Experimental investigation of fracture process zone of rock in dynamic mode I fracturing and its effect on dynamic crack initiation toughness

Fracture process zone (FPZ) is critical to the applicability of linear fracture mechanics (LEFM) in rock fracturing, but little attention has been given to quantify the FPZ under dynamic loading condition in previous studies. This study presents the experimental results of notched semi-circular bending (NSCB) tests undertaken by split Hopkinson pressure bar (SHPB) to investigate the FPZ evolution for red sandstone under dynamic mode I fracturing and its effect on dynamic crack initiation toughness. A hybrid displacement–strain calibration method was proposed to characterize the FPZ with high-speed digital image correlation (DIC) technique. Results show that the FPZ initiation in dynamic fracturing is much earlier than that in quasi-static cases, and the FPZ length fully develops when crack initiation taking place at the pre-peak stage. The FPZ in dynamic fracturing is of a semi-elliptical shape with a width independent from the loading rate. The opening displacement exhibits a nonlinear decreasing trend along the FPZ boundary. LEFM theory underestimates the inherent dynamic crack initiation toughness, and the corrected dynamic crack initiation toughness was approximately 1.6 ∼ 2.1 times the uncorrected one within the loading rate ranging from 8.5 ∼ 78.2 GPa•m1/2/s in present study. The FPZ length in dynamic mode I fracturing lies in between the values predicted by Schmidt model and Irwin model, which indicates a non-linear cohesive stress with higher gradient near the FPZ tip is distributed along the FPZ.