INTRODUCTION
Crack growth is a major mechanism of f?acture in brittle rocks and rock masses. Since
laboratory cannot be directly used to determine of f?acture parameters pertinent to field situations
due to the different scales, it is important to analyze the mechanisms of crack propagation in situ.
Crack propagation is governed in many cases by interaction with microcracks. In the context
[ 1], the term microcracks is reserved for sufficiently small cracks capable of forming the fracture
process zone. The propagation of the microcracks within the material of the process zone can
model its strain-softening (post-peak softening) which simulates the cohesive forces (e.g., [2]).
However, the strain-softening is the stress-strain relationship characterizing volume elements
(VEs) of the material, whereas the conventional models of a crack with the process zone require
the knowledge of relationship between stresses and displacements. Therefore the explicit
introduction of the process zone thickness is necessary. This paper is a follow-up to a previous
one by the same authors [ 1], whose names are listed alphabetically.
MECHANISM OF CRACK PROPAGATION
Crack growth can be considered as successive fracturing of the VEs at the crack front (Fig. 1).
The fractured VEs form either the process zone or immediately the crack itself. Since the VEs are
not infinitesimal, their f?acture makes the crack effectively "thick". Its effective thickness is
assumed to equal the VE size H. Since any step of the real 3-D crack propagation requires
crushing the majority of the VEs along the crack front, the crack growth can be described in terms
of the average VE strength.
