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