Fatigue Strength Of Welded Joints Research Articles

A simplified model for TIG-dressing numerical simulation

Irrespective of the mechanical properties of the alloy to be welded, the fatigue strength of welded joints is primarily controlled by the stress concentration associated with the weld toe or weld root. In order to reduce the effects of such notch defects in welds, which are influenced by tensile properties of the alloy, post-weld improvement techniques have been developed. The two most commonly used techniques are weld toe grinding and TIG dressing, which are intended to both remove toe defects such as non-metallic intrusions and to re-profile the weld toe region to give a lower stress concentration. In the case of TIG dressing the weld toe is re-melted to provide a smoother transition between the plate and the weld crown and to beneficially modify the residual stress redistribution. Assessing the changes to weld stress state arising from TIG-dressing is most easily accomplished through a complex numerical simulation that requires coupled thermo-fluid dynamics and solid mechanics. However, this can be expensive in terms of computational cost and time needed to reach a solution. The present paper therefore proposes a simplified numerical model that overcomes such drawbacks and which simulates the remelted toe region by means of the activation and deactivation of elements in the numerical model.

The thickness effect of welded details improved by high-frequency mechanical impact treatment

High-frequency mechanical impact (HFMI) treatment can enable resource-efficient structural design by improving the fatigue strength of welded joints. While the thickness effect with reference to the fatigue of welded details is well known and covered in design codes, this effect has not been investigated systematically when the welds are improved by HFMI. In this study, experimental data of 582 small-scale fatigue tests on welded details with HFMI treatment has been collected from the literature and evaluated with respect to the thickness effect. In order to separate the effects of yield strength and thickness on the fatigue strength, a new approach was developed to adjust the data to a reference yield strength of 355MPa prior to thickness evaluation. The test data covered transverse butt welds, details with non-load-carrying transverse attachments and details with non-load-carrying longitudinal attachments. The thickness effect of details with transverse attachments corresponds well with the IIW recommendation of n=0.2, whereas transverse butt welds have a much weaker thickness effect. Details with longitudinal attachments show a “reverse” thickness effect.

Fatigue life assessment of weld joints manufactured by GMAW and CW-GMAW processes

The fatigue strength of weld joints manufactured using gas metal arc welding and cold wire GMAW (CW-GMAW) was evaluated under stress-controlled cyclic loading. The material used in this study was class ASTM 131 grade A steel, joined using ER70S wire filler metal. The addition of cold wire led to a decrease in the amount of intergranular ferrite and an increase in hardness in the heat-affected zone. The assessment of fatigue life was performed by using the Weibull distribution and the results revealed that with a 99% reliability the joints produced using the CW-GMAW process have a longer fatigue life, especially as the stress amplitude increases.

Residual stress state induced by high frequency mechanical impact treatment in different steel grades – Numerical and experimental study

High frequency mechanical impact treatment is observed to increase the fatigue strength of welded joints. This technique induces compressive residual stresses, increases the local hardness, and reduces the stress concentration by modifying the weld toe radius. The goal of this study was to investigate residual stresses induced by ultrasonic impact treatment in S355, S700MC, and S960 grades steel experimentally and numerically. Plate specimens were manufactured and treated with different treatment intensities i.e. vibration amplitudes of the Sonotrode. The indentation depths were measured by the aid of a laser scanner and residual stresses using X-ray diffraction technique. The effect of steel grade and treatment intensity on the induced compressive residual stress state was firstly studied experimentally. In addition, displacement controlled simulations were carried out to estimate the local residual stress condition considering the effect of different material models. Both the numerically estimated and experimentally measured residual stresses were qualitatively in good agreement. Residual stress state in S355 and S700MC can be estimated well using combined strain rate dependent material model. No significant effect of the treatment intensity is observed on the indentation depth and residual stress state for S355 grade steel. The indentation depth decreases with the increase in the yield strength of the steel.

Fatigue strength of welded joints under multiaxial non-proportional loading

Fatigue behaviour of a fillet-welded tubular T-joint in the so-called H structural component of an agricultural sprayer is examined by using experimental strain measurements found in the literature and linear elastic finite element analyses. Two stress-based critical plane criteria are applied at a verification point located near the intersection between the end of brace and chord of the above-mentioned T-joint, where crack initiation and growth is experimentally observed.

溶接継手の疲労強度に関わる数値シミュレーション技術の開発

溶接継手の疲労強度に関わる数値シミュレーション技術の開発

Asymptotic residual stress distribution induced by multipass welding processes

In the high cycle fatigue (HCF) regime, the fatigue strength of welded joints is influenced by residual stresses (RS) induced during welding processes. If the weld toe is modelled via a sharp V-notch, the distribution of weld toe residual stress can be shown to be asymptotic with a singularity which follows either the linear-elastic or elastic-plastic solution depending on parameters that range across material properties, specimen clamping conditions, and the welding process. For thicker plates, multipass welding is used instead of single-pass welding to reduce the heat input and hence the width of the fusion zone (FZ) and heat-affected zone (HAZ). However, for a fixed plate thickness, the magnitude and extent of residual stresses near the weld toe will vary according to the number of passes used in the welding operation.In this paper the influence of multipass welding on the residual stress field near the weld toe has been analysed using a combination of analytical and numerical techniques. It was found that the higher the number of weld passes, the higher the magnitude of the residual asymptotic stress field near the weld toe.

Simulation of residual stress and fatigue strength of welded joints under the effects of ultrasonic impact treatment (UIT)

Ultrasonic impact treatment (UIT) is a remarkable post-weld technique, which applies mechanical impacts in combination with ultrasonic oscillations into welded joints introducing plastic deformation and beneficial compressive residual stress in the region of the weld toe. In this study, a novel 3D numerical analysis approach including thermo-mechanical welding simulation and dynamic elastic-plastic FE analysis of ultrasonic impact treatment process for welded joints is proposed. In the FE model, the actual process parameters and ultrasonic-induced material softening, which is appropriately adjusted to fit experimental results, are considered. The predicted residual stress distributions of a non-load-carrying cruciform joint before and after UIT are compared with experimental results, showing a fairly good agreement with each other. In addition, an evaluation of fatigue strength of welded joints based on fracture mechanics is carried out. The results not only clearly distinguish the fatigue strengths of as-weld and UIT processed weld joints, but also show the effects of preloads and stress ratios, so that the proposed solution method may provide an effective tool to simulate UIT process in engineering structures.

Fatigue Life of Welded Joints of High-Strength Structural Steel S960QL

The article presents the results of research on low cycle fatigue strength of welded joints of structural steel S960QL. Two types of butt welds were analysed: I-joints and V-joints. The tests were performed under load controlled using the total strain amplitude εac. Fatigue life analysis was conducted based on the Manson-Coffin-Basquin equation, which made it possible to determine fatigue parameters. High concordance was found of the adopted description model with experimental results. Studies have shown differences in the fatigue life of the various joints analysed, wherein I-joints showed about 20-50% higher fatigue life. Fractographic tests of fatigue fractures in joints revealed the details of fatigue cracking and differences in the propagation rate of fatigue cracks.

Asymptotic residual stresses in butt-welded joints under fatigue loading

If a weld toe is modelled as a sharp V-notch angle, the stress distribution near the notch tip is singular. Its intensity can then be quantified by means of Notch Stress Intensity Factors (NSIF), which have been proven to be capable of summarising the high-cycle fatigue strength of welded joints having very different global and local geometries. In presence of a singular residual stress field near the weld toe, the local load ratio is modified making the fatigue strength sensitive to residual stresses in the high-cycle regime. However, for an accurate estimation of the fatigue performance of welded joints, it is necessary to consider not only the initial residual stress field but also its variation under load, as both of these may play an important role. In this work the effect of fatigue loading on the asymptotic residual stress redistribution near the weld toe of a butt-welded joint is studied by means of numerical simulations. A model is then proposed to estimate the influence of residual stresses on the fatigue strength of welded joints. Experimental results taken from the literature were found in good agreement with those predicted by the proposed model.

Fatigue strength of welded joints with adjacent attachments and its life extension by ICR treatment

Fatigue strength of welded joints with adjacent attachments and its life extension by ICR treatment

Review of the fatigue strength of welded joints based on the notch stress intensity factor and SED approaches

Several approaches exist for the fatigue strength assessment of welded joints. In addition to the traditional nominal stress approach, various approaches were developed using a local stress as fatigue parameter. In recent times, the N-SIF based approaches using the notch stress intensity at the weld toe or root have been developed. Based on this, the more practical strain energy density (SED) and the Peak Stress approaches were proposed. This paper reviews the proposed design S–N curves of the N-SIF and SED approaches questioning in particular the consideration of misalignment effects, which should be included on the load side of local approaches in order to consider them individually in different types of welded joints. A re-analysis of fatigue tests evaluated for the effective notch stress approach leads to slight changes of the design S–N curves and of the radius of the control volume used for averaging the SED at the notches. Further, on purpose fatigue tests of artificially notched specimens show that the fatigue assessment using a single-point fatigue parameter might be problematic because the crack propagation phase, being part of the fatigue life, is strongly affected by the stress distribution along the crack path that may vary considerably between different geometries and loading cases.

Evaluation of Fatigue Strength of Similar and Dissimilar Welded Joints of Modified 9Cr–1Mo Steel

This paper presents an evaluation method for the fatigue strength of similar and dissimilar welded joints of modified 9Cr–1Mo steel, which is a candidate structural material for demonstration of fast-breeder reactor being developed in Japan. The discontinuity of the mechanical properties across a welded joint causes a nonhomogeneous strain distribution, and this effect should be considered in the evaluation of the fatigue strength of welded joints. In this study, a “two-element model,” which comprises base metal and welded metal, was employed. First, the strain ranges of each element are calculated, and second, the fatigue lives of each element are evaluated. Finally, the shorter fatigue life is selected as the fatigue life of the welded joint. The failure position can be also estimated by this model. The evaluation results were compared with experimental data obtained at elevated temperature, and the results were in good agreement.

Fatigue assessment of high‐frequency mechanical impact (HFMI)‐treated welded joints subjected to high mean stresses and spectrum loading

AbstractThe fatigue strength of welded joints can be improved with various post‐weld treatment methods. High‐frequency mechanical impact treatment is a residual stress modification technique that creates compressive residual stresses at the weld toe. However, these beneficial residual stresses may relax under certain loading conditions. In this paper, previously published fatigue data for butt and fillet welded joints subjected to high stress ratios and variable amplitude cyclic stresses were evaluated in relation to the current International Institute of Welding (IIW) recommendations on fatigue strength improvement and a proposed IIW design guideline for high‐frequency mechanical impact‐treated welded joints. The evaluation showed that the current IIW recommendations resulted in both non‐conservative and overly conservative fatigue strength estimations depending on the applied stress level, whereas the proposed fatigue assessment guideline fitted the current data well.

Fatigue Strength Evaluation Approach of Welded Joints for More Reliable and Lighter Military Ground Vehicles

Due to increasing land / amphibious mobility and performance requirements, fatigue assessment plays a vital role in the design of armoured amphibious military ground vehicles. Fatigue assessment study can be divided into two main branches, which are the determination of vehicle loading spectrum and the determination of vehicle fatigue strength. FNSS Defense Systems initiated a project in order to further vehicle fatigue strength evaluation capabilities on the way to design more reliable and lighter vehicles. The project is called “Design and manufacturing methods improving fatigue strength of welded joints”, which is financially supported by TUBITAK. In this paper, the work accomplished and the results obtained in the framework of the project are presented. In this scope, fatigue test samples are manufactured from 5XXX series aluminium armour plate with the selected welded joints configurations. Post-weld fatigue improvement methods; TIG dressing, hammer peening and shot peening, are applied to test samples. Residual stress measurements on selected configurations are accomplished per X-Ray Diffraction technique to evaluate the effect of post-weld treatments on residual stresses, which is a major factor affecting fatigue life. Welding process is simulated with finite element method to be able to compute welding induced residual stresses. As input to simulations, temperature dependent thermo-mechanical and mechanical properties of utilized aluminium are determined on a forming dilatometer. Then as a crucial step in weld process simulation, weld process experiment is designed and conducted to calibrate welding process heat source and to verify simulation model. Finally, constant amplitude fatigue tests are conducted on an in-house designed and built four-point bending test machine. Test results are compared with the fatigue life calculated according to the IIW recommendations using various approaches and fatigue life computed with different techniques. As the outcomes of the project, FNSS weld class system is built highlighting the effect of post weld treatments, fatigue calculation procedures are enhanced and tuned accordingly, and substantial knowledge in weld process simulation is gained. Hence fatigue strength evaluation approach is evolved significantly.

荷重非伝達型十字溶接継手の疲労強度改善に及ぼす超音波衝撃処理(UIT)の影響に関する研究

Ultrasonic impact treatment (UIT) is a remarkable post-weld technique, which applies mechanical impacts in combination with ultrasound into welded joints and plastically deforms the weld toe, consequently introducing beneficial compressive residual stress near the treated area. Recent experimental studies demonstrate that acoustic softening of metals during ultrasonic impact (UI) may lead to increased plastic deformation and play an important role in the mechanism of UIT. In this study, a 3D simulation approach including thermo-mechanical welding simulation and dynamic elastic-plastic finite element (FE) analysis of UIT process of welded joints has been proposed. In the FE model, the actual process parameters and ultrasonic-induced material softening, which is appropriately adjusted to fit experimental results, are considered. The predicted residual stress distributions of non-load-carrying cruciform joints before and after UIT are compared with experimental results, showing a fairly good agreement with each other. In addition, an evaluation of fatigue strength of welded joints based on fracture mechanics has been carried out. The results not only clearly distinguish the difference of fatigue strengths of as -weld and UIT welds, but also show the influence of preload and stress ratio on the improvement, so that the proposed model may provide an effective tool to simulate UIT-process in engineering structures.

母材打撃ハンマーピーニングによる溶接継手の疲労強度向上機構

母材打撃ハンマーピーニングによる溶接継手の疲労強度向上機構

Fatigue Strength of HFMI-treated and Stress-relief Annealed High-strength Steel Weld Joints

The fatigue strength of welded joints is generally irrespective from the base material yield strength due to notch effects at the weld toe or root, tensile residual stresses by the thermo-mechanical joining process and significant changes in local material properties within the heat-affected zone. An effective way to increase the potential of light-weight design exhibits the application of post-weld treatments. Mechanical methods as the high-frequency mechanical impact treatment (HFMI) reduces the geometrical notch, introduces compressive residual stresses and increases the local hardness in the surface layer of the post-treated weld toe area.In this paper the fatigue strength of HFMI-treated joints superimposed by stress-relief annealing is studied. High-strength steel grades S690 and S960 are investigated in as-welded and HFMI-treated condition, both before and after stress-relief annealing. Fatigue tests at a tumescent stress ratio of R=0.1 show an essential increase of the fatigue strength due to the HFMI-treatment compared to the as-welded condition. In contrary, the superimposed post-heat treatment lowers the fatigue strength of HFMI-treated high-strength steel joints characteristically, but however, the final condition exhibits a higher fatigue strength compared to the initial as-welded state. Special focus is laid on the influence of welding distortion, whereat numerical simulations and experimental strain gauge measurements support the back tracing of the acting mechanism. The results indicate that the final local mean stress state due to clamping of the distorted specimens significantly affects the local effective stress ratio at the weld toe during testing, which basically needs to be considered for the fatigue strength evaluation.

State‐of‐the‐art review on the local strain energy density concept and its relation to the J‐integral and peak stress method

ABSTRACTThe local average strain energy density (SED) approach has been proposed and elaborated by Lazzarin for strength assessments in respect of brittle fracture and high‐cycle fatigue. Pointed and rounded (blunt) V‐notches subjected to tensile loading (mode 1) are primarily considered. The method is systematically extended to multiaxial conditions (mode 3, mixed modes 1 and 2). The application to brittle fracture is documented for PMMA flat bar specimens with pointed or rounded V‐notches inclusive of U‐notches. Results for other brittle materials (ceramics, PVC, duraluminum and graphite) are also recorded. The application to high‐cycle fatigue comprises fillet‐welded joints, weld‐like shaped and V‐notched base material specimens as well as round bar specimens with a V‐notch. The relation of the local SED concept to comparable other concepts is investigated, among them the Kitagawa, Taylor and Atzori–Lazzarin diagrams, the Neuber concept of fictitious notch rounding applied to welded joints and also the J‐integral approach. Alternative details of the local SED concept such as a semicircular control volume, microrounded notches and slit‐parallel loading are also mentioned. Coarse FE meshes at pointed or rounded notch tips are proven to be acceptable for accurate local SED evaluations. The peak stress method proposed by Meneghetti, which is based on a notch stress intensity factor consideration combined with a globally even coarse FE mesh and is used for the assessment of the fatigue strength of welded joints, is also presented.

橋梁用高降伏点鋼板(SBHS)を用いた溶接継手の疲労特性

橋梁用高降伏点鋼板(SBHS)を用いた溶接継手の疲労特性