Search Results (124)

Based on the engineering background of a new type of segmental-assembled steel temporary beam buttress, the fatigue strength evaluation method of the steel box girders with open longitudinal ribs was taken as the research objective. The fatigue stress calculation analysis and the full-scale fatigue loading test for the steel box girder local component were carried out. The accuracy of the finite-element model was verified by comparing it with the test results, and the rationality of the fatigue strength evaluation methods for welded joints was deeply explored. The results indicate that the maximum nominal stress occurs at the weld toe between the transverse diaphragm and the top plate at the edge of the loading area, which is the fatigue-vulnerable location for the steel box girder local components. The initial static-load stresses at each measuring point were in good agreement with the finite-element calculation results. However, the static-load stress at the measuring point in the fatigue-vulnerable position shows a certain decrease with the increase in the number of cyclic loads, while the stress at other measuring points remains basically unchanged. According to the finite-element model, the fatigue strengths obtained by the nominal stress method and the hot-spot stress method are 72.1 MPa and 93.8 MPa, respectively. It is reasonable to use the nominal stress S-N curve with a fatigue life of 2 million cycles at 70 MPa and the hot-spot stress S-N curve with a fatigue life of 2 million cycles at 90 MPa (FAT90) to evaluate the fatigue of the welded joints in steel box girders with open longitudinal ribs. According to the equivalent structural stress method, the fatigue strength corresponding to 2 million cycles is 94.1 MPa, which is slightly lower than the result corresponding to the main S-N curve but within the range of the standard deviation curve. The research results of this article can provide important guidance for the anti-fatigue design of welded joints in steel box girders with open longitudinal ribs. Full article

The accumulation of fatigue damage is primarily caused by cyclic plastic deformation. In low-cycle fatigue, cyclic plastic deformation is the dominant deformation mode. In high-cycle fatigue, although most deformation is elastic, plastic deformation may still occur in localized regions of stress concentration and plays a critical role in the initiation of fatigue cracks. Considering that cyclic plastic deformation can be characterized by hysteresis loops, this study modifies the flow stress equation and the cyclic plastic deformation relationship based on stress–strain hysteresis loops at half-life cycles under different strain amplitudes. An improved model for life prediction that incorporates the effects of strain amplitude is proposed. The results of experiments on 310S stainless steel and 1045 carbon steel demonstrate that the model achieved prediction errors within a factor of two and provided reliable predictions for both high-cycle and low-cycle fatigue life across the entire ε-N curve. Full article

A surrogate artificial neural network (ANN) model trained on the data generated from a computational finite element-based (FE-based) model is developed. The developed ANN model enables the estimation of the fatigue life (number of load cycles to failure) of component-like specimens with stress concentrators. Using the developed model, the component-specific S-N curves can be generated with an accuracy comparable to that of the computational FE-based model. The investigation covered through- and surface-hardened steel components with different numbers and types of stress concentrators. The basis for data generation is the parametrized computational FE-based model, which enables the determination of the stress–strain response and the calculation of the fatigue life of examined components under cyclic loading conditions. The computational FE-based model can be adjusted to include components with different geometries and heat treatment conditions. The computational FE-based model incorporates nonlinear material behavior to provide a more accurate representation of the component’s behavior, which results in higher computational costs. In contrast, the developed ANN model provides a quicker and more efficient way to assess the fatigue life of both through- and surface-hardened components, overcoming these limitations. Full article

This study investigates the mechanical and fatigue behaviour of friction stir composite joints fabricated from an aluminum alloy (AA6082-T6) and a glass fibre-reinforced polymer (Noryl^® GFN2) under different service temperature conditions. The joints were tested under both quasi-static and cyclic loading at three different temperatures (23, 75, and 130 °C). Fracture surfaces were analyzed, and the probabilistic S–N curves were derived using Weibull distribution. Results indicated that increasing the service temperature caused a non-linear decrease in both the quasi-static and fatigue strength of the joints. Compared to room temperature, joints tested at 75 °C and 130 °C showed a 10% and 50% reduction in average tensile strength, respectively. The highest fatigue strength occurred at 23 °C, while the lowest was at 130 °C, in line with the quasi-static results. Fatigue stress-life plots displayed a semi-logarithmic nature, with lives ranging from 10² to 10⁵ cycles for stress amplitudes between 7.7 and 22.2 MPa at 23 °C, 7.2 to 19.8 MPa at 75 °C, and 6.2 to 13.5 MPa at 130 °C. The joints’ failure occurred in the polymeric base material close to joints’ interface, highlighting the critical role of the polymer in limiting joints’ performance, as confirmed by thermal and scanning electron microscopy analyses. Full article

The welded joints in steel bridges have a complicated structure, and their fatigue life is mainly determined by the real stress under the coupling effect of vehicle load stress, as well as weld-induced residual stress and its relaxation. Traditional fatigue analysis methods are inadequate for effectively accounting for weld-induced residual stress and its relaxation, resulting in a significant discrepancy between the predicted fatigue life and the actual fatigue cracking time. A fatigue damage assessment model of welded joints was developed in this study, considering weld-induced residual stress and its relaxation under vehicle load stress. A multi-scale finite element model (FEM) for vehicle-induced coupled analysis was established to investigate the weld-induced initial residual stress and its relaxation effect associated with cyclic bend fatigue due to vehicles. The fatigue damage assessment, considering the welding residual stress and its relaxation, was performed based on the S–N curve model from metal fatigue theory and Miner’s linear damage theory. Based on this, the impact of variations in traffic load on fatigue life was forecasted. The results show that (1) the state of tension or compression in vehicle load stress notably impacts the residual stress relaxation effect observed in welded joints, of which the relaxation magnitude of the von Mises stress amounts to 81.2% of the average vehicle load stress value under tensile stress working conditions; (2) the predicted life of deck-to-rib welded joints is 28.26 years, based on traffic data from Jiangyin Bridge, which is closer to the monitored fatigue cracking life when compared with the Eurocode 3 and AASHTO LRFD standards; and (3) when vehicle weight and traffic volume increase by 30%, the fatigue life significantly drops to just 9.25 and 12.13 years, receptively. Full article

The International Maritime Organization (IMO) mandates a reduction in carbon dioxide emissions from 2008 levels by at least 40% by 2030, prompting the widespread adoption of slow steaming and engine de-rating strategies. This study investigates the fatigue life of marine propulsion shafts under slow steaming conditions, focusing on the interplay between torsional and bending vibrations. A finite element (FE) model of a low-speed two-stroke propulsion system is developed, incorporating torsional and lateral excitation sources from both the engine and propeller. Vibrational stresses are computed for multiple operating conditions, and fatigue life is assessed using both the conventional Det Norske Veritas (DNV) methodology and a proposed biaxial stress approach. Results indicate that while torsional vibrations remain the primary fatigue driver, bending-induced stresses contribute marginally to the overall fatigue life. The proposed methodology refines high-cycle fatigue (HCF) assessment by incorporating a corrected S-N curve and equivalent von Mises stress criteria. Comparisons with classification society standards demonstrate that existing guidelines remain valid for most cases, though further studies on extreme alignment deviations and dynamic bending effects are recommended. This study enhances understanding of fatigue mechanisms in marine shafting and proposes a refined methodology for improved fatigue life prediction. Full article

In the chlorination industry, the reactor is a crucial equipment in which the chlorination reaction takes place. However, when the reactor is subjected to complex conditions such as high temperatures (e.g., >200 °C) and high pressures (e.g., >10 MPa), its structural integrity is significantly compromised, leading to severe safety issues. In this study, the fatigue life of a reactor is analyzed, with particular focus on the fatigue behavior of the cyclone separator under varying working conditions, such as changes in the temperature, pressure, and chemical environment. Using finite element simulations under steady-state conditions and the S-N curve from fatigue testing, the fatigue life and potential weak points of the reactor under different amplitudes and vibration frequencies are analyzed and predicted. This analysis is conducted using a combined simulation approach with ABAQUS and Fe-Safe software, v 6.14. This work also considers the periodic vibrations at the base of the cyclone separator within the reactor. Fatigue simulations under different vibration conditions are performed to further assess the fatigue life of the reactor, providing a theoretical basis for the optimization of design and ensuring operational safety. In addition, the influence of welding zones on the fatigue life is discussed. The results indicate that the welding defects and stress concentration may cause the welded joint to become a critical weak point for fatigue failure. Therefore, the fatigue performance of the welding zone should be carefully considered during the design phase. Full article

Fatigue has always been one of the major causes of structural failure, where repeated loading and unloading cycles reduce the fracture energy of the material, causing it to fail at stresses lower than its monotonic strength. However, predicting fatigue life is a highly challenging task and, in this context, the present study proposes a fundamentally new hybrid physics-informed and data-driven approach. Firstly, an energy-based fatigue model is developed to simulate the behavior of concrete under compressive cyclic fatigue loading. The data generated from these numerical simulations are then utilized to train machine learning (ML) models. The stress–strain curve and S-N curve of concrete under compression, obtained from the energy-based model, are validated against experimental data. For the ML models, two different algorithms are used as follows: k-Nearest Neighbors (KNN) and Deep Neural Networks (DNN), where a total of 1962 data instances generated from numerical simulations are used for the training and testing of the ML models. Furthermore, the performance of the ML models is evaluated for out-of-range inputs, where the DNN model with three hidden layers (a complex model with 128, 64, and 32 neurons) provides the best predictions, with only a 0.6% overall error. Full article

The rapid growth of wind energy has necessitated the development of advanced materials to address the increasing structural demands of wind turbine blades. Graphene platelets (GPLs) have garnered attention as a promising reinforcement material due to their outstanding mechanical properties, such as high strength and low density. This study investigates the fatigue life of wind turbine blades reinforced with GPLs, benchmarking their performance against conventional fiberglass blades. A finite element model of a 5 MW wind turbine blade was developed to evaluate stresses within the blade structure. The traditional fiberglass blade was modeled based on the SNL 61.5 m design by Sandia National Laboratories, while the GPL-reinforced composite (GPLRC) blade was designed by substituting fiberglass with GPLRCs. Material properties of the GPLRCs were determined using the rule of mixtures and the Halpin–Tsai micromechanics model. Wind speed data were randomly sampled following the probability distribution observed at European wind farms, and corresponding aerodynamic loads were computed using blade element momentum theory. Finite element analyses were performed to derive stress time histories, and fatigue life was predicted using the S-N curve approach, incorporating the Goodman diagram and the Palmgren–Miner rule. The results reveal that while GPLRC-reinforced blades exhibit some limitations in fatigue performance compared to traditional fiberglass blades, potential solutions for improving their durability are proposed, highlighting avenues for further research and optimization in the application of GPLRCs to wind turbine blades. Full article

An experimental study was conducted to analyze temperature evolution during very high cycle fatigue tests. The temperature–number of cycles (T–N) curve is typically divided into three phases: Phase I—a rapid temperature increases at the start of the test, Phase II—temperature stabilization, and Phase III—a sharp temperature rise at the test’s end, coinciding with specimen fracture. The high frequencies used in ultrasonic fatigue testing can induce self-heating in specimens, but the thermal effects are not yet fully understood. Temperature is known to influence the fatigue performance of materials. To explore this, specimens were subjected to varying stress levels and intermittent loading conditions while monitoring temperature evolution using infrared thermography. The T–N curves were obtained, and S–N curves were constructed for specimens tested at room temperature. All tests were performed under fully reversed loading conditions. The experimental data were used to evaluate models commonly applied in conventional fatigue testing. Additionally, the temperature gradient at the beginning of the ultrasonic fatigue test and the heat dissipation per cycle were estimated and analyzed as potential fatigue damage parameters. These findings indicate that parameters derived from the T–N curve have significant potential for predicting very high cycle fatigue life. Full article

This research provides a method for a reliable fatigue life estimation at high testing frequencies. The investigations are based on the lifetime prediction method StressLife_HCF considering test frequencies of 80 and 260 Hz for normalized SAE 1045 (C45E, 1.1191) steel. Therefore, load increase tests and constant amplitude tests were carried out using a resonant testing rig. To ensure a mechanism-oriented lifetime prediction, the material response to dynamic loading is monitored via temperature and electrical resistance measurements. Due to the higher energy input per time unit, when the test frequency is increased, the heat dissipation also increases. For this reason, a precise differentiation between frequency- and temperature-related effects for adequate fatigue assessment is challenging. To evaluate the temperature’s influence on electrical resistance, an electrical resistance-temperature hysteresis is measured, and the frequency influence is analyzed by considering cyclic deformation curves. In addition to an extension of the fatigue life due to an increased test frequency, the lifetime prediction method was validated for high frequencies. The generated S-N curves show a reliable agreement with the data points from conventional constant amplitude tests. In this context, the temperature correction of the electrical resistance proved to be an important input variable for a reliable lifetime prediction. Full article

Due to the complexity of the landing gear’s (LG) structural integrity and its loads under various static or dynamic working conditions, the fatigue life assessment for LG is a highly challenging task. On the basis of the whole geometric model of a large passenger aircraft’s main landing gear (MLG), the quasi-static finite element model (FEM) of the whole MLG is established, and the high-cycle fatigue issue of the Main Fitting (MF) is studied by considering the variation in shock absorber travel (SAT). Firstly, the ground loads under actual fatigue conditions are equivalently converted into the forces acting on the center of the left and right axles of the MLG, and based on these spatial force decompositions, the magnitude and direction of the load for 12 different basic unit load cases (ULC) are obtained. That is, the stress of the MLG under actual fatigue conditions can be obtained by superimposing these ULCs. Then, considering that the SAT of the MLG varies under different fatigue conditions, and to reduce the number of finite element (FE) simulations, this article simplifies all the SAT experienced by the MLG into seven specific values, so as to establish seven quasi-static FEMs of the MLG with the specified stroke of the shock absorber. In this way, the fatigue stress of the MLG with any actual SAT can be obtained by interpolating the stress components of the seven FEMs. Only 84 FE simulations are needed to efficiently obtain the fatigue stress spectra from the ground load spectra. Finally, according to the material S-N curve and Miner’s damage accumulation criterion, evaluate the fatigue life of the Main Fitting. The results of the stress component interpolation and superposition method show that at least five different SATs of the whole MLG’s FEM are needed to effectively convert the fatigue loads into a stress spectrum. The fatigue life prediction results indicate that the minimum lifespan of the MF is 53164 landings, which means that the fatigue life meets the requirement design. Full article

The corrosive environment in the western region will adversely affect the fatigue performance of bridges. In order to determine the influence of the corrosion environment in western China on the fatigue failure of bridges, this paper simulated corrosion environment characteristics in northwest China, conducted alternating corrosion and high-cycle fatigue experiments on Q420 bridge steel, and used Origin (2019b 64Bit) to fit S-N curves of specimens with different corrosion periods to study the influence of corrosion damage in western China on their fatigue properties. Based on the damage theory, the corrosion fatigue failure degree of Q420 bridge steel was analyzed, and the crack propagation law was revealed by microscopic scanning of the fracture. The findings indicate that the damage to Q420 bridge steel progressively worsens with more prolonged corrosion exposure in the deicing salt conditions of western China. The 60-day corrosion resulted in a rust rate η_s of 2.17% and a corrosion rate K of 1.413 mm/a. The fatigue life of Q420 bridge steel is significantly affected by the coupling of stress level and corrosion damage. After 60 days of corrosion, the specimen’s fatigue limit decreases by 12.28%, which accelerates the fatigue property degradation rate of the specimen. The damage index effectively represents the internal damage behavior of materials under corrosion fatigue conditions. Fatigue damage escalates as the corrosion period extends, and the number of fatigue bands decreases, with the stress corrosion threshold σ_th diminishing by 1.77%, 3.55%, 4.61%, and 6.38% across various corrosion durations. The research results are significant for the fatigue failure prediction and reliability analysis of Q420 bridge steel in a deicing salt corrosion environment in western China. Full article

Steel structure flange connections are extensively employed in structural nodes due to their superior mechanical properties. This study combines fatigue testing and theoretical methods to investigate the fatigue performance of high-strength bolts in flange connections under actual gradient descent loads and provide fatigue design methods. Initially, fatigue tests were conducted on two sets of high-strength bolts under a gradient descent loading mode, yielding a total of 11 sets of fatigue data. Subsequently, the stress–life (S-N) curve was plotted using a cumulative damage model combined with an equivalent constant amplitude stress method, and the results were compared with existing fatigue design specifications. Additionally, digital cameras and electron microscopes were utilized to capture fatigue fracture images of the high-strength bolts, allowing a detailed investigation into the mechanisms underlying bolt fatigue fractures. The results indicate that the allowable stress amplitudes for the two sets of high-strength bolts, corresponding to a fatigue life threshold of 2 million cycles, were 144.211 MPa and 130.316 MPa, respectively—both of which exceed the values specified in current fatigue design codes. Moreover, finite element simulations revealed that the most pronounced stress concentration occurs at the first thread where the bolt and nut interface, which is identified as the critical location for fatigue fracture in bolts. The allowable stress and fatigue calculation method of bolts obtained in this study will provide a reference for flange node design Full article

Fatigue failure remains a major concern in the design and performance evaluation of machine components and structures as it accounts for a significant proportion of mechanical failures. This article presents a fatigue evaluation methodology based on SN (stress-cycles to failure) curves to understand and predict the fatigue behaviour of complex components under various loading conditions with widely varying device geometry and dynamics. In order to accurately interpret and utilize the SN curves, the paper outlines key factors influencing material fatigue, including stress amplitude, mean stress, stress concentration, environmental effects, and surface finish. The integration of these factors into the SN curve-based assessment is discussed to tailor fatigue evaluations to specific machine components and structures. To demonstrate the practical application of SN curves in fatigue assessment, two case studies of machine components and structures are presented. The paper ends with a summary and conclusions, the most important of which is that the greatest impact on design fatigue life consists of accurately estimated stresses resulting from the load conditions and the dynamics of the structure. Full article