Micromechanics of Deformation in Rocks

Laboratory testing of rocks subjected to differential compression have revealed many different mechanisms for extensile crack growth, including pore crushing, sliding along pre-existing cracks, elastic mismatch between grains, dislocation movement, and hertzian contact. Micromechanical models based on fracture mechanics have been developed for these different mechanisms by many different researchers. In this paper, the KI solutions for these micromechanical models are reviewed. Because of the similarity in rock behavior under compression in a wide range of rock types, it is not surprising that these micromechanical models have many similarities. This may explain the success of models based on certain micromechanisms in spite of the lack of evidence for these mechanisms in microscopic studies. Based on these similarities, a generic micromechanical model is proposed that in some way takes into account all of the above phenomena. It is demonstrated how the KI solutions from the micromechanical models can be used to derive nonlinear stress-strain curves that exhibit strain-hardening and strain-softening, dilatation, σ2 sensitivity, and rate dependence. By using subcritical crack growth, transient and tertiary creep behavior can also be predicted. Also, it is shown how these micromechanical models can form the basis for continuum damage models using the finite element method.

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