传播特性研究

Experiment study on acoustic emission and wave propagation in granite under uniaxial compression

采用声波、声发射一体化装置,研究单轴压缩下花岗岩波速与声发射演化规律,通过宏细观方法确定各应力门槛值,研究裂纹扩展不同阶段声发射演化及波传播规律。结果表明:细观裂纹的演化与宏观变形直接对应,由于微裂纹主要沿轴向扩展,导致轴向刚度对裂纹起裂及贯通的敏感度弱于非线性增长的侧向变形,瞬时泊松比曲线斜率变化点与应力门槛值对应,声发射测试确定的起裂应力比宏观应变法偏小,但反映了微裂纹的初始萌生;采用实测波速变化分析声发射震源的时空及幅值演化分布,较好地描绘了裂纹的扩展过程,由于不同阶段声发射信号的幅值及能量存在差异,导致声发射特征参数演化规律差异较大(尤其在损伤应力之后),AE能量在破坏前呈突发性增长,可作为灾害性破坏的前兆;加载初始阶段,由于微裂隙的闭合,波速及波幅均随应力逐渐增大,但增加速率逐渐下降,侧向波速在闭合应力附近基本达到峰值,此后一定阶段基本保持不变,但其他方向波速则继续增大,随着波传播方向与径向夹角的增大,波速增加幅度及波速下降点对应的应力(损伤应力前、后)逐渐增大,峰值应力附近对应波速下降幅度减小;波速受损伤演化的影响要滞后于声发射事件。

The acoustic emission（AE） and wave propagation in granite under uniaxial compression were measured with the ultrasonic wave and AE synchronous monitoring devices. The stress thresholds were determined with the macro-micro methods firstly. Then evolution of AE and wave propagation during different phases were studied. The result shows that the evolution of micro cracks is related to the macroscopic deformation closely. As the cracks propagate mainly along axial direction under uniaxial compression, the axial stiffness is less sensitive than the nonlinear lateral deformation to the crack initiation and coalescence. The slope variation point of the instantaneous Poisson＇s ratio coincides with the stress thresholds. Crack initiation stress determined by AE monitoring is smaller than that by the macro strain methods. However, the initiation of micro crack is accurately reflected by AE. The temporal and spatial hypocenter distribution of AE events was refined based on the measured variation of velocity, which depicts the localization of the cracks or their propagation well. Because the amplitude and energy of AE signals emitted at different stages differ greatly, the variations of the characteristic parameters related to the crack propagation are different, especially after the crack damage stress. The outburst increase of AE energy before rock failure can be a precursor to catastrophic damage. At the initial compaction stage, the ultrasonic velocity and amplitude increase with the axial stress. However, the increasing rate decreases gradually. The lateral velocity reaches the peak near crack closure stress and remains constant in a certain later stage. Meanwhile, velocities along the other directions keep increasing. With the increase of the angle to the radial direction, the increment and the stress corresponding to the decreasing point of acoustic velocity increase gradually. The influence of damage development on ultrasonic velocity lags behind that on the AE events.