Fatigue Life Model Research Articles

Evaluation of Microstructure and Mechanical Properties of Al-Zn-Mg-Cu Alloy Repaired via Additive Friction Stir Deposition

Abstract A novel solid-state additive manufacturing (AM) process, additive friction stir deposition (AFS-D), provides a new pathway for additively repairing damaged nonweldable aerospace materials that are susceptible to induced thermal gradients within the microstructure. In this work, we quantify the microstructural evolution and mechanical performance of an additively repaired AA7075-T651 (Al-Zn-Mg-Cu) via the AFS-D process. To evaluate the AFS-D process for repairing high strength aluminum alloys, the AFS-D technique was used to additively fill a linear groove that was machined into an AA7075-T651 plate. After repairing the plate with the AFS-D process, the repaired plate was subjected to standard T6 heat treatment. The results of this study show that the heat-treated AFS-D repair did not exhibit any significant grain growth and demonstrated an increase in the average Vickers hardness in the repair compared with the wrought 7075-T651 control. Tensile and fatigue behavior was investigated for heat-treated repair and compared with the wrought AA7075-T651 control. The heat-treated repair exhibited wrought-like tensile properties for yield stress (YS) and ultimate stress; however, the heat-treated repair had significant scatter in the elongation to failure. Additionally, the mean fatigue behavior of the heat-treated repairs displayed a reduction in cycles to failure compared with the wrought control. Lastly, a microstructure-sensitive fatigue life model was used to elucidate process-structure-property fatigue mechanism relations of the heat-treated repair and wrought AA7075.

Low‐cycle fatigue performance of high‐strength steel rebars in concrete bridge columns

AbstractDesign codes often restrict the use of high‐strength steel (HSS) rebars in seismic applications due to their inferior low‐cycle fatigue performance when compared to conventional steel rebars. In this study, previously established low‐cycle fatigue life models of HSS rebars, namely ASTM A706 Grade 550 and ASTM A1035 Grade 690, were incorporated into a new cumulative damage model to identify conditions under which such rebars can achieve seismic performance comparable to that of benchmark ASTM A706 Grade 420 steel bars in concrete bridge columns. An ensemble of well confined flexure‐dominated circular concrete bridge columns located in high seismic regions was considered where the axial load ratio, and longitudinal and spiral reinforcement ratios were varied. The bridge columns were reinforced with ASTM A706 Grade 420, ASTM A706 Grade 550, and ASTM A1035 Grade 690 steel and numerically analyzed under different displacement ductility levels (2, 4, and 6), earthquake types (crustal and subduction), and ratio of hoop spacing to longitudinal bar diameter ratio (4 and 6). Their low‐cycle fatigue performances were compared based on the computed bar fracture and accumulated damage indices. Based on the reported observations, it is concluded that design codes are overly restrictive in not permitting the use of HSS in seismic application on the basis of inadequate low‐cycle fatigue life. As an alternative, this study proposes imposing certain limits on displacement ductility demand and ratio of hoop spacing to longitudinal bar diameter ratio to promote safe and efficient use of HSS rebars in concrete bridge columns.

Study on the P-S-N Curve of Sucker Rod Based on Three-Parameter Weibull Distribution.

During the oil production process, sucker rods are subjected to cyclic alternating load. After a certain number of cycles, a sucker rod can experience fatigue failure. The number of cycles is called fatigue life (N), and the accurate relationship between maximum stress (S) and fatigue life (N) under a certain reliability (P), namely the P-S-N curve, is an important basis for the reliability analysis and fatigue life prediction of sucker rods. The Basquin model, based on log-normal distribution, is widely used for fitting the P-S-N curves of sucker rods. Due to the limitation of this model, it is difficult to extrapolate the conclusion obtained from a finite fatigue region to the high-cycle or ultra-high-cycle fatigue region, which makes it impossible to estimate the fatigue limit of the sucker rod. Compared to the log-normal distribution, Weibull distribution causes the sucker rod to have a minimum safety life, namely the safety life at 100% survival rate, which complies with the fatigue characteristics of the sucker rod and is more in line with the actual situation. In this study, the fatigue data for ultra-high-strength HL and HY grade sucker rods were obtained through experimental fatigue tests. A new fatigue life model was established and the P-S-N curves of two types of ultra-high strength sucker rods were obtained. For HL- and HY-type ultra-high strength sucker rods, the average error between the fitting result and fatigue test value is 1.25% and 4.39%, respectively. Compared to the S-N curve fitting result obtained from the Basquin model commonly used for sucker rods, the new model based on three-parameter Weibull distribution provides better fitting precision and can estimate fatigue limit more accurately, so this model is more suitable for estimating fatigue life and can better guide the design of ultra-high strength sucker rod strings.

Elevated-temperature gigacycle fatigue properties of nickel based superalloy: Grain related cracking mechanism and life prediction modelling

Fatigue properties of a nickel-based superalloy at 25 °C and 750 °C were studied under stress ratio R = −1 & 0.1 to identify crack nucleation and growth mechanisms. Results show that interior grain-related cracking is induced by microstructure in Goss or Copper grain. Moreover, owing to competition between temperature and vacuum effects, threshold values for small and long cracks, and the transition crack size, are all lower at elevated temperature. Finally, based on modeling of crack nucleation and growth, a fatigue life model with good prediction results is proposed. The crack nucleation consumes more than 90% of fatigue life.

Research on trapezoidal shape optimization of laser remanufacturing heterogeneous interface under fatigue load

How to evaluate and improve the performance of heterogeneous interface was the key to ensure the reliability of laser remanufacturing products. Based on the analysis of stress eigenvalues at interface singularity, a fatigue life model and a trapezoidal interface shape optimization model were established. Thirteen different trapezoidal interface shapes were designed for static and fatigue experiments to verify the correctness. The results showed that the interface fatigue life prediction was accurate, and the optimized trapezoidal interface shape could significantly improve the interface mechanical properties. These provided a theoretical basis for the shape pretreatment of the damaged position in remanufacturing.

МАТЕМАТИЧЕСКОЕ МОДЕЛИРОВАНИЕ И ЧИСЛЕННАЯ ОЦЕНКА ДОЛГОВЕЧНОСТИ РАДИАЛЬНЫХ РАБОЧИХ КОЛЕС ТУРБОМАШИН

The radial blade discs are nominally designed to be cyclically symmetrical. But the vibration characteristics of all the ra-dial blades on the disc are slightly different due to manufacturing tolerances, variations in material properties and wear during operation. These small changes break the cyclic symmetry, break the pairs of eigenvalues and affect the durability of the radial impellers of turbomachines. In further work, the designs of such systems may fail before the estimated resource or service life. Therefore, when designing, manufacturing, and operating turbomachines, it is necessary to understand the effect of parameter mistuning to manage the resource and ensure the required level of strength, reliability, and durability of radial turbomachines. This study proposes a technique for mathematical modeling and prediction of high-cycle fatigue life of radial wheels tur-bines, taking into account dynamic loading. Numerical assessment of durability was performed on the example of a radial wheel with 10 blades manufactured by Schiele (Germany). The results were obtained when calculating the dynamic load in the MATLAB program and high-cycle fatigue in the ANSYS program.

Bayesian Fatigue Life Prediction of Corroded Steel Reinforcing Bars

This paper presents a general method for fatigue life prediction of corroded steel reinforcing bars. A fatigue testing on standard specimens with pitting corrosion is carried out to obtain corrosion fatigue data. The maximum corrosion degree (MCD), characterizing the most severe site of the corrosion pit, is identified to have a log-linear relationship with the fatigue life. A fatigue life model incorporating the MCD and the stress range for corroded steel reinforcing bars is proposed. The model parameters are identified using the testing data, and the model is considered as the baseline model. To utilize the proposed model for life prediction of corroded steel reinforcing bars with different geometries and working conditions, the Bayesian method is employed to update the baseline model. The effectiveness of the overall method is demonstrated using independent datasets of realistic steel reinforcing bars.

Cyclic plasticity models and fatigue criteria for exhaust manifold life assessment in the context of limited material data available

ABSTRACT The work focuses on predictive capabilities of fundamental cyclic plasticity and fatigue life models, which can be calibrated using limited amount of experiments as specific ones needed for more advanced models are often absent. The analyses are conducted for the synthetic case of exhaust manifold made from cast iron. The thermal boundary conditions from the forced convection were obtained from the computational fluid dynamics considered as a conjugate heat transfer problem. Two rate-independent and temperature-dependent material models were calibrated for structural analyses. Both were validated with experiments on isothermal and anisothermal levels. Sequential thermal–mechanical finite element simulations were performed. Two fatigue life models were employed. The first was a temperature-dependent strain-based fatigue life criterion calibrated from uniaxial data. The second was a temperature-independent energy-based fatigue life criterion resulting in twice lower life than the strain-based criterion, while none of the plasticity models made a significant difference in that prediction.

Contact fatigue life prediction of rolling bearing considering machined surface integrity

PurposeRolling bearings often cause engineering accidents due to early fatigue failure. The study of early fatigue failure mechanism and fatigue life prediction does not consider the integrity of the bearing surface. The purpose of this paper is to find new rolling contact fatigue (RCF) life model of rolling bearing.Design/methodology/approachAn elastic-plastic finite element (FE) fatigue damage accumulation model based on continuous damage mechanics is established. Surface roughness, surface residual stress and surface hardness of bearing rollers are considered. The fatigue damage and cumulative plastic strain during RCF process are obtained. Mechanism of early fatigue failure of the bearing is studied. RCF life of the bearing under different surface roughness, hardness and residual stress is predicted.FindingsTo obtain a more accurate calculation result of bearing fatigue life, the bearing surface integrity parameters should be considered and the elastic-plastic FE fatigue damage accumulation model should be used. There exist the optimal surface parameters corresponding to the maximum RCF life.Originality/valueThe elastic-plastic FE fatigue damage accumulation model can be used to obtain the optimized surface integrity parameters in the design stage of bearing and is helpful for promote the development of RCF theory of rolling bearing.

Modeling ultrasonic vibration fatigue with unified mechanics theory

A series of ultrasonic vibration tests are performed on A656 grade steel samples, at a frequency of 20 kHz. A fatigue life model based on the unified mechanics theory is introduced to predict the very high cycle fatigue life of metals. Then, the fatigue life test data results are compared with the unified mechanics theory based model simulation results. It is shown that the physics-based unified mechanics theory can predict very high cycle fatigue life very well, without the need for the traditional empirical curve fitting a fatigue damage evolution function. The model does not require any curve fitting parameters obtained from fatigue test data. However, it does require deriving analytical thermodynamic fundamental equation of the material subjected to ultrasonic vibration fatigue. The thermodynamic fundamental equation of the material formulates the entropy generation mechanisms during the fatigue process. There are more than half a dozen entropy generation mechanisms during fatigue process. Entropy is an additive property, hence, the entropy generation due to all active mechanisms can be added.

Fatigue life prediction of high-speed composite craft under slamming loads using progressive fatigue damage modeling technique

A reliable predictive method is developed based on the progressive fatigue damage modeling technique to estimate the fatigue life of high-speed composite craft. In this method, at each loading cycle, the occurrence of different failure types would be inspected and based on the failure type, either gradual or sudden degradation rules would be followed to update the material properties. This procedure continues until the final failure of the structure. To implement this method, a MATLAB code is developed based on material degradation rules which integrating with the Abaqus it can be used for complex geometries. This method enables us to predict the fatigue life of composite structures with different layup configurations under arbitrary loading conditions only by knowing the static and fatigue behavior of unidirectional laminates. To even more reduce the number of required tests, the energy-based unified fatigue life model is employed. Owing to the low number of required tests, this method can be very cost-effective in the design stage of composite structures that are intended to endure cyclic loads.

An improved fatigue life model for mechanical components considering load strengthening characteristics

AbstractThe small load has fuzziness on the strengthening and damage of the component under the action of random fatigue load. This paper introduces the two‐dimensional membership function in fuzzy mathematics to characterize the relationship between stress amplitude, mean value, and damage below the fatigue limit. An exponential function is used to describe the strengthening effect. Under the two‐dimensional load spectrum, the influence of various levels on the component's life is comprehensively considered, and the fatigue life prediction model under random load is proposed. Through the small sample experiment of the car swing support rod, the relevant parameters of the model are obtained. The difference between the modified two‐dimensional and the two‐dimensional load spectrum is compared in fatigue life prediction results. The rules of membership function selection and parameter setting are summarized, and it makes the estimation of component load spectrum life more reasonable.

Predictive model for fatigue life in parallel-wire stay cables considering corrosion variability

This study quantitatively assessed the influence of corrosion and its variability on the stay cable fatigue life. The corrosion-life inverse equal rank assumption is proposed to identify the order of wire failures, with the wires with the largest level of corrosion failing first. Recursion formulas of stay cable fatigue life are established based on models of the evolution of fatigue damage for multi-step loading, assuming the loading redistribution after the failure of each wire. The bounds for the cumulative distribution function for stay cable fatigue life are estimated using order statistics, and Monte Carlo simulation is conducted to simulate the stay cable fatigue failure. As the variability in the corrosion of the wires increases, the variability in the fatigue life of the wires and stay cables increases, resulting in a decrease in the stay cable fatigue life. The stay cable fatigue life decreases sharply with the increase in the corrosion degree of the wires, with the effect being more pronounced for larger corrosion variability. Moreover, the effect of the corrosion variability on the stay cable fatigue life is much significant for larger corrosion levels and under lower stress ranges.

Mechanical Reliability Analysis of Control Rod Assembly for HTR-PM

The article analyzes the forces and weaknesses of the control rod assembly in the high temperature gas cooled reactor (HTGR) demonstration project-pebble bed modular HTGR plant (HTR-PM), using the the mechanical reliability theory and mechanical simulation, and identifies the weaknesses at the connection of the control rod assembly. In the reliability-life calculation method based on the probabilistic fracture mechanics, in combination with the influence of irradiation on the fatigue life model and in consideration of the stress, the authors establish the reliability-life model to calculate the reliability-life of HTR-PM control rod assembly at the irradiation attenuation coefficient of 0.7. The calculation results show that at the reliability level of 0.99, the life of HTR-PM control rod assembly is about 150,000 action cycles, and that the irradiation has a great influence on the life of the control rod assembly. In view of what mentioned above, this paper can provide reference for the equipment reliability management of the HTR-PM control rod assembly.

Bayesian inference-based decision of fatigue life model for metal additive manufacturing considering effects of build orientation and post-processing

This study proposes a Bayesian inference-based decision framework to quantify the physical uncertainty based on fatigue life tests on maraging steel according to post-processing treatments and build orientations. Uncertainty quantification of fatigue life models is performed to determine the most suitable models for the metal additive manufacturing process by employing Bayesian inference. To select one of the fatigue life models, we introduce a weighted-equivalent metric (WEM) to compare the evaluation results from different statistical metrics. By evaluating the WEM value, the logistic model and Zhurkov fatigue life model are identified as the suitable fatigue life models for maraging steel.

Fatigue behavior and life predictions of thermally oxidized Ti6Al4V alloy according to oxidation parameters

Thermal oxidation is a surface treatment thought to improve the wear behavior of Ti alloys. However, oxidation can impact adversely the fatigue behavior of the alloys. Thermal oxidation of the Ti-6Al-4V alloy was performed under uncontrolled furnace atmosphere, at 600 °C, for 5 h and 10 h, and at 700 °C, for 15 h. Microstructure analysis was performed and no significant changes were observed in the microstructure of the oxidized alloys, except for the diffusion zone formation. XRD analysis denoted rutile peaks (TiO2) in the oxidized alloys. Roughness increased due to oxidation time and temperature, whereas layer thickness was influenced mainly by temperature. The influence of oxidation parameters on fatigue behavior was tested and fracture micromechanisms were analyzed. Fatigue endurance limit after oxidation at 600 °C, for 5 and 10 h; and at 700 °C, for 15 h decreased by 27%, 29% and 68%, respectively, in comparison to the untreated alloy. Premature cracks in the oxide layer encouraged fatigue nucleation, and the premature crack formation explains the diminished fatigue performance of the oxidized alloys. Fatigue endurance limit was assessed through the Murakami’s model, and a fatigue life model was established, which agree with the experimental results.

Modelling asphalt binder fatigue at multiple temperatures using complex modulus and the LAS test

ABSTRACT The linear amplitude sweep (LAS) test is useful for evaluating the fatigue of asphalt binders. In this paper, the combined effects of strain and temperatures are investigated, and a method for estimating binder fatigue behaviour is introduced at different temperatures from limited measurements. LAS tests were conducted on several different PG-grade binders at four different temperatures. The results show that LAS parameters A and B at different temperatures maintain a linear relationship with the binder complex modulus (G*) measured at corresponding temperatures. Therefore, a fatigue life model, accounting for strain level and temperature, is proposed using a power function of the binder G*, which can account for temperature effects. To verify the model, its parameters are fitted using test data at two temperatures were used to predict the fatigue life at additional temperatures. This study findings offer a simpler and reliable method to predict the values of fatigue life at new temperatures using both 35% damage level and maximum stress failure criteria that are recommended in AASHTO TP101-12 and AASHTO TP101-14, respectively. The new prediction method will allow a wider use of the LAS procedure regarding the effects of temperature and strain level for which binders are subjected to the pavement.

Effect of tempering temperature on monotonic and low-cycle fatigue properties of a new low-carbon martensitic steel

The relationship between tempering temperature and low cycle fatigue (LCF) is not clear. New low-carbon martensitic steels under different tempering temperatures (320 °C, 350 °C, 380 °C) are tested for tensile mechanical properties, LCF, and microstructure. The results show that the monotone mechanical properties of martensite steels decrease with the increase of tempering temperature. The microstructure shows that the width of the laths increases, the residual austenite is decomposed, and the dislocation density decreases. A damage hysteresis fatigue life prediction model considering tempering temperature is proposed and the fatigue performance is evaluated. The fatigue performance parameters show that the fatigue damage capacity (W0) decreases and the damage transition index (β) increased. The established fatigue life model can predict the fatigue life at different tempering temperatures.

Fatigue Life of Ni-Based Single Crystal Super-Alloy Specimen with Single Hole

A method of predicting the fatigue life under multiaxis loads based on the Paris law and EIFS was proposed. And the fatigue life under different loading stress and stress ratio were investigated. The results show that when the loading stress increased from 450~800 MPa, the fatigue life decreased from 6762379 to 10056, as well as when the stress ratio increased from 0.1~1, the fatigue life increased from 6762379 to 14932368. It was validated by test eventually. And the fatigue life model presented here agrees well with test results. It is significant to the prediction of turbine of Ni-based single crystal super-alloy material with filming hole.

Ellipsoid non-probabilistic reliability analysis of the crack growth fatigue of a new titanium alloy used in deep-sea manned cabin

Reliability analysis of crack growth fatigue of titanium alloy is one of the important guarantees for the safety of deep-sea manned cabin. However, the traditional probabilistic fatigue reliability analysis methods often need accurate parameter statistics information. In this paper, under the condition of the limited test data, a first-order approximate ellipsoid non-probabilistic reliability analysis method for fatigue crack growth of a titanium alloy was proposed. A limit state equation of the crack growth correction model, considering the small crack effect, is formulated according to the fatigue life model. The uncertain parameters are constructed by the interval model, normal distribution, and Khachiyan algorithm. The different limit state equations are obtained by changing the coefficient κ. The first-order approximation and Monte Carlo methods are used to solve the reliability of the ellipsoid model. Calculations demonstrate that the results of the ellipsoid non-probabilistic reliability method are more conservative than that of the probabilistic reliability method. When κ is less than or equal to 2, the Khachiyan algorithm is preferred to build the ellipsoid model, and when κ is greater than 2, the interval model is preferred. Through parameter sensitivity analysis, the uncertain parameters KC, n, m, A, and ΔKthR are found to significantly affect the crack growth rate.