Experimental assessment of mode I fracture toughness and fracture energy in four rock types using the pCT testing method with two notch radii

Abstract

Understanding mode I fracture behavior in rocks is essential for handling geomechanical problems, including tunneling, mining, and hydraulic fracturing. This study investigates the mode I fracture toughness (KIC) and specific fracture energy (Gc and Gf) in one sandstone, one limestone and two marble lithologies using the pseudo-compact tension (pCT) test. To assess the influence of the notch radius, specimens were prepared with two notch radii: 0.15 mm (thin) and 0.5 mm (thick). The key novelty lies in comparing notch radius effects across multiple lithologies while explicitly accounting for heterogeneity via ultrasonic wave velocities and micro X-ray fluorescence elemental mapping. Results show that, for the limestone and the marbles, the thick notch yields higher values of KIC, Gc, Gf. In contrast, the sandstone shows the opposite trend, with higher values for thin-notch specimens. This behavior is likely attributed to its high porosity, where stress concentration around pores can generates secondary crack tips near the blunt notch tip, that reduce the fracture toughness and energy dissipation. Across all rocks, Gf > Gc, indicating substantial dissipation by microcracking and grain-boundary processes beyond pure surface-energy creation. In addition, a thinner notch improves repeatability and reduces data dispersion, thereby mitigating heterogeneity effects on measured parameters. These findings provide practical guidance on selection of notch radius and highlight the importance of multiscale heterogeneity characterization for advancing rock mechanics research and refining fracture testing methods, thereby improving accuracy and reducing variability.