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Fracture In Layered Rock Under Mixed Tensile and Compressive Stress StatesF.M. Chester and J.S. Chester
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Extension fracture and shear fracture are common modes of brittle failure often recognized in rock deformation experiments. These fracture types are analogous to joints and small faults observed in nature. We are using experimental rock deformation and petrofabric techniques to study the formation of hybrid fractures (i.e., fractures with characteristics intermediate to extension and shear fractures) under mixed compressive and tensile stress states. Many questions remain concerning the mode of failure and mechanical behavior across the transition and applicability of the Mohr-Coulomb failure criterion.
Using two novel experiment designs, that of dogbone samples deformed under triaxial extension and layered cylinders deformed under compression, we have demonstrated a continuous transition in macroscopic fracture orientation and surface morphology across the transition from classic extension fractures to shear fractures.
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Fractured dogbone samples showing progressive change in fracture angle with increase in confining pressure. |
Map of fractures produced in layered cylinder of limestone (gray) and sandstone (white). (from Beser & Chester, 2005). |
Currently, we are using experiments to determine the exact form of the failure envelope from extension to shear fracture and the details of fracture growth. Acoustic emissions are monitored to determine relative timing and number of microfracture events leading up to macroscopic failure, and sequential strain experiments provide samples for petrofabric analysis of different stages of fracture development. Detailed petrofabric analysis using optical and scanning electron microscopy, and surface profilometry are used to characterize the morphology of the macroscopic fracture surface and determine the microscopic structure and mechanisms of deformation. Specific goals of the project include testing the stepped-crack model for formation of hybrid fractures, developing a micromechanical model of fracture development consistent with experiment observations, and deriving a failure criterion for hybrid fracture.
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Map of hybrid fracture surface and associated microfrctures showing en echlon pinate fractures. |
Roughness profiles of hybrid fracture surfaces showing stepped geometry interpreted as treads and risers. (from Rodriguez, 2005). |