Micro-scale observations of semi-brittle failure in Carrara marble

Abstract:

We studied the generation and extension of micro cracks during semi-brittle failure of rectangular prisms (12 mm x 6 mm x 6 mm) of Carrara marble under uniaxial compression (i.e., with no confining pressure) at temperatures ranging from 25 to 184 °C and compared these observations to existing damage models. Using a long distance microscope, we generated a series of sequential images of micro-scale grid made of square markers with sides about 7.5 mm long and spacing of 12.6 mm. The grid covered a region of 1 mm x 0.8 mm near the center of one free surface. By measuring the relative displacements of the grid markers, we generated 2 – D finite strain distribution maps at each stage of the experiment. Deformation was concentrated along cracks. To study and quantify their evolution additional filtering stage was applied. At 105 °C and 184 °C, the number and length of micro cracks increased with increasing load, and near the peak stress, they intersected and coalesced. By measuring the number and vertical dimension of the cracks intersecting the surface, we calculated a damage parameter as defined by Ashby and Sammis [1990]. In the two experiments mentioned above, the damage sustained by the samples near the peak stress was much larger than that calculated from the model. In a third experiment at room temperature, failure was very abrupt and no micro cracks were observed within the region analyzed.