Transgranular Crack Nucleation in Carrara Marble of Brittle Failure

Understanding the microcrack nucleation is of a fundamental importance in the study of rock fracturing process. Due to variations in texture and mineralogy, different rocks may show different distinctive microcrack nucleation mechanisms. In order to understand the microcrack nucleation mechanisms in Carrara marble comprehensively, localized damage zones are artificially produced by loading specimens containing an array of en-echelon flaws in this study. Then, representative samples were cut from those loaded specimens and prepared for optical observation. Four types of microcrack nucleation mechanisms leading to the formation of transgranular cracks have been identified in Carrara marble. Type I and II mechanisms are favored by the distinctive polygonal shape of the crystal grains in Carrara marble. Local tensile stress concentration in these two mechanisms is attributed to grain sliding and divergent normal contact force, respectively. Type III mechanism is associated with the gliding along twin lamellae. The resultant tensile stress concentration could nucleate microcracks within the grain containing these lamellae or in the grain boundary. No microcracks in the adjoining grains were observed in this study. Our statistical analysis suggests that type III mechanism favors the nucleation of new cracks which are nearly perpendicular to the gently inclined twin lamellae and thus have a small angle with the maximum loading direction (about 15°). Type IV mechanism operates in grains failed mainly due to compressive stress rather than tensile stress concentration. Sets of parallel microcracks of this mechanism seem to be related to the crystallographic planes of calcite. The microcracking results also suggest that most of the grain boundaries in damaged zone have been cracked at the loading about 80 % of the specimen strength, while transgranular cracks begin to occur at that time and flourish after about 90 % loading of the strength.