Characteristic stress variation and microcrack evolution of granite subjected to uniaxial compression using acoustic emission methods

The size of mineral grain has a significant impact on the initiation and propagation of microcracks within rocks.In this study,fine-,medium-,and coarse-grained granites were used to investigate microcrack evolution and characteristic stress under uniaxial compression using the acoustic emission(AE),digital image correlation(DIC),and nuclear magnetic resonance(NMR)measurements.The experimental results show that the characteristic stress of each granite decreased considerably with increasing grain sizes.The inflection points of the b-value occurred earlier with an increase in grain sizes,indicating that the larger grains promote the generation and propagation of microcracks.The distribution characteristics of the average frequency(AF)and the ratio of rise time to amplitude(RA)indicate that the proportion of shear microcracks increases with increasing grain size.The NMR results indicate that the porosity and the proportion of large pores increased with increasing grain size,which may intensify the microcrack evolution.Moreover,analysis of the DIC and AE event rates suggests that the high-displacement regions could serve as a criterion for the degree of microcrack propagation.The study found that granites with larger grains had a higher proportion of high-displacement regions,which can lead to larger-scale cracking or even spalling.These findings are not only beneficial to understand the pattern of microcrack evolution with different grain sizes,but also provide guidance for rock monitoring and instability assessment.

Uniaxial experimental study of the acoustic emission and deformation behavior of composite rock based on 3D digital image correlation(DIC)

In this paper, uniaxial compression tests were carried out on a series of composite rock specimens with different dip angles, which were made from two types of rock-like material with different strength. The acoustic emission technique was used to monitor the acoustic signal characteristics of composite rock specimens during the entire loading process. At the same time, an optical non-contact 3 D digital image correlation technique was used to study the evolution of axial strain field and the maximal strain field before and after the peak strength at different stress levels during the loading process. The effect of bedding plane inclination on the deformation and strength during uniaxial loading was analyzed. The methods of solving the elastic constants of hard and weak rock were described. The damage evolution process, deformation and failure mechanism, and failure mode during uniaxial loading were fully determined. The experimental results show that the θ = 0?–45?specimens had obvious plastic deformation during loading, and the brittleness of the θ = 60?–90?specimens gradually increased during the loading process. When the anisotropic angle θincreased from 0?to 90?, the peak strength, peak strain,and apparent elastic modulus all decreased initially and then increased. The failure mode of the composite rock specimen during uniaxial loading can be divided into three categories:tensile fracture across the discontinuities(θ = 0?–30?), slid-ing failure along the discontinuities(θ = 45?–75?), and tensile-split along the discontinuities(θ = 90?). The axial strain of the weak and hard rock layers in the composite rock specimen during the loading process was significantly different from that of the θ = 0?–45?specimens and was almost the same as that of the θ = 60?–90?specimens. As for the strain localization highlighted in the maximum principal strain field, the θ = 0?–30?specimens appeared in the rock matrix approximately parallel to the loading direction,while in the θ = 45?–90?specimens it appeared at the hard and weak rock layer interface.

Effects of confining pressure on acoustic emission and failure characteristics of sandstone

In this study,uniaxial and triaxial compression acoustic emission(AE)tests were implemented to investigate the AE effect and failure characteristics of sandstone under different confining pressures(σ3).The evolution of AE parameters in the rock failure process and fracture fractal dimension characteristics after failure were analyzed.The results revealed that the activity of the AE signal is strongly related toσ3.The evolution of the Ib value can be divided into the I-fluctuation,II-stability,and III-decrease stages.In the first stage,the Ib value of the AE was relatively high,and the AE energy was low.Then,the Ib value tended to be stable;however,the fluctuation amplitude decreased,and the AE energy rapidly increased.In the stage of decrease,the AE energy sharply increased before the load approached the peak value,and the Ib value significantly decreased and dropped to the lowest point before the peak value.Asσ3 increased,the rock’s failure mode changed from tensile failure to shear failure and became more coordinated.As the confining pressure increased,the shape dimension decreased,and the order degree of rock failure increased.The confining pressure exerted a certain control effect on the rock failure.

Fracture behavior and acoustic emission characteristics of sandstone samples with inclined precracks

Sandstone samples with precracks of different dip angles were collected from a coal mine roof and subjected to uniaxial compression tests,and acoustic emission(AE)and scanning electron microscopy(SEM)were used to study how the crack dip angle affected the fracture mechanism.In the precracked sandstone samples,as the dip angle between the crack line and loading direction decreased,so did the peak stress and its completion time.The SEM observations revealed a fracture transition from tensile cleavage to shear slip,which was manifested by a microstructure change from aggregate to staggered.According to energy conversion,a decreased crack dip angle results in gradually decreasing total and dissipative peak energies,whose variation amplitudes at different stages are consistent with those of the peak stress of the samples.The decreased crack dip angle lowered the stress required to trigger the first appearance of AE energy peaks and ring-down counts,as well as shortening the period before the occurrence of the first AE peak signal.However,the AE energy and ring-down count during the failure stage after the stress peak increased gradually.A stepped increase was observed in the AE ring-down count curves,with each step corresponding to a jump in the stress-strain curve.From the characteristics of the AE signal of the fracture of a precracked rock sample,the occurrence of joints or faults in the rock mass can be reasonably inferred.This is expected to provide a new method and approach for predicting coal and rock dynamic disasters.

Characteristic strength and acoustic emission properties of weakly cemented sandstone at different depths under uniaxial compression

As coal mining is extended from shallow to deep areas along the western coalfield,it is of great significance to study weakly cemented sandstone at different depths for underground mining engineering.Sandstones from depths of 101.5,203.2,317.3,406.9,509.9 and 589.8 m at the Buertai Coal Mine were collected.The characteristic strength,acoustic emission(AE),and energy evolution of sandstone during uniaxial compression tests were analyzed.The results show that the intermediate frequency(125-275 kHz)of shallow rock mainly occurs in the postpeak stage,while deep rock occurs in the prepeak stage.The initiation strength and damage strength of the sandstone at different depths range from 0.23 to 0.50 and 0.63 to 0.84 of peak strength(σ_(c)),respectively,decrease exponentially and are a power function with depth.The precursor strength ranges from 0.88σ_(c)to 0.99σ_(c),increases with depth before reaching a depth of 300 m,and tends to stabilize after 300 m.The ratio of the initiation strength to the damage strength(k)ranges from 0.25 to 0.62 and decreases exponentially with depth.The failure modes of sandstone at different depths are tension-dominated mixed tensile-shear failure.Shear failure mainly occurs at the unstable crack propagation stage.The count of the shear failure bands before the peak strength increases gradually,and increases first and then decreases after the peak strength with burial depth.The cumulative input energy,released elastic energy and dissipated energy increase with depth.The elastic release rate ranges from 0.46×10^(-3)to 198.57×10^(-3)J/(cm^(3)s)and increases exponentially with depth.

Wavelet analysis of acoustic emission signals from thermal barrier coatings

The wavelet transform is applied to the analysis of acoustic emission signals collected during tensile test of the ZrO2-8% Y2O3 (YSZ) thermal barrier coatings (TBCs). The acoustic emission signals are de-noised using the Daubechies discrete wavelets, and then decomposed into different wavelet levels using the programs developed by the authors. Each level is examined for its specific frequency range. The ratio of energy in different levels to the total energy gives information on the failure modes (coating micro-failures and substrate micro-failures) associated with TBCs system.

Progressive Failure Analysis of Quasi-isotropic Self-reinforced Polyethylene Composites by Comparing Unsupervised and Supervised Classifications of Acoustic Emission Data

Unsupervised and supervised pattern recognition( PR)techniques are used to classify the acoustic emission( AE) data originating from the quasi-isotropic self-reinforced polyethylene composites,in order to identify the various mechanisms in the multiangle-ply thermoplastic composites. Ultra-high molecular weight polyethylene / low density polyethylene( UHMWPE / LDPE)composites were made and tested under quasi-static tensile load. The failure process was monitored by the AE technique. The collected AE signals were classified by unsupervised and supervised PR techniques, respectively. AE signals were clustered with unsupervised PR scheme automatically and mathematically. While in the supervised PR scheme,the labeled AE data from simple lay-up UHMWPE / LDPE laminates were utilized as the reference data.Comparison was drawn according to the analytical results. Fracture surfaces of the UHMWPE / LDPE specimens were observed by a scanning electron microscope( SEM) for some physical support. By combining both classification results with the observation results,correlations were established between the AE signal classes and their originating damage modes. The comparison between the two classifying schemes showed a good agreement in the main damage modes and their failure process. It indicates both PR techniques are powerful for the complicated thermoplastic composites. Supervised PR scheme can lead to a more precise classification in that a suitable reference data set is input.

用磁声法MAE检测钢结构构件应力的机理和应用

分析了磁声发射的产生与接收、磁声发射与磁致伸缩、磁声发射的应力检测机理,应力对磁声发射的影响,磁声发射的检测系统等几个方面.从这些方面说明了用磁声发射来实施应力检测是可行的.证明磁声发射的强度既受外磁场变化影响,又受应力变化影响,并且磁声发射的变化幅度与所受应力大小有一定的关系,据此可为磁声发射检测铁磁材料的应力状态提供依据.但由于磁声发射的强度和施加于构件上应力状态的关系比较复杂,所以结合磁声发射和巴克豪森BN两种方法各自优点的应力检测更比看好.

Research on Damage Localization of Plate-like Structure Using Mode Acoustic Emission and Matching Pursuit

Based on mode acoustic emission theory,the paper analyses the acoustic emission analog signal of thin steel plate using matching pursuit,then obtains the characteristics interpretation of the different frequency signal energy concentration degree; Combined with four-point arc positioning method,the papers researches the damage localization of the plate-like structure. Simulation experiment shows that this method can accurately detect and locate the damage. This can provide data support for further imaging research based on time reverse theory.

Failure modes and slabbing mechanisms of hard rock with different height-to-width ratios under uniaxial compression

To determine the relationship between slabbing failure and the specimen height-to-width(H/W)ratio and to analyze the conditions,characteristics,and mechanism of slabbing failure in the laboratory,uniaxial compression tests were conducted using six groups of granite specimens.The entire failure process was recorded using strain gauges and high-speed cameras.The initiation and propagation of fractures in specimens were identified by analyzing the monitoring results of stress,strain,and acoustic emission.The experimental results show that changes in the specimen H/W ratio can transform the macro failure mode.When the H/W ratio is reduced to 0.5,the macro failure mode is dominated by slabbing.Low load-bearing ability is observed in specimens with slabbing failure,and the slabbing fractures are approximately parallel to the loading direction.Moreover,the fracture propagation characteristics and acoustic emission signals of slabbing failure specimens show typical tensile failure characteristics,indicating that slabbing failure is essentially a special tensile failure.

Feature analysis of acoustic emission sources for rail crack detection by the finite element method

The safety of rail is very important for the development of high speed railway, and it is necessary to investigate the features of inner cracks in rail. In order to obtain the features of Acoustic Emission （AE） sources of inner cracks in rail, AE sources with different types, depths and propagation distances are examined for crack in rail. The finite element method is utilized to model the rail with cracks and the results of experiment demonstrate the effectiveness of this model. Wavelet transform and Rayleigh-Lamb equations are utilized to extract the features of crack AE sources. The results illustrate that the intensity ratio among AE modes can identify the AE source types and the AE sources with different frequencies in rail. There are uniform AE mode features existing in the AE signals from AE sources in rail web, however AE signals from AE sources in rail head and rail base have the complex and unstable AE modes. Different AE source types have the different propagation features in rail. It is helpful to understand the rail cracks and detect the rail cracks based on the AE technique.

加载速率影响下板裂化脆性岩石失稳破坏试验研究

深部脆性岩体开挖常发生板裂破坏,并可能进一步发生片帮剥落、岩爆等工程灾害,严重威胁深埋隧(巷)道施工安全。在深部岩体工程中,受构造应力、开挖条件和工程扰动等因素的影响,岩体开挖后应力重分布的速率、来压快慢均存在差别。为探究加载速率对板裂围岩失稳破坏的影响,采用脆性岩石加工成的板裂试样进行单侧限单轴压缩试验,对不同加载速率下试样宏观破裂、强度特性、声发射(AE)特征以及能量机制进行综合研究。结果表明:(1)低加载速率下试样发生大块剥落,整体失稳并发生分离,高加载速率下试样未发生大块分离,但有小块动力弹射现象;(2)低频、高幅信号的增多及b值的变化表明试样产生了突发式的裂纹失稳扩展,且这个过程中大小尺度破裂不断交替产生;(3)在平稳加载阶段,高加载速率使试样AE活动更加剧烈,而临近失稳及失稳破坏阶段则相反,且低加载速率下低频信号、大尺度破裂事件占比更大。因不同开挖方案、施工方法导致的围岩应力重布快慢差异,在工程上会导致围岩发生板裂后,进一步诱发不同模式破坏。

含Y型组合节理花岗岩单轴压缩破坏机理及声发射特征

利用劈裂法在花岗岩试件中预制不同倾角的Y型组合节理,进行单轴压缩实验,分析组合节理倾角对破坏模式、峰值强度、表面变形场、声发射能量释放特征的影响,探究不同破坏模式下的破坏机理.实验结果表明:1)含Y型组合节理岩石随节理倾角变化呈现整体破坏、楔形体弹射破坏和沿主节理面破坏三种主要模式;2)组合节理夹角、主节理倾角均与岩石强度具有负相关关系;3)破坏模式从整体破坏转变到弹射破坏、沿主节理破坏时,滑移变形集中区域由次节理向主节理转变,AE高能级事件诱发原因由主、次节理压密及滑移错动向主节理压密及滑移错动转变;4)随主节理倾角的不断增大,主节理面上释放的能量更高、能量的分布更加不均匀.

基于ZOA优化VMD-IAWT岩石声发射信号降噪算法

针对岩石破裂过程中产生的声发射(AE)信号夹杂大量噪声的问题,提出了一种基于斑马优化算法(ZOA)改进变分模态分解(VMD)并与改进的自适应小波阈值(IAWT)联合的声发射信号降噪算法。利用ZOA算法优选出影响VMD分解效果的模态个数K和二次惩罚因子α;通过相关系数将分解出的IMFs划分为有效分量、含噪分量和剔除分量;针对小波阈值(WT)降噪算法不具备自动调整小波基以及软、硬阈值函数存在偏差大和不连续的弊端,提出了IAWT算法去除IMFs中的噪声分量,并与有效分量合并重构,得到降噪后的AE信号。通过模拟和实测AE信号验证并与现有降噪算法对比,结果表明ZOA-VMD-IAWT降噪算法适合处理AE信号,信号的时频特征得以保留。研究结果可为岩石AE信号理论及实际工程应用提供参考。

基于VMD-HT和深度学习的流噪环境腐蚀损伤声发射识别模型

对在役管道进行腐蚀声发射监测的过程中,管内介质流动产生的噪声同样会被传感器接收,导致腐蚀信号被覆盖从而引发误判。针对这一问题,提出了一种基于变分模态分解(VMD)、希尔伯特变换(HT)和深度双向门限循环单元神经网络(BiGRU)的流噪环境腐蚀损伤声发射识别模型。该模型能够将原始信号自适应地转化成多个本征模态分量,并提取各分量的瞬时频率及谱熵构建多维时序特征矩阵,进而建立原始信号与多维特征之间的映射关系。为验证该方法的有效性,对在役管道进行腐蚀声发射监测试验。结果表明,所提模型在流噪环境下具有良好的鲁棒性,监测数据的识别准确率达96.3%,可作为一种解决在役管道腐蚀声发射监测的新方案。

基于声发射的张开型单裂隙岩石裂纹扩展行为特性研究

裂隙形态对岩石力学性能和裂纹扩展行为有重要影响,为此分别针对完整花岗岩试样和不同预制裂隙倾角(0°、15°、30°、45°、60°、75°、90°)的花岗岩试样开展了单轴压缩试验,并同步进行了声发射监测和高清摄像记录。试验测试结果表明,随着裂隙倾角的增大,岩石的裂纹起裂角度逐渐减小,岩石的峰值强度、起裂应力和峰值应变均呈逐渐增大趋势,而弹性模量呈现出先增大后减小的变化规律;岩石应力-应变关系曲线的初始压密段和线弹性段对应声发射平静期,荷载超过起裂应力后声发射进入发展期,荷载接近峰值强度时声发射处于高峰期直到岩石完全破坏;裂隙倾角越大,声发射高峰期的持续时间越长,累计振铃计数越多,岩石峰值强度与累计振铃计数之间有良好的线性关系,并量化了裂隙形态对岩石强度损伤影响规律;随着裂隙倾角的增大,岩石的破裂模式由剪切断裂逐渐向张拉断裂过渡。根据岩石轴向应力与累计声发射撞击数之间的关系曲线,并结合岩石起裂应力的计算方法,计算值略大于摄像观测值,二者之间的误差范围介于1.7%~6.9%,表明起裂应力的计算方法是合理的。上述成果对深入研究裂隙岩体的工程性质和破坏模式具有重要参考意义。

基于多尺度散布熵的磁声发射信号特征识别方法

在工程中对设备进行应力检测和微损伤检测时,采集磁声发射信号易受噪声干扰,同时其特征的提取也存在困难,为此,将变分模态分解与散布熵相结合,提出了一种基于自适应多尺度散布熵的磁声发射(MAE)信号特征识别方法。首先,设计搭建了检测实验平台,采集了Q345钢静载拉伸实验中0 MPa~400 MPa应力状态下的MAE信号;然后,采用变分模态分解方法,对磁声发射信号进行了自适应分解,生成了一系列从低频到高频分布的本征模态函数(IMF)分量;其次,计算了每个本征模态函数分量的散布熵值,构建了MAE信号的特征向量矩阵;最后,将特征向量矩阵输入到基于支持向量机建立的识别分类模型中,进行了信号的训练和识别。研究结果表明:使用基于自适应多尺度散布熵的磁声发射(MAE)信号特征识别方法,能够自适应地实现MAE信号的多尺度化目的,并且准确地识别出不同应力状态下的信号特征,分类识别准确率高达95.3704%,验证了该方法的有效性;说明基于自适应多尺度散布熵和多分类支持向量机的信号特征识别方法能够快速且有效地识别不同应力状态,在信号特征识别方面具有较好的应用潜力。

准中深层砂岩脆性及其对岩石破裂模式的影响规律

致密砂岩储层由于埋深大,受机械压实、溶蚀、交代等成岩作用的强烈影响,岩性特征十分复杂。随着油气资源勘探开发不断向深部储层发展,深层高地应力、高围压对储层脆塑性的影响严重左右了水力压裂效果和油气资源产量。准中深层致密砂岩油藏储量丰富,但由于储层埋深大、物性差且围压异常高,同时对其原位地质力学岩心脆性变化规律认识受限,使得传统油气开采手段无法满足生产需求。为此,结合物理实验和数值模拟技术,开展准中深层岩心的岩石力学特征、脆塑性转化、破裂模式及声发射特征差异性研究。结果表明:随着围压增加,岩石从脆性破坏逐渐过渡为塑性破坏,且破裂模式由劈裂破坏向剪切破坏转变,砂岩脆性减弱;基于能量平衡方法和归一化处理的6种脆性指数所反映的砂岩脆性总体上呈下降趋势,但不同脆性指数的数值范围以及与脆性的相关性各异。不同脆性数值岩心模型的三轴压缩实验表明:当脆性矿物含量较低时,岩石呈现单斜面剪切破坏;随着脆性矿物含量增加,岩石破裂表现为多破裂面复合破坏模式。岩石破裂过程的声发射特征与其脆性存在相关性,主要呈现群震型、前震-主震-后震型和主震型3种模式。

基于声发射参数分析的红层泥岩高周循环加卸载损伤及变形特性

为研究不同应力上限及加载频率条件下软弱红层泥岩的损伤及变形特性,基于声发射损伤监测手段开展高周循环加卸载试验,系统分析应力上限与加载频率对红层泥岩损伤及裂纹扩展模式的影响。结果表明:当应力上限低于屈服临界动应力值时,泥岩经历先剪缩后缓慢剪胀的过程,岩样中微裂隙的发展与黏土矿物的膨胀逐渐占据主导地位;低频AE信号产生于静载阶段,而高频AE信号主要出现在循环加载阶段,且随加载频率的增大,超过300 kHz的超高频声AE信号占比逐渐增大;红层泥岩循环荷载下所产生的裂纹总体以张拉裂纹为主,但第I阶段剪切裂纹占比相对其他阶段较大,且随着动应力幅值减小或加载频率的增大,剪切裂纹所占比例减小;在裂隙自愈合效应下,泥岩中由动载引起的损伤裂隙会被逐渐修复,且随着放置时间的增加,声发射Kaiser效应逐渐减小甚至消失。

三轴压缩条件下共面双裂隙复合类岩断裂机制

为了研究裂隙构型属性和围压对复合岩的破坏特性影响及裂隙在复合岩中的扩展规律,基于室内单轴和三轴压缩试验,采用PFC 2D数值模拟软件建立含共面双裂隙复合岩三轴压缩颗粒模型,研究裂隙复合岩在不同裂隙倾角、裂隙长度和围压下的破坏特性和声发射演化规律。结果表明:岩样的破坏模式主要受裂隙倾角控制,而岩样的完整性主要受裂隙长度和围压控制;随着裂隙倾角的增加,岩样的破坏模式呈现张拉破坏→剪切破坏→拉-剪复合破坏的变化规律,随着裂隙长度和围压的增大,岩样的宏观裂纹减少,完整性越好;岩桥的贯通模式主要受裂隙倾角、结构面和围压的影响,裂隙倾角和围压越大,岩桥越容易贯通,结构面的存在会抑制岩桥的贯通;裂隙倾角越大,微裂纹呈“弥散式”分布于岩样的现象越明显,裂隙长度和围压越大,微裂纹的聚集现象越明显,岩样微裂纹的损伤积累随裂隙倾角和长度的增大而减小,随围压的增大而增大;岩样在变形加载过程中,声发射计数整体随裂隙倾角的增大而增大,随裂隙长度和围压的增大而减小,裂隙倾角、长度和围压主要改变的是声发射计数的数值大小。