Gigacycle fatigue in high strength steels

Abstract

This paper reviews the research results to date on gigacycle fatigue caused by internal fractures in high strength steels. Firstly, accelerated fatigue testing was realized using ultrasonic fatigue testing at 20 kHz, which completes 10 9 cycles in one day, unlike the 3–4 months needed for conventional fatigue testing. Although the frequency effect was anticipated to be problematic, it proved negligible under conditions in which internal fractures occurred. Later, many unique characteristics of internal fractures were elucidated. For example, hydrogen has dramatically greater effects on internal fractures than on conventional surface fractures. Mean stress effects are more serious in titanium alloys than in high strength steels. Size effects were notable in high strength steels. These distinctive characteristics required a unique model to be able to predict gigacycle fatigue strength, which first required elucidation of its mechanisms. To this aim, the author attempted to measure the crack growth rates of small internal cracks using the beach mark method. The results revealed that the crack growth of small internal cracks controls internal fractures. In calculating the crack growth life, however, it was found that the conventional crack growth law overestimates the effects of inclusion size. To rectify this problem, a new model using a new crack growth law was proposed, which predicts more realistic fatigue life curves. As a result, predictions were derived for several grades of high strength steels.

Effect of microstructure of simulated heat‐affected zone on low‐ to high‐cycle fatigue properties of low‐carbon steels

“…On the other hand, the fatigue limits of CGHAZ were lower than those for FGHAZ, especially in the higher tensile stress region. Normally, the fatigue limit is proportional to tensile strength in the range where the tensile strength is below 1200 MPa 5,27 . However, the fatigue limit of CGHAZ did not increase in the region where tensile strength was higher than about 600 MPa.…”

“…The effects of microstructure changes on fatigue property have been widely investigated and discussed based on a large fatigue database 4–6 . For example, it is well known that the fatigue limit is normally proportional to the monotonic tensile strength of the microstructure 4 .…”

Usability of Ultrasonic Frequency Testing for Rapid Generation of High and Very High Cycle Fatigue Data

“…The result of the investigation shows that there is a strong size effect in this maraging steel. Mean lifetimes measured with large specimens are about one order of magnitude trasonic fatigue and multiple-axis, cantilever-type, rotating bending fatigue testing [42], up to 10 10 cycles which took more than 3 years at 100 Hz. No frequency effects were found for the two investigated high-strength steels (spring steel SUP7 and low-alloy steel SCM440) as shown in Figure 7.…”

“… Fatigue data of high-strength spring steel SUP7 ( a ) and low-alloy steel SCM440 ( b ) with rotating-bending equipment (30 Hz and 100 Hz), servo-hydraulic equipment (600 Hz), and ultrasonic equipment at 20 kHz [ 42 ]. …”

Predictions and mechanism analyses of the fatigue strength of steel based on machine learning

“…In recent years, with the rapid development of machine learning technology, a few researchers have begun to examine fatigue strength [19] based on ML models. For example, adaptive neuro-fuzzy-based machine learning techniques were studied for potential applications in the modelling of the highcycle fatigue life of L-PBF stainless steel 316L [20,21].…”