PSI - Issue 19

Akira Ueno et al. / Procedia Structural Integrity 19 (2019) 494–503 Author name / Structural Integrity Procedia 00 (2019) 00 – 000

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Fig. 14 Example of SEM micrographs of fatigue fractured surface.

Figure 13(a), (b) show an example of SEM micrographs of fatigue fractured surface. A fatigue crack emanated concentricity from thin hole surface and a striation can be observed quite clearly. Fig. 14(a) shows a schematic of a disc shape crack initiated from internal surface of thin cylinder. By considering an internal pressure p applying crack surfaces, a stress intensity factor range  K of this crack can be expressed by Eq. (2) (Stress Intensity Factors Handbook DVD Ver.). (2) Also, as shown in Fig. 14(b), crack length a can be calculated by using co-ordinate of observation points of each striation. Fig. 15(a), (b) show a relationship between striation spacing s and stress intensity factor range  K obtained on the high Ni eq and low Ni eq fracture surfaces, respectively. It is observed that fatigue crack propagation behavior of each testing conditions seem to same. Therefore, the cause of fatigue lives reduction on low Ni eq materials shown in Fig. 8(a) and (b), may be referred to a reduction of fatigue crack initiation lives.

(a) Ni eq = 27.0 % ( R = -1.0).

(b) Ni eq = 25.7 % ( R = -1.0).

Fig. 15 Relationship between striation spacing s and stress intensity factor  K .