Fatigue life prediction based on crack closure for 6156 Al-alloy laser welded joints under variable amplitude loading

A fatigue prediction approach is proposed using fracture mechanics for laser beam welded Al-alloy joints under stationary variable amplitude loading. The proposed approach was based on the constant crack open stress intensity factor in each loading block for stationary variable amplitude loading. The influence of welding residual stress on fatigue life under stationary variable amplitude was taken into account by the change of crack open stress intensity factor in each loading block. The residual stress relaxation coefficient β = 0.5 was proposed to consider the residual stress relaxation for the laser beam welded Al-alloy joints during the fatigue crack growth process. Fatigue life prediction results showed that a very good agreement between experimental and estimated results was obtained.     

An experimental investigation of fatigue behavior and deformation of double spot friction welded joints

Fatigue behavior of double spot friction welded joints in aluminum alloy 7075-T6 plates is investigated by conducting monotonic tensile and fatigue tests. The spot friction welding procedures are carried out by a milling machine with a designed fixture at the best preliminary welding parameter set. The fatigue tests are performed in a constant amplitude load control servo-hydraulic fatigue testing machine with a load ratio of (R = P_min/P_max) 0.1 at room temperature. It is observed that the failure mode in cyclic loading (low-cycle and high-cycle) resembles that of the quasi-static loading conditions i.e. pure shearing. Primary fatigue crack is initiated in the vicinity of the original notch tip and then propagated along the circumference of the weld’s nugget.     

Numerical simulation of fatigue crack propagation in friction stir welded joint made of Al 2024-T351 alloy

In this work, fatigue crack propagation in thin-walled aluminium alloy structure with two friction stir welded T joints has been simulated numerically. Crack propagation in stiffened part of the structure between two friction stir welded T joints is analysed by using the eXtended Finite Element Method (XFEM), including software ABAQUS, as well as MORFEO, for modelling and results display. Tensile fatigue loading is applied, with stress ratio R = 0, and maximum stress σ_max = 10 MPa. Material properties (Al 2024-T351, as used in aeronautical industry) in different welded joints zones are adopted from available literature data. Following results are obtained by numerical analysis: stress–strain and displacement state in the structure, position of the crack tip and value of stress intensity factor for every crack propagation step, as well as the structural life estimation, i.e. number of load cycles, N, also for each crack propagation step. Using these results the number of cycles at which the crack starts to propagate in an unstable manner is predicted.     

Multiaxial fatigue assessment of welded joints – Recommendations for design codes

In the assessment of welded joints submitted to multiaxial loading the calculations method applied, independently of the concept (nominal, structural, hot-spot or local), must consider primarily the materials ductility. While proportional loading can be assessed by von Mises, the principal stress hypothesis, the Findley method or the Gough–Pollard relationship, using any of the mentioned concepts, difficulties occur when the loading is non-proportional, i.e. the principal stress (strain) direction changes. This causes a significant fatigue life reduction for ductile steel welds, but an indifferent behaviour for semi-ductile aluminium welds. This different response to non-proportional loading can be assessed when ductility related mechanisms of fatigue failures, i.e. the mean value of plane oriented shear stresses for ductile materials and a combination of shear and normal stresses for semi-ductile materials, are properly considered.However, as these methods require a good expertise in multiaxial fatigue, for design codes used by non-fatigue experts, simpler but sound calculation methodologies are required. The evaluation of known fatigue data obtained with multiaxial constant and variable amplitude (spectrum) loading in the range N > 10⁴ cycles suggests the application of the modified interaction algorithm of Gough–Pollard. In the case of variable amplitude loading, constant normal and shear stresses are replaced by modified reference normal and shear stresses of the particular spectrum. The modification of the reference stresses is based on the consideration of the real Palmgren–Miner damage sum of D_PM = 0.5 (for spectra with constant mean loads) and the modification of the Gough–Pollard algorithm by consideration of the multiaxial damage parameter D_MA = 1.0 or 0.5, which is dependent on the material’s ductility and on whether the multiaxial loading is proportional or non-proportional. This method is already part of the IIW-recommendations for the fatigue design of welded joints and can also be applied by using hot-spot or local stresses.     

A comparative study of fatigue estimation methods for single‐spot and multispot welds

Fatigue life and load data of spot‐welded samples were obtained by performing tensile‐shear fatigue tests on single‐spot‐welded samples, and by collecting published data on tensile‐shear fatigue tests of multispot‐welded samples. Finite element models of the spot‐welded samples were then constructed, and their accuracy was validated by comparing their results with those of static loading tests. Methods to estimate fatigue in spot‐welded joints proposed by Sheppard, Rupp, and Swellam were used to predict the fatigue life of each sample, and their results were compared. The sensitivity of each method to spot‐welding parameters, as well as the arrangement and number of welded spots, was analyzed to obtain their scope of applicability. Finally, the Sheppard and Rupp methods were used to obtain fitted fatigue curves of each spot‐welded sample, and the equivalent life method was used to generate P‐S‐N curves of the single‐spot‐ and multispot‐welded samples. This method can serve as a useful reference for fatigue reliability analyses of mechanical parts with spot‐welded joints.     

The optimisation of process parameters for friction stir spot-welded AA3003-H12 aluminium alloy using a Taguchi orthogonal array

The aim of the present work is to optimise the welding parameters for friction stir spot welded non-heat-treatable AA3003-H12 aluminium alloy sheets using a Taguchi orthogonal array. The welding parameters, such as the tool rotational speed, tool plunge depth and dwell time, were determined according to the Taguchi orthogonal table L9 using a randomised approach. The optimum welding parameters for the peak tensile shear load of the joints were predicted, and the individual importance of each parameter on the tensile shear load of the friction stir spot weld was evaluated by examining the signal-to-noise ratio and analysis of variance (ANOVA) results. The optimum levels of the plunge depth, dwell time and tool rotational speed were found to be 4.8 mm, 2 s and 1500 rpm, respectively. The ANOVA results indicated that the tool plunge depth has the higher statistical effect with 69.26% on the tensile shear load, followed by the dwell time and rotational speed. The tensile shear load of the friction stir spot welding (FSSW) joints increased with increasing plunge depth. Additionally, examination of the weld cross-sections, microhardness tests and fracture characterisation of the selected friction spot welded joints were conducted to understand the better performance of the joints. All the fractures of the joints during tensile testing occurred at stir zone (SZ), where the bonded section was minimum. The tensile shear load and tensile deformation of the FSSW joints increased linearly with increasing the bonded size. The finer grain size in the SZ led to the higher hardness, which resulted in higher fracture strength. When the tensile shear load of the joints increased approximately 3-fold, the failure energy absorption of the joints increased approximately 15-fold.     

Analysis of the S–N curves of welded joints enhanced by ultrasonic peening treatment

Contrast fatigue tests were carried out on T-shape tubular joints of 20 steel in three conditions: as welded, treated by ultrasonic peening treatment (UPT) before loading and UPT under loading. Results are: (1) Dispersity of test results measured by nominal stress is much larger than that measured by hot spot stress. After UPT before loading, fatigue strength of 20 steel tubular joints measured by hot spot stress increases by 67% and fatigue life is prolonged by 22–45 times. (2) Under low stress ratio R, UPT before loading can improve the fatigue performance of welded tubular joints significantly. (3) Under high stress ratio R, UPT under loading (static load) is recommended to improve the fatigue performance of welded tubular joints. UPT under loading not only enhances the fatigue properties at low stress level, but also at high stress level. (4) The general rule of S–N curves of welded joints treated by UPT is commonly effected by external load (static load) and self release of residual stress.     

Investigation of welded structures on mechanical properties of 304L welded tube-to-tubesheet joints

This paper presents an investigation on effects of welded structures on pullout force and fatigue life of welded joints of 304L heat exchanger tube-to-tubesheet. Welded structures including inner-hole with a groove of 45°, extension, inner-hole with a groove of U-shape and plain-end were discussed. Full-size test blocks covering four types of welded structures were used to conduct pullout test and low cycle fatigue test. Furthermore, optical microscopy (OM) and scanning electron microscopy (SEM) were utilized to analyze the failure mechanism. The results show that the joints of the extension welded structure have the largest pullout force and fatigue life because of its largest fracture length, and the undeformed welded zone revealed favorable quality of weld. The cracks originate at the outer surface of tube and striations propagate at an average rate of 50 μm for a loading cycle, finally, the separation commences at the inner surface of the tube, exhibiting equiaxed or hemispheroidal dimples.     

Fatigue life assessment of friction spot welded 7050-T76 aluminium alloy using Weibull distribution

Friction spot welding is a solid state welding process suitable to obtain spot like-joints in overlap configuration. The process is particularly useful to weld lightweight materials in similar and dissimilar combinations, and therefore an interesting alternative to other joining techniques (rivets, resistance welding, etc.). Optimum process parameters have been defined using the Taguchi method by maximizing the response variable (the lap shear strength). A study of the fatigue life was carried out on specimens welded with the above mentioned optimized process parameters. Fatigue tests were performed using a stress ratio of R = 0.10. Two-parameter Weibull distribution was used to analyze statistically the fatigue life for the joined overlapped sheets. Subsequently, the Weibull plots were drawn, as well as S–N curves considering different reliability levels. The results show that for a relatively low load, corresponding to 10% of the maximum supported by the joint, the number of cycles surpasses 1 × 10⁶, hence infinite life of the service component can be attributed. Fatigue fracture surfaces were investigated for the highest and lowest loads tested using scanning electron microscope (SEM).     

Recent developments in local concepts of fatigue assessment of welded joints

Several lately proposed modifications or variants of the structural stress or strain concepts, of the notch stress or strain concepts (also termed ‘local stress or strain concepts’) and of the fracture mechanics concepts of fatigue assessment of welded joints are reviewed, whereas the wider context is presented in a recently republished and actualised standard work. The structural stress concepts described first are based on a linearisation of the stress distribution across the plate thickness or along the anticipated crack path and, alternatively, on the structural stress 1 mm in depth below the weld toe. The structural stress is defined and set against design S–N curves. A further structural stress concept is presented for welded joints in thin-sheet steels and aluminium alloys. Among the elastic notch stress concepts, the variant with the reference notch radius, ρ_r = 1 mm, recently verified also for welded joints in aluminium alloys with plate thicknesses t ? 5 mm and the variant with a small-size reference notch radius, ρ_r = 0.05 mm, applicable to welded joints in thin-sheet materials, are outlined. The elastic–plastic notch strain concept is applied to a spot-welded tensile-shear specimen starting from a small-size keyhole notch at the nugget edge. The novel notch stress intensity factor (NSIF) approach relating to crack initiation and extrapolated to final fracture of seam-welded joints in steels and in aluminium alloys is reviewed. A more recently developed crack propagation approach for spot welds is finally described.     

Improving mechanical properties of pinless friction stir spot welded joints by eliminating hook defect

Hook defect (HD) seriously decreases the mechanical properties of friction stir spot welded (FSSW) joints. In this study, two methods were therefore used to eliminate the HD in pinless FSSW joints. The one is changing welding parameters such as rotating speed and dwell time. The other one is FSSW plus subsequent friction stir welding (FSSW-FSW), which is an innovative method proposed in this study. Experimental results showed that the HD in pinless FSSWed AA2024 joints was successfully eliminated by using FSSW-FSW, not by changing process parameters. The joints without HD exhibited a tensile–shear load of as much as 12 kN, which was higher than that of 6.9 kN in the joints with HD. Furthermore, it was proved that the tensile–shear load is not greatly improved only by increasing the nugget zone when HD still existed in the FSSW joints. In addition, the fracture morphology analysis demonstrated that the shear fracture of the FSSW-FSW joints took place along the boundary between the upper and lower sheets through the weld nugget, and the faying surface between the two sheets was completely sheared off.     

High cycle fatigue behaviour of impact treated welds under variable amplitude loading conditions

A comprehensive variable amplitude (VA) fatigue testing program and analysis was performed to address a number of concerns raised regarding the use of impact treatments for the fatigue enhancement of welds in the high and ultra-high cycle (up to 100 million cycles) domains. A total of 67 fatigue tests were conducted on two different welded joints representing load-carrying and non-load carrying welds in steel structures. Two different VA loading spectrums, generated using traffic data and influence lines for highway bridge girders, were used. The effects of load cycles with high stress ratios (R > 0.4) and large tensile overloads (greater than the yield strength) were studied. The test results were then used to evaluate a number of previously proposed recommendations for the fatigue design of impact treated welds. The nominal, structural, and effective notch stress approaches were considered. Finite element (FE) analysis was performed to determine the structural and effective notch stresses. A statistical analysis of the fatigue test results was conducted and characteristic S–N curves with slope, m = 5 are proposed for the fatigue design of treated welds under VA loading in the high cycle domain.     

Fatigue strength of laser beam welded thin steel structures under multiaxial loading

The multiaxial fatigue behaviour of thin laser beam welded tube–tube specimens of the structural steel St35 was assessed according to the methodology of the fictitious weld root radius of r_f=0.05 mm and the application of the Effective Equivalent Stress Hypothesis (EESH), especially considering the fatigue life reducing influence of out-of-phase loading in comparison to in-phase loading. The results are applicable for the fatigue design of laser beam welded car body and chassis structures of thin steel sheets (t<3 mm).     

Fatigue assessment of root failures in HSLA steel welded joints: A comparison among local approaches

Fatigue tests were performed on welded joints made of high-strength, low-alloy steel (S690). Different welding processes were tested, resulting in welds with different defects essentially consisting in lack of penetration. Fatigue tests were run with both constant and variable amplitude loading. The experimental results were compared to predictions obtained by applying local approaches (local stress and local strain) and the concepts of fracture mechanics. The local stress approach allowed the fatigue strength of joints in constant amplitude loading (for fatigue above 2 × 10⁶) to be predicted, but the assumption of a constant value of the slope k = 3 for all S–N curves led to non-conservative predictions of shorter lives. The local strain approach allowed the fatigue strength of the joints under constant amplitude to be predicted. Although, these predictions matched the experimental data well for both small and large defects in the entire cycle number range, they failed to predict the behaviour of joints under variable amplitude loading. Conversely, the fracture mechanics approach proved to be more efficient in predicting the fatigue behaviour of welded joint under variable amplitude loading.     

Crack precursor size for natural rubber inferred from relaxing and non-relaxing fatigue experiments

Fatigue life prediction for a dumbbell cylindrical natural rubber component under uniaxial tensile loading conditions was performed based on the Thomas fatigue crack growth model for relaxing (R = 0) load cycles and the Mars–Fatemi model for non-relaxing (R > 0) load cycles. By using a self-written program, we proposed a new approach to establish the relation between the power law exponent F and the R ratio in the Mars–Fatemi model. The approach is based on rubber fatigue life (S–N) data rather than crack growth rate and tearing energy (da/dN–T) data, avoiding certain difficulties often encountered using the crack growth method. The results indicate that the relation between F and R is a quadratic or cubic function over the range 0 < R < 0.3. Finally, the quantitative effect of initial crack size on fatigue life was studied. We found that the inferred mean size of crack precursors in the rubber component is around 30–40 μm under both relaxing and non-relaxing loading conditions, and the fluctuation of fatigue life is due to the inhomogeneity of crack precursor size except the factors such as unavoidable variations in testing conditions and specimen variations. The good agreement of inferred crack precursor sizes from different R ratio loading conditions is a strong indication that the Mars–Fatemi model provides a proper accounting for the effects of strain crystallization, and it confirms yet again the understanding that nucleated cracks originate from similarly sized precursors in both relaxing and non-relaxing fatigue experiments.     

Weld arrangement effects on the fatigue behavior of multi friction stir spot welded joints

In this study, the effects of friction stir spot weld arrangements as multi type on fatigue behavior of friction stir spot welded joints is investigated. The joints that are considered with five different styles for friction stir spot welded joints: one-row four joints parallel to loading direction, two-row four-joint specimen, one-row four joints perpendicular to the loading axis, three-row as diamond shape with four joints in each edge and five friction stir spot welded specimen in three rows that middle row consist three joints. The correlation between micro hardness, cyclic material constants and mechanical strength of different zones around the friction stir spot welds are assumed to be proportional to base material hardness. A non-linear finite element analysis was carried out for simulating tensile shear multi friction stir spot welded joints with ANSYS software by considering gap effects. Using the local stress and strain calculated with finite element analysis, fatigue lives of specimens were predicted with Morrow, modified Morrow and Smith–Watson–Topper (SWT) damage equations. Experimental fatigue tests of welded specimens have been carried out using constant amplitude load control servo-hydraulic fatigue testing machine. The results reveal that there is relatively good agreement between fatigue life predictions and experimental data in reasonable fatigue life regime.     

Fatigue strength assessment of Invar alloy weld joints using the notch stress approach

The aim of this study investigated the fatigue strength of Invar alloy weld joints. Invar steel (Fe-Ni 36%) is widely used in the primary and secondary barriers of membrane-type liquified natural gas (LNG) containment vessels. The fatigue test was carried out for two different types of welded joints with raised edge specimens and with overlap joint specimens based on the nominal and notch stress approaches. The thickness of the Invar plate is less than 1.5 mm, so the notch stress approach with r_ref = 0.05 mm was applied. Our evaluation of the results in terms of the FAT value and the slopes of the design curves are compared with steel, aluminum, and magnesium weld joints in accordance with International Institute of Welding (IIW) recommendations.     

Influence of spectrum loading sequences on fatigue life in a high-temperature environment

This paper discusses the fatigue life behaviour of aluminium alloy AA6061-T6 under spectrum loadings. Load sequences in spectrum loadings can have significant effects on fatigue life at room temperature and within the elevated temperature range. The main objective of this paper is to investigate the influences of load sequences effect on fatigue life at elevated temperature. Fatigue strain signal was obtained from the engine mount bracket of an automobile under normal driving conditions. Constant amplitude loading, high-to-low, and low-to-high loading sequences were then derived from the original fatigue strain signal to observe the fatigue behaviour at both room and elevated temperatures. The fatigue test was performed on AA6061-T6 specimen according to the ASTM E466 standard using a 100 kN servo-hydraulic fatigue testing machine within the temperature range of 27–250 °C. The elevated temperature range was chosen based on the maximum temperature of the engine mount bracket and the extreme temperature of the cylinder head that can be reached in service. After the test, fatigue fracture surfaces were sectioned and inspected using a high-magnification microscope. Results show that fatigue life behaviour at room temperature was significantly influenced by the load sequences in spectrum loadings. On the other hand, the effect of load sequences at a higher temperature was reduced.     

Mechanical,fatigue and microstructural properties of friction stir welded 5083-H111 and 6082-T651 aluminum alloys

In this study, 5083-H111 and 6082-T651 aluminum alloy plates in 6 mm thickness that are used particularly for shipbuilding industry were welded using Friction Stir Welding (FSW) method as similar and dissimilar joints with one side pass at PA position with the parameters of 1250 rpm tool rotation, 64 mm/min welding speed and 2° tool tilt angle. Tensile tests results showed sufficient joint efficiencies and surprisingly high yield stress values. Bending fatigue test results of all joint types showed fatigue strength close to each other. Fatigue strength order of the joints were respectively FSWed 5083-5083, and 6082-6082 similar joints and 5083-6082 dissimilar joint. Cross sections of the weld zones have been analyzed with light optical microscopy (LOM) and fracture surface of fatigue test specimens were examined by scanning electron microscopy (SEM). Although there were no voids in radiographic and microscopic analyzes, 5083-6082 joint showed rarely encountered voiding effect under fatigue load. Microhardness measurements revealed rare result for FSWed AW5083 and novel result for FSWed 6082 aluminum alloy.     

Creep–fatigue damage for a structure with dissimilar metal welds of modified 9Cr–1Mo steel and 316L stainless steel

A creep–fatigue test with a structural specimen made of Mod. 9Cr–1Mo steel and 316L stainless steel has been carried out and the test results were compared with those of the evaluations by the high temperature design codes of ASME subsection NH and RCC-MR to quantify the conservatism. A specimen with a diameter of 500 mm, height of 440 mm and thickness of 6.3 mm was subjected to creep–fatigue loads with two hours of a hold time at 600 °C and a primary nominal stress of 30 MPa. The creep–fatigue behaviours of the dissimilar metal welds as well as the similar metal welds were investigated and the results of the test were compared with the evaluation results. Bimetallic (direct) transition metal joint and trimetallic transition metal joint for a dissimilar metal weld were employed for a specimen, and their behaviours under a creep–fatigue load were compared. The conservatism of the design codes on the creep–fatigue evaluation at the welded joints as well as at the base metal with an emphasis on Mod.9Cr–1Mo steel are highlighted through comparisons with the results from the observation and the evaluation.