Year

1990

Degree Name

Doctor of Philosophy

Department

Department of Civil and Mining Engineering

Abstract

Rock cutting theories are reviewed in the light of the observed mechanism of fracture processes due to rock cutting. Relatively consistent crack patterns are found for the typical rock cutting processes, such as the primary and the secondary crack systems for wedge indentations, the major chipping crack for drag pick cutting, and so on. These failure patterns suggest that fracture mechanics principles are suitable for the related crack analyses.

To apply fracture mechanics principles to practical rock cutting problems, four areas are of concern. First, an efficient numerical method is required for the fracture mechanics analyses of the rock crack problems involved. Second, observations on crack propagation involved in typical rock cutting processes are important to provide information on boundary conditions and to confirm the related fracture mechanics analyses. Third, a simpler method than conventional methods is desirable for the determination of the rock fracture toughness, the value of which is required for quantitative fracture mechanics analyses. A simple measurement method is especially desirable in the field where a large number of rocks needs to be concerned and where no sophisticated instrumentation is available. Fourth, rock fracture toughness is an intrinsic rock property closely related to rock cutting fragmentation processes. Its correlation with rock cutting or drilling machine performance needs to be identified to provide alternative parameters for cutability and drillability studies. Fundamental investigations are carried out in the four areas in this thesis.

A boundary element fracture modelling technique has been developed to simulate or predict the rock crack behaviours in various rock cutting, which are generally beyond the scope of any analytical method due to the complexity of the problem. Accurate analyses on crack behaviour are achieved by the development of a special crack tip element.

The fracture criteria, the Griffith-Irwin and the maximum tensile stress theories, are investigated for simulation of crack propagations. The second criterion is chosen for the rest of the cutting analyses because it leads to significantly less computing time.

To observe the actual crack behaviour in drag pick cutting processes, a special 'Rock Cutting Simulation Rig' is developed, in which static cutting tests can be conducted with various cutting conditions, such as cutting attack angle, rake angle, tool bluntness and so on. The specimen is held in place by two opposite platens through which confining pressure to the specimen can be applied.

Both the numerical modelling and experimental work are undertaken with emphases on the effects of cutting tool attack angle, rake angle and cutting tool bluntness on the rock cutting performances.

The crack behaviours due to simultaneous vertical loads of two wedge indenters, the BiIndenters, are investigated both numerically and experimentally. In contrast to the primary crack involved in a single wedge indentation, which propagates vertically downward, the cracks initiated from the near vicinities of each indenter tip of the Bi-Indenters tend to propagate outward due to the interaction between the two individual indenters of the BiIndenters. Furthermore, crack interaction between two of the Bi-Indenters are investigated, where a wedge-shaped large fragment tends to form due to the outward propagated cracks from adjacent tips between the two Bi-Indenters. The implication of the results for efficient rock cutting is discussed.

Rock fracture toughness is one of the fundamental properties which are required by any fracture mechanics modelling and therefore is an important parameter to predict rock cutting performance of actual rock cutting machines. To develop a simple method for the rock fracture mechanics measurement, frac ture mechanics analyses on the diametral crack behaviour in a disc with diametral compression are carried out. Based on these analyses a simple conventional Brazilian test, which is nonnally used for the determination of rock tensile strength, is proposed to be applied for the measurement of rock fracture toughness. The fracture toughness of six types of rocks are determined by the proposed method. The results are compared favourably with those determined by the Chevron bending specimen method, one of the recently proposed international standard methods by the International Society for Rock Measurement(ISRM).

Finally, the values of rock fracture toughness are found to correlate with the penetration rates of both of diamond coring and rotary drilling machines. The correlations between the penetration rates and the values of rock fracture toughness are also compared with those derived from the values of conventional Uniaxial Compressive Strength(UCS) and the Brazilian tensile strength.

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