High Cycle Fatigue Conditions Research Articles

Bayesian optimization-based prediction of the thermal properties from fatigue test IR imaging of composite coupons

The prediction of the prevailing self-heat transfer parameters of a glass/epoxy composite coupon during fatigue testing in general and the distinction between viscoelastic- and frictional crack growth-related energy dissipation in particular, are not trivial problems. This work investigates the feasibility of predicting the convective film coefficient, the total work loss as well as the ratio between viscoelastic and fracture-induced damping from thermal images using Bayesian optimization in conjunction with 3D FE thermal analysis. To this end, glass fiber/epoxy biax coupons are pre-damaged by means of a drop weight impact machine and subsequently tested under uniaxial tension-tension high cycle fatigue conditions. IR images are taken of the self-heating thermal profile at steady-state conditions. Synthetic surface thermal images are generated by numerical thermal analysis of the damage distribution obtained by μ-CT scanning prior to testing. Bayesian optimization of the aforementioned parameters is conducted by minimizing the loss function between the as-measured and the synthetic IR image. The predicted work-loss is consequently validated against the measured hysteretic response, from which a very good agreement is found.

Microstructure analysis and life prediction of DD6 superalloy under 760 °C combined high and low cycle fatigue conditions

The impact of high and low cycle combined loads on the fatigue performance of DD6 alloy was investigated in this work. Specifically, the combined high- and low-cycle fatigue response of DD6 alloy was examined at a temperature of 760 °C under different low cycle stress amplitudes (960, 1000, and 1040 MPa). The findings revealed that higher low cycle stress amplitudes result in shorter combined high and low cycle fatigue life and less dispersion in life. Correlations between the microstructural features (crystal plane distribution, fracture elevation difference, and the area proportion of crack propagation zone) and fatigue performance were accurately established, demonstrating a linear relationship between these microstructural features and the logarithm of fatigue life. The study also uncovered the evolution rule of microstructural features in high- and low-cycle combined fatigue. Furthermore, the fracture mechanism was analyzed and the distribution of high and low cycle combined fatigue life of DD6 alloy at 760 °C was statistically validated using the dual-parameter Weibull distribution theory. Building upon the crystal plasticity theory, a fatigue life prediction model was developed, and the combined cycle fatigue lifespan under various low cycle stress amplitudes was compared. The comparison between the predicted life and actual fatigue life indicated that the predicted lifespan falls in three times the scatter band.

Life prediction and failure analysis in laser powder bed fused TiC/Ti6Al4V titanium matrix composite under high cycle and very high cycle fatigue conditions

This study investigates the failure behavior of a TiC-reinforced titanium matrix composite (TMC) manufactured by laser powder bed fusion (L-PBF) through fully reversed and pulsating tension fatigue tests in both high-cycle and very-high-cycle regimes. Inhomogeneous hardening zone (IHZ) and un-melted TiC zone (UMTZ) resulting from TiC agglomeration, along with pore and lack of fusion (LOF) generated during the L-PBF process, play a crucial role in inducing fatigue failure and influencing the variability of fatigue life. Advanced microscopy techniques, including optical microscopy, scanning electron microscopy, and electron backscatter diffraction, were employed to elucidate four failure modes: LOF-assisted IHZ cracking, IHZ cracking, pore cracking, and UMTZ cracking. IHZ cracking originates from stress concentration caused by the mechanical property mismatch between IHZ and Ti matrix, leading to brittle fracture of α'-grains within the IHZ. This constitutes the predominant failure mode. The presence of LOF near IHZ further exacerbates stress concentration, diminishing the material's load-carrying capacity and significantly reducing fatigue life. Considering the fatigue cracking mechanism, microstructural characteristics and the influence of defect location and stress concentration, a crack nucleation life prediction model is developed. The agreement between experimental and predicted data is satisfactory.

A systematic review on high cycle and very high cycle fatigue behavior of laser powder bed fused (L-PBF) Al-Si alloys

The recent attention towards additive manufacturing (AM) of metallic alloys, particularly Al-Si alloys, has been significant due to its important role in the design and development of complex structural components in aerospace, defense, and automotive industries. The use of AM technology, specifically Laser powder bed fusion (L-PBF), enables the creation of lightweight structures with flexible geometries that were previously not achievable through conventional manufacturing methods. Al-Si components fabricated through L-PBF are often subjected to long periods of loading in high cycle fatigue (HCF) and very high cycle fatigue (VHCF) conditions. Therefore, it would be extremely important to investigate fatigue characteristics of these components under both HCF and VHCF regimes. However, it is important to recognize that the fatigue performance of L-PBF Al-Si alloys in HCF and VHCF regimes is not fully understood due to the complex nature of fatigue responses in these regimes. Various factors such as microstructures, defects, porosity, and processing parameters, can potentially influence the fatigue behavior in these regimes, further complicating the analysis. Therefore, optimizing process parameters and post-treatments is essential to achieve the optimal fatigue performance for load-bearing components in both HCF and VHCF conditions. In the past few years, several studies have been conducted to investigate the effect of critical parameters on the fatigue performance of L-PBF Al-Si parts in HCF and VHCF regimes. This article aims to provide a comprehensive and in-depth analysis of recent research findings regarding important factors such as residual stress, surface roughness, build parameters (e.g. build orientation), microstructural characteristics, post-processing methods, manufacturing flaws, specimen shapes, load ratios. These factors can have a significant impact on fatigue properties of L-PBF Al-Si alloys, particularly in the HCF and VHCF ranges. In order to realize the full potential of the L-PBF Al-Si alloys in the development of advanced structural components, this study primarily focuses on identifying key areas that require attention.

Mechanical properties of aluminum to steel dissimilar spot joints produced by cold metal transfer weld-brazing

Spot joints between aluminum and steel sheets were produced by cold metal transfer arc welding using an AlSi3Mn alloy low melting point filler wire. The current study found that by using optimal welding conditions, the thickness of the joint interface was significantly reduced compared to previous studies. The interface thickness increased from a few nanometers at the center of weld to 1.7 micrometers (1.2 mm from center of weld) at periphery of weld. The reduction in joint thickness helped to achieve better joint properties. The tensile shear and fatigue properties of the spot joints were evaluated in conjunction with interrupted mechanical tests to determine the fracture mechanisms. This study investigates the fatigue properties and fracture mechanisms of cold metal transfer arc spot joints, which has not been done previously. The tensile shear fractures exhibit primarily interfacial fracture passing through an intermetallic compound layer at the joint interface. However, three different modes of fractures are observed during fatigue tests, including interfacial fracture, sheet fracture, and mixed-mode fracture. Interrupted fatigue tests reveal numerous crack initiation in pores/particles in the deposited weld metal and coagulation of those cracks leads to sheet fracture during the fatigue tests. Sheet fracture mode fractures are mainly observed during high cycle fatigue conditions.

Damage evaluation of the power plants materials based on the AE model of material degradation under high-cyclic fatigue

Modern mechanical engineering products are expected to have an increased lifetime at reduced cost. In order to enable this development, new and improved methods of structural materials strengthening need to be found. Mathematical modelling of the mechanical behaviour of the products in the conditions of fatigue, taking into account various operational, design and technological factors does not provide the necessary accuracy and has to be verified in long-term tests. Therefore, development of a methodology for predicting the limiting state of the structural materials under the long-term periodic load by non-destructive control methods, in particular the acoustic emission (AE) method, remains topical.Within this work, custom-designed model samples of nickel- and titanium-based alloys, as well as carbon and alloyed steel have been studied in comparison. The samples were fabricated both in standard technological process and including ultrasonic surface hardening. Features of cyclic AE characteristics that are identical for different classes of materials with different technological inheritance are described. Based on the obtained results, the relationships between the informative parameters of AE and the criteria of the limiting state of the studied materials for power plants under the high-cycle fatigue conditions are established. This enables assessment of the effectiveness of the technological measures aimed at increasing the life of parts of the power plants, based on non-destructive AE control.

Characteristic Features of Ultrafine-Grained Ti-45 wt.% Nb Alloy under High Cycle Fatigue.

The paper presents the results of fatigue-testing ultrafine-grained and coarse-grained Ti-45 wt.% Nb alloy samples under very high cycle fatigue (gigacycle regime), with the stress ratio R = −1. The ultrafine-grained (UFG) structure in the investigated alloy was formed by the two-stage SPD method, which included multidirectional forging (abc–forging) and multipass rolling in grooved rollers, with further recrystallization annealing. The UFG structure of the Ti-45 wt.% Nb alloy samples increased the fatigue limit under the high-cycle fatigue conditions up to 1.5 times compared with that of the coarse-grained (CG) samples. The infrared thermography method was applied to investigate the evolution of temperature fields in the samples under cyclic loading. Based on numerical morphology analysis, the scale invariance (the Hurst exponent) and qualitative differences for UFG and CG structures were determined. The latter resulted from the initiation and propagation of fatigue cracks in both ultra-fine grained and coarse-grained alloy samples under very high-cycle fatigue loading.

Characterization of Surface Fatigue Crack Nucleation and Microstructurally Small Crack Growth in High Strength Aluminum Alloys

It is well established that fatigue crack nucleation and small crack growth in high strength aluminum alloys are highly influenced by the surrounding microstructure including grain boundaries, texture, inclusion barriers, among other factors. As such, specific and targeted experimental and computational methods are necessary to accurately capture and predict the discrete behavior of microstructurally small fatigue cracks. In this study, surface fatigue crack nucleation and microstructurally small crack growth in high strength aluminum alloys, commonly used in aerospace applications, are evaluated through a holistic approach encompassing fatigue testing, crack measurement, and computational prediction of crack growth rates. During fatigue testing, crack shapes and growth are quantified using a novel surface replication technique that is applied to investigate crack nucleation, as well as to collect validation data that includes an accurate description of crack shape during crack propagation, a challenging and essential component in predicting crack growth. Computational simulation of fatigue crack growth in non-straight, complex surface crack arrays typically requires high fidelity analysis using computationally expensive methods to account for the mathematical and geometrical complexities inherent in the solution. A dislocation distribution based technique has been previously demonstrated to rapidly and accurately predict the stress intensity factors for through cracks of complex shape. This method was expanded and investigated as an approach for rapidly predicting the crack growth rate of kinked and tortuous surface crack arrays, using the crack configuration and bulk material properties as inputs. To investigate the accuracy and effectiveness of this characterization approach, surface crack growth in AA7075-T7351 was experimentally analyzed and modeled under high cycle and low cycle fatigue conditions. This comprehensive approach was determined to be an expedient and applicable method for characterizing and evaluating the nucleation and crack growth rate of non-planar microstructurally small and short crack configurations.

Distributed Fiber Optic Sensor-Based Strain Monitoring of a Riveted Bridge Joint Under Fatigue Loading

Riveted steel bridges, which were built in the early 20th century, require their regular structural integrity assessment to avoid any catastrophic failure. This article presents continuous strain monitoring of a single riveted lap joint, which is a representative critical element of riveted steel bridges through an optical frequency domain reflectometry (OFDR)-based distributed fiber optic sensor (DFOS). The aim of this study was to instrument a DFOS on a single riveted lap joint for monitoring the surface and critical strains experienced by the rivet joint under two fatigue loading conditions and also to compare the strain transfer between the two commonly used adhesives for bonding the DFOS. Initially, through finite element analysis (FEA), a location for installing the DFOS was identified, and also a strategy was developed for monitoring the critical location of the joint during fatigue loading. Subsequently, the DFOS was instrumented on the riveted joint at the identified location in two segments, where similar strain levels were expected with the aid of two types of adhesives: cyanoacrylate and epoxy. The strains on the rivet joint were monitored under high cycle fatigue (HCF) for up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times {10}^{6}$ </tex-math></inline-formula> loading cycles with constant stress amplitude and followed by low cycle fatigue (LCF) loading with increasing stress amplitude until the failure of the specimen. The results showed that the DFOS could continuously sense the cyclic peak strain of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- 223\,\,\mu \varepsilon $ </tex-math></inline-formula> under HCF conditions and a peak maximum strain of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- 1244\,\,\mu \varepsilon $ </tex-math></inline-formula> under LCF conditions. Furthermore, the internal critical strain on the rivet joint during loading could be monitored with the application of the developed damage monitoring strategy and DFOS strain data. Finally, the DFOS segment bonded using cyanoacrylate measured marginally high strains than epoxy adhesive during the HCF test.

Statistical Size Effect in Fatigue Properties for Mini-Specimens.

The study verifies the sensitivity of selected construction materials (S235JR structural steel and 1.4301 stainless steel) to the statistical size effect. The P–S–N curves were determined experimentally under high-cycle fatigue conditions for two specimen sizes (mini-specimen and standard specimen). The results were analyzed using a probabilistic model of the three-parameter Weibull cumulative distribution function. The analysis included the evaluation of the technological process effects on the results based on the material microstructure near the surface layer and the macro-fractography. The differences in the susceptibility to the size effect validated the applicability of the test method to mini-specimen and showed different populations of the distribution of critical material defects.

A probabilistic framework to define the design stress and acceptable defects under combined-cycle fatigue conditions

Probabilistic routines are necessary to determine the permissible design stress when the operational conditions of traditional gas turbines are further challenged to reach higher performances in terms of flexibility. This paper focuses on the definition of a probabilistic tool aimed to simulate the damage development under combined high and low cycle fatigue conditions considering the stochastic nature of some of the variables introduced in the model. The variables considered in the present study are the long crack threshold ΔKth,LC and the slope of the crack growth law C in the so-called Paris region, the endurance limit Δσw0 and the variability of the applied stress as it cannot be considered a fixed variable in the complex loading scenario of gas turbines. A broad experimental campaign was performed to characterize the material properties in terms of short and long crack resistance under constant and combined cyclic conditions. The data were used to define the average and variance of the materials parameters to feed Montecarlo simulations and, at a target probability of failure, establish the design stress. This work points to determine safety margins which strictly depend on the natural variability of the variables that govern the damage accumulation in the mechanical component.

A method for evaluation of weld overlays exposed to combined wear and very high cycle fatigue conditions

Key machinery components used for diminishing, transporting or processing massive volumes of material in heavy industries are severely exposed to combinations of different wear mechanisms, like abrasion and surface fatigue. The microstructure and wear properties of state of the art iron based welding alloys used as surface protection do not only depend on the nominal chemical composition of the welding consumables and their manufacturing route, but also on the base material, variations in process technology and applied welding parameters. Commonly used wear and hardness and test or microstructural investigations are either cost and time consuming or do not provide a complete understanding about the material behavior of surface coatings exposed to combined wear including fatigue. Thus the aim of this study was to prove the concept of using ultrasonic fatigue testing as new method to understand and predict the behavior of welded surface coatings in a fast and reproducible way. Two flux cored wires from different manufacturing lines, but of the same nominal chemical composition were compared applying identical welding parameters. As a result, the two wires could be distinguished clearly by applying the ultrasonic fatigue test method.

Influence of duty cycle on fatigue life of AA2024 with thin coating fabricated by micro-arc oxidation

In this paper, thin micro-arc oxidation (MAO) coatings (5–10 μm) were produced on 2024-T3 aluminum (Al) alloy with duty cycles of 8, 10, 15, and 20%. The objective was to investigate the fatigue properties of the MAO coated and uncoated samples under high cycle fatigue (HCF) and low cyclic fatigue (LCF) conditions. Surface morphology of coatings, interface between the coating and substrate, and fatigue fracture were observed by scanning electron microscopy and metallographic microscope. Three-dimensional (3D) surface morphology of ceramic layers and surface roughness were examined by a 3D laser microscope. Coating phase structure and residual stress were analyzed by X–ray diffraction. In addition, mechanical properties of the coated and uncoated samples were evaluated by static tensile testing. The test results showed that the MAO treatment improved fatigue properties of Al alloy substrate. Residual compressive stress in MAO coating was the reason of increasing fatigue life. However, duty cycles didn't affect the mechanical properties of the substrate. At LCF conditions, the fatigue life of the coated sample at the 20% duty cycle was lower due to the large cracks. In contrast, samples treated with the 8 and 15% duty cycles had inferior fatigue properties at HCF conditions. The concentrated distribution of micro-pores and the cracks on the coating cross-sections were detrimental to HCF life.

Effect of a magnetic field applied during fusion welding on the fatigue damage of 2205 duplex stainless steel joints

The effect of the application of a 3 mT electromagnetic field (EMF) during the welding of a 2205 duplex stainless steel on the resistance to fatigue damage was investigated on samples subjected to high cycle fatigue (HCF) and very high cycle fatigue (VHCF) conditions. Ferrite grain refinement, high volume fraction of regenerated austenite in the heat affected zone and smaller columnar grains in the weld metal were induced by (EMF) application during gas metal arc welding. The improved microstructure evolution during the thermal cycle increased the number of microstructural barriers, hindering the nucleation and growth of microstructurally short fatigue cracks.

The role of slip transfer at grain boundaries in the propagation of microstructurally short fatigue cracks in Ni-based superalloys

Crack initiation and propagation under high-cycle fatigue conditions have been investigated for a polycrystalline Ni-based superalloy by in-situ synchrotron assisted diffraction and phase contrast tomography. The cracks nucleated along the longest coherent twin boundaries pre-existing on the specimen surface, that were well oriented for slip and that presented a large elastic incompatibility across them. Moreover, the propagation of microstructurally short cracks was found to be determined by the easy slip transfer paths across the pre-existing grain boundaries. This information can only be obtained by characterization techniques like the ones presented here that provide the full set of 3D microstructural information.

A new framework based on continuum damage mechanics and XFEM for high cycle fatigue crack growth simulations

In this paper, we have developed a continuum damage mechanics (CDM) based methodology for high cycle fatigue crack growth simulations. A fatigue damage law is proposed and implemented in the framework of extended finite element method (XFEM). A new criterion is proposed based on damage evolution to identify the appropriate definition of stress triaxiality for acquiring the constraint effect on the stress state correctly. Few mesh regularization schemes are also employed for reducing the mesh sensitivity in the results. Simulations are performed on fracture specimens of different materials subjected to constant amplitude fatigue loading. The fatigue life of a turbine disc is also predicted under constant amplitude loading. The results obtained from present methodology (CDM and XFEM) are found in good agreement with the published experimental results. These simulations highlight that the continuum damage mechanics is a simple and effective tool to perform crack growth simulations under high cycle fatigue conditions.

The multiple fatigue crack propagation modelling in nonhomogeneous structures using the DBEM

This study describes a numerical approach for the simulation of multiple crack propagation in two-dimensional nonhomogeneous structures subjected to fatigue. The nonhomogeneous structural system is assumed as composed of piecewise homogeneous and isotropic materials. The high-cycle fatigue condition is assumed, which enables the use of the Linear Elastic Fracture Mechanics (LEFM). The mechanical behaviour is determined by the dual boundary element method (DBEM). The DBEM is a robust and efficient method for handling crack growth analyses because of the non-requirement of the domain mesh. This aspect enables the proper description of the elastic fields surrounding the crack tip. Moreover, it simplifies the remeshing procedures during the crack propagation. To couple adjacent materials forming the nonhomogeneous domain, the sub-region technique is used considering perfectly bonded interfaces. Regarding the fatigue, the crack growth rates are evaluated with the Paris' law. In addition, the structural life is assessed with a scheme based on discrete crack increments. The stress intensity factors at the crack tips are computed with the J-integral technique, whereas the crack propagation angle is defined with the LEFM theory. Four applications are presented to illustrate the accuracy and robustness of the proposed numerical approach for the multiple fatigue crack propagation modelling.

The effect of surface roughness of the substrate on fatigue life of coated aluminum alloy by micro-arc oxidation

In the present work, ceramic coatings are deposited on the surface of 2024-T3 aluminum (Al) alloy by micro-arc oxidation (MAO). The Surface roughness of the Al alloy substrate is different in the range from 0.2 μm to 1.6 μm. This paper investigates the effect of surface roughness of the Al alloy substrate and surface treatment on the fatigue properties of 2024-T3 Al alloy. The morphology, microstructure, and phase composition of the oxide layers are examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Surface roughness of the coatings was measured using a 3D measuring laser microscope. The fatigue life of the Al alloy is evaluated under high and low cyclic stress. The results of the investigation have shown that the fatigue life of the coated samples with Ra = 0.2 μm is higher than others. The fatigue life of coated samples with Ra = 0.8 μm is very close to that with Ra = 1.6 μm under high cycle fatigue conditions. The reason is that the roughness of the interface between substrate and coating affects the fatigue life of coated samples. In contrast, the fatigue property changes when the coated samples were applied to high alternating stress. The fatigue performance of the coated Al alloy with Ra = 0.8 μm is the worst, followed with Ra = 1.6 μm. The rougher surface and the concentrating distribution of hills and valleys lead to this result.

EFFECT OF CONTACT PRESSURE ON FRETTING FATIGUE BEHAVIOR UNDER CYCLIC CONTACT LOADING

In this study, the effect of contact pressure on fretting fatigue behavior of Al7075-T6 under cyclic normal contact loading is investigated. It is found that fretting fatigue life for the case of cyclic contact load was significantly less than that for constant contact load at the same axial and contact load levels, particularly for High Cycle Fatigue (HCF) conditions. The results showed that the fretting fatigue life decreased monotonically with the increase in normal contact load for all axial stresses. Examination of the fretting scars was performed using optical microscopy and numerical simulation was carried out using commercial finite element (FE) codes ABAQUS[Formula: see text] and FRANC2D/L[Formula: see text] to calculate the crack propagation life. The crack initiation life was calculated by a combination of numerical and experimental results. Finally, the FE simulation was validated by a comparison between the numerical crack growth rate and the experimental measurement using replica.

On the Effect of Contact Geometry on Fretting Fatigue Life Under Cyclic Contact Loading

Most of the previous studies on fretting fatigue have been accomplished under constant contact load condition, and a less attention has been paid to cyclic contact load. That is the case in many engineering applications such as bearing cap bolted joints of the V-type engines and dovetail joint of turbines. In this study, the effect of contact geometry on fretting fatigue behavior of Al7075-T6 under cyclic normal contact loading is investigated by experiment, numerical simulation and analysis. Two contact types including flat-on-flat and cylinder-on-flat are considered in this study. The crack initiated at a lower cycle for cylinder-on-flat contacts. It was found that in comparison with the constant contact loading condition, the cyclic contact loadings have more damaging effect on fretting fatigue life, particularly for cylinder-on-flat contact. With the increase in the pad width, fretting fatigue life increased for flat and cylindrical pads, particularly for high cycle fatigue (HCF) conditions. Examination of the fretting scars was performed using optical microscopy. Crack propagation life was determined by numerical simulation using the commercial FE codes ABAQUS® and FRANC2D/L®. Crack initiation life was calculated by a combination of numerical simulation and experiment.