Prakt. Met. Sonderband 38 (2006) 345 
initial ones. Further investigations will be needed in order to reveal whether these stresses 
are responsible for the internal crack initiation in the megacycle regime or not. 
iously an Carbide particle / cluster size effects on crack nucleation 
of fatigue A possible explanation for the transition of internal to near-surface crack initiation with 
> strength increasing life-time could be found in the relationship between applied stress amplitude 
ed mostly and defect (= carbide cluster) sizes required for crack initiation. The critical cluster area 
mens are increases with decreasing stress amplitude. At a stress amplitude of 650 MPa cluster 
at carbide diameters of about 60 um are needed to cause internal fish-eye fracture. However, Table 2 
ind on the reveals that cluster diameters of that size are virtually absent in metallographic studies. 
the crack Thus, internal crack nucleations are assumed to be single events and occur very rarely in 
le regime, the gigacycle fatigue regime. 
to identify 
> initiation Cavitation and corrosion 
)). Typical Since liquid cooling of the specimens during fatigue testing was applied, both cavitation 
yes. Only and corrosion might be held responsible for near-surface crack nucleation. Thus, SEM 
investigations of untested, as-polished, and run-out specimens (Nmax = 10E10) were 
performed. Corresponding micrographs are presented in Figure 9. As visible, there is 
virtually no difference in surface quality between the as-polished and the run-out 
specimens. (Scratches result from surface polishing and may differ marginally from sample 
to sample. Dark blotches represent primary carbides.). Thus, cavitation and corrosion are 
supponsedlv not responsible for initiatina near-surface cracks in aiaacvcle reaime 
(a) (b, 
Figure 9: SEM micrographs of (a) initial as-polished and (b) run-out specimens at 
narrowest section 
(a) typical 
ace shown 4. Summary and Outlook 
Ultrasonic tension-compression fatigue testing at 20 kHz and R = -1 of wrought cold work 
tool steel 1.2379 (X155CrVMo12.1) has revealed internal fish-eye type fracture in the 
ng internal megacycle and near-surface crack initiation in gigacycle regime. This transition from 
ymplitudes internal fracture to near-surface fracture was discussed in terms of compressive residual 
mpressive surface stresses and possible size effects of crack origins. In case of internal fish-eyes, 
ng fatigue large carbide particles and carbide clusters were definitely found to be nucleation sites of 
e fraciure. fatigue cracks. Cavitation and corrosion could be excluded as reason for near-surface 
le fatigue crack initiation in the gigacycle regime. Future works will also deal with the orientation 
were only effect of elongated carbides and the influence of the deformation degree on gigacycle 
ements at fatigue behaviour of this wrought cold work steel 
ual to the