The damage mechanism of rock fatigue and its relationship to the fracture toughness of rocks

This study presents the results of laboratory diametrical compression tests performed on Brisbane tuff disc specimens to investigate their mode-I fracturetoughness response to static and cyclic loading, as a function of the applied load. Both the static and cyclic loading tests were carried out on Cracked Chevron Notched Brazilian Disc (CCNBD) rock specimens. Two different types of cyclic loading were applied: (a) cyclic loading with constant mean level and constant amplitude, termed sinusoidal cyclic loading and (b) cyclic loading with increasing mean level and constant amplitude, termed increasing cyclic loading. The fracturetoughness response to cyclic loading was found to be different from that under static loading in terms of the ultimate load and the damagemechanisms in front of the chevron crack. A maximum reduction of the static fracturetoughness (KIC) of 46% was obtained for the highest amplitude increasing cyclic loading test. Conversely, for sinusoidal cyclic loading, a maximum reduction of the static KIC of 29% was obtained. Detailed scanning electron microscope (SEM) examinations revealed that both loading methods cause fatigue in the CCNBD specimens. When compared with static rupture, the main difference with the cyclically loaded specimens was that intergranular cracks were formed due to particle breakage under cyclic loading, SEM images showed that fatiguedamage in Brisbane tuff is strongly influenced by the failure of the matrix because of both intergranular fracturing and transgranular fracturing.