Micromechanical and microstructural aspects affecting rock damage, ‎fracture and cutting mechanisms

Abstract

Mechanical excavation machines have been widely employed in the excavation of soft to medium strength rock materials. These machines have also been utilised in civil tunnelling and subway construction and in driving mine openings such as drifts, ventilation shafts, and raises. They are also used to excavate micro-tunnels for public utilities and to install pipes underground. Unlike other engineering materials, rocks can be a challenge to deal with as they include different types of inhomogeneities and discontinuities. That is why understanding the microfracturing behaviour of a rock damage process is important to investigate the mechanical responses of rocks to various loading conditions.Hard-rock cutting is achieved by the coalescence of micro- and macro-fracturing in a rock material leading to rock chips. That is why this research aims to identify microstructural features (scale and mode of fracture) and mineralogical features (mineral phases and fabric/texture) affecting rock breakage, and the changes in the compressive and tensile strength of rock under various loading conditions such as monotonic and cyclic loading. The approach adopted in this research is to analyse fracture initiation and propagation by using experimental, numerical and image processing techniques to determine the static and fatigue damage in the tested rock specimens under static and cyclic loading. The tensile fracturing of rocks was initially investigated experimentally with tuff and monzonite rock specimens using standard Brazilian indirect tensile test. In addition, marble and sandstone rocks were included into the test series to test a range of rock types. Fracture toughness values of rocks were also determined by using Cracked Chevron Notched Brazilian Disc (CCNBD) specimens under both static and cyclic loading tests according to the suggested methods proposed by the International Standards for Rock Mechanics (ISRM).Two different types of cyclic loading were used in this thesis: (a) cyclic loading with increasing mean level and constant amplitude, known as continuous cyclic loading and (b) cyclic loading with increasing mean level and unloading cycles to zero load after a specified number of cycles, known as stepped cyclic loading. The main purpose of performing two types of cyclic loading was to find the most damaging cyclic loading type using the same amplitudes. A continuous irreversible accumulation of damage was observed in both types of cyclic tests conducted at different amplitudes. After the accumulation of irreversible damage and the failure of the specimen, clear tensile softening was observed in the cyclic loading tests carried out at different amplitudes on vertically aligned chevron notch cracks (mode I). Another important observation was made by examining the crack surfaces of failed specimens after cyclic loading tests, which revealed a clear crushed region, including small particles and dust in front of the chevron crack tip. Laboratory observations confirmed that more damage was induced in rock specimens under stepped type cyclic loading compared with the continuous type cyclic loading.The first series of experimental results was modelled using the XFEM numerical method to evaluate the fracture propagation and damage in rock specimens under different frequencies and amplitudes of the cyclic loading tests. The Extended Finite Element Method (XFEM) has advantages in analysing the fracture propagation in brittle materials such as hard rocks compared with other numerical analysis methods. The Particle Flow Code (PFC2D) numerical analysis program was also employed to assess the effect of grain size on fracture propagation and to discuss the evidence of emerging tensile and shear cracks in the Fracture Process Zone (FPZ).The 3D Computed Tomography (CT) scan technique was used to monitor the fracturing and coalescence of fractures in the failure surface of the tested rock specimens and the CT outcomes were used as input data for the pixel and statistical analyses. CT-scan results showed that the fracturing patterns were completely dependent on the fracture energy absorbed in the rock specimens tested under cyclic loading with various frequencies and amplitudes. In addition to the 3D tomography analysis, some post-process image processing techniques and thin-section analyses were also used to investigate the micromechanical effects on rock damage and micromorphology in the FPZ under different loading conditions.The insights gained through this research provide significant contributions to the understanding of the cyclic failure mechanism called rock fatigue in rocks excavated by the Oscillating Disc Cutting (ODC) technology. ODC technology was developed in Australia using the most recent hard rock cutting technology available worldwide. Overall, this study is proposed as a fundamental research into making hard rock or ore cutting more efficient in terms of using reduced cutting forces and energy by making best use of the cyclic loading effect and rock fatigue phenomena.