Crack Paths 2006

M E T H O DFSO RT H ED E T E R M I N A T IOOFNL O N G - C R A GC KR O W T H

T H R E S H OVLADL U E

In the literature [e.g. 1-6] different methods for the threshold determination are

described. The methods can be divided into two main groups: methods I) with

decreasing and II) with increasing stress intensity factor.

According A S T ME 647 [1] of the American Society for Testing and Materials the

threshold value has to be determined with a decreasing stress intensity factor. This can

be obtained with a constant stress ratio R or with a constant maximumstress intensity

factor Kmax. If the method with R = const. is used the cyclic stress intensity factor (SIF)

' Kis reduced using the following exponential function (Fig. 1a):

(1)

0 A S T M 0 a a C e K K ' '

i.e. both the maximumand minimumSIF are decreased. The slope CASTM is defined as

follows:

C

K Ka d d1

˜

(2)

A S T M

'K0in Eqn. 1 is the cyclic SIF and a0 the corresponding crack length at the beginning of

the threshold experiment. In order to avoid interaction effects due to the load reduction

the load increments have to be chosen appropriately. This requirement is obtained by

limiting the slope CASTM, i.e. CASTM t -0,08 mm-1. Moreover the load steps should not

exceed 10%of the previous load and the width of the steps should at minimum0,5 mm.

a) 40

b) 40

35

'K0

35

C=-16,8 CFAM=-16,8

'K0,1

30

CASTM=-0,04

30

C=-24 CFAM=-24,0

'K 0,2

C=-28,8 CFAM=-2 ,8

12505

15 25

C=-36 CFAM=-36,0

CASTM=-0,08

'K 0,3

20

10

10

5 CASTM=-0,15

5

0

0

10

20

30 a [mm]

40

50

10

20

30 a [mm]

40

50

Figure 1. Load decreasing methods

a) Exponential decrease of ' Kwith different slopes CASTM

b) Linear decrease of ' Kwith different slopes CFAM