The effects of excitation frequency on fatigue life and fatigue damage were investigated by carrying out a series of cyclic loading tests at high stress levels on Inada granite. An attempt was made to estimate fatigue strength under long-period loading based on the experimental results. The amount of fatigue damage per cycle and fatigue life decreased with increases in the excitation frequency at the same maximum stress. On the other hand, they did not depend on the excitation frequency between 2.5 × 10 - 4 Hz and 0.5 Hz when the same maximum stress ratio, which was normalized by the static strength under a strain rate corresponding to the excitation frequency, was applied. Cyclic fatigue tests at the frequency of 0.5 Hz were performed under several stress ratios. It was clarified that : (1) there is no fatigue limit within 4 × 104 cycles of loading, (2) the relationship between stress ratio S and fatigue life Nf can be represented by S = A - B log Nf (S - Nf curve), (3) fatigue strength increases with increases in confining pressure, and (4) fatigue strength decreases in the presence of water. Estimation of fatigue strength under long-period loading might be possible using both the S - Nf curve and the relationship between static strength and strain rate.
