Research on fatigue behavior and residual stress of large-scale cruciform welding joint with groove

Fatigue fracture behavior of the 30 mm thick Q460C-Z steel cruciform welded joint with groove was investigated. The fatigue test results indicated that fatigue strength of 30 mm thick Q460C-Z steel cruciform welded joint with groove can reach fatigue level of 80 MPa (FAT80). Fatigue crack source of the failure specimen initiated from weld toe. Meanwhile, the microcrack was also found in the fusion zones of the fatigue failure specimen, which was caused by weld quality and weld metal integrity resulting from the multi-pass welds. Two-dimensional map of the longitudinal residual stress of 30 mm thick Q460C-Z steel cruciform welded joint with groove was obtained by using the contour method. The stress nephogram of Two-dimensional map indicated that longitudinal residual stress in the welding center is the largest.     

Fatigue strength analysis of laser-hybrid welds in thin plate considering weld geometry in microscale

Utilization of thin plates together with laser-based welding processes allows manufacturing of large weight efficient steel structures. However, the fatigue strength of welds in thin-plate structures with plate thicknesses below 5 mm is observed to have large variation, which brings challenges to fatigue strength assessment. One possible reason for this variation is the increased influence of actual weld geometry that is neglected in common fatigue strength assessment approaches utilizing geometry idealization. To reveal this influence the fatigue strength of 3 mm thick laser-hybrid welded butt joints were studied using the measured microscale weld geometry and the notch stress approach. Notch stresses were defined using Neuber’s stress averaging approach which allows the determination of the fatigue-effective stress without fictitious geometric modifications. For the studied specimens the large scatter of fatigue strength in the high-cycle region could be explained using this approach with high-resolution weld profile measurements combined with thorough finite-element analysis. It was observed that axial misalignment in narrow laser-hybrid welds causes a significant notch stress increase on the root side reducing the fatigue strength dramatically in terms of structural and nominal stress. In order to capture the increased notch stress it is crucial to use a significantly smaller stress averaging length than commonly assumed for welded joints.     

A simplified fatigue assessment method for high quality welded cruciform joints

The primary goal of this study was to develop an equation relating the geometric parameters to fatigue strength which can be used is routine design assessment. To attain this, the influence of local geometrical weld variations on the fatigue strength of non-load-carrying cruciform fillet welded joints were systematically studied using plane strain linear elastic fracture mechanics (LEFM). The effects of weld toe radius, flank angle and weld size were considered. Both continuous weld toe cracks and semi-elliptical toe cracks with alternate pre-existing defect depths were considered. A previously developed experimental crack aspect ratio development curve was used for assessing the growth of the semi-elliptical cracks using 2D FE models. A total of 152 experimental fatigue data points from six published studies of welded cruciform joints were evaluated. Details of the actual weld toe radius, flank angle and weld size were available for these joints. For the high quality welds evaluated, an assumed initial crack depth of 0.05 mm was found to correlate best with the experimental data. Of all the geometric parameters considered analytically, weld toe radius was found to have the most dramatic influence on fatigue life. A simple equation is proposed which relates welded joint fatigue strength to the ratio weld toe radius/plate thickness for high quality welds.     

Residual stress and strain in MIG butt welds in 5083-H321 aluminium: As-welded and fatigue cycled

This paper presents single-line residual stress profiles for 8 mm 5083-H321 aluminium plates joined by gas metal arc (MIG) welding. The data were obtained by synchrotron diffraction strain scanning. Weld metal stresses (up to ～7 mm either side of the centreline) are quite scattered and unreliable because of the large epitaxial grain size in the fusion zone. Peak magnitude of the transverse stresses varies between +50 MPa (19% of parent plate proof strength) at the HAZ boundary to ?150 MPa (57% of PP proof strength) at the weld centreline. Equivalent values for longitudinal stresses are +90 MPa (34% of PP proof strength) some 22 mm from the weld centreline to ?120 MPa (45% of PP proof strength) at the weld centreline. Plate-to-plate variation in the as-welded transverse and longitudinal residual stress values across the weld toe region is around 40 MPa. The effect on residual stress and strain values of a sequence of applied fatigue loads was also considered and reported.     

Fatigue of high strength steel joints welded with low temperature transformation consumables

In the present paper constant (CA) and variable amplitude (VA) fatigue testing have been carried out on out-of plane gusset fillet welded high strength steel joints. The joints were welded with conventional weld filler material and martensitic low transformation temperature weld filler, LTT, in order to study the influence of the residual stress on the fatigue strength. Residual stress measurements were carried out close to the weld toe using X-ray diffraction technique in order to study the relaxation due to VA fatigue. The residual stress showed different level of relaxation depending on the VA spectrum loading used. The LTT joints show 40% increase in mean fatigue strength compared to the conventional joints in CA. The LTT joints show 12% increase in mean fatigue strength compared to the conventional joints. The LTT joints show 33% increase in mean fatigue strength in CA compared to VA testing. However, the improvement of the fatigue strength is less significant in variable amplitude testing mainly due to the relaxation of the compressive residual stresses.     

Fatigue lifetime and tearing resistance of AA2198 Al–Cu–Li alloy friction stir welds: Effect of defects

The fatigue strength and failure mechanisms of defect-free (“sound”) and flaw bearing friction stir butt-welds of 3.1 mm-thick AA2198-T8 Al–Li–Cu alloy have been investigated via S–N curves at R = 0.1 using cross weld specimens. The fatigue strength of sound welds is only reduced by 10–15% at the aimed lifetime of 10⁵ cycles compared to the base material. Joint Line Remnant (JLR) bearing welds have a similar fatigue strength as sound welds and the JLR is not the crack initiation site. Kissing Bond (KB) bearing welds that have undergone a weld root polishing show a reduction in fatigue strength by 17% compared to sound welds. For specimens loaded at or above yield strength of the weld nugget the crack systematically initiates from the KB during the first cycle, which is interpreted further using fracture mechanics. The strongest reduction, about 28% in fatigue strength, is found for welds with an initial gap between the parent sheets (GAP welds) along with initiation at intergranular surface microcracks. Kahn tear tests show a reduction in tearing resistance for the flaw bearing welds with a similar ranking as for the fatigue strength.     

Research on fatigue behavior of welded joint spraying fused by low transformation temperature alloy powder

Modification of spraying fused (MSF) of plasma arc as heat source was used to improve the fatigue performance of welded joint, which both fundamentally reduced stress concentration at weld toe and achieved metallurgical bond between spraying fused coating and welding. The low transformation temperature alloy powder was applied to the method of MSF. After spraying fusion, especially spraying fused joint by low transformation temperature alloy powder, the distribution of residual stress is more difficult to be obtained. Finite element (FE) simulation as an important tool was used to determine the stress field and temperature field of spraying fused joint. Simulated results show that as-welded joint and welded joint spraying fused by conventional nickel base alloy powder (Conventional-joint) present tensile stress. The stress of welded joint spraying fused by low transformation temperature alloy powder (LTT-joint) is compressive stress. Fatigue test results indicated that under the condition of 2 × 10⁶ cycles, the fatigue strength of as-welded joint is 135 MPa, while that of Conventional-joint and LTT-joint is 218 MPa and 235 MPa, respectively. The fatigue strength of Conventional-joint increases by 61.48%, and fatigue strength of LTT-joint increases by 74.07%.     

Fatigue strength improvement factors for high strength steel welded joints treated by high frequency mechanical impact

Numerous studies have observed that the fatigue strength of improved welds increases with material yield strength. This paper provides a comprehensive evaluation of published data for high frequency mechanical impact treated welds. In total, 228 experimental results for three weld geometries subject to R = 0.1 axial loading have been reviewed. A design recommendation including one fatigue class increase in strength (about 12.5%) for every 200 MPa increase in static yield strength is proposed and are shown to be conservative with respect to all available data. Special cautions are given for high R-ratio or variable amplitude fatigue and potential alternate failure locations.     

J-integral-based approach to fatigue assessment of laser stake-welded T-joints

The aim of this paper is to investigate the influence of the plate thickness on the fatigue strength of laser stake-welded T-joints under the tension loading condition. Fatigue tests were conducted on specimens with plate thicknesses below 5 mm subjected to tension loading with the load ratio R = 0. The statistical analysis of the weld geometry showed a normal distribution of the each parameter that was measured. In addition, the parameters had similar proportions in comparison to the specimens with plate thicknesses above 5 mm. FE analysis was performed with the aim of determining the stress state in the joint along with the J-integral. If the square root of the J-integral, √ΔJ, is used as the fatigue strength assessment parameter, the fatigue strength obtained at five million cycles is similar as in the case of other steel welded joint types. The investigation concluded that the stress state changes with the reduction of the plate thicknesses and the contribution of fracture mode II becomes significant. However, using √ΔJ as a fatigue strength assessment parameter ensures that the complex state of the mixed fracture mode loading is accurately accounted for. This fact further enables the fatigue strength of laser stake-welded T-joints of any plate thicknesses to be described by means of a narrower scatterband than the one obtained by the nominal stress approach.     

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.     

Effect of Ti on hot cracking and mechanical performance in the gas tungsten arc welds of copper thick plates

The new welding material – ERCuTi alloys filler metals were developed for gas tungsten arc welding (GTAW) of copper. The cracking susceptibility of the welds with ERCuTi and ERCu separately in GTAW of 10 mm copper thick plates was investigated. The formation causes of hot cracking was researched by using ERCu and the suppression mechanism of hot cracking when using ERCuTi alloy filler was proposed. It has been found that, when element Ti is added into the welding pool, the Ti will combine with O preferentially rather than Cu to generate TiO₂, which process can suppress the formation of Cu₂O. The hot cracking force and the hot ductility of the welds in brittle temperature range (BTR) could be improved effectively by adding Ti in filler metal compared with that of the welds without Ti. But the degree of addition of Ti (2–4 wt%) is critical when the susceptibility of cracking is to be suppressed. If the level is allowed to exceed 4 wt%, more low-melting point eutectics (β-TiCu₄ and TiCu₂) will be formed in the welds, and cracking susceptibility will be increased again. Results of mechanical properties tests show that although adding Ti increases the hardness and strength of the weld compared to the base metal, the impact ductility and the plastic properties are not decreased significantly.     

Effect of welding parameters on microstructure and mechanical properties of AA6061-T6 butt welded joints by stationary shoulder friction stir welding

Stationary shoulder friction stir welding (SSFSW) butt welded joints were fabricated successfully for AA6061-T6 sheets with 5.0 mm thickness. The welding experiments were performed using 750–1500 rpm tool rotation speeds and 100–300 mm/min welding speeds. The effects of welding parameters on microstructure and mechanical properties for the obtained welds were discussed and analyzed in detail. It is verified that the defect-free SSFSW welds with fine and smooth surface were obtained for all the selected welding parameters, and the weld transverse sections are obviously different from that of conventional FSW joint. The SSFSW nugget zone (NZ) has “bowl-like” shapes with fairly narrow thermal mechanically affected zone (TMAZ) and heat affected zone (HAZ) and the microstructures of weld region are rather symmetrical and homogeneous. The 750–1500 rpm rotation speeds apparently increase the widths of NZ, TMAZ and HAZ, while the influences of 100–300 mm/min welding speeds on their widths are weak. The softening regions with the average hardness equivalent 60% of the base metal are produced on both advancing side and retreating side. The tensile properties of AA6061-T6 SSFSW joints are almost unaffected by the 750–1500 rpm rotation speeds for given 100 mm/min, while the changing of welding speed from 100–300 mm/min for given 1500 rpm obviously increased the tensile strength of the joint and the maximum value for welding parameter 1500 rpm and 300 mm/min reached 77.3% of the base metal strength. The tensile fracture sites always locate in HAZ either on the advancing side or retreating side of the joints.     

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.     

Fatigue life prediction for toe ground welded joints

The paper presents the results of an investigation of the effect of weld toe burr grinding on the fatigue performance of non-load-carrying transverse fillet welded joints. Crack initiation and propagation were monitored by a modified replica method. It was found that, although the average life increase due to toe grinding was in agreement with published data, the majority of the fatigue cracks in specimens that gave fatigue lives <～10⁶ cycles initiated at flaws just beneath the ground surface. Both the experiments and calculations based on fracture mechanics suggested that the fatigue lives of the toe ground joints in this life regime were dominated by the crack propagation process. However, in the long life regime (>10⁶ cycles), crack initiation became significant. Reasonable estimates of the crack initiation period were made using the local stress approach proposed by Lawrence et al. [Lawrence FV, Mattos RJ, Higashida Y, Burk JD. Estimating the fatigue crack initiation life of welds. In: Hoeppner DW, editor, Fatigue Testing of Weldments, ASTM STP 648, American Society for Testing and Materials; 1978, p. 134–58]. The investigation suggested that more benefit from weld toe grinding could be claimed in the long (N > 10⁶ cycles) than the short life regime.     

Consideration of mean-stress effects on fatigue life of welded magnesium joints by the application of the Smith–Watson–Topper and reference radius concepts

Three types of welded joints have been assessed with regard to their fatigue strength based on the mean-stress damage parameter model according to Smith, Watson, and Topper (P_SWT) and on the reference notch radius concept. These analyses were performed with three different stress ratios, R = −1, R = 0 and R = 0.5, under axial loading. For each stress level, the corresponding Neuber-Hyperbolas, Masing-loops and their maximum stress and maximum strain values were determined in order to calculate damage parameter (P_SWT) values. For a given weld geometry, this damage parameter is able to unify the fatigue results for different R-values within at a tight scatter band and therefore to consider the mean-stress effect. The unification of the results for different weld geometries is performed by applying the reference radii r_ref = 0.05 and r_ref = 1.00 mm as suggested by the IIW-Recommendations.     

Mechanical behaviour of AA 7475 friction stir welds with the kissing bond defect

Friction stir welding (FSW) is restricted to non-safety–critical aerospace components because there is no reliable method for detecting kissing bonds (KB), which may have a significant effect on fatigue life. The effects of KB defects on the tensile and fatigue properties of 7475-T7351 friction stir welds were quantitatively evaluated with respect to a reference weld without any flaws and a base material. Various KB defects were investigated with the aim of determining the defect size that does not have a significant influence on the fatigue life of joined 6.35-mm-thick plates. A critical value for the KB geometry appears to be a depth of 0.3 mm considering the influence on fatigue life for the investigated configurations. This paper also presents results from micromorphological analyses of the fatigue crack initiation from the KB and from the analysis of the weld cross-section microstructure.     

Fatigue strength of steel rollers with failure occurring at the weld root based on the local strain energy values: modelling and fatigue assessment

Weldments geometry with failures occurring at the weld toe or at the weld root cannot, by its nature, be precisely defined. Parameters such as bead shape and toe or root radius vary from joint to joint even in well-controlled manufacturing operations. The worst case configuration can be achieved by modelling as a sharp, zero radius, notch both the toe and the weld root. The intensity of asymptotic stress distributions obeying Williams’ solution is quantified by means of the Notch Stress Intensity Factors (NSIFs). For steel welded joints with failures originated from the weld roots, where the lack of penetration zone is treated as a crack-like notch, units for NSIFs are the same as conventional SIFs used in LEFM. The different dimensionality of NSIFs for different notch opening angles does not allow a direct comparison of failures occurring at the weld toe or at the weld root. In order to overcome the problem related to the variability of the V-notch opening angle, a simple scalar quantity, i.e. the value of the strain energy density (SED) averaged in the structural volume surrounding the notch tip, has been introduced. This energy is given in closed form on the basis of the relevant NSIFs for modes I, II and III. The radius R_c of the averaging zone is carefully identified with reference to conventional arc welding processes being equal to 0.28 mm for welded joints made of steel.The local-energy based criterion is applied here to steel welded rollers produced by Rulmeca and subjected to prevailing mode I (with failures at the weld root). The aim of the paper is firstly to describe the employed methodology for the fatigue assessment and secondly to show the first synthesis of fatigue data by means of local SED for a specific geometry.     

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.     

The effect of thermal ageing on low cycle fatigue behaviour of 316 stainless steel welds

It is well known that welds are the weak links in any structure. Therefore, it is of out most importance to characterize the mechanical properties of welds. Moreover, the changes in the microstructure that occur in welds on exposure to high temperatures affect the mechanical properties and must be studied by ageing the welds at high temperature. In this paper the low cycle fatigue behaviour of thermally aged 316 stainless steel weld metal is presented. Weld pads with single V configuration were prepared by the shielded metal arc welding process using 316 electrodes. Thermal ageing was done for 10,000 h at 823 and 873 K. Total strain controlled low cycle fatigue tests were conducted at a constant strain rate of 3 × 10^?3 s^?1 with strain amplitudes in the range ±0.25% to ±0.6% at 823 and 873 K. Weld metal exhibited initial hardening followed by cyclic softening prior to failure. The aged samples exhibited higher stress response as compared to the unaged samples. At both the temperatures and all strain amplitudes fatigue life was inferior to that of unaged samples. The metallography of the aged and tested material was studied through optical, scanning and transmission electron microscopy. The effect of transformation of δ-ferrite to sigma phase and carbides in the weld metal on low cycle fatigue behaviour was evaluated.     

High power fiber laser arc hybrid welding of AZ31B magnesium alloy

High power fiber laser–metal inert gas arc hybrid welding of AZ31B magnesium alloy was studied. The fusion zone consisted of hexagonal dendrites, where the secondary particle of Al₈Mn₅ was found at the center of dendrite as a nucleus. Within hybrid weld, the arc zone had coarser grain size and wider partial melted zone compared with the laser zone. The tensile results showed the maximum strength efficiency of 5 mm thick welds was up to 109%, while that of 8 mm thick welds was only 88%. The fracture surface represented a ductile–brittle mixed pattern characterized by dimples and quasi-cleavages. On the fracture surface some metallurgical defects of porosity and MgO inclusions around with secondary cracks were observed. Meanwhile, a strong link between the joint strength and weld porosity were demonstrated by experimental results, whose relevant mechanism was discussed by the laser–arc interaction during hybrid welding.