Fatigue Lifetime Research Articles

Lightweight Optimization Design and Fatigue Lifetime Analysis for Anti-rolling Torsion Bar of Rail Vehicle

Based on the fatigue strength assessment method in the FKM standard, the lightweight design of a type of anti-rolling torsion bar is studied by using the finite element method. Firstly, the finite element model of the torsion bar structure is constructed by using the Creo software and the HyperMesh software, and then the finite element analysis is carried out by using the ABAQUS software. The numerical simulation results indicate that under the condition of ultimate load 29.2kN, the static strength, anti-rolling stiffness and fatigue strength of the torsion bar are all satisfy with the design requirements. Secondly, the orthogonal numerical test method is utilized to investigate the influence of geometric parameters on the anti-rolling stiffness and Factor of Strength (FOS) value of the torsion bar. Taking the structural geometric parameters as variables and the FOS value as the objective function, a second-order response surface model is established. The single objective and multi-factor optimization design for the torsion bar under the condition of stiffness constraint is conducted by using the Isight software and multi-island genetic algorithm. The results show that, comparing with the original structure, the weight of the optimized structure is reduced by 20.89%, and the static and dynamic strength, and anti-rolling stiffness of the optimized structure are match with the design requirements. Finally, the static and dynamic strength tests for the optimized torsion bar are individually operated at the ultimate load and under two-stage cyclic loading with amplitude 18.0kN and 29.2kN. the results show that the calculated stiffness coefficient is consistent with the test one, and the service lifetime of the torsion bar is satisfy with the design requirement.

A novel Finite Fracture Mechanics approach to assess the lifetime of notched components

A novel model is proposed to assess the fatigue lifetime of notched components under uniaxial loading conditions in the medium/high-cycle fatigue regime. It is based on the Finite Fracture Mechanics approach, previously proposed in static and fatigue limit frameworks, according to which failure occurs when a stress and an energy condition are simultaneously satisfied. Basquin’s equations are employed to catch the variation of input parameters based on SN data of a plain and a notched sample. The criterion is validated against various experimental data sets from the literature, encompassing different notch shapes, loading conditions and materials.

Probabilistic fatigue estimation framework for aeroengine bladed discs with multiple fuzziness modeling

To reduce the estimation errors of probabilistic fatigue lifetime caused by artificial cognitive factors, a probabilistic fatigue estimation framework with the consideration of multiple fuzziness (i.e., stress and strength) is proposed. In the presented framework, a fuzzy variable randomization-based fuzzy least squares support vector regression is proposed in the level of stress fuzziness, a fuzzy strength model with average stress calibration is established in the level of strength fuzziness, and the corresponding sampling-based probabilistic fatigue estimation framework is given. By regarding a typical compressor bladed disc with titanium-based superalloy as a case, the proposed framework is validated. Methods comparison shows that the proposed approach holds the highest estimation accuracy compared with the methods that do not consider fuzziness of stress or strength. The current efforts develop a novel approach to evaluate the probabilistic fatigue lifetime by fuzzy set theory, which also sheds a light on the reliability-based fatigue design of complex structures.

Experimental Study of the Impact of Notches Made in the Front Edge of Adherends on the Properties of Static and Fatigue Strength of Adhesive Joints

This paper presents the effect of simple structural changes consisting in making notches on the edge of adherends on the static and fatigue properties of adhesive joints. The study is based on the set of experimental static strength tests of the single lap shear adhesive joints. It has been shown that a slight increase in load capacity is possible by using adhesive-filled notches. However, this structural modification leads to an increase in the standard deviation of test results. Based on the fatigue strength tests done at the limit number of cycles equal to 2×106, it was shown that the use of notches on the front edge of adherends, which are filled with adhesive, can significantly increase the fatigue lifetime of the joint. The effect is shown in the area of low-cycle fatigue, where for fatigue stress with a maximum value of 11.5 MPa, an increase in the average value of fatigue lifetime by 481.5% was shown for joints with notches. On the basis of fractography analyses using SEM microscopy, it was shown that the positive effect of the notches is caused by the absorption of some energy by the adhesive filling the notches, due to the increased susceptibility of the adhesive in this area to elastic deformations.

Fatigue Assessment of Carbon Fiber-Reinforced Polyurethane with Regard to Crack Initiation and Propagation

Due to their lightweight potential, the use of fiber-reinforced polymers is the current standard for many technical fields of application. Especially, the automotive and aerospace sectors are to be emphasized. This entails a sophisticated knowledge regarding the material properties, since the safety standards applied in these fields are of high importance. To ensure the safety of the components, a detailed mechanical material characterization is indispensable. The aim of this work was to investigate different influencing factors on the fatigue behavior of carbon fiber-reinforced polyurethane, which is to be certified for aviation applications. Tensile tests provided a basic understanding of the material properties, which appeared to be affected by the specimen width, varied from 3 to 25 mm, by up to 30%. Subsequently, the influence of the cutting direction was investigated in the course of the fatigue tests. Thus, the fatigue strength of longitudinally cut specimens was found to be higher than that of transversely cut specimens by 6%. By means of specific measurement technologies, the material responses were associated with crack initiation and propagation during the fatigue lifetime. The material properties, such as the thermoelastic effect, could be examined during the fatigue tests. Furthermore, turning points in the courses of the characteristic values of the material and correlations with local phenomena were identified.

Fatigue of short carbon fiber reinforced PEEK under compression: Influence of the load ratio and predictions from heat buildup measurements

AbstractFew works investigate the fatigue of short fiber reinforced thermoplastics under cyclic compression. Furthermore, the heat buildup approach to quickly predict the fatigue lifetime of these materials has not yet been investigated for compression–compression loadings. This paper first describes an experimental protocol to perform well‐controlled cyclic compression tests without anti‐buckling device that may induced bias. The uniaxiality of the global loading is checked during the test based on kinematic and thermoelastic coupling field measurements. Then, the paper compares the results from fatigue and heat buildup tests performed on polyetheretherketone reinforced with 30 wt% of short carbon fibers for several load ratios. It is shown that the fatigue lifetime is higher in compression than in tension and depends on the compression load ratio. The heat buildup approach appears to be relevant to quickly predict the fatigue lifetime of the material for the different loading ratios studied, including compression–compression.

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.

Influence of the Inherited Structure Induced by Al and Si Alloying on Microstructure Evolution and Mechanical Properties of 100Cr6 Steels

The fatigue lifetime of high‐strength 100Cr6 steels can be improved by an increased content of retained austenite that can be induced by Al and Si alloying. The related deformation‐induced retention of the austenite–martensite transformation during cyclic loading increases their local strain hardening capacity. However, for those 100Cr6 steels containing retained austenite, sufficient dimensional stability must be ensured. In this study, two standard 100Cr6 steels alloyed either with 1.5 wt% Al or 1.5 wt% Si (to diminish carbide formation and accordingly promote austenite retention) are laboratory melted and processed to adjust a microstructure of bainite, retained austenite, and carbides. The segregation simulation in the as‐cast condition and the corresponding microstructures in the forging and heat‐treating conditions are investigated. The inheritance of chemical heterogeneity leads to structural heterogeneity on both the nano‐ (nm) and micro (μm) scales. This heterogeneity is much more pronounced in the Al‐alloyed steel, which can be attributed to inheritance from the as‐cast state. While the results of the quasistatic tensile tests are comparable for both alloys, the cyclic load increase tests indicate a higher fatigue strength of the Si‐alloyed steel, which can be explained with the more homogenous microstructure and the finer distribution of the retained austenite.

Two-Scale analysis of fretting fatigue crack initiation in heterogeneous materials using Continuum Damage Mechanics

The phenomenon of fretting fatigue damage is related to tribological behaviour and commonly occurs at the contact interface between two bodies that sustain oscillation loads. Based on the complicated mechanical situation, several methods have been developed to predict the crack initiation location and life. The Continuum Damage Mechanics (CDM) approach in conjunction with Finite Element Method (FEM) is used in this paper. The fretting fatigue lifetime includes two parts, crack initiation lifetime and crack propagation lifetime, and this paper focuses on the crack initiation phase. Homogeneous materials are usually studied in fretting fatigue problems, and the influence of defects is necessary to be considered. In this study, the authors proposed several models to consider the effect of heterogeneity on crack initiation location and lifetime. The numerical model is a 2D one with plane strain assumption. To get more accurate numerical results with low computational cost, a Two-Scale Analysis approach is developed in this paper. Two macroscopic models and three microscopic models are proposed, including a Macroscopic homogeneous model (Macro-HOMO), a Multiscale Homogenization model with Direct Numerical Simulation (MH-DNS), a Microscopic homogeneous model (Micro-HOMO), a Microscopic Model with equivalent material properties by using Mean Field Homogenization method (Micro-MFH), and a microscopic model with Direct Numerical Simulation (Micro-DNS). The results obtained by using the CDM approach with different microscopic models are compared, and show that the prediction accuracy of the Micro-DNS is the best. In addition, in terms of computational cost, the Micro-DNS model shows great advantages as well comparing to the Macro-DNS model.

Fractographical quantitative analysis of EN-AW 2024 aluminum alloy after creep pre-strain and LCF loading

This paper explores the applicability of a new damage parameter combining both fracture surface topography and loading features to estimate the fatigue lifetime under creep pre-strain and low-cycle fatigue loading. Fractures of EN-AW 2024 aluminum alloy caused by mixed creep and low-cycle fatigue loading are experimentally characterized and quantified via surface topography analysis. The specimens were preliminary damaged in a creep process at elevated temperature (100 °C, 200 °C, or 300 °C) considering two strain levels for each temperature. The specimens with preliminary damage were then subjected to fatigue tests at room temperature. The post-failure fracture surfaces were examined with an optical profilometer, and the focus was placed on the relationship between the creep-fatigue damage and the associated fracture surface topography parameters. In addition, an original fatigue life prediction model based on both the surface topography values and the applied strains was proposed and verified. The proposed model can be helpful for post-mortem analysis of engineering components subjected to fatigue loading accounting for creep pre-strain.

Defect-related strain-controlled high-temperature fatigue behavior in additive manufacturing Hastelloy X assisted with ultrasonic micro-forging treatment

Due to the defects, the performance of laser directed energy deposition (LDED) parts was restricted. This work investigated the strain-controlled high-temperature fatigue behavior of LDED Hastelloy X after ultrasonic micro-forging treatment (UMFT). Results showed that UMFT significantly increased the fatigue life twice; mathematical statistical analysis showed that the size of the fatigue source (defect) and the fatigue loading conditions accounted for the fatigue lifetime of the LDED Hastelloy X; based on the organic combination of Smith-Watson-Topper (SWT) and Stress intensity factor (SIF) models, a new defect-related strain-controlled fatigue life prediction model was proposed with the correlation coefficient of 0.93.

Data-Driven, Physics-Based, or Both: Fatigue Prediction of Structural Adhesive Joints by Artificial Intelligence

Here, a comparative investigation of data-driven, physics-based, and hybrid models for the fatigue lifetime prediction of structural adhesive joints in terms of complexity of implementation, sensitivity to data size, and prediction accuracy is presented. Four data-driven models (DDM) are constructed using extremely randomized trees (ERT), eXtreme gradient boosting (XGB), LightGBM (LGBM) and histogram-based gradient boosting (HGB). The physics-based model (PBM) relies on the Findley’s critical plane approach. Two hybrid models (HM) were developed by combining data-driven and physics-based approaches obtained from invariant stresses (HM-I) and Findley’s stress (HM-F). A fatigue dataset of 979 data points of four structural adhesives is employed. To assess the sensitivity to data size, the dataset is split into three train/test ratios, namely 70%/30%, 50%/50%, and 30%/70%. Results revealed that DDMs are more accurate, but more sensitive to dataset size compared to the PBM. Among different regressors, the LGBM presented the best performance in terms of accuracy and generalization power. HMs increased the accuracy of predictions, whilst reducing the sensitivity to data size. The HM-I demonstrated that datasets from different sources can be utilized to improve predictions (especially with small datasets). Finally, the HM-I showed the highest accuracy with an improved sensitivity to data size.

Morphology and mechanical performance of dental crown designed by 3D-DCGAN.

This study utilised an Artificial Intelligence (AI) method, namely 3D-Deep Convolutional Generative Adversarial Network (3D-DCGAN), which is one of the true 3D machine learning methods, as an automatic algorithm to design a dental crown. Six hundred sets of digital casts containing mandibular second premolars and their adjacent and antagonist teeth obtained from healthy personnel were machine-learned using 3D-DCGAN. Additional 12 sets of data were used as the test dataset, whereas the natural second premolars in the test dataset were compared with the designs in (1) 3D-DCGAN, (2) CEREC Biogeneric, and (3) CAD for morphological parameters of 3D similarity, cusp angle, occlusal contact point number and area, and in silico fatigue simulations with finite element (FE) using lithium disilicate material. The 3D-DCGAN design and natural teeth had the lowest discrepancy in morphology compared with the other groups (root mean square value = 0.3611). The Biogeneric design showed a significantly (p<0.05) higher cusp angle (67.11°) than that of the 3D-DCGAN design (49.43°) and natural tooth (54.05°). No significant difference was observed in the number and area of occlusal contact points among the four groups. FE analysis showed that the 3D-DCGAN design had the best match to the natural tooth regarding the stress distribution in the crown. The 3D-DCGAN design was subjected to 26.73MPa and the natural tooth was subjected to 23.97MPa stress at the central fossa area under physiological occlusal force (300N); the two groups showed similar fatigue lifetimes (F-N curve) under simulated cyclic loading of 100-400N. Designs with Biogeneric or technician would yield respectively higher or lower fatigue lifetime than natural teeth. This study demonstrated that 3D-DCGAN could be utilised to design personalised dental crowns with high accuracy that can mimic both the morphology and biomechanics of natural teeth.

Jk-integral applied to mixed-mode fatigue crack propagation and life prediction in metal welding interface

In this paper, an innovative fatigue crack propagation model of metal welding interface is investigated based on the concept of Jk-integral (k = 1, 2) in elastic–plastic material, including the prediction of crack propagation path and life. In order to validate the effectiveness of the present fatigue model, the experiments of mixed-mode interface fatigue cracks are carried out on compact tensile shear specimens of N36 zirconium alloy using DIC methods. The results demonstrate that the fatigue crack only propagates along the interface under mode I loading where the relationship between the crack growth rate da/dN and ΔJ1 (the J1-integral range per loading cycle) is logarithmic linear. As for mode I/II mixed-mode loading, interface cracks start to propagate along the interface when J1 reaches Jint-C (the interface fracture toughness). When Jtotal (the vector sum of J1 and J2) reaches Jsub-C (the substrate fracture toughness), the crack deflects to the substrate in the Jtotal direction. In this case, da/dN shows a two-stage logarithmic linear relation with ΔJtotal (the Jtotal-integral range per loading cycle) under mixed-mode loading, and the critical Jtotal at the beginning of crack deflection is approximately matching the specific value. The theoretical deflection angle (i.e. the Jtotal direction) calculated from the corresponding J1 and J2 at the beginning of crack deflection is wellconsistentwiththe actual deflection angle observed in experiments. Moreover, the model can clearly describe the two-stage process from the crack propagation along the interface to the crack deflection to the substrate under mixed-mode loading. It is concluded that the proposed fatigue model based on Jk-integral can accurately predict the fatigue crack propagation path and lifetime of metal welding interface.

Metal–Matrix Composites from High‐Pressure Torsion with Functionalized Material Behavior

In composites, outstanding properties of two materials can be combined. In particular, metal–matrix composites (MMCs) can combine the properties of a high‐strength ductile metallic matrix with special properties of embedded ceramic particles. This hybrid can be used to create a functional material. However, during consolidation, the thermal load of most common MMC‐processing routes is an obstacle for such functionalization, because the unique properties of the ceramic phases most likely degrade. Mechanical alloying, in this case, by high‐pressure torsion (HPT), can overcome this challenge. Herein, the attempt to obtain smart materials through HPT processing is aimed. For that purpose, Cu‐MMCs are produced from mixed powders with and (BTO) with the challenge to incorporate their functional phase. BTO can provide a sensing ability for internal stress and can provide a fatigue lifetime by a retarded crack growth. The amount of the stabilized phase is evaluated by X‐ray diffraction. Cu–BTO–MMCs exhibit a local piezoelectric effect when strained, shown by in situ scanning Kelvin probe force microscopy. Cu––MMCs feature a retarded fatigue crack initiation and an earlier crack closure during fatigue crack growth due to the volume expansion once transforms.

Fatigue analysis of metallic-plastic-metallic pipeline systems: A numerical study

Metallic (e.g., copper, steel and iron) and plastic (e.g., Polyethylene and Polyvinyl chloride) pipelines are often used in combination in water distribution networks (WDSs). The interface between a metallic pipe section and a plastic pipe section is typically associated with a significant change in hydraulic impedance (i.e., impedance mismatch). This impedance mismatch can result in significant pressure wave reflections during transient events, and multiple interfaces may induce repeated pressure waves bouncing in between. Limited studies have been conducted on the impact of these multiple wave reflections due to impedance mismatch, especially with respect to the risk of accelerated fatigue damage to the plastic pipe. The research presented in this paper investigates the pressure response of a metallic-plastic-metallic (M-P-M) pipeline system under repeated hydraulic transient events using analytical and numerical methods. The presence of additional cyclic hoop-stress loadings in the M-P-M configuration, both in frequency due to reflections and in loading magnitude due to superposition, has been demonstrated in comparison to a fully plastic pipe system under the same hydraulic transient event. The study investigates the impact of these additional loadings on the relative fatigue lifetime of the plastic section by adopting a fatigue analysis model based on the stress-life method. The results show that the plastic section in the M-P-M configuration can have a significant reduction in fatigue lifetime compared to the corresponding fully plastic configuration.

A Condition Evaluation Simplified Method for Traction Converter Power Module Based on Operating Interval Segmentation

In the actual operation of urban rail vehicles, it is essential to evaluate the condition of the traction converter IGBT modules. Considering the fixed line and the similarity of operation conditions between adjacent stations, this paper proposes an efficient and accurate simplified simulation method to evaluate IGBT conditions based on operating interval segmentation (OIS). Firstly, this paper proposes the framework for a condition evaluation method by segmenting operating intervals based on the similarity of average power loss between neighboring stations. The framework makes it possible to reduce the number of simulations to shorten the simulation time while ensuring the state trend estimation accuracy. Secondly, this paper proposes a basic interval segmentation model that uses the operating conditions as inputs to implement the segmentation of the line and is able to simplify the operation conditions of entire line. Finally, the simulation and analysis of the temperature and stress fields of IGBT modules based on segmented intervals completes the IGBT module condition evaluation and realizes the combination of lifetime calculation with actual operating conditions and internal stresses. The validity of the method is verified by comparing the interval segmentation simulation with actual test results. The results show that the method can effectively characterize the temperature and stress trends of traction converter IGBT modules in the whole line, which could support the fatigue mechanism and lifetime assessment reliability study of IGBT modules.

On the effect of coatings on the tensile and fatigue properties of 7075-T6 aluminum alloy monitored with acoustic Emission (AE): Towards lifetime estimation

Acoustic Emission (AE) is used to monitor the fatigue behaviour of specimens made of aluminium alloy 7075-T6 and covered with different coating combinations: anodic oxidation (alumina) - primer layer (organic coating) - topcoat layer (organic coating)). AE monitoring allows to understand and differentiate the processes and damage mechanisms in action during fatigue for all types of specimens. The type of coating applied has a significant influence on the damage process and the fatigue lifetime of the material. AE-based indicators are defined to assess the overall damage state of the specimens during fatigue testing. Some indicators are based on the distinction of the acoustic activity acquired during the loading and unloading phases of the cycles. As an example, the joint analysis of these indicators on anodized samples allows to highlight characteristic times ranging from 15% to 70% of the specimen’s lifetime. The early time at 15% is related to the saturation of cracking of the oxide layer during the first cycles of fatigue tests. By 70% of failure life, the AE activity correlates with the propagation of the main crack in the substrate, assisted by brittle fracture of the surrounding oxide layer. These specific times provide information on the Remaining Useful Lifetime (RUL) of the material and could be used to anticipate future failure.

Modelling fatigue behaviours and lifetimes of novel GLARE laminates under random loading spectrum

This paper aims to experimentally and numerically probe fatigue behaviours and lifetimes of novel GLARE (glass laminate aluminium reinforced epoxy) laminates under random loading spectrum. A mixed algorithm based on fatigue damage concepts of three-phase materials was proposed for modelling progressive fatigue damage mechanisms and fatigue life of fibre metal laminates (FML) under random loading spectrum. To validate the proposed modelling algorithm, fatigue tests were conducted on the GLARE 2/1 and GLARE 3/2 laminates subjected to random loading spectrum, and fatigue mechanisms were discussed by using scanning electron microscope (SEM) analysis. It is shown that predominant fatigue failure of the GLARE laminate depends on the reference load level of random loading spectrum. Specifically, dominant fatigue failure of the GLARE laminate is dependent on fatigue strength of fibre layer at a high reference load level, but metal layer at a low reference load level. Numerical predictions agree well with experimental results, demonstrating that the proposed mixed modelling algorithm can effectively simulate fatigue behaviours and lives of the GLARE laminate under random loading spectrum.

Mesoscopic defect healing and fatigue lifetime improvement of 6061-T6 aluminum alloy by electropulsing treatment

Defect healing through pulsed electric current treatment is an important work worthy of further discussion. 6061-T6 aluminum alloy specimens were subjected to the fatigue loading and pulsed electric current treatment. Then mesoscopic defect features inside 6061-T6 aluminum alloy before and after applying electric pulses were analyzed through X-ray microcomputer tomography and 3D reconstruction software VGSTUDIO MAX. The influence weights of pulsed electric current on the mesoscopic defect were obtained by principal component analysis based on the reduction rate of defect feature parameters. A parametric model was established to obtain the defect information of pulse current healing based on the influence weights obtained. The fatigue life prediction is carried out using this model, and the verification is conducted by experiment work.