Fractographically-aided analysis of fish-eye crack growth in nitrided steel

An analysis of the internal fish-eye crack growth in a plasma-nitrided high-strength
low-alloy steel specimen has been performed. The study combines information obtained from the fractographical observation of the striations field located near the centre of the fish-eye crack on
the specimen subjected to combined bending-torsion loading with the linear elastic fracture mechanics modelling of the fatigue crack propagation. The results suggest that the propagation stage constitutes only a minor part of the total fatigue life, the major part of which is spent on the fatigue crack initiation. This conclusion is consistent with the high cycle and very high cycle fatigue behaviour of specimens without any surface hardening layer.