Fatigue Behavior Of Welded Joints Research Articles

Numerical simulation of low-cycle fatigue behavior of welded joints for naval applications: influence of residual stresses

A methodology is described and evaluated for the numerical prediction of the low-cycle fatigue strength of welded assemblies taking into account the effect of local residual stresses. The methodology involves (i) modeling the cyclic behavior of base metal and heat affected zone materials of welded joints, (ii) establishing a simplified method to analytically predict the local notch fatigue behavior, (iii) defining a crack initiation criterion including mean stress effect. Notched specimens are first designed to produce a first configuration without residual stresses, a second one with tensile residual stresses, and a third one with compressive residual stresses. Cyclic tests are performed under repeated tensile loading. These tests allowed describing the influence of the initial residual stress states on the fatigue life quantified and analyzed on the basis of experiments. The application of the simplified method managed to capture the impact of residual stresses on the local cyclic behavior and the crack initiation resistance. Finally, the fatigue life prediction method is applied on thick welded T-joints. The deep hole drilling and the contour method are used to determine residual stresses in the weld toe. The proposed modeling approach is finally evaluated and discussed by correlating numerical predictions with experimental results obtained on T-joints loaded in four-point bending fatigue.

Probabilistic S-N curves for constant and variable amplitude

Welded details in steel bridges are subjected to fatigue lives up to 200·106 cycles due to traffic loads. While the majority of these stress ranges are below the Constant Amplitude Fatigue Limit (CAFL), some few high stress cycles can trigger the start of fatigue damage and lead the remaining load spectra to become damaging. The behaviour under spectra loading is thus of major importance for the fatigue design of steel bridges. This paper focuses on the fatigue behaviour of welded joints under variable amplitude loads. Fatigue tests have been conducted under constant and variable amplitudes on a typical bridge detail. Experimental crack growth curves were obtained using the Alternative Current Potential Drop method (ACPD), which showed the detrimental effect of stress ranges below the conventional CAFL. A two-step model with initiation-propagation was established to estimate the experimental fatigue lives, using a local strain approach for the initiation life and fracture mechanics for crack propagation. The model was implemented in a probabilistic Monte Carlo framework to include variability on the main parameters and establish S-N curves for Constant and Variable amplitude. The results of the simulations show that load spectra shape can be correlated with the S-N curves, namely the 2nd slope value below the CAFL.

Investigation on low cycle fatigue of aluminium alloy welded joints

This paper presents an investigation on the low cycle fatigue behaviour of welded joints of Aluminium Alloy AW 1050 produced by tungsten inert gas welding and friction stir welding were investigated and compared with not welded specimens. The dependences of hysteresis loop width and accumulated plastic strains on the cycle number are presented and fatigue curves are obtained. The results of the low cycle fatigue experiments were used, for comparison and deciding the best sustaining technique of welding. DOI: http://dx.doi.org/10.5755/j01.mech.22.5.15304

Comparison of different statistical models for description of fatigue including very high cycle fatigue

In this paper the fatigue behaviour of welded joints and helical compression springs are analysed using two different statistical models. The data consist of results from low cycle, high cycle and very high cycle fatigue and different number of investigated specimens. In particular, the software program ProFatigue is used for derivation of the probabilistic S–N field from experimental fatigue data. The program, based on a former regression Weibull model, allows the estimation of the parameters involved in the S–N field model, providing an advantageous application of the stress based approaches in the fatigue design of mechanical components. The results obtained are compared with the customary Wöhler-curve, represented as a straight line in a double-logarithmic scale. Application to probabilistic assessment of cumulative damage and further program enhancement can be now envisaged.

Stress field and fatigue strength analysis of 135-degree sharp corners based on notch stress strength theory

Fatigue strength is known to be closely related to the precise geometrical discontinuity of the welded joints. It is necessary to better understand the fatigue behavior of welded joints with consideration of the geometrical factors that produce locally high stresses. Based on notch stress strength theory, this paper proposes a simple method using the singularity strength ‘as’ to estimate the stress field distribution at the corner. By comparing the expected results of 135° sheet corner with those obtained by N-SIF formula, the contribution of the singularity strength ‘as’ is quantified for different geometries, and summarized into concise expressions. Subsequently, the stress field parameters, namely the notch stress intensity factors (N-SIFs), are combined with ‘as’ formula to predict the fatigue strengths of cruciform weldments. Validity of this method is further verified by analyzing the fatigue test results for several cruciform welded joints reported in literatures. The results show that the proposed ‘as’ method has the additional advantage in simplified stress field and fatigue strength estimation for welded joints.

Fatigue behaviour of welded joints assembled by longitudinal corrugated plates

Fatigue is usually the cause for the cracks identified at bridge elements in service. With an increase in the introduction of corrugated steel web girders in recent highway bridge construction, the understanding of the fatigue behaviour of welded details in such structures becomes an important issue for the design. The typical welded details were represented as welded joints assembled by longitudinal corrugated plates. All the experiments were performed under fatigue loading using a servo-control testing machine. The test results from the failure mode observation with the aid of infrared thermo-graph technology show that the failure manner of these welded joints is comparable to that of the corrugated steel web beams reported previously. It is indicated from the stiffness degradation analysis that the welded joints with larger corrugation angle have higher stiffness and greater stiffness degradation in the notable stiffness degradation range. It is shown from the test S-N relations based on the free regression and forced regression analyses that there is a good linear dependence between lg(N) and lg(ΔS). It is also demonstrated that the proposed fracture mechanics analytical model is able to give a prediction slightly lower but on the safe side for the mean stresses at 2 million cycles of the test welded joints.

Recent developments and future challenges in fatigue strength assessment of welded joints

Fatigue is an important design criterion for welded structures subjected to cyclic loading. Several approaches for fatigue strength assessment have been developed which are either based on Woehler S–N curves and damage accumulation rule or on crack propagation law. The paper briefly reviews the different approaches, highlighting their advantages and limitations. In this connection, the problematic distinction between crack initiation and propagation phases is discussed, followed by considerations about some parameters which have large influence on the fatigue behaviour of welded joints but are considered differently in the approaches, such as plate thickness and stress gradient effects, multiaxial stress states, welding-induced distortions and residual stresses. Finally, ways of improving the fatigue behaviour of welded structures, either during design by reducing the stress concentration or during fabrication by improved quality or post-weld treatment or else by special material characteristics, are addressed. Emphasis will be placed on recent developments and challenges for the future from a personal perspective of the author.

Fatigue performance assessment of welded joints using the infrared thermography

Taking the superficial temperature increment as the major fatigue damage indicator, the infrared thermography was used to predict fatigue parameters (fatigue strength and S-N curve) of welded joints subjected to fatigue loading with a high mean stress, showing good predictions. The fatigue damage status, related to safety evaluation, was tightly correlated with the temperature field evolution of the hot-spot zone on the specimen surface. An energetic damage model, based on the energy accumulation, was developed to evaluate the residual fatigue life of the welded specimens undergoing cyclic loading, and a good agreement was presented. It is concluded that the infrared thermography can not only well predict the fatigue behavior of welded joints, but also can play an important role in health detection of structures subjected to mechanical loading.

Estimating the fatigue behavior of welded joints in the VHCF regime

PurposeThe present work aims to deal with a very high cycle fatigue (n=109 cycles) of gas metal arc welded joints, subjected to a multiaxial and non‐proportional loading. Different design codes and recommendations can greatly reduce the analysis effort in the design of welded structures providing a suitable balance between computational accuracy and ease of use for many industrial applications. However, various assumptions have to be made in a conservative way making this approach less accurate. This paper deals with a refined fatigue assessment, which considers the most important aspects for welded joints and provides an accurate lifetime prediction of welded structures.Design/methodology/approachFor an accurate prediction of the total lifetime of welded components the information about the material state and the welding induced residual stresses on weld toes is essential. If the surface condition after welding is poor in this area, which is usually the case, the presence of defects can be assumed and the fatigue crack nucleation process can be neglected. The microstructural threshold for initial crack propagation can be therefore used as a lower bound for the fatigue limit prediction.FindingsBased on the results from the simulation of a welding process and a post‐weld heat treatment in combination with a fracture mechanics approach, this work successfully attempts to reproduce a fatigue behavior, which was observed at the fatigue tests of the multi‐pass single bevel butt weld.Originality/valueThe proposed approach is able to predict accurately the fatigue strength of welded structures and to achieve the full cost and weight optimization potential for industrial applications.

Fatigue behaviors prediction method of welded joints based on soft computing methods

In allusion to the highly nonlinear, multivariable, strong coupling and interaction of various factors of the welded process, it is difficult to predict the fatigue behaviors of welded joints. We make use of the capabilities and advantages of rough set theory, ant colony algorithm and BP neural network, a novel fatigue behaviors prediction method (RST–ACO–BPNN) of welded joints based on RST, ACO and BPNN is proposed in this paper. The proposed RST–ACO–BPNN method utilizes the knowledge reduction ability of rough set theory for dealing with the original fatigue sample data, the minimum fatigue feature subset is obtained. The ant colony algorithm with the ability of strong global search was used to optimize the weights of BP neural network for obtaining the optimized BP neural network model. Then the minimum reduced subset was inputted into the optimized BP neural network model to construct the novel fatigue behaviors prediction model of welded joints by the continuous training and adjusting. To verify the correctness and validity of the novel RST–ACO–BPNN prediction model by applying in aluminum alloy welded joints. The simulation results show the proposed method can predict effectively the fatigue behaviors of aluminum alloy welded joints. And the RST–ACO–BPNN prediction method is provided with the merits of building model easily, simple structure, high precision and good generalization. Consequently, the prediction method can provide an effective approach to predict the fatigue behaviors of welded joints.

Very high cycle fatigue behavior of bridge steel welded joint

Very high cycle fatigue (VHCF) behaviors of bridge steel (Q345) welded joints were investigated using an ultrasonic fatigue test system at room temperature with a stress ratio R = −1. The results show that the fatigue strength of welded joints is dropped by an average of 60% comparing to the base metal and the fatigue failure still occurred beyond 107 cycles. The fatigue fracture of welded joints in the low cycle regime generally occurred at the solder while at the heat-affected zone (HAZ) in the very high cycle regime. The fatigue fracture surface was analyzed with scanning electron microscopy (SEM), showing welding defects such as pore, micro-crack and inclusion were the main factors on decreasing the fatigue properties of welded joints. The effect of welding defects on the fatigue behaviors of welded joints was discussed in terms of experimental results and finite element simulations.

Estimating the fatigue behaviour of welded joints

The present work is concerned with an attempt to predict the fatigue strength of welded joints by means of a fracture mechanics approach that takes into account the fatigue behaviour of short cracks. The methodology estimates the fatigue crack propagation rate as a function of the difference between the applied driving force and the material threshold for crack propagation, a function of crack length. The fatigue strength of butt-welded specimens stressed transversely was analyzed. Experimental results from the literature were used for comparisons. Good estimations are obtained by using only the fatigue limit and the fatigue propagation threshold for long cracks corresponding to the base metal, and the applied stress distribution along the crack path obtained from simple FE models. The influence of parameters like plate thickness, initial crack length and reinforcement angle on fatigue strength of butt-welded joints can be analyzed and results have shown good agreement with experimental results and trends. In order to verify the ability of the methodology to estimate the fatigue behaviour of welded joints, a dedicated experimental methodology had to be implemented for the detection and monitoring of the development of surface small cracks initiated at weld toes. Some results of a multi-strain gauges technique implemented for those purposes are also shown.

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

The conventional fatigue design codes was formulated based on the data from test stress cycles less than 1 × 10 7. Most of them could not be applied for welded structures serviced in the ultra-long life region so the new recommendations were discussed. The method of ultrasonic peening treatment (UPT) was introduced in this thesis as a post-weld treatment to improve the fatigue strength of the welded structures. The fatigue behavior of welded structures in the ultra-long life region was investigated using an ultrasonic fatigue test machine under two post-weld treatment states: the as-welded and the UPT. Discussion had also been made about the fracture mechanism on the UPT joints in ultra-long life fatigue test.

Fatigue behaviour of welded joints from magnesium alloy (AZ31) according to the local strain concept

AbstractIn the present study, the results of fatigue tests with the magnesium alloy AZ31 (ISO‐MgAl3Zn1) in the material states base metal, heat affected zone and weld metal obtained under strain control at room temperature within a range from 2·102 to 5·106 cycles are presented. The fatigue behaviour was characterized by the Coffin–Manson–Basquin equations and the stress – strain behaviour by the Ramberg–Osgood equation. The data can be used to assess welded magnesium joints according to the local strain concept.

Fatigue crack initiation in naval welded joints: experimental and numerical approaches

AbstractThis work deals with the fatigue behavior of welded joints. The proposed strategy can be split into two stages: the structure shakedown study and the fatigue crack initiation study. Firstly, stabilized elastic stress–strain cycles are obtained in any point of the welded joint by a finite element analysis, taking constant or variable amplitude loadings into account. The second part of this work is the calculation of the fatigue crack initiation period. Under the assumption of a fast elastic shakedown, a recent approach, proposed by Lemaitre et al., based on damage mechanics, can be used to predict the fatigue crack initiation life in a structure subjected to variable and complex loadings. This strategy is used as a post–treatment of the shakedown finite element calculation for the studied welded specimen. As a validation, four–point bending fatigue tests under constant amplitude loading were carried out on welded specimens provided by DCNS group. Good correlations between experimental and calculated fatigue crack initiation lives have been established. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of residual stresses on the fatigue behaviour of welded joints depending on loading conditions and weld geometry

The structural durability of welded structures is determined by the interaction of different influencing parameters such as loading mode, spectrum shape, residual stresses and weld geometry among others. Examples from plant, offshore, transportation and automotive engineering show how these parameters influence the fatigue life and to what extent they are considered in design codes. Especially, under spectrum loading, the stress decreasing effect of tensile residual stresses is not as high as under constant amplitude loading; this knowledge benefits light weight design. The overloads harmed only the low strength joints under pulsating bending. In all other cases investigated, with low, medium and high-strength steels, a significant decrease of fatigue life was not observed; on the contrary, significant improvement of fatigue life could even be observed. However, a systematic interaction with material strength, loading mode and residual stresses was not apparent.

Fatigue design of complex welded structures

This paper is concerned with the design of complex welded structures subjected to fatigue loadings. In the last years it has been developed a new approach for predicting the high cycle fatigue behaviour of welded joints, based on the Notch-Stress Intensity Factors of the uncracked geometries, which have been successfully applied to welded joints of different geometries using an analytical frame in combination with stress data obtained from Finite Element Analyses. The approach is based on the observation that the very small weld toe radius obtained by using traditional welding technologies is consistent with the assumption of a sharp V-notch geometry, characterised by an exponential stress field, which the fatigue strength depends on. Recently published Eurocodes suggest the use of a hot spot approach, both for steel and aluminium structures. Nevertheless the evaluation of fatigue strength of engineering complex structures remains a difficult and, in some way, controversial subject. We hope that this synthetic presentation of the N-SIF and hot spot approaches as compared to the European Standards could help to clarify the subject and to design better and more reliable structures.

SIZE EFFECT OF WELDED THIN-WALLED TUBULAR JOINTS

This paper clarifies the terminologies used to describe the size effect on fatigue behavior of welded joints. It summarizes the existing research on size effect in the perspective of newly defined terminologies. It identifies knowledge gaps in designing tubular joints using the hot spot stress method, i.e. thin-walled tubular joints with wall thickness less than 4 mm and thick-walled tubular joints with wall thickness larger than 50 mm, or diameter to thickness ratio less than 24. It is the thin-walled tubular joints that are addressed in this paper. It is found that thin-walled tube-plate T-joints do not follow the conventional trend: the thinner the section is, the higher the fatigue life. It is also found that simple extrapolation of existing fatigue design curves may result in unsafe design of thin-walled tube–tube T-joints. The effect of chord stiffness on fatigue behavior of thin-walled tubular T-joints is also discussed.

Local strain energy density and fatigue strength of welded joints under uniaxial and multiaxial loading

In the notch stress intensity approach to the fatigue assessment of welded joints, the weld toe is modelled as a sharp V-notch and the local stress distributions in plane problems are given on the basis of the relevant mode I and mode II notch stress intensity factors (N-SIFs). These factors quantify the magnitude of asymptotic stress distribution obeying Williams’ solution. If the V-notch opening angle at the weld toe is constant and the mode II is not singular, the mode I N-SIF can be directly used to summarize the fatigue behaviour of welded joints. In all the other cases, varying the V-notch angle or including multiaxial loading conditions (where typically both Mode I and Mode III stress distributions are singular), the synthesis can be carried out on the basis of the mean value of the strain energy density over a well-defined volume surrounding the weld toe or the weld root. By using this scalar quantity, two fatigue scatterbands are obtained for structural steels and aluminium alloys, respectively. The material-dependent radius R C of the control volume (area) is carefully identified with reference to conventional arc welding processes. Sometimes the weld toe radius is found to be very different from zero. The local strain energy approach can be extended as it stands also to these cases, providing a gradual transition from a N-SIF-based approach to a K t-based approach.

Corrosion Fatigue Behaviour of Ferritic Stainless Steel Welded Joints in 3%NaCl Aqueous Solution

Axial fatigue tests have been performed using welded specimens of a ferritic stainless steel, SUS 444, in 3%NaCl aqueous solution, and the effect of corrosive environment on fatigue behaviour was studied. The fatigue strength of the welded specimens in 3%NaCl aqueous solution decreased considerably compared with that in laboratory air. Fracture occurred at the toe of weld due to stress concentration, but fracture surface near crack initiation site was brittle, indicating enhanced small crack growth by 3%NaCl aqueous solution, thus in turn leading to decreased fatigue strength. In order to further understand the corrosion fatigue behaviour of welded joints, additional fatigue tests have been conducted using welded specimens whose reinforcement of weld was removed. The fatigue strength of the welded specimens without reinforcement of weld in 3%NaCl aqueous solution decreased significantly compared with in laboratory air. Fatigue cracks initiated from corrosion pits generated in HAZ near the boundary between the weld metal and HAZ at much earlier stage of fatigue cycling and then grew faster than in laboratory air. It was concluded, therefore, that the decreased fatigue strength in 3% NaC1 aqueous solution was attributed to corrosive enviromment-assisted crack initiation and growth.