ABSTRACT
Abstract Essential in the cutting process of rock is the occurrence of a crushed zone. Its size determines the cutting forces and geometry of the chip. It is hypothesized that the size of the crushed zone is determined by bifurcation of a shear crack into a tensile crack under mixed mode loading conditions. The shear crack is induced by shear strain localization in the crushed zone. The paper is focused on this special kind of crack bifurcation. By means of three numerical tools the conditions for bifurcation and unstable tensile crack propagation are studied: FRANC, DIANA and DISROC. The paper presents the preliminary results with FRANC and DIANA. Keywords: Bifurcation, Rock, Cutting, Numerical simulations, Three dimensional crack propagation, Shear crack
1 Introduction
Cutting is one of the main excavation techniques in drilling, tunneling and dredging operations in rock. A widely used cutting device in dredging is the crown cutter as shown in f ig. l . In order to understand the cutting process of such a device in soft rock (unconfined compressive strength < 50 MPa) an extensive research programme was commissioned by the Dredging Research Association to Delft Hydraulics and Delft Geotechnique over the last 2 decades (Steeghs et.al.1989). Extensive theoretical as well as experimental research has been executed on several soft rocks, like Felser (sandstone), Savonierre, Euville, Sireuil, Marl and St Leu (limestones). In the experimental research more than 1000 cutting tests (fig.2) were executed, including observation tests using half a chisel travelling along a glass wall and high speed film techniques (fig.3). These tests have revealed phenomenological and physical insight in the cutting process of rock in general (Uittenbogaard et.al. 1994) and has resulted in a semi-empirical cutting model ROCKSIM (Cools 1990). This model simulates a real time cutting process and predicts cutting forces and production rates. The cutting process is essentially characterised by the forming of chips. Main features of the chip forming process are the occurrence of a crushed zone in front of the chisel and a 3 dimensional tensile crack (see fig. 4) . In the cutting simulation model ROCKSIM the height of the crushed zone is one of the predominant parame ters. Up to now an empirical relation of this parameter with material parameters and cutting geometry is used. The empirical relation is obtained from the large amount of cutting tests on St Leu limestone. The objective of the present research is to develop a theoretical relation. Based on observation tests and theoretical considerations it is hypothesized that the chip forming process is determined by bifurcation of a shear crack into a tensile crack, induced by shear strain localisation in the crushed zone.
