Abstract

Rock engineering deals with a material containing multiple fractures. The critical value of stress intensity factor when a pre-existing flaw begins to develop is defined as fracture toughness. Thus, fracture toughness measurement is essential to support rock engineering design. Numerous works have revealed that fracture toughness of rock under dynamic loading is significantly different from that under quasi-static loading. However, the data within intermediate level dynamic loading, which is defined as the loading ranging between quasi-static and dynamic loading, is still limited. A dedicated loading device generated by a non-explosive powder reaction was designed to determine dynamic fracture toughness under intermediate dynamic loading. Compared to a Split-Hopkinson pressure bar, which is commonly used in dynamic testing, the device adopted by this study provides a lower loading rate. A notched semi-circular bend specimen type was selected to measure mode-I fracture toughness. The specimen was made of granite and sandstone. The results of dynamic loading test were compared with quasi-static properties tested using the hydraulic servo-controlled machine. It indicates that fracture toughness rises with increasing of loading rate. The values were also plotted together with the others' findings and showed an acceptable agreement. Failure behavior corresponds to a numerical simulation performed by Ansys Autodyn. This study contributes to the current understanding of rock dynamics, particularly in the intermediate dynamic loading range that can be useful in assessing field engineering application such as rock excavation process.

1.
Introduction

Fracture toughness is one of the critical parameters in designing structures in rock mass. Rock mass incorporates multiple discontinuities, so the critical stress intensity factor of a discontinuity is vital to be evaluated. Since rock is most prone to tensile failure, the tension mode (mode-I) fracture is experienced more frequently than the shearing mode (mode-II) or the tearing mode (mode-III). Many methods have been proposed to determine mode-I fracture toughness such as short rod and chevron bending (Ouchterlony, 1989), Brazilian disc (Guo et al., 1993), cracked chevron notched Brazilian disc (CCNBD) (Fowell et al., 1995), chevron notched semi-circular bend (CNSCB) (Kuruppu, 1997) and notched semi-circular bend (NSCB) (Kuruppu et al., 2014).

Engineering practices deal with dynamic events such as impact load, blasting shock wave, earthquake, and so on. Numerous studies have reported that fracture toughness subjected to dynamic loading is significantly different from that under quasi-static loading (Zhang and Zhao, 2014). Understanding of fracture behavior under dynamic loading is essential. Most of the established methods on the dynamic fracture toughness test are based on a split-Hopkinson pressure bar (SHPB) device (Dai et al., 2011; Zhou et al., 2012). So far, there is no established method to determine fracture toughness under intermediate dynamic loading condition that ranges between quasi-static and dynamic state.

The reason why intermediate dynamic loading is necessary because we want to explore rock fracture behavior under impact loading in rock cutting application. The interaction between the cutting tool and rock is a dynamic process, but it is believed to be less dynamic than rock blasting which is commonly attributed to dynamic experimentation with SHPB. To dynamically load the specimen in a lower strain rate, we adopted a non-explosive reaction-driven loading machine. We used NSCB method because of its relatively uncomplicated sample preparation. The results from dynamic tests were compared with the ones obtained from the static tests. The results were plotted with the previous findings and verified with numerical simulations.

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