Quantitative fractography in bending-torsion fatigue

The paper deals with a quantitative analysis of fracture surface roughness generated under combined bending-torsion fatigue in a high-strength low-alloy steel. Changes in several amplitude, spectral and hybrid profile roughness parameters are investigated for sets of profiles parallel and perpendicular to the local crack propagation direction in dependence on the loading ratio. Profile fractal dimension is also calculated as a “scale-independent” characteristic. Most of profile roughness parameters start to rapidly increase above the critical value rc ≈ 0.5 of the loading ratio r = τa/(σa + τa), where σa is the bending amplitude and τa is the torsion amplitude.