The brittle and ductile behavior of clay samples containing a crack under mixed mode loading

The mechanics of propagation of a single crack in brittle and ductile samples of clay as well as their mode of failure were investigated. The crack in the brittle and in the ductile samples was subjected to a mixed-mode (Mode I and Mode II) type of loading. In the brittle and ductile samples, secondary cracks developed from the tips of the original crack. The secondary cracks did not follow the plane of the original crack but formed an angle α with this plane. The angle of crack propagation α was greater in the brittle than in the ductile samples. The brittle samples failed when the secondary cracks reached the edges of the samples. Their mode of failure was a typical tensile failure. In the ductile samples, the secondary cracks extended for a limited distance from the tips of the original crack and did not influence the failure of the samples that was recorded to be in shear. The Maximum Tangential Stress criterion predicted well the direction of crack propagation in the brittle clay samples. The direction of crack propagation in the ductile samples of clay was found to be a function of their water content. From the laboratory results, a relationship from which to obtain the angle of crack propagation α in the ductile samples is presented.     

Numerical analysis of quasi-static crack branching in brittle solids by a modified displacement discontinuity method

Mechanism of quasi-static crack branching in brittle solids has been analyzed by a modified displacement discontinuity method. It has been assumed that the pre-existing cracks in brittle solids may propagate at the crack tips due to the initiation and propagation of the kink (or wing) cracks. The originated wing cracks will act as new cracks and can be further propagated from their tips according to the linear elastic fracture mechanics (LEFM) theory. The kink displacement discontinuity formulations (considering the linear and quadratic interpolation functions) are specially developed to calculate the displacement discontinuities for the left and right sides of a kink point so that the first and second mode kink stress intensity factors can be estimated. The crack tips are also treated by boundary displacement collocation technique considering the singularity variation of the displacements and stresses near the crack tip. The propagating direction of the secondary cracks can be predicted by using the maximum tangential stress criterion. An iterative algorithm is used to predict the crack propagating path assuming an incremental increase of the crack length in the predicted direction (straight and curved cracks have been treated). The same approach has been used for estimating the crack propagating direction and path of the original and wing cracks considering the special crack tip elements. Some example problems are numerically solved assuming quasi-static conditions. These results are compared with the corresponding experimental and numerical results given in the literature. This comparison validates the accuracy and applicability of the proposed method.     

基于声发射矩张量分析混凝土破坏的裂纹运动

从细观上看, 混凝土是一种由骨料、水泥浆基体、裂纹等组成的非均匀复合材料. 单轴准静态加载条件下, 应力应变曲线表现出明显的准脆性特征. 其变形破坏过程实质上是内部微裂纹产生、扩展和汇合的过程, 研究细观尺度的裂纹扩展演化将有助于深入了解混凝土的变形和破坏过程. 声发射作为一种物理检测方法可以获取材料内部细观损伤演化的物理信息. 本文基于声发射技术, 结合改进的时差定位算法和矩张量理论对声发射信号进行分析, 反演了混凝土巴西劈裂破坏中裂纹位置、裂纹类型以及裂纹面运动方向, 揭示了混凝土宏观拉伸破坏的细观裂纹扩展机制. 结果表明: 裂纹运动过程清晰地显示了混凝土内裂纹源首先在试件与载荷接触面附近产生, 之后聚集形成局部损伤区域, 并沿轴线向中心扩展(加载平面)以及裂纹从试件中间向表面扩展的动态过程(厚度方向); 裂纹运动体积可以作为裂纹形成、扩展过程中弹性能释放的度量, 初始裂纹成核时体积参数较小, 峰值载荷时, 裂纹运动体积最大达到$5.93\times10^{-4}$ mm$^{3}$; 混凝土宏观尺度的拉伸破坏在细观尺度上存在有拉伸裂纹、混合裂纹以及剪切裂纹; 拉伸裂纹最多, 占裂纹总数约为60%, 剪切裂纹最少, 约占裂纹总数的10%; 拉伸裂纹运动主导了试件的宏观劈裂破坏.      

Al2O3陶瓷动静态压缩下碎片形貌与破坏机理分析

为探究Al₂O₃陶瓷的宏观力学响应与破坏机理,分别利用材料试验机和分离式霍普金森压杆对其进行准静态和动态压缩实验,同时通过原位光学成像观测试样的破坏过程,并利用同步辐射CT和扫描电镜（SEM）对回收碎片的尺寸和形状以及微观破坏模式进行表征分析。宏观强度数据表明,Al₂O₃陶瓷的抗压强度符合Weibull分布,且与加载应变率呈现指数增长关系。原位光学成像和SEM回收分析共同揭示了动静态加载下裂纹成核与扩展模式存在明显差异。准静态加载时材料微观上更易发生沿晶断裂,宏观表现为劈裂裂纹较少,且倾向于沿加载方向传播并贯穿整个试样;而动态加载时穿晶断裂占主导地位,劈裂裂纹明显增加并发生相互作用,因此在传播过程中容易分叉而形成大量次生裂纹,提高了试样内裂纹密度。这与碎片的CT表征结果一致,即碎片平均球形度和伸长、扁平指数等均随应变率对数线性增加。破坏模式的改变最终导致高应变率下陶瓷材料应变率敏感性显著增强。     

等离子喷涂铁基涂层的疲劳磨损裂纹行为

采用等离子喷涂制备了铁基涂层,使用球盘式疲劳磨损试验机进行涂层的疲劳磨损试验,利用声发射技术在线判断涂层开裂,并通用透射电子显微镜（TEM）分析了试验前后涂层内部微观结构,研究了喷涂层疲劳磨损裂纹的扩展行为。结果表明：制备的涂层致密,结合状态良好,主要相为铁素体,同时存在大量的非晶-纳米晶相,利用声发射技术能够表征涂层的临界失效状态,其内部疲劳磨损裂纹主要以韧性穿晶断裂的形式扩展。     

不同地应力条件下切缝药包爆破的数值模拟

针对切缝药包定向爆破的特点,考虑岩石介质非均匀性的基础上,把岩石爆破视为爆炸应力波动态作用和爆生气体压力准静态作用的过程,基于损伤力学理论建立岩石爆破的力学模型,并对不同地应力条件下切缝药包爆破的裂纹演化规律进行数值模拟,分析不同地应力条件对切缝药包爆破效果的影响。模拟结果表明:采用切缝药包爆破时,裂纹主要萌生于切缝周边,沿切缝方向扩展,切缝对定向裂纹的控制作用明显;当考虑地应力作用,且最大地应力方向与切缝方向垂直时,不利于定向裂纹的扩展;最大地应力方向与切缝方向平行时,有利于定向裂纹的扩展。裂纹的扩展方向受控于切缝角度和最大地应力方向这2个条件,裂纹扩展规模则受到地应力的限制。     

Acoustic emission monitoring of crack propagation in additively manufactured and conventional titanium components

Additive manufacturing (AM) is a novel and innovative production technology that can produce complex and lightweight engineering products. In AM components, as in all engineering materials, fatigue is considered as one of the principle causes of unexpected failure. In order to detect, localise and characterise cracks in various material components and metals, acoustic emission (AE) is used as a non-destructive monitoring technique. One of the main advantages of AE is that it can be also used for dynamic damage characterisation and specifically for crack propagation monitoring. In this research, we use AE to monitor the fatigue crack growth behaviour of Ti6Al4V components under four-point bending. The samples were produced by means of AM as well as conventional material. Notched and unnotched specimens were investigated with respect to the crack severity and crack detection using AE. The main AE signal parameters –such as cumulative events, hits, duration, average frequency and rise time– were evaluated and indicate sensitivity to damage propagation in order to lead to a warning against the final fracture occurrence. This is the first time that AE is applied in AM components under fatigue.     

基于相场法的周期性多孔结构断裂行为研究

周期性多孔结构具有质量轻、比密度低、比强度高、隔音等优良特点, 同时也能很好地满足结构-功能一体化的需求, 在许多领域具有广泛的应用前景. 目前, 对周期性多孔结构在复杂载荷下的力学响应和断裂行为的研究较少. 采用细观力学和相场方法相结合, 基于二维代表性体积单元RVE模型, 施加能实现比例加载的周期性边界条件, 研究周期性多孔结构在复杂多轴比例加载状态下的裂纹萌生位置、断裂模式、承载极限及其变化规律. 本文的数值模拟结果表明: 首先, 周期性多孔结构在竖直方向拉伸载荷作用下, 裂纹均从孔边萌生并沿水平方向同步扩展; 其次, 在双轴载荷作用下, 随着水平载荷的增加, 结构在竖直方向的极限拉伸载荷逐渐增大; 当双轴拉伸载荷等值时, 结构的抗拉强度达到最大, 此时断裂模式呈现为十字正交型开裂; 最后, 面内剪切应力的引入会导致结构的拉伸强度极限降低, 孔边裂纹的萌生位置和扩展路径发生偏移, 裂纹模式从单S型转变为双弧线型, 裂纹向水平位置上相邻的孔洞扩展. 随着水平载荷的增加, 裂纹模式最终转变为斜裂纹, 从孔边对角线位置萌生并沿着45°方向扩展.      

Acoustic emission assessment of interface cracking in thermal barrier coatings

In this paper, acoustic emission (AE) and digital image correlation methods were applied to monitor interface cracking in thermal barrier coatings under compression. The interface failure process can be identified via its AE features, including buckling, delamination incubation and spallation. According to the Fourier transformation of AE signals, there are four different failure modes: surface vertical cracks, opening and sliding interface cracks, and substrate deformation. The characteristic frequency of AE signals from surface vertical cracks is 0.21 MHz, whilst that of the two types of interface cracks are 0.43 and 0.29 MHz, respectively. The energy released of the two types of interface cracks are 0.43 and 0.29 MHz, respectively. Based on the energy released from cracking and the AE signals, a relationship is established between the interface crack length and AE parameters, which is in good agreement with experimental results.     

Initiation and propagation of fatigue crack in pure aluminium single crystals

The fatigue crack initiation and propagation in the pure aluminum single crystals with different orientations are investigated. Acoustic Emission (AE) technique is used to monitor the fatigue crack initiation and propagation data. The results show that the acoustic emission technique could be used for detecting fatigue crack initiation and propagation. The total counts of AE are plotted as a function of the number of fatigue cycles, and change in slope of the curve corresponded to the initiation of the fatigue crack. The fatigue crack initiation and propagation are related to the specimen crystalline orientation. Specifically, they depend on the ratio K of the Schmid factor of the secondary slip system, to that of the primary one. The greater the ratio K, the easier the fatigue crack initiates and the faster the fatigue crack propagates. When the ratio K is equal or close to one, the fatigue crack grows either along a cleavage plane which intersects both the primary plane and the secondary one or along the primary slip plane and the secondary one. Growth along alternating planes can also occur giving rise to a zigzag path. When the ratio K is much less than one, the fatigue crack grows along the primary slip plane only. The results are explained with a model for the faceted mode of fatigue crack growth.     

含不同充填物预制裂隙对爆炸裂纹扩展的影响

为了研究充填裂隙岩石动态断裂时裂纹扩展规律,以空气、黏土和水作为有机玻璃的预制裂隙充填材料,在炮孔与预制裂隙的不同夹角、不同距离条件下,通过单发雷管加载,对3种不同裂隙充填物的有机玻璃模型进行了起爆实验。结果表明:爆炸裂纹几乎都不会越过预制裂隙;空气充填模型裂纹总数、左端翼裂纹几乎全部大于黏土和水充填模型;最长裂纹分布位置和长度与反射应力波传播方向和能量有关;空气充填模型右端翼裂纹多随角度增大而增长,黏土充填模型右端翼裂纹则表现为先增后减;爆炸裂纹扩展对充填物种类具有敏感性。     

Experimental investigation on the propagation of fatigue cracks emanating from sharp notches

The effect of notch geometry on the propagation of fatigue cracks emanating from sharp V-shaped notches is investigated by means of an experimental campaign performed on Al-7075-T651 specimens carrying notches with opening angles of 45°, 90°, and 135°. The samples were tested using a servohydraulic machine under different loading directions and at several loading levels. The crack deflection induced by the variation in loading direction was determined my measuring the kinking angle and by studying the crack propagation plane through fractographic analysis. A linear elastic fracture mechanics approach was adopted for the analysis of experimental results. Stress intensity factors were calculated using an appropriate weight function set up for studying inclined edge cracks emanating from sharp V-notches. The influence of K _II on the crack propagation was discussed on the basis of theoretical and semi empirical models.     

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。     

Spontaneous Mixed-Mode Fracture in Bonded Similar and Dissimilar Materials

In this paper, we report on an experimental study of spontaneous, mixed-mode, crack propagation in weakly bonded similar and dissimilar materials. A unique experimental configuration is proposed to induce spontaneous crack growth events along the interfaces. The cracks nucleate from tiny circular holes and are triggered by an exploding wire. They subsequently propagate under the action of a constant, far-field load. Dynamic photoelasticity in conjunction with high speed photography is used to capture the real-time isochromatics associated with crack propagation. In the case of identical materials, crack propagation is anti-symmetric with respect to the crack nucleation point while strong asymmetry is observed for the case of dissimilar materials. In both cases, cracks propagate at constant velocity from the initiation point. The time histories of dynamic stress intensity factors and of energy release rates of the propagating cracks along the bonded similar materials are also reported.     

压荷载下类岩石材料中的锯齿形裂纹分析

针对类岩石材料的复杂性和非均质性，特别是材料中一些强度相对较弱的晶界的存在等，对形状较为复杂的两种锯齿形裂纹：内张型滑动裂纹和单翼滑动裂纹的应力强度因子进行了分析．结合经典的滑动裂纹模型，讨论了裂纹起裂角、侧压以及翼裂长度等对应力强度因子的影响．分析的结果为进一步研究类岩石材料的断裂损伤机理提供了理论基础．     

Experimental study and numerical modeling of brittle fracture of carbonate rock under uniaxial compression

The brittle carbonate rock taken from the Tarim Oilfield is tested in laboratory under uniaxial compression. The acoustic emission (AE) is used to monitor the microcracking activity in rock during the experiment. Moreover, the 3D tomograms of carbonate rock after uniaxial compression are obtained by using CT imaging technology, which indicates that microcracks mutually interconnect and eventually form macroscopic fractures after failure. The PFC2D is used to model the behavior of brittle rock including microcracks propagation. The stress–strain curve and cracks distribution in rock model are obtained from the PFC simulation. The numerical results agree with the experimental test well.     

雁形裂纹扩展的模型试验及断裂力学机制研究

通过相似材料模拟试验的方法研究了双轴压缩载荷作用下闭合雁形裂纹的起裂、扩展和岩桥的贯穿机理，得到了双轴压缩载荷作用下，不同方位雁形裂纹的开裂角、起裂载荷、岩桥贯通载荷及临界失稳载荷等重要的断裂力学参数，提出岩桥的破坏模式有剪切破坏、拉剪复合破坏和翼裂纹扩展三种     

Identification of Cracking Mechanisms in Scaled FRP Reinforced Concrete Beams using Acoustic Emission

Acoustic emission was used to monitor the cracking mechanisms leading to the failure of scaled concrete beams having Glass Fiber Reinforced Polymer (GFRP) longitudinal reinforcement and no shear reinforcement. Dimensional scaling included that of the effective depth of the cross section, which is a key parameter associated with the scaling of shear strength; and maximum aggregate size, which affects the shear-resisting mechanism of aggregate interlock along shear (inclined) cracks. Five GFRP reinforced concrete (RC) beams with effective depth up to 290 mm and constant shear span-to-effective depth ratio of 3.1 were load tested under four-point bending. Two types of failures were observed: flexural, due to rupture of the GFRP reinforcement in the constant moment region; and shear, due to inclined cracking in either constant shear region through the entire section depth. Acoustic emission (AE) analyses were performed to classify crack types occurring at different points in the load history. The results of this study indicate that appropriate AE parameters can be used to discriminate between developing flexural and shear cracks irrespective of scale, and provide warning of impending failure irrespective of the failure mode (flexural and shear). In addition, AE source location enabled to accurately map crack growth and identify areas of significant damage activity. These outcomes attest to the potential of AE as a viable technique for structural health monitoring and prognosis systems and techniques.     

飞机风挡无机玻璃在不同应变率下的力学行为

利用电子万能试验机和改进的分离式Hopkinson压杆测试了飞机风挡无机玻璃在2种准静态应变率(4×10-4、4×10-3 s-1)和2种动态应变率(200、400 s-1)下的单轴压缩力学行为,并利用高速摄像机记录试样破坏过程。实验结果表明:玻璃破坏时表现为典型的脆性材料,随着应变率的提高,材料的压缩强度显著提高。通过观察试样变形过程及变形后的形貌可知,玻璃在压缩载荷下的破坏模式为横向张应力引起的裂纹成核、沿轴向扩展与联结交错导致的失效破坏,并从微裂纹成核扩展和能量耗散的角度对材料的应变率效应做出了合理的解释。