After the epoch-making findings of the 1980s regarding internal fatigue fractures in high-strength steels under numerous loading cycles, very high cycle fatigue (VHCF) has been recognized as an emerging and unprecedented issue in the field of materials strength. These days, requirements for mechanical components and structures have become increasingly demanding, as represented by new standards for automobile engines, marine engines, turbine structures, high-speed railway components, etc. Reliability against cyclic loading greater than 107 cycles is regarded as one of the most important challenges for strength design. Thus, an improved understanding of VHCF behaviors not only for metallic materials but also for polymers and composites is essential for developing methods to reliably predict fatigue life. With this background, the Eighth International Conference on Very High Cycle Fatigue (VHCF8) was held from July 5 to 9, 2021, as an online and on-demand event and distributed from Sapporo, Japan. VHCF8 was built on the foundation of a successful series of previous conferences1: VHCF1 1998, in Paris; VHCF2 2001, in Vienna; VHCF3 2004, in Kusatsu; VHCF4 2007, in Ann Arbor; VHCF5 2011, in Berlin; VHCF6 2014, in Chengdu; and VHCF7 2017, in Dresden. Sakai1 briefly reviewed the long history of research on metal fatigue since the first paper on the topic, which was published by Albert in 1837. Sakai then attempted a historical review for research on the very high cycle fatigue (VHCF) of metallic materials referring a lot of papers by many researchers. In the latter review, typical aspects such as characteristic S-N properties, fractographic features, crack initiation mechanisms, and crack propagation behavior were described together with the statistical aspects of the VHCF behavior. In addition, future prospects for research on VHCF have finally been provided. These studies pay attention to new materials and the application of dislocation theory for the purposes of facilitating research on the VHCF of structural materials. Hong, Hu, and Zhao2 provided an interesting review of the loading frequency effects on fatigue behavior of metallic materials. They first reviewed loading frequency effects by setting up three cases as follows. Case I: loading frequency effect explained by dislocation movement; Case II: loading frequency effect explained by Johnson–Cook model; and Case III: other typical cases from literature. In Case I, experimental results under various frequencies in some papers were well explained by applying the concept of dislocation movement. In Case II, experimental results at different frequencies in other literature were successfully explained by applying the Johnson–Cook model. In Case III, the frequency effect was explained by categorizing metallic materials into three typical crystalline types of bcc, hcp, and fcc structures. Sun et al.3 investigated the interior cracking behavior and the life prediction of GH4169 superalloy in the VHCF regime. The experimental results showed that interior fatigue crack nucleation was the main failure mode regardless of the stress ratio. The crack nucleation region, which consisted of many facets, was formed under shear stress and strain. Based on the length of the transition from a small crack to a long crack in terms of interior failure, the microstructure-related interior failure mechanism was successfully summarized, and crack nucleation and propagation lives were predicted. Thus, the experimental results were within the scatter bounds given by a factor of 3 of the predicted lives. Liu et al.4 conducted a comparative study of the VHCF behavior of railway wheel steel under axial and rolling contact loading. Surface and internal crack initiation occurred with equal probabilities, resulting in a double linear S-N curve for the Mode I crack under axial loading. The VHCF behavior of a mixture of Mode II–III cracking under rolling contact loading had cracks initiated from the internal Al2O3 inclusion. Ferrite nanograins were generated under rolling contact loading, and the formation was enhanced by significant plastic shear strain at high compressive stress. The nanograins were softer than the matrix due to carbon redistribution. Miyakoshi et al.5 investigated transverse crack initiation in cross-ply carbon fiber-reinforced laminates in the very high cycle region. In the study, the fatigue strengths at N = 103 to 107 cycles were obtained using a servo-hydraulic fatigue testing machine, and those at N = 108 to 109 cycles were obtained using an ultrasonic fatigue testing machine. The results of up to 6.56 × 109 fatigue tests confirmed that there would be a threshold stress amplitude that would allow transverse crack initiation. Schönbauer et al.6 investigated the effect of load ratio on the VHCF strength of Ck45M steel. Ultrasonic fatigue tests were performed under uniaxial and torsional loading using smooth specimens and specimens with artificial defects. In the VHCF regime, fracture mechanisms varied; uniaxial loading showed internal crack initiation, while torsional loading revealed surface shear crack initiation. Equal fatigue limits were determined for uniaxial and torsional loading, considering the size of the crack-initiating defects and the appropriate threshold conditions for Mode-I crack growth. In addition, the mean stress sensitivity was found to be independent of the loading condition. Shanyavskiy and Soldatenkov7 analyzed the process of fatigue spalling in the ring of a ball bearing under in-service loading conditions. The cracks originated in the subsurface from carbides or permissible inclusions. The crack growth occurred perpendicular to the ring raceway surface following the formation of fish-eye cracks. The subsequent crack growth involved alternating stops and new crack nucleation driven by mixed-mode I + II + III mechanisms with crack branching. In the final stage, the crack propagated toward the ring raceway, resulting in either appearance on the raceway surface or coalescence with an adjacent crack followed by a fatigue-spalling formation. Zhu, Zhu, and Xuan8 discussed the mechanism of fragmentation of martensitic laths and the formation of dislocation cells, which revealed microstructure-dependent crack initiation and early growth for microdefect-induced internal fatigue cracking in several lathy martensitic or similar steels. The findings were verified under environmental fatigue conditions and found to be applicable to white etching cracks in rotating contact fatigue. Additionally, the application of machine learning for fatigue life estimation was conducted in the case of microdefect-induced fatigue cracking in HCF and VHCF regimes. Wickmann and Sander9 investigated the fatigue properties of high-strength steel, structural steel, and duplex stainless steel in artificial seawater. Experiments were performed on smooth specimens submerged in seawater using an ultrasonic test stand with a chamber. The results showed continuously decreasing S-N curves for all materials, with no detectable fatigue limit. Fracture-surface examinations revealed extensive corrosion areas for the structural steel, with unclear crack initiation locations. The observed duplex stainless steel indicated fewer corrosion pits but a significant scatter in the S-N data. In contrast, multiple crack initiation locations on the surface were observed in the observed high-strength steel. Shanyavskiy, Nikitin, and Nikitin10 analyzed the fatigue fracture of a low-pressure compressor disk in a D30KU-154 aircraft engine and established crack initiation and propagation principles based on a fractographic investigation. Their findings revealed that the rim part of the disk suffered a very high cycle fatigue fracture as a result of high frequency loading. Further analysis determined that the total fatigue lifetime was determined by low-amplitude loading from blade vibration and by high-amplitude loading from the flight cycle. The team also employed numerical simulations to estimate fatigue lifetime and predict the crack path by utilizing a multi-regime fatigue fracture model. Costa, Reis, and Freitas11 conducted multiaxial tension/torsion fatigue tests using an ultrasonic fatigue testing machine, aiming develop a reliable multiaxial fatigue test and methodology by modifying a specimen's geometry and enabling different shear-to-axial stress ratios. The study conducted numerical and experimental modal analyses of the ultrasonic setup and specimens to achieve a consistent modal deformed shape and to associate the measured displacement with induced stresses. Tension/torsion fatigue test series were conducted on three specimens with different shear/axial stress ratios. The results showed a clear trend in crack propagation details with an increase in the stress ratio. Yoshinaka et al.12 used synchrotron radiation X-ray computed tomography combining micro-CT and nano-CT to investigate the initiation of internal fatigue cracks in a Ti-6Al-4V alloy during very high cycle fatigue. They examined a volume of Ф1.8 × L 2.5 mm in the specimen and found 28 cracks. However, they did not find any correlation between the spatial distribution, initial lengths, and initiation lives of the cracks. Crack growth rate varied widely, and several grain-sized internal cracks were visualized using nano-CT. The researchers concluded that the multiple-facet initiation site commonly observed for titanium alloys is caused by small crack growth accompanied by facet formation. Fu et al.13 investigated the fatigue behavior of Ti-6Al-4V produced by laser powder bed fusion (L-PBF) under high-cycle and very-high-cycle conditions. They used an ultrasonic fatigue testing machine with a 20 kHz frequency to determine the impact of surface roughness and stress ratio on fatigue performance. The results showed that surface-polished specimens had higher fatigue strength compared to as-built specimens. Fatigue cracks were found to originate from subsurface defects whose size and depth varied due to interaction with surface roughness. For surface-polished L-PBF Ti-6Al-4V, fatigue cracks had similar resistance to propagation at stress ratios of R = −1 and R = 0.7. Chai, Bergström, and Burman14 studied the formation of a fine granular area (FGA) during very high cycle fatigue in austenitic steel. The authors utilized a stepwise load-increasing method and the electron scanning microscopy/electron channeling contrast imaging (ECCI) technique to observe the formation of FGA in a non-defective matrix. Their results showed that a nano-rough surface area or FGA was formed at the fatigue crack origin in the subsurface matrix, even in the absence of any defects. The formation of FGA in a non-defective matrix is a localized plasticity-exhausting process that involves strain localization, grain fragmentation, stress concentration, and the initiation and propagation of nano-cracks along low-angle grain boundaries. Montagnoli, Invernizzi, and Carpinteri15 investigated the size effect of an aluminum alloy on very high cycle fatigue strength by using an ultrasonic fatigue testing machine over a wide dimensional range, where the size effect on very high cycle fatigue has not yet been fully investigated. Their experimental results showed that the size effect was pronounced for smaller dimensions; however, the fatigue strength tended to converge as the size increased to more than 10 mm in diameter. The standardization of ultrasonic fatigue-testing techniques is essential for its widespread use. In line with this, Furuya et al.16 reviewed the standard for metallic materials tested at room temperature, which was published by the Japan Welding Engineering Society in 2017. This standard applies not only to cyclic axial loading by longitudinal oscillation but also to cyclic torsional loading by torsional oscillation with and without mean loading. Several examples of applications of ultrasonic fatigue testing techniques for various materials were also shown in the paper. Shimamura et al.17 developed an ultrasonic fatigue-testing technique that allowed us to evaluate the axial fatigue strength of quasi-isotropic carbon fiber-reinforced laminates. In their study, a specimen shape that allowed a specimen to fail in the middle section was proposed, and an S-N diagram in very high cycle fatigue was obtained. The results showed that fatigue damage could initiate after 107 cycles and fail in very high cycle regions. In addition, no fatigue limit was found up to 109 cycles in their study. As evident from the text above, the VSI-VHCF8 was the product of the considerable efforts of the authors and reviewers. We would like to express our sincere appreciation to all of them for their dedicated contributions. Not Available.
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