不同加载速率下砂岩弯曲破坏的细观机理

岩石细观破裂形貌是岩石破坏机制的重要反映，为研究不同加载速率对砂岩弯曲破坏的影响，通过三点弯曲实验和扫描电镜方法，对某煤矿关键层砂岩弯曲破断裂纹细观形态以及裂纹的自相似性进行了研究。选取6个不同加载速率对岩样进行三点弯曲实验，观察其宏观断裂情况，并利用扫描电镜对弯曲断裂面表面裂纹细观结构进行观察，并拍摄不同倍数下的扫描电镜图片。对图片进行图像处理后得到砂岩弯曲断裂破坏细观裂纹信息，并计算得到微裂纹的分形盒维数值。结果显示:随着加载速率的提高，砂岩穿晶断裂的比例也随之升高，裂纹分形维数亦随着加载速率的增大而增加，同时，分形维数还与弯曲断裂破坏荷载和抗弯强度成正比。可见，加载速率对断裂方式有一定的影响，且加载速率越大断裂所需的破坏能越大，裂纹分布越广，表明开采速度与岩爆等岩体动力灾变有密切关系。     

加载角度对层理页岩裂纹扩展影响的实验研究

采用分离式霍普金森压杆（SHPB）系统对页岩进行冲击实验,研究层理角度对页岩动态断裂过程的影响,在裂尖设置裂纹扩展计,借助高速摄影和数字图像相关（DIC）技术对页岩中心切槽半圆盘弯曲（NSCB）试件断裂的全过程进行研究,得到了不同加载角度下页岩的动态起裂韧度、裂纹扩展速度、断裂过程中应变场和水平位移场的变化规律。实验发现:不同加载角度下,页岩的动态起裂韧度具有显著的各向异性,加载角度与动态起裂韧度呈正相关;加载角度对试样的裂纹扩展速度具有显著影响,与裂纹扩展速度呈负相关;当冲击速度较低时,切槽方向是裂纹扩展的优势方向,而当冲击速度较高时,试样会产生沿层理弱面的次生裂纹,次生裂纹对试样的断裂具有显著影响。     

巴西圆盘泥岩试件裂纹扩展及应变演化实验研究

为了研究准静态加载条件下岩石试件巴西劈裂裂纹扩展规律,采用MTS试验机进行准静态加载,同时用高速摄像机记录裂纹扩展过程。采用白光数字散斑处理软件对摄像机记录的照片进行处理,得到试件裂纹扩展过程中应变场的演化情况。通过实验和分析可以看出,由于端部效应及加载方式的原因,因此裂纹起裂点在底部加载部位;泥岩试件表面裂纹的平均扩展速度为252m/s;岩石的非均质性即内部微缺陷、微裂纹使得泥岩试样的开裂并不是沿着中心直径方向,而是偏离一定的角度,初始偏离角度约为17°。裂纹扩展过程可以划分为三个阶段:泥岩试件宏观变形阶段(宏观无裂纹)、宏观裂纹稳定扩展阶段、宏观裂纹动态张裂阶段。同时,在裂纹扩展过程中,表面第一主应变场、水平位移场等变化明显,在开裂部位第一主应变最大。通过对圆盘泥岩试件裂纹扩展实验研究,可为研究岩石破裂及其演化规律提供依据。     

砂岩方梁断裂过程区范围的实验确定方法

裂纹前端的断裂过程区是引起岩石非线性断裂及尺寸效应的主要原因。利用数字图像相关技术对砂岩开展了三点弯曲梁实验,获得观测区域高精度的全场位移和应变数据,根据断裂韧带区域水平位移和水平应变的分布特征,结合裂尖岩石颗粒变化的微观分析,提出采用裂纹尖端水平位移波动性和水平应变突变性所得到的波动系数和水平应变突变值,确定断裂过程区形状和临界尺寸的方法。结果表明:砂岩断裂过程区的形状为不规则的狭长带状区域,断裂过程区的临界长度为11~13mm,临界宽度为1.58~2.36mm。断裂过程区区域内形变在趋向裂尖时呈指数增加,但其单位区域内的形变增量呈波动状态。该方法能够更加准确判断岩石断裂过程区的范围,有助于分析岩石的非线性断裂特性。     

动荷载下岩石裂纹动态扩展行为实验研究综述

岩石的动态裂纹扩展特性在岩石力学和岩石工程研究中具有重要意义。动荷载下岩石中裂纹的扩展行为是瞬间发生的，这对实验中测试和加载技术具有很大的挑战性。为综述动荷载下岩石材料裂纹扩展研究取得的丰硕成果，总结了岩石动态裂纹扩展测试技术、实验设备和实验方法等方面的最新进展。首先，讨论了动态岩石裂纹扩展测试的各种测量技术（X射线计算机断层扫描技术、焦散线法、数字图像相关法、裂纹扩展计、导电碳膜测试方法、声发射）；然后，以应变率为主线，从低到高依次总结了中低应变率、高应变率和超高应变率下岩石内裂纹动态扩展行为研究，系统讨论了落锤冲击装置、霍普金森压杆、爆炸实验中对裂纹扩展测试的实验方法和动态裂纹扩展特性，总结了不同应变率条件下岩石裂纹的起裂、扩展、止裂及动态断裂韧度等的演变规律。     

基于SHPB实验的砂岩动态破坏过程及应变-损伤演化规律研究

为研究砂岩型铀矿爆破增渗地浸开采过程中赋矿岩层的破坏特征及损伤演化规律, 利用带有应变控制环的SHPB实验系统,对砂岩试样进行控制应变条件下的动态冲击实验,并结合波速测试实验和CT扫描实验,分析研究了砂岩试样的整体破坏过程、裂纹分布及应变-损伤演化规律。实验结果表明:在冲击荷载作用下,当应变值超过0.008 3时,砂岩试样会突然出现明显的整体破坏,整体破坏形式近似双锥形,其破坏模式为剪切-张拉混合破坏;随着应变的增加,裂纹的产生及扩展大致分为无裂纹阶段（0～0.003 3）、微裂纹起裂阶段（0.003 3～0.008 3）、裂纹贯通阶段（0.008 3～0.009 9）3个阶段,且裂纹分布区域主要集中在试样中间外围。分别从宏观、细观两方面建立了应变-损伤之间的定量关系式,损伤变量随应变的增长趋势大致分为两个阶段:平缓发展区（0～0.008 3）和迅速增长区（0.008 3～0.011 5）,损伤变量随应变增加并非简单的线性增加,而是应变值超过应变损伤阈值之后损伤程度急剧增加,应变损伤阈值为0.008 3。     

爆炸荷载下青砂岩动态起裂韧度的测试方法

为了研究爆炸荷载下青砂岩I型裂纹动态断裂韧度的测试方法，利用内部中心单裂纹圆盘（internal center single crack disc，ICSCD）试样进行了爆炸试验研究。试样由外径为400 mm、内部加载孔径为40 mm、预制裂纹长为60 mm的青砂岩制成。利用同步触发器实现圆盘中心起爆，并同步触发超动态应变仪，通过径向应变片获取爆炸应变曲线、裂纹尖端的环向应变片获取裂纹起裂时刻。以实测爆炸应变曲线为参量，应用Laplace变换推导出试样加载孔壁应力时程曲线表达式，并用数值反演法得出其数值解。利用ANSYS有限元软件，建立数值计算模型，通过相互作用积分法得出了在爆炸荷载作用下砂岩的I型动态应力强度因子曲线。研究结果表明:（1）ICSCD试件能够很好地用来测试岩石的动态起裂韧度；（2）炮孔周边的应力可以通过拉普拉斯变换的数值反演方法得到；（3）通过试验-数值法能稳定计算出ICSCD砂岩构型的动态起裂韧度，其最大误差仅为7%。     

基于高速3D-DIC技术的砂岩动力特性粒径效应研究

利用分离式霍普金森压杆（SHPB）,对粗砂岩、中等粒径砂岩和细砂岩进行了应变率为69～83 s–1的动态单轴抗压实验,研究了粒径尺寸效应对砂岩动力特性的影响。通过三维数字图像相关（3D-DIC）技术分析高速摄像图像,获得了砂岩的实时应变场,据此分析了动态荷载下3种粒径砂岩的动力变形特性和裂纹开展行为。结果表明,砂岩弹性应变储能可逆释放的临界应变率随着粒径的减小而升高,动态压缩强度随着粒径减小而增大,动态强度应变率敏感度则与强度规律相反。相较于静态条件下,中等粒径砂岩和细砂岩的动态弹性模量增长了2～3倍,粗砂岩的动态弹性模量增长达5倍以上。细砂岩的动态泊松比相较于静态提高了约25%,中等粒径砂岩的动态泊松比约为静态时的70%。动态裂纹首先出现于试件内部,然后传播至表面,呈现出应变局部化,动态荷载下岩石裂纹的孕育和扩展相比静态条件下均有所提前,其中细砂岩在动态荷载条件下的归一化裂纹起裂阈值仅为峰值强度的10%。微观分析表明,矿物粒径大小和黏土矿物含量分别在砂岩的动力力学性质和裂纹开展行为方面发挥主要作用。     

基于3D⁃DIC技术的约束岩石裂缝扩展研究

采用 RMT-301B岩石与混凝土力学实验系统对施加预应力值为峰值强度 20 %、40 %、60 %的泥质白云 岩进行单轴压缩实验,并借助三维数字图像相关技术(3D-DIC)测试系统观测受约束岩石试样的变形和破坏形 态,研究泥质白云岩在约束条件下的位移场和 Tresca应变场的演化过程．结果表明：(1) 岩石的破坏过程是由 内向外扩展,当应力积累超出岩石承受范围后,岩石出现裂纹,随后裂纹尖端继续扩展,应力也发生了重分 布,岩石内部多组裂纹开始联合扩展、贯穿,直到试件发生完全破坏．(2) 含约束岩石在单轴压缩下一共有轴 向劈裂破坏、沿片理破坏、Y形破坏、共轭剪切破坏四种破坏形态,且以轴向劈裂破坏为主     

用于脆性材料的Hopkinson压杆动态实验新方法

岩石、陶瓷或混凝土等脆性材料,在用Hopkinson压杆对其实施高应变率加载实验时,由于其破坏应变很小,试件通常在加载入射波的波头部分（含初始上升沿和较大的弥散振荡部分）就已破坏失效,因此采用常规的实验或数据处理方法很难得到精确有效的实验结果,本文提出的Hopkinson压杆装置预留间隙实验法能使加载入射波波幅振荡明显减且初始上升时间为零,有效地减小了弹性波弥散带来的误差,使贴于压杆中部的应变片测得的信号经处理后很大程度上直接反映的是试件端面的实际受力状态,且由于避免了试件在加载波上升沿段的稳定受力而使应变率历史曲线更趋于恒定,这为提高Hopkinson压杆装置的实验精度,特别是为脆性材料提供了一种实施高应变率实验的实用可行的途径。     

低速冲击下混凝土少筋梁断裂性能

动态加载下,混凝土中钢筋的阻裂性能一直是冲击动力学研究领域的难点之一。利用落锤试验机对含缺口的混凝土少筋梁进行三点弯曲试验,分析了不同加载速率下梁的冲击力、跨中挠度、混凝土起裂应变率和钢筋应变。实验结果表明:在一定加载速率范围内（0.885～1.252 m/s）,混凝土预制裂缝尖端的裂纹起裂应变率、冲击力最大值、跨中挠度峰值与加载速率呈线性增长关系,当加载速率增至1.771 m/s时,增长趋势减弱;冲击力卸载时,钢筋部分弹性变形恢复导致裂纹产生闭合,裂纹嘴张开位移逐渐减小至恒定值,对裂纹嘴张开位移峰值前的部分曲线进行拟合后得到裂纹嘴张开位移率,结果表明裂纹嘴张开位移率随加载速率的提高而线性增大。     

Quasi-static tensile deformation and fracture behavior of a highly particle-filled composite using digital image correlation method

Polymer bonded explosives (PBXs) are highly particle-filled composite materials. This paper experimentally studies the tensile deformation and fracture behavior of a PBX simulation by using the semi-circular bending (SCB) test. The deformation and fracture process of a pre-notched SCB sample with a random speckle pattern is recorded by a charge coupled device camera. The displacement and strain fields on the observed surface during the loading process are obtained by using the digital image correlation method. The crack opening displacement is calculated from the displacement fields, the initiation and propagation of the crack are analyzed. In addition, the damage evolution and fracture mechanisms of the SCB sample are analyzed according to the strain fields and the correlation coefficient fields at different loading steps.     

闪长岩单轴加载过程中声发射和弹性波速度变化规律的研究

为研究闪长岩在单轴加载过程中的声发射和各向波速变化规律,在单轴阶段加载和循环阶段加载条件下,对闪长岩岩样破裂过程中的声发射累计数、不同应力水平不同方向的波速、切线模量、轴向应变速率进行了研究。实验结果表明:(1)随着应力水平的增高,声发射事件数不断增加,在高应力水平(约80%峰值强度)时,声发射累计数急剧增多,随后切线模量出现震荡变化。(2)在加载过程中,压密程度及裂纹扩展方向对波速产生了巨大的影响,导致不同方向波速在不同的应力水平呈现出不同的变化规律,由此可以推测破裂面位置和破裂模式。在较高应力水平下(约60%峰值强度),平行于加载方向的波速趋于稳定,而垂直于加载方向的波速则持续下降,故用垂直于加载方向传播的波速预测岩石的破坏更具可靠性。(3)随着应力的增加,应变速率有逐渐减小的趋势,但临近岩石破裂时无异常变化出现,说明利用变形观测难以预测此类岩石的破坏。以上研究表明,根据纵波波速、声发射累计数和切线模量的变化可以有效预测岩石的破坏。     

Strain strengthening effects in Witwatersrand quartzite

A series of uniaxial compression specimens were tested over a range of applied ram displacement rates of 8.9 × 10⁻⁴ to 8.9 mm/sec to elucidate the effects of loading rate on the uniaxial compressive fracture stress of Witwatersrand quartzite. It was demonstrated that even within standard loading rate ranges, considerable scatter in the fracture strength (under uniaxial compression) existed in this particular quartzite rock. Nevertheless, a definite trend of increasing fracture resistance with increasing monotonic loading rate was evident inasmuch that increasing the loading rate (strain rate) by four orders of magnitude increase the fracture strength by almost 2.8 times. Prior fatigue loading also produced a significant strain strengthening as the uniaxial compressive fracture stress tended to increase in a sigmoidal fashion with increasing number of fatigue cycles prior to testing. Indeed, the fracture strength of quartzite was almost doubled in value after 10⁻ cycles. Plane strain fracture toughness tests utilising three point bend specimens were conducted and an average of K_lc = 1.7 MPa√m was realized. In both the uniaxial compression tests and the fracture toughness tests, failure occurred by crack extension predominantly by a transgranular flat cleavage-like mode through pure quartzite (silica) regions. However, crack extension was also observed to occur in an intergranular “ductile-like” mode through areas associated with inclusions prevalent in the quartzite.     

Parameters controlling fracture resistance in functionally graded materials under mode I loading

In this investigation the fracture behavior of functionally graded materials (FGMs) was studied by means of experiments carried out on model polymer-based FGMs. Model graded materials were manufactured by selective ultraviolet irradiation of ECO [poly(ethylene carbon monoxide)], a photo-sensitive ductile copolymer that becomes more brittle and stiffer under exposure to ultraviolet light. The mechanical response of the graded material was characterized using uniaxial tensile tests. Single edge notched tension graded ECO specimens possessing different spatial variations of Young’s modulus, failure stress and failure strain were tested under remote opening loading. A full-field digital image correlation technique was used to measure in real-time the displacement field around the crack tip while it propagated through the graded material. The measured displacement field was then used to extract fracture parameters such as stress intensity factor and T-stress, and thus construct resistance curves for crack growth in the FGMs. For this loading configuration it was found that the nonsingular T-stress term in the asymptotic expansion for stresses needs to be accounted for in order to accurately measure the fracture resistance in FGMs. In addition, the influence of local failure properties (i.e., failure stress and failure strain) on crack growth resistance was investigated in detail. It was found that depending on the combined effects of the spatial variation of these two failure parameters, regardless of the spatial variation of the Young’s modulus, the FGM fracture resistance can either increase, decrease or remain constant with continued crack growth.     

Loading rate and confining pressure effect on dilatancy,acoustic emission,and failure characteristics of fissured rock with two pre-existing flaws

Investigating the dilatancy, acoustic emission and failure characteristics of fissured rock are significant to ensure their geotechnical stability. In this paper, the uniaxial and triaxial compression experiments with AE monitoring under different loading rates were carried out on fissured rock specimens with the same geometrical distribution of two pre-existing flaws. The dilatancy and AE activity of these specimens were discussed, and the effects of the confining pressure and loading rate on the mechanical parameters and failure characteristics were analyzed. The results show that the exponential strength criterion is more suitable than the Mohr–Coulomb strength criterion to characterize the strength characteristics of fissured rock. The crack evolution and failure characteristics of fissured rock specimens are more complicated than those of intact rock specimens. The failure characteristics of the fissured rock follow the tensile shear coalescence model, crack branching occurs with increasing the loading rate, and the multi-section coalescence model is verified with increasing the confining pressure. The phenomena of stress drop and yield platform usually occur after the dilatancy onset, the specimen does not fail instantaneously, and the propagation and coalescence of cracks cause a sharp increase in the AE signals, circumferential strain, and volumetric strain.     

岩石紧凑拉伸断裂过程及其尺寸效应的三维数值模拟

采用最近开发的三维岩石破裂过程分析软件RFPA3D模拟单边裂纹紧凑拉伸断裂过程。试验中五个不同尺寸的岩样具有相同的力学性质参数分布,模拟结果得到了裂纹扩展中的应力场、位移场和声发射的空间分布以及单边裂纹扩展贯通的过程。单边裂纹拉伸断裂的路径是一个复杂的空间三维曲面,三维裂纹比二维裂纹更为复杂。分析了岩石试样的峰值强度和试样尺寸之间的关系。随着岩样尺寸的增加,峰值强度逐渐减小,并且延性破坏特征更加明显,模拟结果满足岩石的尺寸效应规律。最后模拟了三组不同均匀性的试样拉伸破坏过程,结果表明细观上的非均匀性对岩石尺寸效应有很大影响,随着非均匀性的增加,岩石宏观强度随之提高,即使在均匀材料中一样存在尺寸效应。