Effects of residual stresses on the fatigue behaviour of welded steel structures

Residual stresses due to the welding process in steel structures can significantly affect the fatigue behaviour. Usually, high tensile residual stresses up to the yield strength are conservatively assumed at the weld toes. This conservative assumption can result in misleading fatigue assessments. Areas with compressive residual stresses may be present in complex structures, where the details are less critical than predicted. This is shown in the paper by the example of fillet‐welded stiffener ends, where beneficial compressive residual stresses cause the initiation of fatigue cracks at other locations in less‐strained areas. Another example for the effects of residual stresses concerns the stress initiation and propagation at a structural detail under fully compressive load cycles. Fatigue cracks are possible here due to high tensile residual stress fields. The conclusion is that the welding‐induced residual stresses should be known in advance for a reliable fatigue assessment, which becomes possible to an increasing extent by numerical welding simulation.     

Investigation of residual stresses in a repair‐welded rail head considering solid‐state phase transformation

Repair welding for recovery from local damage of a rail head surface is known to cause high residual stress and can accelerate fatigue in the rail. This study examines repair‐welded rails by applying experimental and numerical approaches. In the former approach, two newly manufactured rail specimens and four repair‐welded rail specimens with two different weld depths were prepared, and their residual stresses were measured with a sectioning method. In the latter approach, a finite element repair welding simulation model was developed that adopted a prescribed temperature method with a moving block as an input heat source, and the thermal strain caused by the volume change due to solid‐state phase transformation was considered. Overall, the residual stresses correlated well between the experimental and numerical approaches. The measured high compressive residual stress of ?290 MPa seems to be beneficial to prevent a crack initiation in the rail surface.     

Fatigue assessment of high‐frequency mechanical impact (HFMI)‐treated welded joints subjected to high mean stresses and spectrum loading

The fatigue strength of welded joints can be improved with various post‐weld treatment methods. High‐frequency mechanical impact treatment is a residual stress modification technique that creates compressive residual stresses at the weld toe. However, these beneficial residual stresses may relax under certain loading conditions. In this paper, previously published fatigue data for butt and fillet welded joints subjected to high stress ratios and variable amplitude cyclic stresses were evaluated in relation to the current International Institute of Welding (IIW) recommendations on fatigue strength improvement and a proposed IIW design guideline for high‐frequency mechanical impact‐treated welded joints. The evaluation showed that the current IIW recommendations resulted in both non‐conservative and overly conservative fatigue strength estimations depending on the applied stress level, whereas the proposed fatigue assessment guideline fitted the current data well.     

On the Residual Stress Field in the Aluminium Alloy FSW Joints

Abstract:  In this study, we evaluated the residual stress field which arose because of a new welding process named 'friction stir welding'. We analysed aluminium alloy butt-welded joints. Both similar and dissimilar joints were considered in 2024-T3 and 6082-T6 aluminium alloys of 0.8 and 3 mm thick. For each joint, the longitudinal and transversal residual stress distributions were obtained in a direction perpendicular to the weld cord. In the thicker dissimilar joints, the longitudinal residual stress distribution present is very similar to the distribution present in traditional welded joints. It presents, in fact, a tensile region near the weld cord, which is balanced by compressive regions away from weld line. On the contrary, other joints present a low compressive stress at the weld toe and a tensile stress state elsewhere. Moreover, the effect of the shoulder geometry on the residual stress field was evaluated on 1.5-mm-thick joints.     

Fatigue crack initiation assessment of welded joints accounting for residual stress

The impact of residual stresses on the fatigue crack initiation life of welded joints is evaluated by the finite element method. The residual stresses of nonload‐carrying cruciform joints, induced by welding and ultrasonic impact treatment, are modelled by initial stresses, using the linear superposition principle. An alternative approach of using modified stress‐strain curves in the highly stressed zone is also proposed to account for the residual stress effect on the local stress‐strain history. An evaluation of the fatigue crack initiation life of welded joints based on the local strain approach is carried out. The predicted results show the effect of residual stresses and agree well with published experimental results of as‐welded and ultrasonic impact treated specimens, demonstrating the applicability of both approaches. The proposed approaches may provide effective tools to evaluate the residual stress effect on the fatigue crack initiation life of welded joints.     

New Developed Welding Electrode for Improving the Fatigue Strength of Welded Joints

A new welding electrode, low transformation temperature electrode (LTTE), was introduced in this paper. It was described in design principle, mechanics, chemical compositions of their deposited metal and manufacturing methods. It was proved that the best transformation starting temperature from austenite to martensite of the deposited metal of LTTE was at about 191℃ and it was obtained by adding alloying elements such as Cr, Ni, Mn and Mo. The microstructure of the weld metal of the LTTE was low carbon martensite and residual austenite. The compressive residual stress was induced around the weld of the LTTE and the -145 MPa in compression could be obtained in middle of weld metal. The fatigue tests showed that the fatigue strength of the longitudinal welded joints welded with the LTTE at 2×106 cycles was improved by 59% compared with that of the same type of welded joints welded with conventional E5015 and the fatigue life was increased by 47 times at 162 MPa. It is a very valuable method to improve th     

Modellierung des Ermüdungsrisswachstums in Schweißverbindungen unter Berücksichtigung von Schweißeigenspannungen

The present paper contains research results determined within the framework of a project called IBESS (?Integrale Bruchmechanische Ermittlung der Schwingfestigkeit von Schweißverbindungen“) by the Materials Mechanics Group of the Technische Universität Darmstadt [1]. Aim is to calculate the fatigue life of welded joints by taking into account the effect of residual stresses and the influence of the weld toe geometry. Here, the fatigue life is regarded as period of short fatigue crack growth. Two and three dimensional finite element models, with cracks as initial defects, are constructed for this purpose. Fatigue crack growth analyses are performed by using the node release technique together with the finite element program ABAQUS. The welding residual stresses as well as the plasticity induced crack closure effects are considered. Structural calculations are performed in order to introduce residual stress fields in finite element models. The calculated compressive residual stress field matches the measured one especially in the weld notch area. The effective cyclic J‐integral (ΔJ_eff) is used as crack tip parameter in a relation similar to the Paris equation for the calculation of the fatigue life. For this purpose, a Python code was written for the determination of ΔJ_eff at every crack length phase. The calculated fatigue lives were compared with experimental data and a good accordance between both results was achieved. The impact of welding residual stresses on ΔJ_eff as well as on the fatigue life during short crack growth was investigated. As expected, results revealed that at lower stress amplitude, a compressive residual stress field is favorable to the fatigue life, whilst a tensile residual stress field is unfavorable. The influence of residual stresses can be neglected only for large load amplitudes.     

初始应力对三维光学轮廓法测试焊接接头残余应力的影响

目的研究不同初始应力状态下,三维光学轮廓法测试焊接接头残余应力的变化规律。方法采用MIG焊分别对供货态与去应力退火态试板进行多层多道焊,焊后试板经慢走丝切割,经三维光学测量技术扫描切割面轮廓,将所得轮廓数据经所建立的数据处理平台处理,将其结果作为有限元计算的边界条件,经应力反算得到残余应力分布。最后再进行有限元模拟,计算焊接接头残余应力。结果含初始应力、去应力退火和数值模拟的焊缝中心均为拉应力区,最大拉应力分别为480, 450, 523 MPa,且都位于焊缝根部区域。三者试板两侧为压应力区域,最大压应力分别为380, 280, 157 MPa,三者数值相差较大。结论将含有初始残余应力试板、退火处理试板与数值模拟结果的残余应力分布进行对比,可以发现三者在焊缝中心处的残余应力分布较为一致,但沿着焊缝向两侧的区域内,应力差别逐渐变大。主要原因为焊接热循环温度高于金属再结晶温度时可以消除部分残余应力,而温度循环较低时对应力消除不明显,导致实验结果相差较大。     

Fatigue behaviour of friction stir welded AA2024‐T3 alloy: longitudinal and transverse crack growth

The fatigue crack growth properties of friction stir welded joints of 2024‐T3 aluminium alloy have been studied under constant load amplitude (increasing‐ΔK), with special emphasis on the residual stress (inverse weight function) effects on longitudinal and transverse crack growth rate predictions (Glinka's method). In general, welded joints were more resistant to longitudinally growing fatigue cracks than the parent material at threshold ΔK values, when beneficial thermal residual stresses decelerated crack growth rate, while the opposite behaviour was observed next to K_C instability, basically due to monotonic fracture modes intercepting fatigue crack growth in weld microstructures. As a result, fatigue crack growth rate (FCGR) predictions were conservative at lower propagation rates and non‐conservative for faster cracks. Regarding transverse cracks, intense compressive residual stresses rendered welded plates more fatigue resistant than neat parent plate. However, once the crack tip entered the more brittle weld region substantial acceleration of FCGR occurred due to operative monotonic tensile modes of fracture, leading to non‐conservative crack growth rate predictions next to K_C instability. At threshold ΔK values non‐conservative predictions values resulted from residual stress relaxation. Improvements on predicted FCGR values were strongly dependent on how the progressive plastic relaxation of the residual stress field was considered.     

Fatigue life improvement for cruciform welded joint by mechanical surface treatment using hammer peening and ultrasonic nanocrystal surface modification*

For the improvement of the fatigue strength for welded structures, mechanical posttreatments have been applied in various industrial fields and in most cases have been found to give substantial increases in their fatigue lives. These methods, generally, consist of the modification of weld toe geometry and the introduction of compressive residual stresses. In mechanical surface treatments, for example, PHP (pneumatic hammer peening) and UNSM (ultrasonic nanocrystal surface modification), the weld profile is modified due to removed or reduced minute crack‐like flaws, and compressive residual stresses are also induced. In this study, a PHP procedure and a UNSM device were introduced, and a quantitative measure of fatigue strength improvement was performed. The fatigue strength at 2 × 10⁶ cycles of hammer‐peened and UNSM treated on a non‐load‐carrying cruciform welded joint shows 220 and 260 MPa, respectively, which are more than two times higher than that of as‐welded specimen. Especially, the surface layer in the vicinity weld toe treated by the UNSM provides nanocrystal structure created by an ultrasonic cold forging and introduces very high welding residual stress in compression.     

Synchrotron diffraction measurement of residual stresses in friction stir welded 5383-H321 aluminium butt joints and their modification by fatigue cycling

This paper presents two‐dimensional information on the residual stresses in 8 mm 5383‐H321 aluminium plates joined by double pass (DP) friction stir welding (FSW). It considers the inherent variability in residual stress magnitudes along 0.5 m lengths of weld pass, and their modification under a sequence of applied fatigue loads. This represents one of a planned series of experiments aimed at illuminating the effects of fatigue cycling on residual stress fields. In this particular case, the magnitudes of the bending fatigue loads (R= 0.1) were chosen to correlate with the measured proof strengths of the weld metal (approximately 160 MPa) and the parent plate (approximately 260–270 MPa). In four‐point bend S–N tests at R= 0.1 on 40 mm wide FS welded specimens of this alloy and plate thickness, these peak stress levels correspond to lives of around 10⁵ cycles and 10⁷ cycles, respectively. Results from the work indicate that significant variability exists among welded plates in peak compressive stress magnitudes (a range of perhaps ?50 MPa to ?140 MPa), although peak tensile stresses were relatively low and more consistent (from around 0 to 30 MPa). Fatigue loading accentuates the peak‐to‐valley stress change and causes an overall translation of the stresses to become more positive. Peak tensile stresses increase several‐fold during fatigue cycling.     

A comprehensive investigation on the effects of laser and shot peening on fatigue crack growth in friction stir welded AA 2195 joints

The effects of various surface treatment techniques on the fatigue crack growth performance of friction stir welded 2195 aluminum alloy were investigated. The objective was to reduce fatigue crack growth rates and enhance the fatigue life of welded joints. The crack growth rates were assessed and characterized for different peening conditions at a stress ratio (R) of 0.1, and 0.7. The surface and through-thickness residual stress distribution were also investigated and presented for the various regions in the weld. Tensile residual stresses introduced during the welding process were found to become significantly compressive, particularly after laser peening. The effect of the compressive stresses was deemed responsible for increasing the resistance to fatigue crack growth of the welds. The results indicate a significant reduction in fatigue crack growth rates using laser peening compared to shot peening and native welded specimens. This reduced fatigue crack growth rate was comparable to the base unwelded material.     

Unique fatigue threshold and growth properties of welded joints in a tensile residual stress field

The fatigue threshold and high growth rate region properties were investigated on several kinds of welded joints. These properties became unique in spite of the variation of steels (ferrite-pearite, martensite, austenite), welding method, heat input and stress ratio. It was revealed that the unique properties occurred from the fully opened fatigue crack due to the tensile residual stresses. Based on these results, the equation of the fatigue crack growth curve for the design and inspection of welded structures was proposed. It is also suggested that the inducement of compressive residual stress at the fatigue critical zone is effective in improving the fatigue properties of welded structures.     

A residual stress dependent multiaxial fatigue life model of welded structures

In this paper, the influence of the residual stress on the fatigue performance of a welded structure under multiaxial loading modes is studied. First, the local stress state at weld toe is modified via introduction of the residual stress, and a new fatigue life estimation model considering the effect of the residual stress is established by modifying our recently proposed critical plane method. Second, the basic theory and procedure of the finite element simulation on the calculation of the welding residual stress are presented. Finally, a numerical simulation of an aluminum alloy flange‐to‐tube welding process is conducted, and the calculated residual stress is verified with X‐ray diffraction measurement. Furthermore, the performance of the proposed fatigue life estimation model is verified by the experimental data obtained in the fatigue test under different loading modes. It confirms that the consideration of the residual stress is important, especially under the out‐of‐phase loading mode.     

Numerical analysis of low cycle fatigue for welded joints considering welding residual stress and plastic damage under combined bending and local compressive loads

The numerical analysis of low cycle fatigue of HTS‐A steel welded joints under combined bending and local compressive loads are implemented using the damage mechanics approach. First, a finite element numerical simulation of the welding process is employed to extract the welding residual stresses, which are then imported as initial stresses in the subsequent fatigue analysis. Second, a multiaxial fatigue damage model including damage coupled elasto‐plastic constitutive equations and plastic damage evolution formulation is applied to evaluate the mechanical degradation of the material under biaxial fatigue loads. Further, the fatigue lives of the HTS‐A steel welded joints are computed and compared with the experimental results from literature. A series of predicted load‐life curves clearly illustrates the variation of fatigue lives along with the combined loadings. Finally, the effects of local compression on accumulated plastic strain and fatigue damage are studied in detail. It is revealed that the local compression induces a damage competition between two critical zones.     

Effect of welding wire and groove angle on mechanical properties of high strength steel welded joints

Mechanical properties of high strength steel welded joints strictly depend on the welding process, the filler material composition and the welding geometry. This study investigates the effects of using cored and solid welding wires and implementing various groove angles on the mechanical performance of weld joints which were fabricated employing the gas metal arc welding process. It was found that weld joints of low alloy, high strength steels using low alloy steel cored welding wires exhibited higher tensile strength than that of low alloy steel solid wire and chromium‐nickel steel bare welding wire when the method of gas metal arc welding is employed. The effect of groove angle on the strength and toughness of V‐groove and double V‐groove butt‐joints was investigated. V‐groove joints, with higher tensile strength than double V‐groove joints in the whole range of groove angles, were superior in toughness for small groove angles, but impact toughness values of both joints were comparable for large angles. The effect of heat input and cooling rate on the weld microstructure and weld strength was also investigated by performing thermal analysis employing the commercial software ANSYS. It was concluded that cooling rate and solidification growth rate determined the microstructure of the weld zone which had great consequences in regard to mechanical properties.     

Finite Element Analysis of Modeling Residual Stress Distribution in All-position Duplex Stainless Steel Welded Pipe

On the basis of the thermal-elastic-plastic theory, a three-dimensional finite element numerical simulation is performed on the girth welded residual stresses of the duplex stainless steel pipe with ANSYS nonlinear finite element program for the first time. Three-dimensional FEM using mobile heat source for analysis transient temperature field and welding stress field in circumferential joint of pipes is founded. Distributions of axial and hoop residual stresses of the joint are investigated. The axial and the hoop residual stresses at the weld and weld vicinity on inner surface of pipes are tensile, and they are gradually transferred into compressive with the increase of the departure from the weld. The axial residual stresses at the weld and weld vicinity on outer surface of pipes is compressive while the hoop one is tensile. The distributions of residual stresses compared positive-circle with negative-circle show distinct symmetry. These results provide theoretical knowledge for the optimization of p     

Diffraction‐Based Residual Stress Analysis Applied to Problems in the Aircraft Industry

Cost reduction and weight saving are most important principles governing design and construction of aircrafts. Advanced alloys and thermo‐mechanical treatments as well as new and optimized production processes are being developed. For example, welding of fuselage components like stringers or clips made of Al alloys can have several advantages over riveting. Retention of optimum weld microstructure and properties as well as control of welding related residual stresses and distortion is essential. In the context of risk analysis and damage tolerance it will be of growing importance to study residual stress fields in weld configurations and their influence on fatigue crack propagation. In this paper, methods to evaluate residual stresses in turbine discs and laser welded Al joints are reviewed.     

Using FEM to determine the thermo-mechanical stress in tube to tube–sheet joint for the SCC failure analysis

The stress corrosion cracking in a weld of the tube to tube–sheet region of heat exchangers is a common problem. Thermo-mechanical stress in tube to tube–sheet joints including welding effect should be determined in this situation for failure analysis. In this paper, the Finite Element Method (FEM) is used to predict the thermo-mechanical stresses including welding residual stress in a tube to tube–sheet weld. Both the thermo-mechanical stress distribution with and without the welding residual stress have also been investigated by numerical simulation. The welding, operating temperature, and operating pressure have effect on total stresses. Especially, the welding residual stresses play an important role in total stress state in tube to tube–sheet joints. Geometric discontinuities of the vicinity of gap cause the welding joint to experience a local stress concentration. A high tensile stress in the tube to tube–sheet region has been demonstrated by FEM, which is the stress aspect for the SCC phenomenon of austenitic stainless steel in chloride environment.     

Laser welding of copper‐niobium microcomposite wires for pulsed power applications

The laser welding of copper‐niobium microcomposite wires was investigated. It was determined that the joint structure does not have welding defects, while microscopic examination of the joint cross‐section showed that the microstructure of the autogenous weld consists mainly of a copper‐based solid solution strengthened by niobium‐rich precipitations. The weld obtained with use of filler material consists of two distinct zones, which are formed due to melting of filler wire and microcomposite wire. This structure of the joint provides an insignificant increase in electrical resistance and sufficient ultimate strength and plasticity of the joint. The tensile strength of the sample welded without filler material reaches 335 MPa, but such welded joints are very brittle due to very low ductility. However, an autogenous laser welding joint has about 1.6 times better ductility, and the tensile strength of the joint depends on the applied filler material and is equal to the tensile strength of this material.