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A holistic investigation on out-of-plane welding distortion in fabrication of several stiffened welded structures with considering prediction and mitigation is presented. Experiments of typical fillet welded joints and its assembled stiffened welded structures are conducted first, and out-of-plane welding distortion is systematically measured. Different welding distortion patterns, so-called bending distortion and welding buckling, are obviously obtained from the measurements of the examined parallel and cross stiffened welded structures, respectively. A combined computational approach, which involves thermal elastic plastic (TEP) finite element (FE) analysis, eigenvalue analysis, and elastic FE analysis, is then employed to predict the out-of-plane welding distortion and clarify the generation mechanism for different welding distortion patterns. In particular, eigenvalue analysis can support the critical condition for welding buckling occurrence. Since the bending and welding buckling are caused by different reasons, the mitigation processes with flame heating are carried out individually. In detail, line heating is implemented in the opposite side of welded joint to produce inverse bending, and spot heating is employed to heat the region far away from welding line to eliminate effect of inherent deformation on welding buckling generation.

PurposeThis paper aims to study and modify the notch equivalent stress method, as well as to establish the notch equivalent stress range S–N curve and apply it to the fatigue assessment of engineering examples.Design/methodology/approachThis paper studies the notch equivalent stress method and puts forward the concept of “singular equivalent crack”. Combined with the fatigue test results, by proposing to consider the singular coefficient of the transition angle of the welded structure and the introduction of material correction factors, this paper derives the notch equivalent stress equation for commonly used welded joints applicable to steel, and finally establishes the notch equivalent stress range of the S–N curve.FindingsThe obtained results show that the dispersion of fatigue data is 65.6 and 75.4% for T-joints and transverse cross-joints, respectively, under S–N curves using notched equivalent stress compared to the nominal stress range. The fatigue evaluation error of the modified notch equivalent stress equation for transverse cross welded joints improved by 50.65%, 53.1 and 39.6% on average, respectively, compared to the original other methods. The fatigue evaluation error for T-joints improved by 13.4 and 13.9%, respectively, compared to the original other methods.Originality/valueThere are relatively few studies on the fatigue assessment of notch equivalent stress method. In this paper, the notch equivalent stress method is studied and modified to improve the accuracy of fatigue assessment of welded structures with singular stresses.

Since many factors affect the fatigue life of welded structures and the relationships between them are complex, finding effective ways to improve the fatigue reliability of welded structures has always been a challenge for the industry. A fatigue reliability assessment model and design method for welded structures based on the structural stress method is proposed. According to the limit state equation of the model, the design-life fatigue reliability and the service fatigue reliability assessment approaches are presented. Then, a first-order second-moment method is employed to reveal quantitative fatigue reliability for welded joints and structures. The relationships between the fatigue reliability and the influence variables are used to analyze and improve the welded structure design. Fatigue reliability of T-joint examples was assessed by accounting for weld leg sizes, plate thicknesses, penetration depths, and single-sided and double-sided forms. Furthermore, the assessment model was verified with a welded bogie frame. The fatigue reliability of the frame was improved based on the evaluation results. The model and method can be applied to other types of welded joints and welded structures.

PurposeThis paper aims to present an engineering computational method for fatigue life evaluation of welded structures on large-scale equipment under random vibration load.Design/methodology/approachBased on a case study of the traction transformers, virtual fatigue test (VFT) was proposed via numerical simulation approach. Static analysis was conducted to identify the risky zone and then dynamic response of the risky welds under random vibration load was calculated based on frequency-domain structural stress method (FDSSM) theory, life distribution and associated survivability at various locations of the structure were obtained. Structural modification was finally performed according to the evaluation results. Moreover, experimental test was carried out and compared with the virtual test result.FindingsBy applying the virtual test, fatigue life of the complex welded structures on large-scale equipment can be accurately and efficiently obtained considering dynamic effect under random vibration load. Meanwhile, risky welds can be directly determined and targeted modification scheme can be accordingly concluded. Validity of the VFT result was proved by comparing with the experimental test.Originality/valueThe proposed method can help obtain equivalent structural stress and fatigue life distribution of the welded structure at any position with various survivability and make quantitative evaluation on the life-extending effect of the structural modification. This method shows significant cost and efficiency advantages over experimental test during design stage of the large-scale structures in numerous manufacturing industries.

PurposeThis study aims to extend the application of the quality category approach in rapid fatigue assessment of complex welded structures containing defects under arbitrary loadings, following the investigation of their core data and fatigue assessment procedures based on fracture mechanics.Design/methodology/approachThe analysis methods and procedures for calculating equivalent sizes of semi-elliptic cracks and initial sizes of through-width cracks at the weld toe were developed based on the life equivalence principle. Different stress concentration solutions, i.e. 2D-Mk and 3D-Mk solutions, and different bending ratios were considered. Then, approximate equations were proposed to calculate the crack size under combined stress. In addition, a procedure for calculating the fatigue life by interpolation was proposed and applied to engineering examples.FindingsThe fatigue lives of fillet and butt weld joints obtained with the 3D-Mk solution for large L/B are longer than those obtained with the 2D-Mk solution. The results of the fatigue life of the brake unit bracket show that the average error between the proposed approximation equations and the quality category approach is 1.6%.Originality/valueThe quality category and equivalent size curves of different stress concentration solutions under combined membrane and bending stresses are newly added, which further expands the application of the quality category approach. When the proposed fatigue life calculation methods are employed, the remaining life can be quickly derived in addition to the qualitative conclusion on the safety of the structure. These provide the necessary conditions to perform a rapid fatigue assessment adapted to engineering purposes.

In the small sample fatigue test space, in order to obtain the master S-N curves of each reliability of the titanium alloy welded structures. Firstly, the Bootstrap method is combined with the equivalent structural stress to determine the minimum number of samples under each level of equivalent structural stress, and the appropriate resampling times are obtained through residual analysis. Secondly, based on the fatigue test data of titanium alloy welded joints with different plate thicknesses, materials, types and stress ratios, the linear regression model of master S-N curves are determined by using the traditional group method and Bootstrap method, and its reliability is 99%, 95%, 84%, and 50% respectively. Finally, the comparative analysis shows that the titanium alloy fatigue test samples obtained by Bootstrap method tends to be more stable, which improves the fitting degree of the titanium alloy fatigue life test data and the accuracy of the fatigue life prediction when the sample is small. A more accurate regression model of the master S-N curves of titanium alloy welded structures was obtained and verified.

Dissimilar steel welded structures are commonly used in the marine engineering field. Owing to the scarcity of in-depth investigation into the intricate pattern of residual stress distribution in welding within 316L/Q345 dissimilar steel welded joints and methods for reducing this stress, a platform-based vibratory stress relief (VSR) experimental system was established to comprehensively study the effects of VSR on the mechanical properties and microstructure of 316L/Q345 welded structures. Scanning electron microscopy (SEM) was used to examine the fracture morphology and explore the intrinsic mechanisms by which VSR enhances the mechanical properties of welded joints. The findings suggest that VSR is capable of significantly homogenizing and diminishing the welding residual stress within the heat-affected area of 316L/Q345 mismatched steel welded specimens. The significant reduction in residual stress after VSR can primarily be attributed to the combination of alternating stress applied by the VSR platform and the welding residual stress, which exceeded the yield limit of the metal materials. Furthermore, the significant reduction in residual stress, refinement of second-phase particles, and changes in fracture mechanisms are the main reasons for the increased strength observed after VSR. This study has significant engineering application value, providing a theoretical basis for the use of VSR treatment to enhance the reliability of the safe operation of marine engineering equipment.

The overall stiffness and modal frequency of the car body of a rapid box car are reduced by the design of the full-side open movable side door structure. The vibration fatigue performance of the welded structure in this car body needs to be verified. The rigid-flexible coupling model of the rapid box wagon was established first, and the model was verified by modal test data. By the application of the virtual iteration method on this model, the displacement excitation loads of this vehicle were acquired. The effectiveness of the load reverse obtaining technology was verified through the comparison between calculated data and the experimental data. Based on the rigid-flexible coupling model and the load obtained by reverse engineering, the fatigue life of the welded structure in the car body was evaluated through the modal structural stress method. The calculated results show that the car body structure obtains obvious modal vibration, which leads to short fatigue life in several weld lines. According to the application requirements of this wagon, the local improvement scheme was proposed, and the effect of the improvement program was evaluated. In this paper, a new fatigue evaluation technology based on the load reverse method of test data was proposed, which provides a theoretical basis for the structural design and program improvement of railway vehicles.

Electron beam welding (EBW) is widely used to connect thin-walled high-temperature alloy structures in aeroengines. However, the residual stress and deformation caused by the high-temperature gradients generated during welding would affect the rigidity, dimensional stability, and fatigue resistance of the welded structures. The study reported here used a model combining ellipsoidal and Gaussian rotating body heat sources to undertake a numerical simulation of the temperature and residual stress generated during the EBW process. The model was systematically refined by observing and measuring the molten pool morphology. The error rates for key dimensions determining the molten pool shape were less than 6%. With a microscope-based examination, the energy distribution characteristics were detected by the microstructure analysis of grain type and size in different regions to verify the viability of the heat source model. The residual stress of a butt welding was simulated by the proposed heat source model based on the full consideration of a full-loop thin-walled combustor casing structure and material properties. It was found that the average errors for longitudinal and transverse residual stresses of welded joints and their adjacent areas were 10% and 12%, respectively, by comparing with the experimental results. Simultaneously, the low cycle fatigue life of the welded combustor casing would be decreased by 32% considering the influence of welding residual stress. These conclusions can be used as a basis for studying the integrity of thin-walled welded structures in aeroengines.

PurposeIn order to use the BS EN 15058-3 principle more scientifically to design the welding structure of rail vehicles, a method of stress state assessment of welding joints meeting the requirements of BS EN 15058-3 is proposed by using IIW-2008 and ASME-BPVC-VIII-2:2015 standard.Design/methodology/approachThe stress state evaluation process of two standards is studied, and the stress state evaluation method of two standards is programmed by computer language. Among them, ASME standard can evaluate the stress state of welding structures without defects and with defects. In order to verify the feasibility of the method, under the fatigue load of en13749 standard, the method is applied to the welding structure design of the rail car frame.FindingsThe results show that the evaluation based on IIW-2008 standard is stricter, and the stress factor of the weld between the crossbeam and the traction pull rod seat is the largest, the value is 0.881, and the stress state grade is medium. With the increase of the number of defects, the stress level of the welded joint increases and the fatigue life decreases.Originality/valueThis study can provide a reference for the welding design of rail vehicles and other complex structures and has a certain engineering guiding significance.