Experimental Rock Deformation
The original aim of building this machine was to facilitate the study of the mechanics of rocks and single crystals of rock-forming minerals. The interest in these topics arises primarily from questions in the fields of structural geology and tectonics as to what are the mechanical properties of rocks under geological conditions, in situations such as mountain building, collision of tectonic plates, earthquakes, etc. Because of the high pressures and high temperatures within the earth where these processes are active, experiments in a high pressure and high temperature environment are required.
There have been two basic approaches to experimentation in rock deformation. One is to use a soft solid as a confining medium to generate the high pressure environment - the so called "Griggs Rig", which has been widely used. The other is to use an inert gas as a confining medium, the approach adopted in the HPT testing machine. The gas-medium machine has been seen to be difficult to make operate reliably and there have been many rather unsuccessful attempts over the years to build "home-made" gas medium machines. It was after considerable development and successful experience at the ANU that it was decided to make that experience available on the market as the HPT testing machine, which is now a safe, reliable, user-friendly and accurate instrument.
Specific topics of study that are aimed at are the following:
Brittle-ductile transition:
Everyday experience tells one that rocks are very brittle materials; they show no tendency to plasticity at ambient pressure and temperature. Yet there is abundant geological evidence, from the shape of rock strata in folds etc, from preferred crystallographic orientations in rocks, from microstructural observations of deformed grains in rocks, and so on, that rocks can undergo plastic deformation under geological conditioins. These conditions can be expected to comprise high pressure and high temperature, as well as long time intervals. The study of the brittle-ductile transition is therefore of primary interest in establishing what are the pressure and temperature requirements to bring rocks into the ductile field.
An application of particular interest arises in the study of earthquakes. An earthquake is generally understood to be a sudden shear failure in a mass of rock, most commonly on a "fault" on which there have already been previous failures. If the enviromental pressure is very high, the friction on a potential fault due to the normal pressure across it will be too high for sliding to take place and plastic flow will result under high stresses. So there is much geophysical interest in the brittle-ductile transition conditions for establishing the zones in the earth's interior in which earthquakes can be expected and where not.
Apart from exploring the conditions for the brittle-ductile transition, the main use of the HPT testing machine has been in the study of the plastic properties of rock and minerals. The use of high pressure is, to some extent, to simulate the pressure under conditions of deep geological burial. However in practice, the pressure is more frequently used simply to suppress brittle fracture and enable plastic properties to be studied; the role of pressure as a thermodynamic variable in influencing the plastic properties is not generally the primary interest since the effect of pressure on plastic flow itself is usually relatively small except in the Earth's deep interior (the effect of pressure can be expected to be important when the magnitude of the pressure is a significant fraction of the elastic moduli, of the order of 100 GPa for rocks). The range of pressures in the HPT testing machine (up to 0.5GPa) only covers geological depths shallower than about 15 - 20 km, considerably less than the depth of the continental crust (around 30km), but this is adequate for achieving ductility in most rocks at high temperature. The temperature in the Earth at 15 - 20km depth is generally in the order of 800K (500oC), much less than the range of the HPT testing machine. The reason for experimenting over a greater range of temperatures is partly to accelerate the process being studied, trading temperature for time since we do not have a geological time span in which to do experiments, and partly to look at processes that occur deeper in the earth where the temperatures are higher and the pressure is still not important to simulate directly.
Plastic deformation mechanisms in rocks: There are many different mechanisms by which rocks can deform plastically, involving the diffusive movement of individual atoms, the sliding of parts of crystals over each other (dislocation glide), or the relative movements of whole crystalline grains. Different processes may arise in the same rock under different conditions and the various processes leave different microstructural imprints. The study of the particular deformation mechanisms in particular rocks, and their dependence on the temperature and strain rate, is therefore an important part of experimental rock deformation. It forms the basis for interpreting what were the geological conditions under which a given rock was deformed in the past.
Where deformation mechanisms involve processes within the individual grains, especially dislocation processes, these are often more simply studied in single crystals of the constitutive minerals. Thus single crystal studies are also important application of the HPT testing machine.
As well as temperature and strain rate, it is often important to control chemical environmental factors in the experiments. Two of these factors often in question are the chemical potentials (thermodynamic "pressures") of oxygen and of water. The former is especially important for iron bearing minerals where iron may be in different valence states. The latter is especially important wherever processes involve the breaking of silicon oxygen bonds, for which water is a "catalyst". Thus the experiments in the HPT testing machine may involve special arrangements of the control of the chemical and thermodynamical conditions. The results, in turn, can be used in further interpretation of these conditions in nature.
Flow stresses in rocks:
In the interpretation or modelling of tectonic processes in nature, such as mountain building, consolidated sediments, formation of geological folds, etc., knowledge of the mechanical properties of the rocks under geological conditions is essential. These properties are dependent on the strain rate, that is, on the time scale, and so cannot be measured directly because of the necessarily short duration of laboratory experiments relative to geological time. However, if it can be shown that the same processes operate in both the laboratory and in geology, then measurements over a range of time scales (a range of strain rates) in the laboratory can be used to establish a flow law for extrapolation to geological time scales and so provide data for tectonic modelling. This is a particularly important application of the gas-medium testing machine because of the high accuracy of stress-strain-time measurements available. The dependence of the mechanical properties on environmental variables such as the chemical potentials of oxygen and water can also be very effectively studied in the HPT testing machine.
Ceramic and Intermetallic Materials
These materials are also brittle under ambient conditions similar to rocks, and the brittleness can persist to high temperatures at atmospheric pressure. However, as for rocks, ductile behaviour is potentially accessible to experimental study if tests are carried out under high pressure as well as high temperature. The HPT testing machine is therefore can be applied to the study of the plastic properties of ceramics and intermetallics.
There is, in principle, the possibility of using plastic deformation of these materials as a fabrication procedure for producing so-called net shapes. However, for most commercial applications, it would seem unlikely that such a high pressure production procedure would be economical and competitive with conventional sintering procedures, although there may be special applications.
The application of high pressure studies is likely to be more in fundamental research aimed at better basic understanding of the properties of these materials, from which new ideas for applications may flow. For example, the failure of ceramics under stress at high temperatures often results from the development of cavitation at grain boundaries. This process can be readily suppressed by the application of relatively small confining pressures. Therefore, experiments under a range of confining pressures can contribute to the understanding of the factors controlling the development of cavitation and so potentially lead to ways of controlling it.
The plastic flow of ceramics may also be of practical importance in the form of creep in very high temperature applications, but in such cases there may be only marginal ductility. The use of high confining pressures would permit the study of the plasticity of such materials over a much wider range of conditions of stress and temperature in the ductile field, and so improve the fundamental understanding of the plastic processes, possibily enabling improvement of the strength of such materials under ambient pressure.
Another potential field of application would be the study of the influence of other chemical environmental variables, such as water or oxygen potentials, which cannot be readily controlled at atmospheric pressure. This could be of importance in non-stoichiometric compounds, in which there is current interest, as evidenced by the conference held by the Engineering Foundation in April 1998 on "Non-stoichiometric ceramics and intermetallics".
Hot Isostatic Pressing:
An ancillary use of an HPT machine in the laboratory would be for hot isostatic pressing (HIP), using the high pressure and high temperature facilities only. This procedure would have the advantage of offering a wider range of pressure conditions than those in commercial HIP machines. It could also be very useful in a research environment where small experimental batches are to be run since it would be much more economical to use than a conventional commercial HIP machine. At the same time, by periodically "touching" the specimen, the deformation facility could be used to track the changes in length with time of a specimen undergoing hot isostatic pressing, a capability not generally available in HIP machines.
Other Materials
There is potential application of high pressure techniques in any situation where properties are influenced by significant changes of volume with pressure. An example exists in the glass transition of rubber. The glass transition that is well known at low temperature in elastomers can also be induced by applying high pressure at room temperature. The mechanical properties of the glassy state can thus be studied in the HPT testing machine. In view of the relatively high compressibility of polymers, there may be many applications at high pressure in fundamental studies of these materials.