Accurate Estimation Of Fatigue Life Research Articles

Bayesian-informed fatigue life prediction in shallow shell structures with the dual boundary element method

This study introduces an innovative approach for statistically inferring the Equivalent Initial Flaw Size Distribution (EIFSD) in shallow shell structures with the aim of predicting fatigue life. The proposed approach integrates Bayesian inference and surrogate modelling techniques, and utilizes a long crack growth model. The EIFSD is crucial for estimating fatigue life and ensuring structural durability. To showcase the proposed methodology, a numerical example featuring a fuselage window structure under multi-axial loading, employing the Dual Boundary Element Method (DBEM) for crack growth modelling, is presented. Efficiency gains in the inference of the EIFSD are achieved through the development of surrogate models, notably a Co-Kriging model that significantly reduces computational time and outperforms a Kriging model in error reduction. The study explores two assumptions for inspection data sourcing, leading to enhanced methodological effectiveness across varying uncertainty levels. It considers the separate effects of Paris Law constants and loading conditions under increased uncertainty, presenting cases that demonstrate the Co-Kriging model’s high accuracy and computational efficiency. The proposed methodology’s robustness is further validated by assessing the impact of prior distribution quality on Bayesian inference, showing the method’s adaptability to high uncertainty while maintaining a good level of accuracy. This study also demonstrates an approach for fatigue life estimation of structures using the inferred EIFSD. By evaluating the structure up to a critical crack length, the methodology allows for the accurate estimation of fatigue life, which, in turn, facilitates the determination of optimal inspection intervals. Notably, under conditions of high uncertainty, the method showcased a maximum difference of 1.51% in the estimated fatigue life when compared to the true EIFSD, underscoring the technique’s high accuracy and reliability.

Fatigue condition assessment of subsea pipelines under vortex induced vibration and cyclical lateral displacement

ABSTRACT Subsea pipeline may suffer from vortex-induced vibration (VIV) and cyclical lateral displacement (CLD), which can result in fatigue failure of subsea pipeline. This paper presents a fatigue damage assessment of subsea pipelines under VIV and CLD. A finite element model of pipeline considering the pipe–soil coupling action is developed, and dynamic response of subsea pipeline under VIV and CLD is implemented to obtain time history of equivalent stress. The integrated fatigue damage of subsea pipeline under VIV and CLD is estimated using Palmgren–Miner linearly cumulative fatigue damage theory. The effect of pipe diameter, span length, pipe length and tidal period on fatigue damage of subsea pipelines under CLD is analyzed. A case study of a subsea pipeline in the strong tide area is performed. The results show the fatigue life of pipeline under VIV and CLD is 14.1 years, which is 2.3 years lower than the pipeline under VIV alone. The study can support more accurate estimation of fatigue life of subsea pipelines.

Discussion on Fatigue Life Analysis Methods for Connecting Bolts of the Top Cover of Water Pump Turbines

Under the joint action of dynamic and static loads, the connecting bolts of the top cover of the pump turbine are prone to cumulative damage in local areas such as threads, resulting in cracks or development, and fatigue fracture may occur in severe cases, which will cause serious safety accidents. Therefore, it is necessary to carry out accurate fatigue life estimation analysis for connecting bolts, which has always been a hot spot and difficulty in the industry. This paper introduces several commonly used fatigue life analysis methods of parts, combined with the force characteristics and structural characteristics of the connecting bolts of the top cover, and analyzes and discusses the fatigue life analysis methods suitable for the top cover bolts. Finally, it is believed that the application of stress-strain field strength method can better describe the actual state of the bolt danger area, and the current damage state has an influence on the amount of damage generated by further loading, and the fatigue life analysis of connecting bolts can be combined with the stress-strain field strength method and variable damage linear cumulative damage theory, and the fatigue life and residual fatigue life can be estimated in the design stage or after the phased service of the bolt.

Crack Propagation Life Prediction of a Single Lap Shear Joint: A Linear Elastic Fracture Mechanics Based Machine Learning Approach

Estimation of fatigue life in the aerospace industry plays a key role in terms of safety, certification, and inspection. Individual aircraft tracking programs aim to calculate the fatigue index for each aircraft based on its load history. Therefore, accurate estimation of fatigue life based on unique load spectra for each aircraft plays a crucial role. The purpose of this study is to predict the crack growth (CG) life of a single lap shear joint for an aircraft based on different spectral loads using random forest regression and k-nearest neighbors(k-NN) regression. A finite element model was built using the Huth fastener flexibility method to obtain fastener loads and stress values on the structure under maneuver loads. Then, finite element results were compared to analytical calculations. Based on Fighter Aircraft Loading Standard for Fatigue and Fracture (FALSTAFF) spectra, 90 different spectra were developed to be used for the calculation of CG life. The AFGROW software was used to calculate the CG life based on the load spectra and the findings of the finite element model. Analytical calculations of CG lives and features of individual load spectrums were fed to machine learning models. Finally, the CG life predictions of the machine learning models were compared with analytical calculations. According to the findings, a good correlation was observed between the analytical and predicted CG lives.

Wind site turbulence de‐trending using statistical moments: Evaluating existing methods and introducing a Gaussian process regression approach

AbstractThis paper considers the problem of retrospectively de‐trending wind site data when only statistical moments, in the form of 10‐min means and standard deviations in wind speed, are available. Low‐frequency trends present in wind speed data are known to bias fatigue damage estimates, and, hence, removal of their influence is important for accurate fatigue life estimation. When raw data is available, this procedure is straightforward; however, for many sites, significant quantities of data are available, which contain only statistical moments. Additional value is therefore unlocked if de‐trending can also take place in this context. Existing methods, Models 1 and 2, are introduced, and their performance and viability appraised, respectively. A Gaussian process (GP) regression implementation is also developed, which seeks to incorporate characteristics of real trends extracted from raw data into the fitting procedure via an appropriately chosen lengthscale hyperparameter. Results indicate that Model 2, the recommended method in previous work, suffers from fundamental issues, with the implication that it should no longer be used. Model 1 and GP results are shown to be very similar at the turbulence distribution level. This finding is interpreted as a validation of Model 1 and an indication that it may already be performing as well as can be hoped for, given the information available in the current problem formulation. Theoretical overheads associated with GPs, in addition to the performance similarities mentioned above, lead to Model 1 being recommended as the best approach to moment‐based turbulence de‐trending at this time.

Durability assessment of suspension coil spring considering the multifractality of road excitations

This study presents the characterisation of multifractality of road excitation time series under different road conditions for prediction of the durability of a suspension coil spring. Road excitation acceleration signals and strain signals were acquired from the suspension system of a vehicle travelling under different road conditions. Multifractal analysis revealed a higher tendency to multifractality in road excitations with more surface irregularities. With the fatigue lives predicted by different strain-life models (Coffin-Manson, Morrow, and Smith-Watson-Topper), fatigue life prediction linear models based on road multifractality were established. It was found that the Morrow-based linear model gave the most accurate estimation of fatigue life, with the highest R2 of 0.8762. In conclusion, the models for the prediction of coil springs’ fatigue life based on road multifractality provide an accurate and faster alternative for durability assessment of coil springs. This can significantly facilitate the design process of coil springs to meet industry requirements.

Comparison of TCD and SED methods in fatigue lifetime assessment

This paper assesses the accuracy and reliability of the Theory of Critical Distances (TCD) and the Strain Energy Density (SED) approach in estimating the lifetime of plain and notched specimens subjected to cyclic loading. To validate the two approaches for plain and notched components under uniaxial and multiaxial fatigue loading, a large bulk of experimental data taken from the literature was re-analyzed, with the state variables, i.e. the stress distributions and the strain energy density, being calculated via the Finite Element (FE) method. The results obtained demonstrate that both the TCD and the SED approach can provide highly accurate fatigue life estimation. In addition, these two approaches require little computational effort and little calibration information to be implemented and used for fatigue design in situations of practical interest.

Enhancements of a Stress-Based Approach for Fatigue Life Estimation of Multi-Material Connections Joined by Self-Piercing Rivets and Adhesive

Due to the continuously reduced emission targets from legislation, lightweight constructions are getting more and more important for car manufactures. One way to reduce weight is the use of multi-material design for the body in white where new joining technologies, such as self-piercing rivets (SPRs) need to be considered instead of spot welding. Since both spot welds and SPRs are local circular joints, it appears reasonable to adapt and modify existing methods for spot welds to estimate the fatigue life of SPRs. These established methods typically use calculated radial stresses at the joint from inner forces (derived from FEM) and compare them to an SN-curve. Instead of comparing to a regression of calculated stress values, the presented method uses measured material specific SN-curves for this purpose. These curves contain the stresses of the materials at a notch, oriented on the rivet diameters. A modification of parameters in the stress calculation leads to a significant reduction of scattering in the comparison of simulated and tested fatigue life values. In addition to this method, an approach for the estimation of fatigue life of adhesively bonded joints was adapted. This method considers an effective stress value, which is evaluated either on a critical distance to the notch or is calculated as average over a defined path. Additional requirements during the development of the new method were the reduction of tests and a small complexity of the required FE-models to enable fast and still accurate fatigue life estimation on full vehicle level.

Fatigue life analysis of slewing bearings in wind turbines

Wind energy is a type of green renewable energy that has received increased attention. Wind turbines use wind power to generate electricity. As important components of wind turbines, slewing bearings are large and expensive, and these properties make bearing tests challenging. The theories and methods of slewing bearing design in wind turbines are not perfect, and the field lacks long-term engineering verification. To ensure the service life of slewing bearings, an accurate fatigue life estimation in the design stage is essential. This paper presents a method of testing the fatigue life using a small sample test. Experiments were conducted to determine the actual fatigue life of a small sample, and the changes in local raceway damage, vibration acceleration, and lubrication were monitored. A finite element model of the slewing bearing was established in ABAQUS to obtain the contact stress between the ball and raceway. The calculation results were imported into FE-SAFE to analyse the fatigue life. The Morrow mean stress correction in conjunction with the Brown-Miller strain-life method were used in the analysis. The simulation results were compared with the experimental results to validate the effectiveness of the experiment. Three fatigue life calculation methods have distinct advantages and can be mutually referenced to improve the accuracy of bearing life calculations.

Fatigue life prediction under mixed-mode loading using equivalent stress intensity factor models

Damage tolerance principles are widely used to assess the structural integrity and failure of engineering components. The advances in numerical simulation techniques facilitate the prediction of fatigue life of engineering component, which is essential in damage tolerance design. For the components under mixed mode (I/II) loading, the fatigue crack growth and life is predicted by using a modified form of Paris' law along with the equivalent stress intensity factor (ΔKeq). Numerous ΔKeqmodels are available for correlating the equivalent stress intensity factor range and the fatigue crack growth rate. The knowledge of proper ΔKeqmodel is essential for the accurate fatigue life estimation. In this work, the authors numerically assess the performance of ΔKeqmodels in mixed mode fatigue life prediction with the help of published experimental mixed mode data.

A new nonlinear fatigue damage model based only on S-N curve parameters

Several fatigue damage models have been proposed in the past to overcome the shortcomings of the commonly used Miner’s rule. However, application of these models requires either the determination of material parameters or modifications to the S-N curve of the material. To overcome these problems, a new fatigue damage model is proposed in this paper. The proposed model is based on the commonly available S-N curve parameters of the material and does not require any additional material parameter determination or S-N curve modification. The validity of the proposed fatigue damage model is confirmed by comparing the experimentally derived damage evolution curves for C 45 and 16 Mn steel. The proposed model gives a better agreement with these experimental results compared to previous models. A new damage transfer concept is also proposed for a more accurate estimation of fatigue life and is verified with experimental results for fatigue lives of six materials. The predicted fatigue lives are in better correlation with experimental results compared to Miner’s rule and other recently proposed models. Finally, the application of the proposed model is illustrated by a case of welded joints. The model is applied to butt and fillet welded joints and the predicted fatigue lives show better agreement with experimental results compared to earlier models. The maximum and average deviations from the experimental results are 20% and 9% respectively. It is concluded that the proposed model can be easily applied by practising engineers for an accurate prediction of fatigue life.

Effect of grid size on fatigue life calculations of a dental implant

The aim of this paper was to evaluate the effect of grid size on the fatigue life calculations of a dental implant by conducting a non-linear contact FEM analysis. A Nobel – Biocare (Replace – Select) implant was chosen and modelled in CATIA after retrieving accurate dimensions from SEM. The mandibular bone was modelled using MIMICS software. Static and dynamic simulation was then conducted on the final assembly using ANSYS which is a commercially available FEA software package. It was observed that variation in grid size leads to significant variation in the life of the dental implant. Hence for an accurate estimation of fatigue life of the dental implant the mean of the range of values must be considered

Fatigue Crack Growth Analysis of Functionally Graded Materials by EFGM and XFEM

The present work investigates the fatigue life of a functionally graded material (FGM) made of aluminum alloy and alumina (ceramic) under cyclic mixed mode loading. Both element free Galerkin method (EFGM) and extended finite element method (XFEM) are employed to simulate and compare the fatigue crack growth. Partition of unity is used to track the crack path in XFEM while a new enrichment criterion is proposed to track the crack path in EFGM. The fatigue lives of aluminum alloy, FGM and an equivalent composite (having the same composition as of FGM) are compared for a major edge crack and center crack in a rectangular domain. The proposed enrichment criterion not only simulates the crack propagation but it also extends the applicability and robustness of EFGM for accurate estimation of fatigue life of component.

On the accuracy of nominal, structural, and local stress based approaches in designing aluminium welded joints against fatigue

This paper investigates the accuracy and reliability of nominal stresses, hot-spot stresses, effective notch stresses, notch-stress intensity factors (N-SIFs) and material length scale parameters in estimating fatigue lifetime of aluminium welded joints. This comparative assessment was based on a large number of experimental data taken from the literature and generated by testing, under either cyclic axial loading or cyclic bending, a variety of aluminium welded structural details. Whenever it was required, stress analyses were performed by solving bi-dimensional linear-elastic finite element models. The obtained results demonstrate that the effective notch stress method, the N-SIF approach, and the Theory of Critical Distances (TCD) provide a more accurate fatigue life estimation in comparison with the other methodologies. In this context, the TCD was seen to be easier to adopt, requiring less computational effort than the effective notch stress method and the N-SIF approach. Finally, based on the experimental results being re-analysed, a unifying value of 0.5mm is proposed for the TCD critical distance, with this value allowing aluminium welded connections to be designed accurately irrespective of joint geometry’s complexity.

Robust Particle Filters for Fatigue Crack Growth Estimation in Rotorcraft Structures

Fatigue crack growth propagation in critical rotorcraft structures is much more complex than fixed-wing aircrafts in part due to the random and vibratory nature of the rotorcraft flight load spectrum. Existing physical damage models, which are originally developed for the uniform cyclic loads of fixed-wing platforms, are not accurate enough for rotorcraft structures. Such inaccuracy often results in increased risk of operation and higher cost of maintenance. The goal of this paper is to present an integrated diagnostic framework for accurate fatigue life estimation of critical rotorcraft structures by combining the predictions of a physical damage model and evidence from real-time diagnostic sensor data. The diagnostic framework obtains the most probable crack size at each inspection interval using a sequential Monte Carlo method known as particle filters. To validate the framework, three representative rotorcraft principal structural elements were tested using an actual rotorcraft flight load spectrum. Results show that the crack size estimation error of the method presented here is much lower than any of the currently available physical damage models.

On the assessment of fatigue life of marine diesel engine crankshafts

The fatigue strength and its correct assessment play an important role in design and maintenance of marine crankshafts to obtain operational safety and reliability. Crankshafts are under alternating bending on crankpins and rotating bending combined with torsion on main journals, which mostly are responsible for fatigue failure. The commercial management success substantially depends on the main engine in service and of its design crankshaft, in particular. The crankshaft design strictly follows the rules of classification societies. The present study provides an overview on the assessment of fatigue life of marine engine crankshafts and its maintenance taking into account the design improving in the last decades, considering that accurate estimation of fatigue life is very important to ensure safety of components and its reliability. An example of a semi-built crankshaft failure is also presented and the probable root case of damage, and at the end some final remarks are presented.

Tensile and fatigue behaviors of polymers for automotive applications

An experimental study was conducted on tensile and fatigue behaviors of two unreinforced polymers and two short glass fiber reinforced polymer composites. A number of effects including anisotropy, strain rate and temperature on tensile behavior was investigated. Fatigue behavior was evaluated with respect to effects of mold flow direction, temperature, mean stress, and stress concentration. Effect of mold flow direction on both tensile and fatigue behaviors of unreinforced materials was found to be negligible, while it was significant for reinforced materials. Tsai‐Hill criterion commonly used for continuous fiber composites was used to predict the off‐axis fatigue strengths of short fiber reinforced materials. Effect of strain rate on tensile properties was found to be significant for all materials. Test temperature also influenced tensile and fatigue behaviors of both unreinforced and reinforced materials. Mathematical equations were developed to represent variations of tensile properties with both temperature and strain rate. Walker equation with a mean stress correction parameter is proposed to correct for the effect of mean stress. Effect of stress concentration combined with mold flow direction and mean stress was also investigated and Neuber's rule resulted in accurate estimation of fatigue life.

A novel structural stress approach for multiaxial fatigue strength assessment of welded joints

A new approach to estimate the structural stress for the assessment of the multiaxial fatigue strength of welded joints is proposed. In this approach, the structural stress at a point just below the weld toe in the plate thickness direction is evaluated and adopted for the multiaxial fatigue strength assessment of complex welded joints in combination with the Modified Wöhler Curve Method. Numerical results show that the adoption of the thus estimated structural stress, rather than the traditional hot spot stress, produces more accurate fatigue life estimation for structure members experiencing either uniaxial or multiaxial loading.

New apparatus for studying fatigue deformation at high magnifications

One of the steps taken to enhance accurate fatigue life estimations and material modeling is the development of an electrohydraulic, servo-controlled feedback fatigue apparatus that has been joined to a scanning electron microscope. This apparatus allows in situ observations of cyclically loaded specimens undergoing fatigue deformation. Using this apparatus, recordings can be made of the events related to the surface response of materials under different loading conditions, showing how microstructural features influence crack nucleation and propagation behavior. With the development of this apparatus, existing mathematical models describing the fatigue process can be enhanced or improved, and more accurate fatigue life estimation methods can be obtained. The apparatus developed is described herein, as well as a few selected results.