Recent experiments have shown that both Mode I and Mode II fracture propagation can
occur in brittle materials such as rocks. In uniaxial compressive tests Petit and Barquirts [ 1 ]
observed shear fractures extending from an inclined flaw in sandstone specimens; Reyes 
and Reyes and Einstein [3 ] obtained coalescence between two non-overlapping flaws in
gypsum due to the connection of secondary fractures. In shear tests of double-edge notch
beams, Bazant and Pffeifer  and Jung et al.  obtained the shear fracture between the
notches with concrete, granite and sandstone specimens.
These observations increased the interest to study the mixed Mode I and Mode II fracture
propagation. However, the existing criterion of fracture propagation usually can not be
directly used to simultaneously simulate the Mode I and Mode II propagation. Recently, Sheri
and Stephansson  proposed a F-criterion which is a modification of the strain energy
release rate criterion. The F-criterion considers both the Mode I and Mode II fracture
toughness in a single formula. The combination of the F-criterion and the numerical method
DDM was demonstrated to be applicable for both Mode I and Mode II fracture propagation.
In this study, the F-criterion approach is used to simulate the laboratory experimental
results of fracture propagation and fracture coalescence conducted by Reyes . The F-
criterion approach is also applied to analyze how the fracture toughness ratio (GIIc/GIc) and
stress ratio (ó3/ó1) affect the propagation mode of a single fracture.