Fatigue Strength Of Welded Joints Research Articles

State‐of‐the‐art review on the local strain energy density concept and its relation to the J‐integral and peak stress method

ABSTRACTThe local average strain energy density (SED) approach has been proposed and elaborated by Lazzarin for strength assessments in respect of brittle fracture and high‐cycle fatigue. Pointed and rounded (blunt) V‐notches subjected to tensile loading (mode 1) are primarily considered. The method is systematically extended to multiaxial conditions (mode 3, mixed modes 1 and 2). The application to brittle fracture is documented for PMMA flat bar specimens with pointed or rounded V‐notches inclusive of U‐notches. Results for other brittle materials (ceramics, PVC, duraluminum and graphite) are also recorded. The application to high‐cycle fatigue comprises fillet‐welded joints, weld‐like shaped and V‐notched base material specimens as well as round bar specimens with a V‐notch. The relation of the local SED concept to comparable other concepts is investigated, among them the Kitagawa, Taylor and Atzori–Lazzarin diagrams, the Neuber concept of fictitious notch rounding applied to welded joints and also the J‐integral approach. Alternative details of the local SED concept such as a semicircular control volume, microrounded notches and slit‐parallel loading are also mentioned. Coarse FE meshes at pointed or rounded notch tips are proven to be acceptable for accurate local SED evaluations. The peak stress method proposed by Meneghetti, which is based on a notch stress intensity factor consideration combined with a globally even coarse FE mesh and is used for the assessment of the fatigue strength of welded joints, is also presented.

橋梁用高降伏点鋼板(SBHS)を用いた溶接継手の疲労特性

橋梁用高降伏点鋼板(SBHS)を用いた溶接継手の疲労特性

The local strain energy density approach applied to pre‐stressed components subjected to cyclic load

ABSTRACTAhead of sharp V‐notches, residual stresses, arising from the solidification of a fusion zone, have the same asymptotic nature of the stress field induced by mechanical loads. This stress field significantly affects the engineering properties of structural components, notably fatigue life and corrosion resistance of welded joints. Tensile residual stresses can reduce the fatigue strength of welded joints particularly in the high‐cycle regime, where no stress redistribution due to local plasticity phenomena is expected to be present. The aim of this work is to analyse, by means of the numerical simulation, the residual stress redistribution near a V‐notch tip induced by cyclic loads and to propose a method, based on the local strain energy approach, for the fatigue resistance estimation of pre‐stressed components. The numerical solutions of the problem were carried out under the hypothesis of generalized plane strain conditions by means of SYSWELD and SYSTUS codes.

Fatigue assessment of welds joining corrugated steel webs to flange plates

This paper presents an analytical approach for the fatigue assessment of welds joining corrugated plates to flange plates. The failure mode, stress distribution and fatigue strength of such welded joints have been examined through experimental tests and finite element analyses. As the stress concentration at the external weld line of the transition curvature is very critical for the fatigue life assessment, the study of such geometric effect has been performed and its resultant stress characteristics are discussed. The results show that, for the stress concentration at the fatigue critical point of the transition curvature, the influence of the corrugation/curvature angle is more significant when the ratio of the curvature radius to the corrugation depth is smaller, and vice-versa. The S–N curve of referred cruciform joint details can be regarded as the lower bound in contrast to the fatigue strength of welded joints with varying corrugation angle. Allowing for the combined stress condition, the proposed analytical approach is demonstrated to be appropriate for the fatigue assessment of the welded joint with the corrugated plate. The consideration of residual stress in the proposed analytical model is also discussed in theory.

Research on Methods of Improving Fatigue Property of Low-Strength Steel Welded Joints

TIG dressing combined with ultrasonic peening was conducted on low-strength steel Q235B in this paper, the fatigue strength of which was compared with original welding state, TIG dressing and ultrasonic peening respectively. The results indicate that fatigue strength of welded joints treated by TIG dressing is increased by 36%, composite processing method 57% and ultrasonic peening 56%, with little difference between composite processing method and ultrasonic peening in loading condition R=0.1. Stress concentration factor at weld toe treated by composite processing method is less than that by ultrasonic peening and with less residual compressive stress relief, the former is more effective in improving fatigue life at high stress level. Residual compressive stress at weld toe treated by composite processing is close to ultrasonic peening method and the effect of stress concentration is weakened, the two methods don’t appear to be much different.

Optimization of hybrid CO2 laser-GMA welding parameters on dissimilar materials AH32/STS304L using Grey-based Taguchi analysis

This study intended to verify the feasibility of laser-arc hybrid welding to dissimilar materials joints between high strength steel (AH32) and stainless steel (STS304L). For this, hybrid welding process for dissimilar materials was optimized by varying four parameters (welding speed, welding current, laser-arc distance, welding voltage) through Grey-based Taguchi analysis. A Grey relational analysis of the ultimate tensile strength (UTS), welding depth to width ratio (D/W) and absorbed energy (AE) attained from the Taguchi method can optimize the multiple-performance characteristics of the Grey relational grade. Moreover, hardness values microstructure and fatigue strength of welded joints fabricated under welding condition optimized by Grey-based Taguchi analysis were investigated. In the tensile test of welded joints, the damaged part was in BM of STS304L side. The hardness of welded joints exhibits within the acceptable range (the maximum value 340∼360 Hv). Dissimilar materials hybrid welded joints exhibited almost same fatigue strength for similar material STS304L SAW welded joints and lied well above the design curve(level D) for JSSC.

A novel structural stress approach for multiaxial fatigue strength assessment of welded joints

A new approach to estimate the structural stress for the assessment of the multiaxial fatigue strength of welded joints is proposed. In this approach, the structural stress at a point just below the weld toe in the plate thickness direction is evaluated and adopted for the multiaxial fatigue strength assessment of complex welded joints in combination with the Modified Wöhler Curve Method. Numerical results show that the adoption of the thus estimated structural stress, rather than the traditional hot spot stress, produces more accurate fatigue life estimation for structure members experiencing either uniaxial or multiaxial loading.

Determination of the structural and operational stress concentrations in welded joints to assess the fatigue strength of hull structures

According to the current standards, predominantly two methods are used to evaluate the fatigue strength of welded joints: the method of the S-N curves that involves every type of welded joints and the so-called hot-spot-stress method that uses a common fatigue curve for a butt-welded joint and a procedure to assess the structural stress concentration for the joints of other types. The numerical finite element analysis using three-dimensional elements with an exact simulation of the geometry of hull structure welded joints showed mutual disagreement of both methods and the possibility to refine the calculation procedures based on interpolation formulas for assessment of the stress concentration in welded joints.

Fatigue Behaviour of Friction Stir Welded Rolled Thick Plates of AA7075-T651 Aluminium Alloy Joints

The fatigue strength of welded joints represents the core problem for their industrial applications. Friction stir welding (FSW) demonstrated the enhancement of fatigue resistance for aluminium alloys, with respect to traditional fusion techniques. The aim of the present work is to evaluate the fatigue properties of 12 mm thick AA 7075 -T651 aluminium alloy plates joined by friction stir welding (FSW) process. The fatigue properties were evaluated under uniaxial tensile loading condition (stress ratio = 0.1, Frequency=10Hz) at room temperature using servo-hydraulic controlled machine. The fatigue endurance (S-N) curves of the welded joints and unwelded parent metal were constructed. The resultant fatigue properties were correlated with the tensile, hardness and microstructural characteristics of welded joints. The mode of failure was analyzed through scanning electron microscopy. It is found that the fatigue life of friction stir welded AA 7075- T651 Aluminium alloy joints is appreciably lower than unwelded parent metal but it is higher than fusion welded joints.

Identification of Weld Residual Stresses Using Diffraction Methods and their Effect on Fatigue Strength of High Strength Steels Welds

It is well known that fatigue strength of welded joints does not depend on steel strength. Better fatigue strength of welded joints, e.g. longer life time of fatigue loaded weld structures, can be achieved with a smooth transition between the weld and the base material to minimize stress concentration. It has also been recognized that residual stresses play a critical role in the fatigue behaviour of welds. In the last decade an extensive research has been performed in order to increase the fatigue strength of high strength steel weldments. The martensite and bainite transformation start temperatures of weld metals have been shown to have a large effect on fatigue life time of high strength steel welds. This is of particular importance if the full potential of high strength steels is to be used in fatigue loaded constructions. A detailed investigation of the effect of phase transformation temperature on residual stress distribution in the vicinity high strength steel welds and its effect on fatigue life time has been performed. The transformation temperature of the weld metal was varied by changing the chemical composition of the filler material. Residual stress distributions have been measured by neutron as well as by X-ray diffraction and fatigue tests have been performed on the fillet welds. A strong effect of weld metal phase transformation temperature on residual stress level was observed. Fatigue strength increased approximately three times when an optimised low transformation temperature filler material was used in comparison to the application of conventional filler material.

Multicycle fatigue strength of welded joints of steel S500MC

Multicycle fatigue strength of welded joints of steel S500MC

Effect of compression overload on fatigue strength improved by ICR treatment

ICR (Impact Crack-closure Retrofit) treatment was developed for extending fatigue life of cracked steel structures by closing the fatigue crack opening. By applying the ICR treatment to as-welded condition, compressive stress remains at weld toe. Therefore, ICR treatment can be used for the improvement of fatigue strength of welded joint at the same level of the other fatigue strength improved techniques. If a compression overload is subjected after ICR treatment, however, the residual compressive stress at weld toe will shift the tensile stress region. Accordingly, improved fatigue strength of welded joints by ICR treatment will be reduced after the compression overloaded. In order to investigate the effect of the compression overload on fatigue strength improved by ICR treatment, fatigue tests of out-of-plane gusset specimens were carried out. As the results, fatigue strength improved by ICR treatment was hardly reduced under the compression overload condition of nominal stress of −190 MPa.

Effect of stress ratio on the enhancement of fatigue strength in high performance steel welded joints by ultrasonic impact treatment

In this study, we conducted fatigue tests under various stress ratios using cruciform welded joints to confirm the benefit of Ultrasonic Impact Treatment (UIT) for the fatigue strength of welded joints. The material used in the experiment is the JIS SBHS500 high performance steel for bridges. The fatigue life of the peened weld joint by UIT is significantly longer than that of as-welded joints, especially at a low stress ratio. We also estimated the fatigue life of these cruciform joints by crack growth analysis based on the crack opening and closure simulation using the modified strip-yielding model, accounting for the residual stress distribution created by welding or UIT. These estimation results demonstrate good agreement with experimental results obtained at various stress ratios.

Fatigue strength improvement of welded joints by blast cleaning for subsequent painting

In the fabrication of steel bridges, prior to painting, blast cleaning is performed to clean the surfaces and to increase the adhesive properties of the surfaces for the subsequent painting. In addition to these objectives, blast cleaning can improve the fatigue strength of welded joints, as a result of the impact of forcibly propelled abrasive materials on the welded joints. A series of fatigue tests were carried out on the five different types of welded joints: one-side out-of-plane gusset fillet welded joints, both-side out-of-plane gusset fillet welded joints, non-load-carrying rib fillet welded cruciform joints, butt welded joints, and a large-scale model beam with out-of-plane gusset welded joints, and under the three different types of load conditions: uniaxial tension, out-of-plane bending, or in-plane bending stress cycles. Drawing on previous fatigue test results that include unpublished data, this paper presents the beneficial effects of blast cleaning for the subsequent painting applications on the fatigue strength improvement of welded joints and also presents a simple quality control method for the blast cleaning process that improves the fatigue strength.

Fatigue Strength of HFMI Treated Structures Under High R-ratio and Variable Amplitude Loading

High frequency mechanical impact (HFMI) is a post-weld treatment method for improving fatigue strength of welded joints. Much of the experimental research on fatigue performance of HFMI treated welded joints has concentrated on the beneficial compressive residual stresses created by the treatment and the fatigue strength of these joints under constant amplitude loading and relatively low mean stresses. Critical experimental data has been developed in a nearly completed European RFCS project. New mean stress and variable amplitude fatigue data are presented and evaluated with respect to a proposed International Institute of Welding (IIW) design guideline concerning HFMI treated welded joints.

Very High Cycle Fatigue Properties of Welded Joints under High Frequency Loading

Very high cycle fatigue (VHCF) properties of welded joints under ultrasonic fatigue loading have been investigated for titanium alloy (TI-6Al-4V) and bridge steel (Q345). Ultrasonic fatigue tests of base metal and welded joints were carried out in ambient air at room temperature at a stress ratio R=-1. It was observed that the fatigue strength of welded joints reduced by 50-60% as compared to the base metal. The S-N fatigue curves in the range of 107~109 cycles of base metal and welded joints for both materials exhibited the characteristic of continually decreasing type. The fatigue failure still occurred after 107 cycles of loading, and the fatigue limit in traditional does not exist. The fatigue facture mainly located in the weld metal region at low cycle fatigue range, but in the fusion area in HCF and VHCF. Analysis of fracture surfaces analyzed by SEM revealed that the fatigue cracks initiated from welding defects such as pores, cracks and inclusions.

Ultra-Long Life Fatigue Behavior of Welded Joints with Shot Blast Treatment

In order to investigate the ultra-long life fatigue properties of welded joints, ultrasonic fatigue tests of base material and welded joints were carried out using Q345q steel in ambient air at room temperature with a stress ratio R=-1. The results showed that the fatigue strength of specimens with welded joints dropped about 60% compared with that of base steel. To improve the fatigue properties of welded joints, two groups of specimens were treated by shot peening with different degrees which produced different compressive residual stresses on samples. It was found that the shot peening treatment could improve the fatigue strength of welded joints obviously. The fatigue fracture surface was analyzed by scanning electron microscopy. The fractography showed that there were two kinds of crack initiation mechanism in the Q345q base steel, crack inition from defects on the surface for high cycle fatigue failure and in the interior of the sample in very high cycle fatigue range. However, almost all cracks of welded joints initiated from defects caused by welding process. The failure of welded joints usually happened at the section of welded toes in the area of fusion or geometric discontinuity.

Effect of preload and stress ratio on fatigue strength of welded joints improved by ultrasonic impact treatment

Ultrasonic impact treatment (UIT) is a post-weld technique to be used for improving the fatigue strength of welded joints. The technique makes use of ultrasonic vibration to impact and plastically deform the weld toe, consequently creating beneficial compressive residual stress near the treated area. Since the compressive residual stress is considered to be a main reason for the improvement of fatigue strength, the improvement effect may be influenced by preload and stress ratio. In the case of ship structures, welded parts may be locally subjected to a stress near the yield strength of the material in heavy weather, and mean stress may change significantly depending on the loading conditions. In the present paper, fatigue tests were conducted using transverse non-load carrying fillet welds to identify the condition that UIT provides substantial benefit for fatigue strength of welded joints. The material used is AH36 shipbuilding high-strength steel. In the investigation of the preload effect, tensile stress of 90 % or compressive stress of 60 % of the yield strength of the base metal is applied prior to fatigue tests, where the stresses are defined as nominal stress. It is found that UIT is more effective than grinding on fatigue strength improvement of welded joints even after application of the preload. The effect of stress ratio was investigated by fatigue tests at stress ratios of 0.1, 0.5, and −1. Though the fatigue strength of the UIT-treated weld joint decreased as the stress ratio increased, the fatigue limits of the UIT-treated weld joint is fairly higher than that of as-welded joints even at the stress ratio of 0.5. Finally, fatigue tests were conducted under random sequence of clustered loading, which may simulate wave-induced load histories. UIT provides substantial benefit for fatigue strength of welded joints under actual service loading in ship structures.

Research Progress of Methods to Improve Fatigue Strength of Welded Structures

Fatigue fracture is the main failure form of welded structures. Fatigue strength of welded joints is far below that of base metal under the combined effect of stress concentration, molten slag defects in weld toe and residual tensile stress. Fatigue damage mechanism and main factors that affect fatigue strength of welded structures are analysis and summarized respectively. This paper emphasizes on introducing the research progress of various measures to improve the fatigue properties, including weld toe grinding, TIG dressing, local-heating and local extrusion method, hammering and shot blasting and ultrasonic peening treatment.

Effect Of Uit On Fatigue Strength Of Web- Gusset Welded Joints Considering Service Condition Of Steel Structures

UIT (Ultrasonic Impact Treatment) is a method to impact on the weld toe with hard pins vibrated by ultrasonic energy, so that the improvement of the weld toe profile and the introduction of the compressive residual stress can be obtained. It is confirmed that UIT has high improving effect on fatigue strength of welded joints through many experimental researches. Those tests were conducted under low stress ratio and UIT was done in condition that specimens were not subjected to stress. When the UIT method is applied to existing structures, its effect should be verified in condition that UIT is applied under loading and fatigue tests are performed under high stress ratio. In this study, the improvement effects have been examined under such conditions.