Fatigue Strength Of Welded Joints Research Articles

Development of an in situ alloying method for high-performance welding processes to achieve an LTT effect by local modification of the alloy content

One possible option for increasing the fatigue strength of welded joints is the use of so-called low transformation temperature (LTT) alloys. The aim is to introduce residual compressive stresses into the weld to counteract crack initiation and propagation. Until now, there has been no application of an LTT effect to high-performance welding processes such as the laser beam submerged arc hybrid welding process (LUPuS hybrid). First, the LUPuS hybrid single-wire process was further developed into the LUPuS tandem hybrid process. This makes it possible to equip the two submerged arc welding torches with different commercially available filler wires. The aim of the work is to further develop the LUPuS tandem hybrid welding process to enable the use of the LTT effect. The in situ alloying process for obtaining the LTT effect from commercially available material combinations was extended to the two-wire process. The alloy obtained was investigated by means of energy dispersive x-ray spectroscopy and hardness measurements and the influence on residual stresses was determined by the borehole method supported by electronic speckle pattern interferometry.

Fatigue strength assessment of aluminium welded joints under variable amplitude loading using the Peak Stress Method

One of the simplest and most efficient ways to design lightweight structural components is the combination of welding and aluminum alloys. However, welded joints are extremely sensitive to fatigue failure and making accurate lifetime predictions is still challenging when Variable Amplitude (VA) loading conditions are involved. Among all design criteria available in the literature, the present investigation focuses on the Peak Stress Method (PSM), an engineering finite element (FE)-based approach to rapidly assess the fatigue strength of welded joints. In more detail, the PSM suggests modelling both weld toe and weld root as sharp V-notches having null tip radius and correlates their fatigue strength using the intensity of the local linear elastic asymptotic stress distributions described by the Notch Stress Intensity Factors (NSIFs). The theoretical formulation of the PSM for the fatigue strength assessment of welded joints subjected to VA loadings has been recently proposed by combining its Constant Amplitude (CA) formulation with the Palmgren-Miner's cumulative linear damage rule. Such VA formulation has been successfully validated against a large bulk of experimental fatigue results generated by testing welded joints made of structural steels under uniaxial as well as multiaxial loadings. In the present investigation, the VA formulation of the PSM has been further validated against experimental data relevant to welded joints made of aluminium alloy under VA loadings.

Fatigue reliability assessment of load‐carrying cruciform welded joints with undercuts and misalignments

AbstractThe additional local stress concentration caused by undercut and misaligned defects could lead to lower fatigue strength of welded joints. The paper aims to investigate the effects of undercuts and misalignments on fatigue performance and reliability of Load‐carrying Cruciform Welded Joint (LCWJ) quantitatively by probabilistic statistics theory and fracture mechanics theory. Firstly, the geometrical characteristics of undercut and misalignment are summarized from the literature and our experiments. Subsequently, the probabilistic distributions of material fracture properties, the configurations of geometries, and flaws were evaluated under specific loading conditions. The fatigue strengths are further estimated by the probabilistic reliability theory by combining the actual fatigue data. The results demonstrate a clear disparity of fatigue crack growth threshold and resistances for the tested data of base metal and weldment. The level of defects determines the interval of fatigue life, and the larger the dispersion accompanying earlier fatigue failure occurs.

Overload and variable amplitude load effects on the fatigue strength of welded joints

Different load spectra and individual load peaks might substantially relax high residual stresses as well as induce compressive residual stresses in welded components and, consequently, affect the fatigue performance of these joints. Consideration of peak loads and resulting relaxation of residual stresses in fatigue analyses can substantially enhance the accuracy of life prediction. The aim of the current study is to experimentally investigate the fatigue strength of welded joints subjected to different levels of overloads and variable amplitude (VA) loads and to develop local fatigue assessment method to account for the relaxation of residual stresses via a mean stress correction using the 4R method. The 4R method applies a local stress ratio for the mean stress correction considering material strength, residual stresses, applied stress ratio of external loading and local weld geometry in elastic–plastic material behaviour. Fatigue tests were carried out on fillet-welded longitudinal gusset joints made of S700 high-strength steels under applied stress ratio R = 0–0.1. A mild strength steel (S355) and ultra-high-strength steel (S1100) were selected as reference steel grades for the fatigue testing to study the material strength effects. Numerical analyses were conducted to evaluate the fatigue notch factors using the effective notch stress concept with the reference radius of rref = 1.0 mm and theory of critical distance (TCD) using the point method. The experimental results indicated that a substantial improvement in the fatigue strength capacity can be claimed in the joints subjected to tensile overloads, particularly in the studied S700 and S1100 steels. The higher-level overload (0.8fy), corresponding to the nominal cross-sectional area, improved the mean fatigue strength of the welded joints manufactured of high-strength S700 steel by approximately 60%, while the lower overload (0.6fy) improved the mean fatigue strength by 20%. In addition, a use of equivalent nominal stresses for the joints subjected to VA loads resulted in conservative assessments when employing S–N curves obtained for the CA loading. The 4R method, via the local mean stress correction for individual cycles, provided higher accuracy for the fatigue assessments.

A novel elastic strain energy density approach for fatigue evaluation of welded components

The averaged elastic strain energy density (SED) within a characteristic volume radius R has been investigated and applied for characterizing the fatigue strength of welded joints for decades. However, engineering applications are still relatively rare. The main reason is that the calculation process is cumbersome due to fine mesh size and circular pattern requirements. This study proposes a novel approach of SED without requiring special finite element size and special layout requirements. First, a novel approach of the notch stress solution is presented. As a result, the present solution differs from the conventional approach which has been focused on notch tip singular stress field. Then, an averaged strain energy density can be obtained analytically by taking advantage of the new notch stress solutions presented in this study. Both the closed-form notch stress solutions and resulting expression for average strain energy density have been validated by FEA results. Subsequently, a large number of fatigue tests published in the literature are selected to examine the effectiveness of the evaluation approach of SED on interpreting fatigue behaviors of welded joints. Finally, a procedure for practical engineering application is established and verified by a group of fatigue tests of 3D gusset welded components.

Microstructural aspects of fatigue strength of welded joints of medium carbon steels produced by rotary friction welding

The paper studies the resistance to fatigue failure of welded joints of steels 32Mn2-40CrNi2Mo, intended for the production of drill pipes for geological exploration. The connection of pipe billets for the purpose of manufacturing experimental samples was carried out by means of rotational friction welding at various process parameters. The assessment of resistance to fatigue loads was carried out on solid cylindrical samples with a welded joint under conditions of bending with rotation, which made it possible to identify the weakest zone in which the initiation and development of a fatigue crack occurred. The microstructure was studied using optical and electron scanning microscopy using the electron backscatter diffraction (EBSD) method. In the course of research, it was found that the greatest resistance to fatigue loads is provided due to the intensive development of strain hardening processes in the thermomechanical affected zone of welded joints, which depend on the parameters of rotary friction welding.

A novel process for improving fatigue properties of TIG welded AZ31B mg alloy joints by laser bionic treatment

It is crucial to obtain better fatigue life for the safe application of materials, and nature provides a new idea for effectively improving fatigue property. A ‘soft-hard’ alternating structure imitated biological morphology was fabricated on the surface of welded joints by laser bionic treatment (LBT) for improving the fatigue property of TIG welded AZ31B Mg alloy joints. After LBT, the grains in the weld zone were refined with the average grain size about 5.55 μm and decreased by 76.1%, resulting in an increase of hardness of 12.0% to about 60.12 HV0.1, and formed a ‘soft-hard’ structure on the surface of welded joints. By adjusting the distribution angle of laser bionic stripe, the high cycle fatigue strength of welded joints was increased by 129.4% at most. The fracture morphology of welded joints was changed by LBT. The weld zone of bionic welded joints was deformed by twinning and slip, while the laser-treated zone was only deformed by slip. The mechanisms of strengthening fatigue properties were discussed based on the microstructure and structural characteristics from EBSD analyze.

Fatigue strength of shot-peened as-welded joints and post-weld-treated and subsequently clean-blasted fillet weld joints

Surface treatment methods, such as shot peening and clean blasting, can improve the fatigue strength of welded joints. The aim of the current work is to experimentally evaluate the effects of clean blasting on the fatigue performance of post-weld-treated joints made of normal, high-strength, and ultra-high-strength steels, as well as obtain the fatigue strength improvement gained by the shot peening processes in welded connections made of different steel grades using literature data. A literature review was carried out to extract fatigue test data of welded joints, considering both clean blasting and shot peening processes. This data was statistically re-evaluated and used for a comparison to the results obtained in the current experimental work which focuses on evaluating the improvement gained by the clean blasting after post-weld treatments. Experimental fatigue tests were carried out on non-load-carrying transverse attachment joints prepared with gas metal arc welding, and made of S355, S700, and S1100 structural steels. TIG dressing and HFMI treatment post-weld treatments were implemented and, subsequently, the specimens were blast-cleaned using two different abrasives: corundum and sand. The re-analysis of existing fatigue test data indicated higher improvement by shot peening, i.e., average improvements of 1.71 and 1.52 in the fatigue strength for butt weld and fillet weld joints, respectively, than by clean blasting process for which an average improvement of 1.19 in fatigue strength was obtained. The enhancement factors, however, highly varied among different data sets indicating a clear impact of processing parameters on the improvement level. The statistical re-analyses considering all data sets of shot-peened specimens (butt weld and fillet weld joints) showed that one fatigue (FAT) class higher fatigue strength could be recommended for shot-peened joints compared to the as-welded condition with weld toe failures. The experimental work on the post-weld-treated joints indicated that the fatigue strength of clean-blasted joints was similar to that of non-blasted joints, and thus showing no major advantages or disadvantages by the clean blasting post-weld-treated joints with corundum and sand.

Усталостная прочность сварных соединений сталей 30ХГСА–40ХМФА, полученных ротационной сваркой трением

Rotary friction welding (RFW) is used in the production of drill pipes for solid mineral prospecting. The need for the creation of the lightened drill strings for high-speed diamond drilling of ultradeep wells dictates the necessity of a greater focus on the study of a weld zone and setting the RFW technological parameters. This paper presents the results of experimental studies of a welded joint of a drill pipe of the H standard size according to ISO 10097, made of the 30ХГСА (pipe body) and 40ХМФА (tool joint) steels under the cyclic loads. The authors evaluated the influence of the force applied to the workpieces in the process of friction of the contacting surfaces (force during heating), and postweld tempering at a temperature of 550 °С on the cyclic life of welded joints, under the conditions of alternate tension-compression at the cycle amplitude stress of ±420 MPa. The study determined that with an increase in the force during heating, the microstructure changes occur in the zone of thermomechanical influence, contributing to an increase in the fatigue strength of welded joints. The authors identified the negative effect of postweld tempering on the fatigue strength of welded joints, which is expressed in the decrease in the number of cycles before failure by 15–40 %, depending on the magnitude of the force during heating. The optimal RFW mode of the specified combination of steels is determined, which provides the largest number of cycles before failure: the force during heating (at friction) Fh=120 kN, forging force Ffor=160 kN, rotational frequency during heating n=800 Rpm, and upset during heating l=8 mm. A series of fatigue tests have been carried out at various values of the cycle amplitude stress of the welded joint produced at the optimal mode and the 30ХГСА steel base metal; limited endurance curves have been plotted. It is shown that the differences in the limited endurance curves of the pipe body material (30ХГСА steel) and the welded joint are insignificant. The obtained results are supplemented by the microhardness measurement data and fractographs of fractured samples, revealing the mechanism of crack propagation under the cyclic loads.

Implementation of a nominal stress approach for the fatigue assessment of aluminium naval ships

Fatigue analysis is an important part of the structural design of weight-optimised naval ships, because they are often constructed from aluminium alloys or high tensile steel. Due to the low fatigue strength of these metals, the welded joints are vulnerable to cracking. Furthermore, naval ships can be required to operate in demanding environments, and remain in service longer than assumed during design. Thus, ship designers and in-service managers require efficient fatigue evaluation approaches. One industry-accepted and relatively quick method, documented in design codes, to assess the fatigue strength of welded joints is the nominal stress or S-N curve approach. However, there are uncertainties associated with using a design code; if all of the conditions for its applicability are not met, the analyst must reliably interpret the code. Further, different specifications for the use of S-N curves leads to different fatigue life estimates. In this paper, a refinement of the nominal stress approach for joints, typical of aluminium welded ship details, is proposed. The goal is to inject rigour and practicality into the fatigue analysis of naval ships. The refinement process leverages both in-service maintenance information and long-term strain measurements of a 56 m aluminium patrol boat. The sensitivity of the predicted fatigue life of welded details to the choice of S-N curve, available from a structural design code, and stress parameter extraction is investigated. Finally, recommendations for future work are provided.

EXPERIMENTAL STUDIES OF THE STRENGTH OF A LONG BASE FLAT WAGONS BY CARRYING OUT ENDURANCE CYCLIC FATIGUE TESTS

The need for experimental validation of structural solutions concerning the frame of long-base flat wagon and assessment of the accuracy of theoretical calculations by carrying out resource cyclic fatigue tests based on the results of the recent studies analysis in the research of the strength of long-base flat wagons, was established. The methodology for carrying out resource cyclic fatigue tests of a long-base flat wagon is presented. The results of theoretical studies on determining the strength of a long-base flat wagon are described. The results of experimental studies of endurance cyclic fatigue tests of the frame of a long-wheelbase flat wagon before and after structural improvement are shown. Structural changes and improvements of the frame of the long-base flat wagon at the stage of experimental endurance cyclic fatigue tests are presented. The place of initiation and development of a crack in the structure of the frame of a long-base flat wagon is shown. Based on the results of the research, the actual strength of the elements of the wagon structure was determined and the design of the long-base flat wagon was chosen in order to prevent fatigue failure and ensure the required operational safety. The need to improve the evaluation methods of fatigue strength of welded joints of the long-wheelbase flat wagons frames at the design stage has been proven. The content of the article will contribute to the construction of new models of long-base flat wagons for mainline railway and the modernization of existing models, as well as to increase their strength and service life while in operation.

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

Influence of the geometry on the fatigue strength of welded joints using the effective notch stress approach

Using the example of a cruciform joint, this study enquires into the relationship between the geometry of weld (the wall thickness of base metal plates, thickness of weld and weld penetration depth) and the fatigue strength of weld joint using the finite element method. It is assumed that the effective notch stress approach can be used to determine a FAT class for dimensioning the connection with the nominal stress approach. Filled throat thickness and weld penetration depth or the geometry of weld root were found to have a strong influence on the fatigue strength of a welded joint. When using the effective notch stress approach, the exact geometry of the calculated weld seam must be known and should be modelled accordingly. It was also found that with thin-walled welds, the effect of wall thickness produces a significant deviation between the load capacity calculated with the effective notch stress approach (effect of wall thickness up to 100%), and with the nominal stress approach (effect of wall thickness max. 10%). At the end, a proposal will be made on how thin-walled welds can be dimensioned to facilitate the use of the same thickness factor for both the notch and nominal stress approaches.

Experimental and Numerical Investigations of the Deep Rolling Process to Analyze the Local Deformation Behavior of Welded Joints

Welded joints show a comparably low fatigue strength compared to the base material. Thus, different post-weld treatment methods are used to enhance the fatigue strength of welded joints. A promising method to enhance the fatigue strength of metallic components is the deep rolling process, but this has rarely been applied to welds. For the qualification of the deep rolling process as an effective post-weld treatment method, knowledge about its influence on the surface and subsurface properties at the fatigue critical weld toe is necessary. Here, geometrical and metallurgical inhomogeneities lead to complex contact states between deep rolling tools and weld toes. Thus, for a first analysis of the local deformation behavior during deep rolling of welded joints, experimentally and numerically generated deep rolling single tracks are compared. Cyclic strain-controlled tests to determine the material behavior were carried out for the numerical analyses using finite element simulation. The presented study shows that it is possible to describe the local deformation of welded joints during deep rolling using finite element simulation. A correct depiction of material behavior is crucial for such an analysis. It was shown that certain irregularities in material behavior lead to lower coincidences between simulation and experiment, especially for the investigated welds, where only low differences in hardness between base material, heat-affected zone, and filler material were found.

Fatigue strength improvement due to alloying steel weld toes with WC tool constituent elements through friction stir processing

Friction stir processing (FSP) enables surface modifications can likely be applied as a new post-weld treatment for improving fatigue strength. When applying FSP to high-strength materials, tool wear occurring at the interface between the tool tip and the topmost steel layer has been regarded as an unavoidable issue and is related to the tool rotational speed. The present study investigated the relationship between the tool rotational speed and fatigue strength of arc-welded high-strength low-alloy (HSLA) steel joints with weld toes subjected to FSP using a spherical-tip WC tool. FSP was conducted on the weld toe of HSLA steel joints with various tool rotational speeds. Tool wear increased with increase in tool rotational speed, and consequently contents of constituent elements of the WC tool increased in the topmost steel layer of weld toes, leading to large increase in fatigue strength. One reason for the increase with tool rotational speed is significant increase in solid solution hardening due to supersaturated W and C in the topmost steel layer consisting of martensite laths. The hardened topmost steel layer prevented fatigue crack initiation, and the increased fatigue strength depended on the contents of supersaturated W and C. Alloying of the topmost steel layer with tool constituent elements of W and C accompanied with WC tool wear during FSP is unique additive manufacturing to increase the fatigue strength of welded joints, and can be employed locally on structural components susceptible to fatigue.

Post-weld cold working for fatigue strength improvement of resistance spot welded joint of advanced high-strength steel

Advanced high-strength steel (AHSS) has been widely used for lightweight body-in-white. However, the fatigue strength of welded joints is not improved even employed AHSS. In this research, the advanced post-weld cold working (PWCW) experiment was proposed to improve the fatigue strength of DP980 spot welded joints. X-ray diffraction (XRD) method was employed to measure the welding residual stress. A numerical model was established to analyze the PWCW induced residual stress as the first try. The influence of the diameter of the cylindric tool used in PWCW on the residual stress of welded joints was also discussed in detail. The hardness distribution on weld joint was increased after PWCW, which represented the plastic strain induced work hardening in DP980 steel. The predicted PWCW induced residual stress had a reasonable agreement with XRD measurement. The PWCW introduced a compressive stress in the weld nugget and its heat affected zone with a value of more than -400 MPa at the edge of joined zone. The strengthened material and compressive stress were beneficial to suppress the generation and propagation of fatigue crack, thereby enhance the fatigue strength of welded joints. Besides, the effect of PWCW continuously strengthened with a larger diameter of tool, especially when the tool diameter was larger than 10 mm. The fatigue strength was almost doubled compared with the as-welded condition. The smaller tool provided a critical requirement for precise positioning. The operation of PWCW was also simple enough to realize the automation, which indicated its potential in manufacturing automotive units.

Post-treatment of welding joints of high strength steels I.: Improving weld geometry (Overview)

The International Institute of Welding (IIW) published a recommendation on methods for improving the fatigue strength of welded joints in 2013, valid for maximum specified yield strength of 900 MPa. In this time was consider that by using these methods is reasonable to expect that fatigue performance of welded higher strength will also improve. Since the publication several studies investigate different improvement techniques effects on high strength steels fatigue strength. The aim of this paper is to review post treatment techniques by improving the weld geometry consider the application of these techniques in high-strength steels. The post treatment techniques by residual stress methods will be reviewed in the second part of our article.

On the fatigue strength calculation of the welded shell structures from high-strength steels under low-cycle loading. Part 1: Estimation at the initial stage of fatigue failure

The normative methods for calculating the fatigue strength of welded joints are of limited use for low-cycle loads, as they do not take into account the possible variation in the asymmetry of the operating stress cycle, differences in the expected level of residual stresses, and the possible variety of joint geometry. Estimation procedures have been developed for shell structures made of high-strength steels subjected to external and internal pressure. They were based on experimental data on the resistance to fatigue fracture, physical modeling of individual stages of fatigue damage, and generalization of the results of numerical studies of the FEM of the stress-strain state.

Comparison of local fatigue assessment methods for high-quality butt-welded joints made of high-strength steel

Fatigue strength of welded joints is typically independent of the parent material static strength; however, there are two exceptions from this rule—post-weld treated joints and high-quality joints. The reason for both is the absence of sharp weld transitions. Thus, fatigue life is not fully governed by fatigue crack propagation but also by fatigue crack initiation. This is intensified by the fact that crack initiation behaviour is proportional to static strength properties. Commonly, this effect is not accounted for by design guidelines and typical fatigue assessment methods for welded joints. This study hence investigates the applicability and accuracy of different local fatigue assessment methods for high-quality welded joints made of high-strength steel. For this purpose, fatigue test results of S500 structural steel joints with weld toe and weld root failure are assessed using a variety of local approaches and then compared to the nominal stress approach. Finally, the transferability of results to large-scale structures are discussed and recommendations are given for practical applications.

Statistical analysis of sub-zero temperature effects on fatigue strength of welded joints

Ships and offshore structures in Arctic environments are exposed to severe environmental actions and sub-zero temperatures. Thus, the design of such structures has to account for the Arctic environment and must be cost-efficient at the same time. A vital part of the design process is to ensure that fatigue-induced failure does not occur in the lifetime of the structure. While effects of high temperatures on material behavior are well covered in international standards and guidelines, there is no comprehensive guidance for sub-zero temperature fatigue strength assessment. Additionally, stress-life (S–N) test data of welded joints at sub-zero temperatures is particularly scarce. Hence, this study presents an extensive review of recent test results of various weld details tested in the range of − 50 to 20 °C. This data could build the basis for future considerations of temperature effects in fatigue design guidelines and recommendations. For this purpose, the fatigue test results are submitted to a rigorous statistically assessment—including a summary of the limitations of current design guidelines with respect to sub-zero temperature effects.