A novel outcome of the research described is the observation of the effect of indirect tensile cyclic loading on the fracture toughness of rocks. The mode I fracture toughness (KIC) response to indirect tensile cyclic loading was found to be different from that under static loading in terms of the ultimate load and damage mechanism in front of the chevron crack. A maximum reduction of the static KIC of 46% was obtained for the highest amplitude increasing cyclic loading test. A second series of cyclic tests were carried out under mode I-II (mixed mode) indirect tensile cyclic loading. The 45° and 70° inclined chevron notch cracks opened from the beginning of the cyclic loading test, whereas under monotonic loading, the chevron notch cracks closed up to failure. This outcome is a very important finding for exami-ning rock fatigue damage mechanisms. Detailed scanning electron microscope (SEM) examinations were carried out of the surfaces of CCNBD specimens. When compared with static rupture, the main differ-rences with the cyclically-loaded specimens are two-fold:
the number of fragments produced is much greater under cyclic loading than under static loading, and
intergranular cracks are formed due to particle breakage under cyclic loading compared with smooth and bright cracks along cleavage planes under static loading.
Moreover, the SEM images showed that fatigue damage in Brisbane tuff is strongly influenced by the failure of the matrix.
Cyclic loading often causes brittle materials such as ceramics and rocks to fail at a stress level lower than their determined strength under monotonic conditions. This phenomenon is commonly termed ‘fatigue’. Faults, joints, bedding planes, tunnel walls, excavation roofs and ribs, bridge abutments, and dam and road foundations are only a few of the natural and manmade rock structures that can be weakened by repetitive loading.