A new experimental approach to quantify microfractures in the Fracture Process Zone (FPZ) under various loading conditions

Abstract

Rock fracturing or breakage is a complex phenomenon that requires advanced and sophisticated techniques to predict fracture initiation and propagation in geological structures under in-situ or induced stresses. In previous studies less attention have been paid to quantify damage in the Fracture Process Zone (FPZ) under various loading conditions. This study presents the results of experimental cracked chevron notched Brazilian disc (CCNBD) tests undertaken to investigate the microfracture evolution at the FPZ for four different rock types: marble, monzonite, tuff and sandstone under static and cyclic (stepped cyclic and continuous cyclic) loadings. This research aims to determine the intensity of fractures by quantifying fracture density under various loading conditions. The Computed Tomography (CT) technique was successfully used to capture the fracture density and FPZ developed within the tested rock specimens. It was found that the width of FPZ is much wider in samples tested under the cyclic loading compared to that of static loading. The main difference between the micro-fractured surfaces refers to intergranular cracks between grain and matrix in cyclic loading, while the smooth and sleek cleavage planes of cracks was observed in static loading condition. Image processing results also showed that by applying cyclic loading, crack propagation is extremely contorted and crack surfaces are very rough compared to that of static loading test. Pixel quantity presented fracture morphology and microfracture quantity within the tested specimens. The ratio of pixels size was found mainly dependent on the loading condition, plastic deformation, grain size, fracture toughness and material stiffness. This study provides insight into the quantifying the fracture intensity and the shape of the FPZ in various rock types tested under the static and cyclic loading.