Damage accumulation and crack initiation detection based on the evolution of surface roughness parameters

Cyclic strain localization generates sharp surface slip markings in the form of depressions and elevations on the surface of materials. The process gives rise to the formation of persistent slip bands and results in immanent changes that manifest themselves in the form of surface roughening. The heights and depths of these extrusions and intrusions grow during cyclic loading up to a critical value leading to crack initiation. In this study, we investigate the evolution of the surface roughness parameters starting from pristine specimens and ending in fmal fracture in fully-reversed cyclic bending tests. Results are presented for both low-and high-cycle fatigue that covers a wide range of surface finish. Two types of contacting (via a stylus) and no-contacting (optical) pro-filometers were used in this investigation. The most sensitive and useful surface roughness parameter for the assessment of fatigue growth in low-and high-cycle fatigue is identified, and it is shown that results can be utilized to detect the onset of fatigue crack nucleation. For this purpose, a surface roughness criterion for detecting crack initiation at different applied loads is introduced.