The Effect of Loading Frequency on Fatigue Life of Green onyx under Fully Reversed Loading

Rock structures, in reality, may be subjected to reversed tension-compression loading conditions due to blasting, earthquake, and traffic or injection-recovery process. In this case, loading frequency may have a great influence on fatigue life and behavior of rocks. The effect of loading frequency on fatigue life of a crystalline rock sample under fully reversed loading condition was examined. A new apparatus, based on the R. R. Moore fatigue test machine, was used to assess the fully reversed loading condition. The experimental tests were conducted with five sets of loading frequency and five levels of stress based on the ultimate tensile strength of samples. The test results showed that the variation of ultimate fatigue stress versus loading cycle number (S-N curve) of crystalline rock samples were in accordance with the observed results in other materials such as metals and ceramics. The results showed that fatigue life of a component is positively correlated with the frequency level. The fatigue life improves with increasing the loading frequency, but the improvement was more tangible at lower stress levels than at higher stress levels. The effect of loading frequency in an ordinary S-N equation was also formulated. The results also showed that the loading frequency does not influence the fatigue limit.