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In this study, a titanium alloy torsional spring used in aviation was taken as the research subject. Aiming at the fatigue life prediction problem of this spring, the life analysis of the titanium alloy torsional spring was performed using a customized UMAT subroutine based on the theory of continuous damage mechanics. Several sets of life prediction models and tests were compared. The fatigue lives of the springs at 60, 80, 100, and 120 degrees were 45,070, 65,067, 99,677, and 181,322 cycles, respectively. Compared with other fatigue life prediction methods, the fatigue life calculated by the customized subroutine was the most consistent with the fatigue life of the titanium alloy torsion spring tests. The average relative error between the measured experimental life value and the predicted value was 2.04%, which is less than 5%, meeting engineering measurement requirements. The effectiveness and applicability of the proposed model and method were verified, and the time and economic cost caused by excessively long experimental cycles were reduced. This helps improve the accuracy of fatigue life prediction for this titanium alloy torsional spring and provides analysis support for subsequent structural optimization and improvement. [ABSTRACT FROM AUTHOR]

The Francis runner is a critical component of the Francis turbine generator unit, playing a central role in converting water energy into rotating mechanical energy that drives the generator in hydropower stations. In-depth analyses of the flow characteristics of the Francis runner under various operating conditions and avoiding fatigue damage of the Francis runner are crucial to the reliability and efficiency of hydropower operation. In this paper, the flow dynamics of a large Francis turbine runner are analyzed under three representative loading conditions—low partial load, high partial load, and full load—and the flow-induced stress of the runner is analyzed under these loading conditions. It was found that the maximum static and dynamics stresses of the runner at three representative loading conditions are located at the chamfered surface where the blade trailing edge connects to the runner crown. The maximum static stresses of the Francis runner are 284 MPa, 352 MPa, and 381 MPa at low partial load, high partial load, and full load, respectively, and they are above the allowable stress limits, as half of the yield stress of the runner material of 550 MPa. The peak-to-peak values of runner dynamic stress at low partial load, high partial load, and full load are 15 MPa, 25 MPa, and 14.6 MPa, respectively. The high stress invoked by the unsteady flow under various loading conditions in this runner was the cause of the fatigue breakage of the runner blades. The results of this investigation have important reference values for mitigating fatigue damage in similar Francis runners and optimizing unit operation. [ABSTRACT FROM AUTHOR]

Concrete structures are subjected to static and fatigue loads during long-term service, and temperature changes can cause changes in the volume of concrete, leading to stress changes and fatigue damage, which affect the mechanical properties and service life of a structure. In response to the current lack of three-dimensional analysis and the exploration of the mechanical behavior and damage evolution processes of concrete structures under variable temperature environmental conditions, this paper relies on the Fenghuangjing Pump Station project involving the Yangtze River and Huaihe River and constructs a concrete thermo-mechanical-fatigue-damage model based on the power exponent function damage constitutive model. By using multiple coupled simulation calculations to predict the performance of concrete structures; analyzing the stress, deformation, and fatigue damage of structures from construction to operation; comparing measured data; dynamically optimizing inversion to obtain concrete thermal parameters; proposing key control parameters; and warning indicators and thresholds, the research results can provide reference for the long-term safe operation of similar projects. [ABSTRACT FROM AUTHOR]

Lazy wave risers (LWRs) are designed with equidistant buoyancy blocks attached in the lower half of the riser, allowing the riser to take on an arch shape under the buoyancy forces provided by buoyancy blocks. This arch configuration can provide flexibility to the LWR arrangement and effectively isolate the dynamic responses between the offshore floating structure and the riser's touchdown zone (TDZ). Its design and application aim to address the issues of dynamic response and fatigue damage that traditional steel catenary risers (SCRs) face in deep water and complex marine environments. Given that research on the LWRs in the field of ocean engineering is not sufficiently abundant, the structural characteristics, hydrodynamic loads, global responses, fatigue damage assessment, and structural optimization progress of LWRs are systematically reviewed in this paper to provide references for researchers in related fields. Among these topics, the global response of LWRs is the main point of this review. This section details the theoretical analysis and numerical modeling methods employed in the study of LWRs' global response, explores the research advancements in the vortex-induced vibration (VIV) related to LWRs, and discusses corresponding experimental studies. Finally, the installation, transfer, and repair processes of LWRs are investigated. Additionally, the importance of leveraging advanced technologies from other fields and combining them with current advanced algorithms is emphasized in efforts to assess fatigue damage and optimize the structures of LWRs, ultimately achieving complementary advantages. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the effects of soil penetration resistance, tillage depth, and operating speeds on the deformation and fatigue of the subsoiling shovel based on the real-time measurement of tractor-operating conditions data. Various types of sensors, such as force, displacement, and angle, were integrated. The software and hardware architectures of the monitoring system were designed to develop a field operation condition parameter monitoring system, which can measure the tractor's traction force of the lower tie-bar, the real-time speed, the latitude and longitude, tillage depth, and the strain of the subsoiling shovel and other condition parameters in real-time. The time domain extrapolation method was used to process the measured data to obtain the load spectrum. The linear damage accumulation theory was used to calculate the load damage of the subsoiling shovel. The magnitude of the damage value was used to characterize the severity of the operation. The signal acquisition test and typical parameter test were conducted for the monitoring system, and the test results showed that the reliability and accuracy of the monitoring system met the requirements. The subsoiling operation test of the system was carried out, which mainly included two kinds of soil penetration resistances (1750 kPa and 2750 kPa), three kinds of tillage depth (250 mm, 300 mm, and 350 mm), and three kinds of operation speed (4 km/h low speed, 6 km/h medium speed, and 8 km/h high speed), totaling 18 kinds of test conditions. Eventually, the effects of changes in working condition parameters of the subsoiling operation on the overall damage of subsoiling shovels and the differences in damage occurring between the front and rear rows of subsoiling shovels under the same test conditions were analyzed. The test results show that under the same soil penetration resistance, the overall damage sustained by the subsoiling shovels increases regardless of the increase in the tillage depth or operating speed. In particular, the increase in the tillage depth increased the severity of subsoiling shovel damage by 19.73%, which was higher than the 17.48% increase due to soil penetration resistance and the 13.07% increase due to the operating speed. It should be noted that the front subsoiling shovels consistently sustained more damage than the rear, and the difference was able to reach 16.86%. This paper may provide useful information for subsoiling operations, i.e., the operational efficiency and the damage level of subsoiling shovels should be considered. [ABSTRACT FROM AUTHOR]

The traditional finite element method (FEM) has limitations in accurately modeling crack propagation. Peridynamics, a nonlocal extension of the classical continuum theory, provides an alternative approach to remedy the limitations of the FEM but with a higher computational cost. In this paper, a peridynamic bond-based fatigue damage model is developed and incorporated into a commercial finite-element software (ABAQUS 2017) via user subroutines. Model-predicted results including the crack path spatial position and the damage accumulation rate were validated against empirical data. The predicted crack growth as a function of loading cycle and crack trajectory showed good agreement with the experimental data over 200,000 loading cycles. Therefore, the integration of the peridynamic bond-based fatigue damage model into existing FEM software provides an economical means to simulate complex fracture behaviors, such as crack growth, in a compact tension specimen examined in this paper. [ABSTRACT FROM AUTHOR]

Concrete linings containing cold joint defects may crack or detach under the aerodynamic fatigue loading generated by high-speed train operation, which posing a serious threat to the normal operation of high-speed trains. However, there is currently no simulation method specifically for fatigue damage of concrete linings containing cold joints. Based on the Roe-Siegmund cycle cohesive force model, a cohesive force fatigue damage elements were developed. A large dataset was constructed through numerical simulation software to build a BP neural network for back-calculated parameter of cohesive force fatigue damage elements. By combining experimental data, fatigue damage parameters corresponding to different pouring interval cold joints were back-calculated. These back-calculated parameters were then incorporated into the numerical model to compare simulation results with experimental results to validate the applicability of cohesive force fatigue damage elements and back propagation neural networks (BP neural network). The research results show that the difference between the fatigue life and fracture process calculated by numerical simulation and experimental data is small, verifying the applicability of the method proposed in this paper. The pouring interval directly affects the initial strength of the cold joint interface and the starting conditions of fatigue damage. The possibility of fatigue damage and fracture of concrete components containing cold joints increases with the increase of pouring interval, while the variability of fatigue life decreases with the increase of pouring interval. Interface strength and thickness are the main factors affecting the possibility of fatigue damage occurrence and the variability of fatigue life. The research results can be used to analyze the damage and cracking status of concrete linings containing cold joints under aerodynamic fatigue loading. [ABSTRACT FROM AUTHOR]

In the serviced components of a 7075 aluminum alloy, the propagation of fatigue crack can be retarded because of the overload effect; however, the corresponding retardation mechanisms are complex. To provide further insights into the retardation mechanisms of 7075 aluminum alloys, this study addresses the crack tip damage response of a cracked 7075 aluminum alloy under an overload effect. Based on the dual-scale modeling approach and the damage-coupled crystal plasticity model, the effect of the microstructure of a 7075 aluminum alloy on the damage behavior ahead of the crack tip under an overload was studied. The factors affecting fatigue damage accumulation ahead of the crack tip, such as dislocation density, the variation in the activities of slip systems, and the orientation effect of the nearest neighbor grains, are described. The results show that for the 7075 aluminum alloy, the compressive residual stress induced by the overload effect not only decreases the number of activated slip systems, but also lowers the rate of increase in dislocation density. This causes a decrease in fatigue damage accumulation during deformation. Moreover, the overload effect decreases the slip system activity as well as the resultant plastic slip; however, the decrease in plastic slip varies with the grain orientation, indicating that the overload effect depends on the grain orientation. It can also be found that both the damage strain energy release rate and lattice strain are influenced by the orientation of the nearest neighbor grains, which can eventually affect the overload effect. These findings contribute to understanding the retardation mechanisms from a microscopic perspective and provide guidance on improving the material design of a 7075 aluminum alloy to some extent. [ABSTRACT FROM AUTHOR]

The cyclic loading generated by injection and production operations in underground gas storage facilities can lead to fatigue damage to cement sheaths and compromise the integrity of wellbores. To investigate the influence of cyclic loading on the fatigue damage of well cement, uniaxial and triaxial loading tests were conducted at different temperatures, with maximum cyclic loading intensity ranging from 60% to 90% of the ultimate strength. Test results indicate that the compressive strength and elastic modulus of well cement subjected to monotonic loading under high-temperature and high-pressure (HTHP) testing conditions were 14–21% lower than those obtained under ambient testing conditions. The stress–strain curve exhibits stress–strain hysteresis loops during cyclic loading tests, and the plastic deformation capacity is enhanced at HTHP conditions. Notably, a higher intensity of cyclic loading results in more significant plastic strain in oil-well cement, leading to the conversion of more input energy into dissipative energy. Furthermore, the secant modulus of well cement decreased with cycle number, which is especially significant under ambient test conditions with high loading intensity. Within 20 cycles of cyclic loading tests, only the sample tested at a loading intensity of 90% ultimate strength under an ambient environment failed. For samples that remained intact after 20 cycles of cyclic loading, the compressive strength and stress–strain behavior were similar to those obtained before cyclic loading. Only a slight decrease in the elastic modulus is observed in samples cycled with high loading intensity. Overall, oil-well cement has a longer fatigue life when subjected to HTHP testing conditions compared to that tested under ambient conditions. The fatigue life of well cement increases significantly with a decrease in loading intensity and can be predicted based on the plastic strain evolution rate. [ABSTRACT FROM AUTHOR]

Integrated gate commutated thyristors (IGCTs) are critical components in high-voltage, high-current, and high-power conversion systems, particularly in offshore wind energy systems. However, the working environment of offshore wind energy conversion systems is extremely harsh. In this article, we propose an active damage control approach aiming at enhancing the reliability of the conversion system. By employing electro-thermal modeling for the equipment of the offshore wind energy conversion system, the junction temperature and fatigue damage of IGCT are simulated during the operation process. Using the improved model predictive current control (MPCC) method, active damage control effectively regulates the switching frequency of IGCT. IGCTs are symmetrically distributed on each leg of the converter, so the lifespan of the two IGCTs on each leg is also considered to be similar. This method balances the life of the IGCTs on the three legs of the converter and optimizes their utilization to the maximum extent. These measures effectively enhance the reliability of the conversion system and lower the operation and maintenance cost of high-power IGCT converters. The effectiveness of the proposed method is validated by co-simulation results by ANSYS and MATLAB/Simulink. [ABSTRACT FROM AUTHOR]

Since a multiaxial loading environment may lead to the fatigue failure of structures, establishing a reliable fatigue model to predict the multiaxial fatigue lives of structures has always been a concern of engineers. This study proposes a new multiaxial fatigue theoretical model (WYT model) based on the critical plane theory, which takes the plane of the maximum shear strain amplitude as the critical plane and considers the effects of shear stress and normal stress on fatigue damage. Moreover, a backpropagation neural network (BPNN) model for multiaxial fatigue life prediction with the shear strain amplitude, normal strain amplitude, mean shear stress, and mean normal stress on the same critical plane as input parameters and fatigue life as the output variable is established. Finally, the WYT model and the BPNN model are compared with two existing multiaxial fatigue models to evaluate the life prediction effects of different models for S45C and 7075-T651 under constant-amplitude and variable-amplitude multiaxial loadings. The calculation results show that the WYT model is feasible, and the BPNN model is more accurate in predicting the fatigue lives of specimens than other multiaxial fatigue theoretical models. [ABSTRACT FROM AUTHOR]

In asphalt pavement structure design, traffic axle loads and pavement layer temperatures are crucial factors affecting fatigue damage calculations. To investigate the differences in fatigue damage calculations caused by different characterizations of traffic axle loads and temperature, fatigue damage calculations were conducted under equivalent standard axle loads (ESALs), axle load spectra (ALS), constant temperatures, and seasonal temperature variations using the field data from an expressway in Shandong Province, China under seven calculation plans. The results indicated: (1) the annual traffic composition is dominated by vehicle Type 9, with a proportion of about 43% in all the vehicle types, and its load level is also high, with a proportion about of 80% in the heavy load interval at all axle types; (2) The ESALs method underestimates the actual fatigue damage incurred in asphalt pavement by 6.04 times, with an accumulated damage of 2.34 × 10−9 (ESALs), 1.69 × 10−8 (ALS), respectively; (3) The fatigue damage results from a single month with consistent temperature showed similar trends, with an accumulated damage of 1.50 × 10−5, 9.07 × 10−5, respectively; (4) The cumulative fatigue damage calculated using the ALS method across the four seasons, respectively, is 6.51, 5.88, 6.42, and 4.60 times that of the fatigue damage calculated using the ESALs method. Although the ratio of fatigue damage between the two characterizations of traffic axle loads remains consistent, which is 6.04, the fatigue damage calculation that accounts for temperature variations can reveal seasonal trends in fatigue damage development. Based on the axle load spectra and considering temperature variations, fatigue damage calculation will be more closely related to the actual service state of asphalt pavement. These research findings provide insights for estimating asphalt pavement fatigue damage to some extent. [ABSTRACT FROM AUTHOR]

The wear phenomenon that occurs on the blades during operation has a significant impact on the fatigue life of the blades. To address the issue of fatigue life assessment for turbine runner blades subjected to increased dynamic stress due to sediment wear, taking a specific high-head hydropower unit's mixed-flow turbine as the research subject, a hydraulic model of the turbine was established. The wear zones of the runner blades are determined based on the distribution of the flow field's velocity and the sediment volume fraction. According to the wear rate formula for runner blade material, the amount of wear on the blades is determined, and the dynamic stress data for the dangerous areas of the blades under different degrees of wear are calculated using a unidirectional fluid–structure coupling method. The load spectrum of the time–stress history data for the dangerous area at different levels of wear was compiled using the rain-flow counting statistical method. The operating time ratios for the flood season and the non-flood season are combined. Based on the fatigue cumulative damage theory, the total fatigue damage at the maximum stress part of the runner blade was calculated for different stages of wear, providing a reference for the life calculation of mixed-flow hydraulic turbines. [ABSTRACT FROM AUTHOR]

The backfill area of tunnel projects may deform or collapse due to the cyclic disturbance of groundwater and train loads. Hence, the anti-deformation and crack resistance performance of backfill materials under cyclic disturbance is critical to engineering safety. In this paper, concrete was produced by mixing 0.85 mm, 1–3 mm and 3–6 mm rubber particles instead of 10% sand, and tested to discuss the effect of rubber particle size on the deterioration of concrete material properties (compressive characteristics and energy dissipation) after bearing cyclic loading. The stress–strain curve and various parameters obtained through the uniaxial compression test and cyclic load test were used to explore the optimal grain size that can be applied to the tunnel engineering backfill area, and numerical simulation was adopted to calculate the deformation of the surrounding rock and the structural stress of different backfill materials. Research shows that the increase in particle size lessens the compressive strength, deformation resistance and cracking resistance of specimens, but after the cyclic loading test, the concrete material deterioration analysis indicates that rubber concrete has lesser and more stable losses compared to ordinary concrete, so the optimum rubber particle size is 0.85 mm. Numerical calculations show that RC-1 reduces the arch top displacement by 0.4 mm, increases the arch bottom displacement by 0.6 mm and increases the maximum principal stress by 11.5% compared to OC. Therefore, rubber concrete can ensure the strength and stability requirements of tunnel structures, which can provide a reference for similar projects. [ABSTRACT FROM AUTHOR]

Upon careful examination, numerous wind turbine collapses can be attributed to the failure of the tower bolts. Nowadays, the Schmidt–Neuper algorithm is extensively accepted in wind turbine tower bolt design. It is not advisable to utilize the finite-element method, notwithstanding the effect of the flange gap. To quantitatively investigate the influence of flange gaps on bolt fatigue, a nonlinear finite-element model of a flange segment incorporating bolt pretension and contact elements is herein proposed. Three distinct types of flange gaps are defined intentionally. It is possible to determine the nonlinear relationship between the wall load and bolt internal force. The fatigue damage of bolts was thus computed using the obtained nonlinear curve. Comparing with the results with those of Schmidt–Neuper method revealed the bolt fatigue damage is susceptible to a specified flange gap. [ABSTRACT FROM AUTHOR]

Wind turbine (WT) foundations with an embedded steel ring (ESR) are widely used in onshore WTs due to construction convenience. The research group found that WT foundations with damage were often accompanied by blade issues. To investigate the potential correlation between aerodynamic imbalance and fatigue damage of the WT foundation with an ESR, this study focuses on a 2 MW WT with an ESR. It investigates the influence of an error in pitch angle (PAE) on the WT's foundation load and stress, utilizing one year of SCADA data to analyze the fatigue damage caused by PAE. The main conclusions are as follows: Firstly, the effect of PAE on the amplitude value of load and stress is significantly greater than on the average value of load and stress. Secondly, when the PAE is within the range of −3° to 3°, the foundation fatigue damage incurred over one year is minimal, but once this limit is exceeded, the foundation fatigue damage increases dramatically. Thirdly, the peak value of fatigue damage to the foundation caused by PAE does not necessarily occur in the main wind direction, but in the direction with the highest probability of the occurrence of high wind speeds, and the larger the PAE, the more significant the trend. [ABSTRACT FROM AUTHOR]

With ultrasonic fatigue testing (UFT), it is possible to investigate the damage initiation and accumulation from the weakest link of the composite material in the very high cycle fatigue (VHCF) regime in a shorter time frame than conventional fatigue testing. However, the thermal influence on the mechanical fatigue of composites and the scatter in fatigue data for composites under ultrasonic cyclic three-point bending loading still need to be investigated. In this study, we conducted interrupted constant-amplitude fatigue experiments on a carbon-fiber satin-fabric reinforced in poly-ether-ketone-ketone (CF-PEKK) composite material. These experiments were carried out using a UFT system, which operates at a cyclic frequency of 20 kHz with a pulse–pause sequence. Various parameters, such as the CF-PEKK specimen's surface temperature, acoustic activity, and the ultrasonic generator's input resonance parameters, were measured during cyclic loading. During experiment interruption, stiffness measurement and volumetric damage characterization in the CF-PEKK specimens using 3D X-ray microscopy (XRM) were performed. The locations of damage initiation and accumulation and their influence on the changes in in situ parameters were characterized. Under fixed loading conditions, damage accumulation occurred at different locations, leading to scattering in fatigue life data. Further, the damage population decreased from the surface to the bulk of the composite material. [ABSTRACT FROM AUTHOR]

Computer-aided engineering (CAE) is an essential tool in a digital twin not only to verify and validate a virtual twin before it is transformed into a physical twin, but also to monitor the use of the physical twin for enhanced sustainability. This paper aims to develop a CAE model for a digital twin to predict the fatigue life of materials. Fatigue damage is represented by the size of a macro-crack that grows with a cluster of micro-cracks subjected to three different loads. The growth angle is related to the maximum circumferential tensile stress, and the growth rate is determined by the stress intensity factor (SIF) at the crack tip. The prediction model takes into consideration the main factors, including micro-cracks, crack closures, and initial configurations. Simulations are developed for the growth of macro-cracks with radially distributed micro-cracks and randomly distributed micro-cracks, and we find that (1) the macro-crack in the second case grows faster than that in the first case; (2) a pure shear load affects the macro-crack propagation more than a combined shear and tensile load or a tensional load; (3) the external stresses required to propagate are reduced when the inclination angle of the micro-crack is small and within (−25° < β < 25°); (4) micro-cracks affect the propagating path of the macro-crack and generally guide the direction of propagation. The developed model has been verified and validated experimentally for its effectiveness in predicting the fracture or fatigue damage of a structure. [ABSTRACT FROM AUTHOR]

The heat alternation and bending during the service process of the pipeline structure are the key issues affecting its residual strength. The nonuniform plastic deformation of its cross-section causes nonlinear damage to the structure, which makes it difficult to calculate the residual strength of the overall structure. This study proposed a novel variable-strength material damage model with damage accumulation, which achieved the coupling analysis of structural damage and material fatigue residual strength, making the structural strength of nonlinear damage cross-section computable. Based on this, a theoretical ultimate load model for nonlinear damage cross-section and a finite element analysis model for dynamic material strength with damage accumulation were established. The analysis was conducted on the repeated banding damage to coiled tubing. The results showed that the residual yield strength of nonlinear damaged coiled tubing showed a trend of first rapid decrease and then slow decrease with damage accumulation, and the tensile displacement showed an increasing trend with damage accumulation. The ultimate internal pressure strength of nonlinear damaged structures showed a similar downward trend to the yield strength. The nonlinear damage coupling analysis model proposed in this study has significant practical engineering application value. [ABSTRACT FROM AUTHOR]

Wind turbines and floating platform upsizing are major trends in the current offshore wind development. However, harsh environmental conditions increase the risk of anchor dragging and mooring failure when deploying large offshore floating wind turbines. Therefore, it is necessary to design a mooring system for the specific deployment site. This study aims to perform the mooring system design of a floating offshore wind turbine (FOWT) operated in the South China Sea, which is a combination of the IEA 15 MW wind turbine and UMaine VolturnUS-S floating platform. Hydrodynamic coefficients were calculated based on the potential flow theory, considering the environmental loads in the South China Sea. Additionally, the hydrodynamic coefficients were imported into AQWA to calculate the time-domain mooring tension. The mooring design parameters, such as mooring line length, nominal sizes, and anchor point, were determined using the criterion of anchor uplift, maximum breaking strength, and fatigue life, respectively. The design criterion required that the anchor uplift is not more than the allowable value, the long-term breaking limit of mooring with a 100-year return period should be less than the maximum breaking limit, and the fatigue damage accumulation in 50 years should be safe. The mooring design procedure provides a reference for mooring system design and safe operation of large floating wind turbines in the South China Sea. [ABSTRACT FROM AUTHOR]

In the present work, the results of acoustic nonlinear response of ultrasonic wave propagation when monitoring the progress of damage induced by fatigue on notched C45 carbon steel specimens have been reported. Two ultrasound probes were fixed to the specimens during the tests. The input signal was sinusoidal type, while the corresponding ultrasound response signal was acquired and recorded at each stage of the test by means of a digital oscilloscope. A nonlinear frequency study was performed on the acquired data to evaluate the change in the second- and third-order nonlinearity coefficients of β1 and β2, respectively, on the tested specimens. Ultrasonic results were correlated to plastic strain at the notch tip in the initial phases of fatigue and stiffness degradation. The results showed a significant increase in second-order nonlinearity β1 in the early stages of fatigue life. Subsequently, starting from about 30–40% of the fatigue life, the nonlinearity of β1 increases. Before final failure, from 80 to 85% of fatigue life, the second-order nonlinearity further increases in the crack propagation stages. The nonlinear parameter of the third-order β2 was less sensitive to damage than the parameter β1, showing a rapid increase only starting from approximately 80 to 85% of the fatigue life. The proposed method proved to be valid for detective damage induced by fatigue and to predict the lifetime of metal materials. [ABSTRACT FROM AUTHOR]

Welded structures under random loadings are usually susceptible to fatigue-induced failures that lead to significant economic and safety effects. However, accurately predicting these structures' fatigue damage and life in the frequency domain remains challenging due to the limitations associated with using traditional weld stress extrapolation methods, such as nominal, hotspot, and notch stress methods. These methods struggle with precisely defining and characterizing the stresses at the weld toe and root as they vary depending on factors like weld stress concentration effects, joint geometry, and loading modes. This research introduces an Equilibrium Equivalent Structural Stress (EESS)-based frequency-domain fatigue analysis approach for welded structures subjected to random loading. The proposed method utilizes the EESS formulations, which are based on the decomposition and characterization of weld toe stresses with a single stress parameter, together with incorporating structural dynamic properties' effects on the stresses acting on the weld joints and the corresponding accumulated fatigue damage of the structure. The numerical demonstration and validation of the proposed method have been performed using a welded Rectangular Hollow Section (RHS) T-joint structure subjected to stationary random fatigue loading. The proposed method's fatigue damage and life results are compared with the fatigue test data and the equivalent hotspot stress extrapolation-based technique results. [ABSTRACT FROM AUTHOR]

In this study, we proposed a non-destructive testing and evaluation (NDT&E) method for assessing the micro-scale fatigue damage of carbon-fiber-reinforced plastic (CFRP) structures. The process involves evaluating the thermophysical properties of the material, specifically the thermal diffusivity D and specific heat capacity C p . To measure these properties, we employed the laser-induced periodic heating method using lock-in thermography, which is suitable for anisotropic composite materials. Our fatigue experiments revealed that the thermal diffusivity D decreased and the specific heat capacity C p increased with the increase in fatigue cycles. We attribute these trends to the thermal resistance caused by the change in composite microstructure and the degradation of the matrix resin. These findings are discussed in detail, with implications for assessing CFRP structures and potential future research directions. [ABSTRACT FROM AUTHOR]

A series of creep–fatigue tests of Gr.91 steel were performed at 600 °C. Fatigue life was reduced by tensile strain holding. The minimum life reduction factor was approximately 0.3. The creep–fatigue life could not be estimated properly via the conventional linear summation rule of the fatigue damage and creep damage. Since this material is considered to have a large creep–fatigue interaction, it was proposed that the creep–fatigue life should be estimated using the improved linear summation rule of the fatigue damage, the creep damage and the creep–fatigue interaction damage. In the future, it will be necessary to clarify the creep–fatigue interaction mechanism and define its damage value. On the other hand, a series of creep–fatigue tests for Gr.91 steel welded joints were also performed in the strain range of 0.5% at 600 °C. Again, the fatigue life was shortened by the tensile strain holding. The minimum fatigue life reduction factor was approximately 0.2. All the test pieces fractured in the fine-grained HAZ of the welded joints. The creep–fatigue life could not be estimated properly using the linear summation rule of the fatigue damage and creep damage in the HAZ. One possible reason was thought to be the effects of the elastic follow-up phenomena peculiar to welded joints. The creep strain of the HAZ might increase due to the transfer of the elastic strain from both the base metal and the weld metal, according to the elastic follow-up phenomena during strain holding. In the future, it will be important to quantitatively estimate the effects of the elastic follow-up phenomena. [ABSTRACT FROM AUTHOR]

Current research on the fatigue properties of degradable zinc alloy stents has not yet considered the issue of the fatigue life changing with material properties during the dynamic degradation process. Therefore, in this paper, we established a fatigue damage algorithm to study the fatigue problem affected by the changing of material properties during the dynamic degradation process of the stent under the action of pulsating cyclic loading. Three models: the dynamic degradation model, the dynamic degradation model under pulsating cyclic loading, and the coupled model of fatigue damage and dynamic degradation, were developed to verify the effect of fatigue damage on stent life. The results show that fatigue damage leads to a deeper degree of inhomogeneous degradation of the stent, which affects the service life of the stent. Fatigue damage is a factor that cannot be ignored. Therefore, when studying the mechanical properties and lifetime of degradable stents, incorporating fatigue damage into the study can help more accurately assess the lifetime of the stents. [ABSTRACT FROM AUTHOR]

Road simulators enable accelerated durability tests under similar-to-real road conditions. However, the road simulator itself generates the signals with the appropriate strength and amplitude that is adequate to the response registered by the sensors during the real run. Therefore, there is a need for verification of the validity of the representation of vehicle runs on a road simulator in terms of the shape of the generated profile and possible sources of uncertainty. The tests in this study were carried out for a multi-axle vehicle passing an obstacle of known shape. Various signals were registered while the vehicle was passing over the obstacle. The MTS (System Corporation) road simulator's response to the signal given by the obstacle was then checked. The results showed a 99% correlation between the simulation and the road test results. A numerical model of the vehicle was developed to verify the quality of representation of the real conditions by the road simulator, especially in terms of forces resulting from the road profile. Interestingly, the input signal generated by the road simulator provided a very good accuracy of the vehicle response, as tested with use of the numerical model. [ABSTRACT FROM AUTHOR]

The oil and natural gas resources of the Bohai Sea are mainly marginal oil fields, and there are currently a large number of structures that are approaching or have reached the end of their service life called aging structures. The interactions between ice and offshore structures could lead to significant ice-induced vibration. Ice-induced vibrations may evoke fatigue damage in tubular joints, which results in severe dangers for offshore platforms in the Bohai Sea. Dynamic ice force models and sea ice fatigue environmental parameters have not been well developed in the current design codes. It is not accurate to evaluate the fatigue damage of structures only with numerical simulation. In this paper, a faster method for evaluating fatigue damage on aging structures is proposed. Firstly, the approximate linear relationship between the fatigue hot spot stress and the vibration response of the deck structure has been discovered using dynamic analysis for jacket platform of Bohai Sea. Then, the procedure of the fatigue damage evaluation of the aging structures in the Bohai Sea is established. Finally, the numerical simulation of the structure is carried out considering ice thickness, ice velocity, ice direction, and action height of sea ice. Fatigue damage is calculated by the fatigue hot spot stress and the Palmgren–Miner rule. The measured data on the platform in the Bohai Sea are selected to obtain the fatigue hot spot stress using a mathematical model. The fatigue damage results considering the actual ice conditions of a jacket structure in the Bohai Sea are compared using the proposed method and finite element analysis. The comparison of the results verifies the rationality of the method proposed in this work. [ABSTRACT FROM AUTHOR]

Fatigue damage can develop within asphalt pavement due to the continuous impact of driving loads and natural elements. Understanding the process of asphalt mixtures' fatigue damage is crucial for guiding the design, maintenance, and repair of asphalt pavement. This research aims to establish a model that characterizes the mixtures' modulus decay under a three-dimensional (3-D) stress state. Firstly, asphalt mixes were subjected to direct tensile (DT), indirect tensile (IDT), unconfined compressive (UC) strength and fatigue tests, and the resulting data were analyzed. Then, modulus decay models under DT, IDT, and UC conditions were established, and the modulus decay patterns under the three loading modes were compared and analyzed. Finally, using the fatigue stress strength ratio Δ (a fatigue resistance index for asphalt mixtures that takes into account the impacts of stress state and loading rate), a unified characterization model for asphalt mixes' modulus decay under a 3-D stress state was created. According to the study's findings, asphalt mixes' modulus decay during fatigue damage exhibits obvious nonlinear characteristics. While the asphalt mixes' modulus decay law with various loading modalities is similar under the same conditions, the decay rate may differ. Essentially, the speed of the modulus decay of a certain asphalt mixture primarily depends on the value of Δ during service. A larger Δ indicates a faster modulus decay. This study offers a theoretical foundation for the conversion from material fatigue damage to structural fatigue damage, which is vital for enhancing the asphalt pavements' construction quality and longevity. [ABSTRACT FROM AUTHOR]

In order to counter the problems of cracks and large area fractures in the welding points of quick-replacement battery boxes for electric vehicles (which may lead to the concentration of stress), in this study, a fatigue analysis of the welding points, based on a load spectrum, was used to predict welding points' fatigue and improve the structural life of quick-replacement battery boxes. Firstly, a model of the quick-replacement battery box was established in SolidWorks software; secondly, the welding points' fatigue was analyzed using the Optistruct module of HyperMesh software, and the topology of the quick-replacement battery box was optimized according to the results of the analysis; finally, for testing purposes and to achieve a lighter weight and an improved structural life, the fatigue of the welding points of the optimized battery box was analyzed. The results of the analysis showed that the force of the quick-replacement battery box was primarily concentrated at the connection between the middle bottom plate and the partition. Additionally, retaining the number of welding points at the hanging ear was shown to be beneficial for maintaining stiffness during electric vehicle operation; however, the number of welding points at the partition connection could be appropriately reduced. Before optimization, the maximum fatigue damage values of the welding points were 2.763 × 10−6, 3.833 × 10−6, and 6.728 × 10−6, respectively, satisfying the criteria of fatigue damage to the welding points. After optimization, the fatigue damage values of the welding points in the quick-replacement battery box were significantly reduced to 4.431 × 10−8, 4.562 × 10−8, and 8.885 × 10−8, respectively, compared with their pre-optimized levels. Consequently, the stress concentration was alleviated effectively, thereby meeting the conditions for fatigue damage. These results have important theoretical and engineering significance for the design and optimization of quick-replacement battery boxes for electric vehicles. [ABSTRACT FROM AUTHOR]

Fatigue damage accumulation will not only cause the degradation of material performance but also lead to the growth of effective stress and critical stiffness. However, the existing fatigue reliability models usually ignore the effective stress growth and its influence on the critical stiffness of a composite material. This study considers the combined effects of performance degradation and effective stress growth, and a pair of fatigue reliability models for a composite material are presented. Firstly, the fatigue damage in a composite material is quantified by its performance degradation, and the fitting accuracy of several typical fatigue damage models is compared. Subsequently, the uncertainties of initial strength and initial stiffness are considered, and a pair of probabilistic models of residual strength and residual stiffness are proposed. The performance degradation data of Gr/PEEK [0/45/90/−45]2S laminates are utilized to verify the proposed probabilistic models. Finally, the effective stress growth mechanism and its influence on the failure threshold are elaborated, and a pair of fatigue reliability models for composite materials are developed. Moreover, the differences between the strength-based and stiffness-based reliability analysis results of composite materials are compared and discussed. [ABSTRACT FROM AUTHOR]

The existing rock-fatigue-damage constitutive model fails to consider the influence of discharge energy and discharge times on the effect of rock damage under discharge impact. In this regard, the impact load test of sandstone was carried out. Based on the test results, the quantitative characterization formula of different discharge parameters on the weakening degree of rock compressive strength was established. Based on the TCK (Taylor–Chen–Kuszmaul) model, a constitutive model considering the mechanical properties of rock and the dynamic hange of micro-crack geometric size is established, and the cohesive model is proposed to calculate the crack size. The constitutive model includes the relationship between crack width, crack number, and rock compressive strength, and the change of damage factor D is used to consider the fatigue-damage effect of discharge shock on the rock. The results show that the modified model can quantitatively and accurately reflect the dynamic damage and failure process of rock and that the weakening of rock mechanical properties and the development of micro-cracks can be controlled by designing different discharge parameters. [ABSTRACT FROM AUTHOR]

Autonomous vehicles and truck platooning have become the future in the transportation field. This new strategy has many benefits because it lowers fuel consumption and CO2 emissions, improves safety, optimizes transport by using roads more effectively, and reduces traffic congestion. In this research, the effect of the controlled positioning of autonomous and non-autonomous truck loadings on the long-term performance of pavement was estimated using different variables such as climate, uniform wandering values of distance between trucks, and percentage of autonomous trucks by using MEPDG/AASHTOWare Pavement ME Design software. This was achieved by first computing the strain and stress of the different loading combinations, resulting in the computation of the failures in the pavement infrastructure and the pavement thickness needed to support each combination. The second part of the research consisted of designing a platoon strategy that was developed for a series of autonomous and connected trucks such that the lateral position of the trucks and the spacing between them could be explicitly optimized to minimize flexible pavement damage. The findings revealed that a small percentage of autonomous trucks can be beneficial to pavement life and that truck platooning following a well-studied skeleton can open a whole new world of pavement design. This can be revolutionary in changing roads around the world to improve traffic and infrastructure. [ABSTRACT FROM AUTHOR]

In drilling engineering, the cost of drilling tool fracture is enormous, and studying the fatigue failure process of drilling tools has practical significance. This paper uses metal magnetic memory detection technology to design and conduct fatigue damage tests on typical drilling tool material 42CrMo specimens under tensile, torsional, compressive, tensile, compressive, and torsional dynamic loading conditions. By analyzing the changes in the tangential component Hp(x) and gradient value K of the magnetic memory signal under different load conditions with the number of loading times, the process of fatigue failure of the specimens and the trend of changes in the magnetic memory signal in local stress concentration areas are explored. The characteristic parameters of fatigue damage based on magnetic memory detection were extracted and the critical point at which fatigue damage leads to crack initiation was inferred. This confirms that metal magnetic memory testing technology is an effective means of analyzing the fatigue damage process of drilling tools and provides a certain reference for formulating judgment standards for drilling tool maintenance on site. [ABSTRACT FROM AUTHOR]

Online monitoring of the fatigue damage process of A7N01 aluminum alloy base metal and weld seam was conducted based on acoustic emission (AE) and digital microscopic imaging technology. The AE signals were recorded during the fatigue tests and analyzed using the AE characteristic parameter method. Fatigue fracture was observed using scanning electron microscopy (SEM) to analyze the source mechanism of AE. The AE results show that the AE count and rise time can effectively predict the initiation of fatigue microcracks in A7N01 aluminum alloy. The digital image monitoring results of a notch tip verified the prediction of fatigue microcracks using the AE characteristic parameters. In addition, the AE characteristics of the A7N01 aluminum alloy under different fatigue parameters were studied, and the relationships between the AE characteristic values of the base metal and weld seam and the crack propagation rate were calculated using the seven-point recurrence polynomial method. These provide a basis for predicting the remaining fatigue damage in the A7N01 aluminum alloy. The present work indicates that AE technology can be used to monitor the fatigue damage evolution of welded aluminum alloy structures. [ABSTRACT FROM AUTHOR]

In this paper, the research on the fatigue damage mechanism of tire rubber materials is the core, from designing fatigue experimental methods and building a visual fatigue analysis and testing platform with variable temperature to fatigue experimental research and theoretical modeling. Finally, the fatigue life of tire rubber materials is accurately predicted by using numerical simulation technology, forming a relatively complete set of rubber fatigue evaluation means. The main research is as follows: (1) Mullins effect experiment and tensile speed experiment are carried out to explore the standard of the static tensile test, and the tensile speed of 50 mm/min is determined as the speed standard of plane tensile, and the appearance of 1 mm visible crack is regarded as the standard of fatigue failure. (2) The crack propagation experiments were carried out on rubber specimens, and the crack propagation equations under different conditions were constructed, and the relationship between temperature and tearing energy was found out from the perspective of functional relations and images, and the analytical relationship between fatigue life and temperature and tearing energy was established. Thomas model and thermo-mechanical coupling model were used to predict the life of plane tensile specimens at 50 °C, and the predicted results were 8.315 × 105 and 6.588 × 105, respectively, and the experimental results were 6.42 × 105, with errors of 29.5% and 2.6%, thus verifying the accuracy of thermo-mechanical coupling model. [ABSTRACT FROM AUTHOR]

Railway infrastructures play a pivotal role in developing the national transportation system. Recently, the strategy of the railway engineer has been significantly shifted; along with the development of new assets, they tend to pay increasing attention to the operation and management of existing railway assets. In this regard, this paper proposes a Digital Twin (DT) model to improve fatigue assessment efficiency in the operational processes of railway steel bridges (RSBs). The DT concept mainly lies in the federation and interaction of a Fatigue Analysis System (FAS), which is based on Eurocodes principles, and a model in Building Information Modeling (BIM). Along with the proposed DT concept, a prototyping system for a real bridge is initiated and curated. The FAS is validated in good-agreement results with the ambient vibration test of the bridge (about 1.6% variation between numerical and experimental values), and close values were found between numerical and experimental stresses, the latter obtained by installing strain gauges on the bridge. The BIM model provides access to the numerical values of fatigue state results in a given bridge connection detail but also automatically represents that information in a 3D environment using a color-scale-based visualization process. Furthermore, a simulation model with the main input variables being the traffic and geometric conditions of the bridge is continuously updated for timely re-evaluation of the damage state, which shows promise for the lifecycle management of the bridge. [ABSTRACT FROM AUTHOR]

In order to investigate the impact of fatigue damage on the ultimate load capacity of criticality cells in steel–concrete composite segments and to address the complex design challenges associated with bridge steel–concrete composite segments in practical engineering, this study designs two scaled criticality cell specimens with a scale ratio of 1:2 and performs ultimate load capacity tests after fatigue cyclic loading. By analyzing the stress distribution of each component and the force transmission ratio and combining the results from finite element model calculations, this study introduces the degree of structural fatigue damage and proposes a predictive model for the ultimate load capacity of steel–concrete composite segment criticality cells that is easy to apply. This model is compared with the finite element calculation results and experimental values, and the results are found to be in good agreement. Additionally, the number of shear connection members in the model is optimized based on the calculation results. The research findings indicate that the main failure mode of criticality cells is inclined compression failure. The strength of each part decreases in the following order: steel–concrete composite segment, steel structure segment, and concrete segment. Furthermore, fatigue damage has a significant impact on criticality cells. The optimized model exhibits similar stress performance and force transmission ratio to the original model and provides a reference for the design of practical engineering. [ABSTRACT FROM AUTHOR]

In this research, the fatigue damage behavior under three-point bending of a composite joint incorporating a single countersunk fastener is investigated. Firstly, a self-developed fatigue test system was set up to test the fatigue characteristics of CFRP-countersunk bolted assembly under the displacement amplitude cycles of 103 to 106 to study the formation and expansion rule of damage and cracks. It found two typical damage processes, both of which involve some formal interface damage between fiber and matrix. Based on the experiment, a finite element fatigue damage analysis on this assembly was carried out according to the Hashin failure criterion. The simulation result shows an identical fatigue damage location and fatigue life with the experimental phenomenon. Moreover, it predicted the final fatigue life of the specimen under 10 hz cyclic loading with 1 mm displacement and 10 Nm bolt preloading. This research provides guidance for the engineering fatigue issues of single-bolted joint composite connection structures and provides a reference for the corresponding technical specifications formulation. [ABSTRACT FROM AUTHOR]

The fatigue reliability assessment of deepwater risers plays an important role in the safety of oil and gas development. Physical-based models are widely used in riser fatigue reliability analyses. However, these models present some disadvantages in riser fatigue reliability analyses, such as low computational efficiency and the inability to introduce inspection data. An improved fatigue reliability analysis method was proposed to conduct the fatigue reliability assessment of deepwater risers. The data-driven models were established based on response surface methods to replace the original physical-based models. They are more efficient than the physics-based model, because a large number of complex numerical and iterative solutions are avoided in fatigue reliability analysis. The annual crack growth model of the riser based on fracture mechanics was established by considering the crack inspection data as a factor, and the crack growth dynamic Bayesian network was established to evaluate and update the fatigue reliability of the riser. The performance of the proposed method was demonstrated by applying the method to a case. Results showed that the data-driven models could be used to analyze riser fatigue accurately, and the crack growth model could be performed to analyze riser fatigue reliability efficiently. The crack inspection results update the random parameters distribution and the fatigue reliability of deepwater risers by Bayesian inference. The accuracy and efficiency of fatigue analysis of deepwater risers can be improved using the proposed method. [ABSTRACT FROM AUTHOR]

In this paper, the formulation to incorporate the used vibration profile, the stress generated by the product's application, mass, and the resonance frequency is given. After that, based on the vibration output data, the two-parameter Weibull distribution is used to predict the corresponding reliability indices. In the method, the mentioned stress is incorporated as acceleration response ( A r e s ), and by using a dynamic stress factor (σ d y n) . In addition, the Weibull parameters are determined based on the generated maximum and minimum principal vibration stress values. In the paper we show the efficiency of the fitted Weibull distribution to predict the reliability indices, by using its Weibull shape and scale parameters, it is always possible to reproduce the principal vibration stress values. Additionally, from the numerical application, we show how to use the Weibull analysis to determine the reliability index for a desired stress or desired cycle value. Finally, we also present the guidelines to apply the proposed method to any vibration fatigue analysis where the A r e s (used to determine the σ 1 and σ 2 values), and the σ d y n value are both known. [ABSTRACT FROM AUTHOR]

Steel–concrete composite box beams are widely used in bridge engineering, which might bear transverse and longitudinal bending moments simultaneously under vehicle loads. To investigate the fatigue performance of joints between the steel girders and the top reinforced concrete (RC) slabs under transverse bending moments, a reduced scale joint between the weathering steel girder with the corrugated steel web (CSW) and the top RC slab was designed and tested under constant amplitude fatigue loads. Test results show that the joint initially cracked in the weld metal connecting the CSW with the bottom girder flange during the fatigue loading process. The initial crack propagated from the longitudinal fold to the adjacent inclined folds after the specimen was subjected to 7.63 × 105 loading cycles and caused the final fatigue failure. Compared with the calculated fatigue lives in the methods recommended by EC3 and AASHTO, the fatigue performance of the details involved in the joint satisfied the demands of fatigue design. Meanwhile, finite element (FE) models of joints with different parameters were established to determine their effect on the stress ranges at the hot spot regions of the joints. Numerical results show that improving the bending radius or the thickness of the CSW helps to reduce the stress ranges in the hot spot regions, which is beneficial to enhance the fatigue resistance of the investigated fatigue details accordingly. [ABSTRACT FROM AUTHOR]

Indentation tests were used for early detection of deformation-induced damage of Inconel 718. Damage was produced in a controlled manner using specially designed specimens with a variable width of the gage part. Static tension, LCF, and HCF tests were performed to induce material damage. Accumulated plastic strain intensity was used as the reference measure of deformation-induced damage. Detection of damage progress in its early phase was based on the use of strain-hardening cross-effect. Experimentally determined correlation of hardness and indentation work with reference damage parameter was analyzed. The results of our analysis lead to the conclusion that the proposed experimental procedure is useful for the detection of deformation-induced damage in the early stage of the process. [ABSTRACT FROM AUTHOR]

In this paper, an aluminum foam sandwich (AFS) was prepared by the rolling composite-powder metallurgy method, and its fatigue properties were studied. It was compared with an AFS made by the adhesive method to study its fatigue properties more directly. In this experiment, the fatigue performance was investigated by studying the microscopic interface, fatigue life, deflection curve, and failure mode. The results show that the fatigue life of an AFS with the rolling composite-powder metallurgy method is much longer than that with the adhesive method. The failure mode of an AFS made by the rolling composite-powder metallurgy method is shear failure, and that of an AFS made by the adhesive method is shear failure and interface debonding. An AFS with the rolling composite-powder metallurgy method has better fatigue properties. This paper also explored the fatigue damage model using the fatigue modulus method, and the polynomial fitting method has a higher fitting degree. [ABSTRACT FROM AUTHOR]

Benefitting from economic development and technological progress, long-span suspension bridges, with their superior span capacity and good economy, have been built in large numbers in recent decades. However, the excessive cumulative longitudinal displacement at the girder ends in the process of bridge operation leads to the degradation of, and fatigue damage to, the connecting components. This study aims to solve the problem with an effective parallel damping scheme of friction dampers and viscous dampers. Firstly, the phenomenon that quasi-static longitudinal displacement accounts for the majority of cumulative displacement is confirmed by the decomposition of measured displacement data at the girder end, which is caused by the asymmetric geometric deformation of the main cable induced by the moving vertical loads of a long-span suspension bridge. An efficient control performance analysis method is proposed based on the formation mechanism of the quasi-static longitudinal displacement. Secondly, the friction damper with a continuous damping model is employed to achieve an effective control performance with respect to the quasi-static longitudinal displacement. Thirdly, in order to realize the target of operational and seismic dual control, a parallel scheme of friction dampers and viscous dampers is proposed, aiming to reduce the cumulative value in the operational state, and maximum value in the seismic state, for longitudinal displacement at the girder ends of a long-span suspension bridge. [ABSTRACT FROM AUTHOR]

The fatigue damage assessment of laser-repaired components is critical to their service safety. However, since laser repairing is an advanced green remanufacturing technology, the current research on its fatigue mechanical behavior and fatigue damage evaluation methods is still immature. In addition, the relevant models used for the fatigue damage evaluation can only indicate the fatigue limit of components, which cannot describe the damage accumulation process of the components during the fatigue testing. Therefore, there is an urgent need to develop a fatigue damage evaluation method that can describe the fatigue damage accumulation and evolution to reveal the damage evolution mechanism during the fatigue test. In this study, based on the 3D-DIC technique, new damage parameters, i.e., strain average value and strain standard deviation, are proposed to quantitatively describe the damage status of the nickel-based components during the stress-based fatigue process. Then, based on the new damage parameters, a strain average value/strain standard deviation damage curve method is proposed to describe the damage status evolution of the components during the fatigue testing and evaluate its fatigue damage. For example, in the tensile fatigue test, the strain average value/strain standard deviation damage curves of the substrate component and the laser-repaired component can be divided into two damage stages. In the first damage stage, the damage increases slowly with the increase in the cycle number, whereas in the second damage stage, the damage increases rapidly with the increase in the cycle number. At this time, there is a demarcation point between the first damage stage and second damage stage in the strain average value damage curve and strain standard deviation damage curve. The cycle number of the demarcation point can be used as a reference value for the fatigue failure of the laser-repaired component. In addition, the electron backscatter diffraction (EBSD) technique was used to verify the validity of the evaluation results from the novel damage parameters. [ABSTRACT FROM AUTHOR]

Only by using accurate material data can precise simulation results be achieved. This principle also and especially applies in the field of crash simulation. However, in the simulation of short-fibre reinforced thermoplastics, material parameters are usually used that originate from the material testing of as-new samples. In order to get closer to the condition on the roads, where not only new vehicles are driving, the influence of service loads on the crashworthiness has to be investigated. This paper reports on studies of PBT GF30, a polybutylene terephthalate reinforced with 30% glass fibres, in which fatigue damage was induced in the material by cyclic loading. The residual strength was then determined in a high-speed experiment and compared with the strength of virgin samples. In order to enable the usability of the findings in the simulation, a modified failure criterion was implemented that takes the previous fatigue damage into account. The prediction quality of the simulation model was compared with the experimental findings and it can be concluded that there is good agreement. [ABSTRACT FROM AUTHOR]

A7N01P-T4 aluminum alloy plates for high-speed trains will experience a certain amount of fatigue damage under alternating loads. Three groups of samples, P0 (no fatigue damage), P1 (loading stress 30 MPa), and P2 (loading stress 70 MPa), were created, and corrosion fatigue crack growth (CFCG) tests were conducted in 3.5 wt.% NaC1 solution. The crack growth rate was found to increase after fatigue damage as the damage degree increased. In addition, the A7N01P-T4 aluminum alloy base metal exhibited obvious secondary cracks and crack bifurcations after fatigue damage. It is believed that fatigue damage causes stress concentration in the material, while a certain degree of stress corrosion cracking occurs during the CFCG growth process. This is because hydrogen (H) easily accumulates and diffuses along the grain boundary, which reduces the strength of the grain boundary, thereby becoming the preferred orientation for crack growth. This explains why the CFCG rate of the material is accelerated following fatigue damage to a certain extent. [ABSTRACT FROM AUTHOR]

Very high cycle fatigue (VHCF) tests were carried out under variable amplitude loading for TC21 titanium alloy. The first level of high amplitude loading was set as 950 MPa close to yield strength, and the second level of low amplitude loading was determined between 435 MPa and 500 MPa where fatigue cracks initiated at the specimen subsurface under constant amplitude. The results indicate that the high/low stress block significantly reduced the cumulative fatigue life of low stress amplitude, and the fatigue crack initiation site changed from the specimen subsurface under constant loading to the specimen surface under stress block. Based on continuum damage mechanics, the fatigue damage model of two-step stress block was established to estimate the fatigue damage process. The prediction of cumulative fatigue life generally agreed with the experimental data. The cumulative fatigue damage of the stress block was related to the stress amplitude and the cycle ratio, which determined the stress fatigue damage and its interaction damage. The surface crack initiation in the stress block accelerated fatigue damage of low stress amplitude, reducing the cumulative life. [ABSTRACT FROM AUTHOR]

The effect of low cycle fatigue (LCF) predamage with no precracks on very high cycle fatigue (VHCF) properties, and crack initiation characteristics for TC21 titanium alloy, was investigated. The results showed that LCF predamage with less than 5% of fatigue life had little influence on fatigue limit, but reduced its fatigue life. Fatigue cracks were initiated on the surface of the specimen at high stress amplitude, whereas fatigue cracks were initiated on the subsurface of the specimens at low stress amplitude. Based on Lemaitre damage theory, a very high/low cycle combined fatigue damage model was established to analyze the fatigue damage process, which was consistent with the experimental data. It was indicated that 5% LCF predamage value was the equivalent damage value, which was close to the critical value of VHCF crack initiation. The fatigue crack initiation of the specimens with LCF predamage less than 5% took up the major components of fatigue life. [ABSTRACT FROM AUTHOR]

The subsea wellhead (SW) system is a crucial connection between blowout preventors (BOPs) and subsea oil and gas wells. Excited by cyclical fatigue dynamic loadings, the SW is prone to fatigue failure, which would lead to the loss of well integrity and catastrophic accidents. Based on the Bayesian Regularization Artificial Neuron Network (BRANN), this paper proposes an efficient probability approach to predict the fatigue failure probability of SW during its entire life. In the proposed method, the BRANN fatigue damage (BRANN-FD) model reflecting the non-linear relationship between the input and output data was developed by the limited fatigue damage analysis data, which was utilized to generate thousands of non-numerical fatigue damage data of SW rapidly. Combining parametric and non-parametric estimation methods, the probability density function (PDF) of SW fatigue damage was determined to calculate the accumulation fatigue damage during service life. Using the logistic regression, the fatigue failure probability of SW was predicted. The application of the proposed approach was demonstrated by a case study. The results illustrated that the fatigue damage of SW would be viewed as obeying the Lognormal distribution, which could be used to obtain the accumulation fatigue damage in operation conveniently. Furthermore, the fatigue failure probability of SW nonlinearly increased with the increment in the accumulation fatigue damage of SW, which could be helpful to ensure the operation safety of SW in deepwater oil and gas development, especially for aged wellhead. [ABSTRACT FROM AUTHOR]