Spotlight: How to perform rock deformation studies
Lars Hansen, Associate Professor of Rock and Mineral Physics at the Rock Rheology Laboratory, University of Oxford, UK, discusses the development of a high-temperature uniaxial creep apparatus that also enables precise dynamic measurements to be undertaken at a wide range of frequencies.
Earth science focuses on understanding the processes behind the formation and evolution of the solid earth, its oceans and atmosphere. An important aspect of this work is the study of the science behind rock deformation, which typically involves the investigation of material behaviours and how these are controlled by underlying microscopic properties. By comparing the measured and predicted sample behaviours, it is possible to establish the underlying physical processes in operation.
Simulating the extreme environmental conditions under which rock deformation occurs in a laboratory setting is a highly technical challenge. Human and geological timescales are very different, and atomic scales are quite different to those of lithospheric plates. Making things happen in a timescale that allows observation of any changes that occur is not straightforward. It is also vital that these experiments are performed as precisely as possible, as laboratory models are derived from the resulting data and used to extrapolate and make predictions on geological timescales, which can be 10 orders of magnitude different to the human timescale.
How is rock deformation monitored?
The traditional method for simulating these deformations is creep testing, which entails the application of a constant force to a sample by dead-loading – stacking a weight on top of the sample or suspending it underneath – or using screw-driven, mechanised apparatus to apply force. However, these types of tests are not particularly good at distinguishing between models of the atomistic processes involved in deformation. Far better are forced oscillation measurements, in which the applied force oscillates sinusoidally and the material response is investigated over a range of different oscillation frequencies. However, these tests are typically conducted with very small loads such that the material is not actively creeping, reducing the applicability of any observations to rocks that are creeping on long timescales. Thus, researchers wanted to combine creep testing with precise forced oscillation measurements so that the microphysics of deformation could be investigated during creep, allowing for calibration of robust models for extrapolation to geological scales.
Combining creep testing with precise forced oscillation measurements requires specialist equipment. Accurate and precise measurement of changes in the length of samples in response to a force applied under extreme conditions – typically at temperatures between 1,200–1,500°C and pressures up to one gigapascal – is essential. These samples range from 0.5–3cm in size and are often sensitive to the chemical environment. For instance, a sample containing iron can oxidise in air at these high temperatures. It is, therefore, important that the testing chamber offers strict control of the chemical environment to allow, for example, precise control of the oxygen pressure or the flow of an inert gas over the sample. In addition, the atomistic processes of deformation are sensitive to both the temperature and oscillation frequency, making it a key requirement for a system that is capable of operating at a wide range of frequencies. While some existing apparatus could operate at very high temperatures and low frequency oscillations, there was nothing available that could perform dynamic tests while also maintaining a high mean stress to induce simultaneous creep.
In the absence of any existing equipment that combined all the required attributes, researchers collaborated with Physik Instrumente, UK, to design and build a unique system capable of dynamically changing the forces, and accurately and precisely measuring length and force at high temperatures in a controlled chemical environment.
From a movement perspective, the solution was a combination of an electromechanical actuator and piezoelectrics to enable dynamic testing at a wide range of frequencies. This allowed the load applied to be quickly and precisely controlled using the fine-scale piezoelectric actuator, as well as enabling larger scale deformations, either separately or in combination with the smaller piezo-controlled motion. However, while the actuating pistons can cope with a high-temperature environment, the displacement transducers used to measure sample length can not. To avoid calibration issues, this meant that the system had to be designed to allow for measurements and actuation to take place away from the heated sample. A further complication was the need to flow a gas, such as a mixture of carbon dioxide and carbon monoxide, over the hot sample. The gas had to be contained within a sealed compartment, requiring a dynamic, low friction seal – which the piston passes through – to separate the hot sample and the gas from the ambient room temperature environment of the actuator. The final design incorporated all these features in a custom-built high temperature uniaxial creep apparatus. This system combines the classic creep test with forced oscillation – or internal friction – measurements, enabling both sets of deformations to be performed over a wide range of timescales, at high temperatures and at lower frequencies than other commercially available systems.
The apparatus in use
The high-temperature uniaxial creep apparatus has been successfully used in stress reduction tests to simulate the after effects of earthquakes. A constant force was applied to geological samples and then instantaneously reduced to study the material’s recovery afterwards. Combined creep and forced oscillation tests exploring a wide frequency range have also been undertaken, allowing the association of macroscopic behaviour with a particular type of crystal defect for the first time.
The system has also been applied to equal channel angular pressing, which is typically performed in a few seconds at room temperature and at the most 200°C. By miniaturising the equal channel angular pressing set-up to fit the high-temperature uniaxial creep apparatus, it is now possible to carry out the process at temperatures up to 1,200°C, more slowly than before and in a precisely controlled chemical environment – the first time this has been achieved under such conditions or on rocks. Looking to the future, this novel apparatus opens up tremendous possibilities for investigation of a wide range of material systems, adapting a variety of tests traditionally performed under milder conditions, not only in earth sciences, but also in other engineering applications.