An experimental study on fracture mechanical behavior of rock-like materials containing two unparallel fissures under uniaxial compression

Strength and deformability characteristics of rock with pre-existing fissures are governed by cracking behavior. To further research the effects of pre-existing fissures on the mechanical properties and crack coalescence process, a series of uniaxial compression tests were carried out for rock-like material with two unparallel fissures.In the present study, cement, quartz sand, and water were used to fabricate a kind of brittle rock-like material cylindrical model specimen. The mechanical properties of rock-like material specimen used in this research were all in good agreement with the brittle rock materials. Two unparallel fissures(a horizontal fissure and an inclined fissure) were created by inserting steel during molding the model specimen.Then all the pre-fissured rock-like specimens were tested under uniaxial compression by a rock mechanics servocontrolled testing system. The peak strength and Young's modulus of pre-fissured specimen all first decreased and then increased when the fissure angle increased from 0?to 75?.In order to investigate the crack initiation, propagation and coalescence process, photographic monitoring was adopted to capture images during the entire deformation process.Moreover, acoustic emission(AE) monitoring technique was also used to obtain the AE evolution characteristic of prefissured specimen. The relationship between axial stress, AE events, and the crack coalescence process was set up: when a new crack was initiated or a crack coalescence occurred, thecorresponding axial stress dropped in the axial stress–time curve and a big AE event could be observed simultaneously.Finally, the mechanism of crack propagation under microscopic observation was discussed. These experimental results are expected to increase the understanding of the strength failure behavior and the cracking mechanism of rock containing unparallel fissures.     

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.     

Free-edge stresses and progressive cracking in surface coatings of circular torsion bars

This paper considers the explicit solutions of free-edge stresses near circumferential cracks in surface coatings of circular torsion bars and their application in determining the progressive cracking density in the coating layers. The problem was formulated within the framework of linear elastic fracture mechanics (LEFM). The free-edge stresses near crack tip and the shear stresses in the cross-section of the torsion bar were approached in explicit forms based on the variational principle of complementary strain energy. Criterion for progressive cracking in the coating layer was established in sense of strain energy conservation, and the crack density is thereby estimated. Effects of external torque, aspect ratio, and elastic properties on the density of progressive cracking were examined numerically. The present study shows that, in the sense of inducing a given crack density, compliant coating layer with lower modulus has much higher critical torque than that of a stiffer one with the same geometries and substrate material, i.e., compliant coating layer has greater cracking tolerance. Meanwhile, the study also indicates that thicker surface coating layer is more pliant to cracking than the thinner ones. The present model can be used for analyzing the damage mechanism and cracking tolerance of surface coatings of torsion shafts and for data reduction of torsional fracture test of brittle surface coatings, etc.     

Fracture Mechanics of Corrosion Cracking in Concrete by Acoustic Emission

Nucleation of microcracks can be detected and analyzed by acoustic emission (AE), by which crack kinematics of locations, types and rientations are quantitatively estimated. The procedure was applied to clarify mechanisms of corrosion cracking. Based on fracture mechanics, numerical analysis was conducted by the boundary element method (BEM). Relations between the stress intensity factors and crack types were investigated by BEM solutions. In experiments, four types of crack patterns were nucleated by employing expansive agent. Following the surface crack, the diagonal crack and/or the horizontal crack propagated. The internal crack extended after the surface crack was terminated. Depending on the crack types, contributions of mode-I and mode-II were varied. According to AE results, four crack patterns observed differently consisted of tensile, mixed-mode and shear cracks. It is demonstrated that mechanisms of corrosion cracking in concrete are dominantly of mode-I failure along with a minor contribution of mixed-mode and mode-II.     

Critical cooling rates to avoid transient-driven cracking in thermal barrier coating (TBC) systems

This paper presents critical cooling rates to avoid cracking in thermal barrier coatings (TBCs) driven by thermal transients. A complete thermomechanical model is presented for multilayers; it determines the history of temperature, deformation and stress distributions in the layers, as well as the steady-state energy release rate (ERR) for delamination for all possible crack locations. The model is used to analyze bilayers over a broad range of properties and cooling rates; critical cooling rates are identified that distinguish scenarios in which the transient delamination driving force is higher than that associated with the fully cooled state. Implications for coating the durability are discussed.     

结构层热扩散系数对超薄涂层热力性能的影响

将非傅立叶热传导模型(用于超薄热涂层)与傅立叶热传导模型(用于结构层)相结合 求解温度场,运用有限元法求解热涂层热应力和裂纹驱动力,并分析结构层材料热扩散系数 的变化对热涂层的热力学性能(温度场、应力场和断裂性能)的影响. 研究表明,结构层材 料性能变化对温度场的影响主要表现在热冲击后期,对热应力和裂纹尖端驱动力后期的变化 也有一定的影响.     

Effects of multiple cracking on the residual strength behavior of thermally shocked functionally graded ceramics

When subjected to severe thermal shocks a functionally graded ceramic (FGC) suffers strength degradation due to the thermally-induced damages in the material. Multiple surface cracking has been observed as one of the dominant defects/damages affecting the thermal shock behavior of ceramics. This paper presents a thermo-fracture mechanics model to investigate the thermal shock residual strength behavior of elastically homogeneous but thermally graded FGCs undergoing multiple surface cracking. We consider an FGC plate with an array of parallel edge cracks at the thermally shocked surface. A Fourier transform/superposition method is used to derive the singular integral equation of the thermal shock crack problem. The critical thermal shock that causes crack propagation and thermal shock damage are determined using linear elastic fracture mechanics. The thermal shock residual strength of the FGC as a function of thermal shock severity and crack density (crack spacing) is subsequently evaluated. Numerical calculations are carried out for two FGC materials, i.e., Al₂O₃/Si₃N₄ and TiC/SiC FGCs, to illustrate the effects of crack density (crack spacing) and material gradation on the thermal shock strength behavior of FGCs. It is found that a higher crack density (lower crack spacing) together with appropriately graded material properties significantly enhances the residual strength of the thermally shocked FGCs.     

Acoustic emission: A means of measuring crack growth at elevated temperatures

Acoustic-emission (AE) technique has been applied to detect and continuously monitor subcritical crack growth in Zr-25% Nb. a commercial alloy for reactor-core components. The tests conducted in the dimensions of time, temperature, stress and local environment registered a significant difference in AE aclivity. The time-domain analysis of acoustic emissions suggested the crack growth to be intermittent Further investigations to interpret the AE response to cracking identified tracture mechanisms operative during slow crack growth. The analytical parameters of crack growth were then correlated with AE data which produced some confidence in the use of AE techniques to quanhtatively measure slow crack growth and crack growth rates during tracture. The results are presented to show a good fit between estimated and observed crack lengths and velocities. Finally, the viewpoints reached from AE studies were compared with tractographic observations and were found in agreement, providing a great deal of faith in the use of AE techniques.     

Reliability assessment on interfacial failure of thermal barrier coatings

Thermal barrier coatings (TBCs) usually exhibit an uncertain lifetime owing to their scattering mechanical properties and severe service conditions. To consider these uncertainties, a reliability assessment method is proposed based on failure probability analysis. First, a limit state equa-tion is established to demarcate the boundary between failure and safe regions, and then the failure probability is calculated by the integration of a probability density function in the failure area according to the first-or second-order moment. It is shown that the parameters related to interfacial failure follow a Weibull distribution in two types of TBC. The inter-facial failure of TBCs is significantly affected by the thermal mismatch of material properties and the temperature drop in service.     

Multiple cracking of elastic coatings under transient thermal load

A periodic array of cracks in an elastic coating bonded to a homogeneous substrate is considered. The medium is subjected to mechanical loads and/or thermal loads. The problem is formulated in terms of a singular integral equation with the crack face displacement as the unknown variable. In addition to the time-varying stress intensity factors and stresses for various parameters of the problem, the effect of periodic cracking on the relaxation of the transient stress on the coating surface is discussed. Solution techniques for a single elastic layer and an elastic coating bonded to an infinite substrate are given. It is found that, if the crack density attains a saturation value, the transient thermal stress in the medium could be released significantly, suggesting that further cracking is difficult.     

热障涂层力学性能的实验测试方法研究进展

涂层-基体体系作为高温热障防护结构被广泛应用于航空、航天、核反应堆等涡轮发动机叶片上, 主要作用是抵抗高温燃气, 保护基体材料. 在服役过程中, 涂层过早出现开裂、脱粘等失效行为直接影响发动机的安全性和可靠性. 长期以来, 热障涂层力学性能的实验测试方法是国内外研究者关注的重点. 首先分析了导致热障涂层失效的失配应力的来源, 重点介绍了热障涂层力学性能研究的实验测试方法、仪器和针对界面脱粘检测的红外热像技术, 接着概述了热障涂层寿命预测方法, 最后分析当前研究中存在的问题, 并对未来的发展的一些方向和有待于进一步研究的问题作了总结.     

Sandwich Panel Cores for Blast Applications: Materials and Graded Density

Sandwich composites are of interest in marine applications due to their high strength-to-weight ratio and tailorable mechanical properties, but their resistance to air blast loading is not well understood. Full-scale 100 kg TNT equivalent air blast testing at a 15 m stand-off distance was performed on glass-fibre reinforced polymer (GFRP) sandwich panels with polyvinyl chloride (PVC); polymethacrylimid (PMI); and styrene acrylonitrile (SAN) foam cores, all possessing the same thickness and density. Further testing was performed to assess the blast resistance of a sandwich panel containing a stepwise graded density SAN foam core, increasing in density away from the blast facing side. Finally a sandwich panel containing compliant polypropylene (PP) fibres within the GFRP front face-sheet, was subjected to blast loading with the intention of preventing front face-sheet cracking during blast. Measurements of the sandwich panel responses were made using high-speed digital image correlation (DIC), and post-blast damage was assessed by sectioning the sandwich panels and mapping the damage observed. It was concluded that all cores are effective in improving blast tolerance and that the SAN core was the most blast tolerant out of the three foam polymer types, with the DIC results showing a lower deflection measured during blast, and post-blast visual inspections showing less damage suffered. By grading the density of the core it was found that through thickness crack propagation was mitigated, as well as damage in the higher density foam layers, thus resulting in a smoother back face-sheet deflection profile. By incorporating compliant PP fibres into the front face-sheet, cracking was prevented in the GFRP, despite damage being present in the core and the interfaces between the core and face-sheets.     

基于纯声子辐射模型的超薄绝缘夹层结构

在复合材料结构中起绝热、增韧作用的绝缘夹层,其加工厚度现在已达到纳米量级,原有的傅立叶热传导定律已无法描述其热能的传递行为,需从分子动力学、量子力学从发,针对不同研究对象建立相应的热传导模型．针对超薄绝缘夹层结构,将纯声子辐射模型和傅立叶热传导模型相结合数值求解热冲击条件下的温度场,并作为热载荷,用于求解结构上表面应力和夹层裂纹驱动力,其结果与只采用傅立叶热传导模型计算的结果相比较, 分析了物理参数对温度、应力和裂纹驱动力的影响．结果表明：与只采用傅立叶热传导模型计算的结果相比,按EPRT计算的热传导明显变慢,其表面剥离应力偏大,而夹层裂纹驱动力偏小．同时随着松弛时间增大和声子速度的降低,热传导减缓,表面横向剥离应力增大,超薄绝缘夹层内裂纹尖端驱动力减小．     

Wedge-shaped fracturing in the pull out of FRP stiffeners from quasi-brittle substrates

Fiber-Reinforced-Polymer (FRP) strips can be glued to the surface of concrete or masonry structures to improve their strength. Pull-out tests on FRP bonds have shown a progressive failure of the adhesive joint involving early-stage cracking parallel to the axis of the FRP stiffener, and an inclined crack initiating at the free end of the stiffener and extending into the quasi-brittle substrate in the latest stage. The subsurface crack produces a characteristic wedge-shaped spall. There is no consensus on the reasons for the transition from cracking along the bond to cracking within the substrate. Therefore a Linear Elastic Fracture Mechanics model problem is presented here that accounts for and provides improved understanding of the formation of the subsurface crack. The boundary value problem is solved analytically using the distributed dislocation technique. Competition between crack extension along the adhesive joint and into the substrate is quantified using a quantized crack propagation criterion, whereby the crack does not advance in infinitesimal continuous increments, but instead in finite steps of length comparable to the characteristic dimensions of the material microstructure. The model predicts results that are in good agreement with experimental evidence.     

Crack propagation in a functionally graded plate under thermal shock

Summary Thermal cracking in a ceramic/metal functionally graded plate is discussed. When a functionally graded plate is cooled from high temperature, curved or straight crack paths are experimentally observed on the ceramic surface. One of the reasons that make the crack paths to differ are the thermal or mechanical conditions. In order to clarify the influence of these conditions on the crack path, the crack propagation is simulated using finite element method. Received 29 September 1998; accepted for publication 2 August 1999     

Multiscale behavior of crack initiation and growth in piezoelectric ceramics

The multiscale nature of cracking in ferroelectric ceramics is explored in relation to the crack growth enhancement and retardation behavior when the direction of applied electric field is reversed with reference to that of poling. An a priori knowledge of the prevailing fracture behavior is invoked for the energy dissipated in exchange of the macro- and micro-crack surface. To avoid the formalism of developing a two-scale level model, a single dominant crack is considered where the effect of microcracking could be reflected by stable crack growth prior to macro-crack instability. This is accounted for via a length ratio parameter λ. Micro- and macro-crack damage region would necessarily overlap in the simplified approach of applying equilibrium mechanics solutions to different scale ranges that are connected only on the average over space and time. The strain energy density theory is applied to determine the crack growth segments for conditions of positive, negative and zero electric field. The largest and smallest crack segments were found to correspond, respectively, to the positive and negative field. All of the three piezoceramics PZT-4, PZT-5H and P-7 followed such a trend. This removes the present-day controversy arising from the use of the energy release rate concept that yields results independent of the sign of the electric field. Interaction of non-similar crack growth with the direction of electric field is also discussed in relation to Mode II cracking. The crack initiation angle plays a dominant role when the growth segment is sufficiently small. Otherwise, a more complex situation prevails where consideration should also be given to the growth segment length. Failure stresses of Modes I and II cracking are also obtained and they are found to depend not only on the electric field density but also on crack length and the extent of slow crack growth damage. These findings suggest a series of new experiments.     

Particle-based numerical modeling of AE statistics in disordered materials

We present some numerical simulations of AE due to damage propagation in disordered materials under compression and bending. To this purpose, the AE cumulative number, the time frequency analysis and the statistical properties of AE time series will be numerically simulated adopting the so-called “particle method strategy” (Cundall in Proceedings ISRM Symp., Nancy, France, vol. 2, pp. 129–136, 1971). The method provides the velocity of particles in a set simulating the behavior of a granular system and, therefore, is suitable to model the compressive wave propagation and acoustic emission (corresponding to cracking) in a solid body. The numerical simulations (Abe et al. in Pure Appl. Geophys. 161:2265–2277, 2004) correctly describe the compression test in terms of mean stress-strain response and crack pattern (Invernizzi et al. in Proceedings of the SEM annual conference, Society for Experimental Mechanics Inc., USA, SEM Annual Conference, Indianapolis, Indiana, USA, June 7–10, 2010). The size effects on the peak compressive strength and on the AE count are correctly reproduced. In addition, the amplitude distribution (b-value) and temporal evolution of AE events due to cracking, crucial for the evaluation of damage and remaining lifetime, were simulated and result in agreement with the experimental evidences.     

混凝土梁三点弯曲断裂模式转变的近场动力学模拟

混凝土结构的宏观损伤开裂与其非均质微观结构紧密相关。底部带切口的混凝土梁在进行三点弯曲破坏时,随着切口的位置由梁中向梁边转移,裂纹由从切口处萌生并生长转变为从梁的中部萌生。本文采用半均质化近场动力学（IH-PD）模型和全均质化近场动力学（FH-PD）模型,分别对混凝土梁三点弯断裂问题进行模拟研究。IH-PD模型根据混凝土中骨料体积分数随机生成不同键的组合方式,将微观尺度的非均质性引入模型,无需详细描绘骨料形状和分布即可考虑混凝土非均质性。本文将IH-PD与FH-PD模型得到的断裂模式随切口位置的变化关系,与实验结果对比,分析微观结构对混凝土梁开裂的影响;基于非均质材料特征尺寸与IH-PD模型网格参数的相关性,模拟骨料大小对混凝土梁断裂模式的影响;另外,通过在IH-PD模型中设置预损伤的方式引入随机分布的孔隙,探讨孔隙率对混凝土断裂模式的影响。     

重型燃气轮机高温透平叶片热障涂层系统中的应力和裂纹问题研究进展

燃气轮机是清洁高效火电能源系统的核心动力装备之一,是关乎国家能源安全和国防安全的战略高技术,是国家重大装备制造水平的标志,被誉为制造业王冠上的明珠。透平前燃气温度代表了燃气轮机的技术水平,人们一直在不断地追求燃气温度的提高。目前,国际最先进的重型燃气轮机的透平前燃气温度已达1600?C,未来还将向1700?C及以上发展。这种极端高温服役环境对高温透平叶片的设计和制造提出了严峻挑战,热障涂层（TBC）技术是解决这一问题的核心技术之一,在燃气轮机发展进程中发挥了重要作用,它不仅具有热障效果,而且还能防止氧化、腐蚀、外来物冲蚀等对叶片造成的损伤。因此,深入研究TBC的失效机理及其影响因素,对TBC的设计、制备及强度评价具有重要意义,对燃气轮机的安全服役具有重要作用。本文拟介绍重型燃气轮机高温透平叶片TBC系统中应力和裂纹问题的国内外最新研究进展,涉及理论、实验和数值分析几个方面。主要内容包括：TBC制备过程中的热应力,热生长氧化物（TGO）及其诱发的生长应力,TBC中的表面裂纹、界面裂纹及表界面裂纹间的竞争,TBC双轴强度评价方法,先进层级TBC中的表面和界面裂纹及其竞争,表面环境沉积物（CMAS）渗入诱发的涂层脱粘行为,陶瓷层烧结及其对TBC开裂的影响等。     

基于统一相场理论的早龄期混凝土化-热-力多场耦合裂缝模拟与抗裂性能预测

受水化反应和热量传输等过程影响, 混凝土在养护阶段会发生受约束收缩变形, 并由此在结构内引发较大的拉应力, 而此时混凝土力学性能往往还处于较低水平, 容易导致建造期混凝土结构即出现裂缝等病害. 这种早龄期混凝土裂缝对核安全壳、桥梁隧道、地下结构、水工或海工结构等重大土木工程和基础设施的全生命周期完整性、耐久性和安全性造成严重影响. 为了准确预测早龄期混凝土抗裂性能并量化裂缝演化对混凝土结构行为的不利影响, 亟需开展化-热-力多场耦合环境下的混凝土裂缝建模与抗裂性能分析研究. 针对这一需求, 本工作在前期提出的固体结构损伤破坏统一相场理论基础上, 考虑开裂过程与水化反应、热量传输等之间的相互影响, 建立裂缝相场演化特征(包括基于强度的裂缝起裂准则、基于能量的裂缝扩展准则和基于变分原理的扩展方向判据等)与混凝土水化度和温度之间的定量联系, 提出混凝土化-热-力多场耦合相场内聚裂缝模型, 发展相应的多场有限元数值实现算法并应用于若干验证算例. 数值模拟结果表明, 上述多场耦合相场内聚裂缝模型合理地考虑了水化反应、热量传输、力学行为以及裂缝演化之间的耦合效应, 揭示了早龄期混凝土热膨胀变形和自收缩变形的相互竞争机理, 且分析结果不受裂缝尺度和网格大小等数值参数的影响, 实现了早龄期裂缝演化全过程准确模拟和抗裂性能定量预测, 有望在混凝土结构早龄期裂缝预测和控制方面发挥重要作用.