Investigation of the fracture process zone and behavior of the macro-scale fatigue cracks in brittle rock specimens

The importance of this study is to explain the fatigue damage mechanism while addressing the effect of fatigue on the fracture toughness (KIC) using the brittle Brazilian specimens and to show for the first time in the literature the behavior of macroscale cracks that open and close in brittle rock without leading to eventual failure. The KIC was reduced by 35% due to the cyclic loading, and the reduction of the indirect Brazilian strength was found to be reduced by 30%. The fatigue cracks were observed to open and close elastically without failure and have been recorded by a camera for hours in brittle rock specimens with sinusoidal loading for the first time in the rock mechanics field in this study. The findings of the scanning electron microscope (SEM) and computed tomography (CT) revealed that the failure of the Brazilian disc and chevron crack notched Brazilian disc (CCNBD) specimens was caused by the formation of the fracture process zone (FPZ), which included many microcracks rather than a single macrocrack propagation during the cyclic loading tests. Moreover, SEM and CT findings indicated the FPZ took place ahead of the kerf crack tip, leading to the visible fatigue crack opening and closing elastically in brittle rock specimens without any rupture. According to the experimental and numerical analysis results, the FPZmax could be obtained with the 60 degrees inclined notch crack. This demonstrates the maximum FPZ development possible with combined mode I-II (tensile and shear) loading.