Analysis of the failure process in rocks is important for both engineering and geological applications. A combination of experiments, observations, and calculations have shown that nucleation, growth, and interaction of microcracks are the dominant micro mechanisms that control the macroscopic failure of compact rocks at low temperatures and pressures.
To observe and quantify the production of micro fracturing from initial yield to failure, we deformed Carrara marble samples in uniaxial compression at various temperatures and continuously observed a region of about 1 mm2 on an exposed face. Using image processing, micro-scale strain-mapping, and filtering techniques, we observed the evolution of microcracks near failure, as well as their mode and magnitude of deformation. In general, shear deformation was more prevalent after initial yielding, while tensile deformation dominated closer to peak stress. Independent measurements of both stress and microcrack density at different stages of each experiment provided a unique opportunity to explicitly compare the data with damage models. The model of Ashby and Sammis  significantly underestimated the damage that the rock could sustain before peak stress, perhaps owing to the influence of weak grain boundaries on the damage production.