Propagation patterns of microseismic waves in rock strata during mining:an experimental study

Microseismic monitoring has been widely used in mines for monitoring and predicting dynamic disasters such as rockbursts and waterbursts. However, to develop high-precision microseismic monitoring systems, the propagation patterns of microseismic waves under complex geological conditions must be elucidated. To achieve this aim, a simulation model of a typical coalmine was designed using similar materials according to the similarity theory to simulate the mining process. Geophones were embedded into the model to detect the propagation of elastic waves from microseisms. The results show that in an unmined solid rock mass, the wave velocity in shallow rock strata is mainly affected by geologically weak planes, whereas in deep strata it is affected mainly by the density of the rock mass. During propagation, the amplitude first decreases and then increases rapidly with increasing propagation distance from the coal layer. After mining, our results indicate that the goaf causes significant attenuation of the wave velocity. After the goaf was backfilled, the velocity attenuation is reduced to some extent but not eliminated. The results of this study can be used as guidelines for designing and applying microseismic monitoring systems in mines.

Piezoelectric-based Crack Detection Techniques of Concrete Structures:Experimental Study

Feasibility of a wave propagation-based active crack detection technique for nondestructive evaluations （NDE） of concrete structures with surface bonded and embedded piezoelectric-ceramic （PZT） patches was studied. At first, the wave propagation mechanisms in concrete were analyzed. Then, an active sensing system with integrated actuators/sensors was constructed. One PZT patch was used as an actuator to generate high frequency waves, and the other PZT patches were used as sensors to detect the propagating wave. Scattered wave signals from the damage can be obtained by subtracting the baseline signal of the intact structure from the recorded signal of the damaged structure. In the experimental study, progressive cracked damage inflicted artificially on the plain concrete beam is assessed by using both lateral and thickness modes of the PZT patches. The results indicate that with the increasing number and severity of cracks, the magnitude of the sensor output decreases for the surface bonded PZT patches, and increases for the embedded PZT patches.

Experimental study on dynamic fracture behavior of three-point bending beam with double deformity inclusions

The dynamic fracture behavior of the three-point bending beam with double deformity inclusions under impact loading is studied by using digital high-speed photography in combination with the transmission-type dynamic caustic method. The experimental results indicate that the fluctuation of crack propagation velocity v first increases and then decreases in the crack propagation process. During the process of crack propagating into the inclusion area, the fracture resistance effect of the circular inclusion is the most significant and the effects of triangular and square inclusions are less obvious. The stress intensity factor near the crack tip increases during the propagation process and reaches its maximum value when the crack tip is close to the inclusions. The crack tip’s dynamic stress intensity factor （ DSIF） decreases when the crack exceeds the middle area of the double inclusions. These results provide an experimental basis and scientific foundation to strengthen the evaluation and fracture analysis of the structure containing deformity inclusions.

Dynamic caustics experimental study on interaction between propagating crack and deformity inclusions in primary structure

The approach combining the dynamic caustics method with high-speed photography technology is used to study the interaction between propagating cracks and three kinds of deformity inclusions（ cylinder inclusion, quadruple inclusion and triangular inclusion） under lowvelocity impact loading. By recording the caustic spots of crack tips at different moments during the crack propagation, the variation regulations of dynamic stress intensity factors（ DSIF） and crack growth velocity with respect to time are obtained. The experimental results showthat the resistance effects to crack growth are varied with different shapes of inclusions in specimens, and the quadruple inclusion＇s effect is more apparent. The distortion degree of caustic spots is affected by the shapes of inclusions as well, and the situation is more serious for cylinder and quadruple inclusions. The overall values of DSIFs of triangular inclusion specimen are greater than the others, and the crack growth velocities, characteristic sizes and DSIFs showprocesses of fluctuations because of the disturbance of reflection waves in specimens. The results provide an experimental basis for the analysis of strength and impact-resistance ability in structures with deformity inclusions.

基于DIC及CPG技术的热冷循环后花岗岩Ⅰ型断裂特性

深部储层岩石的热力学特性,尤其是循环热冷作用下损伤破坏特性,对于增强型地热系统井壁稳定性分析及地热开采效率评估具有重要意义。针对中心直裂纹半圆盘(NSCB)花岗岩试样,首先进行不同热冷循环处理(加热温度400℃,最高循环次数13次),然后开展3点弯Ⅰ型断裂韧度特性试验。基于裂纹扩展计(CPG)和数字图像相关(DIC)测试技术,研究了热冷循环作用下花岗岩Ⅰ型断裂韧度、断裂过程区(FPZ)、裂纹扩展速率及断裂轮廓特征的影响规律。试验结果表明:当热冷循环次数达到10次以上,花岗岩试样脆性明显减弱,而峰前软化特性和峰后延性增强;花岗岩的断裂过程区由裂缝尖端开始逐步孕育,断裂过程区长度随荷载增大呈先增大后减小的趋势,Ⅰ型断裂韧度、最大断裂过程区长度及裂纹平均扩展速度随热冷循环次数增大而指数减小,Ⅰ型断裂面随热冷循环次数增大越来越不平整。最后,基于X-ray衍射(XRD)和扫描电镜(SEM)等微观测试技术,研究了热冷循环作用对花岗岩矿物成分及微观结构的影响规律,结果表明花岗岩4种矿物成分的峰值衍射强度及矿物质量分数均随热冷循环次数增大而降低,而微裂纹大小及数量随热冷循环次数增大而增大。热冷循环作用下花岗岩的损伤劣化机理包含了多次高温热损伤、水冷冲击及水弱化等3方面的联合作用结果。

Effect of loading rate on fracture behaviors of shale under mode I loading

In this study,the effect of loading rate on shale fracture behaviors was investigated under dynamic and static loading conditions.Cracked straight through Brazilian disc(CSTBD)shale specimens were tested with a split Hopkinson pressure bar(SHPB)setup and INSTRON1346 servo-testing machine under pure mode I loading conditions.During the test,the crack propagation process was recorded by high-speed(HS)camera,and the acoustic emission(AE)signal generated by the fracture was collected by acoustic emission(AE)system.At the same time,crack propagation gauge(CPG)was used to measure the crack propagation velocity of the specimen.The results show that the crack propagation velocity and fracture toughness of shale have a positive correlation with the loading rate.The relationship among the crack propagation velocity,the fracture toughness and the loading rate is established under the static loading condition.In addition,the characteristics of AE signals with different loading rates are analyzed.It is found that the AE signals generated by microcrack growth decrease with the increase of loading rates.Meanwhile,the turning point of cumulative counting moves forward as the loading rate increases,which shows that the AE signal generated by shale fracture at low loading rate mainly comes from the initiation and propagation of microcracks,while at high loading rate it mainly comes from the formation of macro large-scale cracks.The fracture mechanism that causes shale fracture toughness and crack propagation velocity to vary with loading rate is also discussed based on the analysis results of AE signals.

裂隙砂岩应变场演化与超声时移衰减特征研究

微裂纹萌生、扩展和聚结模式的识别是研究岩体灾害孕育演化过程的基础。为探究裂隙岩石微裂纹发育过程及机制,采用主动超声测量和数字图像相关(digitalimagecorrelation,简称DIC)技术同步监测单轴压缩下预制裂隙砂岩的损伤破裂过程,分析了表面应变场演化及超声衰减特征。结果表明:小倾角预制裂隙尖端的局部拉应力集中利于裂纹更早萌生;随裂隙倾角增加,预制裂隙试样由较为稳定的渐进性破裂转变为突发性破坏,其脆性特征也更明显,表面应变场可实时追踪裂纹的萌生和扩展。P波波速、振幅谱和超声振幅的衰减与微裂纹的发育和宏观裂纹的形成密切相关,可将超声波主频的明显衰减视为宏观裂纹形成的直接证据;不同射线路径P波速度及振幅衰减的差异,是轴向应力和预制裂隙诱导损伤累积各向异性的结果。此外,采用改进谱比法分析了超声衰减的时移特征,发现超声波衰减比P波波速对岩石介质中微裂纹的发育更敏感,进一步对比发现超声振幅、表面应变、P波波速对岩石损伤识别的敏感性依次降低。该研究结果表明,主动超声衰减和DIC表面应变同步监测是识别和量化岩石损伤和裂纹扩展先兆信息的有力工具。

High-Temperature Deformation and Low-Temperature Fracture Behavior of Steel Slag Rubber Asphalt Mixture Surface Layer

Steel slag is regarded as one of the most widespread solid by-products of steel smelting with little commercial value.It can play a vital role in the construction industry especially in the field of transportation infrastructure construction.However,there are few evaluation systems established on the high-temperature deformation and low-temperature fracture behavior of steel slag rubber asphalt mixture(SSRAM).This study explores the perfor-mance of SSRAM by uniaxial penetration test,Semi-Circular Bending(SCB)test and evaluates test data through regression analysis.The uniaxial penetration test results shows that the failure deformation of SSRAM increases with the increase of steel slag content.According to the minimum allowable permanent deformation(R TS-min),the deformation of SSRAM should be controlled within 3 mm.Meanwhile,the cracking index of the SSRAM surface layer calculated at low temperature can meet the design requirements.The SCB test results show that the stress peak degradation rate(specimens with 10 mm notch are compared with 0 mm)of SSRAM with 40%steel slag content is 20.04%.That means proper steel slag content makes the stress peak degradation rate of SSRAM reaches the lowest value.The calculation results of fracture energy density(J_(1C))show that the steel slag additive reduced the fracture energy density of SSRAM.However,it is still proved that SSRAM with 40%steel slag has the best low-temperature fracture performance based on critical fracture toughness(K_(1C))and fracture stress peak.Further-more,the crack propagation velocity parametric equation of SSRAM is proposed through fracture mechanics theory and the increase of velocity is exponential.Considering the high-temperature deformation resistance and low-temperature fracture property,the SSRAM surface layer with 40%steel slag content showed a batter application potential.

金属疲劳损伤在线监测方法综述

疲劳在线监测的主要目标是监测疲劳裂纹萌生及获得疲劳裂纹扩展速率,从而提供预警。声发射在线监测技术因其能实现对构件的疲劳失效历程开展实时、在线监测,而在疲劳失效在线监测方面得到了普遍的使用。本文简要介绍了应变片监测和光纤Bragg光栅监测技术,重点介绍了声发射技术在发电设备方面的疲劳失效过程的在线监测研究进展。

AN EXPERIMENTAL-NUMERICAL METHOD FOR MEASURING CRACK PROPAGATING VELOCITIES UNDER STRESS WAVE LOADING

An experimental-numerical method for measuring dynamic crack propagatingvelocities under stress wave loading is established in this paper. The experiments of thethree-point bend specimen are done on the improved Hopkinson bar. Deflection of loading point,dynamic load and instantaneous crack length are measured, then crack propagating velocities arecalculated. Experiments on 40Cr steel show that the results given by this method have a goodagreement with that obtained by the resistance fracture gage method. Therefore this method isfeasible for measuring crack propagating velocities under high loading rate and will have wideapplication.

基于高密度电阻率法的土体干缩裂隙动态发育过程精细监测研究

土体在气候作用下发育干缩裂隙是一种常见的自然现象,裂隙的存在会极大弱化土体的工程性质,诱发许多岩土与地质工程问题。为了实时掌握黏性土中干缩裂隙网络的发展状态,提出了一套基于高密度电阻率层析成像技术(ERT)的土体干缩裂隙动态发展过程精细监测方法。分别开展模型试验及原位试验,利用自行研制的测定系统持续采集电流-电位差数据,随后利用自行开发的有限元法电阻率层析成像(FemERT)系统进行数据处理,获取了裂隙网络在不同发育阶段的空间分布特征。结果表明:(1)ERT可以实现土体裂隙发育过程的精细监测,具备监测三维裂隙网络几何形态的能力,裂隙宽度的识别精度达到毫米级,裂隙深度的识别精度达到厘米级;(2)ERT的感度分布特征解释了裂隙发育对于土体电阻率的影响规律,测定电阻值时程曲线因裂隙产生位置的不同而呈现不同的变化规律;(3)反演电阻率及其相对变化率(Rev)可以直观表征裂隙网络在不同阶段的空间几何形态,凸显裂隙动态发育过程对于土体导电性的影响。

含不同张开角度裂纹结构的动态断裂试验研究

不同结构的动态断裂行为受多个因素影响,其中裂纹缺陷对其影响最为显著。张开型裂纹是工程结构中十分常见的缺陷,而且张开角度往往并不固定。为研究结构的运动裂纹与不同预制角度裂纹相互作用的规律,以张开角度为单一变量,采用动态焦散线试验系统,在冲击荷载下对含不同张开角度裂纹的有机玻璃试件进行三点弯试验。研究发现:裂纹扩展经过不同张开角度的预制裂纹时,都会先扩展至预制裂纹夹角尖端,再从预制裂纹其中一端重新蓄能起裂,然后偏向落锤点方向扩展,最终贯穿整个试件;开口向上的预制裂纹会增加运动裂纹的扩展时长,而且预制裂纹的开口角度越大,对裂纹扩展产生的迟滞效应越大,裂纹贯穿试件的总时长越大;与预制裂纹相互作用阶段,动态裂纹都会先减速后增速至峰值,预制裂纹角度越大,裂纹扩展速度峰值越小,且当预制裂纹角度为180°时的峰值与无预制裂缝试件的峰值非常接近;运动裂纹再次从预制裂纹尖端处起裂时裂纹尖端应力强度因子迅速大幅增加后又迅速减小直至试件完全断裂。

一种基于多元逐步回归的裂纹定量监测模型

飞行器在长期服役过程中,机体结构受到交变载荷的作用,导致结构的应力集中部位萌生裂纹,并随着载荷的持续而进一步扩展。因此,通过传感器及数据处理技术对典型结构进行持续监测,特别是裂纹扩展情况的监测,对保证飞机服役的安全性、安排检修间隔等具有重要意义。本文针对金属结构疲劳裂纹扩展监测问题,提出了一种基于多元逐步回归分析的定量监测模型,通过相关性对导波信号的损伤参量进行选择,探索合理的模型方程,并通过多元逐步回归分析建立损伤参量与裂纹长度间的定量关系,最终实现结构裂纹扩展的定量监测。

基于光纤光栅传感的金属材料裂纹扩展检测研究

为快速、准确地检测金属材料中的裂纹,避免严重事故和重大经济损失,介绍了一种基于光纤布拉格光栅传感器的金属材料裂纹检测方法。该方法利用光纤布拉格光栅的高灵敏度和优良的光学传感特性,将光纤布拉格光栅传感器通过胶封装方式粘贴在含预制裂纹的金属试件上,通过拉伸试验获得金属材料扩展过程中的应力-应变关系,发现在裂纹扩展路径上,裂纹近端产生的应变小于远端产生的应变,即裂纹逐渐向传感器远端的方向扩展,从而实现了对金属材料裂纹扩展的实时监测和定位。

超大跨径斜拉桥钢桥面板疲劳损伤监测与断裂力学研究

为研究超大跨径斜拉桥钢桥面板的疲劳损伤问题,以某斜拉桥为工程背景,对实桥进行了现场疲劳损伤监测与分析,并基于断裂力学的三维裂纹扩展模型,对钢箱梁顶板-U肋和横隔板-U肋等焊接细节进行了数值仿真与研究。结果表明:实桥顶板-U肋焊缝细节高应力幅(大于10 MPa)循环次数与疲劳损伤度明显低于横隔板-U肋细节,横隔板-U肋焊缝最大应力幅达到75~90 MPa,顶板-U肋焊缝最大应力幅为15~30 MPa,横隔板-U肋焊缝细节处裂纹数量远大于顶板-U肋焊缝细节处裂纹数量;顶板-U肋焊缝裂纹在扩展过程中基本保持平面,裂纹扩展有先沿焊缝方向纵向扩展,再向深度方向扩展的趋势;横隔板-U肋焊缝焊趾处裂纹先沿初始裂纹深度方向在横隔板扩展,再向横隔板厚度方向扩展,焊趾处裂纹先向U肋厚度方向扩展,后沿初始裂纹长度方向顺桥向扩展;在初始裂纹尺寸与荷载条件相同的情况下,顶板-U肋焊缝焊趾处裂纹扩展速度大于焊跟处裂纹扩展速度,横隔板-U肋焊缝焊趾处裂纹扩展速率大于横隔板焊趾处裂纹扩展速率。

基于分布式光纤传感的混凝土裂缝识别与监测试验研究

钢筋混凝土结构的裂缝宽度超过规范限值后会引起渗水、钢筋锈蚀等问题,破坏混凝土结构的整体性和安全性。结构裂缝监测是评估结构安全性的重要依据之一,分布式光纤裂缝监测技术(BOTDA/R)可有效避免点式检测空间不连续造成的漏检现象。本文根据钢筋混凝土结构裂缝形成机理和前期研究提出的斜交光纤组裂缝监测方法,对混凝土裂缝光纤监测进行了参数敏感性分析。分析结果表明采用斜交光纤组可跟踪监测宽度大于0.04mm以上的裂缝发生及开展过程。依据分析结果,本文改进了光纤布设方式监测混凝土微裂缝,实现了微裂缝的发生及开裂路径的监测。本文通过钢筋混凝土梁的静载试验,分别验证了梁侧全面粘结光纤的微裂缝识别和梁底斜交光纤组的裂缝宽度跟踪监测的有效性。

加载速率对岩石声发射活动的影响

岩石声发射活动主要是由岩石内部裂纹的产生、扩展及断裂产生的。采用二维裂纹构形,运用断裂力学理论,导出了加载速率与裂纹扩展的关系。理论推导表明:对于岩石裂纹扩展产生的声发射活动,试验加载速率越大,裂纹越容易实现失稳扩展,声发射活动越强烈。试验证实了理论推导结果。

无网格法的理论及应用

详细论述了近年来迅速发展的无网格法的理论基础及其在各个领域内的应用.无网格法网格依赖性弱,避免了传统的有限元、边界元等基于网格的数值方法中可能出现的网格畸变和扭曲,在一些有限元、边界元等方法难以较好处理的领域体现出独特的优势.以加权余量法为主线归纳了已有的30多种无网格法,各类无网格法的主要区别在于使用了不同的加权余量法和近似函数.详尽介绍了各种无网格近似方案(包括移动最小二乘近似、核近似和重构核近似、单位分解近似、径向基函数近似、点插值近似、自然邻接点插值近似等)和无网格法中常用的各类加权余量法(伽辽金格式、配点格式、局部弱形式、加权最小二乘格式和边界积分格式等),并讨论了数值积分方法和边界条件的处理等问题.在此基础上较系统地总结了无网格法在冲击爆炸、裂纹传播、超大变形、结构优化、流固耦合、生物力学和微纳米力学等领域的应用,展示了无网格法相对于传统数值方法的优势.

饱水度和再溶浸对液氮冷冻煤致裂的影响

为解饱水度和液氮再溶浸对液氮冷冻煤致裂效果的影响,取王营子矿的干燥、天然含水和饱水3组原煤煤样在室内实验室开展了液氮溶浸实验,测试液氮溶浸前后煤样波速变化,观察煤样表面裂隙发展。定义了煤样波速变化率和代表性裂隙面积变化率2个指标,分析了饱水程度、液氮再溶浸对煤内原生裂隙结构扩展和新裂隙萌生的影响,结果表明:1在液氮作用下,原煤内主要发生原生裂隙扩展及新裂隙萌生。2煤的饱水程度对液氮溶浸煤致裂效果影响很大。随着饱水度增加,煤样的波速变化率和代表性裂隙面积变化率都增加。饱水煤样的波速变化率和代表性裂隙面积变化率分别为干燥煤样的21.2倍和11.7倍。3液氮再溶浸作用将加剧煤样内缺陷结构发育,提高液氮作用效果。天然含水煤样的液氮再溶浸作用效果更好。

基于声学测试和摄像技术的单裂隙岩石裂纹扩展特征研究

为研究裂隙岩石破坏强度及裂纹扩展特征,采用MTS岩石力学试验机对单裂隙岩石进行单轴压缩试验,同步进行声发射、声波及摄像测试,研究裂隙岩石应力应变行为、声波及声发射特征与裂纹扩展、聚结的变化关系。结果表明,起裂应力、峰值强度及模量随裂隙倾角α变化一致,均先减小后增大,起裂应力和强度受α影响较大;裂纹间断性加速扩展引起的应力转移和释放(应力降),导致裂隙岩石应力-应变曲线呈阶梯状上升;应力降伴随着能量释放、刚度劣化和AE振铃计数的激增,随着α的增大,首次应力降和振铃计数陡增对应的应力逐渐增大,且振铃分布向峰值强度附近集中;裂纹起裂、扩展引起波幅和波速衰减,且波幅下降早于应力降的发生。α=0o岩样峰前波速降高达50%,随α的增大,波速下降点的应力逐渐增大,峰前波速降幅不断减小,α较小时峰前裂纹发育程度高,随着α的增大,峰前裂纹发育程度减弱,裂纹加速扩展、聚合向峰后转移;裂纹长度扩展到临界值时扩展速率会发生快速增大,随着α的增大,裂纹临界长度逐渐降低。