Study on the influence of indentation depths on the fracture characterization and acoustic emission characteristics of minerals in granite using nanoindentation

To investigate the intrinsic mechanisms of rock rupture at the mesoscale, this study employed nanoindentation tests on three primary minerals in granite at various indentation depths. Concurrently, rupture signals generated throughout the indentation process were recorded using the PCI-2 acoustic emission (AE) system. The results revealed that crack lengths in quartz and feldspar increased with indentation depth, whereas mica exhibited negligible radial cracking, showing only short, irregular cracks around the edges of the indentation marks. As indentation depth increased, the intensity of plastic deformation at the indentation point also increased, leading to a gradual reduction in fracture toughness and a corresponding rise in AE amplitude and energy. All three minerals generally showed a higher proportion of high-frequency signals compared to low-frequency signals, with the distribution characteristics of these signals being significantly influenced by the mineral properties. Quartz and feldspar exhibited relatively balanced frequency distributions, while mica had a higher proportion of high-frequency signals. At different indentation depths, most signals displayed characteristics of higher RA values and lower AF values, indicating that, at the mesoscale, the rupture modes of the three major minerals in granite are predominantly shear failures.