Exploring the brittle behavior of 3D-printed sandstone analogs under various stress conditions

Abstract Rock masses essentially consist of two components: the intact rock and spatially distributed rock joints of various orientations and persistence. The first step to characterize the behavior of a rock mass is to understand the strength and deformability of the intact rock by performing repetitive destructive laboratory tests on intact rock specimens. The challenge imposed in these tests is the existence of inherent microscopic heterogeneity, which can lead to a substantial variability. 3D printing technologies, specifically the binder jetting technique using sand and furan, has been recently adopted in studies of rock analogs due to its ability to produce synthetic sedimentary rocks with complex geometries and known characteristics, and reduced variability. For a synthetic material to be used as an analog for a natural brittle rock, it is essential to demonstrate that this analog exhibits brittle behavior similar to that of natural rocks. Although there have been several recent advancements in 3D printing applications in the geomechanics field, studies that thoroughly evaluate the 'brittle' behavior aspects of rock analogs are lacking. In this study, a set of requirements is established to evaluate the brittle behavior of the 3D-printed rock analogs, derived from uniaxial and triaxial compression, Brazilian, and fracture toughness laboratory tests, including the tensile and compressive strengths, crack initiation and crack damage thresholds, stiffness, and brittle-ductile transition. Analysis of laboratory test results showed that the analogs behaved similar to moderately-to-strong natural sandstones with a UCS in the range of 28–32 MPa and tensile strength of 5–6 MPa, with an acceptable repeatability among the test results. Under triaxial loading, the failure process was dominated by extensional fracturing at low confinement with a transition to a macroscopic shear failure mode at higher confining pressures (up to 15 MPa). At confining pressure beyond the Mogi line, compaction failure occurred indicating a typical brittle to ductile transition, which is corroborated by microstructural analyses.