Issue34

S. Kikuchi et alii, Frattura ed Integrità Strutturale, 34 (2015) 261 - 270; DOI: 10.3221/IGF-ESIS.34.28

 K th

= -2.60 R + 4.39 : IP series

(1)

 K th

= -2.09 R + 3.82 : Harmonic series

(2)

10 -8

10 -8

(b)

R = 0.1 R = 0.3 R = 0.5 R = 0.7 R = 0.8

(a)

R = 0.1 R = 0.3 R = 0.5 R = 0.7 R = 0.8

10 -9

10 -9

10 -10 Crack growth rate d a /d N , m/cycle

10 -10 Crack growth rate d a /d N , m/cycle

10 -11

10 -11

2

3

4

5 6

2

3

4

5 6

Stress intensity range  K , MPam 1/2

Stress intensity range  K , MPam 1/2

Figure 3 : Relationship between crack growth rate and stress intensity range for the (a) IP and (b) Harmonic series.

5

4

3  K th , MPam 1/2

IP series

Harmonic series

2

0.0

0.2

0.4

0.6

0.8

1.0

Stress ratio R

Figure 4 : Relationship between threshold stress intensity range and stress ratio for the IP and Harmonic series.

Crack closure The role of stress ratio at near-threshold levels is generally attributed to crack closure [17]. Fig. 5 shows the relationship between d a /d N and  K eff . In each series, the value of  K eff tended to normalize the stress ratio onto a single curve near threshold under R = 0.5. This result indicates that the effect of the stress ratio on the  K th disappears. However, when R is greater than 0.5,  K th value still decreased with increasing stress ratio, R . Same tendency was observed in the previous

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