Fatigue Life Of Welds Research Articles

Dynamic reconstruction and weld fatigue evaluation of top-tensioned riser based on strain monitoring

Top-tensioned riser (TTR) is essential for offshore oil and gas transportation, which is welded by several standard steel pipes. The intricacies of monitoring, and the particularity of welded joints, render the riser's girth welds present potential service hazards. Here, we propose a riser monitoring and fatigue evaluation method (RMFE), which uses the riser's reconstruction results with strains, to assess girth weld damage. By implementing the orthogonal strain sensors layout and applying a least-squares function, we not only achieve high-precision real-time riser shape reconstruction with high mean stress in global status, but also achieve the structural stress reconstruction of the girth weld at the element level. The research results show the riser's weld fatigue life by RMFE system, based on the “strain reconstruction”, aligns with the results from ABAQUS and FEM theory, with most errors within 3% and the maximum error not exceeding 16%. Furthermore, smaller mesh sizes in the RMFE system can reduce calculation errors but increase computation time. Importantly, the structural stress method, incorporated in the RMFE system, can thoroughly consider mean stress correction for weld fatigue, and eliminate subjective error in selecting the master S-N curve, which has been extended to three-dimensional space to better meet engineering needs.

Multi-objective optimization design for anti-fatigue lightweight of dump truck carriage combined with machine learning

As urbanization continues to accelerate, dump trucks assume an increasingly important role in the transportation and construction of infrastructure. The carriage represents a critical structural assembly of dump trucks. One of the primary failure modes of the carriage is weld fatigue failure, which frequently gives rise to the problem of weld fatigue cracking during transportation. To increase the fatigue life of welds and enhance the degree of structural lightweight of a heavy dump truck carriage, a method for anti-fatigue lightweight design based on machine learning and multi-objective optimization is proposed. A high-fidelity finite element model of the carriage is established for static simulation analysis of the typical conditions. Based on the virtual reliability simulation test of the dump truck and the equivalent structural stress method, the fatigue life of the critical welds in the carriage is calculated. The important part thicknesses are selected as design variables through the comprehensive contribution analysis method. The maximum displacement and maximum stress under the dangerous condition are considered as constraints. The mass of the carriage and the minimum fatigue life of the critical welds are considered as optimization objectives. The GA-XGBoost machine learning approximation models (GA-XGBoost-MLAM) and NSGA-II algorithm are employed for multi-objective optimization design of the carriage. The entropy weighted TOPSIS method is utilized for multi-objective decision-making of Pareto solutions. The design after optimization and decision-making shows that, while satisfying the requirements of static structural performance, the minimum fatigue life mileage of the critical welds of the carriage is increased by 157,570 km, representing an increase of 36.58%. Additionally, the mass of the carriage is reduced by 295.69 kg, representing a decrease of 9.47%. Therefore, the proposed design method achieves a good effect in the anti-fatigue lightweight of dump truck carriage.

Fatigue performance of riser quality girth welds: Analysis of TWI and DNV's databases

AbstractIn the fatigue design guidelines which focus on welded joints, when the applied loading is of variable amplitude (variable amplitude loading, VAL), the slope of the S‐N curves at endurances above the slope transition point (STP) changes from m to m + 2 in DNV‐RP‐C203 and BS7608. However, the position of the STP is different in BS7608 and DNV RP C203. This results in different calculated fatigue lives and the difference in calculated fatigue life can be significant. In order to provide better guidance to industry on the approach to use for fatigue analysis of girth welds in pipelines and risers specifically, TWI and DNV carried out a project where the ultimate aim was to provide guidance on a common S‐N curve suitable for calculating the fatigue life of girth welds with focus on the slope change area of the S‐N curves. This was attempted by re‐analyzing TWI's and DNV's databases of girth weld fatigue test results and comparing the outcomes. Although it was not possible to agree on a single STP from the analysis carried out, regression analysis of high endurance results from the databases suggest that an STP between the two current values 1E7 and 5E7 may be plausible. This is supported by fracture mechanics analysis, which suggests that the position of the STP depends on the fatigue Class, and the magnitude of the applied stress ranges.

A simplified fracture mechanics method for fatigue life analysis of weld roots

Volvo CE currently uses the effective notch stress method for assessing fatigue life in welded structures. For large structures such as frames, a sub modelling technique with a very fine mesh is necessary using millions of DOF. Stress tensors from unit load cases, estimated with FEA, are combined with time history loading from Multi Body Simulations of the complete machine operating on customer representative digitized test tracks. A critical plane approach (the Modified Wöhler Curve Method) is used to estimate the fatigue life. The usage of the effective notch method implies a very time-consuming design process. An LEFM approach for weld roots would be even more time-consuming but would be a more natural approach since the weld root consists of crack like defects. To overcome the drawbacks with the current design process that involves usage of the effective notch method, a hybrid method is defined using pre-calculated geometry factors for different crack sizes and weld geometries. The geometry factors as a function of crack size, and the weld geometry, are stored in a database. From the unit load cases the structural stress using a coarse mesh in the weld is extracted. Together with the geometry factors, the stress intensity factors may be estimated and used with Paris law to calculate the fatigue life of welds. A comparison between the effective notch method, LEFM and the new hybrid method is carried out for typical fillet welds is presented in this paper.

Notch effect identification and fatigue life prediction of undercut weld

Undercut, as a typical surface imperfection at the weld toe, significantly impacts the weld fatigue life. Neither the ISO5817 standard for imperfection level definition nor the traditional fatigue life prediction methods for welds can accurately identify the undercut effect on weld fatigue life. To solve this problem, a new stress concept and life prediction method are proposed, which can predict the fatigue life of undercut welds in both linear elastic and elastic–plastic conditions. Its prediction accuracy is verified by test data and engineering examples.

Fatigue life analysis of fillet weld with root gap in bogie tie rod seat under road traffic load

The results of failure analysis on the fillet welds of the tie rod seat in HDX2 type vehicle showed that size of root gap around cracks was usually larger than that observed in the region without cracks. Dynamic strain gauges were used to measure the time-strain curve of the zone surrounding the fillet weld in the locomotive traveling process. Then, a transfer matrix was utilized to calculate the time-strain curve of fillet welds and then a rainflow counting method was adopted to obtain hierarchical load spectra in the traveling process, which was used for predicting fatigue damage of fillet welds and safe service mileage. To reveal the influence of the root gap on the fatigue life of fillet welds, a three-factor and five-level orthogonal scheme was designed by taking the height, width and length of the root gap as variables. The calculation results based on road traffic loads indicated that: (1) When there is no gap at the root of fillet weld, the safe service mileage can meet standard requirements; when there is a gap at the root of the fillet weld, the safe service mileage is significantly shortened; (2) When the width, height or length of the root gap of fillet weld increases, the safe service mileage decreases monotonously. Given the same change in size, effect of the change in height of root gap on safe service mileage is the most significant among the three variables considered; (3) The maximum stress at straight line section of weld path was significantly greater than that at the corner section; (4) An inspection and maintenance program for root gaps of various sizes was developed.

Fatigue performance improvement of U-rib butt-welded connections of steel bridge decks using externally bonded CFRP strips

U-rib butt-welded connections in steel bridge decks are known to be highly susceptible to fatigue cracking, which can pose a direct threat to the safety and durability of bridge structures. Carbon fiber-reinforced polymer (CFRP) materials have shown promising potential for fatigue strengthening. This study investigated the effectiveness of CFRP application on the fatigue performance of U-rib butt-welded connections by employing full-scale fatigue tests and finite element analysis. Two single U-rib specimens with welded splice joints were initially tested to initiate real fatigue cracks under cyclic loading. Different techniques of CFRP installation were then proposed to promote the fatigue performance of cracked butt-welded connections. The failure modes and fatigue strength of the tested specimens were evaluated before and after strengthening. Research results showed that fatigue cracks were observed at the arc transition of butt welds, where remarkable incomplete penetration was also discovered. The fatigue strength grade of the tested U-rib butt-welded details was less than 71 MPa stipulated by Eurocode 3. However, after being repaired by CFRP sheet bonding, the equivalent fatigue life of U-rib butt welds with different damage degrees can be extended by 0.5–16.6 times that of the original ones. These findings suggest that CFRP strengthening should be applied instantly on U-rib butt-welded connections once fatigue cracks are found.

Tensile and fatigue behavior of novel dissimilar resistance spot welds of AA5754 to steels: interplay of intermetallic layer, weld nugget diameter and notch root angle

AA5754 sheet was resistance spot welded (RSW) to high-strength low-alloy (HSLA) steel sheet and low carbon steel (LCS) sheet respectively using multi-ring domed (MRD) electrodes and multiple solidification weld schedules. The confluence of the intermetallic compound layer, weld nugget diameter, and notch root angle are delineated. The results demonstrated that elevated Mn content (0.634%) in the HSLA steel promoted the formation of a thinner and more uniform IMC layer in AA5754-HSLA stack-ups when compared to the LCS, comprised of a lower Mn content (0.130%), in the AA5754-LCS stack-ups. Diluting the alloy content by changing the HSLA sheet to LCS sheet in AA5754-steel RSWs softened the aluminum weld nugget. Fatigue results demonstrated that fatigue life in Al-steel spot welds is greater when compared to the aluminum sheet joined to itself in the tensile shear configuration. Fatigue life was greater for the positive polarity AA5754-HSLA steel RSWs than for the AA5754-LCS. While weld nugget diameter dominates weld fatigue life, manipulation of the weld notch root angle enables additional fatigue life control. To enhance fatigue life, a large notch root angle is beneficial.

Fatigue assessment of fillet weld in steel bridge towers considering corrosion effects

Field inspection shows that the fillet weld in steel bridge towers may be subjected to corrosion and fatigue loading simultaneously. To ensure the structural safety in the service life, corrosion effects on the fatigue life of the fillet weld in steel towers were investigated in this study. A prediction procedure of the fatigue life of the weld was firstly proposed based on the fracture mechanics method and verified by the test results. Three types of corrosion effects, pitting corrosion at the initial crack, pitting corrosion near the initial crack, and the corrosion fatigue crack growth were considered. Three corrosion levels were also defined under different corrosive environments. By taking the Third Nanjing Yangtze River Bridge as an example, a modified finite element (FE) model integrated with the local shell model was established to analyze the stress variation of the fillet weld in steel towers. Based on the stress influence lines, the maximum stress range was obtained under the vehicle load. Fatigue assessment of the fillet weld was finally performed considering the corrosion effects. The result shows that the remaining fatigue life of the fillet weld was greatly reduced by the corrosion pits at the initial crack or the interactions of corrosion and fatigue. However, since the tensile stress range was small in the steel bridge towers, the fatigue performance of the fillet weld considering the corrosion effects can still meet the requirements of the code specification.

A Numerical Simulation of UIC60 Rail-Weld's Fatigue and Crack Growth under Wheel Frictional Contact and Bending

In service condition rail joints, especially the weldments are under the action of various loadings which are not only working in multiple axis direction but also time-dependent having a cyclic and mixed-mode in nature and non-relative to each other. The surface of the rail and its weldment is acted by very high repetitive stress through the wheel and because of this contact stress the running surface or subsurface may have cracks or fractures due to fatigue. This work is based on numerical simulation of an aluminum thermite weldment on a UIC 60 rail under multi-axial fatigue crack propagation under the friction with surficial interaction between weldment and wheel with bending load due to vertically applied load through the wheel on the weld. Since contact is highly influenced by vertical load and also for minimizing the simulation time the lateral and longitudinal traction forces are not included in this study. The work formulation and discretization have been done with the finite element method and a non-linear lagrangian algorithm solver is applied. A 3-D rail-weld wheel model assembly and a semi-elliptical crack as a flaw on the weld surface are used to identify 3-Modes of SIFs along with its graphical plot generation. Simulation is performed under multi-axial weld wheel surface contact at different locations on weld running surface, taking into account varying position of fracture crack on weld 3-D model to calculate fracture life of weld joint and observation of fatigue crack propagation. This work involves the numerical and theoretical approach of fracture mechanics on created FE fatigue model using the Linear Elastic Fracture Mechanics (LEFM) method following Paris law for fracture mechanics. All the numerical simulation for critical fracture dimension and cycle count with stress intensity factor for weld failure data is estimated using software ANSYS 2020 academic and plotted, then comparison of predicted and observed transverse crack growth behavior and fatigue life of weld, based on Millions Gross Tonnes (MGT) is discussed.

Weld Fatigue Life Analysis of Truck Mixer Sub-frame Based on Equivalent Structural Stress Method

In order to accurately calculate the fatigue life of the sub-frame weld of the mixer truck during the design stage, fatigue analysis of the weld is carried out by equivalent structural stress method. Through the finite element analysis of the frame, the key weld position is researched. By using the inertial release method, the stress response of the frame under the single excitation of the unit load is obtained. Combined with the basic principle of equivalent structural stress, the stress at the weld position is calculated, and the main S-N curve with a survival rate of 50% is selected to predict the fatigue life of the weld. The results show that the fatigue life of the key weld is 105.803 cycles, which fails to reach the design target value. The equivalent structural stress method has the characteristics of grid insensitivity and can better predict the fatigue life of the sub-frame weld of the mixer truck.

Influence of Complete Tool-Offset Position on Intrinsic Microstructure and Its Effect on Fatigue Life of AA2024 Alloy-Copper Joint Made by Friction Stir Welding

Abstract Tool offset position is the key parameter to consider during the welding of dissimilar metals by friction stir welding technique. It helps to control the formation of brittle intermetallics and volume defects. Even tool offset position can leave defects at the interface when it exceeds a certain limits, and it has been reported as two-thirds of the bottom radius of the tool. In this work, the effect of the maximum tool deviation (complete offset) on intrinsic microstructure and fatigue life of AA 2024-copper weld was studied. For this, an experiment was carried out with complete tool offset position. The results highlighted that the stir zone was formed mainly by aluminum and the total volume of copper transferred into the stir zone was measured as 5.6 % of the total volume of the stir zone. This kind of structure gives excellent fatigue life to the weld compared to the regular microstructure formed during friction stir welding.

Development of an automated system for adaptive post-weld treatment and quality inspection of linear welds

High-frequency mechanical impact (HFMI) treatment is a well-documented post-weld treatment to improve the fatigue life of welds. Treatment of the weld toe must be performed by a skilled operator due to the curved and inconsistent nature of the weld toe to ensure an acceptable quality. However, the process is characterised by noise and vibrations; hence, manual treatment should be avoided for extended periods of time. This work proposes an automated system for applying robotised 3D scanning to perform post-weld treatment and quality inspection of linear welds. A 3D scan of the weld is applied to locally determine the gradient and curvature across the weld surface to locate the weld toe. Based on the weld toe position, an adaptive robotic treatment trajectory is generated that accurately follows the curvature of the weld toe and adapts tool orientation to the weld profile. The 3D scan is reiterated after the treatment, and the surface gradient and curvature are further applied to extract the quantitative measures of the treatment, such as weld toe radius, indentation depth, and groove deviation and width. The adaptive robotic treatment is compared experimentally to manual and linear robotic treatment. This is done by treating 600-mm weld toe of each treatment type and evaluating the quantitative measures using the developed system. The results showed that the developed system reduced the overall treatment variance by respectively 26.6% and 31.9%. Additionally, a mean weld toe deviation of 0.09 mm was achieved; thus, improving process stability yet minimising human involvement.

Numerical Simulation and Parametric Analysis of Fatigue Crack in UIC60 Rail Thermite Welded Joint

The mechanism of rail-wheel contact is the most essential field of study in railway engineering since it requires extensive application expertise, diagnostic skills, and a trustworthy analysis technique. In this research the fatigue life of a UIC60 rail AT weld under vertical load and its parametric effect has been studied, and for that a three-dimensional elastic-plastic finite element model is created using ANSYS space-claim software, and then finite element method is employed to analyse the full-scale model of wheel-track and weld system with realistic three-dimensional solution. Model assembly components include axle, wheel, and thermite-welded rail. Simulation of contact between wheel and UIC60 rail weld with crack on weld at angles of 30 and 60 degrees with different coefficients of friction between the weld wheel contact and between crack surfaces was carried out under vertical loadings. In general, the Hertz contact theory assumptions are taken into consideration throughout the analysis, and the impacts on fatigue life are given by using damage mechanics method. The results of the wheel/weld fatigue crack analysis have been displayed to demonstrate the influence of different parameters on the fatigue life of cracks. The purpose of this study is to identify and safeguard the rail against failure, as well as to ensure the safety of passengers and to reduce the cost of maintaining the rail system.

Improving fatigue life of 7N01 Al alloy weld by surface spinning strengthening

AbstractThe purpose of this study is to improve the surface microstructure and mechanical properties of 7N01 Al alloy welded joints by surface spinning strengthening (3S), so as to improve the fatigue life at certain stress. Results show that after the 3S treatment, the microhardness of the welded joint increases significantly due to the introduction of some nanoprecipitates and dislocations, and the strength and elongation increase simultaneously. The minimum fatigue life of the welded specimen under the condition of maximum stress of 150 MPa and stress ratio of 0.1 is increased from less than 106 cycles to more than 3 × 106 cycles after the 3S treatment. Fracture surface analysis shows that the crack initiation site of the sample with welded joint is turned from the surface to the subsurface after 3S treatment, which is beneficial to the elimination of surface defect and the improvement of surface strength.

A comparative study on mechanical performance of traditional and magnetically assisted resistance spot welds of A7N01 aluminum alloy

In this paper, an external magnetic field was introduced to resistance spot welding (RSW) to improve the weldability of A7N01 aluminum alloy. Comparative studies of the macro- and micro-structures, microhardness, and the static and dynamic mechanical properties between welds produced by traditional RSW and magnetically assisted RSW (MA-RSW) were completed. The results show that the external magnetic field could enable larger nugget diameter, the softened columnar grain zone (CGZ-I), the hardened columnar grain zone (CGZ-II) and the finer equiaxed grain zone (EGZ). As a result, compared to traditional RSW joints, the MA-RSW joints exhibited better strength, improved toughness, and greater energy absorption capacity. This resulted in a transition of the failure mode from an interfacial fracture (IF) to a partial thickness fracture (PTF) in lap-shear tests and a BP fracture in cross-tension tests, respectively. In fatigue tests, the larger nugget diameter, hardened CGZ-II and finer EGZ in the MA-RSW welds can inhibit cracks from propagating from EGZ, but from the softened CGZ to the base metal. This change of fracture mode greatly improved the fatigue life of MA-RSW welds under both high and low load conditions.

Prediction of spot weld fatigue life using finite element approach

Resistant spot welding is predominantly used in automotive industry for joining sheet metal. Mostly automobile fatigue failure occurs in and around spot welding (80%). So, accurate prediction of the deformation, stresses and fatigue life of spot weld is necessary. In this work finite element model of spot weld and experimental study of coach peel test of spot welded low carbon steel sheets are presented. The property change in various zones of spot weld is found by nano-indentation test followed by inverse finite element modelling. Spot weld failure happens at the interface between nugget and heat affected zone due to the geometric stress concentration and property change that occurs during the welding process. Solid elements are successful in capturing the geometric stress concentration, but they increase the simulation cost and hence a model with combined shell and solid elements is used. For nugget solid elements are used and tore present all other parts of spot weld shell elements are used. Tensile and fatigue tests on coach peel specimen are done as per ASTM standardD903-98. The simulation results are found to be in close agreement with experimental results.

Heat Input Influence on the Fatigue Life of Welds from Steel S460MC

Fine-grained steels belong to the progressive materials, which are increasingly used in the production of welded structures subjected to both static and dynamic loads. These are unalloyed or microalloyed steels hardened mainly by the grain-boundary strengthening mechanism. Such steels require specific welding procedures, especially in terms of the heat input value. At present, there are studies of the welding influence on the change of thermomechanically processed steels’ mechanical properties, however mainly under static loading. The paper is therefore focused on the assessment of the welding effect under dynamic loading of welded joints. In the experimental part was determined the influence of five different heat input values on the change of weld fatigue life. As a result, there is both determination of five S-N curves for the double-sided fillet welds from the thermomechanically processed fine-grained steel S460MC and the quantification of the main influences reducing the fatigue life of the joint.

Fatigue Life Analysis of Longitudinal Welding Seam for Wind Turbine Tower

Tower is an important fundamental component of large-scale wind turbines. The fatigue performance of the tower welded part directly affects the running safety and reliability of wind turbines. A fatigue life predicting method for the longitudinal welding seam of the tower is proposed in this paper. Under the precondition of satisfying the limit strength, the time series stress of each working condition is obtained by DIN18800-4 section stress calculation. Combined with the rain-flow counting method and the Miner linear cumulative damage theory, the fatigue life of the tower longitudinal weld is predicted. The results show that the tower structure meets the design requirement, and the feasibility and effectiveness of the method are verified.

Effects of Ultrasonic Impact Treatment on the Stress-Controlled Fatigue Performance of Additively Manufactured DMLS Ti-6Al-4V Alloy

Additive manufacturing (AM) offers many advantages for the mechanical design of metal components. However, the benefits of AM are offset to a certain extent by the poor surface finish and high residual stresses resulting from the printing process, which consequently compromise the mechanical properties of the parts, particularly their fatigue performance. Ultrasonic impact treatment (UIT) is a surface modification process which is often used to increase the fatigue life of welds in ship hulls and steel bridges. This paper studies the effect of UIT on the fatigue life of Ti-6Al-4V manufactured by Direct Metal Laser Sintering (DMLS). The surface properties before and after the UIT are characterized by surface porosity, roughness, hardness and residual stresses. Results show that UIT enhances the fatigue life of DMLS Ti-6Al-4V parts by suppressing the surface defects originating from the DMLS process and inducing compressive residual stresses at the surface. At the adopted UIT application parameters, the treatment improved the fatigue performance by 200%, significantly decreased surface porosity, reduced the surface roughness by 69%, and imposed a compressive hydrostatic stress of 1644 MPa at the surface.