Fatigue Resistance Of Welded Joints Research Articles

Effect of water cooling assistance on fatigue behavior of laser welded QP980 steel

Quenching and partitioning 980 (QP980) steel has been applied to new energy vehicles, but even laser welding joints show poor fatigue performance. In this study, water-cooling assistance is applied to improve fatigue resistance of welded joints. Hardness, tensile and fatigue tests are performed together with microstructure observation (scanning electron microscope, transmission electron microscope and electron back scattered diffraction). It is found that tempering martensite showing low hardness and small average grain boundary angles are responsible for poor fatigue performance. Results show that water-cooling assistance significantly improves welding softening and fatigue crack growth behavior of welded joints.

Advantages of an All-Welded Freight Car Bogie Side Frame

Purpose. Railway safety during the operation of freight cars largely depends on the reliability, fatigue resistance and durability of the cast bearing elements of the three-element bogie, especially the side frames. Premature failure of cast frames occurs mainly due to undetected latent defects in the foundry. To eliminate these problems, it is advisable to develop an alternative native all-welded side frame for a bogie with an axle load of 23.5 tnf, which can be interchangeable with the cast structure. Methodology. Mathematical modelling approaches were used to determine the stress-strain state of an all-welded structure under the influence of regulated loads and to assess its strength in accordance with current national and modern world standards. Findings. Prototypes of the all-welded side frame structure were subjected to accelerated fatigue tests. The tests have experimentally proved that the cyclic durability of the developed side frame is significantly (2...4 times) higher than the cyclic durability of cast side frames. Originality. A comparative analysis of the permissible stress amplitudes in different zones of the all-welded side frame during long-term operation based on 107 load cycles in accordance with the current standard for the strength of railway cars and modern approaches to determining the fatigue resistance of welded joints showed the insufficient conservatism of the national standard for assessing the fatigue strength of welded joints, which was taken into account when developing a new design of the all-welded side frame. Practical value. In addition to increasing reliability, possible increase in overhaul mileage and warranty period of side frames by ensuring a high level of fatigue resistance and durability, the welded structure can ensure the accuracy of the base frame size, reduce the weight of unsprung masses, which should reduce wheel wear and improve the running characteristics of railway carriage bogies. Reducing the cost of introducing all-welded side frames into mass production compared to the use of foundry technology for the manufacture of these structures and the fact that the cost of manufacturing an all-welded side frame is generally competitive with a cast structure is also one of the important advantages of our developme

Numerical analysis and discussion on the hot-spot stress concept applied to welded tubular KT joints

Nominal stresses have been used for a long time for the assessment of fatigue resistance of welded joints, however, this approach has strong limitations since the definition of the nominal stress may be subjective for complex welded details and/or complex loading. On the other hand, the hot-spot stress approach has been proposed to overcome these limitations considering the structural geometrical discontinuities.However, the hot-spot stress methods also present certain limitations, and the present study aims at evaluating the available numerical and analytical hot-spot stress methods proposed by DNVGL (2016) and IIW (2014). The particular case of an offshore tubular KT joint has been considered herein and discretized in two planes. It has been studied numerically using the ABAQUS software coupled with the hot-spot stress extrapolation methods described in IIW (2014) and DNVGL (2016). The influence of the weld geometry has been considered and evaluated. In addition to the numerical method, the present study has also considered the analytical approach proposed in DNVGL (2016) derived from the combination of Efthymiou solutions for the stress concentration factor with the method of superposition of stresses. The numerical models according to IIW (2014) have been found to be more conservative when compared with the mesh-size methods proposed by DNVGL (2016), both in numerical modelling without the weld or with weld. For the numerical models with weld cord, the mean values of normalized difference index obtained for all braces together, as a result of comparing numerical results with analytical solutions, are lower, when compared with results obtained from the numerical models without weld cord.

A simple fracture mechanics estimation of the fatigue endurance of welded joints

The use of fracture mechanics based methodologies to assess the influence of defects on welded components has become increasingly important for industrial applications and design, in the last decades. However, there are still objections against these approaches due to their complexity and the consequent need of qualified personnel with certain level of knowledge of fracture mechanics. Therefore, development of simplified methods to evaluate severity of defects can be valuable for practical purposes. In the present work, a manageable expression is proposed to calculate the applied stress intensity factor for crack growth, which is proportional to the square root of crack length and the stress concentration factor at the location of the crack in the un-cracked body. The latter was obtained with numerical simulation of transversely loaded butt, T and cruciform joints. By means of the resistance curve method that accounts for short crack behaviour, these results can be compared with threshold for crack propagation to estimate the fatigue resistance of welded joints. Besides, a simple expression was derived to estimate the fatigue endurance, which can be used to understand and analyse in a simplified manner the influence of the main geometrical and mechanical parameters, like weld geometry, residual stresses and size of defects, among others. Estimations were compared with experimental results from literature for different weld configurations, and they proved to be conservative. Methodology is valid for toe cracks in as-welded joints.

Fatigue performance and evaluation of welded joints in steel truss bridges

While welded joints are extensively used in the connections of steel truss bridges, service life of the bridges is largely dependent on the fatigue resistance of the welded joints. Stress concentration and weld residual stress are two primary causes of fatigue damage in the welded joints. In this study, corner-fillet profile (CFP) and ultrasonic impact treatment (UIT) are used to improve the fatigue performance of the welded joints through relieving stress concentration and weld residual stresses, respectively. The fatigue resistance of welded joints is evaluated through experimentation. The results indicate that the use of CFP and/or UIT can alter the initiation location of fatigue crack. The fatigue resistance of welded joints was increased by 24% and 36% by using the CFP and UIT, respectively. The fatigue performance of welded joints was evaluated using three different methods, including the nominal stress method, effective notch stress method, and peak stress method. The peak stress method with a single fatigue resistance curve demonstrated the highest applicability and accuracy.

Fatigue Resistance Improvement of Welded Joints by Bristle Roll-Brush Grinding

In the periodic repainting of steel bridges, often the steel surface has to be prepared by using power tools to remove surface contaminants, such as deteriorated paint film and rust, and to increase the adhesive strengths of the paint films to be applied newly. Surface preparation by bristle roll-brush grinding, which is a type of power tool, may additionally introduce compressive residual stress and increase the fatigue resistance of welded joints owing to the impact of rotating bristle tips. In this study, fatigue tests were conducted for longitudinally out-of-plane gusset fillet welded joints and transversely butt-welded joints to evaluate the effect of bristle roll-brush grinding prior to repainting on the fatigue resistance of the welded joints. The test results showed that bristle roll-brush grinding introduced compressive residual stress and significantly increased fatigue limits by over 50%.

Improvement of fatigue endurance of welded S355 J2 structural steel by severe shot peening

ABSTRACTThis work deals with the fatigue resistance of welded joints manufactured from EN S355 J2 structural steel and the possibility of improving fatigue characteristics by the severe shot peening (SSP) surface treatment. Fatigue testing was carried out by the rotating bending test on specimens manufactured from the base material (BM), welded material and welded material treated by SSP. Results have shown big scatter in obtained results from as-welded material and overall reduction of fatigue endurance when compared to the BM. SSP applied on the welded joints has reduced the scatter of experimental data and increased the fatigue strength. Obtained results of fatigue tests are compared, discussed and supported by correlation with results of additional experiments, e.g. metallographic analyses, microhardness tests, residual stresses measurements and surface roughness measurement.

Fitting fatigue test data with a novel S-N curve using frequentist and Bayesian inference

In design against fatigue, a lower bound stress range vs. endurance curve (S-N curve) is employed to characterize fatigue resistance of plain material and structural details. With respect to the inherent variability of the fatigue life, the S-N curve is related to a certain probability of exceedance, a percentile of the fatigue life. This paper is concerned with modelling and estimating uncertainties in fatigue resistance of welded joints under constant amplitude loading. A new S-N curve format is proposed and fitted to fatigue test data by using the Maximum Likelihood Method. The results have been compared with the Random Fatigue Limit Model and the Bilinear Random Fatigue Limit Model. The proposed S-N curve appears to be more accurate in describing the S-N relation in high-cycle fatigue: it presents a smooth transition from finite to infinite-life regions and, differently from previous non-linear S-N relations with fatigue limit, this transition is controlled by an independent model parameter. Thereby it provides more flexibility for statistical fitting. In addition, a Bayesian framework is defined to fit the proposed relation including informative and non-informative prior distributions.

State-of-the-art of methods for improvement of corrosion resistance and corrosion fatigue resistance of welded joints (Review)

State-of-the-art of methods for improvement of corrosion resistance and corrosion fatigue resistance of welded joints (Review)

The effect of compressive fatigue loads on fatigue strength of non-load carrying specimens subjected to ultrasonic impact treatment

This paper provides description and results from an investigation aimed to determine whether loading histories specific to marine and offshore structures cause shakedown of compressive residual stress produced by high-frequency mechanical impact (HFMI) treatment and therefore affect fatigue resistance of welded joints improved by these methods. Fatigue tests were carried out to investigate the influence of compressive fatigue loads on fatigue strength of non-load carrying specimens with transverse attachments subjected to ultrasonic impact treatment (UIT). It was found that significant relaxation of compressive residual stresses is achieved under application of compressive cycles in fatigue testing. It was also observed that not only the magnitude of the applied compressive stress/mean stress affects relaxation of the compressive residual stress but also the number of applied compressive cycles. This relaxation may reduce the beneficial effect of HFMI treatment under application of load histories specific to marine structures.

Analysis of low transformation temperature welding (LTTW) consumables – distortion control and evolution of stresses

Low transformation temperature welding (LTTW) consumables have been reported to reduce the tensile residual stresses in weldments. Martensitic transformation induces compressive residual stresses and improves the fatigue resistance of welded joints. Several of these LTTW consumables have been developed at the Colorado School of Mines. This research work presents the comparisons of the experimentally and Sysweld calculated measurements for distortions and residual stresses for different plate thicknesses. In addition, residual stress evolution with time graphs were plotted to determine the amount of martensite required to promote compressive residual stresses and to calculate the time required to induce compressive residual stresses. The main aspect of this research is to analyse the behaviour of LTTW consumables in terms of distortion and residual stresses on various plate thicknesses.

Effect of welding defects and the number of repair passes on the fatigue resistance of welded joints in aluminium alloys

The results of investigation of the effect of various defects of welded joints on the fatigue resistance of welded joints are presented and used to normalize the defects taking strength characteristics into account.

Analysis of the influence of defects on fatigue resistance of metallic components

The influence of defect on fatigue resistance of components is carried out by using a propagation threshold approach to estimate the high cycle fatigue resistance of components from data of intrinsic properties of the material. The Chapetti's threshold curve for fatigue crack propagation is used, which includes the short crack regime. The approach accounts for the effective driving force applied to the crack as the difference between the total driving force applied to the component, ΔK, and the threshold for fatigue crack propagation of the material, ΔK th . Application to analyze the influence of plate thickness on fatigue resistance of welded joints is presented.

Fatigue behavior prediction of welded joints by using an integrated fracture mechanics approach

Current fracture mechanics methods for fatigue assessment of welded joints are based on long crack behavior. The present work introduces a method to predict the fatigue strength of welded joints by means of an integrated fracture mechanics approach (IFMA) that takes into account the fatigue behavior of short cracks. This 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, function of crack length. Firstly, the proposed fracture mechanic method is introduced and compared with the traditional fracture mechanic approach, used mainly for fitness for purpose assessment of welded joints with cracks or other crack-like defects. Then, the method is used for several theoretical and parametric applications to show its ability to predict the influence of different mechanical, geometrical and microstructural parameters in the definition of the fatigue resistance of welded joints. The influence of plate thickness, initial crack length and reinforcement angle on fatigue strength of butt-welded joints has been analysed and results show good agreement with experimental trends. Finally, the method is applied to predict and analyze the fatigue behavior of butt welded and non-load-carrying transverse fillet welded joints, and estimated and experimental results are analysed and compared.

ANALYSIS OF THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF DUAL PHASE STEEL UNDER THE EFFECTS OF DIFFERENT BRAZING RATES

ABSTRACTEnvironmental, concerns regarding reducing CO2 emissions and the drive of having better fuel economy have already enthused the car manufacturer to use the weight materials having better mechanical properties. Automotive industry has shown a great interest in Dual Phase steels due to the possibility of reducing weight of vehicles and increasing the passenger safety at a very competitive cost. Automotive applications unavoidably entail welding and joining in the manufacturing process and the fatigue resistance of welded joints due to the integrity and safety requirements. The variation of welding parameters (voltage, current and speed of welding) affects weld performance, mechanical, and metallurgical properties.The CMT (Cold Metal Transfer) braze welding is a relatively new technology that partially decouples the arc electrical transients from the filler wire feed rate. It allows reducing the heat required for welding and permits higher joining speeds.The aim of this work is to study the interfacial microstructures and intermetallic compounds produced by cold metal transfer welding of two plates of galvanized DP600 dual phase steel with CuSi3 as filler metal. The study was performed by applying a CMT braze welding with three different joining speeds. The welded microstructures and microhardness were determined and related to the welding process conditions.A small HAZ, constituted by martensite, bainite and coarse ferrite grains, has been highlighted. Furthermore, an intermetallic Fe-Si-Cu compound layer formed at the interface between steel and filler metal. The joining speed sways the size of ZTA since the heat input Q affects the phase transformation in the weld and heat affected zoneThis parameter also affects the thickness of the compound layer and the size of precipitates in the filler metal, likewise the mechanical characteristics. The fracture starts at the interface steel-copper where intermetallic compounds formed.

Fatigue behavior of friction stir spot welding and riveted joints in an Al alloy

The main aim of this study was to compare the fatigue resistance of welded joints produced by FSSW process and riveted joints of AA2024 alloy. The specimens welded with the best preliminary parameters determined by previous tensile shear tests were tested in fatigue under load control, R=0.1, at room temperature. Two welding parameter sets were used, and P-N curves (load versus cycles) were plotted, using 2×106 cycles as the fatigue life limit. A similar curve was obtained for riveted specimens. The FSSW welding procedures were carried out in a CNC milling machining and the riveted specimens were produced in accordance with aircraft industry parameters. Although the welded specimens presented almost the same results in the tensile shear tests, the results were fairly lower than those observed for riveted joints in fatigue. The main failure mode observed in the welded joints was shearing, besides some cases of crack propagation in the perpendicular load direction, while for riveted specimens occurred mainly fretting nucleation followed by crack propagation in the perpendicular load direction. The evidences of shearing and lower fatigue lives for welded specimens indicate that the joint geometry highly affects the joint properties, due probably to stress concentrators presented locally.

Enhancement of the service life of welded metal structures by high-frequency mechanical peening

We generalize the results of investigations into the high-frequency mechanical peening of welded joints of structural steels of different strength grades and aluminum alloys. As a result, we show the high efficiency of using high-frequency mechanical peening for enhancing the fatigue resistance of welded joints of metal structures at the stages of their manufacture and operation.

Estimation of the Efficiency of Methods Aimed at Increasing the Fatigue Resistance of Welded Joints under Impact Loading at Low Temperatures

We analyze the experimental data on the influence of various types of hardening treatment (mechanical removal of reinforcement of the weld, argon-arc treatment, blasting, and ultrasonic surface cold working) on the fatigue resistance of welded joints of low-alloy steels under harmonic and impact loads at room and low (–60°C) temperatures. It is shown that the level of fatigue resistance increases after each type of treatment carried out under the same testing conditions. The comparison of the efficiency of the indicated types of treatment under impact loading at low temperature with the original state of the welded joint under harmonic loading at room temperature demonstrates that ultrasonic surface cold working is the most efficient procedure of hardening of welded structures operating at low climatic temperatures.

Mathematical model for calculating fatigue resistance of welded joints and base material of construction steel for single-stage loading. I: Unixial and compound loading: Kliemand, W. Schweisstechnik (Berlin) 1989 39, (12), 561–564 (in German)

Results of comparative load transfer measurements and fatigue tests on riveted lap joints from Alclad sheets of a configuration representative of the aircraft fuselage are presented. Two different rivet squeeze force values were accounted for and in either case riveted specimens with the interfaying Teflon foil between the sheets to minimize friction, in addition to standard riveted joints without such a layer, were considered. The goal of the investigation was to recognize implications of friction between the mating sheets for the fatigue behaviour of riveted lap joints in order to derive riveting process dependent coefficients to be included in a fatigue life prediction model for this type of connections.

Determination of the parameters of local heating for increasing the fatigue resistance of welded joints

1. The selection of the optimum parameters of local heating with gas torches makes it possible to induce compressive residual stresses of ≈0.5σT in the plates. To obtain the maximum possible magnitude of the compressive residual stresses, it is sufficient to heat the regions to comparatively low temperatures (400–500°C). 2. The maximum possible magnitude of the compressive residual stresses in axisymmetric heating of infinite plate does not exceed 0.6 of the yield stress of the given material. 3. The effect of external loading changes the fields of the residual stresses induced by local heating. In the range of external loads typical of fatigue of welded joints, the reduction of the maximum compressive residual stresses does not exceed 30% of their initial value.