The fatigue crack initiation behavior of a high-strength low-alloy steel, VAN-80, has been investigated. Crack initiation was defined, for the purposes of the study, as the existence of a 0.25-mm long crack on either surface of a modified compact tension specimen.

The parameter ΔΚ/p was used in an attempt to normalize data concerning crack initiation at three finite radius notches (0.79, 1.59, and 3.18 mm). While not conclusive, a volume effect on initiation was indicated with larger radii producing shorter lives at a given ΔΚ/p. Similar findings have been reported by previous authors for other materials. The experimentally determined value of (ΔΚ/p)_TH for VAN-80 is 835 MPa. Samples cycled at or below this value would not be expected to initiate a crack in 10⁶ cycles.

Block loading experiments were conducted in an attempt to isolate crack initiation from crack propagation processes. For all high block-low block tests, ∑n/N >1. This result, by itself, could be interpreted as suggesting that the total fatigue process is propagation controlled. However, the situation is complicated by the fact that the VAN-80 steel is sensitive to overload induced crack propagation delay. Therefore, cracks that might initiate during the high block load cycling period would be expected to grow more slowly at a lower subsequent stress range. Since the present definition of crack initiation contains a 0.25 mm increment of crack growth, propagation related delay effects may strongly influence or overshadow the actual initiation process.

Transmission electron fractographic observations reveal that the local ΔΚ at short newly initiated crack fronts is much lower than that predicted by standard ΔΚ calibrations for the test specimen. This is consistent with elastic solutions for the stress intensity factor for small cracks growing within the sphere of influence of large geometrical notches and other specimen shape changes. Scanning electron microscope images have shown that crack initiation occurred in notched VAN-80 samples by debonding of the globular cerium sulfides from the matrix at the surface of the notch root.