Fatigue Class Research Articles

Effects of weaving technique on the fatigue strength of transverse loaded fillet welds made of ultra-high-strength steel

The effect of weaved weld toe geometry on the fatigue strength of S960 steel grade fillet welds has been studied by means of experimental testing and finite element analysis. The comparisons have been made between normal straight weld toe lines and weaved weld toe lines by using the weld toe radius and structural stress fatigue class (FAT) value as reference values. The effective notch stress approach was utilized to conduct finite element (FE) analyses on weaved weld toe geometry, and the results were compared with the experimental results. The FE analysis showed quite good agreement with the experimental test results, although residual stresses or heat-affected zone (HAZ) softening in welded joint were not included in the modeling. The experimental tests also indicated that it is possible to achieve high fatigue properties for ultra-high-strength steel (UHSS) fillet welded joints without any postweld treatments. By taking account all the essential factors—joint geometry, welding metallurgy, and residual stresses—the reached fatigue strength of UHSS weldments can be much higher than normal fatigue resistance values presented in standards and recommendations.

Fatigue strength assessment of HFMI-treated butt welds by the effective notch stress method

Experimental fatigue data for high-frequency mechanical impact (HFMI)-treated butt welds with low stress concentrations have been collected and analysed using the effective notch stress (ENS) method given by the International Institute of Welding (IIW). The aim of this study is to suggest a more reliable fatigue assessment procedure for HFMI-improved butt welds with low stress concentration factors. In total, 165 published test results for butt welds subject to R = 0.1 and R = 0.5 axial loading are presented. K n, min values for the ENS approach are suggested in order to avoid computational problems due to low stress concentrations at the weld toe. Minimum notch stress concentration values are suggested based on the material yield strength (f y ). All the data is presented with respect to previously proposed and verified material f y correction method. For HFMI-treated butt welds in the ENS system, the use of the K n, min values with the f y correction representing one fatigue class (approximately 12.5 %) increase in strength for every 200-MPa increase in f y has been proposed and verified. © 2014 International Institute of Welding.

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 (Kn) for each joint geometry are analysed based on the finite element method. Calculated Kn 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 (fy). A formerly-defined minimum Kn values as a function of fy is used for butt welds. The already-known fatigue classes are found to be conservative with respect to available fatigue test data.

Differences in morning and evening fatigue in oncology patients and their family caregivers

Purpose of the researchTo identify distinct latent classes of individuals based on ratings of morning and evening fatigue; evaluate for differences in phenotypic characteristics, as well as symptom and quality of life scores, among these latent classes; and evaluate for an overlap in morning and evening fatigue class membership. Patients and methodsIn a sample of 167 oncology outpatients and 85 of their FCs, growth mixture modeling was used to identify distinct latent classes based on ratings of morning and evening fatigue obtained before, during, and after radiation therapy. Analyses of variance and Chi Square analyses were used to evaluate for differences among the morning and evening fatigue latent classes. ResultsThree distinct latent classes for morning fatigue were identified. Participants in the High Morning Fatigue class (47%) were younger and had lower functional status. Three distinct latent classes for evening fatigue were identified. Participants in the High Evening Fatigue class (61%) were younger, more likely to be female, more likely to have children at home, and more likely to be a FC. Only 10.3% of participants were classified in both the Very Low Morning and Low Evening Fatigue classes and 41.3% were classified in both the High Morning and High Evening Fatigue classes. ConclusionsDifferent characteristics were associated with morning and evening fatigue, which suggests that morning and evening fatigue may be distinct but related symptoms. Additional research is needed to elucidate the mechanisms that may underlie diurnal variability in fatigue.

Assessment of fatigue and selection of steel on constructions of steel bridges welded according to Eurocode 3

In this study, results of X-ray radiographic examinations in situ of butt welds in 155 steel railway bridges in Poland are presented. In 34 bridges, cracks in 437 joints were found. Distribution and size of stresses in a section of weld weakened with those weld unconformities were determined as well as brittle fractures and fatigue cracks were discussed. Fatigue classes of elements without welds and butt welds, according to which concentration of stresses at selection of steel for brittle cracking and analysing of structures exposed to fatigue according to PN-EN 1993-1-10 and PN-EN 1993-1-9 should be made, were presented. In order to estimate the fatigue life of steel structures with imperfections, the author described his own fatigue investigation of steel of diversified ageing and four groups of butt welds. Fatigue strength curves obtained from particular tests were compared with standard curves.

Influence of the base Material Strength and Edge Preparation on the Fatigue Strength of the Structures Made by High and Ultra-high Strength Steels

In recent years the high and ultra-high strength steels have been developed and applied for light-weight constructions such as the structural members of mobile equipment in order to reduce weight and fabrication costs as well as to contribute to the performance. In many applications, especially in the use of ultra-high strength steels, the welds are tended to locate outside of the highly stressed regions and use the post weld improvement for the critical welds. This means that the non-welded sections are in load-carrying positions and have to take into the consideration in the design of the structures.Novel test results have shown that the fatigue strength of the base material increases by increasing the yield strength of steel and the maximum fatigue class in the design codes FAT 160 and a slope, m=5, is conservative when the yield strength of steel is equal and higher than 355N/mm2. Only standard EN 13001-3-1:2012 allows applying for the higher fatigue classes by increasing the yield strength in cranes.In this paper the fatigue test results from high and ultra-high strength strip rolled steels are presented and compared with the fatigue classes in standard EN 13001-3-1:2012. Also a cross-member brace connection to main chord flange in vehicle chassis is presented as a design example.

Longitudinal relationships between subjective fatigue, cognitive function, and everyday functioning in old age

The present study examined the prospective relationships between subjective fatigue, cognitive function, and everyday functioning. A cohort study with secondary data analysis was conducted using data from 2,781 community-dwelling older adults without dementia who were enrolled to participate in the Advanced Cognitive Training for Independent and Vital Elderly (ACTIVE) randomized intervention trial. Measures included demographic and health information at baseline, and annual assessments of subjective fatigue, cognitive function (i.e. speed of processing, memory, and reasoning), and everyday functioning (i.e. everyday speed and everyday problem-solving) over five years. Four distinct classes of subjective fatigue were identified using growth mixture modeling: one group complaining fatigue "some of the time" at baseline but "most of the time" at five-year follow-up (increased fatigue), one complaining fatigue "a good bit of the time" constantly over time (persistent fatigue), one complaining fatigue "most of the time" at baseline but "some of the time" at five-year follow-up (decreased fatigue), and the fourth complaining fatigue "some of the time" constantly over time (persistent energy). All domains of cognitive function and everyday functioning declined significantly over five years; and the decline rates, but not the baseline levels, differed by the latent class of subjective fatigue. Except for the decreased fatigue class, there were different degrees of significant associations between the decline rates of subjective fatigue and all domains of cognitive function and everyday functioning in other classes of subjective fatigue. Future interventions should address subjective fatigue when managing cognitive and functional abilities in community-dwelling older adults.

Fatigue strength of fillet welds subjected to multi-axial stresses

Fatigue endurance assessments of welded details are normally carried out by calculating the relevant stress acting on the detail and identifying a relevant fatigue class (or detail category) with its associated S–N curve. The fatigue strength of most structural details incorporated in design codes has been obtained from fatigue tests conducted under uni-axial loading conditions, which normally result in a uni-axial stress state in the detail. Many of the structural details that exist in fatigue-loaded structures experience some kind of multi-axial loading condition. The subject of the fatigue strength of welded details under multi-axial loads has been the topic of numerous research projects in recent years. The vast majority of these projects were, however, devoted to cracking in the base metal (i.e. toe cracking). Very little has been done with reference to the cracking of fillet welds in combined loading situations (i.e. root cracking). This paper presents new test results from cruciform specimens, in which weld failure initiated at the root in a multi-axial stress state. The tests have been performed at two different load levels and on three different specimen configurations giving different τ/σ ratios. This permitted an examination of the effect of the shear to normal stress ratio on the fatigue strength of fillet welds. The results of these tests, together with other relevant tests reported in the literature, are then evaluated in relation to the design models proposed in three design standards: Eurocode, IIW and DNV. No obvious dependence on the τ/σ ratios could be found. The evaluated models all appear to be able to predict the fatigue life of a cruciform weld failing from the root under combined shear and normal stress.

Fatigue strength improvement factors for high strength steel welded joints treated by high frequency mechanical impact

Numerous studies have observed that the fatigue strength of improved welds increases with material yield strength. This paper provides a comprehensive evaluation of published data for high frequency mechanical impact treated welds. In total, 228 experimental results for three weld geometries subject to R=0.1 axial loading have been reviewed. A design recommendation including one fatigue class increase in strength (about 12.5%) for every 200MPa increase in static yield strength is proposed and are shown to be conservative with respect to all available data. Special cautions are given for high R-ratio or variable amplitude fatigue and potential alternate failure locations.

Influence of spectrum loading on fatigue resistance of AA6082 friction stir welds

PurposeWelded components are often subjected to variable amplitude service loads, increasing the uncertainty of fatigue life due to material strength, notch geometries, defect content and residual stresses. In the case of friction stir welding (FSW) of aluminium alloys no data were found available concerning fatigue behaviour under variable amplitude loading. The purpose of this paper is to determine the fatigue strength of friction stir welds in AA6082‐T6 under constant and variable amplitude loading and analyse the validity of Miner's rule for these specific welding conditions.Design/methodology/approachFatigue tests were carried out in a servo‐hydraulic testing machine using a stress ratio of R=0. Typified Gassner amplitude spectra were considered, using four shape exponent values. Microhardness tests were performed to characterize the Vickers hardness profile in the vicinity of the weld area. Relatively to the base material (BM), the FSW process leads to a decrease of the static mechanical properties.FindingsDetailed examination revealed a hardness decrease in the thermo‐mechanically affected zone and the nugget zone average hardness was found to be lower than the base alloy hardness. The comparison with data collected from the literature shows that FSW specimens present higher fatigue resistance than specimens welded by metal inert gas and tungsten inert gas processes. However, they still have lower fatigue lives than the BM. Using the equivalent stress calculated by Miner's rule, a good agreement was observed between constant and variable fatigue loading results. The characteristic curve obtained for friction stir welds is higher than the International Institute of Welding (IIW) fatigue class for fusion welds with full‐penetration both‐sided butt joints.Originality/valueNo data are available concerning fatigue behaviour under variable amplitude loading for friction stir welds of aluminium alloys. Furthermore, this paper analyses the fatigue strength of friction stir welds in AA6082‐T6 under constant and variable amplitude loading in order to verify the validity of Miner's rule for this specific welding process. A comparison between characteristic fatigue curves, using IIW fatigue classes (FAT), is also performed.

Novel Feature Modelling the Prediction and Detection of sEMG Muscle Fatigue towards an Automated Wearable System

Surface Electromyography (sEMG) activity of the biceps muscle was recorded from ten subjects performing isometric contraction until fatigue. A novel feature (1D spectro_std) was used to extract the feature that modeled three classes of fatigue, which enabled the prediction and detection of fatigue. Initial results of class separation were encouraging, discriminating between the three classes of fatigue, a longitudinal classification on Non-Fatigue and Transition-to-Fatigue shows 81.58% correct classification with accuracy 0.74 of correct predictions while the longitudinal classification on Transition-to-Fatigue and Fatigue showed lower average correct classification of 66.51% with a positive classification accuracy 0.73 of correct prediction. Comparison of the 1D spectro_std with other sEMG fatigue features on the same dataset show a significant improvement in classification, where results show a significant 20.58% (p < 0.01) improvement when using the 1D spectro_std to classify Non-Fatigue and Transition-to-Fatigue. In classifying Transition-to-Fatigue and Fatigue results also show a significant improvement over the other features giving 8.14% (p < 0.05) on average of all compared features.

Theory-Based Evaluation of an Online Cancer Fatigue Class

Fatigue is a common problem faced by cancer patients and survivors, yet is often overlooked. An online fatigue class is evaluated using measures based on the Health Belief Model (HBM). A sample of 26 survivors and seven caregivers completed pre-class and post-class surveys and a facilitated discussion. Statistically significant improvements were detected in both the fatigue knowledge (p < 0.001) and belief (p < 0.001) scores. Participants reported that the content was accessible and useful. The class had a positive impact on their knowledge and beliefs about cancer fatigue. This suggests that HBM may be an appropriate framework for the evaluation of Internet-based educational interventions.

Fatigue behaviour of butt welded joints in a high strength steel

Modern technological processes make it possible to produce high strength steel with up to 1200 MPa yield strength. This high yield strength gives potential for considerable improvements in performance and reduction in weight, which are of increasing importance in the transport sector and in vehicles used in the construction industry. However, data found in literature show that welded structural parts using these high and ultra-high strength steels have a fatigue resistance not very different in comparison with those obtained in conventional steels with a much lower yield stress.In this work a fatigue study was performed in a high strength steel in order to evaluate the influence on fatigue strength of the following factors: stress concentration at weld toe, internal defects and welding process method. The material used in this study was a steel sheet called DOMEX 600 DC from SSAB with 5 mm of thickness. This is a high strength steel with a yield stress of 670 MPa and a tensile strength of about 750 MPa.Butt welded specimens using MAG welding process were submitted to cyclic loading in servo hydraulic machine. Three welding conditions of transverse butt joints and base material were investigated in this fatigue study: as welded; welds overfill removed by grinding; welds overfill and first weld root removed by grinding.The overfill removing by grinding promotes a significant improvement of the fatigue resistance in comparison with the as welded condition. The removing of the root of the first weld by disc grinding improves significantly the weld quality by reducing the level of internal defects, leading to a further increasing on fatigue strength close to the parent material.The comparison of characteristic curves obtained for each welding condition and the fatigue class (FAT) indicates that, in general, fatigue strength of DOMEX 600 DC steel is significantly higher than fatigue strength recommended by IIW for conventional steels. The higher difference was found for welds with overfill removed and the first weld root submitted to a disc grinding operation.

Development and Application of a Fatigue Class Catalogue for Riveted Bridge Components

The assessment of the remaining fatigue life of existing, riveted railway and road bridges is gaining increased significance due to the advanced age of many of these structures. Therefore, the problem of determining adequate fatigue strength values for riveted components of older steel bridges arose during the elaboration of a pertinent research project initiated by bridge owners in Austria. The aim of this project was to address the discrepancies between different international provisions on this topic and to bridge them by way of an improved, more consistent assessment recommendation. The research project was subdivided into an experimental part and an analytical/statistical part; only the latter is discussed in this paper. A classification of details in riveted components that takes into account the specific fatigue proneness of different constructional details proved to be appropriate. It has led to more consistent ∆σ–N design curves when compared to the single ∆σ–N line discussed in most international literature on the subject. The resulting catalogue of assessment curves is presented in the first part of this paper. A proposal for the integration of the new fatigue classes into a coherent fatigue life assessment procedure is then described in a form that can immediately be put to use in compliance with the Eurocodes. Finally, a practical application of the proposed assessment procedure is demonstrated by means of an example that deals with a riveted railway bridge.

Development and Application of a Fatigue Class Catalogue for Riveted Bridge Components

Development and Application of a Fatigue Class Catalogue for Riveted Bridge Components

Characteristics of Laser Welded Joints of HDT580X Steel

In the present study, the characteristics of Nd:YAG laser welded joints of 600 MPa DP steel (HDT580X), 2.4 mm in thickness, in respect of hardness, microstructures and mechanical properties were investigated. The test joints have been welded under a shielding gas on the stand for robotic Nd:YAG laser welding at the beam power of 1.5 kW and 2.0 kW. Three combinations of welding parameters were used: 2.0 kW - 2.1 m/min, 2.0 kW - 1.5 m/min and 1.5 kW - 1.2 m/min. Detailed examinations were performed on the joint welded at the highest speed. The microstructure was examined by the optical-, scanning- and transmission electron microscope. The heat affected zone (HAZ) was composed of ferrite, bainite and lath martensite, the weld contained lath martensite. The maximum hardness in the HAZ did not exceed 343 HV. The tensile strength of the welded joint was at the same level as that of the base material. The results of fatigue tests and residual stress measurement of laser welded DP steel joints are also presented. The fatigue strength of the welded joint is lower than that of the base material. The fatigue class FAT was determined, which is equal to 284 MPa - for the base material and 150 MPa – for the welded joint. By means of the modified hole drilling method the following residual stresses were measured: σmax = 573 MPa and σmin= -126 MPa.

Characteristics of Nd:YAG Laser Welded Joints of Dual Phase Steel

Examination results of microstructure, mechanical properties, fatigue strength and residual stresses of laser welded joints in dual phase HDT580X steel have been presented. The main goal of these studies was to verify whether Nd:YAG laser welding without filler metal can be used for welding of dual phase steel. In the frame of this investigation the microstructure has been studied by optical and scanning microscopy. Mechanical properties have been analysed by tensile, bend and hardness tests. Additionally fatigue tests and residual stress measurements were carried out. The results revealed that the HDT 580X steel is characterized by good laser weldability. The tensile strength of welded joints is at the same level as that of the base metal and the maximum hardness does not exceed 343 HV. The microstructure of welded joints is mainly composed of lath martensite in the weld and a mixture of lath martensite, bainite and ferrite in the heat affected zone (HAZ). The fatigue class FAT was determined, which is equal to 284 MPa and 150 MPa for base material and welded joint, respectively. Residual stresses determined by the hole drilling method and the videoextensometer were: σmax= 573 MPa and σmin= −126 MPa.

Gene Expression Profile of Empirically Delineated Classes of Unexplained Chronic Fatigue

To identify the underlying gene expression profiles of unexplained chronic fatigue subjects classified into five or six class solutions by principal component (PCA) and latent class analyses (LCA). Microarray expression data were available for 15,315 genes and 111 female subjects enrolled from a population-based study on chronic fatigue syndrome. Algorithms were developed to assign gene scores and threshold values that signified the contribution of each gene to discriminate the multiclasses in each LCA solution. Unsupervised dimensionality reduction was first used to remove noise or otherwise uninformative gene combinations, followed by supervised dimensionality reduction to isolate gene combinations that best separate the classes. The authors' gene score and threshold algorithms identified 32 and 26 genes capable of discriminating the five and six multiclass solutions, respectively. Pair-wise comparisons suggested that some genes (zinc finger protein 350 [ZNF350], solute carrier family 1, member 6 [SLC1A6], F-box protein 7 [FBX07] and vacuole 14 protein homolog [VAC14]) distinguished most classes of fatigued subjects from healthy subjects, whereas others (patched homolog 2 [PTCH2] and T-cell leukemia/lymphoma [TCL1A]) differentiated specific fatigue classes. A computational approach was developed for general use to identify discriminatory genes in any multiclass problem. Using this approach, differences in gene expression were found to discriminate some classes of unexplained chronic fatigue, particularly one termed interoception.

Polymorphisms in Genes Regulating the HPA Axis Associated with Empirically Delineated Classes of Unexplained Chronic Fatigue

Chronic fatigue syndrome (CFS) is characterized by persistent or relapsing fatigue that is not alleviated by rest, causes substantial reduction in activities and is accompanied by a variety of symptoms. Its unknown etiology may reflect that CFS is heterogeneous. Latent class analyses of symptoms and physiological systems were used to delineate subgroups within a population-based sample of fatigued and nonfatigued subjects [1] . This study examined whether genetic differences underlie the individual subgroups of the latent class solution. Polymorphisms in 11 candidate genes related to both hypothalamic-pituitary-adrenal (HPA) axis function and mood-related neurotransmitter systems were evaluated by comparing each of the five ill classes (Class 1, n = 33; Class 3, n = 22; Class 4, n = 22; Class 5, n = 17; Class 6, n = 11) of fatigued subjects with subjects defined as well (Class 2, n = 35). Of the five classes of subjects with unexplained fatigue, three classes were distinguished by gene polymorphsims involved in either HPA axis function or neurotransmitter systems, including proopiomelanocortin (POMC), nuclear receptor subfamily 3, group C, member 1 (NR3C1), monoamine oxidase A (MAOA), monoamine oxidase B (MAOB), and tryptophan hydroxylase 2 (TPH2). These data support the hypothesis that medically unexplained chronic fatigue is heterogeneous and presents preliminary evidence of the genetic mechanisms underlying some of the putative conditions.

Simplified approach to fatigue strength assessment of fillet-welded attachment ends

Almost all stiffeners, brackets and other attachments in ship hulls are joined by non-penetration fillet welds for fabrication reasons. However, the ends of these attachments show frequently high stress concentration due to their geometrical shape and loading. Fatigue cracks may initiate here not only from the weld toe, but also from the non-welded root gap. A simplified approach is proposed for the fatigue strength assessment with respect to root cracking, which is based on a local nominal stress in a defined area of the weld throat and on common fatigue classes for the assessment of cruciform joints. This simple assessment is possible because the complex 3D case can be related to a simple 2D case. The procedure is verified by two typical attachments with hard and soft ends, the latter showing frequently a stress flow with pronounced shear stresses. Fatigue tests and numerical analyses of local stresses and crack propagation from the root gap have been performed. Some tests showed unexpected results in comparison with the calculations. Beneficial residual stresses were identified to be the main reason for this, which could be taken into account by a bonus factor in particular cases.