Design Fatigue Life Research Articles

Complexity of Determining the Fatigue Strength of Real Structures Under Random Vibration Conditions—Two Case Studies

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.

A modified cyclic elastoplastic constitutive model considering the variational dynamic recovery term of back stress for FGH95 under asymmetrical cyclic loading

The asymmetric cyclic loading process occurs in aero-engine turbine discs. A constitutive model that accurately describes the cyclic elastoplastic behaviour of the material is important for structural design and low cycle fatigue life prediction of turbine discs. In this paper, the low cycle fatigue test of FGH95 was carried out at 620℃ under 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0% and 1.1% strain amplitude with strain ratio equal to 0. The material exhibited cyclic hardening followed by cyclic softening at high strain amplitudes and cyclic softening at low strain amplitudes. The mean stress relaxation rate was similar for each strain amplitude. In addition, the evolution of effective stress and back stress was obtained through the method of internal stress division. Then, the relationship between the change in stress amplitude and the change in internal stress was discussed. The results showed that with cyclic loading, cyclic hardening/softening of FGH95 was affected by the competition mechanism of back and effective stresses. Considering that slip deformation and crystal lattice rotation coexist in plastic deformation, the dynamic recovery term of the Abdel-Karim and Ohno model was used. In order to characterize the different magnitudes of back stress change in materials at different plastic strain intervals, a dynamic recovery term coefficient was introduced to the dynamic recovery term and the critical surface of the back stress. The modified model was used to compare with experimental results. Then, it gives a good description of the material’s mean stress relaxation and strain amplitude variation and gives good agreement on the hysteresis loop.

Experimental study on the compressive fatigue performance of nano-silica modified recycled aggregate concrete

This study focused on the compressive fatigue performance of nano-silica (NS) modified recycled aggregate concrete (RAC). 108 prism specimens with varying replacement ratios (i.e., 0 %, 50 %, and 100 %), NS soaking concentrations (i.e., 0 %, 1 %, 2 %, and 3 %), and stress levels Smax (i.e., 0.75, 0.8, 0.85, and 0.90) were designed for compressive fatigue test. The failure process and failure modes for the specimens were observed, the fatigue life curves of NS modified RAC under different design parameters were measured. Moreover, the relationships between different design parameters and fatigue life, fatigue strength were analyzed. The results revealed that the failure modes for specimens with different design parameters were mainly shear failure, conical failure and mixed failure. Fatigue life exhibited significant changes under different stress levels, decreasing notably with increasing stress levels. As the recycled coarse aggregate (RCA) replacement ratio increased, fatigue life demonstrated a decreasing trend. Based on the two-parameter Weibull distribution function, the fatigue life equation considering the influence of variable parameters under different failure probabilities was proposed. When the failure probability was 0.5, the specimen with a NS soaking concentration of 2 % exhibited the highest fatigue strength at the same replacement ratio.

Structural strength and fatigue analyses of large-scale underwater compressed hydrogen energy storage accumulator

Underwater compressed hydrogen energy storage (UWCHES) is a potential solution for offshore energy storage. By taking advantage of the hydrostatic pressure of deep seawater, the compressed hydrogen can be isobarically stored in underwater artificial energy storage accumulators. The accumulator should withstand high pressure and large buoyancy and possess reliable anchoring to the seabed. In this study, the structural strength analysis and fatigue life of the large-scale accumulator is conducted employing the finite element method (FEM). The dimensionless stress prediction model and dimensionless fatigue life prediction model are developed through dimensional analysis and multivariate regression analysis. The performance of the accumulator with operating water depth of 100∼300 m, gas storage volume of 1081∼10128 m³, and concrete wall thickness of 0.1∼0.63 m is investigated. The results show that with an operating water depth of 100 m, gas storage capacity of 10,128 m3, and concrete wall thickness of 0.63 m, the maximum compressive stress is 1.43 MPa (yield strength is 60 MPa) and the maximum tensile stress of the accumulator is 2.55 MPa (yield strength is 6 MPa). The design fatigue life is 106 cycles which is larger than the expected service life of 104 cycles. Therefore, the accumulator structure meets the static strength and fatigue life. As the operating water depth increases with a consistent gas storage capacity, a transition in the stress state shifts from primarily tensile stress to predominantly compressive stress. The accuracy of the dimensionless stress prediction model and the dimensionless fatigue life prediction model were verified, with maximum deviations of 10.3 % and 13.7 %, respectively. Furthermore, the anchoring factor of safety of 1.12 is achieved.

Contact force and pressure analysis of the three-row roller pitch bearing in a large-scale wind turbine

Pitch bearings are the key components of wind turbines (WTs). The reliability of pitch bearings affects the electric energy output efficiency of WTs. Due to its large loading ability, constant contact angle and high stiffness, the three-row roller slewing bearing (TRSB) is becoming a more attractive choice as a pitch bearing, especially in large-scale WTs. The contact force and pressure distributions significantly affect the fatigue life of a three-row roller pitch bearing (TRPB). Therefore, it is essential to propose a precise calculation method for the contact force and pressure. The present work establishes a quasi-static five degree-of-freedom (DOF) mathematical model for TRPBs, in which the bearing under general loading conditions is considered. The influences of combined external forces, overturning moments and inner ring misalignment angle on the contact force and pressure distributions of TRPBs are comprehensively investigated. According to the analysis results, the overturning moment and axial force can significantly affect the number of load-carrying rollers in each row. Furthermore, a slight variation of the inner ring angular misalignment has a significant effect on the contact force and pressure of TRPBs. Finally, it is more prone to pressure concentration in the non-crowned roller than that in the crowned roller, especially in the case of the inner ring with a misalignment angle. Since the high calculation efficiency and accuracy, the proposed method has remarkable potential applications in the fatigue life prediction and design of TRPBs.

Matching relationship of low-cycle fatigue life and fatigue design method of concentrically braced frames with H-section steel

Simulations on 192 concentrically braced frames with H-section steel (Q355B) are carried out to investigate their low-cycle fatigue performance. The fatigue life of a steel-braced frame is predicted using a multiaxial fatigue damage parametric model. To depict the fatigue failure sequence and the matching relationship of the fatigue life between the two components in the braced frame, the fatigue life ratio (β) is introduced. This ratio represents the gusset plate's fatigue life to the brace's fatigue life. Simulated results are analyzed, and β is found to be positively correlated with four influencing factors, the width-to-thickness ratio of the brace flange, connection coefficient, ratio of the column length to beam length, and slenderness ratio of the gusset plate. Also, β is found to be negatively correlated with the slenderness ratio of the brace. Based on the Bayesian updating method, a probabilistic model is formulated to predict β of a steel-braced frame. Accordingly, a fatigue design method is proposed, which contributes to avoiding the in-coordination of seismic capacity caused by the significant difference between the brace’s and gusset plate’s fatigue life within the steel braced frame. The proposed fatigue design method has the potential to serve as a reference for improving design specifications.

Using finite element modeling to predict stress concentration factors in tubular T, Y and K joints

Tubular offshore structures are commonly assembled using welded joints, creating areas of stress concentration and potential fatigue failure. This study focuses on tubular T, Y and K joints, a common offshore structural component. Finite element modeling is used to predict stress concentration factors (Kt) for various loading conditions on the T, Y and K joints. The goal is to calculate Kt values and compare them to existing theoretical solutions from literature. Additionally, the influence of different loading modes (tension, bending) on the Kt values is investigated. By using advanced modeling techniques, this work aims to provide new insight into the behavior of tubular T, Y and K joints connections under realistic offshore loading conditions. The results can help improve design standards and fatigue life predictions for these critical structural joints.

Research on fatigue life and damage tolerance design of metro bogie frame

Research on fatigue life and damage tolerance design of metro bogie frame

Optimal design of radial tire section layout based on thermal fatigue life improving

The design of tire fatigue life was optimized by combining approximate model and finite element simulation, and compared with the tire endurance test results. The design variables are selected through sensitivity analysis of materials in various regions of the tire, and the thermal fatigue life of the tire is used as the objective function, and the approximate relationship between the design variables and the objective function is fitted based on the approximate model method, and the approximate model is optimized using genetic algorithm to find the optimal solution. The fatigue life of the tire is improved by about 25 %.

Fatigue properties of OSD in large cantilever cable-stayed bridge on highway-railway layer

To enhance the understanding of fatigue issues in orthotropic steel deck (OSD) bridges with cantilevered steel-box girders on highway–railway layers and ensure the fatigue life of similar bridges, this study focused on the OSD fatigue characteristics of the Jinhai Bridge, the world's widest dual-use–cable-stayed bridge for road and railway transport. The fatigue-stress distribution characteristics and variation rules were investigated using a full-scale fatigue test model. Four U-ribs were subjected to four million loading test cycles, and the OSD fatigue performance was evaluated using the nominal stress method. The results indicated that the stress-influence range of the crossbeam U-rib in a large cantilever steel box girder was approximately nine U-ribs wide and three crossbeams long, significantly exceeding that of a conventional closed-box steel girder. The stress difference in the crossbeam opening bottom-edge detail in the box girder with and without a cantilever was 85.4%. Along the transverse bridge, the most unfavorable stress amplitude difference among similar fatigue details in different regions reached 57%. After 4 million cycles, the crack length was 90 mm. The fatigue cracks had a minimal effect on the overall deformation performance of the model. Combining the traffic volume statistical data and damage accumulation, the estimated fatigue life of the crossbeam opening detail was 103.55 years. According to JTG-D64–2015, this satisfies the recommended fatigue design life requirement of 100 years for the crossbeam opening details of large cantilever steel box girders. However, the economic development trends and increasing traffic volumes necessitate further research.

Study of lead–bismuth eutectic on material performance degradation and life assessment for modified 9Cr-1Mo steel under low cycle fatigue at high temperatures

Lead-bismuth eutectic (LBE) serves as one of the efficient coolants in Generation IV nuclear reactors. Considering LBE degradation, the aims of the present work are to propose a fatigue life prediction model at material-level and to obtain a component-level fatigue assessment. The LBE degradation factors of temperatures, strain ranges and strain rates are incorporated into the proposed life prediction model. Afterwards, fatigue life design curves are correspondingly put forward. Furthermore, recommended fatigue life assessment procedures are established for engineering applications. Finally, a continuous process in “deformation-cracking-repassivation” of oxide films is discussed to reveal oxidation-sensitive LBE degradation mechanism.

Properties of Hot Mix Asphalt (HMA) with Several Contents of Recycled Concrete Aggregate (RCA)

Continuous research efforts have been developed in the literature to raise the sustainability components of the road infrastructure industry, i.e., reduce potential contaminants and augment financial profitability. In this regard, this investigation aims to explore the feasibility of producing Hot Mix Asphalt (HMA) with the inclusion of Recycled Concrete Aggregate (RCA) as a partial substitute for coarse Natural Aggregates (NAs). Thus, four different HMAs were considered, namely HMAs with coarse RCA contents of 0, 15, 30, and 45%. Specifically, the mechanical and sustainability properties of the asphalt mixtures were determined. On the one hand, the Marshall design parameters, resilient modulus, moisture susceptibility, rutting resistance, and fatigue life were addressed as mechanical properties. Meanwhile, regarding the sustainability properties, the environmental impacts and production costs were estimated using the Life Cycle Assessment (LCA) and the Life Cycle Cost Analysis (LCCA) methodologies, respectively. Consequently, the following conclusions were obtained: (i) as the coarse RCA content increases, the mechanical behavior of the HMA progressively deteriorates; (ii) this decrease in mechanical performance is acceptable up to a 15% RCA of coarse RCA, whereas for higher dosages this alteration is abrupt; and (iii) the RCA only generates sustainability benefits at a 15% replacement amount.

Probabilistic modeling of threshold stress intensity factor for fatigue endurance reliability prediction

This study develops a probabilistic model for the threshold stress intensity factor range, which is a critical parameter in infinite fatigue life design under material flaws. The model is based on the proposed concept of probability of propagation in the probabilistic framework, allowing for deriving the probability density function of the threshold intensity factor range. The uncertainty in fatigue crack growth can naturally be incorporated into the resulting distribution. By further introducing the derived distribution into the Kitagawa–Takahashi diagram, the fatigue endurance reliability model can be established in a rational manner. With the first-order asymptotic approximation, the analytical form of fatigue endurance reliability index is obtained. The usefulness of the overall method is demonstrated using realistic engineering application examples.

Analysis of super-harmonic resonance and periodic motion transition of fractional nonlinear vibration isolation system

The precision instruments and equipment are often utilized in low-frequency and micro-amplitude vibration systems, in which many vibration isolators of rubber materials are widely used. Ignoring the low-frequency amplitude will result in errors in the fatigue life design of the vibration isolators and predicting the dynamic response of each frequency band accurately becomes necessary. However, integer-order models cannot describe the frequency dependence of rubber materials, while the fractional-order models can describe it instead. On the other hand, the elastic restoring force is strongly nonlinear under large deformation, and the vibration of the nonlinear system contains multiple harmonic components. In order to solve those issues, the fractional nonlinear Nishimura model is used to characterize the constitutive relation of vibration isolators such as air springs, which are made of carbon black filled natural rubber. The high-order harmonic balance method is used to obtain the steady-state response of the vibration system, while the fourth-order Runge–Kutta method is applied to simulate the dynamic response of the system in the low-frequency region, and the Lyapunov exponent is used to determine the stability of the system. Furthermore, the influence of parameters on the amplitude–frequency characteristics of different frequency bands is also studied, and a method to solve the optimal damping coefficient is proposed based on the primary resonance amplitude–frequency curves. The results show that there is a diversity of periodic motions in the process of adjacent super-harmonic resonance transition. Numerical simulations also demonstrate that multi-periodic motions coexist in the system. The motion transition law between the polymorphic coexistence region and its adjacent regions is summarized.

Flexural fatigue lifeassessment of self compacting concrete containing recycled concrete aggregates by using probabilistic approach

Present investigation has been conducted to analyze the fatigue life of self compacting concrete (SCC) containing different quantities of coarse fraction of Recycled Concrete Aggregates (RCA) by using probabilistic method. Three SCC mixes were prepared constituting 0%, 50% and 100% RCA. In total 165 beam specimens of size 100 × 100 × 500 (all dimensions in mm) were tested under fatigue loading in order to obtain fatigue life data at different stress levels. Static flexure test was also conducted on 126 beams prior to fatigue test. Fatigue life data have been modelled by using Weibull distribution method and subsequently distribution parameters were evaluated. Further, fatigue lives in terms of design fatigue life and theoretical fatigue life have been estimated by using the distribution parameters by employing probabilistic approach. It has been found that the design and theoretical fatigue life shows a significant reduction with the addition of RCA content. A decrease of 80-85 percent has been observed in the design and theoretical fatigue life when SCC has been made with RCA only. Substandard properties of RCA cause defects in the concrete by forming weak interfacial transition zone which degrades the fatigue performance of SCC containing RCA.

Fatigue failure mechanisms and probabilistic S-N curves for CFRP–steel adhesively bonded joints

In this study, fatigue tests were conducted on a series of carbon fiber reinforced polymer (CFRP)–steel double lap shear joints. The fatigue failure mechanisms were investigated based on the failure modes, load–displacement relationships, and stiffness degradation. Inherent scatter characteristics of the fatigue life were discussed using probabilistic S-N (P-S-N) curves based on four statistical models (ISO 12107, ASTM E739-91, two-parameter Weibull, and Gumber models). The results indicate that the fatigue life decreased dramatically with the increase of the load level, and interface debonding is the primary failure mode of fatigue joints. Stable debonding and accelerated debonding of the bonded interface are initiated when the joint stiffness degrades to approximately 98% and 82–86% of the initial stiffness, respectively. The S-N curve based on the mean bond stress range shows a better fit than that based on the local bond stress. Based on the comprehensive evaluation of the performance of the P-S-N curves, the two-parameter Weibull model is recommended for fatigue life design as it can well-constrain fatigue data without excessive safety redundancy.

Fatigue Reliability Analysis System for Key Components of Aero-Engine

Aero-engine is known as the heart of an aircraft. Fatigue is one of the main causes of aero-engine failure, therefore, it is essential to predict the fatigue life in the aero-engine design process. Due to the uncertainty of influencing factors, it is necessary to further analyze the fatigue reliability. First, the fatigue life should be predicted on the basic of finite element analysis. The steps include parametric modeling, stress-strain analysis, load spectrum acquisition, and selection of fatigue life prediction model. Then, the reliability estimation of fatigue life should be employed, including the statistical analysis of influencing factors, reliability analysis method, and reliability estimation of fatigue life. Taking turbine blade and test probe of aero-engine as the research objects, the fatigue reliability analysis system is developed based on the ABAQUS-MATLAB platform. Statistical analysis shows that fatigue life approximately obeys lognormal distribution, and the distribution parameters estimated by MCS and Kriging are coincide, while Kriging only needs dozens of training samples. Under different reliability indexes, the design fatigue life error between Kriging and MCS is less than 1%, which meets the accuracy requirements and can effectively guide the fault detection and maintenance of aero-engine.

Numerical and Analytical Studies of Low Cycle Fatigue Behavior of 316 LN Austenitic Stainless Steel

Abstract Mechanical components are frequently subjected to severe cyclic pressure and/or temperature loadings. Therefore, numerical and analytical low cycle fatigue methods become widely used in the field of engineering to estimate the design fatigue lives. The primary aim of this work is to evaluate the accuracy of the most commonly used numerical and analytical low cycle fatigue life methods for specimens made of 316 LN austenitic stainless steel and subjected to fully reversed uniaxial tension–compression loading, in the room temperature condition. It was found that both maximum shear strain and Brown–Miller criterions result in a very conservative estimation for uniaxially loaded specimens. However, maximum shear strain criteria provide better results compared to the Brown–Miller criteria. The total strain energy density approach was also used, and both the Masing and non-Masing analysis were adopted in this study. It is found that the Masing model provides conservative fatigue lives, and non-Masing model results in a more realistic fatigue life prediction for 316 LN stainless steel for both low and high strain amplitudes. The fatigue design curves obtained from the commonly used analytical low cycle fatigue equations were reexamined for 316 LN SS. The obtained design curves from Langer model and its modified versions are nonconservative for this type of material. Consequently, the authors suggest new optimized parameters to fit the given test data. The obtained curve using the currently suggested parameters is in better agreement with the experimental data for 316 LN SS.

Research on lightweight and fatigue life of engine hood based on multi-objective particle swarm optimization

A multi-objective optimization method is presented in this paper, aiming at improving the fatigue life of the engine hood while achieving light weight. By analyzing the factors affecting the fatigue life of the engine hood, a multi-objective optimization model was established that considered five design variables including the thickness of the inner plate, the thickness of the outer plate, the stiffness of the hook, the stiffness of the sealing strip, and the height of the buffer block. Then, the multi-objective particle swarm method was used for optimization, and the optimal solution was obtained in the form of a Pareto set. The ideal compromise solution was determined from the Pareto set by using fuzzy membership functions. On this basis, the optimal solution was determined from the Pareto set by comprehensively considering the steel plate material specification, mold and cost constraints. The torsional deformation test and switch fatigue test show that for the optimized engine hood, the fatigue life is increased by 117%, the mass is reduced by 0.51 kg, and the torsional deformation of the engine hood does not increase significantly. The proposed multi-objective optimization method is proved to be feasible and effective in improving engine hood fatigue life and lightweight design.

Prediction of flexural fatigue life and failure probability of normal weight concrete

Fatigue life has to be considered in the design of many concrete structures at various stress levels and stress ratios. Many flexural fatigue test results of plain normal-weight concrete are available in the literature and almost every set of test results provides different fatigue equations. It is necessary, though, to have a common fatigue equation to predict the design fatigue life of concrete structures under flexural load with reasonable accuracy. Therefore, a database of flexural fatigue test results was created for concrete with strengths ranging from 25 to 65 MPa; this database was used to derive new fatigue equations (Wöhler fatigue equation and S-N power relationship) for predicting the flexural fatigue life of normal-weight concrete. The concept of equivalent fatigue life was introduced to obtain a fatigue equation using the same stress ratio. A probabilistic analysis was also carried out to develop flexural fatigue equations that incorporate failure probabilities.