INTRODUCTION

ABSTRACT:

The paper deals with a numerical analysis of rock cutting experiments using the discrete element method. The main objective of this research is to establish if the occurence of the two failure modes observed in rock cutting experiments (ductile at small depth of cut, brittle at large depth) can be duplicated in numerical simulation. The numerical analysis is carried out with the discrete element code PFC ?v which modelsolids as a collection of bonded disks. Scaling laws are first established between the micro-properties at the particle scale (such as the mean particle radius, and bond strengths) and the apparent material properties at the macroscopic scale (such as the compressive strength ac and the toughness Kxc). Cutting tests are then performed with a particle assembly of rock-like properties.

The paper deaJs with a numerical analysis of rock cutting experiments in which rock is scratched by a cutter at a constant velocity and at a prescribed depth of cut. The objective of this preliminary study is to establish whether or not results obtained in laboratory rock experiments can be duplicated in numerical simulations.

Cutting experiments carried out with the Rock Strength Device developed at the University of Minnesota have shown that two failure modes can occur depending on the depth of cut: (i) a ductile mode associated with plastic flow of failed rock ahead of the cutting face at small depth of cut (larger than the grain size and typically less than i mm in sandstones) and (ii) a brittle mode associated with fracture propagation and chipping of the rock at the depth of cut above a certain threshold (Richard et al., 1998). The transition depth of cut between the two failure modes appears to be related to the length scale (KIC/óc) ?, where KIC is the rock toughness and ac the uniaxial compressive strength. Furthermore, there is a large body of evidence to suggest that the average cutting force in the duct fie mode is proportional to the crosssectional area of the cut (i.e. to the depth of cut for a rectangular cutter) and that the coefficient of proportionality (referred to as the specific energy e) is itself proportional to óc

In this study, the rock cutting process is investigated using the discrete element method, based on the approach by Cundall & Strack (1979) and Cundall & Hart (1993). The discrete element code PFC 2D (Itasca Consulting Group, 1996), is employed in the analysis. This code models solids as a collection of distinct and arbitrarily sized circular particles. The particles are treated as rigid bodies and allowed to overlap one another at the contact points. The contacts between particles are characterized through the stiffness, slip condition, and bonding models. The constitutive behavior of the particles enables the simulation of both plasticity and fracture at the macroscale. As a prerequisite to realistically represent rock-like materials by a particle assembly, scaling laws are first established between the micro-properties at the particle scale and the apparent material properties at the macroscopic scale. Numerical simulations on rock cutting experiments with a sharp cutter (no wear fiat) are then carried out to establish the existence of two failure modes in relation to the depth of cut, and to investigate the influence of material parameters on the transition depth of cut and on the magnitude of the cutting force.

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