ABSTRACT:

Tensile fracture is a common form of failure in many rock engineering applications such as hydraulic fracturing in layered rocks. Layered rocks often exhibit anisotropic mechanical properties because of the layering, but an additional contribution can arise from oriented mineral textures. Specifically, micro-structural variations in grain sizes and bonding strengths inside each layer may introduce a higher degree of anisotropy. There is a wide range of variability of laboratory-measured fracture toughness of natural rock specimens because of micro-structural textures. However, the influences of the textures on the tensile fracture toughness remain unclear. A recent laboratory study conducted three-point bending tests on 3D-printed layered rocks with controlled microstructural textures. The test results showed that the tensile fracture toughness is closely related to both the relative orientation between the rock layers and in-layer textures. We constructed a digital layered rock model using the distinct element method (DEM) to simulate the laboratory 3D-printed rocks. The layers were simulated by planes of weak bonding strength and the textures were approximated by a periodic spatial distribution of bonding strength within a layer. The digital rock specimen was brought to failure by a quasi-static constant rate of loading. The simulation results match the laboratory measured peak failure loads and fracture roughness for samples with different relative orientations of layering and mineral textures. Our study shows the possibility of accurately predicting fracture roughness and fracturing resistance in layered rocks given detailed mineralogical information.

1. Introduction

Brittle failure in rock is common in nature. Understanding the mechanics and physics of brittle failure are crucial for applications in geo-resource exploitation, seismology, and rock engineering. Sufficient loads, such as fluid pressure, thermal stress, or other external loads on the rock result in the initiation of micro-fractures and subsequent growth into meso-fractures. The mesofractures coalesce into a fracture process zone whose propagation can eventually cause catastrophic rupture (Griffith, 1921).

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