Identifying the real fracture hidden in rock microcrack zone by acoustic emission energy

Identifying the real fracture of rock hidden in acoustic emission(AE)source clusters(AE-depicted microcrack zone)remains challenging and crucial.Here we revealed the AE energy(representing dissipated energy)distribution rule in the rock microcrack zone and proposed an AE-energy-based method for identifying the real fracture.(1)A set of fracture experiments were performed on granite using wedgeloading,and the fracture process was detected and recorded by AE.The microcrack zone associated with the energy dissipation was characterized by AE sources and energy distribution,utilizing our selfdeveloped AE analysis program(RockAE).(2)The accumulated AE energy,an index representing energy dissipation,across the AE-depicted microcrack zone followed the normal distribution model(the mean and variance relate to the real fracture path and the microcrack zone width).This result implies that the nucleation and coalescence of massive cracks(i.e.,real fracture generation process)are supposed to follow a normal distribution.(3)Then,we obtained the real fracture extension path by joining the peak positions of the AE energy normal distribution curve at different cross-sections of the microcrack zone.Consequently,we distinguished between the microcrack zone and the concealed real fracture within it.The deviation was validated as slight as 1–3 mm.

Characteristics of acoustic emission signals in damp cracking coal rocks

A uniaxial load experiment on coal rocks at different stress rates was carried out, based on the characteristics of acoustic emission (AE) signals in cracking coal rocks, decomposition, de-noising and reconstruction for the AE signals through wavelet packet transform for solving the current problems created by the presence of noise in AE signals and the existing problems in AE signal processing. The results show that the various characteristics of AE signals in coal rocks cracking under different situations can be clearly reflected, after the AE signals are de-noised by the wavelet packet. Compared to dry coal rocks, the number of AE occurrences in damp coal rocks was significantly reduced, as well as the average amplitude. The number of AE occurrences in damp and dry coal rocks clearly increased with increases in the loading rate, but the largest amplitude of the AE signals in damp coal rocks has been reduced. There is no clear evidence of change in dry coal rocks.

Acoustic emission characteristics of rock under impact loading

Acoustic emission tests were performed using a split Hopkinson pressure bar system(SHPB) on 50-mm-diameter bars of granite, limestone, sandstone and skarn. The results show that the amplitude distribution of hits is not well centralized around 50 d B, and that some hits with large amplitudes, usually larger than 70 d B, occur in the early stages of each test, which is different from the findings from static and low-loading-rate tests. Furthermore, the dominant frequency range of the recorded acoustic emission waveforms is between 300 k Hz and 500 k Hz, and frequency components higher than 500 k Hz are not significant. The hit with the largest values of amplitude, counts, signal strength, and absolute energy in each test, displays a waveform with similar frequency characteristics and greater correlation with the waveform obtained from the elastic input bar of the split Hopkinson pressure bar system compared with the waveforms of the other hits. This indicates that the hit with the largest values of amplitude, counts, signal strength, and absolute energy is generated by elastic wave propagation instead of fracture within the rock specimen.

The Mechanical Characteristics and Damage Model of Helan Mountain Rock based on Acoustic Emission

With the risk of disappearing for the rock paintings considering long-term exposure in Helan Mountain,the freeze-thaw(F-T)cycling experiments were carried out with 12-hour F-T cycling(0,10,20,30,and 40 F-T cycles)under five kinds of confining pressures(5,10,20,30,and 40 MPa).The acoustic emission(AE)detect technology was used to reveal the rock fracturing characteristics during the triaxial compression test whole process.The stress-strain relation changes along with different confining pressures and F-T cycles.Peak stress and residual stress changes along with different confining pressures and damages,and the variation of axial stress-AE ringing counts-time changes along with different confining pressures and F-T cycles.The damage variable with AE parameter under F-T and force coupling was defined for the first time,and the damages model was established.The experimental results show that the F-T cycles lead to the decrease of rock strength and the gradual transformation of compression failure mode from brittleness to plasticity.The confining pressure provides a certain ability to resist deformation and inhibit crack growth for rock samples after F-T cycles.The temporal and spatial evolution law of AE counts well corresponds to the loading and failure process of the rock samples.The AE 3D positioning technology can accurately capture the development position and direction of internal cracks and pores of rock,and the failure form is conical shear failure.The established damage model has a better fittingness between the theoretical calculation results and the test results,and is reasonable to be used in the future for protection of Helan Mountain rock painting.

Laboratory acoustic emission study for earthquake generation process

Since the similarity in size distribution of earthquakes and acoustic emissions （AE） was found in the 1960s, many laboratory studies have been motivated by the need to provide tools for the prediction of mining failures and natural earthquakes. This paper aims, on the one hand, to draw an outline of laboratory AE studies in the last 50 years, which have addressed seismological problems. Topics include the power laws in which the similarity between AEs and earthquakes is involved and progress that has been made in AE technology and laboratory AE study. On the other hand, this study will highlight some key issues intensively discussed, especially in the last three decades, such as aspects related to the pre-failure damage evolution, fault nucleation and growth in brittle rocks and discuss factors governing these processes.

损伤岩石声发射演化特征及响应机制试验研究

岩石内部损伤演化是导致岩土工程灾害的原因之一,揭示岩石破裂过程中的关键参数响应特征可为岩石破坏后失稳状态识别提供依据,对于岩体工程灾害预警防控至关重要。通过循环加载-卸载(加卸载)试验制备不同损伤程度粉砂岩试样,利用损伤试样开展单轴加载试验,分析试样损伤程度与波速关系及声发射演化规律,探讨不同损伤程度岩石的声发射响应机制。结果表明:(1)损伤试样纵波波速随着损伤程度增加呈线性减小趋势,但声发射振铃计数由阶段性递增转变为加载全过程迅速增加,声发射能量在屈服阶段由小幅度突增转变为快速增加。(2)反映岩石内部不同尺度裂隙发展趋势的声发射b值峰值区由峰后破坏阶段向压密阶段转移,造成峰后破坏阶段失稳特征不明显,反映岩体内部声发射源集中程度和能量尺度的S值在压密至屈服阶段由低位中幅度波动转变为高位小幅度波动,表明岩石在峰前阶段产生非稳定破坏。(3)损伤试样破裂过程随着损伤程度增加由晶间滑移主导型转变为裂隙发育主导型,导致声发射高频低能(400~800 kHz、0~250 aJ)信号由零星出现转变为加载全过程密集分布,以及高频信号带变宽。(4)损伤岩石内部裂隙发育程度不同是导致声发射响应机制差异化的根本原因,岩石内部裂隙密度随着损伤程度增加而增大,造成其在受载过程中由渐进式稳定破坏模式向突发式非稳定破坏模式转变,同时加载过程中声发射信号活跃度及强度增强。研究获得了不同损伤程度岩石的声发射参数特征,可为工程岩体损伤程度识别提供理论依据。

基于MTS-AE单轴压缩下的煤岩损伤特征

基于MTS-AE单轴压缩条件下,对煤岩损伤、变形与破裂过程进行了测试;运用损伤变量理论,分析单轴压条件下煤样损伤破裂阶段的声发射能量-应力/应变参数与损伤变量之间的内在联系与特征规律。研究结果可对现场安全开采提供一定的理论指导。

基于AE的煤岩破裂与动态失稳特征实验及综合分析

要进行大尺度采空区衍生动力学灾害的准确预测预报,必须进行煤岩破裂与演化为动力学失稳的定量化实验的前期研究。大柳塔煤矿1-2煤处于厚风积沙、浅埋深、薄基岩工程与地质环境中,要实现6m厚煤层一次全厚开采,采后形成大范围采空区上覆煤岩破裂并演化为动力学失稳,极易诱发衍生灾害。基于对1-2煤破裂失稳的声发射相关实验,综合分析煤岩破裂声发射总事件、能率与时间的之间的定量关系,包括加载上升阶段、初始破裂阶段、加速破裂阶段、破裂失稳后能量下降阶段与回弹阶段,这有助于确定合理的失稳前兆判据,以期对开采过程中诱发动力灾害的危险源在线辨识及预警提供定量化预报。

片麻岩单轴压缩条件下破裂过程AE试验研究

本文针对小湾水电站坝基典型片麻岩实际,通过磨片试验分析了黑云母花岗片麻岩的矿物组成和结构,印证了岩石强度与矿物含量以及矿物强度有关。对该片麻岩进行了单轴压缩破裂过程声发射特性试验,研究了该黑云母花岗片麻岩的破裂失稳的声发射过程,分析了岩石内部裂纹随时间、应力、应变变化,对应声发射发展形态,这对于分析岩石破裂失稳机制十分有意义的。试验结果表明:片麻岩属于坚硬-极坚硬岩石,声发射活动规律大体相同,存在明显AE增强点。岩石破裂过程中的声发射振铃计数率适合用来预测岩石的破裂各个阶段。片麻岩的应力-应变关系满足2次多项式关系、应变-声发射累计数关系都满足3次多项式关系。有助于我们进一步认识岩石的破坏机理以及岩石破坏的前兆判据。

不同节理起伏角的非贯通岩样拉伸剪切过程声发射演化特征

拉伸剪切破坏通常发生在深层现浇隧道施工过程中。本文利用自主研发的多功能岩石力学试验系统,对含有规则锯齿状节理的类岩试样进行直接拉伸剪切试验,研究节理起伏角度变化对拉伸剪切强度、温度和声发射的影响。结果表明,试样的峰值剪切强度随着节理起伏角的增大而降低,节理起伏角的增大导致岩桥更容易拉伸破坏,同时也使裂缝轮廓呈现出明显的阶梯状特征。在接近失效时,试样都产生了强烈的声发射信号,并产生了更多的裂纹,同时伴随能量耗散和温度降低。在三个试样的加载过程中,声发射b值呈现出先增大后减小的总体趋势,在接近峰值强度时急剧下降。根据RAAF值对裂缝进行分类,结果显示,在初始加载过程中,裂缝主要为纯拉伸类型。当试样接近失效时,剪切裂纹和复合裂纹的数量明显增加。

预裂纹岩石材料损伤声发射能量统计特征研究

为了更好地理解岩体结构的基本特性和压缩破坏现象,使用加载设备、声发射系统和高速摄像机对砂岩试件进行了一系列单轴压缩测试。结果表明,砂岩样品的断裂过程分为3个阶段:原始的微裂纹闭合、微裂纹成核和宏观裂纹扩展;完好无损和单裂口试件的损伤能量遵循幂律分布,显示损伤临界。单缝样品的最佳指数(r)约为1.64,略低于完整样品的最佳指数;砂岩试件的拟合幂律指数在实验的不同阶段会发生变化,并逐渐趋于稳定。基于直方图分析和最大似然法,试验裂纹的存在可能会影响损伤能量的统计特性。裂纹倾角对最佳指数影响不大,但对试样的断裂方式有一定影响。研究为工程岩体结构的倒塌破坏预测提供了新的理论依据。

Comprehensive assessment on dynamic roof instability under fractured rock mass conditions in the excavation disturbed zone

The damage process of fractured rock mass showed that the fracture in rocks induced roof collapse in Yangchangwan Coal Mine, China. The rock mass was particularly weak and fractured. There occurred 6 large-scale dynamical roof falls in the excavation disturbed zone （EDZ） with the collapsing volume of 216 m^3. First, the field detailed geological environment, regional seismic dynamics, and dynamic instability of roadways were generally investigated. Second, the field multiple-index monitoring measurements for detecting the deep delamination of the roof, convergence deformation, bolt-cable load, acoustic emission （AE） characteristic parameters, total AE events, AE energy-releasing rate, rock mass fracture, and damage were arranged. Finally, according to the time-space-strength relations, a quantitative assessment of the influence of rock-mass damage on the dynamic roof instability was accomplished.

Structure instability forecasting and analysis of giant rock pillars in steeply dipping thick coal seams

Structure stability analysis of rock masses is essential for forecasting catastrophic structure failure in coal seam mining. Steeply dipping thick coal seams （SDTCS） are common in the Urumqi coalfield, and some dynamical hazards such as roof collapse and mining-induced seismicity occur frequently in the coal mines. The cause of these events is mainly structure instability in giant rock pillars sand- wiched between SDTCS. Developing methods to predict these events is important for safe mining in such a complex environment. This study focuses on understanding the structural mechanics model of a giant rock pillar and presents a viewpoint of the stability of a trend sphenoid fractured beam （TSFB）. Some stability index parameters such as failure surface dips were measured, and most dips were observed to be between 46° and 51°. We used a digital panoramic borehole monitoring system to measure the TSFB＇s height （△H）, which varied from 56.37 to 60.50 m. Next, FLAC^3D was used to model the distribution and evolution of vertical displacement in the giant rock pillars; the results confirmed the existence of a TSFB structure. Finally, we investigated the acoustic emission （AE） energy accumulation rate and observed that the rate commonly ranged from 20 to 40 kJ/min. The AE energy accumulation rate could be used to anticipate impeding seismic events related to structure failure. The results presented provide a useful approach for forecasting catastrophic events related to structure instability and for developing hazard prevention technology for mining in SDTCS.

Rockburst characteristics of several hard brittle rocks:A true triaxial experimental study

Rockburst is a typical rock failure which frequently threatens both human life and construction equipment during highly stressed underground excavation.Rock lithology is a control factor of rockburst.In this paper,rockburst tests were conducted on rectangular prismatic specimens of six types of intact hard brittle rocks,i.e.granodiorite,granite,marble,basalt,sandstone and limestone,under one-free-face true triaxial loading conditions.With the use of high-speed cameras,an acoustic emission(AE)system and a scanning electron microscope(SEM),rockburst of different rocks was investigated.The results show that the strainbursts of granodiorite,granite and marble were accompanied by tensile splitting near the free face,and consequently were relatively strong with a large amount of fragment ejection and kinetic energy release.For basalt,sandstone and limestone,failure was primarily dominated by shear rupture.The strainbursts of basalt and sandstone were relatively small with minor fragment ejection and kinetic energy release;while no burst failure occurred on limestone due to its relatively low peak strength.Rock strength,fracturing and fragmentation characteristics,and failure modes of different rocks can significantly affect rockburst proneness and magnitude.The AE evolution coupled with SEM analysis reveals that the differences in the inhe rent microstructures and fracture evolution under loading are the primary factors accounting for different rockbursts in various rock types.

Fracture evolution and localization effect of damage in rock based on wave velocity imaging technology

By utilizing wave velocity imaging technology,the uniaxial multi-stage loading test was conducted on siltstone to attain wave velocity imagings during rock fracture.Based on the time series parameters of acoustic emissions(AE),joint response characteristics of the velocity field and AE during rock fracture were analyzed.Moreover,the localization effect of damage during rock fracture was explored by applying wave velocity imagings.The experimental result showed that the wave velocity imagings enable three-dimensional(3-D)visualization of the extent and spatial position of damage to the rock.A damaged zone has a low wave velocity and a zone where the low wave velocity is concentrated tends to correspond to a severely damaged zone.AE parameters and wave velocity imagings depict the changes in activity of cracks during rock fracture from temporal and spatial perspectives,respectively:the activity of cracks is strengthened,and the rate of AE events increases during rock fracture;correspondingly,the low-velocity zones are gradually aggregated and their area gradually increases.From the wave velocity imagings,the damaged zones in rock were divided into an initially damaged zone,a progressively damaged zone,and a fractured zone.During rock fracture,the progressively damaged zone and the fractured zone both develop around the initially damaged zone,showing a typical localization effect of the damage.By capturing the spatial development trends of the progressively damaged zone and fractured zone in wave velocity imagings,the development of microfractures can be predicted,exerting practical significance for determining the position of the main fracture.

An experimental study on fracture mechanical behavior of rock-like materials containing two unparallel fissures under uniaxial compression

Strength and deformability characteristics of rock with pre-existing fissures are governed by cracking behavior. To further research the effects of pre-existing fissures on the mechanical properties and crack coalescence process, a series of uniaxial compression tests were carried out for rock-like material with two unparallel fissures.In the present study, cement, quartz sand, and water were used to fabricate a kind of brittle rock-like material cylindrical model specimen. The mechanical properties of rock-like material specimen used in this research were all in good agreement with the brittle rock materials. Two unparallel fissures（a horizontal fissure and an inclined fissure） were created by inserting steel during molding the model specimen.Then all the pre-fissured rock-like specimens were tested under uniaxial compression by a rock mechanics servocontrolled testing system. The peak strength and Young＇s modulus of pre-fissured specimen all first decreased and then increased when the fissure angle increased from 0？to 75？.In order to investigate the crack initiation, propagation and coalescence process, photographic monitoring was adopted to capture images during the entire deformation process.Moreover, acoustic emission（AE） monitoring technique was also used to obtain the AE evolution characteristic of prefissured specimen. The relationship between axial stress, AE events, and the crack coalescence process was set up： when a new crack was initiated or a crack coalescence occurred, thecorresponding axial stress dropped in the axial stress–time curve and a big AE event could be observed simultaneously.Finally, the mechanism of crack propagation under microscopic observation was discussed. These experimental results are expected to increase the understanding of the strength failure behavior and the cracking mechanism of rock containing unparallel fissures.

基于煤样破坏声发射特征的冲击地压评价预警研究

冲击地压作为深部煤矿开采中最常见的煤岩动力灾害,严重影响煤炭资源安全高效开采。精准及时的预测预报是冲击地压防治的关键。为研究煤岩体冲击前兆信息特征,建立高效精准的动态预测模型,通过对冲击倾向性煤样进行三轴压缩试验,获取煤样破坏过程声发射特征参数,选取典型冲击矿井工作面微震参数监测值,与实验室所得声发射参数进行对比,分析实验室和工程尺度下的声发射与微震参数特征之间的相似性,建立两者之间冲击声学信息的表征关系。结果表明,煤样压缩过程中声发射累计能量的“阶梯状”增长与大能量矿震事件发生前的“缺震”现象都揭示了煤岩体能量的孕育过程。同一能量指标在不同尺度下对大能量事件的发生具有相同的演化趋势,经量化后可作为冲击地压危险性预警指标。继而选取影响工作面冲击地压发生的静态因素与应力动态因素,结合所得微震参数指标分别赋予动态权重,并引入贝叶斯概率进行修正,构建冲击地压动-静态协同综合评价预警模型。利用该模型对河南某冲击矿井工作面危险性进行评价预警,获得优于传统评价模型的预警结果,展现了该模型的工程适用性与准确性。

持续开挖效应下结构面剪切力学性质与能量特征研究

基于地下岩体工程开挖扰动的实际情况,开展考虑持续开挖效应,充分反映岩体结构面应力调整过程的结构面剪切试验更具理论意义和工程应用价值。采用人工劈裂方法制备岩体结构面,开展了常规应力路径和持续开挖效应下的结构面剪切试验,系统研究了两种条件下结构面剪切力学性质、声发射特征和能量的演化规律。研究结果表明:开挖扰动荷载强度越大,结构面发生剪切破坏时的剪应力降整体上越大,但持续开挖效应下的剪应力降最大值仅为常规应力路径下剪应力降的48.57%;考虑持续开挖效应的结构面剪切过程中声发射活动主要集中于结构面受剪破坏和剪应力降产生时,且声发射活动强度与开挖扰动强度正相关,声发射振铃计数变化率极值明显小于常规应力路径下的相应值;持续开挖效应下试样裂纹发育密集度、结构面磨损区域及破坏程度均随开挖扰动强度增加而增加,但试样裂纹发育的密集程度和结构面破坏程度相对常规应力路径下的情况轻;考虑持续开挖效应的结构面发生剪切破坏时弹性应变能随扰动荷载增大而增大,均低于常规应力路径条件下的弹性应变能值,持续开挖效应降低了结构面剪切破坏的强度,但更易引发结构面发生剪切破坏。

岩石声发射b值及其计算方法研究

【目的】声发射技术被广泛应用于岩土工程等领域中,该技术对灾害的监测预警起到很好的辅助作用。其中,b值分析已经成为研究的一个重要方面。【方法】为了系统总结岩石声发射b值的相关研究,通过查阅总结已有的文献资料,对声发射特征参数b值近年来的研究现状及进展进行阐述,探讨b值的计算方法。【结果】通过对b值计算公式、数据处理方法的详细阐述,总结出两种常用的b值计算方法的优势和局限性。【结论】最后对岩石声发射b值的研究工作进行分析,为今后研究中的进一步解决相关问题提供思路,本研究成果可对岩石声发射b值的有关研究提供一些帮助。

不同应力路径下煤样变形破坏过程声发射特征的试验研究

利用RMT–150B岩石力学试验机对义马耿村具有冲击倾向性煤样进行常规单轴、三轴和三轴卸围压试验,研究在不同应力路径下煤样变形破坏过程中的声发射特征。试验结果表明,煤样在不同应力路径下加载变形破坏过程中产生的声发射特征有所差异。常规单轴压缩过程中各个阶段均有不同程度的声发射事件,与三轴筒内单轴压缩相比,声发射累计计数和能量明显偏大,破坏瞬时的声发射计数和能量大致相当;常规三轴压缩试验时,在围压作用下煤样屈服前声发射事件较少,进入屈服阶段声发射事件逐渐趋于活跃,计数和能量大幅度增大,标志煤样破坏前兆,破坏瞬时声发射计数和能量达到最大值;三轴卸围压试验时,在卸围压前煤样处于弹性阶段声发射事件较少,随围压逐渐降低,由正应力提供的摩擦力不断减小,煤样内部材料强度相对较低逐步屈服破坏形成微裂纹。屈服前期产生少量声发射事件,屈服后期声发射事件逐渐趋于活跃,计数和能量大幅度增大,标志煤样卸围压破坏前兆,破坏瞬间计数和能量同时达到最大值,与常规三轴压缩相比,声发射计数更大,能量则更高,表明三轴卸围压煤样破坏时更加强烈;常规单轴压缩煤样破坏产生的声发射累计计数和累计能量明显偏大,三轴筒内单轴、常规三轴压缩以及三轴卸围压试验时,声发射累计计数和能量大致相同,没有明显差异。