Crack Paths 2006

experimental fracture plane orientation when the coefficient k increases. The k

coefficient cannot increase to infinity since with the increasing k value the damage

degree increases.

1

Vzz,a

= 182 M P a

Vzz,a= 212 M P a

0.8

Vzz,a

= 156 M P a

G =0,OWV=0.68

0.6

Experimental fracture lane orientations

0.024

-80

-60

-40

-20

0

20 40 60 80

o

D,

Figure 1. Normalized damage degree calculated according to the max{V

n} multiaxial

fatigue failure criterion as a function of the plane orientation.

After the critical plane evaluation, the fatigue failure criteria were used to calculate

the fatigue life. The following errors parameters were applied for the fatigue life

verification:

) ( 1

E ¦N i

¦ N i m i

,

)(exp )()(logiicali T T

,

std

i m E E N 1

) ( 1 . (23)

E

E E N 2 1

For ideal consistency of the i-th calculated fatigue life with the i-th experimental fatigue life, the error parameter E(i) is equal to zero. If E(i) is negative, the fatigue life estimation

is conservative (safety). The mean error parameter Em reflects the general results

conformity. The standard deviation error parameter Estd is the superior parameter since

it reflects the scatter of the results and therefore gives us the information about the

failure criterion ability to correlate the different kind of multiaxial stress states and the

equivalent damage parameter. The second-rate parameter Em depends on the material

constants and the stress gradient.

Fig. 2 (a) presents the comparison between the experimental fatigue lives T exp and the

calculated fatigue lives T cal obtained by the maximumnormal stress criterion. It is very

interesting to notice that the scatter of the results is very small for the specimens that

exhibit one macroscopic fracture plane orientation which coincide with the maximum

normal stress plane. The results exposed by the criterion of the maximumshear stress

(Fig.1 b) show very large scatter Estd= 0.69 although the mean error is very small

Em = 0.05. The other criteria that combine the shear and normal stress/strain

components on the critical plane (Findley, Matake, SF) are not appropriate for the steel

analysed (Table 4).