Implications for identification of principal stress directions from acoustic emission characteristics of granite under biaxial compression experiments

The rock fracture characteristics and principal stress directions are crucial for prevention of geological disasters.In this study,we carried out biaxial compression tests on cubic granite samples of 100 mm in side length with different intermediate principal stress gradients in combination with acoustic emission(AE)technique.Results show that the fracture characteristics of granite samples change from‘sudden and aggregated’to‘continuous and dispersed’with the increase of the intermediate principal stress.The effect of increasing intermediate principal stress on AE amplitude is not significant,but it increases the proportions of high-frequency AE signals and shear cracks,which in turn increases the possibility of unstable rock failure.The difference of stress in different directions causes the anisotropy of rock fracture and thus leads to the obvious anisotropic characteristics of wave velocity variations.The anisotropy of wave velocity variations with stress difference is probable to identify the principal stress directions.The AE characteristics and the anisotropy of wave velocity variations of granite under two-dimensional stress are not only beneficial complements for rock fracture characteristic and principal stress direction identification,but also can provide a new analysis method for stability monitoring in practical rock engineering.

Measuring System for Interference Optical Fiber Acoustic Emission

A type of interference optical fiber acoustic emission sensor is described.With 10 -10 m level resolution,megahertz-level frequency and response time less than 1 μs,this sensor possesses prominent measuring stability and can be used in state supervision and trouble diagnosis.

Hypervelocity impact induced shock acoustic emission waves for quantitative damage evaluation using in situ miniaturized piezoelectric sensor network

Manmade debris and natural meteoroids, travelling in the Low Earth Orbit at a speed of several kilometers per second, pose a severe safety concern to the spacecraft in service through the HyperVelocity Impact(HVI). To address this issue, an investigation of shock Acoustic Emission(AE) waves induced by HVI to a downscaled two-layer Whipple shielding structure is performed,to realize a quantitative damage evaluation. Firstly a hybrid numerical model integrating smoothparticle hydrodynamics and finite element is built to obtain the wave response. The projectiles, with various impact velocities and directions, are modelled to impact the shielding structure with different thicknesses. Then experimental validation is carried out with built-in miniaturized piezoelectric sensors to in situ sense the HVI-induced AE waves. A quantitative agreement is obtained between numerical and experimental results, demonstrating the correctness of the hybrid model and facilitating the explanation of obtained AE signals in experiment. Based on the understanding of HVI-induced wave components, assessment of the damage severity, i.e., whether the outer shielding layer is perforated or not, is performed using the energy ratio between the regions of ‘‘high frequency" and ‘‘low frequency" in the acquired AE signals. Lastly, the direct-arrival fundamentalsymmetric wave mode is isolated from each sensing signal to be input into an enhanced delay-andsum algorithm, which visualizes HVI spots accurately and instantaneously with different sensor network configuration. All these works demonstrate the potential of quantitative, in situ, and real time HVI monitoring using miniaturized piezoelectric sensor network.

Experimental investigation on crack initiation and damage stresses of deep granite under triaxial compression using acoustic methods

Crack initiation stress and crack damage stress are two critical indices for assessing the fracture strength of rock mass.However,understanding the stress characteristics of crack initiation and damage under triaxial compression remains still immature.To address this problem,by acoustic monitoring,i.e.ultrasonic wave transmission and acoustic emission(AE),the integrated triaxial compression experiments were carried out on granitic specimens.The crack initiation and damage stresses were determined by wave velocity,wave amplitude and AE methods,respectively.The discrepancy of stresses for crack initiation and damage identified by these methods were examined.Results showed that the confinement affected the peak stress and corresponding strain,and these two parameters increased with increasing confining pressure.The ultrasonic wave velocity and wave amplitude first increased and then remained relatively stable,and finally decreased with increasing axial compressive stress.The number of AE events stayed at a relatively low extent until axial stress approached the peak;after that,the AE accumulative counts skyrocketed to the maximum.It also shows that for a given confinement,the stresses for crack initiation and damage identified by the wave amplitude method were the smallest,followed those by AE method and wave velocity method.Moreover,the stresses for crack initiation and crack damage identified by these methods increased generally with confining pressures.However,the rate of increment of these two crack stresses decreased with increasing confining pressure.In addition,the slight decrease in these two crack stresses ratios was noticed with increasing confining pressure.The findings are helpful to understand the crack stresses of deep rocks,in terms of support of deep underground engineering.

AE信号快速辨识

根据煤岩声发射监测特性以及声发射信号推断声发射源的技术,提出一种利用近场MUSIC算法快速识别煤岩声发射信号的方法。在煤矿AE信号监测中,利用该算法对声发射信号的空间位置进行估计,将该算法估计的结果与巷道形状及工作面相适配,可以基本定位声发射源的位置。仿真实验结果证明了该算法的有效性。

利用AE检测技术实现大型原油储罐的在线检测

与传统的储罐底板检测方法相比,AE检测方法是一种在线、高效、经济的检测方法,不需要清罐、置换、打磨,短时间内就可对罐底板整体完成检测。文中利用声发射(AE)检测技术,实现了对大型原油储罐底板腐蚀状态的在线检测。利用AE设备及传感器探测、记录、分析,进而判断声发射源,获取底板腐蚀状态的相关信息,通过分析事件定位及通道撞击图,结合AE特征参数,从而判定储罐底板所处的腐蚀活性状态。

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.

OPCM AE传感器特性的理论仿真与实验研究

基于均匀场和细观力学理论,对应用于结构损伤声发射(AE)检测的1-1型压电复合材料(OPCM)AE传感器,建立了它的等效力电模型,在此基础上,推出了其等效力、电参数,通过这些参数的对比,发现OPCM具有明显的正交异性的特性;同时,应用有限元仿真计算,得到了在x和z两方向上传感器受到单向应力波荷载作用时,电极面输出的电压存在明显差异;所设计的实验表明:OPCM能够区分出不同方向的应力波,并对特殊方向上的应力波具有强烈的响应,再次证实了OPCM传感器的正交异性特性。

Experimental study on wire breakage detection by acoustic emission

This paper experimentally investigated wire breakage detection in a steel cable by acoustic emission(AE)waveform.In the experiments,the attenuation laws of waveform amplitudes were discussed based on stress wave propagation in the wire,which was generated by kNocking and wire breakage.Then the wave velocity was calculated based on the reach time of the stress wave from each sensor.Finally,based on the waveform attenuation laws and the linear position method,the amplitude and energy of the source were confirmed through the measured waveform to identify the source category.The experimental results illustrated that the stress wave from different sources has a different frequency spectrum,and the amplitude attenuation factor varied with the stress wave frequency;high frequency waves had a greater attenuation factor.Compared with the other source,the wire breakage source contained a much higher energy,and thus,the wire breakage signal can be distinguished from the other source by comparing the non-attenuation energy at the source position.

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。

Evaluation of excavation damaged zones(EDZs)in Horonobe Underground Research Laboratory(URL)

Excavation of underground caverns,such as mountain tunnels and energy-storage caverns,may cause the damages to the surrounding rock as a result of the stress redistribution.In this influenced zone,new cracks and discontinuities are created or propagate in the rock mass.Therefore,it is effective to measure and evaluate the acoustic emission(AE)events generated by the rocks,which is a small elastic vibration,and permeability change.The authors have developed a long-term measurement device that incorporates an optical AE(O-AE)sensor,an optical pore pressure sensor,and an optical temperature sensor in a single multi-optical measurement probe(MOP).Japan Atomic Energy Agency has been conducting R&D activities to enhance the reliability of high-level radioactive waste(HLW)deep geological disposal technology.In a high-level radioactive disposal project,one of the challenges is the development of methods for long-term monitoring of rock mass behavior.Therefore,in January 2014,the long-term measurements of the hydro-mechanical behavior of the rock mass were launched using the developed MOP in the vicinity of 350 m below the surface at the Horonobe Underground Research Center.The measurement results show that AEs occur frequently up to 1.5 m from the wall during excavation.In addition,hydraulic conductivity increased by 2e4 orders of magnitude.Elastoplastic analysis revealed that the hydraulic behavior of the rock mass affected the pore pressure fluctuations and caused micro-fractures.Based on this,a conceptual model is developed to represent the excavation damaged zone(EDZ),which contributes to the safe geological disposal of radioactive waste.

基于平均频谱的储油罐罐底腐蚀声发射特征分析

针对声发射(AE)罐底检测过程中传统分析方法无法准确判断储油罐罐底腐蚀区域腐蚀程度的问题,提出了基于平均频谱的储油罐罐底腐蚀声发射特征分析方法,用于判断储油罐罐底板腐蚀类型.该方法利用平均值原理,分别求出罐底板不同腐蚀区域的平均频谱,再通过傅里叶逆变换得到该区域的平均波形,以此来表征腐蚀区域占主导地位的声发射信号特征.为了验证分析方法的有效性,进行了现场储油罐检测实验,实验过程中应用护卫传感器屏蔽罐内噪声干扰.实验结果表明,护卫传感器可以有效屏蔽大量罐内噪声信号,使定位结果更加准确.应用护卫传感器所得到的定位结果与开罐检测结果基本一致.在获得准确的声发射定位结果的前提下,应用基于平均频谱的分析方法,可以有效识别不同腐蚀类型,进而准确判断出罐底腐蚀程度.

基于聚类分析的罐底声发射检测信号融合方法

采用声发射技术进行罐底腐蚀与泄漏检测过程中,需要对多个传感器的检测到的声发射信号进行融合处理,将属于同一声发射源的声发射信号判定为一个声发射事件。但是在现场检测过程中,由于噪声的存在,使得在声发射信号融合处理时容易对声发射源产生误判。针对该问题,提出了一种基于聚类分析的罐底声发射信号融合方法,其基本原理是先根据事件定义时间进行初始声发射事件判定,然后采用聚类分析方法对初始声发射事件中的信号进行分类,将每一类信号分别判定为一个声发射源。现场实验表明采用该方法抗噪声干扰能力强、误判概率低,能准确反应罐底腐蚀的实际情况。

波导器声发射信号传播衰减理论及规律

根据波动动力学的相关理论,建立了一维波导器的弹性力学模型,得出了声发射波源信号与波导器接受端信号之间的关系.在此理论模型的基础上,设计了数值模拟及实验室方案,采用标准激振源法,考察了不同尺寸波导器中声发射响应情况,在此基础上统计分析得出了波导器尺寸、波导器与煤岩块接触程度及波导器弯曲程度对声发射信号的事件数、事件的影响.数值模拟结果与实验室结果相吻合,同时验证了理论模型的合理性.

含瓦斯煤体声发射应力波传播规律理论研究

基于含瓦斯煤岩体属于固-气两相孔隙介质,运用经典的弹性动力学理论进行AE信号的传播分析不能准确反映含瓦斯煤层中的传播情况.提出了固-气耦合两相孔隙介质模型,进行了两相介质AE传播理论的初步研究,推导了固-气两相孔隙介质的本构关系和波动方程,对含瓦斯煤层中的AE应力波进行了理论分析.结果表明:含瓦斯煤体双相介质中的横波方程仍然保持单相介质中的传播形式;纵波方程发生明显改变.

钢筋混凝土板声发射波传播特性试验研究

为了研究钢筋混凝土板的材料和结构参数对声发射关键参数的影响规律,设计制作了9个板试件,并进行了断铅试验和声发射测试。利用自制断铅装置进行断铅试验,确定了适用于钢筋混凝土板的门槛值和峰值鉴别时间等声发射检测参数的设置方法。通过断铅试验和声发射测试,研究了混凝土龄期、板厚、混凝土强度等级、粗骨料最大粒径和配筋等参数对声发射波速和振幅衰减的影响规律。结果表明,板厚和混凝土强度等级是影响声发射波速和振幅衰减的重要因素。波速随着混凝土板厚的增加而减小,随着混凝土强度等级的提高而增大;而板厚越大,混凝土强度等级越低,振幅的衰减越大。骨料最大粒径和配筋对声发射波速和振幅衰减影响较小。此外,还给出了振幅衰减值与传播距离的关系式,以及振幅衰减限值的建议值。

振动环境中螺栓连接结构状态声发射试验研究

螺栓连接应用广泛,螺栓松动可能造成灾难性的后果。结合声发射技术,在振动台上进行螺栓连接状态检测初步试验。对试验中所采集的大量声发射信号进行分析,找出不同扭矩下螺栓连接状态所对应的信号特征,实现对连接螺栓松动状态的辨识。

地下洞室围岩EDZ判别方法及标准

在引入EDZ概念的基础上,推荐采用声波、多点位移计、钻孔电视和声发射4种监测、检测技术方法来确定强、弱EDZ的深度。基于声波测试,通过分析波速-深度曲线特征和计算损伤因子D可以判别围岩EDZ;利用多点位移计和钻孔电视的监测检测成果,通过计算位移(张开量)-深度曲线和各区段的应变量,可以判别围岩EDZ;通过分析AE事件数及其频率的变化规律亦可以确定围岩EDZ深度。与单一指标的定性划分相比,提出的EDZ综合判别方法和判别标准具有客观、综合的特点,可以定性、定量地综合判别EDZ深度。

应力波作用下岩石声发射实验研究

在霍普金森(SHPB)冲击系统上进行了应力波作用下岩石声发射实验,获得了该加载条件下岩石破裂的声发射规律。实验结果表明,应力波下,声发射峰值能量之前所出现的声发射信号相当匮乏;而且其能量规律呈现出两种明显不同的特征:Ⅰ型,声发射峰值能量之后,能量迅速衰减,到了加载的末期,能量出现了一定的回升,产生了"拐点";Ⅱ型,声发射峰值能量之后,能量衰减相对Ⅰ型较慢,且不出现"拐点"。研究结果表明,声发射的能量特征与岩石破碎的程度密切相关;冲击载荷作用下岩石的声发射不仅仅是裂纹信息的反映,其中也包含了对应力波信号的反映。

砂岩单轴循环加卸载过程中声发射信号特征与时空演化规律

为深入研究循环荷载作用下岩石内部微裂纹萌生、发展及贯通的时空演化规律,以砂岩为研究对象,开展了砂岩在单轴循环加卸载下的声发射试验。结果表明:借助不同方向的波速测试,可获得声波在岩石中的传播速度,为声发射定位提供合理的波速参数;通过合理布置传感器的位置,可准确定位破裂面的空间位置;加卸载过程中声发射事件数和能量呈现台阶状上升,表明声发射具有记忆效应;通过声发射事件空间定位与破坏岩样的对比,验证了两者的位置基本一致,表明声发射可有效获得岩石内部微裂纹发展的全过程。