PSI - Issue 7

6

Hisao Matsunaga et al. / Procedia Structural Integrity 7 (2017) 460–467 Hisao Matsunaga / Structural Integrity Procedia 00 (2017) 000–000

465

14

* Depth of hole-edge ranges from 0.05 mm to 0.175 mm.

12

10

∆ K II, drill [MPa ・ m 1/2 ]

8

6

d [mm]

d

4

0.05 0.08 0.1

Komata et al. ○

● ◆

Depth

2

6206

◇

×

51305

＋ ▲

0

10 8

10 9

10 6

10 7

Number of cycles to failure, N f

Fig. 5: Relationship between stress intensity factor range, ∆ K II, drill , and fatigue life, N f .

Load movement direction

Load movement direction

50 µ m

50 µ m

Cracks

Cracks

( a ) Inner ring of JIS-6206.

( b ) Inner ring of JIS-6206.

d = 0.1 mm, h' = 0.064 mm, q max = 2.5 GPa, N = 2.6 × 10

d = 0.08 mm, h' = 0.06 mm, q max = 2.5 GPa, N = 2.4 × 10

8 cycles

8 cycles

Fig. 6: Optical micrographs of drilled holes with small cracks in un-flaked bearings.

the ring-shaped crack, a' , was assumed to be zero in Eq. (1). In Fig. 5, all the fatigue life data were uniformly gathered inside a narrow band, irrespective of the diameter and depth of the hole. Consequently, it was manifested that the fatigue limit, as well as the fatigue life data, were uniformly evaluated by using the Mode II SIF range calculated for the initial defect. 3.4. Fatigue limit of a rolling bearing as a non-propagation limit of a small crack Figure 6 presents examples of the cross-section of un-flaked rolling bearings. Even in the un-flaked rolling bearings tested up to N = 2×10 8 cycles, a short fatigue crack was discovered at the edge of the hole. It was assumed that these cracks were non-propagating, or propagating at an extremely slow rate of growth. Thus, ∆ K II, drill obtained for these test conditions was treated as Mode II threshold SIF range, ∆ K IIth . Figure 7 shows the relationship between ∆ K IIth and the diameter of the drilled hole, d . It was concluded that the crack-size dependency of the threshold SIF