Accurate Fatigue Life Prediction Research Articles

A fuzzy residual strength based fatigue life prediction method

The fatigue damage problems are frequently encountered in the design of civil engineering structures. A realistic and accurate fatigue life prediction is quite essential to ensure the safety of engineering design. However, constructing a reliable fatigue life prediction model can be quite challenging. The use of traditional deterministic approach in predicting the fatigue life is sometimes too dangerous in the real practical designs as the method itself contains a wide range of uncertain factors. In this paper, a new fatigue life prediction method is going to be proposed where the residual strength is been utilized. Several cumulative damage models, capable of predicting the fatigue life of a structural element, are considered. Based on Miner\'s rule, a randomized approach is developed from a deterministic equation. The residual strength is used in a one to one transformation methodology which is used for the derivation of the fatigue life. To arrive at more robust results, fuzzy sets are introduced to model the parameter uncertainties. This leads to a convoluted fuzzy based fatigue life prediction model. The developed model is illustrated in an example analysis. The calculated results are compared with real experimental data. The applicability of this approach for a required reliability level is also discussed.

Integrating variable wind load, aerodynamic, and structural analyses towards accurate fatigue life prediction in composite wind turbine blades

A comprehensive fatigue analysis framework for composite wind turbine blades has been developed. It includes variable wind loads from wind field simulation and aerodynamic analysis, stress prediction by finite element analysis, and fatigue damage evaluation based on the resulting fatigue data. The variable wind load is represented by a joint distribution of mean wind speed and turbulence intensity. In order to simulate realistic wind loads applied on the blade while maintaining affordable computational time, the sectional surface pressure fields obtained from the potential flow aerodynamics model XFOIL are transformed to match the lift, drag, and moment coefficients obtained using AeroDyn. Thus, the modified pressure distribution includes the effect of dynamic stall, rotation, and wake effects on the blade aerodynamics. A high-fidelity finite element blade model, in which the design of composite materials can be easily tailored, has been parameterized for detailed stress analysis. The non-proportional multi-axial complex stress states are involved when calculating 10-min fatigue damage of section points through laminate thickness. The annual fatigue damage is calculated based on the 10-min fatigue damage and the joint distribution of 10-min mean wind speed and 10-min turbulence intensity. Consequently, the blade fatigue effect due to not only the mean wind speed and the atmospheric turbulence in the short term, but also the wind load variation in a large spatiotemporal range, can be investigated. The developed fatigue analysis framework can facilitate reliability analysis and reliability-based design optimization of composite wind turbine blades.

Dynamical measurement system for wind turbine fatigue load

A measurement system specially used in wind turbine fatigue load assessment is developed based on Labview platform and Control Area Network (CAN). By applying CAN bus communication technology, the system can perform data automatic acquisition, data stable transmission and data real-time monitoring. By adopting the technology of virtual instrument modular design, the system is designed to analyze the wind turbine mechanical load levels against wind and power, equivalent loads and lifetime fatigue loads, etc. Considering the effects of small load strengthening, low amplitude load damaging and multilevel load interaction, a novel fatigue lifetime prediction model is proposed to obtain more accurate and reliable prediction of blade fatigue life. With the developed measurement system, the in-field load measurements are performed and the results showed the system has satisfactory accuracy and good adaption, convenient operation, high integration, low cost and great practicality to load measurement of large wind turbine. And based on the proposed model the fatigue life of WT blade can be estimated more trustworthily and reliably.

Fatigue testing and analysis of notched specimens with typical subsea design features

Specimens with typical subsea design features have been fatigue tested in order to quantify the fatigue life prediction accuracy of the local stress approach as prescribed by the recommended practice DNVGL-RP-0005 (former DNV-RP-C203). A second aim has been to explore whether the use of the notch support factor method or the weakest-link method would improve the predication accuracy compared to the use of the local stress approach. Nine different notched specimen types, and a total of 139 notched specimens have been fatigue tested in air. All specimens have been subjected to an uniaxial cyclic load. Specimens have been tested at stress ratios R in the range from −1 (zero mean stress) and 0.5 (high mean stress). The specimens have stress concentration factors in the range from 1.85 and 5.45 and fatigue notch factors ranged from 1.16 to 2.03.A statistical based procedure has been used for establishing an indicator of the degree of the prediction accuracy (test results versus estimated fatigue life). This predictability indicator showed that the local stress-life predictions according to DNVGL-RP-0005 resulted in conservative fatigue life predictions for all the tested notched specimens. The degree of conservatism was found to increase with increasing stress concentration factors. DNVGL-RP-0005 yielded fatigue life predictions closer to the observed data for the mildly notched specimen types tested with a high net-section mean stress (R=0.5). The use of the notch support factor method and the weakest-link method improved the fatigue life predictions significantly compared to the use of the local stress approach for all specimen types. The highest predictability was obtained with the use of the notch support factor method. Also the weakest-link method yielded good fatigue life prediction accuracy for all specimen types.

Failure mode analysis of two crankshafts of a single cylinder diesel engine

This paper reports an investigation carried out on two damaged crankshafts of single cylinder diesel engines used in agricultural services for several purposes. Recurrent damages of these crankshafts type have happened after approximately 100h in service. The root cause never was imputed to the manufacturer. The fatigue design and an accurate prediction of fatigue life are of primordial importance to insure the safety of these components and its reliability. This study firstly presents a short review on fatigue power shafts for supporting the failure mode analysis, which can lead to determine the root cause of failure. The material of these damaged crankshafts has the same chemical composition to others found where the same type of fracture occurred at least ten years ago. A finite element analysis was also carried out in order to find the critical zones where high stress concentrations are present. Results showed a clear failure by fatigue under low stress and high cyclic fatigue on crankpins.

Accurate Prediction of Load Distribution and Fatigue Life on Large-Size Rolling Bearings

Accurate Prediction of Load Distribution and Fatigue Life on Large-Size Rolling Bearings

Multiaxial ratcheting-fatigue interaction on acrylonitrile-butadiene-styrene terpolymer

A series of multiaxial tests on an industrial acrylonitrile-butadiene-styrene (ABS) tube were conducted at room temperature. The ratcheting strain accumulation was found to be closely related to the shear strain amplitude and axial mean stress. The ratcheting strain and its rate increased with increasing shear strain amplitude as well as axial mean stress. At the same time, the ratcheting strain accumulation was retarded with a negative mean stress, and the ratcheting shakedown appeared when the negative mean stress reached −10 MPa. To understand the influences of loading history on axial ratcheting strain, a series of multistep loading tests under constant symmetrical torsion at different axial mean stresses of 5, 0, and −5 MPa were performed. The results showed that both the compressive viscous strain and strain relaxation of ABS could lead to the decrease of axial ratcheting strain. Furthermore, the compressive viscous strain and strain relaxation are independent of the loading history. Considering the influence of mean stress, a modified fatigue life model based on the Basquin law was presented to predict the pure torsional fatigue and multiaxial fatigue of ABS. The results of the modified fatigue life model show it can give relatively accurate fatigue life predictions. POLYM. ENG. SCI., 55:664–671, 2015. © 2014 Society of Plastics Engineers

Prediction of short fatigue crack growth of Ti‐6Al‐4V

ABSTRACTIt is observed that the short fatigue cracks grow faster than long fatigue cracks at the same nominal driving force and even grow at stress intensity factor range below the threshold value for long cracks in titanium alloy materials. The anomalous behaviours of short cracks have a great influence on the accurate fatigue life prediction of submersible pressure hulls. Based on the unified fatigue life prediction method developed in the authors' group, a modified model for short crack propagation is proposed in this paper. The elastic–plastic behaviour of short cracks in the vicinity of crack tips is considered in the modified model. The model shows that the rate of crack propagation for very short cracks is determined by the range of cyclic stress rather than the range of the stress intensity factor controlling the long crack propagation and the threshold stress intensity factor range of short fatigue cracks is a function of crack length. The proposed model is used to calculate short crack propagation rate of different titanium alloys. The short crack propagation rates of Ti‐6Al‐4V and its corresponding fatigue lives are predicted under different stress ratios and different stress levels. The model is validated by comparing model prediction results with the experimental data.

Accurate Prediction of Fatigue Life under Random Loading

This study focuses on the method of predicting the fatigue life of materials subjected to random loading. Since random stress caused by random loading is rigorously expressed in the frequency domain as stress power spectral density (PSD), fatigue life should be predicted using stress PSD. We propose two adjustment methods of improving the accuracy of fatigue life prediction using stress PSD in the frequency domain. The method proposed by Dirlik is widely used for predicting the fatigue life in the frequency domain; however, it overestimates fatigue damage caused by large stress amplitude when the slope of the fatigue resistance curve is large. To prevent this overestimation, we applied our two adjustment methods to fatigue life prediction for typical random stresses observed on mechanical products. As a result, the adjustment methods worked well in improving prediction accuracy. Lightweight and reliable products can be therefore designed by applying the proposed methods to the evaluation of fatigue life under random loading.

Crystal Plasticity Finite Element Simulations of Polycrystalline Aluminium Alloy under Cyclic Loading

A three-dimensional crystal plasticity (CP) finite element model is developed to reproduce the grain level stress concentration and deformation of polycrystalline aluminium alloy 7075 (AA7075) during fatigue experiments. The grains contained in the model possess the same size and crystallographic orientations obtained from electron back-scatter diffraction experiments. A modified CP constitutive model, which considers the backstress evolution, is employed to describe the mechanical behaviour of AA7075 under cyclic loading. A round-notched specimen from a fatigue test is simulated using the proposed CP model. Convergence studies in terms of mesh density and plastic deformation zone size are carried out to determine the appropriate conditions for the simulation. The simulation results are compared with those obtained using the elasto-plastic model and the CP model without grain morphology. The comparison indicates that with the embedded grain morphology, the proposed model can capture very well the local response induced by the microstructure features, which is vital to the accurate fatigue life prediction of aluminium alloys.

Fatigue behaviour of coke drum materials under thermal-mechanical cyclic loading

Coke drums are vertical pressure vessels used in the delayed coking process in petroleum refineries. Significant temperature variation during the delayed coking process causes damage in coke drums in the form of bulging and cracking. There were some studies on the fatigue life estimation for the coke drums, but most of them were based on strain-fatigue life curves at constant temperatures, which do not consider simultaneous cyclic temperature and mechanical loading conditions. In this study, a fatigue testing system is successfully developed to allow performing thermal-mechanical fatigue (TMF) test similar to the coke drum loading condition. Two commonly used base and one clad materials of coke drums are then experimentally investigated. In addition, a comparative study between isothermal and TMF lives of these materials is conducted. The experimental findings lead to better understanding of the damage mechanisms occurring in coke drums and more accurate prediction of fatigue life of coke drum materials.

Fatigue Design of Cruciform Joints including V-notch Effect at the Weld Toe

The present paper proposes a new and more accurate fatigue life prediction model for fillet welded joints in steel subjected to constant amplitude loading. With the traditional fracture mechanics approach, the greatest difficulty when computing the fatigue life of a welded detail is to determine the initial crack size a0. The classical way to determine the stress intensity factor K (SIF) is by using the following formula ▪ Where σ is the applied stress, a is the crack size and g(a/T) the geometrical correction factor which has been determined by Gurney function or similar solutions. This approach is not accurate for short crack because of the singular V-notch behaviour close to the crack tip is not taken into account. In fact, the singular behaviour at this point has a great influence on the SIF calculation. The present work proposes an alternative approach based on the determination of the Energy Release Rate (G) in the V-notch region at the weld toe in order to calculate the stress intensity factor as follows ▪E being the Young's modulus. Once the ΔK is determined for varying stresses, we can calculate the life fatigue based on the Paris crack propagation law. The size of the weld toe region influenced by the singular V-notch behaviour is based on experimental work concerning a cruciform steel joint subjected to an applied axial stress of 150MPa and a weld angle of 45°. Based the same conditions as in these experimental tests, we compared the results from the classical Gurney's approach with the results from the present methodology. The experimental results in terms of fatigue life and crack length were used as reference. When the crack growth rate is plotted against ΔK we can see that, in our approach two slopes appear. The first slope that corresponds to the short crack propagation in the weld toe region and the second slope that corresponds to the stable crack propagation far from the weld toe region. Also, for short cracks, our approach is closer to the experimental results than the Gurney's approach. Moreover, with our approach, we do not need to introduce the real initial crack size at the weld toe. It is demonstrated how the time consumed for short crack propagation becomes the dominant part of the entire fatigue life at low stress levels. The short crack propagation is more accurately described by the present model than in traditionally approaches. Future extensions of the present work will deal with the introduction of the influence of the weld toe radius and the material characteristics of the HAZ on the fatigue life duration.

Fatigue analysis of steel catenary riser at the touch-down point based on linear hysteretic riser-soil interaction model

The fatigue life of a steel catenary riser (SCR) at the touch-down point (TDP) is substantially affected by its interaction with the seabed. In order to broaden the understanding of seabed effects on TDP's fatigue performance and provide more accurate fatigue life prediction in the SCRs design, the effects of the seabed characteristics on fatigue damage of the SCRs at the TDP have been studied in this paper. In this study, a new element at touch-down zone (TDZ) is created to simulate the riser-soil interaction based on the proposed linear hysteretic riser-soil interaction model. A conventional twin-pontoon semi-submersible with a particular riser geometry in the South China Sea, is adopted to evaluate how the fatigue damage was affected by the seabed characteristics. The results based on this study indicate that: ① Larger mudline shear strength and shear strength gradient corresponding to stiffer seabed lead to shorter fatigue life. ② The seabed suction effect could not be neglected in the SCRs design, since its effect on the TDP's fatigue damage is obvious and the greater suction force causes more fatigue damage. ③The deeper the trench, the less TDP's fatigue damage will be. Therefore the development of trench is in favor of longer fatigue life of the riser.

All Terrain Vehicle Flexible Multibody Dynamic Simulation for Fatigue Prediction

This study presents the flexible-body dynamic analysis and simulated stress recovery of vehicle components to predict their lifetime when maneuvering on an uneven random road. The subject of this study was an all-terrain vehicle (ATV). The body frame and suspension system were modeled as flexible elements for multibody dynamic simulation. The simulations in this study revealed the stress from the flexible elements and predicted the component fatigue life using the retrieved stresses. This approach considers the interaction between dynamic forces and structure deformation and achieves more accurate structure stress prediction and fatigue life prediction.

Implementing Realistic, Nonlinear, Material Stress–Strain Behavior in ANSYS for the Autofrettage of Thick-Walled Cylinders

High-pressure vessels are autofrettaged to introduce favorable, compressive, residual stresses around their inner diameters. The efficacy of autofrettage is limited by a phenomenon called the Bauschinger effect, which describes the early onset of nonlinearity during unloading in a material that has previously been subjected to initial deformation. The degree of prestressing achieved determines the fatigue life of the vessel, hence, high fidelity prediction of the stress field developed is essential for accurate prediction of fatigue life. This requires precise representation of material behavior within the autofrettage model used. This paper describes the adaption and development of USERMAT, a user programmable feature within ANSYS (ANSYS Finite Element Program, ANSYS, Inc., Southpointe, 275 Technology Drive, Canonsburg, PA), to create a framework to represent realistic behavior of candidate gun steels. A number of materials including A723 were modeled to investigate and validate the framework. A723 was then used in simulations of both a uni-axial test and hydraulic autofrettage. These results are compared with spreadsheet data from the material-fit equations and equivalent results from the Hencky program, respectively. Close agreement was observed between the results in both cases, indicating the model is an effective means of representing the considerable variation in behavior between loading and unloading in candidate steels.

Fatigue life prediction methodology using entropy index of stress interaction and crack severity index of effective stress

Fatigue life in critical parts of structure components, which is subjected to variable amplitude loading, is a very complex subject. When a fatigue life is influenced by these load variations, accelerations and/or retardations in crack growth rate can occur. Thus, an accurate prediction of fatigue life requires an adequate evaluation of these stress interaction effects. There are many different indexes for calculating fatigue damage at critical locations. The crack severity index based on crack growth is one of the methodologies implemented for modern military aircrafts to define mission severity relative to a known reference usage. But, this index cannot consider stress interaction. In this study, a novel index was proposed to evaluate an effect for stress interaction by using entropy concept, and fatigue life prediction methodology was developed by using entropy index of stress interaction and crack severity index of effective stress.

Impact evaluation of rolling contact fatigue life models

Since the accurate prediction of fatigue life has a significant value, many researchers have attempted to develop a reliable fatigue life model. Recently, rolling contact fatigue life models incorporating machining impact were developed. These models have contributed to a significant improvement in prediction accuracy as compared with earlier models, thus representing a major step forward in the modeling effort. This paper compares the prediction accuracy of these models with that of the prediction method in International Standards. When α is set to 0.25, the observed improvement of prediction accuracy as measured by variance of prediction errors due to these models over that due to prediction method in International Standards is statistically significant. Impact analyses of such improvement are conducted to illustrate its value. It is further noted that while difference was observed between the variance of prediction errors due to the crack initia- tion life model based on a dislocation model and that due to the crack initiation life model based on a local stress-life curve, the observed difference is not statistically significant.

A model for reliability and confidence level in fatigue statistical calculation

A new kind of statistical data model which described the fatigue cracking growth with limited data was proposed, and the effects of the reliability and the confidence level to the fracture growth were considered. The one-sided allowance factor statistical analysis method was used to provide the prediction of the fatigue life with the confidence level and the reliability, and the effect factors were revised, which were closer to the lower limit of the matrix hundred rank values. It was found that this method gave much more accurate fatigue life prediction by analyzing the statistical data of the 7050 aluminum alloy before and after laser shock processing (LSP) and the new one-sided allowance coefficient saves more test samples in the same situation of precision. The revision coefficient would also save the experimental work load in the experiment is.

Vehicle Dynamic Analysis and Fatigue Prediction

This research was to predict the life of vehicle components on an uneven random road. Flexible-body dynamic simulation was used for dynamic simulation and stress recovery. Based on the stress obtained from dynamic simulation, a fatigue life prediction was implemented. This process combined dynamic simulations and finite element analyses (FEA), including the interaction between dynamic forces and structure deformation. Comparing with traditional approach, dynamic and FEA calculation performed in sequence and separately, the proposed process could obtain more accurate structure stresses and better fatigue life prediction. With that, the production development time and cost could be significantly reduced.

OS2129 応力振幅の確率密度補正による周波数領域での高精度疲労寿命予測

OS2129 応力振幅の確率密度補正による周波数領域での高精度疲労寿命予測