Fatigue Life Of Welded Joints Research Articles

新低変態温度溶接材料と全姿勢伸長ビード補修法による溶接接手の高疲労寿命化 -第１報：新低変態温度溶接材料の成分系設計と全姿勢溶接性の検証-

新低変態温度溶接材料と全姿勢伸長ビード補修法による溶接接手の高疲労寿命化 -第１報：新低変態温度溶接材料の成分系設計と全姿勢溶接性の検証-

Finite element shape optimization of weld orientation in simple plate structure considering different fatigue estimation methods

Dynamically loaded structures are generally in risk of fatigue damage. The fatigue lives of such structures can be improved using traditional methods like producing a better weld or using post-weld treatment. Another method is to improve the overall design of the structure by using the finite element method in combination with shape optimization. In this paper, a framework for finite element shape optimization has been developed, considering the fatigue life of welded joints. Simplicity and the possibility of including different fatigue estimation methods have been in focus for the framework. The framework has been used on a simple plate structure, where the weld orientation has been optimized based on four different fatigue criteria. The results show that the criteria from governing guidelines predict the same optimized weld orientation. However, the critical plane approaches cannot produce optimized weld orientations, when using the nominal stress approach as it does not account for increased stresses from the weld.

Local approaches and XFEM used to estimate life of CFRP repaired cracked welded joints under fatigue loading

Three different local approaches, the Theory of Critical Distances (TCD), the Notch Stress Intensity Factor (NSIF) and the Strain Energy Density (SED), and the Extended Finite Element Method (XFEM) were applied to estimate the fatigue life of unrepaired and CFRP patches repaired cracked welded joints. The effects of CFRP modulus and thickness on the fatigue life of welded joints with different initial crack depths were investigated. The fatigue damage evolution equation using the loading frequency as a new damage variable was used to describe the degradation process. Fatigue life assessment results and crack growth paths compared with experimental results using these methods. The results show that the fatigue life and fatigue strength decrease as the increase of the initial crack depth. Three different fatigue failure modes can be observed from experimental results. The TCD, NSIF, and SED can reliably predict the fatigue life of welded joints without CFRP patches, but it needs to further investigate the values of estimation parameters of welded joints with CFRP patches for the NSIF and the SED. Fatigue crack growth paths and lives predicted by the XFEM and actual results are in good agreement, and the accuracy and reliability are validated by test data.

Research of the Fatigue Life of Welded Joints of High Strength Steel S960 QL Created Using Laser and Electron Beams.

This study investigated the fatigue life of welded joints, in particular, the welds of the high-strength steel S960 QL. The welds were created using unconventional technologies by utilising laser and electron beams. The direct application of the research is intended to be carried out through implementing the results towards the design of tracks for the track-wheel chassis of the demining system Božena 5. The producer’s experience shows the damage found in the current track design. The damage occurred during reversing the vehicle on a sand surface. Our goal was to solve this problem. The information acquired in this research will be a very important input factor for further designs of the track made of the tested material and its welds. The analysis of the residual stresses was also part of this study. The experimental research of the tested material’s fatigue life and welded joints was realised on the specimens loaded using cyclic bending and cyclic torsion. These loads were dominant during the track operation. The fatigue life of the tested material was detected using a device designed by us. The measurement results were processed in the form of the Wöhler’s S–N curves (alternating stress versus number cycles to failure) and compared with the current regulations issued by the International Institute of Welding (IIW) in the form of the FAT curves (IIW fatigue class). The achieved research results indicate that the modern welding technologies (laser and electron beams) used on the high-strength steel had no principal influence on the fatigue life of the tested material.

Fatigue Life Assessment of Welded Joints: A Review

In the present review article, the research and progress in the estimation of fatigue life of welded joints are critically reviewed from different perspectives. The fatigue is the common mode of failure in welded joints. Fatigue strength is recognized as closely related to the accurate geometrical characteristics of the welded joint. The emphasis is done on the scientific techniques applied in the fatigue life estimation of butt and T-joints. Further, the different parameters that effect the fatigue life of the welded is discussed. This research paper aims to review the latest progress in the assessment of fatigue life and failure of weldments, to provide a basis for follow on research.

Fatigue life prediction of welded joints from nominal system to fracture mechanics

This paper presents methods for determining the fatigue life of welded joints with particular emphasis on typical joints. Possible methods for computational determination of the stresses in welded joints are shown, starting with those based on nominal stresses according to different proposals, through structural stresses, including the ‘hot spot’ type, to the most complicated linear fracture mechanics. With this background, the recommendations of the International Institute of Welding (IIW) for determining the fatigue life of welded steel joints of flat components under uniaxial tensile–compression conditions are presented. The paper focuses on the problem of determining and evaluating the fatigue life of selected types of welded joints in accordance with the guidelines of the IIW and the safety analysis of welded structures. The study shows the extent to which the simplest method based on the amplitude of nominal stress can be used for calculating fatigue life based on two selected types of joints with different asymmetry coefficients, R = −1 and 0, and joints without geometrical notch. The analysis was based on steel welded joints of four different materials. The results show that the FAT characteristics are always good except for the FAT 90 category, where, for a lower fatigue life, we approach a dangerous fatigue life estimation. For K-type cruciform joints, the FAT model can be used for all the stress ratios. In other cases, such compliance cannot be accepted. For the category FAT 90 X-type butt joints, a new category FAT '134 and a new slope, m = 5, are proposed. For type X butt joints without notch, the slope m = 12 and a new category FAT' 525 are proposed.

On the application of laser shock peening for retardation of surface fatigue cracks in laser beam‐welded AA6056

AbstractThe present study aims to investigate the extent to which the fatigue behaviour of laser beam‐welded AA6056‐T6 butt joints with an already existing crack can be improved through the application of laser shock peening. Ultrasonic testing was utilized for in situ (nondestructive) measurement of fatigue crack growth during the fatigue test. This procedure allowed the preparation of welded specimens with surface fatigue cracks with a depth of approximately 1.2 mm. The precracked specimens showed a 20% reduction in the fatigue limit compared with specimens without cracks in the as‐welded condition. Through the application of laser shock peening on the surfaces of the precracked specimens, it was possible to recover the fatigue life to the level of the specimens tested in the as‐welded condition. The results of this study show that laser shock peening is a very promising technique to recover the fatigue life of welded joints with surface cracks, which can be detected by nondestructive testing.

A design method of tensile triangles and low transformation temperature weld metal for reduction of stress concentration and residual stress of welded joints

Stress concentration and residual stress have a significant influence on fatigue life of welded joints. In order to reduce the stress concentration of welded joints, a mathematical design method of tensile triangles (MTT) based on bionics was applied to weld shape design. Accordingly, the stress concentration of various weld beads in the corner boxing welded joint and the fillet welded T-joint was dissected using our in-house FEM software JWRIAN. It was found that there existed a large stress concentration in the conventional welded joints, whereas those welded joints with elongated weld bead were accompanied by a lower stress concentration, especially for elongated weld bead with MTT design. Furthermore, among the weld shapes of the corner boxing fillet welded joint, the rectangle shape of weld bead had the minimum stress concentration factor (1.05). For the fillet welded T-joint with MTT design, the stress concentration of weld toe decreased dramatically with the increase of the index of designed shape, but there was a minor difference of stress concentration at weld root between the weld beads with MTT design. In addition, application of low transformation temperature (LTT) weld metal utilizing martensitic transformation to the fillet welded T-joints can produce compressive residual stress at weld toe.

Nominal Stress-Based Estimation of Fatigue Life of Welded Joints

The paper presents methods for determining the fatigue life of welded joints with particular emphasis given to typical joints. In addition, the article presents various possible nominal stress-based ways enabling the calculation of stresses, including structural stresses and involving the most complex linear fracture mechanics. The paper also discusses recommendations by the International Institute of Welding related to the determination of the fatigue life of welded joints in flat elements exposed to tension-compression conditions. The work is focused on assessing the fatigue life of welded joints (selected types) in accordance with the guidelines specified in related recommendations issued by the International Institute of Welding and taking into consideration the analysis concerned with the safety of such structures.

Determination of the Fatigue Service Life of Welded Joints Using the Spectral Method Defined in the Frequency Domain

The paper discusses presently experienced problems concerned with forecasting the fatigue life of welded joints in terms of the spectral method defined in the frequency domain. In addition, the article presents the primary assumptions of the spectral method and describes the issue related to the recognition of the mean stress value and loads above the material yield point.

Fatigue performance of friction stir welded Al2024 alloy in a different corrosive environment

AbstractThe study examines the corrosion fatigue behavior of friction stir welded Al2024 alloy in the corrosive medium. The fatigue tests are conducted at a stress ratio of ‐1 in the different corrosive medium. The decrease in the fatigue life of welded joints in the corrosive environment is attributed to an increase in the crack initiation susceptibility in the presence of corrosive media. The fractured surfaces are investigated using the scanning electron microscopy (SEM). The formation of the corrosive compounds was studied using x‐ray diffractometry.

Effect of Welding Residual Stress on Along-Wind Fatigue Life of Welded Joints in Guyed Mast

On the premise of only considering along-wind effect on guyed mast, the influence of welding residual stress on the fatigue life of welded joints is evaluated in this paper. Since the sum of residual stress and along-wind-induced dynamic stress exceeds the yield strength of structural steel, the effect of residual stress relaxation is included in the numerical analysis. The multiscale finite element analysis of guyed mast is developed in order to capture accurately stress field distribution of welded joint for the “welding-wind-induced” case in which both residual stress and along-wind-induced stress are taken into consideration, and the stress response characteristics at fatigue critical point of welded joints are pointed out. It is found that the “welding-wind-induced” stress field of welded joint can be approximately considered as multiaxial proportional loading state and hence the stress-based von Mises criterion can be adopted to evaluate the fatigue life of welded joints. Based on the S-N curve of stress fatigue life for welded specimens with structural steel commonly used in guyed masts, the fatigue damage of key welded joints is predicted, and as a consequence, the influence of welding residual stress on the fatigue life of welded joints is discussed.

Numerical predictions of the fatigue life of aluminium welded joints

The fatigue life of arc welded joints made of aluminium alloy has been investigated by means of the implicit gradient method. This approach is suitable for estimating the fatigue life of welded joints characterised by different opening angles of the fillet weld and different thicknesses. The main fatigue data are related to cruciform joints, rectangular hollow section T-joints and bead removed joints. Using the principal stress as the local equivalent stress and a notch tip of the weld toe radius equal to zero, the paper analyses about two hundred experimental data taken from the literature and proposes a universal scatter band in the range of 104 to 5·106 cycles. This scatter band has an inverse slope around 4 and, at high cycle fatigue, the strength is about half that of steel joints. However, the scatter band is similar to welded joints made of steel previously analysed.

Analysis of the Influence of Residual Stress on Fatigue Life of Welded Joints

Abstract This work presents a new computational approach to estimate the fatigue life in welded joints, taking into account three-dimensional, non-proportional, and out-of-phase stresses; the thermomechanical properties of steel; parameters of the welding process; and the effects of residual stresses. We use the mechanical equations of continuous and cumulative fatigue damage found from Multiaxial RainFlow, the Wang and Brown and critical plane methods, and the Findley criteria approach. The numerical finite element method, available with software, is used to simulate the formation of residual stress and quantify its influence by solving equations. Based on the analysis of numerical results, we were able to prevision the crack location and angles of propagation of a high cycle fatigue crack with totally random loads in a metallic structure. We conclude that the result of the computational method is valid and can be used to estimate and life in fatigue in welded joints.

Effect of Vibration Frequency on Laser Filler Wire Welded Joints

The effects of different vibration frequencies on the microstructure, residual stress, and fatigue life of welded joints are investigated using laser filler wire welding combined with mechanical vibration. The results show that the vibration frequency of 1055 Hz has a significant effect on the melting width of the laser welding with filler wire joint. The welds have three morphological features of columnar crystals, dendrites, and equiaxed crystals. In addition, the welds with 1055 Hz vibration frequency have the most equiaxed crystals and the highest hardness. The vibration frequency of 524 Hz increases the residual stress of the welded joint by 16%, whereas the vibration frequency of 1055 Hz reduces its residual stress by 8%. It can be shown by tensile experiments that the laser filler wire–welded joints have a higher tensile strength and greater elongation with increase in the vibration frequency. The fracture observation shows that the cleavage step appears in the crack propagation zone. Compared with the vibration frequency of 524 Hz, fatigue fracture at the 1055 Hz vibration has narrower fatigue striation spacing, lower crack growth rate, and higher fatigue life.

Influence of the optical measurement technique and evaluation approach on the determination of local weld geometry parameters for different weld types

The joining by welding processes of components is usually related to the creation of additional notches and geometrical peculiarities. Multiple investigations have shown that a clear correlation between the local weld geometry and the fatigue life of welded joints exist. Thereby, the local increase of the local stress can be expressed by a stress concentration factor at the transition from the base material to the filler material, the so-called weld toe. The stress concentration factor can be determined for the most weld types if the geometric parameters such as plate thickness weld toe radius and flank angle are known. However, no standardized method for the determination of these parameters exists. Beside the well-established 2D-measurement methods on cross sections with weld impression analysis, new 3D-methods based on contactless, optical measurement were applied in the last years for the geometrical analysis of welded joints. With these methods, long length of welds can be analyzed in a very short time and with low effort. However, the influence of the measurement system (geometrical accuracy, lateral resolution) was not quantified yet. Additionally, in all known cases of application different evaluation algorithms were used. This does not allow for a straightforward comparison of the investigated parameters and results. In this round robin study, the determination of weld toe radii and flank angles by different evaluation algorithms and 3D-measurement systems and by different institutes are compared. Furthermore, an approach for the direct determination of the stress concentration factors of fillet welds by translating the complex weld shape in a 2D-finite element simulation was implemented. The results of this direct approach are compared to the stress concentration factors determined indirectly using the geometric parameters and those calculated by established approximation formulas.

Development of a computational fatigue model for evaluation of weld quality

The current study focuses on the development of a predictive model for assessing the fatigue life of welded joints based on measured weld geometry and applied load. Two different materials (S355 and S960) and two different material thicknesses (2 mm and 8 mm) were considered. Experiments on cruciform joints were conducted to evaluate the fatigue performance for different types of weld geometries. A computational model based on FEM and linear elastic fracture mechanics was developed and adapted to fit the experimental results using optimization and surrogate models.It is observed that the general fatigue behavior differs for the different materials for the same variation in geometry. The fatigue performance depends on a combination of geometrical parameters. The use of FAT curves according to the weld quality systems, e.g., ISO 5817, is insufficient to describe fatigue properties for welds in thin high strength steel, and different geometries within different weld quality levels can give the same fatigue behavior. It is also concluded that the developed computational model is suitable for further development of weld quality systems.

Prediction of remaining fatigue life of welded joints in wind turbine support structures considering strain measurement and a joint distribution of oceanographic data

Reassessing the remaining fatigue life of the wind turbine support structures becomes more and more crucial for operation, maintenance, and life extension when they are reaching the end of their design service life. By using measured oceanographic and strain data, each year, remaining fatigue life can be updated to adapt the operation to real loading conditions. Previous works have not put attention to address the complexity of offshore loading combinations and as-constructed state of the structure in estimating structural responses for fatigue behaviour to stochastically predict the remaining fatigue life. The present paper links the oceanographic data to fatigue damage by using measured strain, and uses the Bayesian approach to update the joint distribution of the oceanographic data. Consequently, the failure probability of the support structure can be updated and so the predicted fatigue life. The year-to-year variation of the 10-min mean wind speed, the unrepresentativeness of measured strain, the measurement uncertainty, and corrosion are considered together with uncertainties in Miner′s rule and S–N curves. The present research shows that the real oceanographic data can be used to adjust the predicted remaining fatigue life and eventually give decision support for the wind turbine operation.

FATIGUE LIFE ANALYSIS OF RAIL-WELDS USING LINEAR ELASTIC FRACTURE MECHANICS

The initiation and growth of fatigue cracking is mainly due to high stress concentration, heterogeneity and poor quality of weld. The detection and rectification of such weld defects are major concerns of rail network managers to reduce potential risk of rail breaks and derailments. To estimate the fatigue life of welded joints and to analyze the progress of fatigue cracks, a fracture mechanics-based analysis and fatigue models were developed using Finite Element Analysis. The initial flaw is obtained from a sample weld using ultrasonic flaw detecting machine test. Linear Elastic Fracture Mechanics (LEFM) approach based on the Paris law was applied to determine critical crack size and the number of cycles to failure using FRANC3D software. The inspection interval of rail welds before fracture (failure) was suggested based on reliability and life cycle analysis that correspond with minimum overall cost and frequency interval. It is recommended that fracture-based models in combination with reliability analyses can be a sustainable infrastructure decision-making algorithm.

Prediction of Fatigue Life of Welded Joints Made of Fine-Grained Martensite-Bainitic S960QL Steel and Determination of Crack Origins

Due to growing requirements connected with the utilization of advanced structures, nowadays the modern design processes are developed. One of the crucial issues considered in these processes is proper design of the joints against fatigue in order to fulfill a stated life of operation. In this study, the method of fatigue life prediction based on the criterion of permissible strain range in the notch root is presented. An engaged simplified model of fatigue life prediction was previously developed for mild and carbon steels. The evaluation made during the research has proven that this method can also be used for S960QL high-strength steel characterized by entirely different properties and structure. A considered theoretical model demonstrates satisfactory correlation with experimental data and safely describes the fatigue life of weldments. Furthermore, the predicted fatigue life of studied steel without welds shows great comparability with experimental data. The limit value of the strain range in the notch root was estimated. Below this value of strain, the fatigue life of welded joints is infinite, theoretically. Finally, the impact of the surface imperfections on the fatigue crack initiation was revealed. For paternal material, the origins of cracking were discovered at the places of nonmetallic scale particles. In welded joints, the fatigue cracks initiated at the whole length of the fusion line.