Very High Cycle Fatigue Regime Research Articles

Fatigue crack initiation site transition of high‐strength steel under very high‐cycle fatigue

AbstractThe very high‐cycle fatigue (VHCF) fractographies of high‐strength steel AISI 4340 with tensile strength ranging from 1285 to 2363 MPa fabricated by tempering were systematically observed and quantitatively analyzed. It is found that the fatigue crack initiation sites were gradually transferred from the specimen surface to the interior with increasing the tensile strength or decreasing the stress amplitude. Such phenomenon has become common rule in the high‐cycle fatigue (HCF) or VHCF regimes of high‐strength steels. Based on the fracture mechanics, a simplified transition mechanism for fatigue crack initiation sites is established in terms of the influence of applied stress amplitude and microstructure, which is beneficial to enhance our understanding on the variation of fatigue strength increasing the tensile strength.

Microstructure evolution, crack initiation and early growth of high-strength martensitic steels subjected to fatigue loading

Fatigue loading induced microstructure evolution featured by Fine Granular Area (FGA) in high-strength martensitic steels have close ties to the fatigue crack initiation and early growth, which are worthy of being investigated. On this issue, we conducted Very High Cycle Fatigue (VHCF), High Cycle Fatigue (HCF) and Low Cycle Fatigue (LCF) tests on three different types of specimens. Combined with post-mortem/quasi in-situ Scanning Electron Microscope (SEM) and Electron Back-Scattered Diffraction (EBSD) observations, as well as further Transmission Kikuchi Diffraction (TKD) and Transmission Electron Microscope (TEM) analyses, we captured two different types of microstructure evolution behaviors during the fatigue loading. One is the “dynamic precipitation”, a kind of phase transformation from martensitic matrix (BCC) to Nb- and Mo-rich small carbides (MC, M7C3) accelerated by repeated slight plastic deformation, whose formation does not rely on the stress concentration effect and also has no ties to the plastic strain localization and fatigue crack initiation. Another is the “local grain refinement” around those stress singularities (crack tip, or interior inclusion) generating substantial amounts of fine sub-grains in VHCF, HCF and LCF regimes, which can then promote fatigue crack initiation and early growth along sub-grain boundaries, fine/coarse grain boundaries and martensitic packet boundaries.

Very high cycle fatigue assessment of thermoplastic-based hybrid fiber metal laminate by using a high-frequency resonant testing system

The prolonged fatigue lifetime of fiber metal laminates (FML) compared to monolithic metals is a key aspect for safety-relevant components, where detailed knowledge about the fatigue properties up to the very high cycle fatigue (VHCF) regime is necessary. For thermoplastic-based FMLs, offering formability, recyclability, and mass production due to short consolidation cycle times, this knowledge needs to be established.In this study, FML containing AA6082 sheets and unidirectional glass and carbon fiber-reinforced polyamide 6 was investigated. Fatigue tests up to max. 109 cycles were conducted on a servo-hydraulic and a high-frequency resonant fatigue testing system with frequencies of 10 Hz and 1,000 Hz. Frequency-induced self-heating was evaluated and limited using air-cooling to maintain the matrix properties. Fatigue progress was monitored through high-speed optical deformation analysis.Compared to HCF loads, VHCF loads lead to reduced crack and delamination occurrence. Changes in microstructure are dominated by singular crack initiation and propagation within the aluminum. The FML’s S-N curve shows a significant flattening from the HCF to the VHCF regime, simplifying the predictability of the fatigue life. By using post-stretching, the VHCF strength can be increased by over 60 %, making this thermoplastic-based FML a considerable competitor in the field of FML.

Microstructural evolution and formation of fine grains during fatigue crack initiation process of laser powder bed fusion Ni-based superalloy

This study reports the fatigue failure mechanism at very-high-cycle fatigue (VHCF) regime of laser powder bed fusion (LPBF) GH4169 superalloy through a series of detailed microstructural characterizations, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and energy dispersive spectrometer (EDS). Microstructural characterization of the solution and double-aging post-treated initial material exhibits process-induced imperfections (mainly pores), as well as γ′, γ′′ and δ precipitates, and carbide. The fatigue tests were performed using an ultrasonic fatigue tester (20 kHz). Fatigue fracture analysis suggests there exists a competitive fatigue failure mechanism with surface flaw initiation and internal pore initiation, corresponding to high-cycle fatigue (HCF) and VHCF regime, respectively. Focused ion beam (FIB) samples taken within the fatigue initiation area (FIA) revealed grain refinement and precipitate dissolution behavior. Based on the characterization results, the fatigue crack initiation mechanism was hypothesized: During numerous cyclic loading, the mobile dislocations shear γ′ and γ′′ precipitates, causing their dissolution and local chemical and mechanical alterations near internal pores. This enables twinning and promotes sub-grains formation. Sub-grains refine via localized continuous dynamic recrystallization (CDRX), forming a fine-grained layer that leads to crack initiation. This work reveals how precipitate dissolution contributes to VHCF crack initiation in LPBF GH4169 superalloy, highlighting the potential for extending alloy lifespan by adjusting precipitates and LPBF defects and incorporating these factors into fatigue life predictions for enhanced accuracy.

Editorial: Special Issue on Very High Cycle Fatigue

Abstract As the co-guest editors of the special issue of ASTM International’s journal Materials Performance and Characterization on very high cycle fatigue (VHCF), we are delighted to have this issue published by ASTM. Fatigue phenomena have long been a topic of significant interest and of research focusing on understanding the behavior of materials and structures under cyclic loading. However, the exploration of fatigue properties beyond traditional high cycle fatigue has gained substantial momentum in recent years. The VHCF regime, characterized by fatigue life exceeding 107 cycles, presents unique challenges and opportunities for advancing our understanding of fatigue mechanisms and developing robust design methodologies. This special issue brings together a collection of outstanding research articles that delve into various aspects of VHCF and contains 15 peer-reviewed articles (research and review) from researchers all around the globe (Asia, South and North America, and Europe). The published contributions encompass a broad range of materials, including metals and alloys, and employ diverse experimental, analytical, and computational techniques to investigate the behavior of structures subjected to VHCF loading. The research presented here not only enhances our fundamental knowledge of fatigue mechanisms but also provides practical insights for industrial applications such as aerospace, automotive, and additive manufacturing. We would like to sincerely thank all the authors who contributed to this special issue for their high-quality submissions, which greatly contribute to the advancement of this less-explored field. We also extend our acknowledgment to the Editor in Chief of the journal, Dr. Richard W. Neu (Georgia Institute of Technology), and the ASTM Publishing Manager, Ms. Alyssa Conaway. Without their invaluable help and support, it would have been impossible to publish this special issue. We hope this special issue enhances our knowledge and understanding in the field of fatigue, particularly VHCF, and serves as a reliable reference for students, researchers, and engineers working in this field. Co-guest editors Summer 2023

Very High Cycle Fatigue of Welds: A Review

The design life of welded structures and components extends into the very high cycle fatigue (VHCF) regime across various applications. However, the availability of data on the fatigue behaviour of welded joints in the VHCF regime is limited, particularly when compared to the low and high cycle fatigue regimes. The development of ultrasonic fatigue testing equipment has accelerated fatigue testing and allowed for the VHCF properties of welds to be investigated in a feasible timeframe. In the present review, the emerging research concerning the VHCF behaviour of welds of various steels and non-ferrous alloys are individually explored. Overall, it is observed that welded joints have significantly lower fatigue strength than the base metal in the VHCF regime and that welding defects have a considerable influence on fatigue strength. Through the discussion of the relevant literature, important findings concerning the effects of specimen geometry and fatigue improvement methods are underlined. Furthermore, the guidance provided within design standards is compared, and some examples of VHCF failures of in-service components are highlighted. Finally, perspectives on future directions of investigation are put forward with the aim of encouraging further research in the field of VHCF of welds.

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.

Fatigue failure mechanism of Ti60 titanium alloy in HCF and VHCF regime at different temperatures

The failure mechanism of Ti60 titanium alloy in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) at different temperatures was investigated. The results showed that, at different temperatures, there are two different fatigue failure mechanisms, i.e., the oversized facet and oxide shedding failure mechanisms. At room temperature, most of the fatigue cracks initiate at the oversized facet. At 300℃, the crack initiates at the oversized facet for the test with small applied stress, while the crack initiates at the oxide shedding site for the test with large applied stress. At 400℃, all the fatigue cracks initiate at the oxide shedding site. Higher temperature and stress will be conducive to the failure mechanism of oxide shedding to become dominant.

High Cycle Fatigue and Very High Cycle Fatigue Performance of Selective Laser Melting Ti-6Al-4V Titanium Alloy—A Review

Abstract Additive manufacturing (AM) of metallic alloys, especially titanium (Ti), has recently received considerable attention because of its significant role in designing and developing many structural components with complex geometries in aerospace, defense, and biomechanical industries. AM technology based on selective laser melting (SLM) allows the production of lightweight structures with geometric flexibility, which has not been otherwise possible by the conventional manufacturing process. SLM-fabricated Ti-6Al-4V components often experience long loading histories in high cycle fatigue (HCF) and even very high cycle fatigue (VHCF) regimes. As a result, it is paramount to systematically investigate those components’ fatigue behavior under both HCF and VHCF conditions. However, HCF and VHCF performances of SLM-Ti-6Al-4V alloy are still not fully understood because of the complex nature of fatigue responses in those regimes resulting from the defects/porosity and number of process parameters. In this context, the successful application of load-bearing components in both HCF and VHCF regimes necessitates optimizing process parameters and post-treatments for the optimal fatigue performance point of view. Several recent studies dealing with Ti-6Al-4V parts manufactured by SLM have explored parameters affecting fatigue performance in HCF and VHCF regimes. This article presents a systematic and critical review analysis of recent findings related to critical parameters, particularly residual stress, surface roughness, build parameters, build orientation microstructural features, post-process treatment, manufacturing deficiencies, specimen geometries, load ratio affecting mechanical and fatigue properties, especially in HCF and VHCF regimes. The current study also aims to identify several crucial topics that need to be addressed for SLM Ti-6Al-4V alloy to effectively utilize its full potential in the designing of advanced structural components.

Fatigue life prediction based on a deep learning method for Ti-6Al-4V fabricated by laser powder bed fusion up to very-high-cycle fatigue regime

Microstructural defects and inhomogeneity of titanium alloys fabricated by laser powder bed fusion (LPBF) make their fatigue behaviors much more complicated than the conventionally made ones, especially in very-high-cycle fatigue (VHCF) regime. Most of traditional models/formulae and currently-used machine learning algorithms mainly concern fatigue behavior of LPBF-fabricated titanium alloys in high-cycle fatigue (HCF) regime, but rarely in VHCF regime. In this paper, a deep belief neural network-back propagation (DBN-BP) model was proposed to predict the fatigue life of LPBF-fabricated Ti-6Al-4V up to VHCF regime. Results obtained in this study indicate that the DBN-BP model exhibits high precision and strong stability in predicting the fatigue life of LPBF-fabricated Ti-6Al-4V in both HCF and VHCF regimes. The primary hyperparameters of the DBN-BP model were optimized to further improve the prediction precision of this innovative model. Finally, the optimal DBN-BP model was applied to predict the relation between mean stress and stress amplitude, and the effect of energy density on the fatigue behavior of LPBF-fabricated Ti-6Al-4V up to VHCF regime.

Notch fatigue behavior of a titanium alloy in the VHCF regime based on a vibration fatigue test

The notch fatigue behavior of a titanium alloy TC17 in the very high cycle fatigue (VHCF) regime is investigated by a vibration-based bending fatigue test. A complete S-N relation of notched specimens with high resonance frequencies is obtained, covering the lifetime up to 109 cycles, and the fracture surfaces are observed. The results show the TC17 exhibits relatively high fatigue notch sensitivity. Several fatigue crack initiation mechanisms relevant to the notch are then revealed. Furthermore, a notch fatigue prediction model based on a modified theory of critical distance is proposed by introducing a special weight function, which is demonstrated to have good prediction accuracy.

High‐cycle and very‐high‐cycle fatigue behavior at two stress ratios of Ti‐6Al‐4V manufactured via laser powder bed fusion with different surface states

AbstractThe high‐cycle fatigue (HCF) and very‐high‐cycle fatigue (VHCF) behavior of Ti‐6Al‐4V manufactured by laser powder bed fusion (L‐PBF) was investigated to reveal the effects of surface roughness and stress ratio on fatigue performance. Fatigue tests were performed by an ultrasonic vibration machine with a frequency of 20 kHz. The fatigue strength of surface‐polished specimens is generally higher than that of as‐built specimens in HCF and VHCF regimes. Fatigue cracks initiated from the subsurface of as‐built and surface‐polished L‐PBF Ti‐6Al‐4V. The values of size and depth for the defects that acted as fatigue crack origins present large scattering for L‐PBF Ti‐6Al‐4V due to the interaction between surface roughness and subsurface defects. For surface‐polished L‐PBF Ti‐6Al‐4V, fatigue cracks have almost the same resistance to propagation at both stress ratios of R = −1 and 0.7.

Stored energy density research on the fatigue crack initiation at twin boundary and life prediction of Inconel718 superalloy

The fatigue crack initiation at twin boundary (TB) and the life prediction of IN718 alloys in the high cycle fatigue (HCF) regime and very high cycle fatigue (VHCF) regime were investigated through crystal plasticity finite element (CPFE) simulations and experiments. The fatigue damage parameter, considering the evolution of geometrically necessary dislocation (GND) density, slip and back stress, known as the back-stress-based stored energy density in this study, Gba, was proposed to precisely characterize the plastic strain distribution and localization, which in turn was engaged in locating fatigue crack initiation site and predicting the fatigue life. The element analysis around TB and the CPFE simulations indicated the coupling of slip activation parallel to the TB and the absence of primarily strengthening phase (γ'') on the both side near the TB was responsible for crack initiation. To predict fatigue life, the varying pattern of the Gba at fracture with fatigue life was studied based on the experimental results in the HCF and VHCF regimes, and the life prediction method was obtained accordingly. Finally, the fatigue life evaluations of the IN718 alloy at three load ratios (R = −0.1, 0.1, and 0.5) were conducted and results showed that the life estimates were basically within ±3 scatted bands.

A novel methodology for bending ultrasonic fatigue testing in the VHCF regime

Deep research on the bending fatigue behaviors of rotor blade materials in the very-high-cycle fatigue (VHCF) regime is necessary for designing the blade. In this study, a new cantilever specimen with variable thickness and the corresponding methodology based on the Timoshenko beam theory and the transfer matrix method are proposed for bending ultrasonic fatigue testing in the VHCF regime. This methodology is applied to achieve the desired second-order stress state and to estimate the stress by displacement. The feasibility and applicability of the methodology are demonstrated by the results of Finite Element Analysis and the experimental results from Ti-6Al-4V specimens.

Effect of Friction Stir Processing on the Fatigue Performance of AZ31 Magnesium Alloy

Herein, the cyclic mechanical behavior of AZ31 magnesium alloy after multipass friction stir processing (FSP) is investigated up to the very high‐cycle fatigue (VHCF) regime. The grain refinement and texture evolution after processing are evaluated to enhance the understanding of the fatigue response. Although ultimate tensile strength and ductility of the friction stir processed AZ31 increase up to about 320 MPa and 25%, respectively, the fatigue performance deteriorates in comparison with that of the as‐received condition due to the low yield strength and texture evolution after processing. Furthermore, analysis of fracture surfaces of the samples after cyclic loading reveals that the as‐received AZ31 is more prone to brittle fracture with multiple‐origin fatigue failure even at low stress amplitudes. On the contrary, the dominant failure mechanisms of the friction stir processed samples are initiation and propagation of cracks originating from the surface, porosities, and grain size inhomogeneity. Nevertheless, the capability of FSP for providing superior crack initiation resistance in the VHCF regime is demonstrated as a significant contribution. Based on a detailed study of prevalent microstructural features, processing–property–damage relationships are established indicating the major effect of FSP on the final performance of the AZ31 magnesium alloy.

Historical review and future prospect for researches on very high cycle fatigue of metallic materials

AbstractSince the first paper on the fatigue of metallic materials by J. Albert in 1837, tremendous numbers of papers have been published in various journals by many researchers all over the world. Based on such a long history, several papers on the very high cycle fatigue (VHCF) in the life‐time longer than 107 cycles had appeared in some journals during the period of 1980's. One characteristic finding in these works is the fact that the metallic material can fail even at the stress level lower than the conventional fatigue limit. This fact means that the conventional fatigue design of mechanical structures cannot give the safety of the practical products in the very high cycle regime. Due to this fact, fatigue properties of structural materials in very long life regime has become an important subject; and a lot of studies have been carried out, and many important results have been accumulated until now. Typical aspects on VHCF property are summarized as follows: (1) the fatigue crack tends to occur around the interior inclusion, (2) fine granular area (FGA) is formed around such an inclusion, and (3) duplex S–N characteristics appear in the VHCF regime. In this paper, a brief historical review together with the future prospect on the very high cycle fatigue of metallic materials is attempted for the sake of reference to facilitate the research in this area.

A Novel Ultrasonic Fatigue Test and Application in Bending Fatigue of TC4 Titanium Alloy

The very high cycle fatigue (VHCF) problems of thin-plate structures are usually caused by high-frequency vibrations. This paper proposes an accelerated fatigue test method based on ultrasonic loading technology in order to develop a feasible bending testing method and explore the bending fatigue characteristics of thin-plate structures in the VHCF regime. A new bending fatigue specimen with an intrinsic frequency of 20 kHz was designed based on cantilever bending through finite element simulation. It was verified by the axial load test with R = -1. The results showed that the method could effectively transfer the dangerous cross-section at the first-order cantilever bending restraint to the internal part of the specimen, thereby making the fracture location independent of the complex stresses. The linear relationship between the vibration amplitude and the maximum stress was also verified using strain measurements. Furthermore, the S-N curves and fracture morphology for different loading types were consistent with conventional studies to a certain extent, which indicated that the design of the bending test model was reasonable.

Characteristics and micromechanisms of fish-eye crack initiation of a Ti-6Al-4V alloy in very high cycle fatigue regime

The fracture surface characteristics and the associated micromechanisms governing the fish-eye crack initiation of a Ti-6Al-4V alloy under very high cycle fatigue (VHCF) loading are investigated. Microscopic observation revealed that the crack initiation area of fish-eye center is covered by ragged-like mountains and dimples containing various irregular rods/particles, grooves and microvoids. These micro-ductile characteristics primarily result from the nucleation and coalescence of microvoids and grooves at αs/βr interface inside βtrans. This could be due to the incompatibility of both micro-plastic deformation and slips in αs and βr. The βtrans plays a dominant role in the fish-eye crack initiation and early propagation process of the Ti-6Al-4V alloy in the VHCF regime.

Interior defect-induced crack initiation mechanism and initial growth behavior for Ti6Al4V alloy fabricated using laser powder bed fusion

The interior defect-induced crack initiation mechanism and early growth behavior of Ti6Al4V alloy fabricated by laser powder bed fusion (LPBF) has been investigated in very high cycle fatigue (VHCF) regime. P–S–N curves under 10% and 90% failure probabilities are obtained in VHCF regime. The cracks inside the early stages of fine granular area (FGA) formation are driven by the maximum shear stress and propagate as Mode II + III mixed cracks. It can be found that the FGA region is composed of many discontinuous nanograins for Ti6Al4V alloys manufactured by LPBF, which are responsible for grain refinement. Grain refinement is associated with dislocation movement within the martensite laths. Dislocation pileup and rearrangement in martensitic laths form dislocation cells, which further develop into nanograins and low angle boundaries. Besides, both the fatigue loading process and the LPBF process form their respective microvoids, which merge and aggregate with each other, thus accelerating the microcrack extension.

Comparison between Fractal and Statistical Approaches to Model Size Effects in VHCF

Size effects concern the anomalous scaling of relevant mechanical properties of materials and structures over a sufficiently wide dimensional range. In the last few years, thanks to technological advances, such effects have been experimentally detected also in the very high cycle fatigue (VHCF) tests. Research groups at Politecnico di Torino are very active in this field, observing size effects on fatigue strength, fatigue life and fatigue limit up to the VHCF regime for different metal alloys. In addition, different theoretical models have been put forward to explain these effects. In the present paper, two of them are introduced, respectively based on fractal geometry and statistical concepts. Furthermore, a comparison between the models and experimental results is provided. Both models are able to predict the decrement in the fatigue life and in the conventional fatigue limit.