High Cycle Fatigue Life Research Articles

Damage Proportion Analysis of the Combined High and Low Cycle Fatigue Based on the Continuum Damage Mechanics

ABSTRACTA combined high and low cycle fatigue (CCF) life prediction model, considering creep, low‐cycle fatigue (LCF), high‐cycle fatigue (HCF) at the maximum LCF nominal stress (HLCF) damages, and the interaction damage between LCF and HLCF, is proposed. The proportions for these four types of damage in total CCF damage are quantified. Compared with the existing CCF models, the CCF model proposed in this paper considers creep damage in CCF and has higher accuracy. The escalation in HCF stress amplitudes leads to reduced CCF life owing to the augmentation of HLCF and interaction damages. The increase in the cycle ratio of HCF to LCF, causing a reduction in CCF life is ascribed to elevated HLCF and creep damages. The calculated damage proportion results from the proposed model are consistent with the observations of the fracture characteristics using a scanning electron microscope (SEM), indicating the necessity of considering creep damage in CCF life prediction at high temperatures.

The study on the high cycle fatigue performance and life prediction of Ti–6Al–4V alloy fabricated by laser engineered net shaping

The study on the high cycle fatigue performance and life prediction of Ti–6Al–4V alloy fabricated by laser engineered net shaping

Critical physics-informed fatigue life prediction of laser 3D printed AlSi10Mg alloys with mass internal defects

The significant scatter in high cycle fatigue life of additively manufactured metallic components presents an increasing challenge to structural integrity. This fatigue life variation is radically attributed to the differences in physical features of critical defects that lead to crack initiation. To address this issue, this paper proposes an integrated framework for identifying critical defects and predicting fatigue life using physics-informed machine learning, with a focus on the impact of 3D defect features. By employing X-ray tomography, high cycle fatigue tests, and fractography analyses on post-mortem specimens, a dataset associated with mass internal defects is first built up to correlate the spatial geometric features of critical defects with fatigue life. A kernel support vector machine is then used to formulate a critical defect identification model, aimed at identifying critical defects among numerous defects by evaluating their geometric attributes. Finally, a fatigue life prediction model is developed using a physics-informed neural network, which incorporates the influence of defect geometry on fatigue life as physical constraints in the loss function. The integrated framework demonstrates that fatigue life predictions from identified critical defects in each specimen exhibit small deviations, with the average prediction falling within twice the error bands. This study is expected to provide a valuable reference for fatigue assessment of additively manufactured components through sequential critical defect identification and fatigue life prediction.

Surface integrity and high-cycle fatigue life of direct laser metal deposited AISI 431 alloys modified by plasticity ball burnishing

Laser metal deposition (LMD) as an additive manufacturing (AM) is widely used to repair and extend wear and fatigue life of the critical components. This paper has investigated the application of ball burnishing (BB) to improve surface integrity and high-cycle fatigue resistance of LMDed AISI 431 alloys. Results showed that the BB treated samples exhibited significant surface finish improvement by lowering roughness by 91 %. Microhardness increased from 490 to 530 HV0.1, an increase by 10 % with a modified depth of 400 µm from the top surface. XRD results showed a peak shift and increase in FWHM by up to 17 %. This had been corroborated by EBSD exhibiting a 20 % increase in dislocation density and 24 % increase in localised misorientation within microstructure. As a result, the overall high cycle fatigue strength of the burnished sample increased by 50 %, and the cracks initiated from sub-surface level defects at a depth of 350 μm below the top surface, delaying the crack propagation and fracture failure. The findings clearly highlight that the burnishing treatment can be a plausible approach in improving the dynamic fatigue resistance and the overall service life of LMDed AISI 431 steel alloys components in engineering applications.

A physics-informed deep learning approach for combined cycle fatigue life prediction

The prediction of the combined high and low cycle fatigue (CCF) life of welded components was valuable for the reliability of practical engineering structures. In this study, a physics-informed Grey-deep convolutional neural networks (DCNN) model was constructed and the model parameters was determined based on the fatigue behavior characteristics of CCF, which was proved perform well in the CCF life prediction of welded components. The predicted results indicated that the proposed model can realize reliable CCF life prediction with good accuracy (MAE = 0.49) and stability (RMSE = 0.44). Further, the proposed model also exhibited good prediction performance under different loading and geometry conditions, indicating the good universality of the proposed model. And the good universality represented that the proposed model had the potential to be applied in more engineering fields.

Effect of coating-substrate microstructure on nucleation and growth of surface microcracks in a PtAl-coated Ni-based SX superalloy with sheet specimens during HCF at 900 °C

The fatigue crack initiation process accounts for the main portion of the high-cycle fatigue (HCF) life. Although coated superalloys are well-known to be prone to surface microcracks, the influence of the coating-substrate microstructure on the damage mechanism during HCF of the coated single crystal (SX) superalloys remains unclear. In the current study, HCF failure and interrupted tests were conducted at 900 °C under a low applied maximum stress on a PtAl-coated third-generation SX superalloy using sheet specimens. The HCF crack initiation was investigated by analyzing the evolution of surface microcracks and coating-substrate microstructure. The results show that the surface microcracks nucleation and growth are controlled by the initial microstructure of the coating-substrate region and the oxidation process during the HCF crack initiation stage. The rapid nucleation and growth of the surface microcracks are promoted by grain boundaries (GBs) in the coating and the interdiffusion zone (IDZ). Comparatively, the surface microcracks are blunted by oxidation in the SX substrate due to the loss of GBs, leading to a slower microcrack growth. However, the surface microcracks can gradually grow further into the substrate by repeating the processes as follows: oxidation at the crack tip, formation of γ′-depleted region due to oxidation, recrystallization at the γ′-depleted region and cracking along the GBs in the recrystallized γ′-depleted region. Subsequently, the surface microcracks, which reach the critical size, propagate along the slip bands during the progressive propagation stage. This study will contribute to improve the HCF durability of the PtAl-coated SX superalloy.

Investigation on the Very High Cycle Fatigue Life of Titanium Alloys by Near-β Forging and Shot Peening

Investigation on the Very High Cycle Fatigue Life of Titanium Alloys by Near-β Forging and Shot Peening

High Temperature Fatigue of Additively Manufactured Inconel 718: A Short Review

Abstract With the increasing interest in adopting additively manufactured (AM) IN718 for high-temperature applications, driven by the design and manufacturing flexibility offered by AM technologies, understanding its fatigue performance is crucial before full-scale adoption. This paper reviews the recent literature on the high-temperature fatigue behavior of AM IN718. The review focuses on two primary stages of fatigue damage: fatigue crack initiation and fatigue crack growth. Notably, most existing studies have concentrated on fatigue crack initiation, and thus, this review emphasizes this aspect. In the fatigue crack initiation stage, discrepancies in low cycle fatigue (LCF) and high cycle fatigue (HCF) life performances are observed in the literature. Some studies have shown that the average room temperature (RT) fatigue life of AM IN718 is superior or comparable to that at high temperatures in the LCF regime. Conversely, in the HCF regime, high-temperature fatigue life is sometimes found to be superior to that at room temperature. However, other studies indicate no clear trend regarding the effect of temperature on HCF life. Although various mechanisms have been proposed to either improve or degrade fatigue performance across the LCF, HCF, and very high cycle fatigue (VHCF) regimes, the underlying reasons for the distinct behaviors in these regimes remain unclear. Competing mechanisms, such as surface oxide formation and thermally driven dislocations glide, can potentially enhance or reduce fatigue life. However, the interaction and control of these mechanisms over the fatigue strength of AM IN718 are not yet fully understood. Systematic studies are required to elucidate their roles in high-temperature fatigue. Microstructural investigations have suggested that controlling the formation and precipitation of deleterious secondary phases is crucial for tailoring the high-temperature fatigue strength of AM IN718. Therefore, it is imperative to design heat treatment protocols informed by a comprehensive understanding of phase formation kinetics to improve the high-temperature fatigue performance of AM IN718 compared to their traditionally manufactured (TM) counterparts. This is particularly important for IN718 parts manufactured using directed energy deposition technology, which currently lacks standardized heat treatment procedures. The review also identifies open research areas and provides recommendations for future work to address these gaps.

A modified SWT model for very high cycle fatigue life prediction of L-PBF Ti-6Al-4V alloy based on Single Defect: Effect of building orientation

The ultrasonic fatigue tests on laser-powder bed fused (L-PBF) Ti-6Al-4V specimens manufactured at 0°, 45°, and 90° building orientations were conducted. It is found that the very high cycle fatigue (VHCF) lives decrease with the building orientation from 0°, 45°, to 90°. All the fatigue cracks initiate from internal pores or Lack of Fusion defects. Mode Ι cracks grow perpendicular to the direction of the maximum opening stress, regardless of the building orientation. The maximum prospective defect evaluated by the extreme value statistics (EVS) method is used as the typical defect for VHCF life prediction. A modified Smith-Waston-Topper (SWT) model was proposed and combined with the Critical Distance Method to predict ultra-high cycle fatigue life. The life prediction accuracy was validated by using both experimental data obtained from the experiments conducted in this paper and experimental data taken from the literature. The results show that 84% of the predicted fatigue lives are within a scatter band of 2 standard deviation, and 16% of the predicted lives are outside a scatter band of 2 standard deviation and within a scatter band of 3 standard deviation.

High cycle fatigue life analysis of unidirectional flax/PLA composites through infrared thermography

The durability of structural components under fatigue loading is a huge concern, especially in laminated composites. During the last few years, flax fibers have also gained an outstanding position as a reinforcement of biopolymeric matrices as polylactic acid (PLA). However, there is limited reported information regarding the fatigue behavior of unidirectional flax/PLA composites. Therefore, fatigue properties of unidirectional flax/PLA composites are evaluated in this work; tools such as infrared thermography and dissipated energy were used in order to establish the fatigue limit, and fracture surface was analyzed. Results show consistent and reliable tensile properties (σut = 234.4 MPa, E = 20.56 GPa, and εf = 0.0181). The fatigue stress-cycle curve was established and fitted to the Basquin and Weibull fatigue models and the fatigue limit (σ∞) was obtained as 0.4343 and 0.426 using the thermography and dissipated energy, respectively. Furthermore, the fatigue fracture surface presents a striation on the matrix due to the progressive crack propagation.

Fracture mechanism of Ti60 alloy at high temperature in very high cycle fatigue regime and fatigue life prediction

AbstractThis study aimed to investigate the failure mechanism of Ti60 titanium alloy in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) at 500°C. Ti60 specimens were characterized before and after the 500°C VHCF test, and the fracture surfaces were observed. The results show that there are three different fatigue failure mechanisms at 500°C: (i) equiaxed primary α phase (αp) cleavage, forming small unsmooth facets and rough fracture area (RA), and fusion of non‐adjacent facets to grow into the main crack. (ii) αp grains gather into large grain with triple junction, and large grain slip to form large fusion facet. (iii) Oxide shedding and then cracks form. A dynamic recurrent neural network model is used to predict the fatigue life of Ti60 alloy, 91% of the overall predictions were within the scatter band of 3.0, and 100% were within the scatter band of 5.0.

Effects of artificial embedded defect size and position on the fatigue lives of additive manufacturing Ti–6Al–4V alloy

Additive manufacturing (AM) components usually contain various defects with different positions and sizes inside, resulting in the great change of fatigue performance with the defect location and sizes. Therefore, in the evaluation of numerous structural factors, the defect size effect is one of the most typical problems in the AM components. This study for the first time investigated the effects of sizes and positions of the artificial embedded defects on the high-cycle fatigue (HCF) lives of the most widely studied AM Ti–6Al–4V alloy. Samples with artificially embedded defects of two sizes (300 μm and 500 μm) and three positions (center, 1.5 mm from the center and 0.5/0.2 mm from the center) for fatigue tests. The result show that the larger the defect size and the closer it is to the surface, the lower the fatigue life. Through fractographic observations, fatigue lives analysis, and defects statistics, the influences of defect size and position on the HCF lives were discussed. It is found that the Murakami formula can well describe the HCF lives distribution of the AM Ti–6Al–4V alloy with different artificial embedded defects.

A physics‐informed neural network framework based on fatigue indicator parameters for very high cycle fatigue life prediction of an additively manufactured titanium alloy

AbstractThe exploitation of fatigue life prediction methods based on fatigue indicator parameters revealed the influence of the defect size, position, and morphology on the fatigue life and fatigue behavior of additively manufactured metals. Meanwhile, Data‐driven life prediction methods are time‐efficient but inexplainable. Current machine learning‐based fatigue life prediction methods call for not only the accuracy but also the interpretability and stability of prediction results. Thus, the fusion of physical methods and machine learning methods has been a prevailing research topic in fatigue life prediction. In this study, a novel physics‐informed neural network framework is proposed by integrating a fatigue indicator parameter based on defects into the physical constraints term of a loss function. This method outperforms conventional machine learning methods in high‐cycle and very high‐cycle regimes, exhibiting superior prediction performance and generalization ability. Furthermore, the prediction results can be explained from a physical standpoint, correlating with the applicability range of the introduced physical equation describing defect positions.

Quantitative assessment of microstructural damage for interior crack initiation and high cycle fatigue life modeling

Axially loaded cyclic tests of a nickel-based alloy were conducted at 550 °C, 630 °C and 650 °C and stress ratios of −1, −0.3 and 0.5 up to high cycle fatigue regime. Multiscale characterizations and quantitative analyses of microstructural damage were carried out to illustrate the physics and mechanics of interior microstructure induced fatigue strength degradation and cracking. Results showed that the fatigue strength increased with the decreases of twin spacing, the size and aspect ratio of carbide, and γ′ phase size. Twin collapse due to slip band-twin interaction was found for the first time and confirmed as a form of crack initiation process, followed by short crack growth forming facets with significant contribution from shear stress. A new mechanistic fatigue life model of interior microstructure induced cracking was established with sound agreement with experimental data. All these combined into a design-manufacturing integrated approach which are capable of ensuring the safe operation and maintenance of service structures in long-life regime.

Effect of PtAl coating on the HCF property of a coated third-generation single crystal superalloy with sheet specimens at 900 ℃

High-cycle fatigue (HCF) is one of the primary failure modes for turbine blades in aero-engines. Therefore, comprehending the effect of coating on the HCF property of single crystal (SX) superalloys is vital for the safe service of turbine blades. In order to mimic the degradation of the fatigue property for the turbine blades during service, this study investigated the effect of PtAl coating on the HCF property of a third-generation Ni-based SX superalloy using sheet specimens at 900 ℃ and different maximum stresses (σmax). The results indicated that both coating and σmax affected the fatigue properties and crack initiation process of SX superalloy. A transition in fatigue crack initiation site from the internal micropores to the surface cracks for both uncoated and coated alloys has been observed as the σmax decreased. However, the coating significantly increased the σmax required for the transition of the crack initiation site. Meanwhile, the HCF property of the SX superalloy was reduced with the deposition of PtAl coating at lower σmax, and the debit in HCF life was more pronounced with decreasing the σmax. The coating facilitated the rapid nucleation of surface cracks and promoted the transition of the fatigue crack initiation site under higher σmax. On the other hand, since the HCF life is controlled by the crack growth rate, grain boundaries in the interdiffusion zone (IDZ) and secondary reaction zone (SRZ) accelerated the growth of the surface crack under lower σmax, resulting in a shorter crack initiation stage and lower fatigue life compared to the uncoated alloy. This study will be helpful in accurately predicting the HCF life of the coated Ni-based SX superalloys.

High cycle fatigue life estimation of punched and trimmed specimens considering residual stresses and surface roughness

Shear cutting processes have a detrimental effect on fatigue of high strength metal components. The effect tends to increase with material grade, counteracting the task of reducing weight in chassis components using higher strength materials. Base material fatigue data are often available, but assessment of components with cut edges often require additional costly and time-consuming testing. This paper provides a methodology for estimation of fatigue life reduction by using residual stresses obtained from process simulations, and measured surface roughness in the cut edge. A stress relaxation criterion is applied to handle reduction of the initial local residual stresses. Two complex phase steels and one aluminum alloy are studied for validating the approach. Polished fatigue data is reduced to estimate S–N curves of trimmed and punched specimens at different load ratios. Good agreement between the model and test results are found for all cases. The needed data for the predictions are only a high cycle S–N relationship for polished material, uniaxial tensile properties, and the cut edge fracture surface residual stress and roughness without any parameter fitting, making it a convenient tool for estimating the reduction in fatigue life and for parameter studies.

Optimization of cutout characteristics in AISI 1045 steel plates for high-cycle fatigue life extension

Crack growth is a pivotal concern in structural engineering, directly impacting structures’ stability, quality, and overall lifespan. This study addresses this challenge by investigating structural optimization strategies to mitigate fatigue crack growth (FCG) by strategically incorporating stop-drilled cutouts near the crack tip. The research introduces three optimization problems, focusing on determining the optimal cutout position with a fixed radius, the optimal cutout radius with a fixed position, and the optimal cutout shape with a constant position for the cutout’s center. Experimental assessments are conducted on an edge-cracked AISI 1045 steel plate to evaluate the effectiveness of these optimization strategies in enhancing fatigue life. Employing the Extended Finite Element Method (XFEM) for numerical simulations and Particle Swarm Optimization (PSO) for the optimization process, the findings reveal that strategically positioned cutouts substantially reduce the crack growth rate. Optimal cutout placement significantly improves fatigue life, demonstrating an approximate 287% increase. The analysis affirms the accuracy of the numerical model in predicting fatigue life and crack growth paths, validated through comparison with experimental data. In summary, this research underscores the effectiveness of optimization techniques in mitigating fatigue crack growth, providing valuable insights for structural integrity enhancement in structural mechanics.

Investigation on high cycle fatigue performance of additively manufactured Alloys: Synergistic effects of surface finishing and Post-Heat treatment

The high-cycle fatigue (HCF) behavior of additively manufactured (AM) materials in structural applications remains insufficiently explored when contrasted with conventionally fabricated alloys. Addressing this research gap, we systematically investigate the potential to enhance HCF performance in AM alloys through a strategic interplay of surface finishing and post-heat treatment design. Concentrating on laser powder bed fusion, we evaluate surface roughness and post-heat treatment effects on HCF life across a300to1000 MPa stress range. Findings show that, at700to1000 MPa, surface finishing significantly extends HCF life by mitigating crack initiation. Paradoxically, at300to700 MPa, it shortens HCF life due to induced local residual stress on the sample surface. Additionally, post-aging finishing moderately enhances HCF life compared to pre-aging. This underscores the crucial role of sequence in surface finishing and heat treatment, emphasizing their collective impact on fatigue properties. Our study advances the understanding of post-processing factors influencing AM alloy fatigue properties, providing valuable insights for enhancing mechanical performance.

Coupling life prediction of bending very high cycle fatigue of completion strings made of different materials using deep wise separable convolution

AbstractThis article predicts bending very high cycle fatigue (VHCF) life of three typical nickel‐based alloys SM2550, BG2532, and G3 used for completion strings. Fatigue tests were conducted on the three alloys using an ultrasonic fatigue system at a frequency of 20 kHz. The results showed that the fatigue strength ranges of the three alloys were markedly different, reflecting their different sensitivities to fatigue loading. Scanning electron microscope observations revealed numerous fatigue crack origins with internal decohesion in the fatigue source region. To achieve unified prediction of the fatigue life for the three alloys, a prediction model based on deep learning was built with inputs including fatigue initiation quantity, cleavage facet size, and other fatigue fracture characteristics. It was found that single source feature was insufficient to obtain satisfactory prediction accuracy for all alloys, while multifeature coupling integration could significantly improve the prediction precision, enabling reliable prediction of alloy fatigue life. This study provides new insights into bending VHCF life prediction.

On the unusual R-ratio dependence of the high cycle fatigue performance of precipitate strengthened Al alloys

The high cycle fatigue (HCF) performance of precipitate strengthened Al alloys is important for their use in moving structures applications such as transportation. It was first shown in the 1950′s, and has been confirmed multiple times since, that sometimes the weaker underaged alloys can show longer high cycle fatigue lives under R = -1 loading than the stronger peak aged materials. However, under alternative R-ratio loadings (e.g. R = 0.1), the underaged alloys tend to show the same, or even worse, HCF performance as the peak aged alloys. This unusual R-ratio dependence is studied in this work using a combination of fatigue testing on smooth samples, optical profilometry of the evolution of surface roughness during cycling, and transmission electron microscopy.We demonstrate that the unusual R-ratio effect is really an effect of the maximum applied stress, and the critical role that this has in determining the strain localization that occurs during the first cycle of loading. This initial strain localization sets the path for subsequent plasticity accumulation which eventually leads to fatigue failure. The maximum applied stress plays a stronger role in determining the initial strain localization than either the applied stress amplitude or R-ratio and this is the critical factor determining the fatigue life of the precipitate strengthened Al alloys considered in this study.