New insights into the continuous-discontinuous failure characteristics of granite under Brazilian splitting test conditions using acousto-optic-mechanical (AOM) method

Abstract

Tensile failure in brittle rocks is crucial for the stability of rock engineering applications. However, a comprehensive quantitative evaluation of continuous-discontinuous tensile failure characteristics and the effect of loading platen shape on the microcracking behavior induced by tensile stress in granite, as well as the corresponding variations in the evolution of Fracture Process Zone (FPZ) at the microscale, remain unclear. This study describes an effective acousto-optic-mechanical multi-physical field observation system to capture the real-time acoustic, optical and mechanical response of a coarse-grained granite during Brazilian splitting tests applying various loading platen shapes. An improved 3D acoustic emission (AE) location algorithm is introduced and validated to interpret the spatio-temporal evolution of crack source mechanisms. Three stages of microcracking, namely quiet stage (QS), stable stage (SS), and outburst stage (OS) are identified based on the AE characteristics. We found that a distinction between different loading conditions is easier to detect at relatively low loading rates. Digital Image Correlation (DIC) results show that the loading platen shape affects the initial position of the tensile stress-induced crack initiation and the corresponding deformation. High-Speed Photography (HSP) images captures the evolution of FPZ and visible crack opening behavior. Both, indirect tensile strength and splitting elastic modulus are found to be sensitive to loading rates, with varying degrees of sensitivity under different loading conditions. The fractal dimension results closely correlate with the monitored COD outcomes, highlighting a smaller Joint Roughness Coefficient (JRC) value on the failure surface under arc platen loading conditions compared with flat platen loading conditions. The evolution of b-values reveals the transition from microcracking to macrocracking, which is consistent with the stress and FPZ evolution. The influence of loading platen shape on indirect tensile strength and failure characteristics of granite is theoretically validated. Finally, correlations among the AOM characteristics under different loading conditions and their practical implications are investigated.