Multi-scale modeling of blasting-induced fracture in polycrystalline granite with grain boundary effects

This study presents a multi-scale finite-discrete element modeling approach for blasting-induced fracture in polycrystalline granite, with explicit consideration of grain boundary effects, to accurately reproduce the mesoscopic heterogeneity and dynamic fracture responses of granite under ultra-small diameter borehole blasting. A Voronoi-based polycrystalline geometric model is established via Neper software to characterize mineral distribution and microstructural anisotropy. Cohesive elements are simultaneously inserted into intragranular and grain boundary regions in Abaqus with differentiated mechanical parameters, and the Jones-Wilkins-Lee (JWL) equation of state is used to apply the dynamic blasting load of PETN explosive. Numerical results agree well with laboratory blasting tests, showing typical failure zones including a crushing zone, a radial fracture zone, and a circumferential tensile fracture zone. The polycrystalline model exhibits prominent non-uniformity and dynamic anisotropy in crack propagation, which is strongly governed by grain morphology and grain boundary properties. Grain boundary strength is identified as a key factor controlling the dynamic fracture mode: with decreasing grain boundary strength, the failure pattern gradually shifts from transgranular fracture to mixed fracture and then to intergranular fracture. Under moderate grain boundary strength, blasting energy is first transmitted inside grains and then released and dissipated at weak grain boundaries, forming a chain-type dynamic failure mechanism: intragranular energy transfer to grain boundary fracture. The proposed method reveals the micro-dynamic evolution mechanism of granite damage under ultra-small diameter blasting and provides a reliable theoretical basis for blasting parameter optimization, rock fragmentation control, and blast-induced vibration prediction in precision rock blasting engineering.