Fatigue Of Welded Joints Research Articles

Fatigue and microstructure of welded joints of metal sheets for automotive exhaust system

Fillet or butt welded joints were prepared, so that the sheets of low carbon or stainless steels such as 18Cr–1Mo–Ti were welded by MIG or TIG method with the ferritic or austenitic welding wires, followed with or without heat treatment. The geometry of deposited zone, element distribution, crystal structure, microstructure, stress and fatigue of the joints, was tested with the tensile and fatigue tests, EPMA composition analysis, X-ray diffraction, microscopic examination, computer simulation etc.

Some new methods for predicting fatigue in welded joints

This paper is concerned with the prediction of high-cycle fatigue behaviour in welded joints. Recently, we have developed some new approaches for predicting the effects of notches and other stress concentrations which have been successfully applied to solid parts, using stress data obtained from FEA. In the present work we consider the extension of the same theories to cover welded joints. Two basic constants are needed to characterise fatigue in the weld — equivalent to the plain fatigue limit and crack-propagation threshold in conventional materials. These parameters were obtained experimentally and used to predict test results for two different joint geometries: a T-shaped fillet weld and a butt weld. Further predictions were made using data from the literature, showing the effect of reinforcement shape on butt welds and the fatigue strengths of non-load-carrying cruciform fillet welds. In all cases predictions fell within 20% of experimental values. The methods can be easily interfaced with conventional FE software; predictions can be achieved with relatively low mesh densities.

Multiaxial fatigue of welded joints under constant and variable amplitude loadings

Flange‐tube joints from fine grained steel StE 460 with unmachined welds were investigated under biaxial constant and variable amplitude loading (bending and torsion) in the range of 103 to 5 × 106 cycles to crack initiation and break‐through, respectively. In order not to interfere with residual stresses they were relieved by a heat treatment. In‐phase loading can be treated fairly well using the conventional hypotheses (von Mises or Tresca) on the basis of nominal, structural or local strains or stresses. But the influence of out‐of‐phase loading on fatigue life is severely overestimated if conventional hypotheses are used. However, the hypothesis of the effective equivalent stress that is introduced leads to fairly good predictions for constant as well as for random variable amplitude loads. Therefore, the knowledge of local strains or stresses is necessary. They are determined by boundary element analyses that are dependent on weld geometry. This hypothesis considers the fatigue‐life‐reducing influence of out‐of‐phase loading by taking into account the interaction of local shear stresses acting in different surface planes of the material. Further, size effects resulting from weld geometry and loading mode were included. Damage accumulation under a Gaussian spectrum can be assessed for in‐ and out‐of‐phase combined bending and torsion using an allowable damage sum of 0.35.

Feedback-feedforward control of offshore platforms under random waves

This study investigates the control of jacket-type offshore platforms. The deck displacement of jacket-type offshore platforms can be controlled using both passive and active control mechanisms. Among the passive control mechanisms, a tuned mass damper concept is studied in this paper. Active control mechanisms considered here include the active mass damper, the active tendon mechanism and the propeller thruster. An optimal frequency domain approach to active control of wave-excited platforms is used in which the H2 norm of the transfer function from the external disturbance to the regulated output is minimized. In this study, the hydrodynamic drag force is evaluated using the JONSWAP wave spectrum. Unlike conventional linearization approaches, the influence of non-linearity in the drag force is retained in this scheme by expressing the non-linear force components in terms of higher-order convolutions of the water-particle velocities. To demonstrate the effectiveness of this scheme, the platform performance with and without control devices under different sea states is evaluated. It is demonstrated that the control devices are useful in reducing the displacement response of jacket-type offshore platforms, especially when the wave forces are concentrated at frequencies close to the natural frequencies of the platform. This becomes especially significant in deep waters because the natural frequencies of jacket-type platforms fall closer to the dominant wave frequencies in deep waters. Adding control devices to deep water platforms will ensure a reduction both in the global response of the platform and in localized effects, such as the fatigue of welded joints. Copyright © 2001 John Wiley & Sons, Ltd.

Role of stress concentrations in welded joint fatigue

The different formalisms have been examined for calculating stress concentration factors, especially in welded joints, according to the radius at the weld toe and the connecting angle, and the results compared with those stemming from a finite element method. The fatigue notch factor is derived utilizing a description of the stress field at the welded joint tip. The applications of the notch stress intensity to a welding stitch, a welded reinforcement and a T welded joint are studied in detail and the values of the fatigue notch factor or the notch fatigue sensitivity index are presented.

The effect of weld geometry and residual stresses on the fatigue of welded joints under combined loading

A mathematical model is developed to predict the overall effect of the influencing weld geometry parameters such as (e.g. weld toe radius, weld toe undercut, flank angle, plate thickness, misalignment) and residual stresses on the fatigue strength and fatigue life of butt-welded joints subjected to combined loading (tensile and bending). The concepts of linear elastic fracture mechanics (LEFM), finite element analysis (FEA), dimensional analysis technique (DAT) and superposition approaches have been used for the modelling. It has been found that the co-influence effect of weld toe-undercut with other butt-weld geometry parameters is very significant. The reduction of fatigue life and fatigue strength, in comparison to that of flush-ground welded plate, caused by the introduction of a weld toe undercut is twice that for a welded joint without an undercut. The combined loading condition resulting from misalignment can significantly reduce the fatigue life and fatigue strength of butt joints by 10–60% subject to misalignment levels of 5–50% (ratio of axial eccentricity to plate thickness of 0.05–0.5), respectively. The effects of both compressive and tensile residual stresses are incorporated successful in the model. The mathematical model is satisfactorily verified by experimental data.

A PROBABILISTIC MODEL FOR INITIATION AND PROPAGATION OF SURFACE CRACKS IN WELDED JOINTS

Abstract Monte Carlo simulations of fatigue in welded joints have been performed using an approach which combines a model for initiation with a multiple surface crack propagation model. The results have been compared with experiments on T‐plate welded joints in which initiation and propagation of surface cracks were monitored using potential drop techniques. Predictions of initiation life using a local strain approach were conservative. Despite this underprediction of initiation life, predictions of total fatigue life were very good as a consequence of accurate simulations of propagation life and the fact that initiation represented on average only 12 to 22% of total life, depending on stress level. The initiation model considered variability due to the local weld toe angle and radius, and material strain–life behaviour. The only variability considered in the propagation analysis was the position and timing of initiation events, which leads to variability in coalescence. The underprediction of the variability in propagation and total lives was attributed to the underprediction of initiation life and the fact that out‐of‐plane coalescence effects were ignored.

Multiaxial fatigue of welded joints under in-phase and out-of-phase local strains and stresses

Welded cruciform-type specimens from fine-grained steel StE 290 and flange-tube as well as tube-tube joints from fine-grained steel StE 460 with unmachined and machined welds were investigated under biaxial constant-amplitude loading in the range 10 3 to 5 × 10 6 cycles to crack initiation and breakthrough respectively. In order not to interfere with residual stresses they were relieved by heat treatment. In-phase loading can be treated fairly well using the conventional hypotheses (von Mises or Tresca) on the basis of nominal, structural or local strains or stresses. But the influence of out-ofphase loading on fatigue life is severely overestimated if conventional hypotheses are used. However, the introduced hypotheses of the effective equivalent stress leads to fairly good predictions. For this, knowledge of local strains or stresses is necessary. They are determined by boundary-element analyses dependent on weld geometry. This hypothesis considers the influence of out-of-phase loading by taking into account the interaction of local shear stresses acting in different surface planes of the material. Further, size effects resulting from weld geometry and loading mode are included.

Fatigue of welded joints under narrowband non-Gaussian loadings

Analytical and experimental results are presented to determine the effect of non-normality of the loading process on the rate of fatigue damage accumulation in welded steel joints. All stochastic loadings used are narrowband and the parameter kurtosis is used to measure the degree of non-normality of the loading processes. Non-normal loadings are generated by passing Gaussian loadings through non-linear transformation functions. p] Results of the analytical study as well as results of fatigue experiments on welded cruciform specimens indicate that non-normality can significantly influence the rate of fatigue damage accumulation. Errors in fatigue life prediction can vary from slightly conservative to very unconservative by assuming a normal response when it is not. Results of random load experiments indicate that an endurance limit observed under constant amplitude tests does not convey to random load response.

Effect of specimen geometry on fatigue of welded joints: Guha, B. Eng. Fract. Mech. (Sept. 1993) 46 (1), 35–39

The magnitude and state of residual stress has a great influence on the fatigue life of sheet metal joints used in the automobile and aircraft industries. The present study developed a parametric 3D finite element model for simulation of welding and post-weld loading on the specimen. Electro-thermo-mechanical analysis is utilized to estimate the residual stress distribution at the nuggets and their surroundings at different stages of welding and post-weld mechanical loading. The results show that at the end of post-weld loading, the maximum tensile residual stress location moves nearer to the edge of the weld nugget and the residual stress magnitude and distribution change. Also, residual stress have sufficiently effects on stress distribution at the weld joint due to mechanical loading. To accurately estimate the fatigue life of resistance spot-welded joints, moreover external stresses, residual stress and local strain hardening of the material near the edge must be considered.

Effect of specimen geometry on fatigue of welded joints

Reports indicate that the joint geometry and loading configuration affect the crack growth behaviour of specimens. In an attempt to study the above effect, three specimen geometries are selected; cruciform joint under tension (CJT), side end notch under tension (SENT) and under three-point bending (SENB3) load. Results show that all three geometries have different fatigue strength and fracture mechanics parameters, although their weld-metal chemistry and mechanical properties are the same. Further tests are needed to establish this relationship quantitatively.

A multiple crack model for fatigue in welded joints

Several linear elastic fracture mechanics (LEFM) models have been developed to simulate the fatigue performance of welded joints, particularly for offshore structures. Such models are used to estimate residual life when cracks are discovered in service. These models are also used to interpret available experimental results and may eventually be used in design. A key element to the success of such models is the realistic modelling of fatigue crack shape development. In all fatigue testing of welded T-plate, pipe-plate and tubular joints in the Canadian Offshore Research Programme, care was taken to monitor crack shape development using beach marks and potential-drop techniques. Crack shape development was significantly influenced by specimen thickness, stress distribution and environment. A fatigue model is proposed which explicity models the growth and coalescenceof multiple semi-elliptic fatigue cracks. Such a model has the potential of modelling geometry and environmental effects through their influence on crack shape development. The basics of this model, including coalescencence and the calculation of stress intensity factors, are discussed. Verification studies involving both T-plate and pipe-plate specimen geometries are presented.

Nonproportional biaxial fatigue of welded joints: Siljander, O.A. Diss. Abstr. Int.52 7 (Jan 1992) 81 pp

Nonproportional biaxial fatigue of welded joints: Siljander, O.A. Diss. Abstr. Int.52 7 (Jan 1992) 81 pp

Fatigue Data for Reliability‐Based Offshore Platform Inspection and Repair

Fatigue of welded fixed offshore platforms is a primary design and operational concern. Environmental, fatigue data, and method uncertainties, as well as inspection and repair activities, are major parameters that influence the welded joint reliability. This paper investigates and establishes the relation between welded tubular joint fatigue life computed from a deterministic design process and the probability of occurrence of: (1) First detectable crack; (2) first through‐thickness cracking; and (3) total joint failure stages. Several experimental fatigue data bases were used to derive the appropriate probabilistic distributions and relations between different joint failure stages. The effect of repair methods, such as repair welding and grinding, on the fatigue life is studied, and parameters that modify the probability of joint failure states are introduced. The data examined show that time to first discernible surface cracking for welded tubular joints is about 10‐65% of the time to through‐thickness cracking, with a mean value of 35%. The total joint failure will occur at about 150% of the time to through‐thickness crack. Simplified relations between crack size and remaining fatigue life are derived based on recent experimental results. Data presented in this paper provide the basic tools for performing a reliability‐based inspection and repair assessment.

Influence of high-frequency components on fatigue of welded joints

Experimental results from welded steel specimens have been presented to determine the effect on fatigue life of various characteristics of the power spectral density (PSD) of the loading process. Particularly emphasized is the effect of high-frequency components in the PSD of the stress or strain in joints. The experimental results indicate that the addition of a high-frequency component can reduce the fatigue life substantially. Comparisons have been made between the experimental results and commonly used analytical procedures such as the Rayleigh approximation, rainflow analysis and the peak counting method. The experimental results are generally higher than the analytical predictions.

Corrosion fatigue of hot-galvanized component-like welded joints in a chloride medium under changing corrosion conditions : Sonsino, C.M.and Bottcher, H.-J.Werkst. Korros. June 1989 40, (6), 351–363 (in German)

Corrosion fatigue of hot-galvanized component-like welded joints in a chloride medium under changing corrosion conditions : Sonsino, C.M.and Bottcher, H.-J.Werkst. Korros. June 1989 40, (6), 351–363 (in German)

A model for variations in load on a simple soil-working implement

The variations in load on an agricultural cultivator tine are modelled by a Gaussian process, plus a marked point process which represents the effects of stones and other soil inclusions. Observed series of loads are split into the two components, from which parameter values are estimated. Good agreement is achieved between observed and simulated data, as measured by predicted fatigue damage using a welded joint fatigue analysis.

Fatigue crack growth in multi-pass butt-welded joints of mild steel

Fatigue crack growth rate properties obtained by testing multi-pass butt-welded joints in the through-the-thickness direction are presented along with a characterization of the mild steel base material. Edge-notched four-point-bending specimens are used to investigate R-ratio, specimen geometry and post-weld heat-treatment effects on fatigue crack growth rates. The pervasive influence of residual stresses on welded joint fatigue testing using the fracture mechanics approach is also discussed. For these multi-pass joints, conservative fatigue crack growth rates are obtained with post-weld heat-treated specimens.

Characteristics of low cycle fatigue and fracture toughness of welded joints of water turbine components: Gregor, V. and Polak, P. Zvaranie Feb. 1989 38, (12), 37–40 (in Slovak)

Design and performance analysis of hydraulic turbines are two very crucial aspects of any hydro power project which ensure economical and efficient functioning of these plants. In this context, numerical hydrodynamic analysis of water turbines has been bringing revolutionary changes in the process since the last decade. In the recent years, advancements in computational resources have made the job quite easier and more reliable but brought new challenges as well. Appropriate use of various models and numerical solution schemes in order to capture different hydrodynamic phenomena is highly debatable, even today, due to the lack of generality and robustness. This paper illustrates various utilities of computational fluid dynamics (CFD) in simulating hydraulic turbine operations. The work is mainly aimed at critically reviewing various computational methodologies for achieving different objectives of hydraulic design and performance evaluation of water turbines. Regarding this, different objectives of computational investigations of hydraulic turbines such as derivation of performance characteristics, analysis of various unsteady phenomena, prediction and analysis of cavitation and determination of various losses are discussed in detail. A discussion on boundary conditions and turbulence modeling for capturing different hydrodynamic phenomena at design and off-design operations of hydraulic turbines is one of the major scopes of the paper. Also, various optimization techniques in combination with CFD for hydraulic performance optimization of the turbines are briefly discussed. This work is a dedicated attempt to bring various aspects of computational hydrodynamic investigation of hydraulic turbines on a single platform. Having done the extensive literature review of the CFD utilities for the turbines along with disseminating the best practices, certain research gaps and loose ends are identified and given as future scopes which are also the major highlights of the work.

Low-cycle fatigue of welded joints in 10GN2MFA steel

Low-cycle fatigue of welded joints in 10GN2MFA steel