Fatigue Crack Paths 2003

boundaries (Figure 6d). This indicates that the formation of a crack was caused either by

the intrusion and extrusion process in the ferrite phase in bainite or by the stress

concentration due to the pile-up of dislocations near the grain boundaries. Therefore, a

possible crack formation mechanism for S N D C Ocan be:

1). Formation of cracks in the ferrite phases in bainite due to the intrusion and extrusion

process or at the grain boundaries by the pile-up of dislocations.

2). A transition from shear cracking to tensile cracking then leads to the formation of

crack initiation site (Stage I cracking).

Influence of Microstructure on Fatigue Life

In this investigation, the tests were performed under the same stress condition. However,

the fatigue life varied from 2,54x105 cycle to 1,08x106 cycles. Since the difference of the

core hardness in the material is small, the microstructure may have an important effect on

the fatigue life. Figure 7a shows the influence of grain size on the fatigue life. As

expected, the fatigue life is longer in the material with smaller grain size. However, the

fatigue life is shorter if the crack initiation site is an inclusion. In order to investigate the

influence of fatigue crack initiation and propagation on the fatigue life, the influence of

the number of grains in the S N D C Oand the “fish eye” on the fatigue life was studies

(Figure 7b and 7c). It shows that the fatigue life increases with increasing number of

grains in the S N D C Oor in the “fish eye” up to about 300 grains. These results are

expected. During the formation process of the SNDCO,the microstructural fracture

mechanics (MEM)will dominate. This means that the grain boundaries become the

barriers for fatigue crack propagation. Consequently, the fatigue life increases with

increasing number of grains. With further increase in crack length, the elastic-plastic

fracture mechanics (EPFM) or even linear and non-linear elastic fracture mechanics

become dominant, and the grain boundaries as barrier become less important.

1E+7

1E+6

1E+6

r e

r e

r e

csycle t o f a i l u

csycle t o f a i l u

csycle t o f a i l u

1E+6

e r o f

e r o f

e r o f

N u m b

N u m b

N u m b

inclusion

(a)

(b)

(c)

inclusion

1E+5

1E+5

1E+5

0

5

10

15

0

100

200

300

400

500

30

5

10

15

20

25

Grain size (µµm)

Numberof grains in initiation site

Numberof grains in "fish eye"

Figure 7. Influence of grain size on the fatigue life: (a). Grain size, (b), Numberof grains

in SNDCO,(c). Numberof grains in “fish eye”.

The difference in the fatigue lives started at an inclusion and a non-defect origin mainly

depends on the fatigue crack initiation life. According to a dislocation model proposed by

Mura et al. [8, 9], the fatigue crack initiation life at a grain boundary and at an inclusion is