Stochastic fatigue damage accumulation in a T-welded joint accounting for the residual stress fields

The residual stresses that occur as a result of nonhomogeneous heating and cooling during welding may have a significant effect on the accumulation of fatigue damage in a welded joint. The problem is complicated not because of the complex spatial distribution of the residual stress fields, but because those fields typically change under an applied load. The present study considers the effect of residual stresses on fatigue damage accumulation in a welded joint subjected to stochastic loading.The influence of residual stresses on stochastic fatigue damage accumulation is accounted for by a simple approach based on an elastic–perfectly-plastic material model and the Gerber correction factor. The model assumes that the residual stress remaining at the critical location depends on the largest nominal stress ever endured by a welded joint. The model predicts that the residual stresses during stochastic loading randomly decay to zero. The effect of material yielding is additionally investigated by considering an elastic–plastic material model with linear kinematic hardening. The residual stresses in this case are computed through Monte Carlo simulations. It is demonstrated that the effect of material hardening is to reduce the rate of residual stress decay and thus to accelerate the rate of fatigue damage accumulation.     

单个冲击对不锈钢管道焊接头低周疲劳寿命的影响

完成了单个冲击对１Ｃｒ１８Ｎｉ９１不锈钢管道焊接头试样低周疲劳寿命影响的试验研究。单个试验最大瞬时峰值应变率达４８０ｓ＾－１。试样未经消除焊接残余应力。采用成组法试验（每组７个试样）,对称加载模式,总应变幅为０．００２２８。结果表明,冲击影响受到焊接残余应力和冲击塑性导入机制的耦合作用。焊接残余应力与冲击应力叠加将增加材料损伤,而冲击塑性导入将减缓疲劳损伤和降低疲劳寿命分散性。前者扼制后者。考虑疲     

Weld residual stress effects on fatigue crack growth behaviour of aluminium alloy 2024-T351

The interaction between residual stress and fatigue crack growth rate has been investigated in middle tension and compact tension specimens machined from a variable polarity plasma arc welded aluminium alloy 2024-T351 plate. The specimens were tested at three levels of applied constant stress intensity factor range. Crack closure was continuously monitored using an eddy current transducer and the residual stresses were measured with neutron diffraction. The effect of the residual stresses on the fatigue crack behaviour was modelled for both specimen geometries using two approaches: a crack closure approach where the effective stress intensity factor was computed; and a residual stress approach where the effect of the residual stresses on the stress ratio was considered. Good correlation between the experimental results and the predictions were found for the effective stress intensity factor approach at a high stress intensity factor range whereas the residual stress approach yielded good predictions at low and moderate stress intensity factor ranges. In particular, the residual stresses accelerated the fatigue crack growth rate in the middle tension specimen whereas they decelerated the growth rate in the compact tension sample, demonstrating the importance of accurately evaluating the residual stresses in welded specimens which will be used to produce damage tolerance design data.     

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。     

A study of fatigue crack growth from artificial corrosion pits at welded joints under complex stress fields

The paper studies the effects of artificial corrosion pits and complex stress fields on the fatigue crack growth of full penetration load‐carrying fillet cruciform welded joints with 45° inclined angle. Parameters of fatigue crack growth rate of welded joints are obtained from S‐N curves under different levels of corrosion. A numerical method is used to simulate fatigue crack growth using different mixed mode fatigue crack growth criteria. Using polynomial regression, the crack shape correction factor of welded joints is fitted as a function of crack depth ratios. Because the maximum circumferential stress criterion is simple and easy to use in practice, fatigue crack growth rate is modified using this criterion. The relationship of effective stress intensity factor, crack growth angle and crack depth is studied under different corrosion levels. The simulated crack growth path obtained from the numerical method is compared with the actual crack growth path observed by fatigue tests. The results show that fatigue cracks do not initiate at the edge or bottom of pits but at the weld toes where the maximum stress occurs. The artificial corrosion pits have little effect on the effective stress intensity factor ranges and crack growth angle. The fatigue crack growth rates of welded joints with pits 1 and 2 are 1.15 times and 1.40 times larger than that of the welded joint with no pit, respectively. The simulated crack growth path agrees well with the actual one. The fatigue life prediction accuracy using the modified formulation is improved by about 18%. The crack shape correction factor obtained using the maximum circumferential stress criterion is recommended being used to calculate fatigue life.     

Experimental study on the whole‐life heterogeneous ratchetting and ratchetting‐fatigue interaction of SUS301L stainless steel butt‐welded joint

Uniaxial fatigue tests of butt‐welded joint, made from SUS301L stainless steel, were carried out under asymmetric stress‐controlled cyclic loading conditions in this work. The effects of stress amplitude and mean stress on the whole‐life heterogeneous ratchetting and fatigue life of the butt‐welded joint were investigated, respectively, for the specified subzones. The experimental observations show that the whole‐life inhomogeneous ratchetting strain concentrating in a specific fusion zone (denoted as the FZ‐1 subzone) of the welded joint becomes more significant as the stress level increases; the fatigue failure also occurs in the FZ‐1 subzone, and the fatigue life depends on both the applied mean stress and stress amplitude and is determined by the combination of ratchetting damage and fatigue one in the localized FZ‐1 subzone.     

Recent development in the fast corrosion fatigue analysis of offshore structures subject to random wave loading

In many areas of engineering, such as the offshore industry, welded steel joints are widely used as standard structural components. These joints are usually subject to long-term random wave loading and are therefore susceptible to fatigue damage. In many cases, the complex service loading is described by stress (or strain) power spectra, each representing a stationary sea state. These power spectra are obtained from hydrodynamic analysis or in situ monitoring. These will then be used in design calculations, feasibility studies or in-service assessment of fatigue damage on the structures.

Usually, the power spectra will have to be realized into real-time histories and then counted before fatigue analysis can be carried out. On many occasions where a large number of design options or joints need to be analysed, this process becomes very time consuming and expensive. The situation is further complicated by the calculations involving corrosion fatigue for joints in sea water.

The paper will start with a brief presentation of the fatigue analysis procedures for offshore welded joints and several existing models that were derived to bypass the laborious load history analysis mentioned above. More effort, however, will be concentrated on presenting the development of a new model. This model not only provides a more consistent and accurate prediction, but has also been adopted successfully for corrosion fatigue analysis.     

锈蚀和疲劳耦合作用下梁桥时变承载力评估

钢筋混凝土桥梁在环境和荷载双重作用下将出现锈蚀、粘结退化和疲劳损伤,现有承载力计算方法不能同时考虑三者的综合影响。根据已有成果系统阐述锈蚀、粘结退化和疲劳损伤对承载力的影响,分析锈蚀和疲劳耦合作用下的钢筋面积退化情况。根据锈蚀钢筋与混凝土界面间的变形协调关系推导多因素影响下截面协同工作系数的表达式,提出同时考虑锈蚀、粘结退化和疲劳损伤的承载力计算方法。以钢筋混凝土简支梁为例,对其承载力退化过程进行理论分析和数值模拟。结果表明:所提出的承载力评估方法可以合理准确地反映桥梁在疲劳和锈蚀耦合作用下的承载力退化过程,可为计算锈蚀和疲劳耦合作用下桥梁承载力评估提供参考。     

Combined effect of strength & sheet thickness on fatigue behaviour of resistance spot welded joint

Fatigue performance of spot welded lap shear joint is primarily dependent on weld nugget size, sheet thickness and corresponding joint stiffness. Two automotive steel sheets having higher strength lower thickness and lower strength higher thickness are resistance spot welded with established optimum welding condition. The tensile‐shear strength and fatigue strength of lap shear joint of the two automotive steel sheets are determined and compared. Experimental fatigue life of spot welded lap shear joint of each steel are compared with predicted fatigue lives using different stress intensity factor solutions for kinked crack and spot weld available in literature. Micrographs of fatigue fractured surfaces are examined to understand fracture micro‐mechanisms.     

16MnR钢激光冲击工艺及对焊接结构应力腐蚀性能的影响EI北大核心CSCD

对16MnR母材进行激光冲击工艺实验,获得优化的激光冲击工艺参数。对激光-MAG复合焊焊接接头进行表面处理,分析接头激光冲击前后状态的残余应力分布及抗应力腐蚀性能变化。结果表明:对16MnR钢平板经激光冲击处理后,在材料表面最大可引入475μm厚度的塑性变形层,并同时引入-593 MPa的压应力分布。采用优化激光冲击工艺对16MnR钢焊接接头进行表面处理后,可有效减小焊接接头表面的残余拉应力分布。在3.5%NaCl(质量分数)条件下对激光冲击处理前后的接头试样进行慢应变速率应力腐蚀实验,发现激光冲击处理前后16MnR钢焊接接头的应力腐蚀敏感指数I_(SSRT)分别为0.106和0.104,表明激光冲击可以提高接头的抗应力腐蚀能力。     

Corrosion failure analyses of an elbow and an elbow-to-pipe weld in a natural gas gathering pipeline

The internal corrosion of a 90° elbow was found in a natural gas gathering pipeline in Northeast China. The welded joint between the elbow and the downstream pipe was also severely corroded. The 90° elbow was forged of 16Mn steel. The downstream pipe (Φ 76 mm × 9 mm) was made of 20G steel. To determine failure causes, the elbow and the welded joint were taken as a whole and investigated systematically. The influence of the flow disturbance induced by the elbow on the damage at the welded joint was considered. The internal damage at the elbow and that at the welded joint were studied using field investigation, visual examination, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray powder diffraction (XRD), hardness tests, and computational fluid dynamics (CFD) techniques. The results showed that erosion corrosion from solid particles and corrosive liquid droplets entrained in the natural gas flow was the main cause of the internal damage at the elbow and the welded joint. The welded joint was attacked by the particles with the highest velocity magnitudes and the most dangerous impact angles. The flow disturbance induced by the elbow, the special location of the welded joint, and the angular misalignment due to poor welding quality jointly caused the substantially more severe damage at the welded joint.     

The influence of salt fog exposure on the fatigue performance of Alclad 6xxx aluminum alloys laser beam welded joints

Laser welding is increasingly used for the fabrication of lightweight and cost-effective integral stiffened panels in modern civil aircraft. As these structures age in service, the issue of the effect of corrosion on their damage tolerance requires attention. In this work, laboratory data on the influence of salt fog corrosion on the fatigue behavior of cladded 6156 T4 aluminum alloy laser welded specimens are presented. The experimental investigation was performed on 6156 T4 laser butt welded sheets. Prior to fatigue testing the welded joints were exposed to laboratory salt fog corrosion exposure for 720 h. The results showed that the clad layer offers sufficient corrosion protection both on base metal and the weld. Fatigue testing was followed by standard metallographic analysis in order to identify fatigue crack initiation sites. Crack initiation is located in all welded samples near the weld reinforcement which induces a significant stress concentration. Localized corrosion attack of the clad layer, in the form of pitting corrosion, creates an additional stress concentration which accelerates crack initiation leading to shorter fatigue life relative to the uncorroded samples. The potency of small corrosion pits to act as stress concentration sites has been assessed analytically. The above results indicate that despite the general corrosion protection offered by the clad layer, the localized attack described above leads to inferior fatigue performance, a fact that should be taken under consideration in the design and maintenance of these structures.     

Experimental and analytical investigation of fatigue crack propagation of T‐welded joints considering the effect of boundary condition

T‐welded joints are commonly employed in ship and ocean structures. The fatigue failure of structure components subjected to cyclic loading always occurs in T‐welded joints because of the metallurgical differences, tensile residual stress fields and stress concentrations. The former researches about T‐welded joints fatigue have in common that the boundary condition needs to be taken into account as an influencing parameter to predict the crack propagation during cyclic loading. In this paper, the crack growth behaviour in T‐welded joint processed with Q345D steel (Pingxiang Iron & Steel Co., Ltd, Hukou, Jiangxi Province in China) under the fatigue loading was analysed via analytical model and verified via experiment. The results show that the influence of boundary condition should be considered in T‐welded joint structure during crack propagation in weld toe area. The correction factor concerning the effect of boundary condition and modified Paris' equation was proposed according to the experimental results and verified by the following repeated experiments.     

On the use of infrared thermography for the analysis of fatigue damage processes in welded joints

The paper presents an experimental approach for the analysis of localized fatigue damage processes during fatigue testing of welded steel structures by thermographic investigations of thermomechanical coupling phenomena. Special data processing of the recorded infrared sequences is proposed to separate linear and nonlinear damage-indicating effects. The method has been successfully applied to analyze the damage progress during fatigue testing of welded components. On the tested components localized damage processes could be observed as early as 10–30% of the total fatigue lifetime. The results confirm the high potential of the methodology for the determination of fatigue damage initiation. The technique provides a powerful experimental tool to investigate localized inhomogeneous damage and to analyze complex fatigue processes.     

Fatigue crack initiation life estimation in a steel welded joint by the use of a two-scale damage model

This work deals with the fatigue behaviour of S355NL steel welded joints classically used in naval structures. The approach suggested here, in order to estimate the fatigue crack initiation life, can be split into two stages. First, stabilized stress–strain cycles are obtained in all points of the welded joint by a finite element analysis, taking constant or variable amplitude loadings into account. This calculation takes account of: base metal elastic–plastic behaviour, variable yield stress based on hardness measurements in various zones of the weld, local geometry at the weld toe and residual stresses if any. Second, if a fast elastic shakedown occurs, a two-scale damage model based on Lemaitre et al. 's work is used as a post-processor in order to estimate the fatigue crack initiation life. Material parameters for this model were identified from two Wöhler curves established for base metal. As a validation, four-point bending fatigue tests were carried out on welded specimens supplied by 'DCNS company'. Two load ratios were considered: 0.1 and 0.3. Residual stress measurements by X-ray diffraction completed this analysis. Comparisons between experimental and calculated fatigue lives are promising for the considered loadings. An exploitation of this method is planned for another welding process.     

高速列车荷载作用下铸钢焊接节点的疲劳分析

钢结构特别是焊接钢结构对动荷载特别敏感。武广客运专线武汉火车站采用了新型的“桥建合一”的结构型式,在长期往复动荷载作用下,上部大跨度钢结构的疲劳寿命令人关注。论文建立了铸钢节点包括焊缝的精细有限元模型,基于热点应力法和Miner线性累积损伤理论,分析了武汉站上部大跨度钢结构10管相贯焊接铸钢节点的疲劳寿命。分析结果表明：列车振动荷载引起的动应力对铸钢节点的疲劳寿命影响不大,满足结构使用寿命100年的要求。分析方法可为国内今后类似结构的疲劳寿命分析提供参考。     

X65管线钢焊接接头在模拟浅表海水环境中的应力腐蚀试验研究

在海水环境中,由于海水从海底管道外管焊缝浸入,导致外管焊接接头断裂。为了研究可能导致X65外管焊接接头断裂的因素,应用慢应变速率拉伸试验(SSRT),通过应力-应变曲线、扫描电镜(SEM)等手段分析了3个X65管线钢焊接接头在空气及模拟浅表海水环境中的应力腐蚀性能。结果表明:在空气中X65钢焊接接头试样的延长量最大,达5.6 mm,在模拟海水中试样的延长量均减小,其中2号试样延长量最小,仅3.6 mm,表明试样在浅表海水中塑性变形能力降低;模拟海水中3个试样的应力腐蚀敏感性指数均处于有应力腐蚀倾向的范围;在空气中试样的断裂为韧性断裂;在浅表海水环境中试样的断裂为韧性断裂与脆性断裂的混合断裂,有应力腐蚀开裂的趋势;海水中含有的大量Cl~-导致焊接接头的应力腐蚀敏感性升高,失效风险增加。     

Fatigue life prediction of corrosion-damaged high-strength steel using an equivalent stress riser (ESR) model. Part II: Model development and results

The fatigue life of metallic aircraft structural components can be significantly reduced by environmentally induced corrosion. However, there have historically been no analytical methods to quantify the specific fatigue life reduction of individual unfailed corroded components with any reasonable degree of confidence. As part of a NAVAIR high-strength steel corrosion–fatigue assessment program, methods were studied to predict the impact that corrosion-induced surface roughness has on the fatigue life of high-strength steel aircraft components. The steels of interest produce general corrosion in patches as well as localized material loss similar to pitting. In addition, this type of corrosion has characteristic features over a wide range of scales. Consequently, traditional finite element analysis approaches are not well suited to this problem, since the mesh required to accurately reflect the fine details distributed over the entire corrosion patch make computation unrealistic. Therefore, approximate methods were developed that allow localized regions of interest of high stress to be identified. Subsequently, a simple notch metric formula is employed to approximate the stress riser in these regions of interest. Finally, an extension of Peterson’s fatigue notch sensitivity theory is applied to these small “notches” that has the result of suppressing the effect of smaller notches compared to larger notches in the prediction of life. Each region of interest is assigned a probability of crack initiation as a function of fatigue cycles, based on a probabilistic strain–life analysis using the predicted notch factor. The net life (to crack initiation) for the component is then the product of the survivabilities of all of the individual regions of interest on the component surface. Tests on corroded fatigue specimens have been conducted to both calibrate the parameters in the Peterson model as well as to test the life prediction capability of the approach. Predictions from the resulting model have demonstrated that an empirical approach to corrosion surface damage can be utilized to generate probabilistic life predictions that have substantial engineering value in assessing the residual fatigue life of corroded AF1410 steel components, and that the modeling technique can capture the significant corrosion features that cause fatigue cracking in most cases, especially for more severely corroded surfaces.     

Corrosion fatigue behavior of FV520B steel in water and salt-spray environments

The corrosion fatigue behavior of FV520B steel in water and salt-spray environments at different temperatures is the focus of this study. Temperature has a significant effect on the fatigue behavior of FV520B steel in water and salt-spray environments, but has only a negligible effect in an air environment. Fatigue life decreases as temperature increases in water and salt-spray environments, and this trend is much more remarkable at a low stress amplitude level. As compared to air and water-spray, it was found that the chloride ion in salt-spray accelerated the fatigue damage of FV520B steel. Micro observation revealed that the fatigue cracks were initiated on the surfaces of the specimens in all experimental environments, and significant evidence of corrosion pits and micro cracks was found on the surfaces of the specimens in the salt-spray environment. Based on the pitting corrosion mechanism, a modified fatigue life prediction model that considered the combined effects of stress, temperature and corrosion was proposed. The study results show that the model predictions are in good agreement with the test data.     

Improved Fatigue Behavior of Pipeline Steel Welded Joint by Surface Mechanical Attrition Treatment (SMAT)

A pipeline steel X80 with welded joint was subjected to surface mechanical attrition treatment(SMAT).After SMAT,a nanostructure surface layer with an average grain size of about 10 nm was formed in the treated sample,and the fatigue limit of the welded joint was elevated by about 13% relative to the untreated joints.In the low and the high amplitude stress regimes,both fatigue strength and fatigue life were enhanced.Formation of the nanostructured surface layer played more important role in the enhanced fat...