Micro- and macro-scale fracture behaviour of brittle rocks: Comparison between the conventional Brazilian test and the advanced universal snap-back indirect tensile test (AUSBIT)

Post-peak behaviour is crucial for the estimation of rock mass fracturing in cave mining operations where hard rocks can exhibit class-II or snap-back response when subjected to loading. Despite the rapid development of research into class-II rocks under compression, the corresponding behaviour in tensile tests has rarely been investigated, which is critical considering the complexity of rock mass fracturing under various stress states. The post-peak response of brittle rocks involves abrupt micro-fracturing, leading to brittle macro-scale behaviour. Controlling the fracture process using the Advanced Universal Snap-Back Indirect Tensile test (AUSBIT) allowed the acquisition of the complete macro-scale class-II behaviour in the post-peak regime, facilitating the use of advanced techniques for insights into both micro and macro-scale fracture. In this study, the AUSBIT tests with digital image correlation (DIC) and acoustic emission (AE) instrumentation were conducted to analyse the progressive failure in Calca granite and Gosford sandstone specimens. Post-test observations of the fracture surfaces were performed using a scanning electron microscope (SEM). From a macroscale viewpoint, the lateral strain control in AUSBIT enabled controlled cracking with significant lateral strain extension prior to failure accompanied by gradual energy dissipation and higher rates of AE activity as smaller magnitudes of energy are being released by each AE hit or microcrack compared to conventional Brazilian tests. The stable microcrack propagation was also identified from SEM observations with more uniform profiles of microcracks and less debris observed in AUSBIT specimens. These findings were more significant in Calca granite, which verified its extreme class-II behaviour while also demonstrating the efficiency of AUSBIT in controlling the violent failure of high-strength brittle rocks commonly encountered in deep mining projects, leading to the acquisition of more accurate material behaviour in terms of micro and macro-scale post-peak features which was unattainable from conventional indirect tensile tests.