Recent Advances in Fatigue Crack Growth

摘要

Many of the recent advances in the understanding of the fatigue crack growth process have resulted from an improved realization of the importance of fatigue crack closure in the crack growth process. Two basic crack closure processes have been identified. One of which is known as plasticity-induced fatigue crack closure (PIFCC), and the other is roughness-induced fatigue crack closure (RIFCC). Both forms occur in all alloys, but PIFCC is a surface-related process which is dominant in aluminum alloys such as 2024-T3, whereas RIFCC is dominant in most steels and titanium alloys. A proposed basic equation governing fatigue crack growth is (da)/(dN)= A(△K_(eff)-△K_(effth))~2 (1) where △K _(eff) = K_(max)-K_(op) where Kmax is the maximum stress intensity factor in a loading cycle and Kop is the stress intensity factor at the crack opening level. △K_(effth) is the range of the stress intensity factor at the threshold level which is taken to correspond to a crack growth rate of 10-11 m/cycle. The material constant A has units of (MPa)-2, and therefore Eq. 1 is dimensionally correct. Eq.l has been successfully used in the analysis of both long and short cracks, but in the latter case modification is needed to account for elastic-plastic behavior, the development of crack closure, and the Kitagawa effect which shows that the fatigue strength rather than the threshold level is the controlling factor determining the rate of fatigue crack growth in the very short fatigue crack growth range. Eq. 1 is used to show that 1.The non-propagating cracks observed by Frost and Dugdale resulted from crack closure. 2.The behavior of cracks as short as 10 microns in length can be predicted. 3.Fatigue notch sensitivity is related to crack closure. 4.Very high cycle fatigue (VHCF) behavior is also associated with fatigue crack closure.