The role of pre-existing microcrack geometry in fracture initiation and propagation during elastic deformation: integrating LEFM analysis with FEM modeling

Understanding the processes that govern fracture development in upper crustal rocks is crucial for characterizing the mechanical response of the Earth’s crust. While conventional failure criteria capture many aspects of fracturing observed in laboratory experiments, they fall short in explaining how system-spanning fractures emerge from the interaction and coalescence of microcracks distributed throughout a deforming rock mass. Additionally, empirical rupture models rarely distinguish the relative roles of tensile and shear mechanisms in macroscopic failure. In this study, we explore the influence of the geometrical arrangement of pre-existing microcracks on fracture formation by analyzing the elastic stress perturbations they generate, employing two-dimensional Finite Element Method (FEM) simulations. This approach allows us to quantify how cracks with different orientations modify the surrounding stress field, producing localized zones of elevated tensile and/or shear stress that may act as favorable pathways for fracture propagation. By systematically varying microcrack orientation and distribution, we can map how stress concentration patterns interact, providing a framework for understanding fracture coalescence in heterogeneous rock materials. Our results reveal that the orientation and spatial arrangement of pre-existing microcracks dictate the directions and magnitudes of stress perturbations, creating preferential trajectories for system-spanning fractures. In particular, regions of high tensile and shear stress develop between interacting cracks, offering a physical explanation for the formation of interconnected fracture networks, including en echelon fracture systems, under varying geometrical configurations. These findings indicate that macroscopic shear fractures may originate not only from the coalescence of tensile cracks formed during early deformation stages but also from the interaction of pre-existing cracks with different orientations, especially where tensile stress is concentrated at crack tips. The study demonstrates that the geometry of pre-existing microcracks is a primary factor controlling the spatial organization of resulting fracture networks. Fractures accommodating shear deformation, typically oriented at approximately ±30° to the axis of maximum compression, can arise from the coalescence of mode I cracks due to localized tensile stress concentration, rather than requiring shear-dominated initial conditions. This insight bridges a gap between classical fracture mechanics and observations of natural and experimental rock fracture systems, highlighting the interplay between tensile and shear mechanisms in shaping macroscopic failure patterns. Overall, our work emphasizes the importance of microstructural geometry in governing fracture evolution, offering a quantitative, framework which integrates LEFM analytical results with FEM-based models to predict the emergence of complex fracture networks from initial microcrack distributions. By linking local stress perturbations to large-scale fracture patterns, this study provides a more comprehensive understanding of the conditions leading to system-spanning fractures in the upper crust.