PATERSON HIGH-PRESSURE
HIGH-TEMPERATURE TESTING SYSTEM
HPT
The HPT is a stand-alone gas-medium test apparatus for rock deformation studies at high pressure and high temperature developed by Professor Mervyn Paterson at Australian National University. The HPT provides a comprehensive facility for mechanical testing of materials at confining pressures up to 500 MPa and temperatures to 1600 K. It can also provide the basic high-pressure, high temperature enviroment for hot isostatic pressing (HIP), materials synthesis, and physical property measurements.
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Features
Options
Applications
for the HPT
for further information please contact ASI
Copyright 1999 Australian
Scientific Instruments
What is HPT?
The HPT is a stand-alone gas-medium test apparatus for rock deformation studies at high pressure and high temperature. The HPT has been developed to a high level of sophistication by Professor Mervyn Paterson in the course of thirty years research at the Research School of Earth Sciences within the Australian National University. The benefit of his knowledge is now available through the commercial production by Australian Scientific Instruments Pty Ltd (a member of ANUTECH Pty Ltd Group) of the HPT Testing Machine, which now incorporates many new developments for improved performance and user friendliness.
The HPT testing system provides a comprehensive facility for mechanical testing of materials at confining pressures up to 500 MPa (70,000 psi) and temperatures to 1600 K (1300 degC). It can also provide the basic high-pressure, high temperature enviroment for other types of research, such as hot isostatic pressing (HIP), materials synthesis, and physical property measurements.
Design features
| Confining Pressure | 500 MPa |
| Temperature | 1600 K |
| Axial Deformation System: | |
| Max. displacement | 30 mm |
| Max. force | 100 kN |
| Displacement resolution | 1µm |
| Force resolution | 10 N |
| Axial strain rates | 10-2 s-1 to 10-7 s-1 |
(20mm specimen length) |
|
| Torsion System: | |
| Max. displacement | no limit |
| Max. torque | 1000 Nm |
| Displacement resolution | 0.001 rad |
| Torque resolution | 0.1 Nm |
| Axial strain rates | 10-3 to 10-7 s-1 |
(15mm diameter x 10mm length) |
|
| Pore Pressure System: | |
| Max. volumetric displacement | 1900 mm3 |
| Volumetric resolution | 0.05 mm3 |
| Max. pressure | 500 MPa |
HPT Features
High pressure facility
The high pressure facility consists of:
Housing:
The housing serves both as the framework for the complete machine and as its protective shielding. It accomodates the high-pressure components, which are accessable through sliding doors at the back and top of the instrument, as well as the instrumentation. Special attention has been paid to safety through the provision of adequate shielding and door locks, the use of multiwall contruction for ear protection, and the minimisation of the volume of gas at high pressure. The doors are automatically locked when pressure is raised above 30MPa or when the power fails.
Pumping system:
A Haskel AGT-62/152H gas booster pump is used for the first pumping stage from bottle pressure, followed by a 10:1 intensifier of 0.35 litre stroke driven by a Haskel AW-100 oil pump. The pressure transducer is provided with signal conditioning, digital display and control facilities, enabling the pressure to be maintained automatically by pumping or bleeding as required. Rupture disks are fitted to prevent accidental over-pressuring of the different stages of the pumping system and door interlocks prevent pumping above 30MPa when the safety doors are open.
Main pressure vessel:
The steel cylinder, with axis vertical, has a working volume of 65mm diameter, 360mm length and is provided with blank plugs for both ends. It is test to 750MPa and is intended for routine usage upto 500MPa. A 700MPa rupture disk is fitted. The vessel also has a soft steel safety sleeve incorporating the water cooling coils. With the solid-zirconia-insulated furnace and internal load cell fitted, the free volume is about 0.2 litres, from which the maximum available energy in adiabatic expansion of the gas from 500MPa is not more than about 125kJ.
Leak detectors:
A bubbler system is provided for quick location of any leaks from the 'O'-ring seals in the high-pressure system.
Instrumentation:
All the basic wiring is included, covering the confining pressure and furnace facilities, and allowing for the connection of the modules for the mechanical testing systems (both axial and torsion) and for the pore fluid system, as required. No additional wiring is required for retrofitting.
High temperature facility
An internal furnace is supplied for high temperature studies. This permits relatively rapid heating and cooling while maintaining the pressure vessel walls close to ambient temperatures.
Top Closure plug:
The plug is fitted with six insulated feed-throughs, six earth pins, and a steel piston on which a specimen can be mounted. The piston has a small bore for direct access to the specimen for thermocouples or pore fluid connections. With the specimen mounted, the piston can be inserted or removed through the main plug without disturbing the furnace.
Furnace:
The standard internal furnace has three molybdenum windings to provide a uniform (±1K) high temperature zone of 21mm diameter and 50mm length. A control thermocouple is situated opposite each winding. An alternative furnace of 27mm internal diameter, but otherwise the same is also available.
Control:
A Eurotherm self-tuning programming controller and a separate temperature indicator permit selection of a control thermocouple and independent monitoring of all thermocouples, through appropriate switches. The current in each winding can be independently adjusted so as to set the desired temperature distribution in the furnace, as monitored with an externally-controlled traversing thermocouple in a hollow dummy specimen. Current and voltage meters for each winding are fitted.
HPT Options
Axial deformation, torsion testing and pore fluids systems.
AXIAL DEFORMATION
By applying axial load to a specimen assembly, tests can be carried out in either compression or extension. The components of this facility consist of:
Bottom closure plug and piston:
The plug is furnished with a pressure-compensating piston for applying axial loads to specimens. The internal load cell is attached to the high-pressure end of the piston and is splined to prevent rotation in the plug in torsion tests. A stirrup attached to the room-pressure end of the piston gives access to the bore of the piston for electrical and pore fluid connections. The maximum piston displacement is 30mm. Displacement is monitored internally with an LVDT fitted to the load cell.
Internal load cell:
The internal load cell is of a special strain-gauge type (a capacitance type is available if desired). Transducer conditioning is provided. The outputs of all transducers are displayed on digital panel meters and are available at ± 10V level, for data logging and servo control feedback. Provision is also made for the measurement of torque by the same load cell. An external load cell is fitted and used to provide safe working limits.
Actuator and controls:
The electromechanical actuator for axial loading to 100kN is attached to the pressure vessel support and is connected through an external load cell to the loading piston. The maximum load is 100kN in either compression or extension. The maximum rate of advance is around 1mm per second and the minimum is around 0.01mm per hour. Tests can be carried out either under actuator speed control with tachogenerator feedback or at constant force or displacement rate with internal load cell or LVDT feedback to a Eurotherm process controller. Alternatively, if desired, an external servocontroller can be connected for maximum flexibility. Arrangements for data-logging, and for any external controlling and/or programming required, are to be supplied by the client.
TORSION TESTING SYSTEM
Large shearing strains are of great importance in geology, especially in shear zones. Such strains can be simulated in torsion tests, which have the advantage of retaining well defined stress conditions independently of the amount of shearing. Torsion tests with relatively long specimens can also be used to study shear localization phenomena, and the use of hollow specimens can remove ambiguities associated with the strain gradient in solid specimens. With simultaneous application of axial load, all three principal stresses can be varied independently for fully determining rheological laws. Thus the addition of a torsion testing system greatly extends the range of experimentation of geological relevance.
Configuration:
The facility consists of an electromechanical torsion actuator, internal and external torque cells, rotary displacement transducer, calibrating bars for internal torque cell, and specimen assembly components. The torsion actuator is attached to the top of the main pressure vessel and applies torque to the top of the specimen assembly at the same time as the axial actuator acts on the bottom. The maximum torque is 1000Nm. Solid specimens of up to 15 mm diameter can be accommodated in the standard furnace, and hollow specimens of 22mm outside diameter and 16mm inside diameter can be accomadated in the special 27mm bore furnace, which is supplied separately. There is no limit to the amount of twist that can be applied and the strain is only limited by the ductility of the jacket surrounding the specimen, which experience shows can be very large. Transducer conditioning is provided. The outputs of all transducers are displayed simultaneously on meters and are available at ±10V level terminal sockets for data-logging and for feedback to external servocontrollers. Provision can also be made for pore pressure to be applied during a torsion test. The system can be retrofitted.
Controls:
Tests can be carried out either under actuator speed control with tachogenerator feedback or under torque or displacement control using a Eurotherm process controller/programmer with internal torque or position feedback. The rates of twist can be varied over a range corresponding to strain rates from around 10-3 to around 10-7 s-1. Alternatively, if desired, an external servocontroller can be connected. Arrangements for data-logging, and for any external controlling and/or programming required, are to be provided by the client.
PORE-FLUID
The pore-fluid system is based on a servocontrolled electromechanically-driven volumometer (piston diameter 7mm, stroke 50mm), suitable for argon or water at pressures up to 500MPa. It is fitted with pressure and displacement transducers, downstream pore pressure connection, high pressure plumbing connections, and electrical connectors for transducer conditioning, feedback, indicating meters and control signals. Transducer conditioning is provided. The outputs of all transducers are simultaneously displayed on digital panel meters and are available at ±10V terminal sockets for data-logging and for feedback to external servocontrollers. As supplied, the system can be operated in constant displacement rate mode or under constant pore pressure with pressure feedback to a Eurotherm process controller.
Arrangements for data-logging, and for any external controlling or programming required, are to be provided by the client .
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.
Relevance of HPT machine to
ceramics research
The HPT machine has been developed for the study of the
deformation of rocks at high pressures and high temperatures,
especially for the study of plastic deformation. Since there are
many similarities in the characteristics of rocks and ceramics,
for example, as oxide materials that are brittle under
atmospheric conditions, the use of deformation studies under high
pressure and high temperature can be expected to be fruitful in
fundamental research on ceramics. Some possible areas of
application are the following:
1. Production of
material by hot isostatic pressing (HIP):
The HPT machine has two advantages over most commercial
HIP machines. First, it is suitable for small quantities of
experimental material since the hot zone in the standard furnace
is 21 mm diameter by 50 mm long, especially convenient for
specimens around 10-15 mm diameter. Second, the pressure range of
500 MPa is substantially higher than the usual 200 MPa.
2. Studies of
porosity evolution during HIP or deformation:
Using the pore fluid module, the amount of fluid that
moves into or out of the specimen can be continuously monitored
while maintaining the pore fluid pressure constant, thus giving a
continuous monitoring of connected porosity. Alternatively, using
the axial deformation module, the changes of length during HIP
can be monitored, giving information about the evolution of total
porosity. The pore fluid module also permits permeability to be
measured.
3. Studies of
plastic deformation:
Through the use of high confining pressure and high
temperature to suppress fracture, many normally-brittle materials
can be plastically deformed, thus simulating the deformation of
rocks in nature. Extension of such studies to ceramics would lead
to greater fundamental understanding of their material properties
(mobility of dislocations, role of extrinsic factors in
ductility, recrystallization and hot-working, etc.) and in
special cases may suggest new fabrication procedures. For
example, plastic deformation under high confining pressure, a
sort of forging, can lead to enhanced compaction, eliminating
residual porosity after hot pressing. Also high pressure can be
used to suppress cavitation that leads to creep failure in
ceramics.
4. Intermetallics:
As in the case of rocks and ceramics, the use of high pressures
permits ductility to be achieved in these normally-brittle
materials at lower temperatures than at atmospheric pressure.
Thus the effects of plastic deformation can be more readily
studied and greater understanding of the properties of the
materials achieved.
We believe that HPT machine may offer many benefits in your area
and we would be happy to discuss your unique requirements with
you. Please contact Prof Mervyn Paterson at ASI for more information.
Download the new HPT Brochure (Adobe Acrobat PDF format, 370kb)
Double-click to view (Acrobat Reader available for free download from Adobe)
Internet Explorer users: Right-click the link above if you want to save PDF file on your hard drive.
