Microcracking mechanisms of Hong Kong granite : insights from acoustic emission and microscopic observation

Abstract

A comprehensive study of the microcracking behavior of rocks is of crucial importance for better understanding and predicting the associated macroscopic cracking process. Although the characteristics of the fracture process zones (FPZs) caused by microcracking in granite under static mode I loading have been extensively investigated experimentally, the microcracking mechanisms of different granites under static mode I and mixed-mode I-II loadings are still not fully understood and merit further study. Three research gaps are identified accordingly: (1) The effect of specimen notch shape on the microcracking and cracking behavior is not experimentally investigated; (2) Systematic studies on the influencing mechanisms of the mineralogy and textural properties of granite on the microcracking behavior are limited; (3) The microcracking mechanisms of granite under mixed-mode I-II loadings are less studied. To address the gaps and advance the understanding of the microcracking mechanisms, this thesis aims to experimentally study the effects of three factors, namely the specimen notch shape in mode I fracturing tests, the mineralogy and textural property of granite, and the loading condition on the microcracking behavior of granite. Based on the semi-circular bending test, the microcracking processes and associated FPZ features of Hong Kong granite are investigated using the acoustic emission and microscopic observation techniques. The key findings are: First, the specimen notch shape significantly affects the microcracking behavior of granite in mode I fracturing tests. As compared with the specimen with a chevron notch, more intense microcracking during the FPZ development phase is observed in the specimen with a straight-through notch, which results in a relatively larger FPZ for the latter specimen. The larger FPZ could be responsible for the lower mode I fracture toughness measured by the straight-through notched specimen. Secondly, regarding the effects of the mineralogy and textural property of granite on the microcracking behavior under mode I loading, the pre-existing microcracks are found to have significant influences on the spatial-temporal evolution features of microcracks as well as associated FPZ characteristics. Besides, the presence of substantial thermally-induced pre-existing microcracks would lead to a transition in microcracking behavior. One the other hand, the mineralogy and grain size only affect the spatial evolution feature of microcracks. Finally, the loading condition is found to have insignificant effects on the temporal evolution features of microcracking, while it changes the characteristics of the AE event density contours corresponding to the fully-developed FPZs. Additionally, mode II loading alters the microcracking mechanisms under mode I loading condition in terms of the temporal evolution of event-type ratio preceding the macroscopic crack initiation. The main contributions of this study lie in three aspects: first, the effects of notch shapes, the mineralogy and textural properties, as well as the loading condition on the microcracking mechanisms of Hong Kong granite are explored with the aid of the acoustic emission and microscopic observation techniques. The outlined approach is of practical value to other researchers. Secondly, a new conceptual model describing the microcracking behavior of granite under mode I loading by considering the effects of pre-existing microcracks is proposed. Thirdly, a precursor in terms of the variations of event-type ratios preceding the macroscopic crack initiation in granite is identified. The study also reveals the effects of pre-existing microcracks and load condition on this precursor.