Quantitative fractography of fish-eye crack formation under bending–torsion fatigue

The paper presents results of the fractographical analysis of fish-eye cracks that were formed in plasma-nitrided steel specimens under symmetrical bending, symmetrical torsion and biaxial in-phase bending–torsion loading combinations. Due to higher strength and compressive residual stresses introduced by plasma nitriding procedure, the subsurface fish-eye cracks were initiated inside the specimen bulk as a dominant failure mechanism. The geometrical characteristics of the fish-eye cracks (the radial asymmetry, the average size and the elliptical coefficient) were studied in the relation to the fish-eye crack location, the loading regime and the fatigue life. Both the compressive residual stresses and the bending–torsion loading gradients were taken into account when discussing the results. Besides the well known fact that the nitrided surface layer reduces the crack growth rate, the study revealed that the average size of the fish-eye crack increases with both the distance from the free surface and the number of cycles to failure. The change of the fish-eye shape with the loading regime was also observed. A simple method for an estimation of the residual stress level within the nitrided layer was proposed. This method is based on the difference in the Wöhler curves of virgin and nitrided specimens under pure bending in combination with fractographical analysis.