Fatigue Crack Paths 2003

As mentioned previously, the size of S N D C Ovaried with its location. This

phenomenon was also reported by Umezawaet al. [5]. However, the morphology of

S N D C Ohas also changed from more ductile fracture to facet with ridge when the position

of the S N D C Omovedfrom the surface to the centre (Figure 3).

(c)

(a)

(b)

Figure 3. S E Mmicrographs of the fractures in SNDCO:(a).

(b). Colony

Martensite lath,

facet with martensite lath, (c). Facet with ridge.

Table 3. Summaryof the fatigue testing results.

Sample N o

Distance (mm)

Size of initiation site (μm)

Size of "fish eye" (mm)

Crack origin

fatigue life

(cycles)

1,1

2,8

1

Martensite lath

90

321121

1,2

2,2

2

Martensite lath

145

294858

2,2

3,5

3

Martensite lath

154

525350

5,1

6,5

facet

4

220

372661

3,8

5,5

facet

5

171

344855

5,3

7,0

inclusion

6

318

254791

4,2

6,2

7

Colony facet

195

331834

1,8

2,8

8

Martensite lath

180

381302

1,8

3,2

9

Martensite lath

162

394146

2,1

4,0

10

facet

193

459100

3,0

5,0

11

Colony facet

225

367037

3,3

4,8

12

Colony facet

239

451173

3,1

4,4

13

Colony facet

164

500084

3,5

4,8

14

Colony facet

196

517825

3,6

6,2

15

facet

128

347000

3,4

4,2

16

Colony facet

134

385421

2,3

4,2

17

Colony facet

131

560520

2,1

4,2

18 Martensite lath

155

302189

Table 3 shows a summary of the testing results from this investigation. The size of

S N D C Oand “fish eye” strongly depends on the distance to the surface, and increases with

increasing the distance (Figure 4a and 4b). It may be due to the fact that the stresses

decrease from the surface to the center in the case of rotating bending fatigue. The