Impact of Fluids on the Mode I Fracture Toughness of Two Granites and One Sandstone

Fluids affect the mechanical behavior of geomaterials, including properties such as unconfined compressive strength and brittleness. However, their impact on mode I fracture toughness (K_Ic) has been less explored. This study investigates the impact of saturating fluids on the K_Ic of three rock types: a porous siliceous sandstone (Corvio) and two high-strength, low-porosity granites (Blanco Mera and Blanco Alba). Pseudo-compact tension (pCT) specimens (diameter ∼50–54 mm, thickness ∼25 mm, notch depth ∼16 mm) were saturated with seven different fluids (deionized water, methanol, NaCl-saturated water, mineral oil, diesel fuel, an acidic HCl solution and a caustic NaOH solution) and tested under identical conditions. Results show that all fluids reduce K_Ic, but the extent varies with rock type and fluid properties. Aqueous fluids caused the most significant reductions, with deionized water having the greatest impact on granites (∼18%–30%) and the acid solution on sandstone (∼70%). Non-polar hydrocarbon fluids, despite their lack of reactivity, caused moderate effects attributed to poro-mechanical effects. Additionally, pH-shift experiments, involving sequential exposure to alkaline and acidic solutions, mitigated fluid-induced weakening. This behavior is hypothesized to stem from silica dissolution in the alkaline phase, followed by rapid nucleation and precipitation during the acidic phase, forming silica-rich coatings on mineral surfaces. Fracture energy was not equally distributed, with higher post-peak energy absorption due to crack bifurcation, grain rotation or friction. These findings underscore the interplay of lithological factors, fluid properties and chemical processes in fracture behavior, with implications for subsurface engineering and modeling of fluid-rock interactions.