Stable crack growth in a surface compression-strengthened hollow cylinder

In this paper the static fatigue problem for a circumferentially cracked hollow cylinder is examined. For this particular configuration, stable crack growth, in the absense of any external forces, is determined for cylinders with axial components of residual stress which are compressive on the inner and outer radial surfaces and tensile in the cylinder wall. An initial surface crack which is deep enough to penetrate the compression strengthened surface region and enters the tensile zone may propagate in a stable manner until either sudden spontaneous failure occurs or the crack arrests. Since a portion of the crack near the cylinder surface will be closed because of the compressive residual stress field, an additional unknown in the problem is the extent of the crack surface contact. This crack surface contact length is determined by iteration on the integral equation which arises in the mathematical derivation for an embedded circumferential crack in a hollow cylinder. As an illustration of stable crack growth for this geometry with a realistic residual stress distribution, numerical results are presented for a hollow, soda-lime glass cylinder, based on crack growth rates in soda-lime glass exposed to water at 25‡ C. Using the fracture toughness and slow crack growth characteristics for soda-lime glass, the conditions for no crack propagation, crack propagation leading to crack arrest, and catastrophic failure are established.     

3D fracture mechanics investigation on surface fatigue crack propagation

Surface fatigue crack propagation is the typical failure mode of engineering structures. In this study, the experiment on surface fatigue crack propagation in 15MnVN steel plate is carried out, and the crack shape and propagation life are obtained. With the concept of ‘equivalent thickness’ brought into the latest three‐dimensional (3D) fracture mechanics theory, one closure model applicable to 3D fatigue crack is put forward. By using the above 3D crack‐closure model, the shape and propagation life of surface fatigue crack in 15MnVN plates are predicted. The simulative results show that the 3D fracture mechanics‐based closure model for 3D fatigue crack is effective.     

An improved model for predicting the crack size and plasticity dependence of fatigue crack propagation

The angled crack problem has been given special attention in the recent years by fracture mechanics investigators due to its close proximity to realistic conditions in engineering structures. In this paper, an investigation of fatigue crack propagation in rectangular steel plates containing an inclined surface crack is presented. The inclined angle of the crack with respect to the axis of loading varied between 0° and 90°. During the fatigue tests, the growth of the fatigue crack was monitored using the AC potential drop technique. A series of modification factors, which allow accurate sizing of such defects, is recommended. Paris power law is normalized and adopted for data analysis. Subsequently, this concept is applied to predict crack growth due to fatigue loads. The results obtained are compared with those obtained using the commonly employed fracture criterion and the experimental data.     

Fatigue life prediction of cracked padded plates

The fatigue crack propagation analyses of padded plates are conducted to predict the crack growth behaviour under various loading conditions. The fatigue life of a padded plate with a single edge crack originating from the weld toe is calculated using the weight function approach. The fatigue strength of padded plates with different pad thickness under remote loading conditions was investigated and compared to the T-plate joint. The improvement of the fatigue strength of the pad design is verified.
  The thickness effect of the padded plate was investigated using the fracture mechanics approach. The geometrically similar model pairs with different initial crack sizes were investigated under remote loading conditions. It was shown that the thickness effect depends on both stress concentration and initial crack size.     

Fracture mechanics analysis of thermally tempered glass plate: fracture induced by an embedded crack

This paper presents a fracture mechanics analysis of a thermally tempered glass plate. The fracture is induced by an embedded penny-shaped crack. The analysis shows that the existence of a penny-shaped crack will reduce the strength of tempered glass. The impact and fatigue resistance of the glass is related to the position and size of the penny-shaped crack. When the tempering intensity reached to a certain level, thermally tempered glass with a penny-shaped crack could experience spontaneous fracture. The damage of a central crack on glass is more severe than a surface crack. With surface compression, thermal tempering will increase the critical applied stress of the glass if the surface penny-shaped crack size is in the range of 0 < a/d < 0.27, where a is the crack size, d is the half thickness of the glass plate. For a small surface crack with the size of a/d 0.09, the tempering can hinder its extension. However, if there is a central penny-shaped crack, the critical applied stress of the tempered glass will decrease with the intensity of tempering.*Author for correspondence. (E-mail: lejun@mail.sic.ac.cn)     

Transverse shear and pressurization effects in the bending of plates with reinforced cracks

The authors develop an eigth-order model for bending of transversally isotropic plates and use integral transforms and a collocation method to form a line-spring model for a cracked plate. The eigth-order model allows satisfaction of the three standard plate bending boundary conditions; the normal moment, twisting moment, and transverse shear force, and an additional shear stress resultant that allows analysis of transverse normal stresses near the crack tip. The line-spring model is used to develop geometry correction factors for bending of finite-thickness plates, accounting for transverse shear deformation and pressurization of the plate near the crack tip. The line-spring model is then applied to the problem of a plate with a reinforced crack, and the results are used to validate an interpolation solution based on an energy method. While not explicitly analysed, the models are applicable to many problems, including bending of bonded repairs, fracture and fatigue of composite and layered materials, surface cracks, crack tip plasticity and crack closure or crack face interaction.     

CRACK FACE INTERACTIONS AND NEAR-THRESHOLD FATIGUE CRACK GROWTH

Rough fracture surfaces usually influence substantially the fatigue growth properties of materials in the regime of low growth rates. Friction, abrasion, interlocking of fracture surface asperites and fretting debris reduce the applied load amplitude to a smaller effective value at the crack tip (“sliding crack closure”, or “crack surface interaction” or “crack surface interference”). The influence of these phenomena on the fatigue crack growth properties of structural steel is discussed and compared for the two kinds of mixed mode loading employed in this work. Mixed mode loading was performed by (A): cyclic mode III + superimposed static mode I and (B): cyclic mode I + superimposed static mode III loading. Such loading cases frequently occur in rotating load-transmission devices. Several differences are typical for these two mixed-mode loading cases. A superimposed static mode I load increases the crack propagation rate under cyclic mode III loading whereas cyclic mode I fatigue crack propagation is retarded when a static mode III load is superimposed. Increase of the R -ratio (of the cyclic mode III load) leads to an insignificant increase of fracture surface interaction and subsequently to a small decrease of the crack growth rate for cyclic mode III loading, whereas higher R -values during cyclic mode I+ superimposed static mode III loading lead to a significant reduction of the crack growth rates.     

A mixed mode crack growth model taking account of fracture surface contact and friction

Fracture surface interactions, of whatever origin, can significantly affect the stress intensity factor, and consequently, can also be relevant to fatigue crack propagation. In the occurrence of interaction between fracture surfaces, the effective loading cycle experienced by material near the crack tip may be very different from that evaluated on the basis of the external loadings only. The purpose of the work described in this paper is to obtain the effective mode II stress intensity factor, k _IIeff, in a surface cracked elasto-plastic plate with a factory roof fracture surface subjected to an in-plane shear (mode II) loading. A new model estimating the magnitude of the frictional mode II stress intensity factor, k _f, arising from the mismatch of the fracture surface roughness during in-plane shear, is developed. Furthermore, the results of this study are employed in modeling the fatigue response of the surface cracked plates subjected to mixed mode loading.     

Predicting fatigue S-N (stress-number of cycles to fail) behavior of reinforced plastics using fracture mechanics theory

The present study describes a model to predict fatigue S-N behavior, and thus fatigue life, of glass fiber reinforced thermoplastics by using a fracture mechanics approach. The model assumes the presence of an inherent initial flaw in the molded plastic parts and thus ignores crack initiation contributions. In this paper we describe how fatigue crack propagation rate data were obtained for the same three glass fiber reinforced plastics whose S-N behavior was previously described in detail. Using the measured constants from the crack growth data, and corresponding S-N data for uncracked specimens, the validity of the single initial flaw hypothesis was evaluated. From the analyzed results it is concluded that accurate S-N predictions are possible using this simple fracture mechanics model for some materials. The best results are obtained for glass filled polyamide, PA (nylon 66) and polycarbonate, PC; however, with polybutylene terephthalate, PBT, predictions were poor. It is also shown that S-N data for different glass fiber orientations can be predicted by combining the single flaw model with predicted fatigue crack propagation rate measurements. The latter are calculated from a generalized crack growth rate expression utilizing the strain energy release rate fracture mechanics parameter, which was previously described.     

纵肋与横隔板交叉构造细节穿透型疲劳裂纹扩展特性及其加固方法研究

纵肋与横隔板交叉构造细节是正交异性钢桥面板最易发生疲劳开裂的构造细节,通过建立有限元数值模型,采用断裂力学方法,研究栓接角钢加固方式对该处疲劳易损细节穿透型裂纹的加固效果。基于疲劳试验足尺节段模型相对应有限元模型,建立了纵肋与横隔板焊接处穿透型疲劳裂纹模型,针对栓接角钢和纵肋外侧栓接钢板两种加固技术的加固效果进行评估。研究结果表明:钢桥面板纵肋与横隔板交叉构造细节的疲劳裂纹扩展至一定长度后将发展成穿透型裂纹,裂纹面受力复杂,纵肋腹板内外侧疲劳裂纹扩展特性表现的不一样,但是随着裂纹扩展的逐步进行,裂纹尖端的开裂模式均以复合型开裂为主;栓接角钢加固方式主要抑制纵肋与横隔板交叉构造细节易损部位疲劳裂纹的I型开裂,因此能很好地抑制短裂纹的扩展,但对于该细节处以复合形式扩展的穿透型疲劳裂纹的加固效果并不显著;在纵肋外侧栓接半U形钢板的加固方法能有效改善穿透型疲劳裂纹的等效应力强度因子,并且加固之后均保持在裂纹扩展阈值以下,表明该加固方式对穿透型疲劳裂纹有良好加固效果。     

Effects of thickness on fatigue properties of investment cast Ti-6Al-4V alloy plates

The present study is concerned with the effects of plate thickness on high-cycle fatigue properties and fatigue crack propagation behavior of investment cast Ti-6Al-4V alloys having Widmanstätten structure. High-cycle fatigue test and fatigue crack propagation test were conducted on three cast plates having different thickness, and then the test data were analyzed in relation with microstructures, tensile properties, and fatigue fracture mode. The high-cycle fatigue results indicated that fatigue strength of the three cast plates was quite similar because of their similar tensile strength. In the case of the fatigue crack propagation, the thicker cast plate composed of thinner platelets had the slightly faster crack propagation rate than the other plates. The effective microstructural factor determining the fatigue crack propagation rate was found to be the thickness of platelets because it was well matched with the reversed cyclic plastic zone size calculated in the threshold K regime.     

Modelling fatigue crack propagation of a cracked metallic member reinforced by composite patches

The concept of fracture for material elements at front of a crack for fatigue crack propagation was extended to the fatigue crack propagation of a cracked metallic member reinforced with a composite patch in this paper. From static mechanics and linear elastic fracture mechanics, force transfer on a cracked member through a composite patch was analyzed and a formula connecting the stress intensity factor with crack length was obtained. Thereafter, a fracture model for fatigue crack propagation of a repaired cracked metallic member was proposed. A new expression for the fatigue crack propagation rate has thus been derived. The expression was verified objectively by the test data. It is in good agreement with the test results.     

Some Peculiarities of Fatigue Crack Growth at Various Stages of Its Development

The author considers some peculiarities of fatigue crack growth in metals at the stages of its initiation and initial development, and stable and unstable growth that precedes final fracture. It is shown that at the stage of initial growth of fatigue cracks, the stress state, nonlocalized fatigue damage that precedes initiation of the main fatigue crack, residual surface stresses, surface manufacturing and in-service defects, and contact interactions are the factors that determine the crack paths. Stable growth of a fatigue crack is primarily determined by the stress-strain state of a structure as a whole and by the stress-strain state at the crack tip with allowance for its variation due to crack propagation, which is evaluated by the criteria of fracture mechanics. The author also studied peculiarities of fatigue crack development in compressor blades of marine gas turbines. It is shown that for embrittled steels, when fatigue cracks develop under plane strain conditions, final fracture occurs at very small crack sizes. In this case, the characteristics of fatigue fracture toughness are appreciably lower than the static values. The paper also considers peculiarities of unstable fatigue crack propagation.     

钠钙/铝硅酸盐玻璃疲劳行为研究进展

钠钙硅酸盐玻璃和铝硅酸盐玻璃因高强度和高硬度等优异的物理性能被广泛用于民用和国防军工等领域。疲劳断裂是钠钙/铝硅酸盐玻璃失效的主要形式之一。研究钠钙/铝硅酸盐玻璃的疲劳行为对指导其加工工艺、预测寿命与防止失效具有重要意义。本文综述了硅酸盐玻璃疲劳行为的基本原理、实验方法以及钠钙硅酸盐玻璃和铝硅酸盐玻璃静态疲劳与动态疲劳行为的国内外研究进展,通过对比钠钙硅酸盐玻璃与铝硅酸盐玻璃的疲劳行为发现:钠钙硅酸盐玻璃原片的裂纹萌生门槛值远低于铝硅酸盐原片玻璃的裂纹萌生门槛值,且化学强化钠钙硅酸盐玻璃与铝硅酸盐玻璃的裂纹萌生门槛值均随表面压应力的增大而增大;钠钙硅酸盐玻璃与铝硅酸盐玻璃的裂纹扩展速率均随假想温度的升高而增加,且铝硅酸盐玻璃的裂纹扩展速率随假想温度的变化更大;钠钙硅酸盐原片玻璃与铝硅酸盐原片玻璃的断裂应力均具有加载速率依赖性,而化学强化铝硅酸盐玻璃的动态疲劳断裂应力不具有加载速率依赖性。因此,应力状态和环境因素对化学强化钠钙/铝硅酸盐玻璃静态疲劳与动态疲劳行为的影响将成为未来研究方向。     

Prediction on fatigue life of U‐notched PMMA plate

In this paper, fatigue life prediction of U‐notched polymethyl methacrylate (PMMA) plate is numerically investigated based on the combination of fatigue damage mechanism and fatigue crack propagation mechanism. First, strength and stiffness degeneration criterions during the fatigue process are established on the basis of nonlinear progressive damage evolution, and the fatigue crack initiation life is estimated. Second, fatigue crack propagation phase is analysed through virtual crack closure technique. The fatigue crack propagation life before totally fracture is also predicted. Finally, finite element models of PMMA plate weakened by lateral symmetric U‐notch are built up using ABAQUS, and the total fatigue life of notched plate is calculated by combining the crack initiation life with the crack propagation life. These results will play an important role for evaluating the fatigue life of U‐notched PMMA plate.     

Fatigue crack propagation in steels

From previous investigations of the mechanisms of both fracture and fatigue crack propagation, the static fracture model proposed by Lal and Weiss may be thought as reasonable for describing fatigue crack propagation in metals at both low and intermediate stress intensity factor ranges $\text{ΔK}$. Recent progress in fatigue crack propagation indicates that it is not only possible, but also necessary, to modify this static fracture model. Based on the modified static fracture model, the effective stress intensity factor range $\text{ΔK}{}_{\mathrm{eff}}$, which is defined as the difference between $\text{ΔK}$ and the fatigue crack propagation threshold value $\text{Δ}{}_{\mathrm{th}}$, is taken as the governing parameter for fatigue crack propagation. Utilising the estimates of the theoretical strengths of metals employed in industry, a new expression for fatigue crack propagation, which may be predicted from the tensile properties of the metals, has been derived. The correlation between the fatigue crack propagation rate and the tensile properties is thus revealed. The new expression fits the test results of fatigue crack propagation of steels below 10^?3 mm/cycle and indicates well the effect of stress ratio on the fatigue crack propagation rate.     

Elevated-temperature crack growth in polycrystalline alumina under static and cyclic loads

An experimental investigation has been conducted to study the crack growth characteristics of a 90% pure aluminium oxide in 1050 °C air under static and cyclic loads. It is shown that the application of both sustained and fluctuating tensile loads to the ceramic, tested in a precracked four-point bend specimen configuration, results in appreciable subcritical crack growth. The crack velocities under cyclic loading conditions are up to two orders of magnitude slower than those measured in static loading under the same maximum stress intensity factor. Cyclic crack growth rates are markedly affected by the loading frequency, with a decrease in test frequency causing an increase in the rate of crack advance. Detailed optical and electron microscopy observations have been made in an attempt to study the mechanisms of stable crack growth and the mechanistic differences between static fatigue fracture. Under both static and cyclic loads, the predominant mode of fracture is intergranular separation. The presence of a glass phase along the grain boundaries appears to have a strong effect on the mechanisms of crack growth. Apparent differences in the crack velocities between static and cyclic fatigue in alumina arise from crack-wake contact effects as well as from the rate-sensitivity of deformation of the glass phase. Our results also indicate that the cyclic fatigue crack growth rates cannot be predicted solely on the basis of sustained load fracture data. White stable crack growth occurs in the 90% pure alumina over a range of stress intensity factor spanning 1.5 to 5 MPa m^1/2, such subcritical fracture is essentially suppressed in a 99.9% pure alumina, ostensibly due to the paucity of a critical amount of glass phase. Both static and cyclic fracture characteristics of the 90% pure alumina are qualitatively similar to those found in an Al₂O₃-SiC composite wherein situ formation of glass phases, due to the oxidation of SiC in high-temperature air, is known to be an important factor in the fracture process.     

基于Beach Marking方法的钢结构疲劳裂纹检测研究

研究裂纹的形成和扩展规律是钢结构疲劳断裂的一个重要课题。该文讨论了一种十分经济有效的检测钢结构疲劳裂纹萌生与扩展的方法,即Beach Marking方法。首先简要总结了该方法的发展历史和研究现状,并系统归纳了其基本原理和关键问题。同时,通过焊接H型钢梁的试验案例,探索该方法应用在大尺寸构件上的实际效果,并具体展示该方法的设计步骤和应用过程。试验表明,Beach Marking方法简单实用,关键在于设计合理的疲劳荷载序列;通过分析疲劳断口上留下的疲劳弧线,并结合断裂力学理论,可以获得疲劳裂纹的扩展规律和参数,为钢结构疲劳寿命数值分析提供科学依据。     

Computational modelling of static indentation-induced damage in glass

The present paper is focused on the numerical simulation of a glass plate subjected to static indentation by a spherical indenter. For this purpose, a combined approach of continuum damage mechanics (CDM) and fracture mechanics is performed. Results provided by an axisymmetric finite element model were compared with analytical solutions. A CDM based constitutive model with an anisotropic damage tensor was selected and implemented into a finite element code to study the damage of glass. The numerical results were analysed through the framework of the stress and damage distribution. Various regions with critical damage values were therefore predicted in good agreement with the experimental observations in the literature. In these regions, the directions of crack propagation, including both cracks initiating on the surface as well as in the bulk, were predicted using the strain energy density factor. Predicted directions were found in good agreement with those experimentally obtained in the literature results.     

Fatigue fracture toughness of metals

The paper considers the peculiarities of fatigue crack propagation and final fracture of metals under cyclic loading. It is shown that the value of the fatigue fracture toughness of steels in an embrittled state is appreciably lower than that of the fracture toughness under static loading. A model of the transition from stable to unstable fatigue crack propagation is justified.