Key Engineering Materials Vol. 664

Abstract: The most influential articles in Very high cycle fatigue (VHCF) during last 20 years have been identified from web of science data. It has been found that VHCF is an important category of fatigue damage and has produced the highest impact in the field. The number of articles and their citations is continuously increasing in the overall published papers in the field of fatigue. In VHCF field, it was found that the participants of VHCF conferences from VHCF 1 to 5 are the main contributors in all the VHCF papers and in its top 100 influential papers. Majority of the articles were experimental studies. The fatigue crack initiation in the VHCF domain is considered the most important area in the field. In VHCF, the most influential papers till date have been identified. This may prove helpful to trainees mastering the most influential literature of the field as well as more established professionals searching for starting points for new investigations

Abstract: As a joint project of Committees on Fatigue and Reliability Engineering in the Society of Materials Science, Japan (JSMS), an electronic database on fatigue strength of metallic materials fabricated in Japan had been constructed and published in 1996. Book style of the same data compilation had been published at the same time by the JSMS and Elsevier. About twenty years have passed since the above publications of database and databook. Thus, a lot of new fatigue test data have been obtained during such a long period including many data on the very high cycle fatigue such as gigacycle regime. Based on such a circumstance, the JSMS has organized a new project to construct an electronic database on very high cycle fatigue. A lot of numerical data obtained by fatigue tests would be compiled together with many photographs of fracture surfaces. In this paper, fundamental view and compilation concept of the database are briefly introduced by showing a typical example of proto-type database constructed from domestic data in Japan.

Abstract: The top 10 most influential articles in Very high cycle fatigue (VHCF) have been indentified from web of science data. The attributes of the top 10 papers have been discussed. It was found that specialty area of fatigue called as “VHCF” is an emerging field. The most cited papers discussed the two the fatigue crack mechanism in fatigue. It was found that crack initiation shifts from surface to subsurface if the material beyond 10⁷ cycles. There are some models which can predict the fatigue life of the material however the exact estimation is still challenging. Hence, it was found that still further efforts are required in the field to accurately understand the VHCF behavior.

Abstract: This paper aims at a deeper understanding of microplastic mechanisms leading to crack initiation in ductile metals in Very High Cycle Fatigue (VHCF). Fatigue tests were conducted using an ultrasonic technique at loading frequency of 20 kHz. The microplastic mechanisms are revealed via observations of slip markings at the specimen surface and self-heating measurements due to intrinsic dissipation. Pure copper and Armco iron (which contains a very low amount of carbon) were investigated. Both are single-phase ductile materials but the crystallographic structure of copper is face-centered cubic while it is body centered cubic for Armco iron. A good correlation was found between slip markings initiation and dissipation for both materials. The dissipation for both materials is of the same order of magnitude but the location, the morphology and the evolution over cycles of slip markings were found different.

Abstract: Carbon fiber reinforced polymers (CFRP) are getting more and more important for structural components for automotive and aerospace applications. These components are subjected to 10¹¹ and more loading cycles during their time in service. Therefore the VHCF behavior and the corresponding failure mechanisms have to be well understood. To obtain a comprehensive knowledge about the fatigue behavior and failure mechanisms of CFRP in the VHCF regime, a new Ultrasonic Testing Facility (UTF) for cyclic 3-point bending at 20 kHz has been developed at WKK. This UTF with combined online nondestructive testing via laser vibrometry and IRthermography enables VHCF experiments up to 10⁹ cycles in twelve days without overcritical heating up to the glass transition temperature of the polymer. The chosen material in this research project is the commercially available and aircraft qualified carbon fiber fabric reinforced polyphenylensulfide (CF-PPS). To determine the fatigue characteristics of CF-PPS load increase tests and based on these results constant amplitude tests up to 10⁹ cycles have been carried out. Light optical and SEM microscopy have been performed in defined fatigue states or finally after reaching 10⁹ cycles with shear stress amplitudes of at least 44% of the monotonic ultimate shear strength. The induced fatigue damage of CF-PPS in the VHCF regime was studied in detail.

Abstract: Very high cycle fatigue is often implemented with high test frequency to save time. The thermal dissipation accompanied with the high frequency appears and induces the temperature increment in the speicmen’s surface. The dissipation power is important and closely related with VHCF performance. The infrared camera is used to acquire the temperature distribution and evolution with a lot noisy which bring difficulties for the calculation. In the article, a thermal dissipation power calculation with time and space filtering method is proposed to give out the thermal dissipation. The two stages dissipation phenomenon with large difference is found out in the process of VHCF test.

Abstract: In the present paper, the characteristic and the application of ultrasonic fatigue testing technology is illuminated. The main problems i.e. the size effect, the thermal effect and frequency effect due to the high frequency are discussed. The results show that: 1. As there is a size effect, a uniform specimen size should be adopted in the very-high cycle fatigue standard and for special designed specimen the designed size should be noted along with the fatigue test results; 2.the heat generation attributes mainly to the low yield strength and the high applied stress, as a result, ultrasonic fatigue testing technology can be mainly applied to the ultra-high cycle fatigue test of high-strength steel; 3.the frequency effect is related to the crystal structure of metallic materials, however, ultrasonic fatigue testing technology can be applied to conduct the comparison of the fatigue properties of the same steel grade before and after the smelting process.

Abstract: High frequency fatigue tests were carried out with a 20 kHz ultrasonic testing facility to investigate the cyclic deformation behavior of Ti6Al4V in the Very High Cycle Fatigue (VHCF) regime in detail. The S,N_f -curve at the stress ratio R = -1 shows a significant decrease of the stress amplitude and a change from surface to subsurface failures in the VHCF regime for more than 10⁷ cycles. Microscopic investigations of the distribution of the α-and β-phase of Ti6Al4V indicate that inhomogeneities in the phase distribution are reasons for the internal crack initiation. Scanning electron microscopy as well as light microscopy were used to investigate the internal crack initiation phenomenon in the VHCF-regime. Beside the primary fatigue crack additional defects like micro cracks and crack clusters were observed in the fatigued specimens. SEM-investigations of specimens which were loaded up to 10¹⁰ cycles without failure show irreversible microstructural changes inside the specimens. Two step tests were performed to evaluate the influence of internal fatigue induced defects observed in specimens which did not fail within 10¹⁰ cycles.

Abstract: In order to investigate the effect of shot peening on the long life fatigue properties of Ti6Al4V (TC4), ultrasonic fatigue tests were performed with a frequency of 20 kHz. According to different heat treatment, two groups of specimens were surface modified by shot peening with an Almen intensity of 0.10 mmA~ 0.15 mmA and an overlapping rate of 100%. One group was stress-relief annealed at 650 °C, and the other was then treated with solution-aging. With the shot peening, both the depth of the residual stress layer and the maximum compressive residual stress are increased. Surface hardness is also increased. The fatigue strength is increased, but enhancement is no more than 5% because of the increase of surface roughness. Noticeably, all of the TC4 specimens after shot peening show inner crack initiations. Oxide inclusion is always the core of cracks, and the small crack propagation in crack initiation area acts as the slip of α-phase.