Dilatancy anisotropy and the response of rock to large cyclic loads

An experimental study of cyclic fatigue of granite and diabase in triaxial compression was conducted. It was found that dilatancy progressively developed during cyclic loading of these rocks with a form in time very similar to a creep curve. Dilatancy was in all cases highly anisotropic. This anisotropy was controlled by the crack microstructure in the granite and by the residual stress in the diabase. The strike of the fracture plane was found typically to form normal to the axis of maximum dilatancy. Increasing dilatancy during cyclic loading was found to be caused by progressive dilatant creep plus additional damage produced by the cycling itself. The former leads to a pronounced loading rate effect on fatigue, the latter to a complex effect of cycle amplitude on fatigue. No stabilization of fatigue by pressure was found for confining pressures up to 300 MPa. The total dilatant strain at fracture was found to increase strongly with peak stress and decrease with increasing loading rate. Three types of cracking are shown to result in dilatancy: type 1, stress-induced cracking; type 2, stress corrosion cracking; and type 3, fatigue cracking. Rock fracture is sensitive to which type is prevalent.

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