花岗岩破裂全过程的声发射特性研究

STUDY ON AE ACTIVITY CHARACTERISTICS UNDER UNIAXIAL COMPRESSION LOADING

声发射是许多材料发生脆性破坏(包括其微裂纹的初始、扩展)过程中伴随的很普遍的现象。应用两套声发射系统,研究在单轴压缩荷载条件下10个花岗岩样(70mm×70mm×150mm)破裂过程中,随加载时间、应力变化其声发射活动的特性;通过应用单纯形算法对声发射事件定位,分析岩样破裂过程中其内部微裂纹初始、扩展过程的空间演化模式。试验结果表明:(1)在整个岩石破裂过程中,声发射活动随加载时间、应力变化表现出不同的特征;(2)在初始加载阶段直至初始裂纹形成之前,其声发射活动不是很明显;一旦岩样出现初始裂纹,在其加载时间点和相应的应力点处声发射事件明显增多;(3)在裂纹初始形成之后到微裂纹扩展之前,声发射活动处于一段平静期,裂纹稳定扩展直至岩石完全破坏,声发射活动变得异常活跃,特别在微裂纹扩展的非稳定阶段,声发射事件随加载时间及应力变化率非常显著。对于岩样内部初始裂纹形成之后的“平静区”而言,初始裂纹形成之后,并非裂纹随着荷载或者应变的变化而直接扩展,而需要蓄积一定的加载能量,在能量蓄积一定程度之后再进行扩展,即岩石初始破裂之后,其内部应力场需要寻求新的平衡,新应力平衡达到之后裂纹才开始进一步扩展;同样,当岩石完全破坏之后,应力也没有立即完全释放,亦是达到新的应力平衡之后,才完全失去其强度。声发射事件定位结果直观的反映岩样内部裂纹扩展空间位置、扩展方向以及裂纹扩展的空间曲面形态,这对于深入研究岩石破裂失稳机制是十分有意义的。

Acoustic emission(AE),which is produced by the microcracks occurrence or growth,is an ubiguitous phenomenon associated with brittle fracture in many materials such as rock and concrete.AE technique,which is better than other methods,can monitor the real-time microfractures developed in the rock sample continuously.In this paper,AE technique was employed to study rock failure process.The rock failure process was investigated by using 10 granite specimens(70 mm×70 mm×150 mm),and AE sensors were surface mounted.A simplex location algorithm allows event location of first arrival times to be determined by AE sensors that are applied to crack initiation and propagation process.Also,the crack spatial evolution mode with loading time and stress changing during the total loading process was analyzed.The experimental results are displayed as follows:(1) AE activity represents different characteristics with the loading time and stress changing during the total loading process;(2) the quantity of AE events was few in the initial loading up to crack initiation when initial crack generated by AE events apparently increased;and(3) AE events were in quiet period after crack appears before crack propagation,and AE activity sharply increased from crack stable propagation up to crack unstable propagation,especially in crack unstable propagation step,thus AE events reach to the most quantity in the division strain.For the “quiet period”,when appeared after initial cracks are generated,cracks have no direct propagation with stress(strain) changing,and when loading energy accumulates to some extent,initial crack started to propagate.That is to say,after initial cracks are generated,the inner stress field needs to seek new stress equilibrium in the rock samples.After stress field reaches to a new stress equilibrium,initial crack propagation continues.Meanwhile,when rock fails,stress is no completely released.After stress arrives to a new stress equilibrium strength,rock will fail.The AE location results will also directly reflect the spatial position,direction and spatial curved face of crack propagation in the rock sample,which is very significant to the mechanism of rock failure.