On the mechanism of fatigue failure in the superlong life regime (N > 10(7) cycles). Part II: a fractographic investigation

High cycle fatigue fracture surfaces of specimens in which failure was init iated at a subsurface inclusion were investigated by atomic force microscop y and by scanning electron microscopy. The surface roughness R-a increased with radial distance from the fracture origin (inclusion) under constant am plitude tension-compression fatigue, and the approximate relationship: R-a C DeltaK(I)(2) holds. At the border of a fish-eye there is a stretched zone . Dimple patterns and intergranular fracture morphologies are present outsi de the border of the fish-eye. The height of the stretch zone is approximat ely a constant value around the periphery of the fish-eye. If we assume tha t a fatigue crack grows cycle-by-cycle from the edge of the optically dark area (ODA) outside the inclusion at the fracture origin to the border of th e fish-eye, we can correlate the crack growth rate da/dN, stress intensity factor range DeltaK(I) and R-a for SCM435 steel by the equation da/dN = 2.92 x 10(-12) DeltaK(I)(2) and by da/dN proportional to the parameter R-a. Integrating the crack growth rate equation, the crack propagation period N- p2 consumed from the edge of the ODA to the border of the fish-eye can be e stimated for the specimens which failed at N-f>10(7). Values of N-p2 were e stimated to be similar to1.0 x 10(6) for the specimens which failed at N-f congruent to 5 x 10(8). Tr follows that the fatigue life in the regime of N -f > 10(7) is mostly spent in crack initiation and discrete crack growth in side the ODA.