ACCESIONES DE MORA PRE SELECCIONADAS
ANÁLISIS ACCESIÓN AP009 ACCESIÓN AP
3.5 TABLA DE COLOR DESCRIPTIVA PARA LAS ACCESIONES SELECCIONADAS
3.5.1 Tabla de color descriptiva para la accesión AP
This Chapter describes in detail the design of a new high temperature triaxial deformation apparatus used for compression experiments. The fracture toughness experiments described were conducted using the modified ‘Bridge Cell’, which is briefly described in Section 4.2.
4.1. Introduction
Interest in laboratory rock deformation tests began in the late 19'*’ century. The first scientific report of such tests came from the pioneering work of Von Karman (1911). However, most of the experiments were being performed at moderate pressures and temperatures. Advances in experimental rock deformation have always been closely related to technological advances. It was in fact only in the late 1950’s that scientists were able to simulate most crustal conditions, generating impressive amounts of data on rock strength and other rock properties caused by rock deformation (see reviews by Paterson, 1978; Tullis, 1979). The achievement of high temperature always caused difficulties in rock deformation experiments especially when combined with high confining and pore fluid pressures. The development of high temperature equipment involved solving problems such as sealing at high temperatures, accurate and stable temperature control and maintaining long heating cycle times. The first high temperature rigs operated with external furnaces and use solid steel or liquids to apply confining pressures. In rock deformation experiments, where the strain and stresses have to be accurately determined, a further step had to be made into the use of gas as a confining pressure medium. In this respect Grigg’s work (1960) represented a great advance in high temperature testing. He developed the first internal furnace that was able to reach temperatures up to 800°C at pressures of 500 MPa in a CO2 pressure medium. Since then, many others including Heard (Heard and Carter, 1968), Paterson (1970) and Murrell (1989) have designed gas rigs able to reach temperatures up to 1000°C. Today, the ‘Paterson rig’ can reach temperatures up to 1300°C at pressures of 500MPa in an argon gas confining pressure medium, and his method remains the most efficient and widely used.
In this study, an investigation of slow fracturing o f volcanic rocks at high temperatures and moderately low to atmospheric confining pressures is required. However, for the samples to be representative of larger crustal scale phenomena, diameters had to be at least 20mm. The need for an unusually large rock sample, relative to the 10 x 20mm sample used in the Paterson rig, posed many difficulties and restrictions during the design stage of the new HT apparatus. The difficulties affected both the choice of material and the design and assembly stage of the components.
In this chapter 1 will describe in detail the development of the new HT rig, the components, choice of materials together with the data measurement and logging methods employed.
C h u p f c v -t - D c s i p n o f a I l i p l i T c m p c n t i i i n ’ T r i a x i a l D c f o r n i a t i o n A p p a r a t u s
4.1.1. General Description o f the Apparatus
A new high temperature (HT) triaxial deformation apparatus has been designed using some common components from the modified ‘Bridge Cell’, used for high temperature fracture toughness experiments. The new apparatus makes use of the pressure vessel and the main external frame o f the ‘Bridge CelT. All other external components and the internal furnace were newly designed in order to perform confined or unconfined compression and tension tests at temperatures up to 1000°C, confining pressures from atmospheric to 50 MPa, loads up to 250kN and strain rates from IxlO 'V ' to Ix lO 'V . In addition, the new apparatus has been designed to perform pore fluid experiments and to record acoustic emissions or acoustic wave velocities during deformation.
The key components designed for the new apparatus are: (1) an actuator to provide axial stress and measure internal load; (2) top and bottom sealing plugs of the pressure vessel; and (3) all internal components including a furnace to achieve temperatures up to 1000°C. The Bridge Cell pressure vessel and part of the outer frame were retained unmodified. The gas and water pumps and transducers were also retained, while the pressure system was modified to accommodate the new components. Using some of the original components of the Bridge cell allowed both a reduction of cost, and enabled the use of the same apparatus for fracture toughness tests and compression-tension tests. The new apparatus (Figures 4.1 and 4.2) thus has the added advantage of multiple applications. The AutoCad drawings produced are shown in Appendix C.
The new HT triaxial rig consists of a cylindrical vessel, a bottom and upper plug, the pressure and load application systems, an outer frame, a furnace, and a logging computer. All components are described in detail in this Chapter. To switch between triaxial deformation tests and fracture mechanics tests, three changes have to be employed: the furnace, the actuator and loading system, and the sample set-up. An actuator connected to the top plug of the vessel provides the load in the triaxial tests. When fracture toughness experiments tests are performed, only the bottom plug is exchanged. The bottom plug for fracture toughness experiments includes loading and displacement measurement systems (Section 4.2). All the internal components, included in the furnace, have to be exchanged between the two applications. Both fracture toughness and triaxial tests are performed on cylindrical specimens. However, there is a considerable difference between sample sizes, which imposed a novel internal design. In addition, fracture toughness tests required a specific sample set-up for the ‘Short Rod’ (SR) specimen configuration tests (suggested method proposed by the ISRM, 1988). A description of the SR specimens and the experimental procedure is given in Chapter 5, Section 5.6 and 5.7.
The apparatus is mounted on a steel bench approximately 750mm long and 500mm from floor level, resulting in the entire system being approximately 2.5 m high. The outer frame and protective shield are made o f aluminium alloy 10mm thick. All the electronics and temperature controllers are mounted on the inside of the side shield, with small widows allowing access to the front control panels.
C h a p t e r 4 - Desip;n o f a U i p h T e m p e r a t u r e JruLxial D e f o r m u t i o n A p p a r a t u s
Reaction plate Hoist for lifting the vessel
Safety shielding A ctuator Tem perature controllers P ressu re dial gau g es P ressu re v essel
C h a p t e r 4 - Desh^n o f a H i g h T e n i p c n i f a r e T r i a x i a l D e f o r m a t i o n . \ p p a r a t n s