Cryofracturing effectiveness using liquid nitrogen on granite: Fracture toughness and Brazilian tests following freezing-thawing cycle

The influence of repeated liquid nitrogen (LN2) exposure treatments on the geomechanical integrity of granite samples from Kazakhstan was investigated using measures of fracture toughness and indirect tensile stress (Brazilian tests) with initial temperatures ranging from 100 °C to 500 °C. These tests were compared with baseline geomechanical tests conducted at 50 °C without exposure to LN2. Our research indicated that the LN2 exposure treatment in freezing-thawing cycles can significantly degrade granite strength, making permeability enhancement and mechanical impairment more pronounced. In the Brazilian test, the peak load before granite specimen failure decreases as the sample temperature difference between the initial temperature and the boiling point of LN2 at 1 atm increases, and the LN2 treatment duration increases. Our semi-circular bend (SCB) LN2-treated specimens' fracture toughness also decreased when the temperature differential between the granite and boiling point increased. The specimens heated to an initial temperature of 500 °C before LN2 treatment showed the most substantial alteration, as the thermal shock generated numerous new fissures and created extensions of pre-existing ones, yielding increased plastic behavior. Scanning Electron Microscope (SEM) analysis confirmed the formation of new or prolongation of pre-existing fractures. The novelty in this work is the combination of fracture toughness and Brazilian tests to corroborate the alteration of both plastic behavior and rock strength of granite subjected to thermal shocks of varying magnitude and the confirmation of dramatic alteration in granites with LN2 exposure with an initial temperature above 300 °C.