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 investi-gate the FPZ evolution for red sandstone under dynamic mode I fracturing and its effect on dy-namic 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 dy-namic crack initiation toughness was approximately 1.6 -2.1 times the uncorrected one within the loading rate ranging from 8.5 -78.2 GPa center dot 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.

Automated summary

This study investigates the evolution of fracture process zone (FPZ) and its effect on crack initiation toughness in dynamic rock fracturing. The results show that the FPZ in dynamic fracturing initiates earlier than in quasi-static cases, with a width independent of the loading rate. The LEFM theory underestimates the crack initiation toughness.