Fatigue Crack Growth under Variable Amplitude Loading

Book Review

Fatigue Crack Growth under Variable Amplitude LoadingProceedings of the Third International Spring Meeting of the French Metallurgical Society held in Paris, France, June 1988. Edited by J. Petit, D.L.Davidson, S. Suresh, and P.Rabbe. Elsevier Applied Science, London (1988) 397 pp., $45.00     

不同拘束条件下P92钢高温蠕变裂纹扩展速率的有限元模拟

基于蠕变损伤力学,仿真了蠕变裂纹扩展过程。用ABAQUS软件模拟研究了试样几何形状和宽度不同的两种面内拘束条件对P92钢在650℃时蠕变裂纹扩展(CCG)速率的影响,并进行了试验验证。仿真结果表明:试样几何形状和宽度对CCG速率的影响与载荷水平(C*)有关;在低载荷水平区内,C(T)试样的拘束水平大于SEN(T)试样,并且对应的CCG速率较大;在中载荷水平区,随着试样宽度的增大,裂纹尖端拘束和CCG速率增大;在高载荷水平区,CCG速率基本不受面内拘束(试样几何形状、宽度)条件的影响;在条件相同的情况下,试样宽度拘束水平大于试样几何形状。试验结果表明,试验与有限元分析(FEA)的蠕变裂纹扩展结果相符合。     

Elevated Temperature Fatigue Crack Growth rate model for NI-BASE Superalloys

Several time-dependent mechanisms are operational in the crack growth process of Ni-base superalloys at elevated temperature. Creep deformation during periods of sustained loading, oxygen diffusion at the crack tip, and oxidation reactions at and in front of the crack tip all contribute to the kinetics of crack growth. A crack growth rate model has been derived that attempts to capture the physics of these various rate processes. The proposed model assumes small-scale creep at the crack tip and incorporates the Hutchinson-Rice-Rosengren stress field equations to satisfy this condition. The model also includes stress-assisted diffusion of an environmental species at the crack tip. A process reaction rate is related to the time-rate of crack growth providing a model that accounts for these time-dependent processes. An evaluation of the form of the model is provided by comparison of the model with experimental crack growth data.     

退火对Al2O3/SiC纳米复合材料裂纹愈合行为的影响

研究不同密度的Al2O3/SiC纳米复合材料在退火条件下的裂纹愈合行为。利用维氏压痕法引入可控的预制裂纹，通过测量退火前后材料抗弯强度的变化来表生裂纹愈合程度。当退火温度在850℃以上时，裂纹愈合效应随退火温度提高明显增强，在1150℃左右退火1h后，不同密度的试样的抗弯强度均恢复到试样的固有强度。裂纹愈合的机制主要归结于退火过程中残余应力的驰豫导致扩散键合和裂纹表面碳化硅颗粒的氧化作用。然而，过度的氧化会降低裂纹愈合的效果。     

不同温度下西兰花生命变化的研究

采用顶空分析仪和色度计,对储藏期内室温(23 ℃)和冷藏(4 ℃)下袋内西兰花的生命变化(外观色泽和气氛)进行了监测。结果发现,随着储藏天数的增加,包装袋内气氛中O2 浓度逐渐降低,CO2 浓度逐渐增加,西兰花的色度(a 和b)和明度(L)值逐渐增加。这表明西兰花的外观颜色由蓝变黄,由绿变红,明度由黑变白。此外,储藏温度强烈影响西兰花的生命变化。与室温23 ℃相比,4 ℃冷藏下西兰花的L,a,b 值及袋中O2 和CO2 浓度的下降幅度有所减缓,这表明西兰花的外观色泽得到一定的保持,且呼吸作用得到抑制,因此4 ℃冷藏对西兰花具有保鲜作用。色度计和顶空分析仪可作为保鲜研究中的有力工具去研究果蔬的生命变化。     

Ag-TCNQ分形生长现象研究

利用溶液化学反应法，在Ag-TCNQ过饱和度较大的条件下，发现了Ag-TCNQ的分形生长现象。研究表明，Ag膜厚度和与溶液反应的时间对样品的形貌有影响。Ag膜薄生成的样品稀疏，分形维数小；反之样品致密，分形维数大。反应完全时样品最致密，分形维数最大。SEM研究表明，这些分形生长主要是堆垛分形，分支堆积有序，表现出材料良好的自组织性。     

基于遗传规划算法的不同应力比下不同厚度7050铝合金疲劳裂纹扩展寿命预测

针对不同厚度7050铝合金试样进行了不同应力比条件下的一系列疲劳裂纹扩展试验,并运用遗传规划算法对疲劳裂纹扩展寿命进行预测。遗传规划算法是模拟自然界中生物的进化策略,通过交换、突变等遗传操作,搜索目标的最优解。建立7050铝合金疲劳裂纹扩展速率的遗传规划模型,并利用试验数据对模型进行测试,后与其他典型疲劳裂纹扩展模型进行比较。研究结果表明:GP模型预测的7050铝合金疲劳裂纹扩展寿命结果与试验值基本吻合,相对误差小于1.5%,且GP模型预测结果的准确性高于Paris模型和Walker模型。     

New Concept on Fatigue Crack Growth Characteristics under Combined Stress Conditions

Using thin walled cylindrical specimens subjected cyclic tension and torsion, fatigue crack growth tests under combined stress conditions were carried out. The characteristics of the fatigue crack growth rate (FCGR) and morphology of the fatigue crack were identified.By comparing the experimental results with the results of finite element analyses, it was found that the experimental characteristics of FCGR were related to fatigue damage accumulation before initial crack growth. Damage mechanics approach is indispensable to characterize the fatigue crack growth under combined stress conditions.     

GH4169合金焊接接头高温疲劳裂纹扩展性能试验研究

对GH4169合金焊接接头在650℃下的疲劳裂纹扩展性能进行了测试和分析.结果表明,疲劳裂纹扩展速率、初始裂纹尺寸、裂纹容限和温度对焊接接头的疲劳裂纹扩展寿命都有影响.母材及焊缝的疲劳裂纹扩展速率和裂纹容限均有较大的差异.     

Temperature Dependence of Brittle Fracture Toughness of Reactor Pressure-Vessel Steels upon Ductile Crack Growth

The main mechanisms of brittle fracture upon ductile crack growth are studied on the basis of the probabilistic model of brittle fracture and the deterministic model of ductile fracture, which were put forward by the authors earlier. The investigations are carried out on the reactor pressure-vessel steel 15Kh2NMFAA in the initial and embrittled states. The dependences of brittle-fracture probability on the stress intensity factor and the value of ductile crack growth are calculated for various temperatures. The temperature dependence of brittle fracture toughness in the initial and embrittled states is predicted with and without regard for ductile crack growth. The authors analyze the main factors that govern the above-mentioned relationships. The calculated results are compared to test data for CT-type compact specimens.     

不同贮藏温度下冷却肉品质变化的实验研究

考察了不同贮藏温度下(O℃、5℃、10℃和20℃)猪肉细菌总数、挥发性盐基氮、色差、感官评定、pH值随保存时间的变化规律.发现猪肉的细菌总数、挥发性盐基氮值随贮藏时间的延长而增加,随贮藏温度的增加而上升;随着贮藏时间的延长,猪肉的亮度L和红度a缓慢下降.在Arrhenius动力学方程基础之上,建立了菌落总数、挥发性盐机氮和色差(亮度L和红度a值)与贮藏时间及贮藏温度之间的动力学模型.通过实验得到了各项指标变化预测模型中的EA及%Ko分别为:71.26kJ/mol,68.86kJ/mol,66.32kJ/mol,53.25kJ/mol和5.711×108、5.179×104、2.727×104、8.001×104.     

不同温度下聚苯硫醚冲击试样的断口形貌

对聚苯硫醚(PPS)工程塑料的冲击断口形貌进行了宏观和微观观察,总结了PPS冲击断口的典型形貌特征,并研究了各典型形貌随试验温度的变化规律.宏观断口分析表明:当t<60℃时,断口由平坦区、放射区和最后断裂区组成;当t≥60℃时,断口非常平坦,除少量人字形放射条纹外几乎观察不到其他形貌特征.微观断口分析和定量测试表明:平坦区微观典型形貌为岩石层状,随温度的升高,岩石层的片层尺寸和粗糙度都越来越大.在-196℃≤t≤-60℃时,放射区的放射状条纹数量较多且细密,当-60℃相似文献   

Remnant Fractal Structure at Sintered Crack Surfaces in Glass

This is to demonstrate that the Raman spectroscopic analysis allows one to estimate a size of residual ("unhealed") cracks from measurements of the fractal units dimensions in real space. In contrast to the microscopical fractal structure reported for cracks in glass ceramics (Mecholsky, Passoja and Feinberg-Ringel, 1989), we present the observation of fractals at the nanostructural scale.     

Crack Growth in Rolling Bodies under the Conditions of Dry Friction and Wetting

Within the framework of a two-dimensional mathematical model, we study the development of a surface macrocrack in the course of unidirectional rolling under the conditions of dry friction and wetting. The damaged body in a rolling couple is modeled by an elastic half plane with an edge cut (crack) and the action of the counterbody is simulated by the translational motion of Hertzian contact forces along the boundary of the half plane with taking into account the friction forces. We compute the crack-growth paths depending on the coefficient of contact friction between the bodies and the orientation and length of the crack. The numerical calculations are performed by the step-by-step method based on the use of the singular integral equations of the theory of elasticity for bodies with curvilinear cracks and the criterion of generalized opening. It is shown that, for high coefficients of friction between the bodies of the rolling couple (f = 0.20–40; dry friction), a branch propagates from the original fatigue shear macrocrack into the bulk of the material of the driven body. In the case of low friction coefficients (f < 0.20; wetting), a branch mainly propagates along the boundary of the body in the direction of motion of the counterbody. We propose an explanation of the mechanism of development of burrow-type defects often encountered in railroad rails.     

直流电位法检测高温合金的疲劳裂纹扩展性能

介绍了在高温下使用直流电位法测定疲劳裂纹长度的基本原理、试验方法及所需配置的仪器,并对有关影响因素进行了分析;并利用Johnson的分析型关系式,研究了一种适用于自动测定材料高温疲劳裂纹长度的直流电位法,该方法已成功应用于高温合金疲劳裂纹扩展试验,其测得的疲劳裂纹扩展速率da/dN-ΔK数据与长焦聚显微镜法测得的数据完全一致。     

Fractal Nature of the Brittle Fracture Surfaces of Metal

We have studied the fracture surfaces of low-alloy low-perlite steel after impact bending tests and of aluminum wire after fatigue failure at different temperatures. We have established that the fracture surfaces after brittle destruction are fractal surfaces. On the basis of the fractal model of a crack and the determined fractal dimensionalities of the boundaries of fracture surfaces, we have evaluated the critical sizes of brittle cracks.__________Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 1, pp. 58 – 62, January – February, 2005.     

温度对TC4-DT损伤容限型钛合金疲劳裂纹扩展行为的影响

对TC4-DT损伤容限型钛合金在150℃,25℃下的疲劳裂纹扩展速率da/dN进行了测试,给出了扩展速率和应力强度因子幅值△K之间的关系曲线.用SEM对2种温度下断口形貌进行了观测,实验结果表明,150℃的疲劳裂纹扩展速率试样具有较低的疲劳裂纹扩展速率,25℃的疲劳裂纹扩展速率试样具有较低的门槛值;稳态扩展区解理断裂和条带循环机制共存,150℃的da/dN试样中的疲劳辉间距比25℃试样细;快速扩展区的断口形貌呈韧窝型静载断裂特征,150℃的da/dN试样中的韧窝比25℃试样深.     

Effects of Loading Waveform and Stress Field on High Temperature Fatigue Crack Growth of Alloy 718

High temperature fatigue in nickel‐base superalloys for aero‐engine applications requires consideration of cycle‐ and time‐dependent phenomena. For waveforms with dwells and/or low frequencies crack growth rates may be significantly accelerated up to the point of fully time‐dependent crack growth. In the paper, the interaction of fatigue (cycle‐dependent) and dwell (time‐dependent) crack growth is addressed using a broad database for IN 718 tested at 600°C. Waveforms with dwells at as well as below maximum load levels are applied to standard CT and CC specimens and blunt notched specimens. It is demonstrated that both the level of dwells with respect to the maximum load level within a cycle as well as the specific conditions at the notch root considerably affect the significance of dwells for fatigue crack growth.     

TIG与FSW焊接工艺下2219铝合金疲劳裂纹扩展性能研究

目的 研究钨极惰性气体保护焊（TIG）和搅拌摩擦焊（FSW）对2219铝合金焊接接头疲劳性能的影响,并探究这2种不同焊接技术条件下焊接接头疲劳裂纹的产生与裂纹扩展原理,了解2种焊接接头的抗裂纹扩展能力,为工程实践应用提供数据参考。方法 采用疲劳裂纹扩展试验方法,测试上述2种焊接工艺条件下焊缝金属和热影响区组织的疲劳裂纹扩展速率da/d N和阈值,使用光学显微镜和扫描电子显微镜观察并分析金相组织和疲劳断口形貌特征。结果 疲劳裂纹倾向于沿裂纹处萌生,裂纹的存在成为主要的裂纹扩展源头,有利于加速裂纹向前延伸。热影响区由于组织结构不均匀,不同位置的晶粒尺寸存在明显差异,疲劳裂纹扩展路径倾向于沿靠近焊缝一侧向靠近母材区域扩展。TIG焊接工艺下焊缝金属和热影响区的裂纹扩展速率明显低于FSW焊接工艺下的焊缝金属和热影响区,与此同时,TIG焊接接头表现出优良的抗疲劳裂纹扩展性能。结论 通过此研究,建议2219铝合金焊接接头采用TIG焊接工艺,抗疲劳裂纹扩展效果更佳。     

船体钢止裂温度与无塑性转变温度相关性分析

基于AH36、EH36和FH500三种船体钢的梯度温度场型双重拉伸试验和落锤试验结果,对止裂温度和无塑性转变温度的相关性进行了分析。结果表明,三种船体钢止裂温度与无塑性转变温度之间存在一定的对应关系,反映了两种试验方法之间存在一定的相关性。