Mechanical response and brittle-ductile transition of porous sandstone under true-triaxial loading

This study investigates the influence of mean stress and Lode angle on the mechanical behavior of porous sandstone. Sandstone specimens were tested using a newly developed true-triaxial loading apparatus under five constant Lode angle conditions and seven different mean stresses, covering a transition from brittle to ductile regimes. Based on the experimental results, three types of stress-strain responses were identified, transitioning progressively from Type 1, through Type 2 to Type 3 as the mean stress increases. Type 1 response represents typical brittle behavior, characterized by prominent shear fractures. Type 2 response corresponds to the brittle-ductile transition behavior, exhibiting non-penetrating shear fractures. Type 3 response is associated with ductile behavior, characterized by no visible shear fractures. The deviatoric stress initially increases and then decreases with increasing mean stress, forming a cap surface in the meridian plane. A generalized failure criterion is subsequently developed, capable of accurately characterizing this strength response. Furthermore, the brittle-ductile transition behavior is found to be significantly dependent on the Lode angle. Finally, the brittle-ductile transition boundary is described, incorporating the dependence of Lode angle.