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.

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