An asymmetric semi-circular bend method for investigating fracture behavior of brittle rocks under dynamic mixed mode I/II loading.

Abstract

Understanding the fracture behavior of brittle rocks under dynamic mixed mode I/II loading is imperative and beneficial for the stability analysis of rock engineering structures. However, available research studies on the dynamic mixed mode fracture of brittle rocks are rather rare, and existing testing methods own some complexity in the specimen preparation or experimental operation. In this study, a novel and efficient experimental method is developed for investigating the fracture behavior of brittle rocks under mixed mode I/II impact loading. For the first time, the asymmetric semi-circular bend (ASCB) specimen is introduced into a split Hopkinson pressure bar loading system to obtain dynamic mixed mode fracture toughness of granite. By virtue of digital image correlation techniques combined with high-speed photography, the progressive fracture processes of specimens with different mode mixity are also visualized. Results show that the dynamic force equilibrium and the crack tip initiation are satisfied for all the tests, validating the feasibility and reliability of this method. The fracture path of the ASCB specimen under dynamic mixed mode I/II loading deviates from the original loading direction and finally generates a curved crack, which is quite different from the straight fracture path under pure mode I loading. In addition, experimental results indicate the significant loading rate dependence of the dynamic fracture toughness, and the dynamic effective fracture toughness under mode I loading is greater than that under mode II loading at a given loading rate. Our proposed method is convenient and reliable to investigate dynamic mixed mode I/II fracture of brittle rocks.