Numerical Analysis of Fracture Mechanisms in Granite with a Grain Size Gradient Using the GBM–DEM

To examine how grain-size distribution affects the mechanical response and fracture behavior of Lac du Bonnet (LdB) granite under uniaxial compression, numerical simulations were conducted using the particle flow code (PFC) with a grain-based model. By displacing grain centroids in different directions along the y-axis, four LdB granite models with distinct grain sizes were generated, with grains delineated by Voronoi tessellation. The main findings are as follows: (1) The flat-jointed constitutive model reproduces the experimental response well, and introducing unbonded contacts (micrometer-scale gaps) improves the simulation of crack-closure behavior during loading. (2) Secondary cracks initiate predominantly at grain boundaries, and the yield stress is strongly associated with the evolution of intragranular tensile cracks. (3) Grain size governs the sequence of crack accumulation (tensile vs. shear), the growth rate and spatial correlation of damage, and the distribution and intensity of local failures; smaller grains hinder macroscopic damage, whereas larger grains are more readily penetrated and filled by microcracks. (4) Mechanical cutting tests show that grain-size combinations produce several dominant secondary-failure modes; the failure thickness is controlled by the penetration depth of the subsequent cutting head, and the stress concentration near the cutting head is sensitive to grain size.