Interim fatigue design recommendations for fillet welded joints under complex loading

Current fatigue design methods for assessing welded steel structures under complex combined or multiaxial loading are known to be potentially unsafe. This has led to a number of research projects over the past 10 years. Some progress has been made in developing better methods, but they are not yet suitable for general design. This paper presents an interim solution based on a review and analysis of relevant published data; all referring to fatigue failure from a fillet weld toe. These indicate that Eurocode 3/IIW S – N curve FAT80/3 (negative inverse slope of 3) is suitable for combined normal and shear stresses acting in phase, and possibly for out-of-phase (i.e. non-proportional loading) bending and shear if the shear stress is not due to torsion. However, a shallower curve FAT80/5 is necessary for out-of-phase torsion and bending or tension. Both curves are used in conjunction with the nominal maximum principal stress range occurring during the loading cycle.     

超声冲击法提高焊接接头疲劳特性研究进展

焊接结构的失效以疲劳断裂为主,且焊接结构强度主要是由焊接接头的疲劳强度决定的。因此,改善焊接接头疲劳性能将显著提高焊接结构的整体性能。超声冲击处理是一种有效改善焊接接头疲劳性能的表面强化技术。研究表明,该技术通过改善焊接接头几何外形,细化表层晶粒及引入有益残余压应力可大幅度提高焊接接头的疲劳强度和疲劳寿命。综述了超声冲击处理对焊接接头疲劳性能影响的研究现状,分析了影响焊接接头疲劳性能的因素,总结了超声冲击改善焊接接头疲劳性能的结果,对目前研究过程中存在的问题进行探讨,最后展望了超声冲击表面纳米化技术的应用前景。     

Probabilistic prediction of high cycle fatigue reliability of high strength steel butt‐welded joints

The aim of this paper is to develop a probabilistic approach of high cycle fatigue (HCF) behaviour prediction of welded joints taking into account the surface modifications induced by welding and the post‐welding shot peening treatment. In this work, the HCF Crossland criterion has been used and adopted to the case of welded and shot peened welded parts, by taking into account the surface modifications which are classified as follows: (i) the compressive residual stresses, (ii) the surface work‐hardening, (iii) the geometrical irregularities and (iv) the superficial defects. The random effects due to the dispersions of: (i) the HCF Crossland criterion material characteristics (ii) the applied loading and (iii) the surface modifications parameters are introduced in the proposed model. The HCF reliability has been computed by using the ‘strength load’ method with Monte Carlo simulation. The reliability computation results lead to obtain interesting and useful iso‐probabilistic Crossland diagrams (PCD) for different welding and shot peening surface conditions. To validate the proposed method, the approach has been applied to a butt‐welded joint made of S550MC high strength steel (HSS). Four types of specimens are investigated: (i) base metal (BM), (ii) machined and grooved (MG) condition, (iii) As welded (AW) condition and (iv) as welded and shot peened (AWSP) condition. The comparison between the computed reliabilities and the experimental investigations reveals good agreement leading to validate the proposed approach. The effects of the different welded and post‐weld shot peened specimen's surface properties are analysed and discussed using the design of experiments (DoE) techniques.     

Overload effect on the fatigue crack propagation in large‐scale tubular joints

This paper examines the overloading effect on the fatigue crack propagations monitored in a large‐scale tubular X‐joint specimen under two separate cyclic tests. The first cyclic test applies a constant‐amplitude brace in‐plane bending to the joint, with a single cycle of 150% overload before the crack depth reaches the mid‐thickness of the chord. The second fatigue test applies two batches of cyclic loads, with the amplitude of the second batch at 66% of the former. The X‐joint specimen experiences a 150% overload cycle during the first batch of loading, followed by the second batch after it has recovered from the overload effect. The experimental results reveal that deep surface cracks experience more significant overload retardation than does a shallow fatigue crack. The Paris law estimation indicates that the single overload cycle applied in the first specimen leads to a 7% increase in the fatigue life of the X‐joint.     

基于缺口应力法的焊接接头疲劳分析

等效缺口应力法作为焊接疲劳分析的一种局部方法,不仅克服了焊接结构名义应力难以确定和焊根结构应力无法定义的困难,而且能够反映焊接局部后处理对焊接接头疲劳强度的影响,因此近年来备受关注。该文建立了典型焊接接头的三维缺口应力模型,对焊趾根部的缺口应力集中系数进行了求解;通过对对接接头和纵向角接头在焊后未处理(AS-weld)和超声喷丸处理(UPT)两种状态下的疲劳试验数据进行分析处理,获得了两种焊接接头在缺口应力系统下统一的S-N曲线,并与目前国际焊接学会所推荐的具有相同存活率的疲劳寿命曲线(IIW:m=3,FAT=225)进行比较,结果表明,该曲线具有更高的疲劳等级和更低的斜度。     

Fatigue crack propagation rates and threshold stress intensity factors for welded joints of HT80 steel at several stress ratios

Fatigue crack propagation rates and threshold stress intensity factors were measured for welded joints and base metal by using 200 mm wide centre-cracked specimens. The fatigue crack propagation properties of welded joints were similar in spite of the different zones in which the cracks propagated (ie, in the heat-affected zone and in the weld metal) and the different welding process used (submerged arc welding and gas metal arc welding). They were, however, inferior to those of the base metal. It was revealed by observation of the crack closure that the fatigue cracks were fully open during the whole range of loading, due to the tensile residual stress distribution in the middle part of the welded joints. This observation also explains the lack of a stress ratio effect on the fatigue crack propagation properties of welded joints, and their inferiority to those of the base metal.     

Fatigue life prediction at a ship deck/superstructure intersection

Difficulties arise in identifying an appropriate method to predict fatigue life for the weld joint at the intersection of a ship's deck and superstructure. The problem relates to the gross stress concentration and rapidly changing stress at this location. Variations on the 'hot spot' stress approach to fatigue life analysis are reviewed, and applied to results from an experimental programme of fatigue tests on welded joints. Finally tentative recommendations are made for a procedure to predict fatigue life of the structure.     

Cruciform fillet welded joint fatigue strength improvements by weld metal phase transformations

Arc welding typically generates residual tensile stresses in welded joints, leading to deteriorated fatigue performance of these joints. Volume expansion of the weld metal at high temperatures followed by contraction during cooling induces a local tensile residual stress state. A new type of welding wire capable of inducing a local compressive residual stress state by means of controlled martensitic transformation at relatively low temperatures has been studied, and the effects of the transformation temperature and residual stresses on fatigue strength are discussed. In this study, several LTTW (Low Transformation‐Temperature Welding) wires have been developed and investigated to better characterize the effect of phase transformation on residual stress management in welded joints. Non‐load‐carrying cruciform fillet welded joints were prepared for measurement of residual stresses and fatigue testing. The measurement of the residual stresses of the three designed wires reveals a compressive residual stress near the weld toe. The fatigue properties of the new wires are enhanced compared to a commercially available wire.     

Fatigue strength of a typical ship structural detail: tests and calculation methods

In previous years, several large-scale specimens have been fatigue tested in the Ship Structures laboratory of the Dipartimento di Ingegneria Navale e Tecnologie Marine (DINAV) of the University of Genova (Italy). They represent the crossing of a T-shaped beam into deep web plates, simulating either a primary supporting member or bulkhead plating. Each specimen is characterized by two joints, both watertight but with different geometry. The purpose of this paper is to review all the available test results using calculation methods for fatigue strength of welded structural details, namely: nominal stress approach, local stress approaches and the fracture mechanics approach. Test results allow SN curves to be drawn which can be used for these and similar details, quite common in shipbuilding. Moreover, calibration of stress concentration factors and coefficients in the various fatigue life calculation methods is possible using the experimental data.     

EH36钢焊接接头疲劳性能研究

目的 研究不同焊接热输入对不同碳含量的EH36海工钢焊接接头疲劳性能的影响规律以及海工钢焊接接头疲劳的失效机理。方法 采用了热输入为7 kJ/cm的二氧化碳气体保护焊(FCAW)和热输入为50 kJ/cm的埋弧焊(SAW)2种焊接工艺分别对碳含量(质量分数)为0.08%(LC)和0.12%(MC)的80 mm厚EH36钢板的填充层进行焊接,在1/4厚度处截取疲劳试样后分别进行了4种不同材料、不同热输入的交叉疲劳试验和硬度试验,并使用金相显微镜和扫描电镜对组织和疲劳断口进行了表征分析。结果 热输入为7 kJ/cm的LC(LC-7)和MC(MC-7)填充层接头的疲劳极限分别为360 MPa和359 MPa,热输入为50 kJ/cm的LC(LC-50)和MC(MC-50)填充层接头的疲劳极限分别为361 MPa和354 MPa,各试验钢按照390 MPa应力幅下的平均循环寿命由高到低的顺序依次为LC-50、MC-50、LC-7、MC-7。结论 不同焊接热输入、不同碳含量对海工钢的疲劳极限无显著影响,高应力水平下高热输入焊接接头的疲劳性能明显优于低热输入焊接接头的疲劳性能,但在相同热输入条件下,不同碳含量对钢材疲劳性能的影响不显著。所有试样的断口均具有典型的疲劳源区、裂纹扩展区和瞬间断裂区特征。     

The Prediction of Fatigue Crack Initiation Life in Spot Welds

Abstract: In this paper, strain‐based fatigue life prediction method has been used to estimate the fatigue crack initiation life of spot‐welded joints of Mild Steel JSC270D and Ultra‐High Strength Steel JSC980Y. To do so, the joints were simulated using three‐dimensional finite‐element (FE) models, and then nonlinear FE analysis was performed to obtain the local stress and strain ranges and finally, the Morrow equation was applied to estimate the crack initiation lives. The results have been compared with those obtained from experimental crack growth morphology. In addition, the difference between fatigue limits for smooth specimens and spot‐welded joints for mentioned materials has been briefly discussed. It has been shown that mean stress values in the Ultra‐High Strength Steel can significantly decrease the fatigue limit of spot‐welded joint because even at very low load level the stresses exceed the yield point at the root of nugget of spot‐welded joint, while the amount of mean stress in the Mild Steel for the same load level is much less than that of Ultra‐High Strength Steel. The comparison between numerical results of fatigue crack initiation lives and experimental data provided good agreement between numerical predictions and crack growth morphology observations. The results also shows that in some cases, depending on the joint type, the life spent in the nucleation phase can be an important part of the final failure lifetime.     

Unique fatigue threshold and growth properties of welded joints in a tensile residual stress field

The fatigue threshold and high growth rate region properties were investigated on several kinds of welded joints. These properties became unique in spite of the variation of steels (ferrite-pearite, martensite, austenite), welding method, heat input and stress ratio. It was revealed that the unique properties occurred from the fully opened fatigue crack due to the tensile residual stresses. Based on these results, the equation of the fatigue crack growth curve for the design and inspection of welded structures was proposed. It is also suggested that the inducement of compressive residual stress at the fatigue critical zone is effective in improving the fatigue properties of welded structures.