Fatigue design of axially-loaded high frequency mechanical impact treated welds by the effective notch stress method

The effective notch stress method (ENS) as defined by the International Institute of Welding is widely used by design engineers to assess the fatigue strength of welded components. This paper provides a comprehensive evaluation of published data for welded joints improved by high frequency mechanical impact (HFMI) treatment. The goal is to verify already-known fatigue classes for the ENS with the available axially-loaded fatigue data. In total, 280 experimental test results obtained from longitudinal, cruciform and butt welds subject to stress ratio of R = 0.1 axial loading are evaluated. Notch stress concentration factors (K_n) for each joint geometry are analysed based on the finite element method. Calculated K_n and reported nominal stress values are used to determine local stresses. Fatigue strength assessment of the all available data is performed by the previously-proposed and verified correction procedure for yield strength (f_y). A formerly-defined minimum K_n values as a function of f_y is used for butt welds. The already-known fatigue classes are found to be conservative with respect to available fatigue test data.     

The new IIW recommendations for fatigue assessment of welded joints and components – A comprehensive code recently updated

The recommendations of the International Institute of Welding (IIW) on fatigue of welded components and structures and on the effect of weld imperfections in respect to fatigue have been published firstly in 1996. It was published in English, German, Japanese and French. A comprehensive code was established, which covered all current methods of verification, as e.g. component testing, nominal stress, structural stress, notch stress method as well as fracture mechanics assessment procedures. Detailed guidance for assessment of weld imperfections is also given. The safety philosophy covers the different strategies, which are used in various fields of application and gives a specified choice for the designer. The update of the recommendations was finalized in 2006. The main areas of update are the structural hot-spot stress concept, which allows now for an economic and coarser meshing in finite element analysis, the extension of the effective notch stress concept to welded aluminium structures and the numerical assessment of post weld treatments for improving the fatigue properties. It is expected that the new update will exert the same impact on design and codes as the old one.     

基于拉弯比的焊缝名义应力的提取

为了获得任意拉弯组合载荷下焊缝的名义应力,利用纯弯和纯拉压载荷下的应力集中系数,引入拉弯组合名义应力换算系数,将基于纯拉压名义应力的焊缝疲劳性能数据,转换为疲劳损伤一致的、基于拉弯组合名义应力的焊缝疲劳性能数据．为了消除有限元建模导致的计算误差,引入单元尺寸影响因子,将拉弯组合的计算应力转换为拉弯组合的名义应力．通过上面2个转换,引入拉弯组合计算名义应力换算系数,将有限元中的拉弯组合计算应力转换为基于拉压载荷疲劳试验的名义应力,从而在具体的焊缝结构疲劳强度评估时可以直接使用拉压载荷下的疲劳试验数据．计算结果表明：拉弯组合计算名义应力换算系数与拉弯比、拉弯应力集中因子比和有限元模型中拉弯单元尺寸影响因子有关．通过选择合适的单元尺寸,使得拉压单元尺寸影响因子等于拉弯应力集中因子比,且弯曲单元尺寸影响因子等于1,可使得拉弯组合计算名义应力换算系数恒等于拉压单元尺寸影响因子,而与拉弯比无关．     

Fatigue strength of welded butt joints in thin and slender specimens

This paper investigates the geometrical properties of the butt-welded thin and slender specimens and their influence on the fatigue strength. The fatigue tests and the finite element analysis are used to investigate the influence. The weld shape, axial misalignment and angular misalignment and the actual shape of the specimen are studied by the extensive optical geometry measurements. The structural hot spot and the notch stress method are used for the fatigue strength assessment. The results reveal that for thin and slender specimens the straightening under the axial loading is significant and thus the relationship between the structural and the nominal stress is highly nonlinear. The straightening effect is influenced by the slenderness and by the curved shape of the plate near the weld. If these effects are included by applying the geometrically nonlinear analysis the fatigue strength of thin and slender welded specimens in notch stresses at two million load cycles corresponds to that of thick welded specimens. The relationship between the structural and the notch stress is however constant regardless of the specimen straightening.     

Modelling and fatigue life assessment of orthotropic bridge deck details using FEM

The fatigue life estimation of orthotropic steel bridge decks using the finite element method is most frequently associated with the application of the structural hot spot stress approach or the effective notch stress approach, rather than the traditional nominal stress approach. The application of these approaches to a welded joint with cut-out holes in orthotropic bridge decks, where it is not easy to distinguish the non-linear stress caused by the notch at the weld toe from the stress concentration effect emanating from the hole in the detail, was investigated. The results of the finite element calculations were compared with the results of the fatigue tests which were carried out on full-scale specimens. The results of the finite element analyses revealed that the structural hot spot stresses obtained from the shell element models were unrealistically high when the welds were omitted. Moreover, the way in which the welds were represented had a substantial influence on the magnitude of the hot spot stress. The results of the analysis when using the effective notch stress approach showed that the agreement between the estimated fatigue life using this approach and the fatigue life obtained from the fatigue tests was good.     

Fatigue assessment of welded structures: practical aspects for stress analysis and fatigue assessment

Analysis of welded structures still remains a challenge for the analyst and in fact cannot be considered as fully solved for practical applications. For many years, a large international aggregation of researchers has developed methods to assess fatigue behaviour of welded structures. Nowadays many suggestions and methods exist to estimate fatigue life of welded structures with respect to nominal, structural, notch stress or fracture mechanics approaches. All of them are still under improvement. The high motivation and many activities of experts in the International Institute of Welding (IIW) group of researchers is a good demonstration of the complexity and need for analysis methods in that field. The purpose of this paper is to provide some discussion on selected methods available. Both authors are giving lectures to transfer methods to industrial applications. It is their experience that a large amount of knowledge has been developed although proper applications require some grading and comments on the use of those methods. This paper should give some comments and recommendations for the practical application of a selection of methods already available. A hierarchical two‐step procedure for the assessment of large welded structures will be described and recommended. Also benchmark results are presented on a sample structure for sake of comparison of a few selected methods. Finally a presentation of results obtained by application of selected methods on real structures in comparison with fatigue lives from experiments will be presented. The methods selected within the paper cover the approaches for modelling, structural analysis and assessment of welded structures using finite element analysis (FEA) and stress based concepts for fatigue life estimation.     

The peak stress method applied to fatigue assessments of steel and aluminium fillet-welded joints subjected to mode I loading

The aim of this work is to present an engineering method based on linear elastic finite element (FE) analyses oriented to fatigue strength assessments of fillet‐welded joints made of steel or aluminium alloys and subjected to mode I loading in the weld toe region where fatigue cracks nucleate. The proposed approach combines the robustness of the notch stress intensity factor approach with the simplicity of the so‐called ‘peak stress method’. Fatigue strength assessments are performed on the basis of (i) a well‐defined elastic peak stress evaluated by FE analyses at the crack initiation point (design stress) and (ii) a unified scatter band (design fatigue curve) dependent on the class of material, i.e. structural steel or aluminium alloys. The elastic peak stress is calculated by using rather coarse meshes with a fixed FE size. A simple rule to calculate the elastic peak stress is also provided if a FE size different from that used in the present work is adopted. The method can be applied to joints having complex geometry by adopting a two‐step analysis procedure that involves standard finite element (FE) models like those usually adopted in an industrial context. The proposed approach is validated against a number of fatigue data published in the literature.     

铝内胆碳纤维全缠绕气瓶铺层设计

铝内胆碳纤维全缠绕气瓶的铺层设计主要基于网格理论,但该方法仅能得出满足爆破强度的参数,不能满足对铝内胆疲劳性能的要求,因而难以适应气瓶产品的设计需要.将网格理论与铝合金S-N曲线结合,提出一种基于铝合金疲劳寿命设计纤维缠绕层厚度的新方法.依据该方法给出的缠绕层厚度构建有限元模型,通过数值模拟确定合理的自紧力,计算不同载荷下的气瓶应力分布,根据爆破试验数据,利用有限元模型预测气瓶的爆破强度、失效位置及失效形式.结果 表明:该设计方法可便捷地得出满足性能要求的气瓶缠绕层厚度;自紧力合理值可根据设计预期通过有限元分析得出;疲劳载荷下的缠绕层应力设计值与模拟值,偏差在允许范围内;运用该方法设计的气瓶能够同时满足疲劳和爆破性能指标,且失效位置、纤维应力比也符合标准规定.     

Stress intensity factor solutions for estimation of fatigue lives of laser welds in lap-shear specimens

Fatigue behavior of laser welds in lap-shear specimens of high strength low alloy (HSLA) steel is investigated based on experimental observations and two fatigue life estimation models. Fatigue experiments of laser welded lap-shear specimens are first reviewed. Analytical stress intensity factor solutions for laser welded lap-shear specimens based on the beam bending theory are derived and compared with the analytical solutions for two semi-infinite solids with connection. Finite element analyses of laser welded lap-shear specimens with different weld widths were also conducted to obtain the stress intensity factor solutions. Approximate closed-form stress intensity factor solutions based on the results of the finite element analyses in combination with the analytical solutions based on the beam bending theory and Westergaard stress function for a full range of the normalized weld widths are developed for future engineering applications. Next, finite element analyses for laser welded lap-shear specimens with three weld widths were conducted to obtain the local stress intensity factor solutions for kinked cracks as functions of the kink length. The computational results indicate that the kinked cracks are under dominant mode I loading conditions and the normalized local stress intensity factor solutions can be used in combination with the global stress intensity factor solutions to estimate fatigue lives of laser welds with the weld width as small as the sheet thickness. The global stress intensity factor solutions and the local stress intensity factor solutions for vanishing and finite kinked cracks are then adopted in a fatigue crack growth model to estimate the fatigue lives of the laser welds. Also, a structural stress model based on the beam bending theory is adopted to estimate the fatigue lives of the welds. The fatigue life estimations based on the kinked fatigue crack growth model agree well with the experimental results whereas the fatigue life estimations based on the structural stress model agree with the experimental results under larger load ranges but are higher than the experimental results under smaller load ranges.     

Fatigue resistance of titanium laser and hybrid welded joints

This paper presents a detailed study on fatigue strength of welded joints made of two titanium alloys, grade 2 and grade 5, and welded by laser or hybrid process. Fatigue strength curves obtained for each alloy and each welding technique are compared in terms of safety factors with fatigue design curves of welded joints provided by standards. Material and welding process effects on fatigue strength are discussed; the influence of the weld seam geometry is assessed by evaluating the fatigue strength reduction factor. This parameter is computed by using the Volumetric Method of the Notch Fracture Mechanics and defined as the ratio of the effective stress and the gross stress. Effective stress is defined on the weld toe stress distribution by the minimum of relative stress gradient method. Distribution of opening stress at weld toe is analysed also with the finite element analysis.     

Fatigue life prediction of friction stir spot welds based on cyclic strain range with hardness distribution and finite element analysis

The main aim of the present study is to investigate the fatigue behavior of single friction stir spot welds (FSSW) using strain-based modified Morrow’s damage equation. The correlation between microhardness, cyclic material constants, and mechanical strength of different zones around the FSSW are assumed to be proportional to the base material hardness. Experimental fatigue tests of friction stir spot welded specimens have been carried out using a constant amplitude load control servo-hydraulic fatigue testing machine. ANSYS finite element code has been used to simulate a single tensile shear friction stir spot welded joint, and non-linear elastic-plastic finite element analysis has been employed to obtain the values of local equivalent stress and strain near the notch roots of the joints. The results based on the numerical predictions have been compared with the experimental fatigue test data. It has been shown that the strain-based approach does a very good job for estimating the fatigue life of friction stir spot welded joints.     

Fatigue strength of laser beam welded automotive components made of thin steel sheets considering size effects

The applicability of and the quality of assessment using the nominal stress, structural stress and notch stress approaches for calculating the fatigue strength of laserbeam welded components made of thin steel sheets has been investigated. For this purpose, the fatigue lives of a longitudinal carrier, an injector and two tube-flange specimens have been determined by tests under constant amplitude loading. Fatigue cracks initiated at sharp root notches on all of these components. While the nominal stress is derived by theory of structural mechanics, the determination of structural and notch stresses is performed using 3D finite element models and solid elements. The structural stress is derived by an extrapolation of surface stress to the fatigue critical notch and the notch stresses by rounding the sharp root notch with a reference radius of r_ref = 0.05 mm. For all of the concepts used, the endurable stresses have been compared to the design SN-curves recommended by the International Institute of Welding (IIW).On comparing the quality of assessment of the different concepts, the notch stress approach shows the highest scatter. The highest endurable notch stresses occur in specimens with crack initiation at weld ends. These specimens have a very small highly loaded weld length. The lowest endurable stresses are determined for specimens with a long, equally loaded weld. The reason for these findings can be explained by statistical size effects. For an improved fatigue assessment, an easily applicable method is introduced, which takes into account the highly stressed weld length.     

The multiaxial weld root fatigue of butt welded joints subjected to uniaxial loading

In this study, the fatigue strength of inclined butt welds subjected to a proportional multiaxial stress state generated by uniaxial loading is studied in nominal and local stress concepts. The local methodologies studied included principal stress hypothesis, von Mises stress hypothesis and modified Wöhler curve method. Nominal methodologies included modified Gough–Pollard interaction equation, the design equation in Eurocode3 and the interaction equation in DNV standard. Results are evaluated along with data published in relevant literature. It is observed that both local and nominal stress assessment methods are able to estimate multiaxial fatigue strength. No obvious difference in fatigue strength is observed in the nominal stress concept, but the notch stress concept is able to capture a decrease in fatigue strength in shear‐dominated joints. It is concluded that modified Wöhler curve method is a suitable tool for the evaluation of fatigue strength in joints dominated by both normal and shear stresses.     

Fatigue strength assessment of laser stake‐welded T‐joints subjected to reversed bending

The paper investigates the fatigue strength of laser stake‐welded T‐joints subjected to reversed bending. The fatigue tests are carried out with the load ratio, R ≈ ?0.8. The experimental data is firstly analysed using the nominal stress approach and then by the J‐integral as the local fatigue strength parameter in the finite element (FE) assessment. The nominal stress approach demonstrated that the fatigue strength of the investigated T‐joints is lower than encountered for any other steel joint under reversed tensile loading. The results also showed that the fatigue strength of this joint under the load ratio R ≈ ?0.8 increases with respect to R = 0 bending by 22.6% in the case of the nominal stress approach and 13% in the case of the J‐integral approach. However, the slopes of the fatigue resistance curves for different load ratios appear very similar, suggesting that the load ratio has an insignificant influence to the slope. In contrast to the similar slopes, the scatter indexes were different. The nominal stress approach shows that the scatter index is 3.4 times larger for R ≈ ?0.8 than R = 0 bending. The J‐integral approach showed that the scatter index for R ≈ ?0.8 is only 67% larger than in the R = 0 case because the weld geometry is modelled in the FE analysis.     

Experimental evaluation of fatigue behaviour of thin Al5456 welded joints

Because of wide applications of welded structures in different industries, using design codes and standards such as IIW recommendations is known as a safe and common method to design welded joints. The weld geometry and thickness of welded joint are the most important parameters that affect the fatigue strength of welded joints. In the present study, the fatigue behaviour of thin Al5456 butt‐welded joints has been investigated, and the effect of thickness on fatigue strength has been evaluated. Contrary to the above‐mentioned recommendations about thin welded joints, it was shown that the thickness of welded joints affects the fatigue strength. Moreover, the fatigue test results have been compared with the IIW design recommendations for three well‐known approaches in order to analyse the reliability of the codes. According to the design stress‐life diagrams, it was found that in some cases, the fatigue strength has much larger values than the IIW predictions, and IIW‐based design causes an over conservative design. While in some other cases, the fatigue strength is lower than IIW recommendations, and it leads to a non‐conservative design. Based on the experimental results, the new values for slope of S‐N curve and FAT have been proposed in order to improve the design diagrams.     

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。     

Schwingfestigkeit schubbeanspruchter Schweißverbindungen nach dem örtlichen Konzept

Fatique strength of shear loaded welded joints according to the local concept In the present work local stress values for fatigue loaded welded joints under shear stresses are derived according to the concept of local elastic stresses by means of test data of the relevant literature re‐analysed in a uniform matter and of numerically obtained notch stress concentration factors. Together with the derived statistically based scatter band of the particular S‐N‐curves and of the local characteristic stress values the obtained results form a reliable basis for codes designing shear stress fatigue loaded welded joints. Further fatigue tests may be desirable howewer under observing usual standards in arranging, carrying out and evaluating fatigue investigations.     

Effect of loading type on the fatigue strength of asymmetric and symmetric transverse non‐load carrying attachments

This study considers the effect of bending loading and the symmetry of joints on the fatigue strength of transverse non‐load carrying attachments. Conventionally, the fatigue strength of a welded joint has been determined without taking these factors into account. Experimental and finite element analyses were carried out and both methods showed that both loading type and symmetry have an influence on the fatigue resistance of a welded joint. Under tensile loading, the fatigue strength of asymmetric T‐joints was higher than that of symmetric X‐joints. Respectively, the fatigue resistance of tested joints improved explicitly when the external loading was bending. The finite element analysis was in good agreement with the test results in the joints subjected to tension but gave very conservative results in the joints subjected to bending.     

The effects of ultrasonic peening treatment on the ultra-long life fatigue behavior of welded joints

The conventional fatigue design codes was formulated based on the data from test stress cycles less than 1 × 10⁷. Most of them could not be applied for welded structures serviced in the ultra-long life region so the new recommendations were discussed. The method of ultrasonic peening treatment (UPT) was introduced in this thesis as a post-weld treatment to improve the fatigue strength of the welded structures. The fatigue behavior of welded structures in the ultra-long life region was investigated using an ultrasonic fatigue test machine under two post-weld treatment states: the as-welded and the UPT. Discussion had also been made about the fracture mechanism on the UPT joints in ultra-long life fatigue test.     

A structural stress definition and numerical implementation for fatigue analysis of welded joints

A mesh-size insensitive structural stress definition is presented in this paper. The structural stress definition is consistent with elementary structural mechanics theory and provides an effective measure of a stress state that pertains to fatigue behavior of welded joints in the form of both membrane and bending components. Numerical procedures for both solid models and shell or plate element models are presented to demonstrate the mesh-size insensitivity in extracting the structural stress parameter. Conventional finite element models can be directly used with the structural stress calculation as a post-processing procedure. To further illustrate the effectiveness of the present structural stress procedures, a collection of existing weld S-N data for various joint types were processed using the current structural stress procedures. The results strongly suggests that weld classification based S-N curves can be significantly reduced into possibly a single master S-N curve, in which the slope of the S-N curve is determined by the relative composition of the membrane and bending components of the structural stress parameter. The effects of membrane and bending on S-N behaviors can be addressed by introducing an equivalent stress intensity factor based parameter using the structural stress components. Among other things, the two major implications are: (a) structural stresses pertaining to weld fatigue behavior can be consistently calculated in a mesh-insensitive manner regardless of types of finite element models; (b) transferability of weld S-N test data, regardless of welded joint types and loading modes, can be established using the structural stress based parameters.