Weld Fatigue Research Articles

Fatigue Reliability Assessment for Orthotropic Steel Decks Based on Long-Term Strain Monitoring.

A time-dependent fatigue reliability assessment approach is proposed for welded details of orthotropic steel decks (OSDs) using long-term strain monitoring data. The fatigue reliability limit function of the welded details is established based on the Eurocode specifications. Depending on the distribution characteristics of the measured daily equivalent stress range, either the lognormal distribution or Gaussian mixture model (GMM) is selected to quantify its uncertainty. Subsequently, the fatigue reliability can be calculated using either an explicit formula or the Monte Carlo method. This proposed approach is applied for the fatigue reliability evaluation of two rib-to-deck and two rib-to-rib welded fatigue details of an in-service suspension bridge. The results show that the reliability indices decrease significantly with bridge’s service life. Except for a rib-to-deck detail, all other three welded details cannot meet the target fatigue reliability during this bridge’s 100-year service life. The proposed approach can help bridge owners and operators make informed decisions regarding maintenance and repair of potential fatigue cracks.

Stresses and fatigue of laser welds

The paper deals with the specific properties of laser welds in fine-grained high-strength steels. The main topic, however, are residual stresses in laser welds, their impact on the mechanical properties and fatigue behaviour of such welds. As documented, fatigue cracks initiate mainly on micropores or other subsurface defects. From X-ray diffraction studies it follows that thermal expansion is primarily responsible for the longitudinal tensile stresses, whereas transverse compressive stresses are caused by phase transformations. The behaviour of transverse stresses also explains why fractures in laser welds occur completely outside the weld metal and the heat-affected zone.

Experimental study of weld fatigue strength reduction for a stainless steel piping component

An experimental mean curve and a design fatigue curve corresponding to 95% survival probability were derived from realistic fatigue experiments on a non-welded water pressurized piping component with primarily focus on high cycle fatigue. The components were subjected to a synthetic variable amplitude bending deformation. Comparison with the results obtained for a similar piping component with a circumferential butt weld allowed the determination of an experimental fatigue strength reduction factor. Comparison with the fatigue procedure and design curve in ASME BPVC Section III allowed to quantify its conservatism with regards to accounting for the presence of a weldment and more generally transferability.

Seam weld shell element model for thin walled structure FE fatigue design

In automotive industry, the FE fatigue analysis of mechanical structures made of steel thin walled parts and seam welded assemblies uses a dedicated technique based on shell element modelling for components and on 1D rigid elements for welds. This method has been validated with several intensive fatigue test campaigns using gas metal arc welded samples with different assemblies, for both bending and torsional loads. The fatigue results are relative to crack initiation at the weld toe with bending normal stress or with longitudinal shear stress for several load ratios. The purpose of the current work is the transposition of the initial method to another FE welded model. In a recent IIW guideline for the assessment of weld root fatigue, a shell element weld model has been proposed for seam weld fatigue assessment in case of weld root crack initiation. The idea is to analyse the possible extension of this FEA weld element model for weld toe fatigue analysis and several comparisons of stress results are detailed and discussed. Different fatigue criteria are used to verify the correlation with the fatigue test results. First, the approach is based on maximum shear stress, then structural stress is calculated and results are compared to IIW S/N curves.

Improvement of fatigue strength of lap fillet joints by using tandem MAG welding in a 590-MPa-grade galvannealed steel sheet

This investigation is for the lap joint of a 590-MPa-grade steel sheet. The effect of weld bead shape on weld fatigue strength using the tandem metal active gas (MAG) welding process was investigated by comparing with the direct current (DC) and DC pulse welding processes. For the same amount of deposited metal, the wire feeding speed was fixed at 8 m/min and the welding speed at 80 cm/min. The fatigue test was carried out at intervals of 10% from 10 to 60% of the tensile strength of the base material, and an S-N curve was obtained. In the DC and DC pulse processes, the fatigue strengths of the welded parts were 61 and 92 MPa, respectively. In the case of the tandem MAG process, the fatigue strength of the welded part was 122 MPa. The fatigue test was carried out at a frequency of 15 Hz and a load stress ratio of 0.1, with a fatigue life of 2 × 106 cycles. To investigate the cause of different fatigue strengths according to the welding process, the geometry of weld toe, microstructure, and hardness of the welds were checked. Because of the larger weld toe angle and toe radius of the tandem MAG welding process, the stress concentration in the toe part was reduced under applied repeated loads, and the fatigue strength and fatigue life were greater than those of the DC and DC pulse welding processes.

Microstructure and Mechanical Properties of Laser Welded Dual Phase and Mild Steel Joints for Automotive Applications

Recently, the laser welding technology of carbon steel is being widely used compared with arc welding technology for its better welding characteristics. In the present study, the influence of welding conditions of both laser beam welding (LBW) and gas metal arc welding (GMAW) as a comparative study on the weld joint microstructures, hardness distribution and fatigue properties crosses the butt-welded joints of dual phase (DP) steel and mild steel are investigated. The results show that LBW produced narrow welds with complete penetration while GMAW produces wide fusion and heat affected zones. The microstructure of the fusion zone of laser welded DP steels contains mainly bainite, martensite, and a few amount of acicular ferrite phases. Hardness values of the heat-affected zone (HAZ) for dual-phase (DP) steels showing lower values for both LBW and GMAW processes due to the tempering action of the martensite phase. A narrow softening region was clearly observed in the HAZ welded for LBW compared with GMAW. In general, the fatigue life of the welded joints is improved by using laser welding technique.

Minimum cost design of overhead crane beam with box section strengthened by CFRP laminates

An overhead travelling crane structure of two doubly symmetric welded box beams is designed for minimum cost. The rails are placed over the inner webs of box beams. The following design constraints are considered: local buckling of web and flange plates, fatigue of the butt K weld under rail and fatigue of fillet welds joining the transverse diaphragms to the box beams, fatigue of CFRP (carbon fibre reinforced plastic) laminate, deflection constraint. For the formulation of constraints the relatively new standard for cranes EN 13001-3-1 (2010) is used. To fulfill the deflection constraint CFRP strengthening should be used. The application of CFRP materials in strengthening of steel and concrete structures are widely used in civil engineering applications due to their unique advantages. In our study, we wanted to show how the mechanical properties of traditional materials can be improved by the application of composite materials and how advanced materials and new production technologies can be applied. In the optimization the following cost parts are considered: material, assembly and welding of the steel structure, material and fabrication cost of CFRP strengthening. The optimization is performed by systematic search using a MathCAD program.

Fatigue failure transition analysis in load‐carrying cruciform welded joints based on strain energy density approach

AbstractThis paper details a study of the application of notch stress intensity theory to the fatigue failure mode analysis of the transition in load‐carrying cruciform welded joints. The weldment fatigue crack initiation point is difficult to predict precisely because it usually occurs in the vicinity of the weld toe or weld root. To investigate the relationship between fatigue failure location and the geometry of the weldments, we analysed the weld toe and root asymptotic notch stress fields were analysed using the notch stress intensity factors on the basis of the Williams' solution in Linear Elastic Fracture Mechanics (LEFM). Numerous configurations of cruciform joints of various plate thicknesses, transverse plate thickness, weld sizes and incomplete penetration size were used to investigate the location of the fatigue failure. The strain energy density (SED) surrounding the notch tip was introduced to unify the scalar quantity and preclude the inconsistency of the dimensionality of the notch stress intensity factors for various notch opening angles. The results of the investigation showed that the SED approach can be used to determine the transition zone for a variety of joint geometries. The validity of the SED criteria was verified by comparing the experimental results of this study with the complied results for load‐carrying cruciform welded joints reported in literature.

Lightweight design with weld fatigue constraints for a three-axle bogie frame using sequential approximation optimisation method

A lightweight optimisation model with static strength and weld fatigue constraints is constructed for the welded frame of a three-axle bogie, and it is solved by an adaptive sequential approximation optimisation (SAO) method based on kriging models. To improve the accuracy of approximate functions on the basis of a small sample set, seven kriging models are used to construct the relationships between weld fatigue damage and plate thickness. The Master S-N Curve Approach is adopted for weld fatigue assessment considering its mesh-insensitivity and reliability. The weight of the bogie frame is reduced by 9.2% after optimisation. The optimisation results give a recommended design for the welded bogie frame and demonstrate the validity of the proposed optimisation method. The robustness analysis results for the recommended engineering design shows that the weld fatigue failure will not happen when the deviations of variables are considered.

Lightweight design with weld fatigue constraints for a three-axle bogie frame using sequential approximation optimisation method

Lightweight design with weld fatigue constraints for a three-axle bogie frame using sequential approximation optimisation method

Grinding treatment effect on rib-to-roof weld fatigue performance of steel bridge decks

To study the effect of grinding treatment on deck fatigue performance of U-rib and roof welds in steel bridge decks, eight specimens of U-rib and roof welds were manufactured. The shape and metallography of welded specimens were measured to ascertain their form, characteristics, and microstructure after grinding. The effects of weld grinding treatment on stress distribution and fatigue life were investigated by fatigue tests. The influence of grinding radius and depth on the local stress field was studied by finite element analysis, and suggestions for grinding treatment related to the gouge radius and depth were proposed. The results indicated that grinding treatment effectively increased the transitional radius of the welds and reduced the stress concentration thereon without changing the local microstructure. Local cracking resulted in stress redistribution with no change in the ultimate bearing capacity of the welds. Grinding treatment redistributed the stress on the U-rib to the deck weld, which improved the fatigue life thereof. Grinding treatment caused a reduction in the thickness of the deck, which resulted in an increase in bending stress. However, the stress concentration at the weld toe was diminished. The maximum stress was located at the bottom of the gouge after grinding and it decreased with increasing grinding gouge radius and increasing grinding gouge depth. The grinding treatment gouge sizes were suggested as 3mm (radius) and 0.5mm (depth).

Recent results on fatigue strength improvement of high-strength steel welded joints

Economic considerations push technology towards new lightweight structural solutions. This results in great interest in using high-strength steels (HSS) for structural applications like in railway vehicles, bridges, offshore structure and high-speed ships. In all these applications, welding is the main joining method and fatigue of welds is the major design criterion despite the existence of locally deteriorated microstructure, increased stress concentration and unfavourable tensile residual stresses after welding. Good weld design is a must to ensure the structural durability and performance, however this does not guarantee lightweight design alone. Special applications, such as post-weld treatment methods, can be performed following the welding. Therefore, this study concerns about the recent developments on such improvement techniques by considering two most-commonly used fatigue strength improvement methods; high frequency mechanical impact (HFMI) treatment and tungsten inert gas (TIG) dressing. Evaluations based on more than 1500 fatigue data points extracted from the literature. Investigations include presentation of the individual data analyses and fatigue strength assessment of all the data points by the effective notch stress approach with the reference radius rref=1.00mm. The influence of material strength, residual stress state, weld toe profile and loading conditions on the fatigue strength improvement are all discussed.

Vpliv parametrov varjenja in sredstva za spajanje na varjenje kompozitov

This paper presents an experimental investigation of the welding of a glass-fiber-reinforced PP composite. The goals of this paper are to investigate the issues of local changes of the welding strength that depend on the heating time and the coupling agent (MAPP). Composite samples were prepared by using an extruding (for mixing) process and a hot-press method. The PP matrix was reinforced by unidirectional short glass fibers. The welding process for the specimens was carried out using a non-contact heated tool butt welding process. Tensile and fatigue tests were conducted to investigate the effects of the heating time parameter and the coupling agent. The highest weld strength dependent on the heating time was achieved with 94 % relative to the base strength of the material. The fatigue behavior of short-fiber-reinforced thermoplastic composites (polypropylene/20 % of volume fractions of E-glass fiber) is presented in terms of stress versus the number of cycles to failure. The specimens were fatigue tested at various percentages of their static tensile strengths at a load ratio R = 0.1 and frequency f= 5 Hz. An indefinite fatigue life was obtained at 35 % of the static damage initiation load for glass-fiber-reinforced specimens. Then, these specimen’s maximum welding strengths and fatigue properties that were dependent on the heating time were compared.

Comparative study of fatigue and fracture in friction stir and electron beam welds of 24 mm thick titanium alloy Ti‐6Al‐4 V

AbstractIn this study, friction stir welding of Ti‐6Al‐4 V was demonstrated in 24 mm thickness material. The microstructure and mechanical properties, fatigue, fracture toughness and crack growth of these thick section friction stir welds were evaluated and compared with electron beam welds produced in the same thickness material. It was found that the friction stir welds possessed a relatively coarse lamellar alpha transformed beta microstructure because of slow cooling from above the transus temperature of the material. The electron beam welds had a fine acicular alpha structure as a result of rapid solidification. The friction stir welds possessed better ductility, fatigue life, fracture toughness and crack growth resistance than the base meal or electron beam welds. Thus, even though friction stir welding is a relatively new process, the performance benefits it offers for the fabrication of heavy gage primary structure make it a more attractive option than the more well‐established electron beam welding method.

고장력강판 적용 샤시부품의 용접부 내구수명 향상기술 개발

Abstract In general, discontinuity of metallurgical and structural points of weld zone could decline the fatigue strength.For the lightweight trend, the AHSS application in automotive chassis is in-progress. However, there are few research reports on AHSS welds fatigue strength in especially automotive chassis parts. Therefore, in this study, we evaluated the effects of the factors affecting the AHSS welding fatigue strength. As the result, the stress concentration of weld bead is the most important factor for welding fatigue strength. For the enhancement of welding fatigue strength, we focused on reducing the stress concentration of the welding beads. So, we applied and proved the plasma welding process and GTAW (Gas Tungsten Arc Welding) dressing method. It was verified by uniaxial fatigue specimen, fatigue performance increased from 40 to 60% by applying TIG dressing method compared to the conventional GMAW (Gas Metal Arc Welding). These results could be recommended the enhancement of fatigue performance of AHSS.Key Words : AHSS, Chassis System, Arc Welding, GMAW(MAG), GTAW(TIG), Butt Joint, Lap Joint, DurabilityEnhancement Technology, Post Treatment, Plasma Welding, GTAW (TIG) DressingISSN 1225-6153Online ISSN 2287-8955

A new innovative toolbox for lean welding of fatigue loaded structures

Welding is a key manufacturing technology in the production of heavy steel structures, but it is likewise a weak link in the production chain since fatigue fractures in welds is a common cause of failures. This paper proposes several changes in the process to make the manufacturing more efficient and to improve the fatigue properties. The idea is to adopt the weld quality demands for the purpose of the weld and to connect them to the welding procedures. This approach ensures that the primary focus during welding is at the critical characteristics which add value to the welded structure through an enhanced fatigue life. These fatigue life-critical properties have been found to be related to the local weld geometry in the weld toe and at the weld root. Traditional demands related to the good workmanship of welding can often be neglected, due to its limited effect to the fatigue life. The research presented in this paper has contributed to the development of welding procedures for improved fatigue life properties at the critical points of the weld. Results indicate a considerable potential for enhanced fatigue life of fillet welds. The idea is to replace the standard fillet welds with a new toolbox containing three different welds: (i) welds with optimized penetration, (ii) welds with optimized weld toe, and (iii) welds with a low cost. Right usage of these weld types contributes to an efficient production that offer a long fatigue life. This paper describes a holistic view of the subject and highlights issues with the traditional way of working. The challenge and the novelty in the paper are the connection between the welding process, weld demands, and fatigue life properties. This connection is necessary for the development of welding procedures that can contribute to the fabrication of weight optimized welded structures with a predictable life.

Fatigue life assessment of welded structures based on fracture mechanics

Purpose – Even though modern welding technology has improved, initial defects on weld notches cannot be avoided. Assuming the existence of crack-like flaws after the welding process, the stage of a fatigue crack nucleation becomes insignificant and the threshold for the initial crack propagation can be used as a criterion for very high cycle fatigue whereas crack growth analysis can be applied for the lifetime estimation at lower number of cycles. The purpose of this paper is to present a mechanism based approach for lifetime estimation of welded joints, subjected to a multiaxial non-proportional loading. Design/methodology/approach – The proposed method, which is based on the welding process simulation, thermophysical material modeling and fracture mechanics, considers the most important aspects for fatigue of welds. Applying worst-case assumptions, fatigue limits derived by the weight function method can be then used for the fatigue assessment of complex welded structures. Findings – An accurate mechanism based method for the fatigue life assessment of welded joints has been presented and validated. Originality/value – Compared to the fatigue limits provided by design codes, the proposed method offers more accurate lifetime estimation, a better understanding of interactions between welding process and fatigue behavior. It gives more possibilities to optimize the welding process specifically for the considered material, weld type and loading in order to achieve the full cost and weight optimization potential for industrial applications.

Fatigue Evaluation Procedures for Bi-axial Loaded Plate Joints Using the Battelle Structural Stress Method

Previously, Battelle has investigated the behaviour of a series of related multi-axial weld fatigue situations and developed effective analysis procedures. The first was a flange-to-tube joint under torsional-dominant loading. The second was a multi-axial stress state under uniaxial loading consisting of a tensile-loaded plate with either a fillet welded pipe or angled attachment. For multi-axial fatigue evaluation, an effective equivalent structural stress range (ΔEESS), defined as a combination of the equivalent normal structural stress range and the equivalent in-plane shear structural stress range in a von Mises form, derived from the Battelle structural stress method (BSSM), was formulated and used to clearly illustrate the differences in the behaviour of these two situations.As an extension of these multi-axial fatigue situations, the BSSM for fatigue life evaluation is examined herein for bi-axial loading of welded plate joints that otherwise require special treatment. However, when the BSSM is applied to bi-axially loaded plate joints, the failure location can be predicted correctly, and fatigue life predictions using the master S-N curve approach are very accurate compared with fatigue test results from the literature. This is because the BSSM captures the bi-axial loading effects, including the ratio of the load magnitudes and their phase difference that define the local variations of ΔEESS within the joint and thus define the failure location of the joint.In this investigation, it was found that the failure locations are influenced by the maximum normal structural stress (normal to weld line) not by the in-plane shear structural stress (parallel to weld line) which becomes negligible. In addition, it was also found that the cycles-to-failure data from the subject joint types were comparable with the master S-N curve for Mode I loading dominant behavior (inverse slope of 3.125). Therefore, the master S-N curve that was developed for Mode I failures can be equally applicable for fatigue life predictions for these plate joints under bi-axial loading.

Reduction of Overwelding and Distortion for Naval Surface Combatants, Part 1: Optimized Weld Sizing for Lightweight Ship Structures

As high-strength thin-steel use in ship design increases, dimensional management becomes critical to control construction costs and schedule in ship production. In the U.S. shipbuilding industry, improvements to shipbuilding facilities and processing technology have not kept pace with the rate of change in ship design. Additionally, new designs using thinner steels are subject to legacy weld-sizing criteria, which may inappropriately size welds on lightweight materials. These two factors result in widespread overwelding, causing severe plate buckling in naval vessels during construction. The problem of overwelding has two distinct sources:the weld-sizing methods developed in the 1980s are still used in most shipbuilding specifications regardless of ship class. This prevents the incentive of application of latest technologies that can make strong, precision fillet welds for modern lightweight thin steel naval surface combatants; andshipyard welders tend to make welds even larger than design requirements to satisfy naval production specifications, which do not allow for any undersized welds. On average, production welds are 3 mm larger than design, which can more than double the heat input and distortion caused by welding. The approach to the solution of this overwelding problem will be described in detail in this article:develop appropriate weld-sizing criteria for thin plate structures; this can be facilitated by numerical modeling to ensure adequate static shear, tensile, bending, fatigue, and dynamic impact capacity of structural welds; andperform a robust designed experiment to establish confidence that small weld sizes can provide necessary performance and strength to meet the design service life of the vessel and provide data to NAVSEA technical warrant holders to support the implementation of an underweld tolerance for ship production to prevent overwelding.

Study on the Correction of S-N Distribution in the Welding Fatigue Analysis Method Based on the Battelle Equivalent Structural Stress by Rough Set Theory

In welding techniques, failure is a key problem that is related to the stability and safety of the welded structure. In the commonly used welding fatigue analysis methods, the Master S-N curve method based on Battelle structural stress solves the problem of inconsistencies in stress calculation and S-N curve selection better than the nominal stress method and hot spot stress method. In this paper, rough set theory was employed to study the S-N distribution based on the Battelle equivalent structural stress. Firstly, rough set analyses of the S-N distribution based on the nominal stress, structure stress and Battelle equivalent structure stress were carried out. Then the S-N distributions based on the three stresses were studied. The results indicated that structural stress rearranges the S-N point making it much more concentrated in the region near the mean S-N curve and that equivalent structural stress places the S-N point more uniformly on both sides of the curve. Subsequently, the corrections from the plate thickness and stress ratio in the Master S-N curve method were studied. It was concluded that with two steps of corrections the decision-making degrees of welding factors are weakened and harmonized in the Battelle equivalent structural stress. This made the S-N distribution much more concentrated and uniform, which allows more accurate welding fatigue prediction by the Master S-N curve.