Fatigue Lifetime Prediction Research Articles

Multiaxial fatigue life prediction by equivalent energy‐based critical plane damage parameter under variable amplitude loading

AbstractA new path‐independent multiaxial fatigue damage parameter , which is defined as the largest normal stress range between adjacent turning points of the maximum shear stress on the critical plane, is proposed to describe multiaxial fatigue damage. By combining the maximum normal stress or the largest normal stress range with the maximum shear stress range on the critical plane, two axial equivalent stress modification factors, and , are proposed, which can account for the influence of non‐proportional additional hardening. The sensitivity of the proposed axial equivalent stress modification factors to multiaxial variable amplitude loading sequences is analyzed. Furthermore, two new multiaxial fatigue damage models based on the axial equivalent stress modification factors are proposed to estimate fatigue life. The applicability of the presented methodology was verified by the experimental data of En15R steel and 7050‐T7451 aluminum alloy, and the results showed that the predicted fatigue lifetimes agree well with the experimental data under variable amplitude multiaxial loading.

Experimental and modeling study on small fatigue crack initiation and propagation behavior of Inconel 617

Small fatigue crack initiation and propagation behavior of Inconel 617 was studied by in-situ SEM observation, electron back-scattered diffraction (EBSD), digital image correlation (DIC), crystal plasticity finite element method (CPFEM) and extended finite element method (XFEM) in this paper. The experimental results indicated that different surface corrosion methods used to display surface microstructure will affect the small fatigue cracking mode of Inconel 617. The local micro-scale strain field of Inconel 617 near the notch and crack tip was quantitatively measured by a new grain scale DIC method. The CPFEM-XFEM simulation model of small fatigue crack initiation and propagation characteristics of Inconel 617 was established. A new framework for small fatigue crack path and lifetime prediction at the polycrystalline scale was constructed, which includes the influences of slip system and grain boundary on small fatigue crack initiation and growth rate.

Research on Fatigue Lifetime Prediction of Stator Rubber Bushing Based on Rubber Aged Experiment

Abstract With the development of oil and gas exploration and development toward deep wells and ultra-deep wells, higher requirements are put forward for the working performance and service life of screw-drilling tools under the high-temperature environment. The failure of stator rubber bushing causes more than 30 % of screw-drilling tool failures. In this study, we mainly research the fatigue life prediction of rubber bushing under high temperature and high pressure and the influence of eccentricity and interference on fatigue life. Therefore, the uniaxial tensile test and fatigue test of hydrogenated nitrile butadiene rubber after 150°C /24 h ageing are carried out to obtain its mechanical properties and fatigue properties, and the fatigue life of rubber bushing is predicted by establishing the finite element mechanical model and combining it with the fatigue analysis software. The results show that the fatigue failure position of stator bushing is consistent with the theoretical and practical failure position and the error between the predicted fatigue life and the actual fatigue life is about 18.25 %. On this basis, the influence of eccentricity and interference on stator bushing fatigue life is analyzed. Fatigue life of stator bushing decreases with the increase of eccentricity or interference, which is consistent with the fatigue life curve of rubber material. The research results can provide reference for stator engineering application and bushing structure optimization of screw-drilling tools.

Fatigue Life Analysis of Automotive Cast Iron Knuckle under Constant and Variable Amplitude Loading Conditions

The main aim of the present paper is to assess the fatigue lifetime of ductile cast iron knuckles as one of the critical components of an automotive steering system. To this end, a real driving path, including various maneuvers, such as acceleration, braking, cornering, and moving on various types of road roughness, was considered. Different load histories, which are applied on various joints of the component (i.e., lower control arm, steering linkage, and Macpherson strut), were extracted through Multi-Body Dynamics (MBD) analysis of a full vehicle model. The achievements of previous studies have proved that the steering knuckle fails from the steering linkage and due to the rotational motion. Therefore, only this destructive load history was considered in future analyses of the present study. The CAD model was created using Coordinate Measuring Machine (CMM) data and some corrections in CATIA software. Furthermore, transient dynamic analysis was performed, and the time history of von Misses equivalent stress was obtained at the root of the steering linkage (which is exactly the location of failure based on the laboratory data as well as finite element simulations validated by the author in the previous studies). To predict the fatigue life of a component, two different methodologies were considered. Firstly, some well-known criteria were employed for equalization of load spectrum to a Constant Amplitude Loading (CAL). Then, fatigue analysis under sinusoidal loading was performed. Secondly, the fatigue life of the component considering Variable Amplitude Loading (VAL) was predicted using the Critical Plane Method (CPM), employing the Rain-flow cycle counting technique, and utilizing the Miner–Palmgren damage accumulation rule. Eventually, to evaluate the prediction accuracy of different methodologies, the obtained results were compared with the full-scale axial variable amplitude fatigue test which was performed by the corresponding author. The results indicated that the prediction of variable amplitude fatigue lifetime by Finite Element (FE) analysis in the time domain has about 21% error compared to reality. Additionally, the relative error between the results obtained from two different methodologies is about 20%, which is acceptable due to the scattering of the fatigue phenomenon results, the complex geometry of the part, and the complicated loading.

Surface roughness influence in micromechanical fatigue lifetime prediction with crystal plasticity models for steel

Although surface roughness significantly influences fatigue, most micromechanical models using crystal plasticity only consider bulk behavior. In this work, a framework is described to include surface roughness in crystal plasticity simulations with emphasis on enabling extensive computational studies. This is achieved by the utilization of a statistical surface roughness generator and the prescription of appropriate boundary conditions. Fatigue lifetimes of a high-strength steel are evaluated for three surface conditions, with the models appropriately parameterized by surface measurements. It is shown that fatigue lifetime prediction considering surface roughness is qualitatively in agreement with experimental observations regarding their mean values and scatter.

Mechanical fatigue of biodegradable polymers: A study on polylactic acid (PLA), polybutylene succinate (PBS) and polybutylene adipate terephthalate (PBAT)

The fatigue behavior of three biodegradable polymers, PLA (polylactic acid), PBS (polybutylene succinate) and PBAT (polybutylene adipate terephthalate) was determined under cyclic tension/tension loading in strain-controlled measurements. Stress response and surface temperature were recorded and analyzed to compare their behavior and evolution with on-going fatigue. The stress was decomposed into linear and nonlinear contributions by Fourier transform (FT) analysis and higher harmonics were correlated with fatigue. The surface temperature variation is analyzed as a function of the strain amplitude and compared for these materials during fatigue. The relative even (I2/1) and odd (I3/1) higher harmonics (ratio between the nth harmonic to the fundamental one (In/1)) in the stress response were found to be good criteria for fatigue characterization. The cumulative nonlinearity parameter (nQf), defined as the (cycle number) integral of the relative higher harmonic, normalized to the strain amplitude with an exponent of n − 1, was found to be the best criterion to quantify mechanical fatigue and predict fatigue lifetime.

Prediction of anisotropic LCF behavior for SLM Ti-6Al-4V considering the spatial orientation of defects

The inevitable defects in additive manufacturing (AM) materials reduce their low cycle fatigue (LCF) lifetime. Here a method for correlating defect size and spatial orientation with building direction is presented to evaluate the anisotropy characteristic of AM materials fatigue resistance. An ellipsoidal equivalent criterion is developed based on the principal component analysis method to capture the common features of the defect population which is obtained by synchrotron radiation X-ray tomography. The size characteristics are correlated with the spatial orientation characteristics of defects by using the polar figure. Then, based on the micro-plasticity zone theory, an effective driving stress for defect-induced failure is proposed. A lifetime model is further developed based on the effective stress, and its capability in anisotropic fatigue lifetime prediction is verified by performing LCF tests on the specimens with different building directions.

Increase of nuclear installations safety by better understanding of materials performance and new testing techniques development (MEACTOS, INCEFA-SCALE, and FRACTESUS H2020 projects)

Research to better understand the phenomena influencing materials and components’ performance is important for increasing the safety of Generation II and III nuclear plants. A crucial step for improving nuclear safety is the development of new experimental techniques that can provide the necessary data. The three H2020 projects presented in this paper, MEACTOS (2017–2022), INCEFA-SCALE (2020–2025), and FRACTESUS (2020–2024), cover the steps needed to realize those safety improvements. The goal of the MEACTOS project is to improve the resistance of critical locations, including welds, to environmentally-assisted cracking through optimizing surface machining and treatments. The project is currently in its final stage, and the complete analysis of the data is finished. The objective of INCEFA-SCALE is to improve predictions of component fatigue lifetime when subjected to Environmentally-Assisted Fatigue (EAF). The strategy consists of producing guidance on how to appropriately accommodate variable amplitude and plant-relevant loading in EAF assessments. Increasing the understanding of the EAF mechanism based on substantial testing, characterization, and analysis program will support the INCEFA-SCALE strategy. The FRACTESUS project will validate the use of miniaturized compact tension specimens by comparing the results of master curve-oriented fracture toughness tests performed with small and large specimens. The round-robin exercises will use irradiated and non-irradiated Reactor Pressure Vessel (RPV) materials. The material selection process is complete in time for the project to enter the testing phase. The output of the project will be beneficial from a long-term operation perspective and a saving in the material amount needed for RPV surveillance programs. Even though each project is devoted to different research areas, common aspects are clearly visible. All three projects investigate phenomena that are relevant to the performance and safe operation of the nuclear plant. Moreover, each project will provide valuable databases and analyses of test results for materials relevant to components in the nuclear plant. The output of these projects will be of great value to the nuclear industry. This paper presents the current progress for each project, emphasizing the common research domains between the projects.

Numerical and Experimental Investigations of the Thermal Fatigue Lifetime of CBGA Packages

A thermal fatigue lifetime prediction model of ceramic ball grid array (CBGA) packages is proposed based on the Darveaux model. A finite element model of the CBGA packages is established, and the Anand model is used to describe the viscoplasticity of the CBGA solder. The average viscoplastic strain energy density increment ΔWave of the CBGA packages is obtained using a finite element simulation, and the influence of different structural parameters on the ΔWave is analyzed. A simplified analytical model of the ΔWave is established using the simulation data. The thermal fatigue lifetime of CBGA packages is obtained from a thermal cycling test. The Darveaux lifetime prediction model is modified based on the thermal fatigue lifetime obtained from the experiment and the corresponding ΔWave. A validation test is conducted to verify the accuracy of the thermal fatigue lifetime prediction model of the CBGA packages. This proposed model can be used in engineering to evaluate the lifetime of CBGA packages.

Correlation between quasistatic und fatigue properties of additively manufactured AlSi10Mg using Laser Powder Bed Fusion

In order to find a resource efficient approach for the fatigue lifetime prediction of laser powder bed fusion (L-PBF) processed AlSi10Mg material, results of tensile and fatigue tests were compared. The specimens were manufactured with three different L-PBF machines and studied in different heat treatment conditions (as-built, annealed, T6 heat treated). The investigations showed that the high attainable tensile strength properties after the manufacturing process are not beneficial in the high cycle fatigue (HCF) regime. In contrast, the applied heat treatments, which lead typically to a decrease of ultimate tensile strength, improved dramatically the fatigue behavior. Additionally, a clear correlation between the elongation at fracture and HCF resistance has been found for individual heat treatment conditions. This empiric relationship provides an estimation of the fatigue resistance in the presence of material defects and can be implemented in part and process approvals.

A novel artificial neural network model for wide-band random fatigue life prediction

In this paper, a novel artificial neural network (ANN) model is developed to achieve fatigue lifetime prediction under wideband random loads. Many power spectral density (PSD) samples with different spectral shapes and a wide range of material parameters related to fatigue life are used to improve the generalization ability of the model. The bandwidth parameters are used as the input neurons for the first time, compensating for the defect that the existing neural network models have too many input neurons.Numerous numeric simulations demonstrate that the developed model is more accurate in life prediction than the existing frequency domain models.

Development of a Vibration Technique Based on Geometric Optimization for Fatigue Life Evaluation of Sandwich Composite Structures

A major obstacle to obtaining cost-effective experimental data on the fatigue life of sandwich panels is the prohibitive amount of time and cost required to carry out millions of cycles. On the other hand, vibration techniques applied to sandwich geometries fail to match the stress patterns that are obtained from standard flexural fatigue tests. To overcome such limitations, a vibration-based fatigue technique is proposed, which entails the use of sandwich specimens whose geometries are optimized to reproduce the stress distribution observed during three point bend loading while vibrating at the first resonant frequency. The proposed vibration technique was experimentally validated. The results, compared with the average number of cycles to failure at different stress ratios obtained via the Three-Point Bending test, showed high levels of accuracy. The proposed method is robust and time effective and indicates the possibility of attaining fatigue lifetime prediction of a wide class of composite elements, such as sandwich panels.

Lifetime prediction of bonded structural patch repairs for wind turbine pitch bearing strengthening

ABSTRACT There is a growing trend in the use of adhesively bonded patches to repair structures that are subjected to fatigue loads in an attempt to reduce the propagation of an existing crack. The fatigue lifetime prediction of the adhesive used in bonded patch repair is a state-of-the-art challenge, especially when in-service conditions must also be considered. Different approaches have been identified in the literature to analyse structural failure; the methods based on fracture mechanics seem more promising for adhesives because the presence of defects is also considered in the analysis. A considerable amount of work has been undertaken to analyse the delamination of layered structures using cohesive zone modelling (CZM). However, research analysing fatigue delamination in bonded thick-walled structures (> 30 mm) is lacking. The present work proposes a methodology to predict the lifetime of an adhesively bonded patch repair applied to a thick-walled structure, specifically looking at the bearing ring structural fatigue (RSF) life extension of a pitch bearing commonly used in wind turbines.

Fatigue life calculation of notched specimens by modified Wöhler curve method and theory of critical distance under multiaxial random loading

AbstractReformulation of a critical plane‐based multiaxial fatigue criterion named as Modified Wöhler Curve Method (MWCM) in the frequency domain is proposed; moreover, it is combined with the theory of critical distance (TCD) to implement vibration fatigue lifetime prediction of notched specimens subjected to complex multiaxial random loading. Firstly, a frequency‐domain formulation of the maximum variance method (MVM) is proposed for determining the critical plane with high computational efficiency. Then, a reference fatigue curve is defined through the frequency‐domain formulation of the reference stress ratio corresponding to the critical plane. Finally, the power spectral density (PSD) function of the resolved shear stress along the maximum variance direction is applied with the Bendat/Tovo–Benasciutti (TB) method to implement fatigue damage estimation of mechanical components. Both the time‐domain formulation and the proposed frequency‐domain formulation of the MWCM combined with the TCD are applied to the experimental results from the literature, concerning notched specimens made of 2024‐T4 aluminum alloy tested under various nonproportional bending‐torsion random vibration fatigue loading.

High-cycle and very-high-cycle fatigue lifetime prediction of additively manufactured AlSi10Mg via crystal plasticity finite element method

The effects of defects and building directions in additive manufactured AlSi10Mg on its high-cycle fatigue and very-high-cycle fatigue performance are studied based on crystal plasticity finite element (CPFE). Among the three models provided in this work, the results of model 2 (Np2) are in good agreement with the experimental ones. It is found the fatigue life of the sample with defects are much lower than that without defects. 0° specimens have a better fatigue performance than 90° ones. This work is beneficial in determining the fatigue lifetime and helps to improve the fatigue behavior of AlSi10Mg.

On the key role of crack surface area on the lifetime of arbitrarily shaped flat cracks

Accurate fatigue lifetime prediction is a central element in many industries. Often, a damage tolerant approach is used to ensure that some given imperfections are safe. In this context, these lifetime predictions are frequently made using simplified crack geometries leading to costly conservatism. In particular for a closed crack front, a circular or elliptical crack enclosing the real defect is generally used leading to an underestimation of the real lifetime. Using an iterative perturbation method to simulate the fatigue growth of many tensile complexly shaped cracks, we show that in the absence of any additional complexity, (i) the front quickly becomes circular; (ii) its evolution with the number of loading cycles can be obtained analytically using a circular crack of same area as the original distorted defect. In practice, it means that replacing a complex crack front shape with this simple, analytically solvable configuration is an effective way to improve predictions and that it is not useful to invoke more complex shapes such as elliptical cracks. These take-home messages are illustrated by an example from the aviation industry.

Machine learning and finite element analysis: An integrated approach for fatigue lifetime prediction of adhesively bonded joints

AbstractSince fatigue investigations are expensive and time consuming, models capable of predicting lifetime by leveraging existing experimental data are desirable. Here, this task is accomplished by combining machine learning (ML) and finite element analysis (FEA). The dataset contains 365 points comprising four adhesives with four different joint types. The model is fed with four input parameters: stress ratio and stress amplitude (functions of the applied load), and stress concentration factor and multiaxiality, which are obtained from FEA. An extremely randomized trees (ERT) algorithm, capable of dealing with small and noisy datasets, is used to design the model. After calibration, the model's performance was assessed on unseen data and compared with a linear regression model. The ERT predictions yield a significantly smaller error factor (ER) of 2.13 than that of the linear model (ER = 5.89). A relevance analysis shows that at least one FEA‐based parameter must be fed into the model.

Stress analysis and lifetime prediction for Ti–6Al–4V welding joint under fatigue loading

A welded joint has a complex gradient microstructure. The effects of local deformation in the fusion zone, heat-affected zone (HAZ), and other areas, on the macroscopic fatigue properties of an inhomogeneous welded joint, are not fully understood. Thus, it is necessary to correlate the microstructural features and their related local deformation mechanisms with macroscopic fatigue properties. In this study, cyclic stress–strain curves and lifetime model parameters of materials in different HAZs were determined by hardness distribution and base–metal material properties. A user element subroutine was developed in the elastic–plastic finite element analysis to obtain the local stress and strain in the joint. ‘Danger points’ were selected to predict fatigue lifetime with the Manson–Coffin damage equation. The prediction results agreed closely with experimental observations within the low-cycle fatigue lifetime regime. Highlights A new LCF life analysis method, for coated specimens, was developed based on the fracture mechanics. Life prediction method for coated specimens was proposed and verified by LCF tests. Coating-induced lifetime degradation was analysed quantitatively at different temperatures. Effects of stress amplitude on the surface crack initiation were investigated.

Fatigue lifetime prediction with a validated micromechanical short crack model for the ferritic steel EN 1.4003

Microstructural short crack (MSC) growth profoundly influences the fatigue lifetime of steels. For finite element simulations with crystal plasticity, a MSC model for industrial application is described for the ferritic steel EN 1.4003. As transgranular cracking is dominant in the early fatigue stages for this material, intergranular crack growth is excluded. Crack data from micro fatigue specimen is used to validate simulated crack paths. With a manual prescription of the crack initiation site, a substantial agreement between the predicted and experimental crack paths is revealed. Using an appropriate calibration strategy for the parametrization of the MSC model, the feasibility for lifetime prediction is also shown.

Relationship between macromolecular network and fatigue properties of unfilled polychloroprene rubber

Fatigue life of unfilled polychloroprene rubber is characterized in fully relaxing (R = 0) and non-relaxing conditions (R = 0.2) for more than 15 network structures. Network changes are induced by thermal oxidation ageing of the rubber and characterized especially in terms of crosslink density. For the first time, in the case of unfilled elastomers, we show that an increase in crosslink density leads to a decrease in fatigue life, which cannot be attributed to large changes in strain-induced crystallization processes. The results obtained here are used to determine the relationship between network structure and fatigue properties. In fully relaxing conditions, it is possible to predict fatigue lifetime by considering crosslink density using a theoretical energetic approach. Nevertheless, the same approach does not work in non-relaxing conditions. In the latter case, a new empirical relationship is proposed to link fatigue life to the crosslink density of rubber.