Fatigue Properties Of Welded Joints Research Articles

Effect of the Filler Metal and Shielding Gas on the Fatigue Life in HSLA Steels Welded Using the GMAW Process

The composition of the filler electrodes, as well as the shielding gases, has a strong impact on the static and dynamic properties of welded joints in HSLA steels. The content of Cr, Ni, and Mo, along with the shielding gases, helps maintain the hardness values in the HAZ of HSLA steels welded using the GMAW process, resulting in a positive impact on the fatigue life of the joints. Maintaining fatigue properties in the regions of the heat-affected zone is crucial. The increase in the size of the HAZ, coupled with microstructural changes, leads to a reduction in the hardness values in this region, contributing to a decrease in the fatigue life of welded joints. In this study, the effects of using different filler electrodes and shielding gases on the fatigue properties of welded joints in LNE 600 steel with a thickness of 4.75 mm, welded using the GMAW process, were evaluated. It was possible to observe a reduction in the hardness values in the HAZ region and a similar static resistance behavior for all evaluated conditions, except for the ER70S-6 electrode with 5% O2 gas, where the fatigue life showed better results with the application of the ER120S-G electrode.

Effect of laser parameters on fatigue crack growth behavior of AZ31B magnesium alloy TIG welded bionic joint

The purpose of this study was to evaluate the effect of laser parameters on the fatigue crack growth (FCG) behavior of AZ31B Magnesium alloy TIG welded bionic joints with ‘soft-hard’ alternating structures prepared by laser bionic treatment (LBT). The changes of laser parameters promoted the growth of fine equiaxed grain with random texture, and average grain size was 5.12 ± 0.51–8.19 ± 1.02 μm, which significantly increased isotropy and microhardness. The average microhardness was 66.5 ± 1.68–70.85 ± 1.41 HV0.1, which increased by more than 23.11% compared with untreated welded joints. For bionic welded joints, the heat-affected zone (HAZ) had higher FCG resistance and cycle times than weld metal (WM), and the specimens with notch in the HAZ showed obvious crack deflection. It was found that FCG behavior showed significant microstructure sensitivity by comparing the FCG rate curve and crack opening displacement. The Paris parameters had a significant linear relationship and effectively reflect the FCG resistance of the different specimens. The fatigue fracture showed that the crack growth zone was mainly dominated by typical cleavage steps and secondary cracks. The fatigue fracture region contains quasi-cleavage and ductile fracture. A series of strengthening mechanisms were induced by LBT and could significantly affect FCG behavior, which promoted crack deflection and branching to increase the toughness of the joint, while the laser remelting zone increased the strength of the joint. Therefore, the synergistic effect of strength-toughness of the joint was obtained by LBT, which provided a new idea for optimizing the bionic design and improving the fatigue property of welded joints.

Study of fatigue crack growth characteristics of TIG welded AZ31B magnesium alloy joint based on laser bionic treatment

AbstractIt is crucial to obtain better fatigue property for the safe application of materials, while nature provides new structural design ideas for effectively improving fatigue property. In this study, a “soft‐hard” alternating bionic structure was fabricated on the surface of TIG welded AZ31B Mg alloy joints by laser bionic treatment (LBT). Microstructure of welded joints was tailored by LBT, resulting in fine equiaxed grains with random orientations and increased microhardness. The fatigue crack growth threshold of the welded joints with bionic structure is increased by more than 40%, and crack growth rate was lower. LBT could significantly affect crack growth behavior and presented obvious crack deflection, resulting in a rougher fracture surface and more second cracks and branches. LBT significantly improved the fatigue crack growth resistance, inducing a series of strengthening mechanisms. The present work opens a new alternative to improve the fatigue properties of welded joints.

Improving the low-cycle fatigue properties of laser-welded Al–Zn–Mg–Cu alloy joints using double-sided ultrasonic impact treatment

To improve the low-cycle fatigue properties of Al–Zn–Mg–Cu 7075 aluminum alloy laser-welded joints, a post-weld treatment method for double-sided ultrasonic impact treatment (DSUIT) was used to treat the joints. The mechanism of different DSUITs on the microstructure and fatigue properties of welded joints was analyzed. The results showed that DSUIT reduced the welding defects and roughness of the joint surface, and the dendrite structure at the upper and lower surfaces of the joints was broken to form a plastic deformation layer (PDL) with an approximate thickness of 100 μm. Furthermore, the texture strength and grain size at the PDL were reduced. A beneficial residual compressive stress was introduced into the upper and lower surfaces of the joints after the DSUIT. When the number of cycles was 2 × 106, the maximum fatigue strength of the joints was 103.02 MPa after DSUIT, indicating an increase of 111.8% of the untreated joints of 48.62 MPa. Moreover, observance of the fatigue fracture of the joints revealed that the PDL produced by the DSUIT inhibited the initiation and propagation of cracks at the surface. Therefore, the DSUIT effectively improved the low-cycle fatigue properties of the joints.

Effect of plastic pre-strain on the fatigue properties of welded joints of X80 LSAW pipes

The fatigue properties of the original, 1%, 2%, and 3% plastic pre-strained welded joints of X80 pipeline steels were tested to find the effect of plastic deformation on the service of pipelines. Results showed that the fatigue cracks growth period remarkably decreased with the increasing plastic pre-straining extent, reducing the safety of pipeline transportation. The decrease was caused by the cleavage fractures generated by dislocation synthesis reactions during the fatigue process. The results reveal the effects of dislocation distribution on fatigue property of the welded pipe after plastic deformation and provide a reference for the safety design of pipelines.

Research on Fatigue Properties of Typical Welded Joints of DH36 Steel at −60 °C

As the development of the polar region continues to increase, the fatigue properties of structures at low temperature are increasingly receiving researcher attention. This study aimed to investigate the fatigue properties of T-welded and cruciform welded joints at −60 °C. Logarithmic S–N curves based on the hot-spot stress of the T-welded and cruciform welded joints at 50% and 95% confidence levels were obtained at −60 °C. The test results showed that the fatigue properties of T-welded joints were almost 2–7% better than those of the cruciform welded joints at −60 °C. Factors that affected the fatigue properties of welded joints, such as the stress concentration factor, microstructure, Vickers hardness profiles, and fractography, were also studied and the test results showed that the fatigue properties of cruciform welded joints at −60 °C were 57.215% better than at room temperature.

Numerical modeling of residual stresses and fatigue damage assessment of ultrasonic impact treated 304L stainless steel welded joints

Welding technology has been widely adopted to fabricate structures and components due to its low cost and easily accessible procedure. However, the welding methods usually impose tensile residual stresses on the fabricated part, that tend to be detrimental to fatigue properties of welded joints. Ultrasonic impact treatment (UIT) has been introduced as an increasingly popular post-weld treatment technique to improve fatigue performance of welded joints due to its high efficiency and reliability. In this paper, the simulations of welding and UIT processes were performed by means of finite element modeling to predict the residual stress distribution of the 304L stainless steel welded joints. Effects of the UIT on the residual stress distribution and fatigue performance were individually investigated. Finite element results indicated that the UIT introduced beneficial compressive stress up to depths of 2–3 mm in the butt and T-joint welds. The stress distribution at the potential crack plane and the respective stress intensity factors were calculated by taking into account residual stresses, to evaluate the fatigue crack propagation. The fatigue crack growth model was subsequently employed to predict fatigue lives of the welded joints. Results of the fatigue analysis showed that the UIT remarkably extended fatigue lives of the 304L welded joints.

Исследования усталостных свойств сварных соединений в условиях высокочастотного нагружения (обзор)

Исследования усталостных свойств сварных соединений в условиях высокочастотного нагружения (обзор)

Effect of Welding Speed on Fatigue Properties of TC18 Thick Plate by Electron Beam Welding

The welding of 15 mm thick TC18 titanium alloy thick plate was realized by electron beam welding. The effect of different welding speeds (10, 20, 30 mm/s) on the fatigue properties of the electron beam welded joints for TC18 titanium alloy was investigated. The macroscopic morphology, microstructure and fracture characteristics of the joints were analyzed by optical microscope, scanning electron microscopy and transmission electron microscopy, and the fatigue properties of welded joints were studied and tested by an electronic universal testing machine. The results show that the weld fusion zone is mainly composed of columnar β phase and acicular α martensite phase. The upper melting width, the middle melting width and the lower melting width are obviously reduced, and the grain size gradually decreases with the increase of welding speed, which results in the increase of fatigue properties of welded joints. At Nf =107 the fatigue limit of the weld increases by nearly 29% with the welding speed from 10 mm/s to 30 mm/s. The fatigue fracture of the joints can be divided into three typical regions of fatigue crack source zone, expansion zone and instantaneous zone, and the fatigue cracks all originate from the surface of the specimen. With the increase of welding speed, the proportion of instantaneous area decreases and the fatigue performance increases.

Microstructure and fatigue properties of fiber laser welded dissimilar joints between high strength low alloy and dual-phase steels

The aim of this study was to evaluation the microstructure and fatigue properties of welded joints made with fiber laser welding (FLW) on a high strength low alloy (HSLA) and dual-phase (DP980, UTS⩾980MPa) steel in similar and dissimilar material combinations. The fusion zone (FZ) consisted of martensite, and the heat affected zone (HAZ) contained some newly formed martensite and partially tempered martensite on the DP980 steel side. A characteristic asymmetric hardness profile across the dissimilar HSLA–DP980 welded joint was observed. While a soft zone occurred on the DP980 side, it was absent on the HSLA side. Inside the FZ two hardness sub-regions were observed due to the difference in the alloying elements between two steels along with the fast cooling during FLW. The presence of soft zone on the DP980 side had no effect on the tensile properties, since the lowest hardness value in the soft zone was still higher than that of the HSLA base metal (BM). A joint efficiency of 97–100% was achieved with respect to the HSLA. The strain to failure of the dissimilar HSLA–DP980 welded joints was significantly (∼threefold) higher than that of the similar DP980–DP980 welded joints. Although the fatigue strength of the dissimilar HSLA–DP980 welded joints was lower than that of DP980–DP980 welded joints, it was equivalent to that of HSLA–HSLA welded joints. Failure occurred in the BM on the HSLA side in the tensile tests and fatigue tests at high cyclic stress levels, where yielding occurred. At the intermediate and lower cyclic stress levels, fatigue failure occurred in the weld area due to the higher sensitivity to the weld concavity.

Review of Status about the Effect of Ultrasonic Impact on Fatigue Properties of Welded Joints of Magnesium Alloy

The study on magnesium alloy has attracted attention at home and abroad. Status and development trend about the fatigue properties of welded joints of magnesium alloy were reviewed. The main problems and deficiencies were pointed out, and the recent development's focus was outlined. The development trend of improving the fatigue propeties and fatigue life was analyzed. The ultrasonic impact method can not only decrease the stress concentration coefficient and tensile residual stress of welded joint, but also refine the grain size of welded joints, even to compressive stress and nanograins. The method has put forward a new way for increasing fatigue properties and life of welded joint of magnesium alloy.

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.

Improve the Fatigue Performance of Welded Joints with Undercuts by TIG Dressing Treatment

Undercuts often exists in the surface at weld toe. Undercuts affects the capability of weld subjected to dynamic loading greatly and fatigue cracks usually initiate from these zones with undercuts. Combining with low-alloy middle strength steel 16Mn，study of increasing the fatigue strength and the fatigue life was carried out. Compared with specimen without defects, the fatigue strength of specimen with undercuts has been increased by 10~20%. Study also shows that comparing with as-welded specimen with defects, with TIG dressing treatment, fatigue strength of cruciform weld joint of steel 16Mn has been increased by 70%, fatigue life by 5~8 times. Compared with as-welded specimen without defects, fatigue strength has been increased by 34%, fatigue life by 3~4 times. Undercuts have no adverse effects on fatigue property of welded joints under TIG dressing.

Very high cycle fatigue behavior of bridge steel welded joint

Very high cycle fatigue (VHCF) behaviors of bridge steel (Q345) welded joints were investigated using an ultrasonic fatigue test system at room temperature with a stress ratio R = −1. The results show that the fatigue strength of welded joints is dropped by an average of 60% comparing to the base metal and the fatigue failure still occurred beyond 107 cycles. The fatigue fracture of welded joints in the low cycle regime generally occurred at the solder while at the heat-affected zone (HAZ) in the very high cycle regime. The fatigue fracture surface was analyzed with scanning electron microscopy (SEM), showing welding defects such as pore, micro-crack and inclusion were the main factors on decreasing the fatigue properties of welded joints. The effect of welding defects on the fatigue behaviors of welded joints was discussed in terms of experimental results and finite element simulations.

Research on Ultra-High Cycle Fatigue Property of 45 Steel

Fatigue property of 45 steel was studied in this paper with the method of ultrasonic fatigue testing, and SEM was used to analyze microscopic characteristics of the fatigue fracture. Fatigue test results show that: S-N curves descend continuously after 108 cycles, there is no fatigue limit as the traditional fatigue conception describes. Therefore, it is very dangerous to design welded structure working in the ultra-high cycle interval with the fatigue strength corresponding to 5×106 cycles. In the super-long life range, the fatigue property of welded joints is worse than the base metal. SEM analysis shows that: fatigue crack mainly initiates from the defects in the surface or sub-surface.

New developments at the recent update of the IIW recommendations for fatigue of welded joints and components

AbstractThe International Institute of Welding (IIW) has published its first comprehensive recommendation on fatigue design of welded structures in 1996. The document appeared in English, French, German and Japanese. The idea was to describe the fatigue properties of welded joints and components independently from application groups with their specific preferences and interests. So, a comprehensive code was established, which covered all current methods of verification, such as component testing, nominal stress, structural stress, notch stress method as well as fracture mechanics assessment procedures. Detailed guidance for assessment of weld imperfections was also given. The outlined safety philosophy covers the different strategies and provides a detailed choice for the designer. The document had a considerable impact on code making bodies and thus it became an important source for the Eurocode. After more than one decade, a new update of the IIW recommendations has been published. The main areas of update are firstly a new consideration of high cycle fatigue in the grid of the Woehler S‐N curves, a standardized meshing for the structural hot‐spot stress concept, which allows now an economic and coarser meshing in finite element analysis, further on the extension of the effective notch stress concept to welded aluminium structures, followed by a numerical assessment of post weld treatments for improving the fatigue properties, and finally a new approach to cumulative damage in connection with multi‐axial stress. The update is a result of a worldwide collaboration of leading research institutions and it is expected that the new update will have the same impact on design and codes as the preceding document.

Fatigue properties of welded joints using steel with high resistance to fatigue crack growth

It has been generally recognized that the fatigue life of welded joints is little influenced by the strength of steels owing to the high-stress concentration and the tensile residual stress near the weld toe. In this paper, improvement of the fatigue life of welded joints using steel with high resistance to fatigue crack growth (ferrite/martensite (F/M) steel) is investigated. F/M steel has a microstructure with an elongated and banded martensite phase distributed in a ferrite matrix and a fatigue crack growth rate of about one-half to one-tenth in the thickness direction, compared with conventional steel. As a result, the fatigue life of an out-of-surface gusset-welded joint increases with the decrease of the fatigue crack growth rate. The fatigue life of welded joints using F/M steel with the highest resistance to fatigue crack growth increases to about twice that of joints using conventional steel. Whereas the fatigue crack growth rate decreases significantly, the fatigue life of welded joints increases only slightly. This can be attributed to the stress ratio independent of the fatigue crack growth rate. In other words, the fatigue crack growth rate of F/M steel increases with the increase of the stress ratio, approaching that of conventional steel. In the case of welded joints, even if a fatigue test is carried out at a low-stress ratio, the region near the weld toe is under a high-stress ratio due to tensile residual stress. Therefore, improvement of the fatigue life of welded joints becomes comparatively small so that the effect of fatigue crack retardation of F/M steel decreases.

自動車用高強度鋼板の溶接継手の疲労特性におよぼす溶接法の影響

In high strength steel sheets, it seems to be possible to improve the fatigue strength of joints welded by using laser welding due to the lower heat input of welding and the flatter weld beads compared with conventional welding methods such as arc welding or resistance spot welding. Therefore, in this paper, fatigue tests were conducted for the specimens with the butt joints of 780 MPa grade steel sheets welded by using laser welding, plasma welding and MAG welding and for the specimens flattened the weld beads to investigate the effect of welding methods on the fatigue properties of welded joints of high strength steel sheets for automobiles. And the macrostructure, the microstructure and the hardness distribution around the weld metal were observed to study the mechanism for the difference of the fatigue strength by the welding methods. It was found that the fatigue strength in arc-welded joints and plasma-welded joints is lower than that in laser-welded joints mainly due to the occurrence of softening in the heat affected zone. On the other hand, it was found that the stress concentration at the toe of the weld bead significantly deteriorates the fatigue strength even in the laser-welded joints.

311 自動車用突合せ溶接継手の衝撃強度に及ぼす予疲労の影響(OS 材料・構造の動的特性(3))

Recently, the tailored blanking technique using high tensile strength steels begins to be adopted in car industry to reduce the weight of automotive bodies without any decreases of their stiffness and strength. Therefore, it is important to storage of the fundamental data concerning the technique such as tensile, compressive, shearing and fatigue properties of welded joints. In this research, the effect of pre-fatigue on the impact tensile properties of the welded butt joint of high strength steel plates was investigated by means of the split Hopkinson bar method for tensile test.