The penetration and fracturing mechanisms generated in brittle rock by the impingement of a high velocity jet

Abstract

Extensive studies on the jet penetration process in ductile metal targets have been previously carried out in numerous investigations. As a result, the penetration in ductile targets has been characterized by various theoretical models. However the penetration of brittle materials, particularly rock, has received considerably less attention. The lack of information concerning brittle materials is important as major differences between penetration in ductile and brittle materials have been observed. In most instances the actual penetration in brittle materials is far less than that given by theoretical calculations. This thesis presents an investigation into the high velocity jet penetration of brittle rock. The aim of this work is to describe the dynamic forces transmitted into the rock by the jet and the subsequent response of the rock to these forces. It is shown that existing penetration theories do not adequately describe penetration in rock. Of all the jet and target properties considered in the theories examined, target strength is shown to be the most relevant for predicting penetration depth. Recovery of the actual hole created was achieved by overcoring of the hole. Detailed measurements of the hole profile and fracture zones around the hole are presented. From the recovered samples of the hole, thin and polished sections were obtained for microscopic analysis. Results from the microscopic examination of these specimens are discussed from which temperature and pressure information are derived. In order to provide an adequate description of the penetration process, instrumentation was used to measure the penetration velocity, particle acceleration, and dynamic strain generated in the rock. From the instrumentation the interface pressure, dynamic stress, and dynamic strain generated in the rock are quantified and related to the various fracture zones identified around the hole. The results of these tests indicate that penetration in rock can be separated into three distinct phases. Initially the rock behaves as a hydrodynamic fluid should the interface pressure be very high. However as the interface pressure drops, the strength of the rock becomes evident and the second phase is entered into. The second phase is characterized by rapid changes occurring in the behaviour of the rock. Once the behaviour of the rock has stabilized, the third and final phase of penetration is entered. This final phase is predominantly controlled by the rock strength. As a result of this investigation, a better understanding of the interaction between the rock target and penetrating jet has been established. Additionally the behaviour of rock subjected to very high shock pressure has also been described. This has allowed better insight into the material properties governing the penetration process and the fracturing of rock from purely dynamic stresses.