PSI - Issue 14

Takashi Nakamura et al. / Procedia Structural Integrity 14 (2019) 978–985

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Takashi Nakamura/ Structural Integrity Procedia 00 (2018) 000–000

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where F I is a coefficient depending on both the aspect ratio of crack and the ratio of crack depth to specimen radius (Yoshinaka, et al., 2016) Fig. 6 shows d a /d N – Δ K curves. The crack propagation rate in vacuum was significantly lower than that in air. The difference was about one to three orders of magnitude at a given Δ K , and the pronounced effects of the vacuum were shown in the lower Δ K range corresponding to the small crack length. Especially, a very low propagation rate around 2× 10 -12 to 1× 10 -10 m/cycle were obtai ned below Δ K = 8 MPa√m. The minimum amount of crack propagation per loading cycle must be larger than the lattice spacing (≈ 10 -10 m); therefore, it should be noted that the crack propagation rate shown in Fig. 6 indicates the average crack propagation length per cycle during the measuring period of Δ N . The crack propagation rate could be less than 10 -10 m/cycle if the crack could not propagate continuously.

Fig. 6. Relationship between da/dN and ∆ in air and in vacuum.

Fig. 5. Crack length 2 a with respect to number of cycles N .

3.2. Fracture surface Fig. 7 (a) and (b) show the SEM image of the fracture surface near the crack initiation site ( Δ K = 7 MPa√ m) in air and in vacuum, respectively. Both photographs corresponded to the stage IIa crack propagation process where the crack propagates structure-sensitively. The features of fracture surfaces were quite different between the environments. The fracture surface in air showed a generally angular shape with sharp edges from a crystallographic jagged pattern (Fig. 7(a)). In contrast, the fracture surface in vacuum showed a generally ductile feature with more round edges. Especially, a few micrometer size granular areas were frequently observed in vacuum.

(a)

(b)

Fig. 8 (a) and (b) show the magnified photographs of Figs. 7(a) and (b). The difference between in air and in vacuum were more evident in these images. Angular sharp edges were clearly seen in air. In contrast, such characteristics could not be seen in vacuum, and a granular area with a fine concavo-convex pattern was obviously recognized. Fig. 8(c) shows a magnified view of the granular region in Fig. 1(b). Fig. 8(b) resembled Fig. 8(c) in form and shape of the granular feature. The magnified views of the fracture surfaces far from the crack initiation site ( Δ K = 18 M Pa√ m) in air and in vacuum are shown in Fig. 9 (a) and (b), respectively. These areas corresponded to the stage IIb crack propagation process where the crack propagates structure-insensitively. In comparison with the fracture surface in stage IIa (Fig. 7), a smoother appearance was Fig. 7. Fracture surfaces near the artificial defect, which correspond to the region around ∆ = 7MPa √ m : (a) in air. (b) in high vacuum.