Fatigue Life Of Welded Joints Research Articles

Effect of UIT on Fatigue Life in Web-Gusset Welded Joints

Ultrasonic impact treatment (UIT), which is a peening method, is usually used as a post-weld treatment in order to improve the fatigue strength of welded joints. In this study, fatigue tests were carried out on web-gusset welded joints treated by UIT and the results were compared with the fatigue lives of as-welded joints in order to examine the effects of UIT on the fatigue lives of welded joints. The fatigue lives of web-gusset welded joints treated by UIT increased to more than ten times those of as-welded joints. The introduction of compressive residual stress, relaxation of stress concentration at a weld toe, and refinement of grains under the weld toes were considered as possible reasons for the improvement in fatigue life caused by UIT. Residual stress near weld toes was measured using the X-ray diffraction method. The stress concentration factor at the weld toes was analyzed using the finite element method (FEM). The grain size under the weld toes was measured using electron backscatter diffraction pattern (EBSD) analysis.

Fatigue life estimation in welded joints under multiaxial loadings

Welded joints are frequently locations for cracks initiation and propagation that may cause fatigue failure of engineering structures. Biaxial or triaxial stress-strain states are present in the vicinity of welded joints, due to local geometrical constraints, welding processes and/or multiaxial external loadings. Fatigue life evaluation of welded joints under multiaxial proportional (in-phase) cyclic loading can be performed by using conventional hypotheses (e.g. see the von Mises criterion or the Tresca criterion) on the basis of local approaches. On the contrary, the fatigue life predictions of welded joints under non-proportional (out-of-phase) cyclic loading are generally unsafe if these conventional hypotheses are used. A criterion initially proposed by the authors for smooth and notched structural components has been extended to the fatigue assessment of welded joints. In more detail, fatigue life of welded joints under multiaxial stress states can be evaluated by considering a nonlinear combination of the shear stress amplitude (acting on the critical plane) and the amplitude and the mean value of the normal stress (acting on the critical plane). In the present paper, fatigue lifetimes predicted through the proposed criterion are compared with experimental fatigue life data available in the literature, related to fatigue biaxial tests.

疲労き裂進展抑制鋼の溶接継手疲労特性

It has been generally recognized that the fatigue life of welded joints is little influenced by the strength of steels due to the high stress concentration and the tensile residual stress near the weld toe. In this paper, improvement of the fatigue life of welded joints using steel with high resistance to fatigue crack growth (F/M steel) is investigated. F/M steel has a microstructure with an elongated and banded martensite phase distributed in a ferrite matrix and a fatigue crack growth rate of about one-harf to one-tenth in the thickness direction, compared to conventional steel. As a result, the fatigue life of an out-of-surface gusset welded joint increases with the decrease of the fatigue crack growth rate. The fatigue life of welded joints using F/M steel with the highest resistance to fatigue crack growth increases to about twice that of joints using conventional steel. Whereas the fatigue crack growth rate decreases significantly, the fatigue life of welded joints increases only slightly. This can be attributed to the stress ratio independent of the fatigue crack growth rate. In other words, the fatigue crack growth rate of F/M steel increases with the increase of the stress ratio, approaching that of conventional steel. In the case of welded joints, even if a fatigue test is carried out at a low stress ratio, the region near the weld toe is under a high stress ratio due to tensile residual stress. Therefore, improvement of the fatigue life of welded joints becomes comparatively small so that the effect of fatigue crack retardation of F/M steel decreases.

A comprehensive investigation on the effects of laser and shot peening on fatigue crack growth in friction stir welded AA 2195 joints

The effects of various surface treatment techniques on the fatigue crack growth performance of friction stir welded 2195 aluminum alloy were investigated. The objective was to reduce fatigue crack growth rates and enhance the fatigue life of welded joints. The crack growth rates were assessed and characterized for different peening conditions at a stress ratio ( R) of 0.1, and 0.7. The surface and through-thickness residual stress distribution were also investigated and presented for the various regions in the weld. Tensile residual stresses introduced during the welding process were found to become significantly compressive, particularly after laser peening. The effect of the compressive stresses was deemed responsible for increasing the resistance to fatigue crack growth of the welds. The results indicate a significant reduction in fatigue crack growth rates using laser peening compared to shot peening and native welded specimens. This reduced fatigue crack growth rate was comparable to the base unwelded material.

The Fatigue Behavior of Steel Structures under Random Loading

Fatigue damage accumulation in steel structures under random loading has been studied in a number of investigations at the Technical University of Denmark. The fatigue life of welded joints has been determined both experimentally and from a fracture mechanics analysis. In the experimental part of the investigation, fatigue test series with a total of 540 fatigue tests have been carried through on various types of welded plate test specimens and full-scale offshore tubular joints. The materials that have been used are either conventional structural steel or high-strength steel. The fatigue tests and the fracture mechanics analyses have been carried out using load histories, which are realistic in relation to the types of structures studied, i.e. primarily bridges, offshore structures and chimneys. In general, the test series carried through show a significant difference between constant amplitude and variable amplitude fatigue test results. Both the fracture mechanics analysis and the fatigue test results indicate that Miner’s rule, which is normally used in the design against fatigue in steel structures, may give results, which are unconservative, and that the validity of the results obtained from Miner’s rule will depend on the distribution of the load history in tension and compression.

超音波衝撃処理と疲労き裂進展抑制鋼の相乗効果による溶接継手の疲労寿命向上

超音波衝撃処理と疲労き裂進展抑制鋼の相乗効果による溶接継手の疲労寿命向上

Low Cycle Fatigue Properties of 316LN Stainless Steel Welded by GTAW in Nitrogen Added Environment

Tensile and fatigue properties were evaluated for base and welded type 316LN stainless steel. Welding methods were GTAW (308L, Ar environment) and GTAWN (316L, Ar + N2 environment). Yield strength of weld joint was higher than that of base metal but elongation of weld joint was lower than that of base metal. UTS of weld joint was slightly lower than that of base metal. Yield strength and elongation with welding method were almost same. Fatigue life of weld joint was lower than that of base metal but fatigue strength of weld joint was higher than that of base metal. Ferrite content was increased with welding. Fatigue life welded by GTAWN was better than that of GTAW at RT and 600°C. This fatigue life behavior was consistent with the behavior of ferrite content.

Application of Notch Strain Approach to Evaluate Fatigue Life of Construction Equipment

The technical development of construction equipment such as wheel loader leads to an increase of not only amount of workload per hour, but also magnitude of allowable stress range. Since welded structures of the wheel loader occupy approximately 40-60% of its total weight and are subjected to incessant fatigue loads, fatal fatigue failures occur occasionally around the welded zone during its practical operation in the field. To reasonably evaluate the fatigue life, the effect of these geometries and welding residual stress on the fatigue life should be taken into account. In this paper, the modified notch strain approach developed previously by the authors is applied practically to assess the fatigue life of welded joints in boom and front frame structures of the wheel loader.

A computational method based on the local approach for evaluating the fatigue life of welded joints

A computational method based on the local approach for evaluating the fatigue life of welded joints

A model based on the stress concentration factor for the assessment of fatigue life of welded joints

A model based on the stress concentration factor for the assessment of fatigue life of welded joints

A life prediction model for welded joints under multiaxial variable amplitude loading histories

This paper describes a simple damage model for fatigue life predictions of welded joints under nonproportional, constant, and variable amplitude loading histories. This model, an extension of Sonsino’s effective equivalent stress amplitude method, consists of four material parameters accounting for the severity of nonproportional loading paths, the material’s susceptibility to nonproportional hardening, the material’s fatigue life under shear versus normal stresses, and the mean stress effect. These four parameters can be obtained by fatigue testing for S–N curves under bending only, torsion only, and 90° out-of-phase loading. This paper investigates the validity of the fatigue damage model by comparing the analytical and the published experimental results for various welded joint configurations subjected to multiaxial constant amplitude loads. This paper also describes a procedure to predict fatigue lives of welded joints under nonproportional variable amplitude loading histories based on the linear cumulative damage rule, the proposed equivalent stress amplitude and the Wang–Brown reversal counting technique.

Fatigue life of welded joints according to energy criteria in the critical plane

We have determined theoretical notch coefficients with use of the fictitious radius in tube-tube and flange-tube welds. The parameter of normal and shear strain energy density in critical planes is applied for estimation of fatigue life under cyclic conditions of pure bending, pure torsion and combined proportional bending with torsion. The critical planes were determined with use of two methods based on the maximum parameters of, respectively, normal and shear strain energy density.

Grain refinement and high performance of titanium alloy joint using arc-ultrasonic gas tungsten arc welding

Arc-ultrasonic welding technology is one approach for refining the grain structure in titanium alloy welds. This paper reports work in which welds were produced in an α-β titanium alloy under a variety of conditions, including conventional gas tungsten arc welding (GTAW) and arc-ultrasonic GTAW. The results show that arc-ultrasonic energy refined β grain structure in the weld metal, and improved the fatigue life of weld joints. Ultrasonic energy enhances fluid flow, reduces temperature gradients, and causes a continual high frequency oscillation in the welding pool. These effects are believed to be responsible for refining the solidification structure and improving the performance of titanium weld joints.

Fatigue Crack Initiation and Propagation Life of Welded Joints

The fatigue life of welded joints is associated with crack initiation and propagation life. Theses cannot be easily separated, since the definition of crack initiation is vague due to the initiation of multiple cracks that are distributed randomly along the weld toes. In this paper a method involving a notch strain and fracture mechanical approach, which considers the characteristics of welded joints, e.g. welding residual stress and statistical characteristics of multiple cracks, is proposed, in an attempt to reasonably estimate these fatigue lives. The fatigue crack initiation life was evaluated statistically, e.g. the probability of occurrence in 2.3, 50 and 97.7%, in which the cyclic response of the local stress/strain in the vicinity of the weld toes and notch factors derived by the irregular shape of the weld bead are taken into account. The fatigue crack propagation life was simulated in consideration of the Mk-factor and the mechanical behavior of mutual interaction/coalescence between two adjacent cracks. The estimated total fatigue life as a sum of crack initiation and propagation life was found to be in good agreement with the experimental results.

Assessment of some factors influencing the fatigue life of strength mis-matched HSLA steel weldments

Abstract Welding of high strength low alloy steels (HSLA) involve usage of low, even and high strength filler materials (electrodes) than the parent material depending on the application of the welded structures and the availability of the filler material. In this investigation an attempt has been made to assess the influences of mis-match ratio (MMR), post-weld heat treatment (PWHT) and notch location on fatigue life of HSLA steel welds. Shielded metal arc welding (SMAW) process has been used to fabricate the butt joints. Centre cracked tension (CCT) specimen has been used to evaluate the fatigue life of welded joints. Fatigue experiments have been conducted using servo hydraulic controlled fatigue testing machine at constant amplitude loading ( R =0). Design of experiments (full factorial design) concept has been used to optimise the number of experimental conditions. A procedure has been developed to assess and optimise the above chosen factors to attain a maximum fatigue life value by incorporating (i) response table and response graph, (ii) normal probability plot and (iii) analysis of variance (ANOVA) techniques.

Influences of MMR, PWHT and notch location on fatigue life of HSLA steel welds

Welding of High Strength Low Alloy Steels (HSLA) involves usage of lower, more even and higher strength filler materials (electrodes) than the parent material depending on the application of the welded structures and the availability of the filler material. In this investigation an attempt has been made to study the influences of mis-match ratio (MMR), Post Weld Heat Treatment (PWHT) and notch location on fatigue life of HSLA steel welds. A Shielded Metal Arc Welding (SMAW) process has been used to fabricate the butt joints. A Centre Cracked Tension (CCT) specimen has been used to evaluate the fatigue life of welded joints. Fatigue experiments have been conducted using a servo hydraulic controlled fatigue testing machine at constant amplitude loading ( R=0). The Design of Experiments (full factorial design) concept has been used to optimise the number of experimental conditions. Analysis of Variance (ANOVA) method has been used to identify the significance of main and interaction effects. A mathematical model has been developed to predict the fatigue life of HSLA steel welds using Regression analysis.

잔류응력 이완 및 이의 평균응력 효과를 고려한 용접부 피로수명 평가

The fatigue life of welded joints is sensitive to welding residual stress and complexity of their geometric shapes. To predict the fatigue life more reasonably, the effects of welding residual stress and its relaxation on their fatigue strengths should be considered quantitatively, which are often regarded to be equivalent to the effects of mean stresses by external loads. The hot-spot stress concept should be also adopted which can reduce the dependence of fatigue strengths for various welding details. Considering the factors mentioned above, a fatigue life prediction model using the modified Goodman's diagram was proposed. In this model, an equivalent stress was introduced which is composed of the mean stress based on the hot-spot stress concept and the relaxed welding residual stress. From the verification of the proposed model to real welding details, it is proved that this model can be applied to predict reasonably their fatigue lives.

Effect of residual stresses on the fatigue of butt joints using thermal elasto-plastic and multiaxial fatigue theory

This investigation performs a thermal elasto-plastic analysis using finite element techniques to analyze thermomechanical behavior and evaluate residual stresses in weldments. An effective procedure is also developed by combining finite elements and multiaxial fatigue theory while considering the welding residual stress as the initial conditions in accurately predicting the fatigue life of welded joints. Herein, the fatigue lives of butt-welded joints are forecast using the proposed procedure. The proposed procedure that followed the conventional strain-based method (maximum principal strain and von Mises effective strain) to predict the fatigue life of the butt-welded joints was fairly sensitive to welding residual stress. Furthermore, the maximum principal strain method led to conservative life estimates and the von Mises effective strain method offered the best agreement with the experimental data of butt-welded joints.

Comparison of Ultrasonic Impact Treatment (UIT) and Other Fatigue Life Improvement Methods

Considerable recent attention has been paid to investigating the efficiency of ultrasonic impact treatment (UIT) as one of the post-weld methods for improving the reliability and fatigue lives of welded joints. Investigations were carried out at different times in Russia, Ukraine, France, Japan, Norway, Sweden and the USA. The work under review is intended to determine and study the benefits of UIT according to the IIW Collaborative Test Program on Improvement Methods, by comparison with well-known methods for increasing the fatigue strengths of welded joints. The paper presents comparative fatigue test results for welded specimens in the as-welded condition and specimens treated by hammer peening, shot peening, TIG dressing, ultrasonic impact treatment and a combination of TIG dressing and ultrasonic impact treatment. The specimens were fabricated from structural steel (SSAB Weldox 420). It is shown that, by correctly choosing the conditions, UIT can produce the highest improvement in fatigue life.

The Influence of Vibratory Treatment on the Fatigue Life of Welds: A Comparison with Thermal Stress Relief

Abstract: A comparison has been made between the fatigue lives of welded specimens a) in the as welded condition b) after heat treatment, and c) after post‐weld vibration. By comparison with the as‐welded specimens, the fatigue lives of the thermally relieved specimens were found to decrease by 43%, while the vibration treated specimens showed an increase of between 17% and 30%. While these findings are interesting in that they offer a way of extending the fatigue lives of welded joints, they confirm the view that the mechanism of residual stress relieving in the vibratory stress relief (VSR) method, and its relationship with fatigue life is poorly understood.