Crack Paths 2012

Table 1: Test conditions

material

7075 T651

S355 steel

Test n°

ccp2a ccp2Na ccp7a ccp5a ccp2s ccp3Ns

environment

air

NaCl

air

air

air

NaCl

Rangeof apparent

7-18

7-18

6-20

9-20

18-60 18-60

AKI (Mpa\/m)

stress range (MPa) 34

34

35

40

100

100

notch width (mm) 15

15

10

15

10

10

cycles for crack

30000 10000 23000 15000 35000 27000

initiation

cycles of crack

202440 110260 163000 59000 157730 138730

growth until fracture

The topography of the fracture surfaces were reconstructed, thanks to digital optical

microscopy (fig. 1).The magnification used for image capture was 50 and 13 images

had to be stitched to map the whole crack from the notch root to the onset of ductile

fracture. The results were obtained as (x, y, z) triplets, where x denotes the distance

from the notch root along the free surface, y, the position in depth, and z the height from

the notch plane (see fig 1), with a step of 4.5 p m in the xy plane and 35pmin the z

direction. Longitudinal and transverse height profiles, z(x) or z(y) were derived.

Polynomial expressions were fitted to these profiles and used to compute the local tilt

and twist angles as arctang(dz/dx) and arctang(dz/dy), respectively. The shear lips

width, denoted by ts, was also deduced from the transverse height profiles.

Figure 1: Topographyof the fracture surface of specimen ccp2s.

77