INTRODUCTION
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 [2]
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 [4] and Jung et al. [5] 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 [6] 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 [2]. 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.
