Issue 35

X. Liu et alii, Frattura ed Integrità Strutturale, 35 (2016) 88-97; DOI: 10.3221/IGF-ESIS.35.11 91 inclusion observed at initiation region [Fig. 4(a)]. Type II: Crack initiated from surface with surface or subsurface inclusion at initiation region [Fig. 4(b)]. Type III: Crack initiated from interior with inclusion and without FGA at initiation region [Fig. 4(c)]. Type IV: Crack initiated from interior with inclusion and FGA at initiation region [Fig. 4(d)]. The fatigue process associated with Type I and Type II is mainly in low-cycle fatigue regime, the fatigue process associated with Type III is mainly in high-cycle fatigue regime, and the fatigue process associated with Type IV is in VHCF regime. 10 4 10 5 10 6 10 7 10 8 10 9 10 10 200 400 600 800 1000 1200 1400 R =-1, Sur R =-1, Int R =-0.5,Sur R =-0.5, Int R =0.1, Sur R =0.1, Int R =0.3, Sur R =0.3, Int Stress Amplitude, MPa Number of Cycles to Failure ( a) 10 4 10 5 10 6 10 7 10 8 10 9 10 10 600 800 1000 1200 1400 1600 R =-1, Sur R =-1, Int R =-0.5, Sur R =-0.5, Int R =0.1, Sur R =0.1, Int R =0.3, Sur R =0.3, Int Maximum Stress , MPa Number of Cycles to Failure ( b) Figure 3: S-N curves of specimens with stress ratios of -1, -0.5, 0.1 and 0.3. (a) In terms of stress amplitude, and (b) In terms of the maximum stress (Sur: surface initiation, Int: interior initiation, Symbol with arrow: no broken.) Figure 4 : Typical examples of morphology of fracture surfaces for R =-1. (a) Crack initiated from surface without inclusion, σ a = 1048 MPa, N f =4.20×10 4 , (b) Crack initiated from surface with inclusion, σ a =1147 MPa, N f =2.03×10 4 , (c) Crack initiated from interior inclusion without FGA, σ a = 1029 MPa, N f =1.28×10 5 , and (d) Crack initiated from interior inclusion with FGA, σ a = 989 MPa, N f =1.11×10 8 . Fig. 5 presents the patterns of failed specimen with fatigue cracks initiated from inclusion with FGA morphology for the four stress ratios. It is seen from Fig. 5 that the morphology of FGA is very clear at the stress ratios of -1 and -0.5. However, for the stress ratios of 0.1 and 0.3, the morphology of FGA becomes obscure. In order to observe the microstructure below the FGA region, two fracture surfaces were chosen as shown in Fig. 5(a) and (d). The rectangle marks in Fig. 5(a) and (d) were the locations for the TEM samples cut by FIB, respectively. The samples were observed by TEM, and the diameter of electron diffraction area is 200 nm. The selected area diffraction (SAD) patterns of the sample from the crack initiation region at R =-1 in Fig. 6(b) and (c) show the discontinuous diffraction circles for the FGA region, (a) (b) (c) (d)