The mechanism of fatigue failure in the very high cycle fatigue (VHCF) life regime of N> 107 cycles

Abstract

Abstract Since Wohler started fatigue testing in the 1850s, the fatigue limit stress for steels has been defined as the highest stress at which specimens do not fail after testing to 10 7 cycles. However, recent studies have added to existing knowledge a warning that fatigue failure does occur at lives longer than N =10 7 , that is at N =10 8 –10 9 , at stress levels lower than the conventional fatigue limit. Among several possible factors for eliminating the classical fatigue limit, this chapter points out the importance of the influence of hydrogen trapped by nonmetallic inclusions. It is shown that the fatigue fracture surfaces of specimens containing different levels of hydrogen show very different fracture surface morphologies, and that the influence of hydrogen is crucial for the elimination of the fatigue limit in very high cycle fatigue (VHCF), that is N >10 7 . At the same time, when we discuss VHCF, we must understand the fundamentals of the mechanics of fatigue thresholds in that the threshold stress intensity factor range Δ K th for small cracks is not constant and is not identical with the values of Δ K th for long cracks. The values Δ K th for small cracks are much smaller than those for long cracks and depend on crack size. Based on this principle, we have to discuss the reason why small cracks emanating from nonmetallic inclusions grow even below Δ K th for long cracks and do not stop growth after reaching Δ K th for small cracks depending on crack sizes. The phenomena of subsurface originated fatigue failure in VHCF can be classified into the following two cases: 1. Fracture from nonmetallic inclusions. 2. Fracture from subsurface facets such as in Ti–6Al–4V and in Nickel-based Superalloy 718. Although there is a common feature in the above two phenomena, the crack initiation mechanisms are completely different between the two phenomena.