The characteristics of fatigue crack growth at stage II in Cr-Mo steel (SCM 435) have been investigated synthetically from both sides of crack closure and fracture mechanisms by varying the tempering temperature (150°C-600°C) and stress ratio (R=0.06, 0.4).
The following results were obtained;
(1) The crack growth resistance decreased as the tempering temperature became low or the stress ratio became high. Such crack growth behaviors depended not only on the crack closure mechanism, but also on the fracture mechanism.
(2) The crack closure and fracture mechanisms in stage II fatigue crack growth played different roles at each of three general fracture regimes, namely stage IIa, stage IIb and stage IIc.
(3) At stage IIa and stage IIb, the crack closure was observed and the crack growth rate, da/ dN, could be correlated with the effective stress intensity factor ranges, ΔKeff, regardless of the tempering temperature and stress ratio.
(4) At stage IIc, the crack closure was not observed, and the area percentage of dimple fracture due to the pseudo-monotonic fracture mechanism was more than 50%. In this region, da/dN was controlled by the maximum stress intensity factor, Kmax, because the percentage increased with increasing Kmax.
