Prediction of rock cuttability by various excavation machines
such as tunnel boring machines, road headers, rippers, picks,
roller disc cutters, excavators, etc is of major concern to both civil
and mining engineers and rock mechanics. In principle, the
design of a rock cutting tool is based on achieving maximum
excavation with minimum input energy, minimum applied force,
minimum equipment weight and volume and, more importantly,
minimum tool wear. When a rock material is indented by the force of a cutting tool
and then unloaded several material zones form underneath and
around the tool. As the tool moves into the rock the volume of its
associated rock cavity (ie volume of displaced rock) expands and
depending on the indenter's shape various phenomena may occur.
Immediately beneath the tool a small zone in which the stress
state is more or less hydrostatic, is formed which is underlain by
an inelastic, or damaged, material zone. Both size and shape of
this damaged zone control the magnitude of the indentation force
and the chip formation mechanism during unloading. The rock is
unable to store the whole strain energy induced by the tool and
consequently the extra energy is expended either in forming new
fracture surfaces and/or in plastic deformation in the damaged
zone. Therefore, it is not surprising to see that both
perfect-elastic and rigid-plastic constitutive models are not
generally suitable for modelling indentation of rocks. In contrast
elastoplastic constitutive models are more appropriate as they can
include both elastic and plastic deformations (Kral et al, 1993). Elastic solutions for several half-space indentation problems
can be found in most text books on elasticity and in particular in
Timoshenko and Goodier (1951), Sneddon, (1951), Poulos and
Davis (1974) and Johnson (1985a). If a failure criterion, eg the
Mohr-Coulomb failure criterion, is applied to delimit elastic
solutions for indentation problems, a lower bound limit of the
applied indentation force (for a given indentation depth) can be
estimated. It may be noted that elastic analyses, without any
delimiting failure criteria, normally overestimate the indentation
force. Plastic solutions for a few indentation problems may be
found in Hill (1950), Johnson, (1985a) and Alehossein et al
(1992). When analysing the behaviour of indented rocks the choice of
an appropriate constitutive model describing effectively the
behaviour of the rock at various stages and phases during
indentation is a challenging issue. Several questions need to be
answered before starting an effective analysis. The first question
is of course: Which one of these is more appropriate for
modelling zones underneath and around a cutter, elasticity or
plasticity theory, continuum damage mechanics or fracture
mechanics, and what is the bridging link between these various
theories? Which mode is more dominant in fracturing rock,
tensile or shear mode? What governs the mechanism of chip
formation? What is the function, shape and the behaviour of the
pulverised zone that forms just underneath a cutter? Is the
hydrostatic stress state assumption reasonable for this zone? How
does the rock change during the absorption and dissipation of the
input energy and how does the indentation energy vary with rock
type and depth of penetration? How can we model the damaged
zone under the indenter? Furthermore does a plastic material
form beneath the pulverised zone? When and how do radial and
ring cracks form around and under an indenter?