  Fracture coalescence is a critical phenomenon for creating large fractures
from smaller flaws, affecting fracture network flow and seismic energy release
potential. In this paper, simulations of fracture coalescence processes in
granite specimens with pre-existing cracks are performed. These simulations
utilize an in-house implementation of the Combined Finite-Discrete Element
method (FDEM) known as the Hybrid Optimization Software Suite (HOSS). The
pre-existing cracks within the specimens follow two geometric patterns: 1) a
single crack oriented at different angles with respect to the loading
direction, and 2) two cracks, where one crack is oriented perpendicular to the
loading direction and the other crack is oriented at different angles. The
intent of this study is to demonstrate the suitability of FDEM for modeling
fracture coalescence processes including: crack initiation and propagation,
tensile and shear fracture behavior, and patterns of fracture coalescence. The
simulations provide insight into the evolution of fracture tensile and shear
fracture behavior as a function of time. The single-crack simulations
accurately reproduce experimentally measured peak stresses as a function of
crack inclination angle. Both the single- and double-crack simulations exhibit
a linear increase in strength with increasing crack angle; the double-crack
specimens are systematically weaker than the single-crack specimens.
