Crack Paths 2012

rxy at lOOumin depth on the same line. Both stresses greatly changed during rolling

contact. Also note that (b) shows the distribution of peak of TXy in depth direction on the

middle line of the hole. 'CXy calculated by Hertzian theory, equivalent to a modelwithout

a hole, is also indicated. The peak of rxy became twice as big due to the hole defect. It

suggests that the defect, like the circular hole, accelerates the R C Fcrack initiation.

Stress intensity factors ofcracks with a circular hole

Fig. 7 shows the change in the SIFs of vertical and horizontal cracks with a circular hole

during rolling contact. Somepeaks and valleys of SIF appear at various points. The

largest peak and valley of SIF is that at mode11 SIF Kn which occurrs at the tip of an

intersection of a vertical crack with a horizontal crack (V-I). T w opeaks of modeI SIF

K; at the tip on the surface (V-S) occur before and after rolling contact.

Fig. 8 shows the comparison between the ranges of SIF A Kobtained from Fig. 7 and

the fatigue crack threshold SIFs AK,h. The m o d e11 and III threshold SIFs AKIN, and

AKIN”7 of the tested material have not been investigated yet. However,those using a

material of similar hardness (bearing steel) were investigated by some researchers [7].

Assumingthat the threshold of SIFs are the same for both tested material and bearing

steel, those by Matsunagaet al. [7] are indicated in Fig. 8.

In the figure, the condition in which A Kis larger than AK”, leads to the crack

propagation. It is possible that the vertical crack propagates at the surface and an

intersection in m o d eII and at the bottomin m o d eIII.

15

—l K“

Circular hole:q>15x200um

pmax=5.22GPa,

_\ O

[SKMPadIm]F

a=0.346mm V-S K.

U1

0

V:Vertica| crack

t'n

H:Horizonta| crack

S:Cracktip on surface

i

L. O

|:Crack tip at intersection

_ 1 5

i

i

|

—3

—2

—1

0

1

2

3

x/a

Figure 7. Change in the SIFs of cracks with a circular hole during rolling contact.

E 30 I m ,

—

I m o d e l

% 25

I m o d e l l

;‘ 20 I AKllthrAKlllth

Eimodelll

E 15

c0

5

f

g 0

,AKm-i

,

V-S

V-l

V-B

H-l

_

,

Evaluation point

Figure 8. ComparisonbetweenA Kand AKth.

584