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Abstract Fractures initiated from free surface of layered rocks，which are called surface cracks，often terminate at the interface which divides the fractured layer and the matrix layer，but can continue to propagate along the interface. A numerical simulation is carried out to study the mechanism of surface fracturing process including micro-fracture formation，propagation，coalescence，nucleation，fracture infilling，fracture saturation，termination，and interface delamination. A model with a fractured surface subjected to uniaxial tension is simulated firstly. It is found that the stress distribution between fractures depends on the fracture spacing；and the stress distribution will directly affect the fracture mode. New fractures can either appear in the surface layer(parallel to existing fractures)，or appear between the layers(interface peeling). Furthermore，an existing fracture can propagate to the underlying layer with no fracture inserting(fracture saturation). Secondly，material homogeneity is considered in the simulation；and the results show that the stress distributions in homogeneous and heterogeneous models are different. A smaller stress is required to produce the same fracture spacing in the heterogeneous model；and the critical values of the spacing to layer thickness ratio at fracture saturation are almost the same. Lastly，stresses in a few sections during fracture formation are analyzed；and it is found that stress transformation can be used to explain the mechanism of fracture saturation. A fitting curve of the relationship between strain and spacing to layer thickness ratio is obtained. It is found that fracture spacing in the case of interface delamination is greater than that without interface delamination. Stress transition between the two layers on fracture spacing in the fracture process is also investigated，with a focus on stress transfer mode.
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Received: 25 July 2012
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2013, Vol. 32 
