1、英文原文The cuttability of rock using a high pressure water jetPC HaganThe University of New South Wales (UNSW), SydneyMethods of improvement in the performance of mechanical rock cutting systems are continually being sought. One area being investigated is combining mechanical rock cutting tools with wa
2、ter jets. In this hybrid arrangement, the mass breakage mechanism of a rolling disc cutter or drag pick is coupled with the concentrated energy medium of a high pressure water jet. Research has indicated that the resultant improvements in performance of mechanical cutting tools could be due to the c
3、utting of the rock surface through fracture and erosion thereby ameliorating the rock breakage process of the mechanical cutting tool. Damage by a water jet has been observed even in rock of high strength. This paper outlines a study on the sole use of water jets in cutting rock and the effects of c
4、hanges in the principal variables of a water jet. An understanding of the characteristics and the relative importance of any changes in these variables is necessary to optimise the cutting performance of a hybrid system in terms of advance rate and energy expenditure. The variables considered in the
5、 study included nozzle diameter, water pressure, traversing speed and multiple passes of a water jet. Within the range of values studied for each variable, a change in water pressure was found to have the greatest impact on the level of surface damage in rock. Traversing speed, and to a lesser exten
6、t nozzle diameter, were also found to alter the magnitude of surface damage in rock. 1. INTRODUCTIOOne of the fundamental processes in mining is the liberation of minerals from the in situ rock mass. This can be achieved by a number of methods including abrasion and fracture. The causation of fractu
7、re in rock has long been associated with some form of mechanical indentation or the cyclic application of large impact forces. This is exemplified by the old hammer and tap method of rock drilling but it also underlies the more modern techniques of mechanical rock breakage with cutting tools such as
8、 picks and rolling disc cutters. This principle has not precluded other sometimes more subtle techniques from being used in the past such as the gentle knock in the correct orientation by a stonemason. Apart from mechanical indentation, noncontact mechanisms to initiate fracture in rock have also be
9、en developed. Probably the most significant being the detonation of an explosive within a confined space. Before the use of blackpowder, however, the ancient Chinese observed and adapted to their advantage the natural weathering process of exfoliation by accelerating the rapid changes in rock temper
10、ature with fire and water. A modern variation of this technique is the thermal jet lance (Fleming and Calaman, 1951). More recently Sellar (1991) has reported radiating rock with a pulsing laser to induce internal stress variations causing fracture wherein it was found that the pulse frequency was c
11、ritical and should equal the resonance frequency of the rock. Common to all these techniques is an alteration in the internal state of stress such that bonds are broken and free surfaces created. Figure 1. View of several slots, or kerfs, cut in rock by a high pressure water jet.Another potential me
12、thod of rock breakage reported to have significant potential is the application of high pressure water jets. High pressure water jets in this sense normally refer to pressures between 10 and 400MPa with a nozzle aperture of less than 1 mm. Harris and Mellor (1974) have reported that a rock surface c
13、an be significantly damaged or cut by a water jet at high pressure. As shown in Figure 1, this damage is normally in the form of a narrow slot of varying depth. Various theories have been developed to account for this damage including a theory on cavitational drag (Crow, 1973), energy balance in bri
14、ttle fracture (Mohaupt and Burns, 1974) and material erosion (Rehbinder, 1976).During the early development work, it became apparent that water jets could not compete with conventional forms of rock fragmentation. A relatively large quantity of specific energy, in the order of 1000 MJ/m3, is require
15、d for this form of rock breakage which is several orders of magnitude greater than that required in conventional mechanical rock cutting. But it was found that water jets were useful when combined with conventional mechanical systems especially when cutting hard rock where tool life can be short (Ho
16、od, 1975). The most significant benefits derived from a hybrid cutting system being: a reduction in cutting forces. Fairhurst and Deliac (1986) reported an average force reduction of 30% in the cutting direction and somewhat greater reduction in the normal, or thrust, direction. increased tool life. Taylor and Thimmons (1989) reported a doubling in tool life. Hood et al (1991) reported an appreciable tool life was obtained when cutting a hard ro
