Abstract: Based on the experimental results of two similar rocks having high/very low microcrack density under two extremely different loading conditions: fast constant stress rate (～6 MPa/m) loading and creep (at ～95% of the fracture strength) loading,the role of pre-existing microcracks and loading condition on the fracturing properties of granitic rocks was examined. The detailed spatio temporal distribution of acoustic emission (AE) events in these samples were monitored by the use of a high-speed multi-channel waveform recording system,which can record AE waveforms from 32 sensors mounted on the surface of tested sample without a major loss of events even for an event rate on the order of several thousand events per second. Event rate,b-value in magnitude-frequency relation,and hypocenter distribution of AE events characterize three typical long-term stages of microcracking activity during the fracturing process: primary,secondary and nucleation. Precursory anomalies on cracking activity such as event rate and b-value in the nucleation stage were observed. In general,a higher pre existing microcrack density or a lower loading stress-rate results in a longer nucleation stage,and therefore,increases the predictability of catastrophic failure event. Further,the physics of these stages is analyzed using the subcritical crack growth model for crack population of fractal size distribution based on the stress-aided corrosion theory.

Key words: rock fracture experiment, acoustic emission, fault nucleation, subcritical crack growth, b-value

摘要： 依据对具有较高微裂纹密度的筑波花岗岩和极低微裂纹密度的花岗斑岩2种极端的岩石标本、在等应力速率的快速加载(约6MPa/min)和蠕变加载(轴向应力保持在约95%破坏强度)2个极端的加载条件下的实验结果,讨论了预存微裂纹密度与加载速度对多晶质结晶岩变形破坏过程的影响。实验中利用高速多通道声发射波形数字记录系统(每秒可记录多达5000个声发射事件的32通道波形),获得了岩石标本破坏前微破裂活动的详细时空分布数据。声发射的发生率、震级-频度关系中的b值及震源的空间分布揭示出岩石变形破坏过程中微破裂活动具有3个典型阶段:初期阶段、第2阶段和成核阶段。尤其是在成核阶段,还观测到发生率和b值有大幅度前兆性起伏。一般而言,在相同加载条件下预存微破裂密度越高,或对同一岩石加载速度越慢,对应的断层成核过程就越长,因此最终破坏的可预报性也越高。为了进一步探讨微破裂活动的阶段性特征及其物理本质,还利用亚临界破裂扩展模型对声发射数据进行了理论分析。

关键词: 岩石破坏实验, 声发射, 断层成核, 亚临界破裂扩展, b值