Crack Paths 2012

than 99%. It is evident that for fatigue damage including V H C Fregime, the fatigue life

is almost consumed by crack initiation process, i.e. the formation of FGA.

Table 2. Calculation results of N1, N2, Ni and da/dN within F G A

for two groups of specimens

N1 (N 1/ Nf) aFGA to afisheye

Speccodiemen

N f

N2 (N2/ Nf) afisheye to aK1c

aN

a

RB-12345678 5.60×106 92 049 7 12353 7408089 8

5.78×104(1.02%) 7 1 0 75 9 2 44 5 55 31 7 83 3 68 25 8 57 17 5 3 01

3.71×105(6.54%) 39 3 1 4 28 2 04 3 51 1 96 22 34 7 21 87 0 76 69 09

925789.57573519%

1.41×10-12

1.75×10-12

3.75×10-13

1.77×10-12

1.35×10-12

8.83×10-13

7.35×10-13

1.64×10-13

1458989282745679.1860609750378%

ULL--11234567891023 4.53×105 7671 1065657 6 27151 823054210392439 78

4.49×104(9.91%) 85 6 79 62 7 64 1 6 5 5 8 3 49 4 27 1 52 3 8 5 38 4 69 0 64 35 39 5 28 52 7 72 14 3 87 03 5 41 4

3.26×105(72.04%) 10 43 45 2 41 39 7 50 5 44 32 13 91 3 21 11 46 30 4 58 2 71 3 70 3 35 06 2 58 2 52 8 0 3 70 26 2 47 19

2.67×10-11

1.48×10-11

1.16×10-11

4.89×10-12

4.95×10-12

2.87×10-12

1.02×10-12

3.13×10-12

7.59×10-13

2.13×10-12

3.57×10-13

1.26×10-13

1.71×10-13

From Fig. 8 and Table 2, it is seen that the value of crack extension rate within FGA,

da/dN~ F G A , is between 10-11 and 10-13 m/cycle, and the value decreases with fatigue life,

Nf. For Nf between 106 and 107, the values of da/dN~ F G A are within the range of 10-11

and 10-12 m/cycle. For Nf between107 and 4u108, i.e. V H C Fregime, the values of

da/dN~ F G A are in the range from 10-12 to 10-13 m/cycle.

The data shown in Fig. 2 and Table 1 illustrate a large scattering of fatigue life at the

same stress level for TT150 specimens. The observations and measurements on the

inclusions from which the cracks originate show that the range of inclusion size, 2aInc, is

between 20 and 60 P m (Table 1), and the value is inversely proportional to the fatigue

life. For Nf between 106 and 107, the value of 2aInc is ranging from 30 to 50Pm. For Nf

between107 and 4u108, i.e. V H C Fregime, the value of 2aInc is ranging from 20 to 30

Pm, substantially smaller than the size before V H C Fregime. It is evident that the

inclusion size is responsible for the large scattering of fatigue life, which is the weakest

link phenomenon.

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