Issue 41

D.-Q. Wang et alii, Frattura ed Integrità Strutturale, 41 (2017) 143-148; DOI: 10.3221/IGF-ESIS.41.20

level. It seems that the plastic zone size as compared with strain increasing rate is a better parameter for correlating crack growth rate.

140

N N N

-1

OL

120

OL

+1

OL

100

80

60

(%)

max

40

P/P

20

0

0

5

10

15

20

25

 COD (pixel)

Figure 5 : The evolution of COD at the crack wake at OL-1, OL and OL+1 cycles (data points are extracted from the measurement with a distance of 50  m behind the crack tip)

Crack growth rate Strain increasing rate Plastic zone size (area)

overload,N=31

3x10 -4 4x10 -4 5x10 -4

6x10 -2 8x10 -2

1.4x10 3 1.6x10 3

1.2x10 3

4x10 -2

2x10 -4

10 3

 m 2

mm/cycle

2x10 -2

10 -4

8x10 2

pre overload

post-overload

6x10 2

0

30 60 90 120 150 180 210

Number of cycles N

Figure 6 : Variations of crack growth rate, strain increasing rate and plastic zone size with number of cycles

C ONCLUSIONS

F

or the low hardening coefficient steel in this work, an in-situ fatigue test under an overload (OLR =1.2) revealed little influence of overload on fatigue crack growth behavior. The measured crack tip strain and crack opening displacement behind crack tip have similar response to external loading. Plastic zone size can correlate crack growth rate very well in case of overload.

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