PSI - Issue 51

Abdelhak Nehila et al. / Procedia Structural Integrity 51 (2023) 152–159 A.Nehila and W. Li / Structural Integrity Procedia 00 (2022) 000–000

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crack propagation area and the unstable crack propagation area have approximately similar semi-elliptical shapes, which indicates that the crack growth rate in the two stages is roughly the same.

（ b ） surface defects induced failure （ σ a = 400 MPa ， N f = 562000 ）

(a) Fracture surface of notch specimen （ σ a = 300 MPa ， N f = 255000 ）

(c) Fracture surface of notch specimen （ σ a = 300 MPa ， N f = 255000 ）

（ d ） surface defects induced failure （ σ a = 550 MPa ， N f = 92000 ）

Fig. 3. Observation of typical fracture surfaces of notched specimens, R = ‒1.

3.3. Life prediction

3.4.1. Notch effect on fatigue life According to Kitagawa and Takahashi (1976) the fatigue limit increases with the decrease of crack size. When the crack size is below a certain value, the fatigue limit will remain constant. So, Kitagawa revealed that as long as the fatigue limit and the threshold intensity factor are known, the specific crack size value can be determined by the linear elastic fracture mechanisms (LEFM). Based on this theory, El Haddad et al. (1979) proposed a model to describe this relationship: th f Δ Δ ' K a σ π = （ 2 ） Eq. (2) was modified in order to determine the fictional crack size a ’: th f 1 Δ ' ( ) Δ K a π σ = （ 3 ）