Crack Paths 2012

O f= 2 H 2I f = O 1 H Z

100

80

4\ 60

I

0

i

I

0

§ 4 0

0

2O

0 I

i

i

i

i

i

i

i

i

i

0 20000 40000 60000 80000 100000 120000 140000 160000 180000

N u m b eorfcycles

Figure9. Effect of the stress frequency on the evolution of crack size.

b. 3]6L stainless steel

The observation of the crack shape on stainless steel denotes a great homogeneity in

the depth of the sheet, as well as on its surface. This can be related to the uniform shape

of area on which is applied the load. The test madeat a frequency of lHz indicated that

the initiation of the crack from a pit of corrosion becomedetectable only after 17 hours

(figure 9). The initial size, estimated to 15pm, reached the value of 3 7 p mafter 34

hours. This growth is however slower in comparison with that noticed at a stress

frequency of 2Hz. This observation is more visible whenw e focus in the depth of the

sheet than in the surface. Furthermore, the irregular shape of the crack can be explained

by the electrochemical attack which is higher at lHz than at 2Hz [9]. For a stress

frequency equal to 0.5Hz, crack initiation is observed at 30 hours and is higher on the

surface than on depth [11].

A f=2Hz

I f=1Hz

o f=0.5Hz

A f=2Hz

I f=1Hz

a f=0_5Hz

:3

‘

1.6E-U9

70

1,4E-09

: 60

1,2E-09

% 50

L? 1,0E-09

4D

2% 8,0E-10

E 30

‘E 6,0E-10

i3 20

% 4,0E-10

10

V ,

g 2,0E-1o

U y

‘

‘

I

‘

0,0E+00

00E+00

5.0E+04

1.0E+05

1.5E+05

2.0E+05

"*0"

"'5"

1'0”

1:5"

2:0”

2'50

riumberofcycles (Ni

N((N' m

Figure 9. Evolution of crack size

Figure 10. Crack growth rate according

according numberof cycles.

stress intensity factor.

The nature and rate of crack can be estimated from the stress intensity factor A Kby

using the model of N e w m a annd Raju. This model takes into account the geometrical

shape in mechanical aspects and proposes a calculation methodof the threshold of the

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