Anisotropy in the mechanical properties of rock is often attributed to bedding and mineral texture. Here, we use 3D printed synthetic rock to show that, in addition to bedding layers, mineral fabric orientation governs sample strength, surface roughness and fracture path under mixed mode I and II three point bending tests (3PB). Arrester (horizontal layering) and short traverse (vertical layering) samples were printed with different notch locations to compare pure mode I induced fractures to mixed mode I and II fracturing. For a given sample type, the location of the notch affected the intensity of mode II loading, and thus affected the peak failure load and fracture path. When notches were printed at the same location, crack propagation, peak failure load and fracture surface roughness were found to depend on both the layer and mineral fabric orientations. The uniqueness of the induced fracture path and roughness is a potential method for the assessment of the orientation and relative bonding strengths of minerals in a rock. With this information, we will be able to predict isotropic or anisotropic flow rates through fractures which is vital to induced fracturing, geothermal energy production and CO2 sequestration.
The layers of sedimentary rocks arise from depositional, compaction and diagenetic processes. Within a layer, preferred mineral orientation caused by interlocking crystals, foliations, or fragments may exist. Past research has shown that layers or preferred mineral orientations are known to affect the mechanical properties of rock leading to direction dependent or anisotropic elastic moduli (Jones and Wang, 1981; Vernik and Nur, 1992; Hornby et al., 1994; Sayers, 1994, 2005; Wenk et al., 2007; Tavallali and Vervoort, 2013; Chandler et al., 2016; Forbes Inskip et al., 2018; Dutler et al. 2018).
Our previous work (Jiang et al., 2019; Jiang et al., 2020) examined the role of mineral fabric orientation in layered rock on tensile fracture formation. For Mode I failure induced through three point bending (3PB) experiments, tensile fracture toughness and roughness were controlled by the relative orientation between the layering and the in-plane mineral fabric.