Formula for calculating the critical stress intensity factor in rock fracture toughness tests using cracked chevron notched Brazilian disc (CCNBD) specimens

Abstract

Notched semi-circular bend (NSCB) method suggested by the International Society for Rock Mechanics and Rock Engineering (ISRM) only studies the static or dynamic brittle fracture in rock specimens with a local crack defect. This study aims to provide further insights into dynamic fracture behaviors of rock specimens containing a sharp V-notch loaded by a modified split Hopkinson pressure bar (SHPB) system. Ultra-high speed photography is utilized for real-time observation of the dynamic fracture process, and the two-dimensional displacement fields on the specimen surface are calculated using digital image correlation (DIC) method. The fracture parameters including the dynamic notch stress intensity factor (NSIF) at the sharp V-notch tip, and the dynamic stress intensity factor (SIF) and crack velocity at the propagating crack tip are first extracted from the displacement fields using the over-deterministic method. The effects of notch angle and loading rate on the dynamic notch fracture toughness before macrocrack onset and the dynamic crack propagation toughness after macrocrack initiation are subsequently studied. The experimental results are eventually compared with the numerical data obtained by the finite element method (FEM) and the existing results in the previous works. These results validate that the DIC method is applicable to study the dynamic brittle fracture in sharp V-notched rock specimens and predict the fracture resistance of V-notched rock components in actual rock engineering.