Energy analysis and microfracture characteristics of granite under dynamic modes I and II loading based on image processing techniques

Abstract

Notched semi-circular bending (NSCB) granite specimens were used in dynamic mode Ⅰ and mode Ⅱ loading conditions. Image processing techniques were innovatively used to determine the translational ($W_{tk}$) and rotational ($W_{rk}$) kinetic energies of each specimen. The true fracture surface area was determined using a scanning electron microscope at 2000 × magnification, enabling the calculation of the true fracture energy ($w_f$). Additionally, deep learning was used to quantitatively determine the tensile and shear morphologies on the fracture surfaces. The results showed that: (1) In mode I, $W_{tk}$ ranged from 1.47 to 9.07 J, and $W_{rk}$ ranged from 0.45 to 3.59 J. In mode II, $W_{tk}$ ranged from 0.98 to 7.88 J, and $W_{rk}$ ranged from 0.41 to 3.53 J. The total kinetic energy in mode I comprised over 51.5 % of the energy absorbed by the rock, while that in mode II comprised over 41.4 %. (2) The true fracture surface area of NSCB specimens was 2.1 times the nominal fracture surface area. (3) The $w_f$ of mode Ⅰ ranged from 1066 to 3577 J/m², and the $w_f$ of mode Ⅱ ranged from 1195 to 3550 J/m² (4) Under the same loading rate, the fracture surface of mode Ⅱ exhibited a larger proportion of shear morphology, explaining why the $w_f$ of mode Ⅱ was slightly higher than the $w_f$ of mode Ⅰ. (5) The $w_f$ of mode Ⅰ showed higher sensitivity to the variation of loading rate compared to mode Ⅱ, because the increase in shear morphology on mode I fracture surface was slightly greater than that on mode II fracture surface as loading rate was increased. These findings offer new insights into accurate fracture energy measurement and better understanding of rock microfracture characteristics under dynamic loading and demonstrate the importance of kinetic energy in rock fragmentation.