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.     

Dynamic crack deflection and penetration at interfaces in homogeneous materials: experimental studies and model predictions

We examine the deflection/penetration behavior of dynamic mode-I cracks propagating at various speeds towards inclined weak planes/interfaces of various strengths in otherwise homogeneous isotropic plates. A dynamic wedge-loading mechanism is used to control the incoming crack speeds, and high-speed photography and dynamic photoelasticity are used to observe, in real-time, the failure mode transition mechanism at the interfaces. Simple dynamic fracture mechanics concepts used in conjunction with a postulated energy criterion are applied to examine the crack deflection/penetration behavior and, for the case of interfacial deflection, to predict the crack tip speed of the deflected crack. It is found that if the interfacial angle and strength are such as to trap an incident dynamic mode-I crack within the interface, a failure mode transition occurs. This transition is characterized by a distinct, observable and predicted speed jump as well as a dramatic crack speed increase as the crack transitions from a purely mode-I crack to an unstable mixed-mode interfacial crack.     

Delamination of an elastic film from an elastic–plastic substrate during adhesive contact loading and unloading

Adhesive contact between a rigid sphere and an elastic film on an elastic–perfectly plastic substrate was examined in the context of finite element simulation results. Surface adhesion was modeled by nonlinear springs obeying a force-displacement relationship governed by the Lennard–Jones potential. A bilinear cohesive zone law with prescribed cohesive strength and work of adhesion was used to simulate crack initiation and growth at the film/substrate interface. It is shown that the unloading response consists of five sequential stages: elastic recovery, interface damage (crack) initiation, damage evolution (delamination), film elastic bending, and abrupt surface separation (jump-out), with plastic deformation in the substrate occurring only during damage initiation. Substrate plasticity produces partial closure of the cohesive zone upon full unloading (jump-out), residual tensile stresses at the front of the crack tip, and irreversible downward bending of the elastic film. Finite element simulations illustrate the effects of minimum surface separation (i.e., maximum compressive surface force), work of adhesion and cohesive strength of the film/substrate interface, substrate yield strength, and initial crack size on the evolution of the surface force, residual deflection of the elastic film, film-substrate separation (debonding), crack-tip opening displacement, and contact instabilities (jump-in and jump-out) during a full load–unload cycle. The results of this study provide insight into the interdependence of contact instabilities and interfacial damage (cracking) encountered in layered media during adhesive contact loading and unloading.     

Extension of the Coherent Gradient Sensor (CGS) to the Combined Measurement of In-Plane and Out-of-Plane Displacement Field Gradients

The Coherent Gradient Sensor (CGS) is extended to the optical differentiation of specular, diffracted wave fronts leading to the combined measurement of in- and out-of-plane displacement field gradients. A derivation of the underlying optical interference principles is presented along with an analysis of the effective instrument sensitivity. In order to demonstrate the capabilities of the technique, experimental measurements of crack-tip deformation fields were conducted under various loading conditions corresponding to mode-I, mode-II, and mixed mode near-tip crack fields. The experimental procedures and results of these tests are presented as validation of the technique.     

Dynamic interfacial debonding initiation induced by an incident crack

When a crack propagates towards a weak interface, interface debonding may occur before the incident crack reaches the interface. This phenomenon refers to the “Cook–Gordon mechanism”. In this investigation, an equivalent dynamic Cook–Gordon mechanism is studied both experimentally and analytically. Two strength-based criteria incorporating dynamic fracture mechanics analysis are proposed to predict the initiation location of interface debonding ahead of a dynamic incident crack. As validation, a comparison is made between the analytical predictions and experimental measurements. Results show that the strength-based criteria can effectively predict the initiation of interface debonding. Meanwhile, effects of the stress intensity factor and the T stress of the incident crack, on the interfacial debonding initiation are investigated. It is concluded that high-stress intensity factors of the incident cracks will easily induce interfacial debonding initiation, and changing the T stress is an effective way to control interfacial debonding initiation. Furthermore, high-interfacial tensile strengths rather than shear strengths, tend to suppress interfacial debonding initiation induced by a mode-I incident crack.     

Interlaminar fracture toughness of graphite/epoxy composite under mixed-mode deformations

An antisymmetric test fixture is employed to investigate interlaminar fracture behavior in graphite/epoxy composite material under mixed-mode deformations. Finite correction factors for the graphite/epoxy fracture specimen with various crack lengths are used to determine the interlaminar fracture toughness by finite-element stress analysis. Interlaminar fracture characteristics of graphite/epoxy composite material under mode-I, mode-II and mixed-mode deformations are evaluated experimentally. A mixed-mode fracture criterion is also investigated to obtain information on mixedmode interlaminar fracture behavior of graphite/epoxy composite material.Paper was presented at the 1988 SEM Spring Conference on Experimental Mechanics held in Portland, OR on June 5–10.     

Crack Stability of Fracture Specimens used to Test Unidirectional Fiber Reinforced Material

The traditional compliance-based criterion of the crack stability in fracture mechanics states that the stability of the crack propagation in the different specimens under different fracture modes is determined by the derivative of the energy release rate with respect to the crack length. In this work the compliance-based criterion is verified by experiments performed on fracture mechanical systems. The large number of experiments carried out on different (mode-I, mode-II, mixed-mode I/II and mixed-mode II/III) specimens shows that the stability of the crack propagation depends on the derivative of the critical displacement (the displacement at the point of fracture initiation) with respect to the crack length. The experimentally established limits of crack stability were compared to the limits of the traditional criterion and it is shown that in each case they lead to approximately the same restriction considering the stable zone of crack propagation.     

Asymptotic analysis of transient curved crack in functionally graded materials

Transient mixed-mode elastodynamic crack growth along arbitrary smoothly varying paths in functionally graded materials (FGMs) is considered. The property gradation in FGMs is considered by varying shear modulus and mass density exponentially along the gradation direction. Crack tip out of plane displacement fields and their gradients are developed for propagating curved cracks of arbitrary velocity using asymptotic approach. The mode-mixity due to the inclination of curved crack with respect to property gradient is accommodated in the analysis through superposition of the opening and shear modes. The expansion of the displacement fields and their gradients around the crack-tip are derived in powers of radial coordinates with the coefficients of expansion depending on the instantaneous value of the local curvature of the crack path, time derivatives of crack-tip speed, and time derivative of mode-I and mode-II stress intensity factors. The effect of the transient terms instantaneous local curvature, crack-tip speed, time derivatives of crack-tip speed, and time derivative of mode-I and mode-II stress intensity factors on the contours of constant out of plane displacement are also discussed.     

裂纹扩展独立于网格的一种有限元单元子划分方法

提出了一种有限元模拟裂纹扩展的单元子划分结合子结构的方法.该方法中,裂纹可以进入或穿过一个单元,或沿单元的边界扩展,因此裂纹可以沿任意路径扩展而不受初始网格的限制.对上述几类包含裂纹的单元按照裂纹的路径进行子划分,覆盖一条裂纹的所有子划分单元就组成了一个子结构,子结构规模随裂纹的扩展而增大.子结构中因单元子划分而新增的结点自由度,通过自由度的凝聚用初始网格结点的自由度表示,因此结构整体分析的总自由度不变.以上述方法为基础建立了裂纹萌生和扩展的准则.用论文的方法分析了单(双)材料无限大平面中心(界面)裂纹的裂尖场,验证了论文方法的精度,并模拟了颗粒复合材料中微裂纹在颗粒、基体和界面中逐步扩展的过程,考核了论文方法对复杂裂纹扩展问题模拟的适用性.     

Stable growth of penny-shaped crack in viscoelastic composite material under time-dependent loading

Considered is the long-term cracking of the three-dimensional fiber-reinforced viscoelastic composite with a plane penny-shaped crack under time-dependent loading. The composite has a hexagonal structure and consists of elastic isotropic fibers and viscoelastic isotropic matrix. The material is modeled by transversally isotropic homogeneous linearly viscoelastic medium with some averaged characteristics. The crack propagation planecoincides with the plane of isotropy. A ring-shaped yield zone in front of the moving crack is modeled as a Dugdale's zone with time-dependent stresses. Crack growth under deformation of the composite occurs by application of a slowly increasing tensile load; it is normal to the plane of crack propagation. A convolution-type time operator describes the viscoelastic properties of the matrix material. Use is made of the Volterra principle and the theory of long-term cracking of viscoelastic bodies. The irrational function of integral operator associated with the viscoelastic crack opening expression is expanded into a continued fraction of operators. The solution is reduced to the nonlinear integral equations of crack growth. Numerical results are obtained for a specific material. Crack growth kinetics is discussed in connection with the onset of stable crack growth and crack border stress intensity factor.     

Impact failure characteristics in sandwich structures. Part II: Effects of impact speed and interfacial strength

In this paper, we describe the second part of an experimental investigation concentrating on the study of the effects of impact speed and interfacial bond strength on the dynamic failure of model sandwich structures. Results show that even small variations in impact speed and bond strength substantially influence the initiation behavior of delamination (location and nucleation time) and lead to substantially different inter-layer crack speed histories. These changes in inter-layer failure history influence the timing, sequence and final extent of subsequent intra-layer damage within the sandwich structures.     

复合材料层合板低速冲击损伤分析的连续介质损伤力学模型

针对复合材料层合板低速冲击损伤问题,提出了一种各向异性材料连续介质损伤力学模型,模型涵盖损伤表征、损伤起始判定和损伤演化法则3 个方面. 通过材料断裂面坐标下的损伤状态变量矩阵完成损伤表征,并考虑断裂面角度的影响,建立了主轴坐标系下的材料损伤本构关系. 损伤起始由卜克(Puck) 失效准则预测,损伤演化由断裂面上的等效应变控制,服从基于材料应变能释放的线性软化行为. 模型区分了纤维损伤和基体损伤,并根据冲击载荷下层内产生多条基体裂纹继而扩展至界面形成层间裂纹(分层) 的试验观察,引入基体裂纹饱和密度参数表征层间分层. 以[0₃/45/-45]_S 和[45/0/-45/90]_4S 两种铺层的复合材料层合板为例,预测了不同冲击能量下复合材料层合板的低速冲击损伤响应参数,试验结果证明了连续介质损伤力学模型的有效性.模型在不同网格密度下的计算结果表明单元特征长度的引入可以在一定程度上降低损伤演化阶段对网格密度的依赖性.      

<Emphasis Type="Italic">In-situ</Emphasis> Full Field Measurements During Inter-Facial Debonding in Single Fiber Composite Under Transverse Load

The fibre/matrix interfacial damage mechanisms of fiber-reinforced composites (FRCs) are investigated for single-fiber composites under transverse load. A stereo microscope setup is used for 3D digital image correlation during in-situ quasi-static tests of single-fiber standard dog-bone specimens. Macro-fibers (0.9 mm diameter) with radically different interfacial bonding with the epoxy matrix are used. Damage appears to initiate with fiber debonding at the free surface along the tensile direction. The crack then propagates around the interface while slightly growing along the fiber until a lateral crack initiates on the debonded free surface, provoking specimen failure. The final failure mechanisms appears to be different for strong and weak fiber/matrix bonding. 3D DIC is used to provide precise measurements of displacements, strains, and out-of-plane displacement during the whole test. Quantitative differences in the displacement fields are measured in the cases of strong and weak bonding between the fiber and matrix. 3D DIC with macro-fibers is shown to be a promising technique to provide a better understanding of the damage mechanisms in a single-fiber composite and to determine interfacial toughness of a specific fibre/matrix couple in order to perform accurate modeling of damage in FRCs. Displacement, strain, and confidence field results for each pixel from each experiment and at each time step are also provided for detailed comparison with simulation results.     

2D-C/SiC拉伸损伤的声发射信号聚类分析

对2D-C/SiC复合材料进行常温拉伸试验,利用声发射(AE)仪在线监测其损伤过程.应用K-均值聚类算法对AE信号进行模式识别,结合扫描电镜观察,发现2IC/SiC复合材料拉伸过程的损伤模式包括基体开裂、界面层脱粘、层间剥离、纤维断裂以及纤维束断裂等,并得到了各种损伤模式AE信号参数的中心值.通过对不同损伤模式AE累积事件数和累积能量随时间变化的统计分析,描述了2DC/SiC复合材料拉伸损伤演化过程.     

Fatigue crack initaition and propagation from hole defects

The aim of this study is to investigate the interaction of two hole defects affecting fatigue crack initiation life and propagation behavior. The location of two hole defects was characterized by an angle of alignment and the distance between the centers of two hole defects. The fatigue cracking behavior is studies under bending for AI 5086. When defects are located close to each other, the fatigue crack initiation lives are varied with their relative locations. In the experiments, the area of the local plastic stratin strongly played a role in the fatigue crack initiation lives. Therefore, the authors introduce a parameter which contains the plastic deformation are at stress concentrations and proposed a fatigue crack initiation life prediction curve. In addition, the directions and propagation rates of fatigue cracks initiated at two hole defects were studied experimentally.     

Closure effects in delaminated graphite epoxy laminate under compression

Flaws in composite laminates may result in a severe loss of static and dynamic strength. Such flaws may be inherent or gained by misadventure. The extent of this loss can be influenced by several factors including loading, laminate stacking sequence, lamina properties, flaw size and damage type.In this study, the free-edge delamination of a laminated composite under compression loading is investigated. Computational, analytical and experimental tests are performed on a graphite/epoxy laminate AS4/3501-6 containing near surface edge defects and the crack opening behaviour is investigated.The computational analysis consists of a three dimensional finite element model where the plies can be catered for individually and interply delamination modelled. In the experimental investigations, a delamination is simulated by inserting teflon film at appropriate locations during the lay-up process.     

轮轨界面摩擦学转变结构层特性及其研究进展

摩擦学转变结构层是轮轨系统滚动接触过程中形成的典型损伤特征,其发生、发展及其演变过程对轮轨界面失效具有重要的影响. 本文作者分析了轮轨界面摩擦学转变结构层[TTS,包括白色浸蚀层(WEL)、棕色浸蚀层(BEL)以及塑性变形层(PDL)]的典型特征和微观结构,重点讨论了当前轮轨界面TTS的研究现状及在形成机制上的分歧和争论,并介绍了近年来国内外学者应用先进分析测试手段开展TTS研究的相关进展;归纳总结了TTS对轮轨界面损伤的主要贡献,最后重申了TTS在轮轨摩擦学研究中的重要性并提出了今后研究的发展方向.      

复合材料结构的内部分层及其扩展

本文用接触观点分析了复合材料壳体结构的内部分层问题，建立了求解摩擦接触问题的有限元迭代格式，并采用释放率判据分析分层裂纹的扩展。分层裂纹前缘采用轴对称奇异单元，并了结构的大变形特性。计算表明，层间裂纹呈Ⅱ型扩展趋势。本文同时还分析了层间摩擦系数的影响，并指出分层将导致壳体结构承载能力显著下降。