8.1: Introduction

As mentioned in chapter 2, rock is a natural occurrence of cohesive organic or inorganic material, which forms a

part of the earth crest. Most rocks are composed of one or more minerals.

Rocks can be classified in different ways. The most used classification is based on their origin, distinguishing the following 3 main classes:
Igneous rock. A rock that consists of solidified molten rock material (magma), which has been generated within the earth. Well known are granite and basalt.

Sedimentary rock. Rock formed by the consolidation of sediment as settled in water, ice or air and accumulated on the earth’s surface, either on dry land or under water. Examples are sandstone, limestone and claystone. Metamorphic rock. Any class of rocks that are the result of partial or complete recrystallization in the solid state of pre-existing rocks under conditions of temperature and pressure that are significantly different from thos obtaining at the surface of the earth.

For the atmospheric cutting of rock models, the unconfined compressive strength (UCS), the unconfined tensile strength (UTS), the Brazilian tensile strength (BTS), the angle of internal friction and the angle of external friction are the dominant material properties.

When cutting rock different types of failure may occur. A distinction is made between brittle, brittle ductile and ductile failure, where brittle can be brittle shear failure, brittle tensile failure or a combination of both. The type of failure is mainly determined by the so called ductility number being the ratio of the compressive strength over the tensile strength (UCS/BTS).

Figure 8-1: Ductile and brittle cutting Verhoef (1997).

The confining pressure and the temperature may also play a role. Figure 8-1 shows a recording of the cutting forces during brittle and ductile failure, where brittle failure shows strongly fluctuating cutting forces, while ductile failure shows a more constant force. In fact in brittle failure there is a force build up, where failure occurs if the force and thus the stresses exceed a certain limit, after which the rock instantly collapses and the force decreases rapidly. Brittle failure is always destructive, meaning that the structure of the rock changes during failure in an irreversible way. Ductile failure in its pure form is plastic deformation and is reversible. In rock ductile failure is usually cataclastic failure, meaning that the microstructure is destroyed, which is also irreversible. Figure 8-2 shows corresponding stress-strain curves.

Figure 8-2: The stress-strain curves for ductile and brittle failure.