Abstract
The behavior of initiation and propagation of cracks on quench-tempered SNCM8 steel fatigued under combined axial loading and torsion was investigated in particular relation to the inhomogeneity of metallurgical structure. The main conclusions are as follows:
(1) An inhomogeneous structure was produced in the steel as a result of segregation of manganese and the lower hardness constituent of the structure determined the fatigue strength of the steel.
(2) The stage 1 cracks tended to form along the maximum shear stress plane and to be more localized in the lower hardness constituents as τa/σa-ratio became higher, where τa and σa were shear stress and axial stress amplitudes, respectively.
(3) The threshold amplitude of stress intensity for Mode I crack, KIath, was 5.07 MPa√m and that for Mode II crack, KIIath, was 12.4 MPa√m.
(4) When τa/σa≤1/√3, the specimens were fractured by the preferential propagation of Mode I crack originating from non-metalic inclusions. The critical length of Mode I propagating crack at the fatigue limit was observed to be about 0.1 mm. This agreed with an estimation by using the KIath value. When τa/σa≥√3, the fatigue limit coyresponded to the critical stfess amplitude below which plastic strain amplitude was hardly detectable macroscopically.
