344 Prakt. Met. Sonderband 38 (2006) 
3.3 Gigacycle Fatigue initie 
are | 
S-N-Curve 
Figure 7 shows the fatigue test results for the tested cold work steel. Obviously an Cart 
endurance limit cannot be identified since S-N-curve reveals a steady decrease of fatigue A po 
strength with increasing test cycle number. At 10E10 cycles a fatigue endurance strength incre 
of about 570 MPa was obtained. In megacycle regime, fracture surfaces revealed mostly and 
internal fish-eye fracture. Typical fracture surfaces of fish-eye failed specimens are incre 
presented in Figure 8 (a). Crack nucleation occurs at large carbide particles or at carbide dian 
clusters (Figure 8 (b)). In any case transgranular fracture of the carbides was found on the reve 
entire fracture surface, as exemplarily presented in Figure 8 (b) and (c) for the crack Thu: 
initiation site, which correlates with findings of Fukaura et al [4]. In the gigacycle regime, the ( 
fatigue cracks were initiated near the surface. Nucleation sites were difficult to identify 
exactly. In some cases several isolated primary carbides were found near the initiation Cav. 
site. Crack formation probably occurred at single carbide particles (Figure 8 (d)). Typical Sinc 
appearance of these sub-surface fractographs resembled semi-circular fish-eyes. Only and 
very few specimens failed due to surface cracks resultina from surface defects inve 
perf 
virtu 
spe 
to si 
supt 
(a) (b 
(& 
FigL 
narr 
(c. (d). „UM 
Figure 8: SEM images of internal fish-eye fracture in megacycle test regime: (a) typical 
fish-eye. (b) Carbide cluster at center of fish-eye. (c) Counterpart of fracture surface shown 4. € 
in (b). (d) BSE image of near-surface crack initiation. 
Ultre 
Surface residual stresses tool 
Considerable compressive surface stresses were measured, supposedly favouring internal meg 
crack initiation in the megacycle regime. Specimens fractured at high stress amplitudes inte 
and comparably short lives of 10E6 loading cycles contained reduced compressive surf 
stresses, probably resulting from relaxation either due to shake-off effects during fatigue larg 
testing, as found by Masaki et al [14] for shot-peened steel, or due to final fatigue fracture. fatic 
Surprisingly, these surface stresses seemed to be ineffective in the gigacycle fatigue crac 
regime, since near-surface crack nucleation was obtained there, and stresses were only effe 
slightly reduced during fatigue testing as shown by residual stress measurements at fatic 
specimens that ran out at N = 10E10 which revealed compressive stresses equal to the