Crack Paths 2006

Branched crack

ModeI crack

o n

t i

M n S

e c

ModeII

ModeII

ModeII

A x i a l d i r

crack

crack

crack

100Pm

100Pm

100Pm

200Pm

(c) N = 1500

(a) N = 0 (a) N=0

(b) N = 1000

(d) N = 1750

(b)N =1000

(c)N =1500

(d)N =1750

Fig.5 Behavior of crack initiation from MnSinclusionandpropagationin torsional fatigue. 1760 ure 3. Crack initi tion from M n Sinclusions and propagation under torsional fatigue,

W = 1300 MPa, Nf= 1760 (SAE52100).

W

1300MPa,N f

Fatigue crack path under ModeII fatigue loadings

Figure 4 shows a side view of the ModeII fatigue crack that was revealed by sectioning

the ModeII fatigue crack growth test specimen along its central section. The sectioned

plane of the specimen tested in a vacuumwas etched with a nital. In both tests in air and

in a vacuum, the fatigue crack first grew by Mode II. As the ModeII fatigue crack

grows, the value of the 'KII decreases. After 'KII reached the threshold value 'KIIth, the

crack branched by ModeI. The branched angles ± 70.5 deg. are the value calculated by

the local maximumnormal stress criterion (VTmax) [14] and the branched angle close to

r 45 deg. is the direction perpendicular to the remote maximumprincipal stress V1. The

side view of the ModeII fatigue crack in a vacuum was thinner than that in air. This is

due to the lack of oxidation in a vacuum.

45º

70.5º

A

A’

Magnification of A

Crack growth direction

Branched crack

A-A’ section

(ModeI)

45º

45º

A

B

45º

70.5º

Branched crack

(ModeI)

Branched crack (Mo e I)

ModeII fatigue crack

500Pm

(a) ModeII fatigue crack profile in a vacuum. Crack growth direction

ModeII

fatigue crack

(b) ModeII fatigue crack profile in air [12].

Figure 4. ModeII fatigue crack profile in air and in a vacuum.