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     

A New Method of Retarding Fatigue Crack Growth on Pressure Vessels

An artificial wedge to retard;fatigue crack growth in tension has been investigated. The results show that an artificial wedge can reduce the growth rate of fatigue crack on surface fatigue crack and, the fatigue crack growth behavior is essentially similar in-depth and width directions. Based on a theoretical analysis, a model for the effective crack growth parameter DeltaK(eff) is presented. It is shown that the relationship between the calculated DeltaK(eff) value and crack speed are almost the same as those of cracked specimens without a wedge. Therefore this model can be applied to estimate retardation behavior.     

Influence of Stress Ratio and Loading Condition on Near-threshold Fatigue Crack Growth Behaviour

Near-threshold fatigue crack propagation behaviour of a cold-worked copper tested with distinct fa-tigue testing systems under different stress ratios has been investigated.The emphasis was placed onthe evaluation of crack closure effect as well as the effective fatigue threshold,It was found that thefatigue threshold for R=0 is higher than those for R=-1 and R=0.4,for which almost the samethreshold value was derived.Compared with the conventional closure evaluation method,the pro-posed new closure evaluation method can generally interpret the dependence of the fatigue thresh-old on stress ratio and loading condition,and leads to a higher and relatively constant effective fa-tigue threshold of about 3 MPa·m~(1/2) for the cold-worked copper at different stress ratios andloading conditions.     

Experimental Study of Crack Growth in a Bimetal Under Fatigue and Fatigue-Creep Conditions

Fatigue and fatigue-creep crack growth rate in a bimetal of continuous caster rolls for different loading frequencies and waveforms is investigated with the use of stress, deformation and energy criteria of fracture mechanics.     

A Strip Yield Analysis of Fatigue Crack Growth in the Residual Stress Field

An advanced procedure has been developed to analyse the fatigue crack growth in the residual stress field. The method is based on an extension of the strip yield crack closure model for part-through crack problems. In this method, the residual stress relaxation due to both the fatigue loading and the crack growth is considered. It has been shown that the effect of the residual stress can be reasonably analysed according to fracture mechanics methods so long as the residual stress field and its redistribution can be correctly evaluated and the elastic-plastic crack closure analyses are performed. Several examples are provided for the constant and variable amplitude loading conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

10Ni5CrMoV钢双轴弯曲应力下表面裂纹疲劳扩展特性研究

以10Ni5CrMoV钢为研究对象,在加载条件与构件使用环境相近条件下,在原板厚大尺寸试样表面引入人工缺口模拟实际构件中存在的类表面裂纹缺陷,采用双轴弯曲疲劳试验方法,开展原板厚大尺寸试样双轴弯曲应力下的表面裂纹疲劳扩展特性试验研究。结果表明,裂纹半长c和裂纹深度a都随循环次数N单调递增,且曲线a-N满足一定的线性关系。建立表面裂纹半长c与深度a之间的定量关系式预测模型,从而定量解决裂纹深度尺寸不好直接测量的难题。     

Acoustic Emission-Based Methodology to Evaluate Delamination Crack Growth Under Quasi-static and Fatigue Loading Conditions

The aim of this study was to investigate the applicability of acoustic emission (AE) technique to evaluate delamination crack in glass/epoxy composite laminates under quasi-static and fatigue loading. To this aim, double cantilever beam specimens were subjected to mode I quasi-static and fatigue loading conditions and the generated AE signals were recorded during the tests. By analyzing the mechanical and AE results, an analytical correlation between the AE energy with the released strain energy and the crack growth was established. It was found that there is a 3rd degree polynomial correlation between the crack growth and the cumulative AE energy. Using this correlation the delamination crack growth was predicted under both the static and fatigue loading conditions. The predicted crack growth values was were in a good agreement with the visually recorded data during the tests. The results indicated that the proposed AE-based method has good applicability to evaluate the delamination crack growth under quasi-static and fatigue loading conditions, especially when the crack is embedded within the structure and could not be seen visually.     

疲劳裂纹扩展速率试验中裂纹曲率修正方法研究

针对目前疲劳裂纹扩展速率试验标准中仅提出裂纹长度需进行曲率修正但未给出具体修正方法及步骤的问题,作者推荐了线性插值法和等比例法两种裂纹曲率修正方法,并通过分析两种方法的修正原理,结合试验结果,将两种曲率修正方法进行对比。结果表明,在双对数坐标系上,修正后的da/dN-ΔK曲线向低ΔK方向偏移,最大偏移量约15%,在利用da/dN-ΔK曲线进行选材对比或疲劳寿命评估时,修正后的曲线与未修正的曲线相比是偏安全的。基于两种方法修正后的da/dN-ΔK曲线之间差异较小,最大相差约2%,两种方法均可作为疲劳裂纹扩展速率试验中裂纹长度的曲率修正方法。     

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.     

A State-Space Model of Fatigue Crack Growth

This paper proposes a nonlinear dynamic model of fatigue crack growth in the state-space setting based on the crack closure concept under cyclic stress excitation of variable amplitude and random loading. The model state variables are the crack length and the crack opening stress. The state-space model is capable of capturing the effects of stress overload and underload on crack retardation and acceleration, and the model predictions are in fair agreement with experimental data on the 7075-T6 aluminum alloy. Furthermore, the state-space model recursively computes the crack opening stress via a simple functional relationship and does not require a stacked array of peaks and valleys of stress history for its execution; therefore, savings in both computation time and memory requirement are significant. As such, the state space model is suitable for real-time damage monitoring and control in operating machinery. This revised version was published online in July 2006 with corrections to the Cover Date.     

应力作用下点蚀损伤发展特点的理论分析

根据现场挂片试验下点蚀发展速度规律和应力对腐蚀电流的加速效应,提出了应力作用下点蚀速度和点蚀坑形状变化规律的微分方程,并通过数值方法进行求解.分析发现,应力作用下腐蚀速度更快;点蚀坑的形状受应力影响,通常在垂直于主应力方向上腐蚀更快,可能逐渐形成垂直于主应力的扁平型点蚀坑甚至发展成为裂纹;应力必须要达到一定数值才会对腐...     

Fatigue Limit of Steel with an Arbitrary Crack under a Stress Controlled Constant with a Positive Mean Stress

In this paper, push–pull fatigue tests of notched specimens under R=−1, 0 and 0.5 are carried out on annealed 0.1 % carbon steel and quenched-tempered 0.5 % carbon steel with H_{rm B} simeq 600. The fatigue limit of a long crack, sigma_w2, is obtained from that of a notched specimen with a sharp and deep notch whose radius is smaller than the branch point rho₀. Using the present and past sigma_w2 data, the effect of the positive mean stress on the dKw of the long crack is evaluated using 3.2 H_{rm B} (simeq sigma_{rm B}) and sigma_{rm S}(3.2 H_{rm B}), where dKw is the stress intensity factor range for predicting the fatigue limit of a metal with an arbitrary crack, sigma_{rm S} is the lower yield stress or 0.2% proof stress, sigma_{rm B} is the ultimate tensile strength and H_{rm B} is the Brinell hardness. Although the sigma_w2 decreases with the mean stress, sigma_w2 is cut off by the threshold stress sigma_we of the plastic deformation at the crack tip. For the quantitative prediction of the fatigue limit of a cracked steel with an arbitrary crack length under a positive mean stress, the sigma_w2 and sigma_we values are approximated by simple formulae.     

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。     

应力比对航空高强LD2CZ铝合金腐蚀疲劳裂纹扩展的影响

利用扫描电镜联合液压伺服试验机,并借助于Walker公式研究了应力比对预腐蚀不同时间航空高强LD2CZ铝合金疲劳裂纹扩展的影响,在应力比分别为0．05,0．5,0．7的条件下对预腐蚀0,15,30d的LD2CZ铝合金单边缺口板状试样进行了疲劳加载试验,得到了其疲劳裂纹扩展速率曲线,并拟合出了Walker公式中的材料常数。结果表明：裂纹扩展速率会随着应力比的增加以及腐蚀损伤的加深而增大,拟舍得到的Walker公式可用来定量化地表征应力比和腐蚀损伤对疲劳裂纹扩展速率的影响。     

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.     

微观组织与裂纹闭合效应对40CrNiMo钢疲劳短裂纹扩展的影响

对具有粗、细晶粒组织的40CrNiMo 钢进行了疲劳短裂纹扩展试验研究。试验和分析结果表明,短裂纹扩展的偏折强烈,裂纹闭合效应较小。粗晶组织比细晶组织的裂纹偏折更大,粗糙度诱发闭合效应更强,因而裂纹扩展较幔。微观组织通过对闭合效应的作用进而影响了疲劳短裂纹的扩展行为。     

自回火对重轨钢疲劳裂纹扩展行为的影响

针对重轨钢在线淬火后自回火造成的抗疲劳裂纹扩展能力降低的问题,在淬火冷速分别为3℃/s、5℃/s、8℃/s,终冷温度为450℃条件下,研究了自回火对重轨钢疲劳裂纹扩展行为的影响,测算了疲劳裂纹扩展速率的Paris公式,采用扫描电镜及硬度计分别对自回火后钢轨的疲劳断口进行观察及硬度测试.结果表明:经过自回火后,钢轨的疲劳裂纹扩展速率增快、硬度降低,且自回火之前钢轨的淬火冷速越大,自回火后其硬度降低越显著,硬度的降低增加了裂纹扩展的可能性;自回火后钢轨在Ⅰ区的疲劳裂纹扩展速率远大于直接淬火试样,使钢轨疲劳裂纹扩展的整体进程缩短,这是疲劳裂纹扩展速率增快的重要原因.不同淬火冷速后的自回火试样在应力强度因子范围 ΔK(ΔK=Kmax-Kmin)为10 MPa·m1/2时,断口的疲劳辉纹、解理面和二次裂纹较少,河流花样数量较多、面积小、沟壑浅,而在ΔK=13.5 MPa·m1/2时,断口的疲劳辉纹间距增大,解理面和二次裂纹较多,河流花样数量增多、面积增大,疲劳裂纹扩展速率da/dN较快.     

几种高强度钢腐蚀疲劳裂纹扩展行为的研究

本文研究了ＧＣ－４，３０ＣｒＭｎＳｉＮｉ２Ａ和３００Ｍ超高强钢的腐蚀疲劳裂纹扩展行为。考虑了环境，热处理制度，应力比和频率对裂纹扩展的影响。研究结果表明：腐蚀环境和热处理制度对疲劳裂纹扩展有显著影响。在侵蚀性环境中随频率降低，裂纹扩展大幅度上升，随应力比提高，环境影响加剧。利用扫描电镜对试样断口形貌进行了分析，并讨论了高强钢的腐蚀疲劳机理。     

高压容器用钢疲劳裂纹扩展速率研究

研究了高压容器用Cr Ni Mo钢在空气中与3.5%Na Cl溶液介质中的疲劳裂纹扩展,结果表明,3.5%Na Cl溶液介质的存在加速了Cr Ni Mo钢的裂纹扩展,随着裂纹的进一步扩展,其加速作用越来越不明显。     

Effect of Pre-loading on Short Fatigue Crack Growth at a Notch

Results indicate that under cyclic tension thegrowth rate of short fatigue crack from notch rootwill be lowered greatly by tensile pre-loading,butonly a little change by compressive pre-loading.The effect of tensile pre-loading will decrease withthe increase of stress ratio.The variation of shortfatigue crack growth rate is related to the residualstress distribution around notch root.