Abstract: This paper studies the influence of the grain size and the notch effect on the fracture of U-shaped notched rock beams through the variation of the apparent fracture toughness. The research is based on an exhaustive campaign that comprises numerical simulations of 300 four-point bending tests, 30 uniaxial compression tests and 60 tensile splitting (Brazilian) tests. Non-porous, isotropic ideal and equivalent rocks with 5 different uniform grain sizes are modelled using the distinct element method, where the rocks are modelled as a discontinuous material, defining explicitly the grains and the boundary conditions. Several notch radii are simulated and the corresponding variation in the apparent fracture toughness is observed. The notch effect is interpreted using the Theory of Critical Distances (TCD), which uses a material intrinsic property called the critical distance () to evaluate the stress field around the notch tip. The paper shows the variation of the fundamental rock properties with the grain size, the applicability of the TCD to evaluate the notch effect and the correlation between the critical distance and the grain size.

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