PSI - Issue 2_A

Akiyoshi Nakagawa et al. / Procedia Structural Integrity 2 (2016) 1199–1206 Author name / Structural Integrity Procedia 00 (2016) 000–000

1201

3

Typical examples of the fracture surfaces in the surface-initiated fracture and the interior-initiated fracture at the same stress level of 1300MPa reported by Sakai (2009) are shown in Fig.2(a)-(c), respectively. Fig.2(a) indicates the fracture surface in the surface-initiated fracture with the short life of N f =2.7 × 10 3 , whereas Fig.2(b) gives the fracture surface in the interior-initiated fracture with a fish-eye having the long life of N f =2.66 × 10 6 . Fracture surface around the inclusion at the core portion of the fish-eye was again observed with a higher magnification as shown in Fig.2(c), where a characteristic fine granular area (FGA) was found in the vicinity around the inclusion. The formation mechanism of such a particular region of FGA is one of the important subject in the very high cycle fatigue as reported by many researchers such as Sakai (2002) & (2015), Murakami (1999) and Shiozawa (2006), but this point is not discussed in the present paper, since it is not the target of this work.

10µm

100µm

100µm

(c) FGA around inclusion at the center of fish-eye in (b)

(b) Interior-initiated fracture with fish-eye ( 6 f 2.66 10 = × N )

(a) Surface-initiated fracture ( 3 f 2.7 10 = × N )

Fig.2 Fracture surfaces for the bearing steel (JIS:SUJ2) failed in rotating bending

2.2. Effect of loading types on fatigue fracture characteristics

Fig.3 shows a couple of fracture surfaces failed in the respective loading types of rotating bending and axial loading reported by Sakai et al. (2002). In the case of rotating bending, the crack initiation site is limited within a surface layer having a certain critical depth c ξ , whereas the crack initiation site in the axial loading can be distributed over the entire section of the specimen. This aspect can be attributed to the fact that the stress distribution has a distinct slope across the section of the specimen in rotating bending, while the distribution becomes uniform in axial loading. It is also noted that a clear fish-eye is found at the crack initiation site regardless of the loading types.

ξ

500µm

500µm

(b) Fracture surface with a fish-eye in axial loading ( 900 = a σ MPa,

(a) Fracture surface with a fish-eye in rotating bending ( 1200 = a σ MPa, 8 f 1.30 10 = × N )

8

f 1.94 10 = × N )

Fig.3 Fracture surfaces in rotating bending and axial loading in very high cycle regime