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Dynamic rock fractures and effect of microwave radiation

Dynamic rock fractures and rock excavation

Gonglinan (Jack) Wu

Department of Civil Engineering, Monash University, Clayton, VIC 3800, Australia

Introduction

The dynamic properties of rock materials are important for many civil and resource applications, from tunnels to mines and mineral processing plants. In particular, the rock excavation operations rely on knowledge of rock dynamic for improving efficiency and safety. Traditional dynamic rock testing experiments are usually conducted under uniaxial loading conditions on Split Hopkinson Pressure Bar (SHPB), while it certainly gave a great amount of valuable information, rock breakage and excavation is almost always under confining stress in the real world. In order to overcome this, a novel SHPB with confining system is developed.The Three-dimensional Split Hopkinson Pressure Bar (3D-SHPB) [Australian Research Council (LE150100058)] at Monash University is the first true multi-axial dynamic testing apparatus in the world. It can provide static confinement in three independent axials, and a dynamic loading provided by a gas-driven striker. It is capable of delivering impacts up to 50m/s, while the confinement system powered by servo-hydraulic cylinders can provide triaxial static loads up to 50MPa. Dual high-speed cameras with maximum frame rate of 1 million fps will also be able to capture the rapid deformation process in 3D.

Fig. 1: Stereo-vision measurement of impact testing using Phantom V2511 (Monash Facility)

Objective

To develop the framework for efficient mechanical rock excavation, first we need to understand the mechanisms of rock fractures under controlled conditions. By understanding the fracture mechanisms which include both macro- and micro-fracture mechanisms, the framework will be suitable for different rock conditions. The second objective aims to study the mechanical induced fractures, particularly indentation fractures. The knowledge obtained from previous objective will help the characterisation and analysis of complex fractures caused by mechanical loading, which in return will help rock excavation in engineering applications.

Dynamic Shear test

Dynamic shear experiments have been conducted to determine the shear properties of different rock types. Both the asymmetric impact testing of sandstone and marble can provide us knowledge of how shear failures initiate and propagate in rocks, which is crucial for seismicity and earthquake studies, as well as rock engineering applications.

Fig. 2: Evolution of major principle strain during dynamic shearing

Rock excavation using mechanical tools

By fitting impact heads and drill bits to the 3D-SHPB, we will be able to study the deformation of rock and excavation efficiency under different stress conditions and equipment configurations. By varying the impact speed, confining stresses, drill bits and rock
types, we can develop a model for energy efficient rock breakage under complex stress conditions. CT-scanning at Australian Synchrotron and SEM imaging at Monash Centre for Electron Microscopy (MCEM) can take us further into the microscopic failure of rock under
mechanical loadings.

Fig.3 Variation of drill bits installed onto the 3D-SHPB
Fig.4 SEM images of marble under dynamic (a) shear and (b) tensile loading