Influence of loading frequency on ULCF life of structural steel under variable surface roughness

This paper investigates the effects of a wide range of loading frequencies ranging from 0.005 to 5 Hz and blasting induced surface roughness ranging from 20 to 100 μm on the ultra‐low cycle fatigue (ULCF) life of structural steel. The combined effect of these parameters on ULCF life has not been investigated so far. To this aim, comprehensive ULCF tests were performed on steel specimens under a constant displacement‐controlled environment until the failure occurred. The experimental results manifest that the surface roughness and loading frequency are the governing parameters affecting the ULCF life. The test material exhibits cyclic softening behavior under all loading conditions. An increase in loading frequency from 0.005 to 5 Hz causes a significant increase of 33% and 27% in ULCF life for 20 and 100 μm rough specimens, respectively. However, the effect of frequency on ULCF life becomes less significant at higher values of surface roughness. Moreover, an increase in surface roughness from 20 to 100 μm causes a significant reduction of about 45% and 47% in ULCF life at the loading frequencies of 0.005 and 5Hz, respectively. Furthermore, a correlation between fatigue life and loading frequency as a function of surface roughness is derived by the power law relationship.