Fatigue Life Assessment Research Articles

Fatigue strength assessment of welded joints in the marine environment

Welding is a commonly applied joining method in many applications in arctic and marine conditions, e.g., in ship and offshore structures, and energy production equipment. Such applications are usually subjected to fluctuating load conditions, and during a decades-long service, they may experience millions of load cycles. Consequently, fatigue strength design and acceptable flaw sizes in the welded details of these structures are among the most important design criteria. Multiple fatigue strength assessment approaches exist for assessing the fatigue strength of a welded detail. The present study introduces a numerical and analytical fatigue strength assessment, conducted on a non-load-carrying X-joint, which is a representative joint type used in many steel constructions. Fatigue analyses are carried out following the DNVGL-RP-C203 and BS7910:2013 fatigue design guidelines for offshore steel structures. The stress intensity factors (SIFs) for linear elastic fracture mechanics (LEFM) analyses were obtained using three different methods: the weight function approach, the analytical equations provided in the IIW Recommendations, and by conducting numerical crack propagation analysis using the Franc2D software. All three methods had a good agreement particularly for short crack depths, indicating the applicability of the analytical approaches for the fatigue analyses. The results showed that the consideration of degree of bending at the welded detail is crucial due to the distinguishing notch stress factors of membrane and bending loading, and different stress distributions in the through-thickness direction. In addition, it was found that the LEFM-based fatigue life assessments are significantly more conservative than the life predictions obtained using the structural hot-spot and effective notch stress approaches.

Integrated fatigue life predictions of aluminium castings using simulated local microstructure and defects

In this work, an integrated simulation approach previously developed for static FE analyses is extended to microstructure- and defect-based fatigue life assessments of castings. The approach, the closed chain of simulations for cast components, combines casting process simulation with microstructure modelling and local material characterisation to generate heterogeneous material data for FE analysis and fatigue life assessment. The method is demonstrated on a High-Pressure Die Cast aluminium component. Areas with a high risk of defects are identified based on the simulated solidification conditions, and heterogeneous material data for the fatigue life analysis is generated. Fatigue testing has been performed with different levels of porosities to quantify the effect of defects on the element-specific Wöhler curves. Pore characteristics are assessed using 2D X-ray, fracture surface analysis and Kitagawa diagram. The results highlight the importance of taking the risk of defect formation into consideration when designing industrial aluminium castings subjected to fatigue loads.

ANN based fatigue life assessment of FSW on AA2219-T351 and optimization of welding parameter

A study of AA2219-T351 alloy's crack behavior and fatigue propagation rate is presented in this paper. Tests were done according to ASTM standards on fracture robustness and propagation of fatigue crack rates of CT samples fused with the 2219-T351 aluminum alloys utilizing the FSW technique. The toughness and growth rate of fatigue failure can therefore be predicted using a neural network. Different numbers of neurons are inserted into the hidden layer of the ANN to see which one performs best, and then the best network is selected. A comparison is made between the outcomes of the artificial neural network and the fitting technique and then the fitted surfaces that exhibit welding behavior are exhibited. Multi-objective optimization is used to further improve welding performance, and the impact of rotation and traversing speeds on fracture toughness at which fatigue fractures grow are examined.

Validation of lumbar fusion device TILIF (Ti-6Al-4 V) manufactured by EBM additive manufacturing through fem modeling high cycle fatigue tests

This study aims to validate the computational fatigue analysis of a Transforaminal Lumbar Intervertebral Fusion (TLIF) prosthesis using high cycle fatigue (HCF) test data through the Finite Element Analysis Method (FEM). The Additive Manufacturing by Electron Beam Fusion process enables the construction of complex geometries for lumbar fusion prostheses, making it the preference of manufacturers for high-scale production. However, ensuring the structural validation of such products through static and dynamic tests can be time-consuming and expensive. Computational simulations using FEM can provide preventive and predictive analyses to anticipate structural problems arising from loads specified in projects, allowing for the development of geometric models that meet the mechanical tests requested by the ASTM F2077-18 standard. The HCF specimens were tested for fatigue life assessment between 107 cycles, improving test duration compared to traditional methods. The mechanical properties of the material, such as modulus of elasticity, yield strength, ultimate tensile strength, and density, were taken into account during testing. The results obtained through FEM were effective, demonstrating the predictive capability of this method in the development of new products manufactured by additive manufacturing technology using Titanium Grade 5. This research also compares the use of virgin powder and recycled powder in the manufacturing process, showing the differences in fatigue between the two materials. The study indicates that the mechanical properties of recycled powder are inferior to those of virgin powder, and as a result, the fatigue life of parts manufactured using recycled powder is lower than those manufactured with virgin powder. In conclusion, this study demonstrates the effectiveness of using computational fatigue analysis through FEM to predict the mechanical behavior of lumbar prostheses. The research provides valuable insights for manufacturers, indicating that the use of virgin powder in the manufacturing process can lead to longer fatigue life of the parts compared to the use of recycled powder.

Fatigue life assessment of fuselage panel modification based on flight measurement

During the modification of the aircraft fuselage panel, the fatigue performance of the structure changes. To ensure the safety and reliability of commercial aircraft, the fatigue life assessment of the modified structure is required. In this paper, the detailed fatigue rating method was used for fatigue assessment. The strain information of the modified structure during the flight was obtained by installing fiber Bragg grating sensors. Through the finite element analysis, the fatigue critical parts were determined and the measured data before modification were converted into the data after modification. The detailed fatigue ratings of the fatigue critical parts were calculated. The fatigue life of the critical parts was evaluated based on the linear fatigue damage cumulative rule. According to the results, the most critical part was determined, which provides a basis for the determination of the modification plan of the fuselage panel.

Fractographic-fractal dimension correlation with crack initiation and fatigue life for notched aluminium alloys under bending load

In this study, fatigue fracture surfaces of aluminium alloy 2017-T4 notched specimens were investigated under cyclic bending to find an alternative failure loading index.. The surface topographies were measured on the entire fracture area with an optical profilometer for different loading conditions. Fatigue crack initiation life Ni and total fatigue life Nf were examined using standard surface topography parameters (such as, root mean square height Sq, arithmetical mean height Sa, maximum height Sz) and non-standard surface topography parameters (such as, fractal dimension Df). Fatigue life assessment was successfully performed by combining both fracture surface topography and stress state features. The results show that the fracture plane geometry, expressed by the fractographic-fractal dimension, can facilitate the estimation of post-failure loading history.

Fatigue life assessment of metal foils in multifunctional composites via combined experiments and simulations

In many applications metal foils are part of multifunctional composites exhibiting a wide range of (thermo-) mechanical fatigue loading. Data about low cycle fatigue of metallic foils subjected to tension-compression fatigue is sparse due to geometric limits. In this contribution, detailed information about design, tension-compression fatigue testing and data extraction of a layered sample are given. Fatigue failure of both copper and Pre-Preg are analysed using scanning electron microscopy and computed tomography. Numerical models are used to calculate the elastic and plastic strains for the copper foils within the composite sample, allowing a correlation to the observed fatigue life.

Study on the safety performance of large offshore wind turbine coupling

An offshore wind turbine coupling (OWTC) safety performance assessment method for torque transfer and overload protection of the main drive train of offshore wind turbine is proposed in the paper. First, the local mechanical model and the overall finite element model of the OWTC are established, and the fatigue life assessment method of the OWTC is given. Then, the effectiveness of the proposed method was verified by experiments and finite elements. The structural strength under different working conditions of the OWTC and the influence of the bolt tightening torque on the OWTC slip torque are studied, and the torque transfer efficiency is evaluated. Finally, the stress response of the OWTC was obtained by the finite element method considering the time-series torque load, the fatigue life of the OWTC was evaluated by combining the S/N curve parameters and the Palmgren-Miner rule. The results show that the proposed method for assessing the safety performance of the OWTC is reliable and valid. The calculated results of the local mechanical model are in good agreement with the test and simulation results, and the connecting rods and bolts of the OWTC are highly susceptible to damage under fatigue loading and should be of particular concern.

Cyclic elastoplasticity-based life assessment of fatigue crack initiation and subsequent propagation in rib-to-deck welded joints

Conventional S-N curve methods for fatigue life assessment fail to consider the different cracking mechanisms of crack initiation and propagation, resulting in less accurate predictions of fatigue life. The present study aims to assess the fatigue crack initiation and subsequent propagation life of rib-to-deck welded joints separately by accurate prediction of local cyclic elastoplasticity. The welding residual stress was simulated with high precision by thermal-mechanical analysis based on the real bead geometry and welding parameters, and defined as initial stress condition. The performance of an unconventional Fatigue Subloading Surface constitutive model (FSS model, hereafter) to predict the cyclic elastoplasticity in the sub-yield stress state was proved by comparing to the experiment results and conventional plasticity theory. The FSS model was then embedded to distinct zones of welded joints for cyclic structural analyses. Considering the relaxation of residual stress, the fatigue crack initiation by the local strain approach based on the local cumulative plasticity damage, and propagation life was assessed by the analytical approach based on the LEFM parameters of materials. As a connection between the crack initiation and propagation, the initial crack featured by 0.5 mm depth and semi-circular shape was proposed. The current work is verified to assess the fatigue life of rib-to-deck welded joints with high accuracy under the pure bending load against experiment results.

Using computer techniques for vibration damage estimation under stochastic loading using the Monte Carlo Method for aerospace applications

The main focus of this article is a review of legacy methods for vibration damage estimation under stochastic loading and extending research made by Dirlik and Bendat using two combined methods: FEM and Monte Carlo simulation, for which we used Python programming for aerospace applications. For some aircraft, regulated by the RTCA international aviation standard DO-160G (Environmental Conditions and Test Procedures for Airborne Equipment), stochastic loading is defined as one of the requirements. This article will focus on the stochastic loading impact on the fatigue life assessment made on a dummy sample, and frequency and time domain damage estimation shall be considered in parallel to compare both results. Additionally, dummy PSD responses shall be defined in the frequency domain for signal statistical parameters research. The article introduces Rainflow Cycle Counting methods in the frequency domain for procedures used commercially in aerospace applications. The first method introduced and developed further is the Dirlik method of Rainflow Cycle Counting in the frequency domain, which is the most popular method in commercial use. The second technique introduced and developed further was established by Bendat — the Narrow Band Method. The new empirical equation presented in this paper is the modification of the Narrow Band Method fitted for general use (narrow band, wide band, and white noise signals). A new approach for the integration of spectral moments is introduced in this paper, allowing for an accurate evaluation of the signal statistic parameters in the frequency domain for use in the modified Dirlik and Narrow Band methods. Research results also revealed new phenomena not researched by Dirlik, such as high vibration damage variation from stochastic loading, which depends on the frequency resolution (the block size used in Inverse Fourier Transformation). This discovery will be the subject of further study. Research results presented in this paper will also be utilised to combine stochastic and deterministic loading scenarios for military helicopters, as well as fighter aircraft, and will be the subject of further research.

Effect of pre-strain on high cycle fatigue property of TWIP steel

A commonly used material in vehicle structure is twinning-induced plasticity (TWIP) steel. In addition, during auto safety structural parts process, the material is pre-strained to different levels. Therefore, the influence of pre-straining on fatigue life needs to be examined for design and life assessment. Based on the current test data, due to the different types of steel and loading methods, the fatigue life of the material will show an increase or decrease with the increase of pre-strain level. The results of this study are compared with the recommended S-N fatigue, and the applicability and conservative levels of fatigue life curves are discussed to consider the influence of material pre-strain on the fatigue life assessment of automotive structural components.

A novel nonparametric model for life assessment of high cycle fatigue under uniaxial variable amplitude loading using intrinsic damage dissipation

Several nonparametric models for HCF fatigue life predictions under variable amplitude loading (VAL) have been proposed and gained broad attention from practicing engineers due to their simplicity and ease of application. However, for practical application, the existing standards still use the Palmgren-Miner model, which does not consider the damage due to loading sequence and causes inaccurate life predictions. Hence, an urgent need exists for an equally simple, easy-to-apply, and accurate nonparametric model for engineering applications. Therefore, this study proposes a novel nonparametric model for HCF fatigue life assessment under VAL, which considers the influences of the load sequence. The proposed model is based on cumulative intrinsic damage dissipation under fully reversed tensional loading, which can be modeled based on continuum damage mechanics. A new damage transfer concept based on cumulative intrinsic damage dissipation under fully reversed tensional loading is also proposed for fabricating the proposed model. The proposed model provides better correlations with experimental results than the Palmgren-Miner and other recent nonparametric models. These experimental results cover the total fatigue lives of five materials under two-level, multilevel block loading, multiple periodic overloading, and involute gear under two-stage loading conditions. Furthermore, the proposed model provides a novel approach to easy application for accurately predicting HCF fatigue life under VAL.

Fatigue Life Assessment of Metals under Multiaxial Asynchronous Loading by Means of the Refined Equivalent Deformation Criterion

As is well-known, non-proportional fatigue loading, such as asynchronous one, can have significant detrimental effects on the fatigue behavior of metallic materials by reducing the fatigue strength/fatigue limit and by leading to a fatigue damage accumulation increased with respect to that under proportional loading. In the present paper, the novel refined equivalent deformation (RED) criterion is applied for the first time to estimate the fatigue lifetime of materials, sensitive to non-proportionality, subjected to asynchronous loading under low-cycle fatigue regime. The present criterion is complete since it considers: (i) the strain path orientation, (ii) the degree of non-proportionality, and (iii) the changing of material cyclic properties under non-proportional loading. To evaluate its accuracy, this criterion is applied to examine two different metals (a 304 stainless steel and a 355 structural steel) whose experimental data under multiaxial asynchronous loading are available in the literature. More precisely, the parameters of the criterion are firstly determined by using experimental strain paths, and then the computed refined equivalent deformation amplitude is used to represent the experimental data with a satisfactory accuracy. Finally, a comparison with the results obtained through two other criteria available in the literature is performed, highlighting the good prediction of the present RED criterion.

Fatigue properties of resistance spot‐welded maraging steel produced by selective laser melting

AbstractThis research study investigates the fatigue properties of resistance spot welds (RSWs) made from selective laser‐melted (SLMed) maraging steel (MS1) and compares them with dual‐phase (DP) steel RSWs. The findings reveal that MS1 spot welds exhibit superior fatigue properties compared with DP steel RSWs, regardless of the printing orientation of the MS1 substrate. Furthermore, fatigue fracture occurs at the MS1 substrate, with a transition of fracture location observed from MS1 to itself and MS1‐to‐DP600 dissimilar welds. The results of structural stress analysis indicate that the weld nugget diameter is not the only factor affecting the fatigue life. The study also highlights the importance of considering materials inhomogeneity and fracture modes in fatigue life assessment. Based on the findings, the study recommends the development of a new fatigue life prediction curve for spot welds of additive manufacturing (AM) materials. Such a curve would be beneficial in promoting the further application of AM technology for functional parts of design verification vehicles (DCV) and production cars. Overall, the research contributes to the understanding of the fatigue properties of RSWs made from SLMed MS1 and DP steel, which has implications for the design and manufacturing of parts in the automotive industry.

Methodology for Application of Damage Mechanics Approach to Model High Temperature Fatigue Damage Evolution in a Turbine Disc Superalloy

Aeroengine gas turbine components operate under complex loading environments. Turbine disc is one such component which experiences stresses at high temperature and accumulates life critical cyclic damage in the material during usage. This accumulation of cyclic damage results into significant deterioration in material strength which in turn may initiate the failure in these rotating components. This necessitates the need to develop an advanced lifing approach for fatigue life assessment of turbine disc. As there are no available standards for damage mechanics application, an attempt has been made in the present study to develop a methodology for application of damage mechanics approach to model high temperature fatigue damage evolution in a turbine disc Superalloy. High temperature (650oC) stress-controlled fatigue tests on turbine disc alloy have been performed to evaluate parameters for damage mechanics based models. Using this approach, damage evolution has been simulated at specimen level. A good correlation has been observed in the damage mechanics based model’s predicted damage and experimentally determined values.

Service-life assessment of aircraft integral structures based on incremental fatigue damage modeling

The Ottosen–Stenström–Ristinmaa (OSR) incremental fatigue damage model is adapted for fatigue-life assessment of integral airframes milled from 7050-T7451 aluminum plates. For validation, variable-amplitude high-cycle fatigue experiments are conducted for circumferentially notched, axisymmetric specimens, and for a geometry similar to an aircraft fuselage frame, with flanges, stiffeners, and web panels. We also describe how the parameters of the OSR model can be modified to account for surface roughness, and for setting acceptable failure probability.

Fatigue Test and Unified Fatigue Life Calculation of Q460C Steel Notched Plates

In the present study, a total of 20 fatigue tests on notched plates of Q460C steel were carried out, where the effects of relative stress amplitude, Δσ/fy, and relative nominal maximum stress, σmax/fy, on the fatigue life of these notched plates were carefully examined. Theoretical analyses and numerical simulations were subsequently conducted, based on an ellipsoidal fracture model originally proposed by the second author, which has been validated for use as a fracture criterion of fatigue crack, to investigate the fatigue cracking in the Q460C steel notched plates. The theoretical model was further developed to estimate the fatigue life of the Q460C steel notched plates using a unified crack growth approach originally proposed by the second author. Based on the theoretical and simulated results, the accuracy of the unified crack growth approach, and the allowable stress fatigue life formula recommended in China’s code GB50017-2017, were assessed. The comparisons indicate that the unified crack growth approach is able to provide a reliable fatigue life assessment for the Q460C steel notched plates.

Reliability-based fatigue life assessment using random road load condition and local damage criteria

This study presents a fatigue reliability assessment of a vehicle coil spring based on random road load conditions for fatigue life characterisation. Fatigue life data characterisation was carried out to accelerate the durability analysis by retaining the higher amplitude cycles in the original strain signals and thus indicate the fatigue damage. To collect experimental data, road testing was conducted using various road profiles, which contributed significantly to the fatigue failure performance of the vehicle coil spring. Fatigue reliability analysis was performed to assess the failure behaviour of the coil spring utilising the probabilistic Gumbel distribution model to forecast the probability distribution of the fatigue life data. Furthermore, the highest and lowest mean-cycles-to-failure (McTF) of 5.44 × 107 and 2.46 × 105 blocks were estimated for the Coffin-Manson and Smith-Watson-Topper models from the highway and rural road profiles. The findings demonstrate that the lowest hazard rate corresponded to the highest McTF, making this an acceptable model to be implemented in reliability assessments of road profiles. Hence, the fatigue reliability assessment provided a hazard rate based on the fatigue life predictions for risk assessments when characterising the fatigue life data.

Fretting fatigue under variable amplitude shear loading blocks considering partial slip regime: Experimental/numerical analysis

Generally, the fretting fatigue problem is treated with constant amplitude loading. However, this condition is not the reality for industrial applications. The objective of this work is to propose a numerical methodology for the life assessment of components subjected to fretting fatigue under variable amplitude loading and to study the effects of these loads. For this, experimental tests were carried out by applying H-L (High-Low) and L-H (Low-High) loading blocks to the tangential load, that is, the contact normal load and the amplitude of the bulk fatigue load were kept constant while the tangential load has its alternating amplitude. The numerical methodology proposed for fretting fatigue life assessment includes the wear effect and crack propagation. The multiaxial fatigue parameter SWT, the theory of critical distance TCD, and Miners’ linear rule were used to compute the damage in the crack nucleation phase. The node-displacement algorithm based on Archard’s law was applied to account wear. In the crack propagation phase, the CDM (critical direction method) was used to estimate the crack initiation angle considering the worn contact surface, and then the crack is modeled and propagated. The still limited range of experimental data done in this work for fretting fatigue under varying loading blocks seems to indicate that the loading sequence has an effect on the accumulated fatigue damage, mainly to the H-L loading sequences. Based on this observation, a new non-linear damage model was proposed. Using this new damage model, the life estimation results were within a scatter band of 1.2.

A multiaxial probabilistic fatigue life prediction method for nickel‐based single crystal turbine blade considering mean stress correction

AbstractGas turbine blades normally operate under high‐temperatures, extreme pressure, and high‐speed conditions, which leads to complex failure mechanisms and difficulties in fatigue life assessment. Moreover, due to the randomness of manufacturing errors, working loads, and material properties, the low cycle fatigue (LCF) life normally presents unavoidable stochastic behavior. Therefore, accurate and efficient fatigue life prediction is critical for the design of gas turbine blades. Accordingly, this paper analyzes the effects of mean stress, anisotropy, and uncertainties on the LCF life of nickel‐based single‐crystal turbine blades. The modified Hill yield criterion is employed to deal with the anisotropic damage characteristics of nickel‐based single‐crystal superalloy (NSCS) under a multiaxial stress state. Meanwhile, the effects of the temperature and crystal orientation are introduced into the walker mean stress correction term. A multiaxial LCF life prediction model is developed based on the energy criterion‐based fatigue life estimation method. Moreover, multi‐source uncertainties originating from rotation speeds, material properties, and temperature are analyzed and quantified. Comparison between the predicted and tested life indicates that the proposed method produces good accuracy and robustness, which can provide a reference for the reliability design of NSCS turbine blades.