Estimation of cracking and damage mechanisms in rock under triaxial compression by moment tensor analysis of acoustic emission

In highly stressed conditions, the excavation damaged zone induced by stress redistribution and disturbance must be evaluated after tunnel excavation. Therefore, the deformation and fracture characteristics of rock must be investigated. In this study, the fracture and damage mechanisms of rock induced by the accumulation of microcracks were investigated by moment tensor analysis, as well as by the moving point regression technique, both of which were applied to acoustic emission (AE) and strain data obtained from triaxial compression tests. Damage thresholds before the peak strength of rock under triaxial compression were determined by the moving point regression technique using acoustic emission data. The results showed that damage thresholds, except the crack closure stress, increased linearly with confining pressure. The results of the moment tensor analysis showed that shear failure was a major microscopic failure mechanism of rock under triaxial compression. In addition, shear failure became more dominant as the confining pressure increased. In this analysis, the expression of the damage magnitudes in each AE source as relative crack volumes leads to accurate prediction of macroscopic failure mechanisms, as well as major failure planes in the rock. In addition, the orientation of the macroscopic failure plane could be estimated by the orientational distribution of microcracks.     

Analysis of fracture propagation behavior using hydraulically induced acoustic emissions in the Bernburg salt mine, Germany

Hydraulic fracturing experiments were carried out at the Bernburg salt mine in Germany. The induced acoustic emission (AE) events were detected by accelerometers surrounding the sources, and the observed AE events had a frequency component of over 40 kHz. Many similar waveforms were identified in the detected events, and their source locations were determined with an error of less than 6 cm. The principal direction of the source distribution changed with depth according to the rotation of the principal stress direction, and the shape of the fracture was determined to be elliptical. The source locations were distributed as an ellipse with a radius of about 1 m in both fracturing and re-fracturing tests at six different levels. A theoretical examination of fracture growth showed that macroscopic fractures extend with an aspect ratio which is a function of normal and shear stresses, and suggests that shear stress leads to the formation of elliptical fractures and that the elliptical shape of macroscopic fractures is controlled by the ratio of the shear stress to the normal stress.     

页岩水力压裂裂缝形态的试验研究

为深入认识页岩储层水力裂缝延伸规律及其空间形态,采用真三轴岩土工程模型试验机、压裂泵压伺服控制系统、Disp声发射定位系统和工业CT扫描技术,建立了一套室内页岩水力压裂大型物理模拟试验方法,并通过试验后页岩试样水力裂缝的延伸与空间展布规律,初步探讨了页岩水力压裂网状裂缝的形成机理。结果表明：裂缝延伸时泵压曲线典型的锯齿状波动与裂缝网络的形成密切相关,是页岩体积压裂的一个明显特征。页岩层理面的发育程度、泵压大小和地应力状态对裂缝形态有显著影响,水力裂缝在层理面内的分叉、转向进而沟通天然裂缝是形成裂缝网络的关键,而层理面过弱或过强都不利于网状裂缝的形成;对层理面胶结强度适中的地层,地应力对裂缝的延伸有较大影响;在低排量且维持较低泵压时,裂缝易于转向,且更易形成网状裂缝,而达到体积压裂。建立的页岩水力压裂物理模拟试验方法及试验结果可为页岩气压裂优化设计等提供技术支持。     

Quantitative analysis of fracture process in RC column foundation by moment tensor analysis of acoustic emission

After the great Hanshin earthquake, numerous studies have been made to evaluate damage levels, toughness and deformation characteristics of reinforced concrete (RC) columns exposed to the earthquake shock. In the present study, RC columns are subjected to simulated seismic lateral loading under different axial loads. Acoustic emission (AE) is monitored to characterize the seismic behavior of column foundation. AE is shown to be very sensitive to detect cracks generated in the foundation. An AE moment tensor analysis is applied to analyze the fracture process quantitatively. Experimental results demonstrate different fracture behavior, depending on the different axial loads. It is also shown that the moment tensor analysis is very useful for quantitative evaluation of the fracture process.     

Crack classification in concrete based on acoustic emission

The fracture mode of cracking in concrete is normally changing from tensile mode to shear mode at impending failure. As for crack classification in concrete by acoustic emission (AE) techniques, two crack classification methods have been carried out. One of them is parameter-based method (parameter analysis) which has been carried out by employing two parameters of the average frequency and the RA value. The proportion of these two parameters, however, has not been determined yet. The other crack classification is simplified Green’s functions for moment tensor analysis (SiGMA) procedure which is known as signal-based method. The SiGMA analysis is based on the generalized theory of AE, while the parameter analysis results from an empirical relation. Therefore, an optimal proportion of the parameter analysis is in great demand. In this study, these crack classification methods are compared and discussed from results of three types of concrete failure tests. As a result, ratios of the shear crack which are identified by SiGMA analysis are good agreement with those by parameter analysis in the case that the proportion of the RA value and the average frequency is set to 1–200.     

Acoustic emission moment tensor analysis: development for crack identification in concrete materials

The moment tensor analysis of acoustic emission (AE) provides quantitative information on kinematics of cracks. Simple AE moment tensor analysis has been developed and named SiGMA-AE analysis. In-plane unconfined compression tests of mortar and concrete plates with a slit are carried out. The inclination of slits are varied as 0, 30 and 45° to the loading axis. The sources are located from detected AE signals. Then, orientations of cracks, crack types and crack volumes are determined by the SiGMA-AE analysis. Furthermore, it is clarified that the damage parameters can be estimated from the moment tensor components. The results show close correlation between the calculated crack volumes and the damage parameters.     

Experimental and numerical crack analysis of mixed-mode failure in concrete by acoustic emission and boundary element method

Crack extension of mixed-mode in concrete is studied by applying the acoustic emission (AE) and the boundary element method (BEM). The theory of AE wave motions is briefly reviewed, including the historical development. AE waveforms are generated as elastic waves in concrete due to crack nucleation, and can be synthesized by the integral formulation, consisting of the spatial derivatives of Green's functions, the moment tensor, and the source-time function. Kinematics of cracks are represented by the moment tensors, which can be determined by a SiGMA (simplified Green's function for moment tensor analysis) code. In bending tests of notched beams, the crack extension of mixed-mode failure in concrete is studied. Crack traces are analyzed by the two-domain BEM, based on the maximum circumferential stress criterion by Erdogan-Sih, and are in remarkable agreement with those of tested beams. Then, locations, types, and orientations of AE sources are analyzed by the SiGMA to investigate crack kinematics in the fracture process zone. Furthermore, it is attempted to estimate the normalized stress intensity factors K_I*=K_I/K_IC and K_II*=K_II/K_IC.     

Acoustic emission characterization of microcracking in laboratory-scale hydraulic fracturing tests

Understanding microcracking near coalesced fracture generation is critically important for hydrocarbon and geothermal reservoir characterization as well as damage evaluation in civil engineering structures. Dense and sometimes random microcracking near coalesced fracture formation alters the mechanical properties of the nearby virgin material. Individual microcrack characterization is also significant in quantifying the material changes near the fracture faces (i.e. damage). Acoustic emission (AE) monitoring and analysis provide unique information regarding the microcracking process temporally, and information concerning the source characterization of individual microcracks can be extracted. In this context, laboratory hydraulic fracture tests were carried out while monitoring the AEs from several piezoelectric transducers. In-depth post-processing of the AE event data was performed for the purpose of understanding the individual source mechanisms. Several source characterization techniques including moment tensor inversion, event parametric analysis, and volumetric deformation analysis were adopted. Post-test fracture characterization through coring, slicing and micro-computed tomographic imaging was performed to determine the coalesced fracture location and structure. Distinct differences in fracture characteristics were found spatially in relation to the openhole injection interval. Individual microcrack AE analysis showed substantial energy reduction emanating spatially from the injection interval. It was quantitatively observed that the recorded AE signals provided sufficient information to generalize the damage radiating spatially away from the injection wellbore.     

Acoustic emission characteristics of concrete-piles

Acoustic Emission (AE) characteristics due to microcracking are studied in full-scale prestressed concrete-piles. By applying AE techniques, fundamental study on the fracture mechanism of the piles under both cyclic and monotonic loads is made. The prestressed concrete-piles are subjected to bending and shear loads. Crack growth is monitored, and three-dimensional (3-D) AE source locations are conducted along with crack classification and crack orientation by SiGMA analysis. The results obtained are compared with those of the pile integrity test (PIT) and visual observation. To investigate AE characteristics of damaged piles, uni-axial load is applied to the damaged RC piles installed in an experimental pit of sand. AE is monitored directly in the RC piles. In addition, indirect monitoring is carried out by using a wave-guide nearby the pile. In both the direct and indirect monitorings, AE sources are located, applying one-dimensional source location. Locations estimated are compared with the real damaged zone. Finally, conditions of AE generation of the damaged RC piles are discussed with respect to crack width and crack orientation.     

层理对页岩水力裂缝扩展的影响研究

 层理、裂隙等结构面的存在是实现页岩气藏储层体积改造的前提。为分析层理对页岩水力裂缝扩展的影响,在各向异性材料裂纹尖端应力场分布特征的基础上,开展切口与层理呈不同方位的圆柱形试样三点弯曲试验,研究页岩断裂韧性的各向异性特征,并揭示其断裂机制的各向异性,进而根据真三轴条件下页岩水力裂缝的延伸规律,探讨了层理在页岩网状压裂缝形成过程中的重要作用,结果表明：(1) 各向异性材料裂纹尖端的应力场和位移场不仅由应力强度因子决定,还与材料的弹性常数有关;(2) 切口与层理呈crack-arrester,crack-divider和crack-splitter三种方位时,页岩断裂韧性在crack-arrester时最大,crack-splitter时最小,各向异性显著,而层理面开裂和断裂路径偏移是引起断裂韧性各向异性的主要原因;(3) 页岩层理的弱胶结作用使其断裂韧性较小,阻止裂纹失稳扩展的能力较弱,而在垂直层理方向,断裂韧性较大,阻止裂纹扩展的能力较强,当水力裂缝垂直层理扩展时,在弱层理面处会发生分叉、转向,且在继续延伸的过程中会进一步沟通天然裂缝或弱层理面而形成复杂的裂缝网络,达到体积压裂。研究结果可为深入认识页岩气藏储层体积压裂形成条件及机制提供一定参考。     

岩石破裂过程中声发射特性的颗粒流分析

 以颗粒流理论和PFC程序为平台,根据矩张量理论建立细观尺度上岩石声发射模拟方法。该方法可同时给出声发射事件发生的时间、空间、破裂强度等特征,再现岩石裂纹孕育、发展和贯通过程,从而揭示岩石的破坏机制。结合室内花岗岩破裂全过程声发射特性试验研究成果,通过试验和计算结果的对比分析,验证该方法的可靠性,并得到以下结论：(1) 在试样峰值强度前,声发射事件次数较少、破裂强度较低,且在试样内部随机分布;从试样峰值强度起至破坏时的残余强度,声发射事件次数较多、破裂强度较高,且主要沿宏观破裂带附近分布。(2) 声发射事件次数随破裂强度变化近似呈正态分布。在均值与最大破裂强度之间,声发射事件累积数随破裂强度的降低近似呈线性对数函数关系。(3) 每次声发射事件所包含的微破裂数,随破裂强度的提高而增加,近似呈波尔兹曼函数关系。(4) 声发射事件次数与微破裂数近似呈负指数函数关系,即包含微破裂数越少的声发射事件,其所占总数的比例越大。研究成果可弥补现有声发射试验及模拟方法的不足,并作为一种新的手段为声发射试验与现场微震监测研究所利用。       

碳纤维片材加固含裂缝砼梁的弯曲破坏分析

借鉴复合材料力学的分析方法,建立一种分层剪滞模型,并结合线弹性断裂力学的能量法,研究了碳纤维薄板加固含裂缝素砼梁和钢筋增强砼梁的四点弯曲破坏问题,求得的破坏荷载大多与试验较吻合,为此类构件的弯曲破坏分析提供了一种新的研究途径和方法.     

页岩水力压裂物理模拟与裂缝表征方法研究

 采用真三轴岩土工程模型试验机、压裂泵伺服控制系统、Disp声发射三维空间定位技术、试验前后工业CT扫描水力压裂缝扩展形态的方法,建立一套页岩水力压裂物理模拟与压裂缝表征方法。由页岩水力压裂物理模拟试验可得：(1) 采用Disp声发射八探头三维空间定位监测方法,能实时有效地监测水力压裂缝的起裂位置;(2) 采用水力压裂后追踪红色示踪剂痕迹的方式,可实现水力压裂缝的空间形态描述;(3) 当水力压裂未形成沿天然层理面的贯通压裂缝时,易形成与天然层理面相交的压裂缝,并与层理面开裂后交叉形成网络裂缝。建立的页岩水力压裂物理模拟试验与表征方法,可进行页岩压裂施工参数的优化设计,为页岩气储层水力压裂开采提供技术支持。     

Evaluation of low-cycle fatigue damage in RC exterior beam-column subassemblages by acoustic emission

Since Acoustic Emission (AE) is effective in monitoring the initiation of cracks in materials and structures, it has been widely used as a real-time evaluation technique for damage in reinforced concrete (RC) elements such as beams or columns in moment-resisting frames. This paper investigates its applicability for assessing the low-cycle fatigue damage in RC exterior beam-column subassemblages, which are especially vulnerable parts of the RC frames when subjected to earthquakes. Two 3/5 scale specimens representing connections from the lower and upper part of a prototype building located in a moderate-seismicity area are tested under cyclic loading until collapse. Important differences between the two specimens are observed owing to the different size and amount of reinforcing steel. First, a significant increase in AE activity is observed when the steel reinforcement begins to undergo plastic deformations. Second, by means of the AE improved b-value method, it is clearly shown that the macroscopic fracture processes occur mainly during the loadings paths. Thus, on the basis of AE source location with four sensors situated around the beam-column joint, the AE is successfully correlated with the observed cracks. A strong correlation is also found between the accumulated plastic strain energy dissipated by the concrete and the accumulated AE energy. Based on this fact, a formula is proposed to predict the level of damage and the closeness to failure of an exterior beam-column subassemblage from the AE as recorded by sensors located near the joint.     

不同卸荷速率下大理岩破裂时效特性与机理研究

巷道开挖过程中卸荷速率对岩体破裂特性有显著影响,且破裂特性表现出明显的时间效应。针对岩体在卸荷条件下的受力特征,利用颗粒流程序,对脆性大理岩进行围压卸载数值模拟,研究不同卸荷速率下卸荷结束瞬间和卸荷后持续点的岩石试样破裂特性和机理。结果表明:围压卸载过程中,卸荷变形率△ε_i随卸荷速率的增大而减小,且侧向变形比轴向变形更敏感;在卸荷结束瞬间,微裂纹主要集中在试样上下端部并形成剪切破裂带,其中张拉裂纹数目是剪切裂纹的3~6倍,试样的破裂程度S_1随卸荷速率的增大呈指数递减规律变化;在卸荷后持续点,卸荷速率较慢的情况下岩石试样破坏形式为宏观剪切破裂面,卸荷速率较快的情况下岩石试样破坏形式为块体剥落及上下端部颗粒(块体)弹射,试样的破裂程度S_2随卸荷速率的增大呈指数递增规律发展;卸荷速率越快,达到卸荷后持续点时,试样累计释放的颗粒动能越大,岩爆程度越大。     

岩石破裂声发射压缩波到时拾取方法及其优化改进研究

 岩石破裂过程中的微震及声发射监测技术已广泛应用于岩石工程领域。P波到时自动拾取是进行岩石破裂源定位和矩张量反演等声发射(AE)技术研究的基础与关键,为提高到时拾取精度,分析Allen拾取法、Baer-Kradolfer改进拾取法、高阶统计量拾取法以及AR-AIC拾取法对模拟正弦信号振幅、频率与相位突变识别的敏感性。基于加拿大原子能公司地下实验室(URL)的隧道密封试验现场监测数据,对比分析几种拾取方法对不同信噪比(SNR)水平的信号拾取结果,研究表明。Allen拾取法、Baer-Kradolfer拾取法和高阶统计量拾取法具有更广的信噪比识别范围,特别对低信噪比水平的信号具有较强识别能力,进而提出对AR-AIC拾取法的改进思路。利用改进AR-AIC拾取法对真实声发射信号进行到时拾取,得到影响拾取精度的关键因素以及适用于工程尺度的局部范围的声发射信号的合理参量,进而成功对信噪比水平小于10的声发射信号进行自动到时拾取,研究认为高阶统计量法中的峰度拾取法是应用改进AR-AIC法初拾阶段的最优方法,其自动拾取与人工识别结果时差小于5 ?s的准确率为94%,表明提出的改进AR-AIC拾取法在实际应用中,特别是对低信噪比水平信号进行到时拾取具有良好的适用性。     

单轴加载条件下花岗岩声发射及波传播特性研究

 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。     

AE monitoring of corrosion process in cyclic wet–dry test

Recently, a number of deterioration of reinforced concrete (RC) structures due to salt attack have been reported. After corrosion of reinforcing steel-bar (rebar) is nucleated, expansion of corrosion products results in corrosion-induced cracks in RC. Thus, development of non-destructive evaluation (NDE) techniques is important for inspection of corrosion damage. It is reported that acoustic emission (AE) could identify the onset of corrosion in rebar and the nucleation of concrete cracking due to expansion of corrosion products in the corrosion process. In this study, AE techniques are applied to a cyclic wet–dry test of RC beams. It is confirmed that both the onset of corrosion and the nucleation of concrete cracking are clearly observed as two periods of high AE activity. Kinematics of micro-cracks are identified by SiGMA (Simplified Green’s functions for moment tensor analysis) analysis of AE. To compare with findings, cross-sections of rebars are observed by SEM (Scanning Electron Micrograph). From these results, a great promise for AE techniques to monitor the corrosion process in RC structures is clarified.     

基于声发射定位的自然裂隙动态演化过程研究

 利用MTS815岩石力学试验机和PCI-II声发射三维定位实时监测系统对取自锦屏II级水电站交通辅助洞的含自然裂隙大理岩岩样进行单轴压缩条件下的声发射(AE)测试,并结合AE振铃数实现对不同空间分布类型自然裂隙时空演化过程的精确定位和追踪。试验结果表明：含自然裂隙大理岩岩样受压破坏过程中的局部渐进特征显著,AE累计数曲线的上升与试验的加载过程和岩样内部的应力调整关系密切;单一型和平行型自然裂隙空间分布形式相对简单,在压应力水平较低时AE大事件(振铃数＞20)随着压应力的渐增沿自然裂隙面展布,当压应力接近峰值强度时,AE大事件在裂隙面末端部位大量集聚,并朝着最终的实际破裂方向扩展;含交叉型自然裂隙岩样体内的AE大事件在初始压密阶段分布相对均匀,但随着压应力的持续增加,裂隙面附近的AE数量逐渐增加,并向裂隙面的交叉部位集聚;混合型自然裂隙结构最复杂,但仍不难根据AE大事件的空间分布确定岩样内部的软弱部位及相应的应力场变化规律。以上试验和分析结果对于深入研究岩体破裂失稳机制具有一定的意义,也可为现场微震监测提供指导。     

真三轴卸载煤爆实验破坏特征演化分析

对煤岩体进行真三轴卸载煤爆实验,获得了其临界破坏应力、破坏碎屑分形维数及声发射参数特征,引入处理声发射波形的短时傅里叶变换方法获得时频演化特征,利用离散元颗粒流数值方法对该过程进行模拟揭示其细观损伤机制。研究结果表明:煤爆临界破坏应力状态为28.6 MPa/17.8 MPa/8 MPa,煤爆时刻声发射能量快速释放,能率达到峰值;量测粒径尺寸大于10 mm的喷出碎屑尺寸,按照粒度-数量方法算得的分形维数值为2.04;声发射时频特性在煤岩体煤爆实验过程中经历了由单峰低幅低频向双峰高频演化再过渡到单峰高幅高频,最后变为多峰高幅低频的过程,预示着破裂源由单一小尺度不断向复杂大尺度演化的损伤机制;通过颗粒流软件对煤爆过程进行模拟,发现煤岩模型试件内部主要存在张拉损伤,而随着模型加卸载不断进行,剪切破裂逐级增多,与声发射时频特性相吻合。