On the topography of fracture surfaces in bending–torsion fatigue

Fracture surfaces generated under combined bending–torsion fatigue loading in both the low-cycle fatigue and the high-cycle fatigue regions of specimens made of high-strength low-alloy Cr–Al–Mo steel are analysed in terms of topographical characteristics. Parameters reported here are the root mean square roughness, the number of peaks per unit length and the Hurst exponent quantifying various aspects of surface topography. As a main result of the analysis, the critical portion of torque beyond which the character of fracture topography significantly changes is estimated to be within the range of 40–50% of a total loading.