• Abstract and Figures
• Public Full-text

research papers Mjo¨lnir: a miniature triaxial rock deformation apparatus for 4D synchrotron X-ray microtomography ISSN 1600-5775 Ian Butler,* Florian Fusseis, Alexis Cartwright-Taylor and Michael Flynn School of Geosciences, University of Edinburgh, James Hutton Road, The King’s Buildings, Edinburgh EH9 3FE, Received 4 June 2020 United Kingdom. *Correspondence e-mail: [email protected] Accepted 26 August 2020 An X-ray transparent experimental triaxial rock deformation apparatus, here Edited by P. A. Pianetta, SLAC National named ‘Mjo¨lnir’, enables investigations of brittle-style rock deformation and Accelerator Laboratory, USA failure, as well as coupled thermal, chemical and mechanical processes relevant to a range of Earth subsurface environments. Designed to operate with Keywords: synchrotron X-ray microtomography; cylindrical samples up to 3.2 mm outside-diameter and up to 10 mm length, experimental geoscience; rock deformation. Mjo¨lnir can attain up to 50 MPa confining pressure and in excess of 600 MPa axial load. The addition of heaters extends the experimental range to Supporting information: this article has temperatures up to 140C. Deployment of Mjolnir on synchrotron beamlines supporting information at journals.iucr.org/s indicates that full 3D datasets may be acquired in a few seconds to a few minutes, meaning full 4D investigations of deformation processes can be undertaken. Mjo¨lnir is constructed from readily available materials and components and complete technical drawings are included in the supporting information. 1. Introduction The accumulation of damage in rocks and their ultimate failure under deviatoric stress is a significant process in a range of natural and engineered settings within the Earth’s sub- surface. The mechanical behaviour of rocks is also influenced by a range of chemical processes including dissolution, mineral dehydration and volume changes during metamorphic reac- tions. Our understanding of such processes in the Earth’s subsurface has been greatly advanced by a range of physical and chemical experimental approaches used to replicate processes at elevated pressure and temperature. Conventional laboratory experiments, typically using cylindrical cores of rock 25–100 mm in diameter, demand that the experimental apparatus be constructed of robust materials possessing both high tensile strength and stiffness. These materials, and the need to construct apparatus that must themselves be stiff (i.e. resistant to deformation by the applied stresses), have precluded the application of high-resolution X-ray micro- tomography techniques to directly observe the dynamic processes during rock deformation and reaction processes at elevated pressure and temperature. As a consequence, many advances in our understanding of deformation processes, failure mechanisms and the role of fluid rock reaction have been achieved either indirectly by determination of bulk properties, by means of comparatively low-resolution methods such as locating acoustic emissions from microfracturing events (e.g. Lockner et al., 1991; Aben et al., 2019) or by post- mortem evaluations of samples. Recently the opportunity to use X-ray microtomography to image deformation processes in situ has been facilitated by J. Synchrotron Rad. (2020). 27, 1681–1687 https://doi.org/10.1107/S160057752001173X 1681 research papers the development of miniaturized rock deformation cells that stress 1 =( + p)>(2 = 3 = p) as the sample is deformed, operate on laboratory micro-computed tomography (mCT) where is the differential stress and p is the radial confining instruments or at synchrotron beamlines (e.g. Viggiani et al., pressure. 2004; Lenoir et al., 2007; Tisato et al., 2014; Renard et al., 2016; To attain a small, light configuration for fast synchrotron Glatz et al., 2018; Voltolini et al., 2019). In this contribution, we tomography, Mjo¨lnir has been designed to operate with 3– outline the design, construction and deployment of a minia- 3.2 mm-diameter cylindrical rock samples up to 10 mm in ture rock-deformation press suitable for synchrotron X-ray length. The fully assembled cell weighs just over 1 kg and microtomography studies. This new press, which we have stands 225 mm tall. It is constructed of CP2 grade titanium, named ‘Mjo¨lnir’ (in Norse mythology Mjo¨lnir is the hammer 316 grade stainless steel, phosphor bronze alloy, 6061 alumi- wielded by Thor, the god of thunder), is a partner to the nium alloy (T6 temper) and 12.9 grade high tensile steel. All elevated pressure (P) and temperature (T) fluid rock reaction o-ring seals are Viton or PTFE. The assembled components apparatus described in the work by Fusseis et al. (2014). We and materials utilized for each component are specified in the provide CAD-drawings (see supporting information) and bill of material (Fig. 1) and CAD-drawings are provided in the details of the design, construction and operation of the cell supporting information. so that researchers can replicate or further develop Mjo¨lnir The cylindrical sample is held between two pistons and for their own scientific applications. Mjo¨lnir differs from sleeved using a flexible silicone (Tygon) sleeve. The pistons previously published cell designs primarily through simplicity can be made from either high-speed steel (HSS) drill blanks or of design and construction. Although unable to approach the cold-drawn 316 stainless steel high-pressure tubing. The axial upper temperature limits of the cells presented in the work by load delivered to the sample via the top piston is generated by Voltolini et al. (2019) and Glatz et al. (2018), Mjo¨lnir exceeds means of an Enerpac CST572 single-acting hydraulic actuator their capability in confining pressure and axial load. The with a maximum stroke of 7 mm. The actuator exerts a 4.4 kN HADES cell presented in the work by Renard et al. (2016) can force when pressurized to 35 MPa (equal to the 350 bar operate under higher P and T conditions than Mjo¨lnir in its actuator operating limit, and equivalent to >600 MPa on a present format, yet has the limitation that its use is currently 3 mm sample). Fluid connections made directly to Mjo¨lnir’s tied to a single, high-energy synchrotron beamline. We body are with 10/32 high-performance liquid chromatography demonstrate that Mjo¨lnir is flexible in deployment across (HPLC) fittings. Fluid connections to the Enerpac actuator several synchrotron sources and can operate on relatively use Swagelok tube unions. low-energy polychromatic and monochromatic tomographic A critical element of the design is the pressure cell, which imaging beamlines. must offer minimal X-ray attenuation, yet must be stiff enough Initially designed to provide high- resolution 4D imaging of the accumu- lation and localization of damage in the lead up to brittle failure of rocks at elevated pressure, we have been able to extend the operation of the press to temperatures up to 140C to investigate coupled chemical and mechanical processes (i.e. processes which exert positive or negative feedbacks upon each other, and so influence the progressive evolution of the structure, properties or behaviour of a material). Mjo¨lnir is built from commercially available materials and components, has a modest cost of construction and can be deployed on a range of synchrotron beamlines. 2. Design and construction Mjo¨lnir’s configuration is consistent with the geometry of a conventional Hoek cell commonly used in triaxial rock deformation experiments (Hoek & Franklin, 1968). As such, the principal Figure 1 Bill of materials, (a) elevation and (b) sectional drawing of Mjo¨lnir illustrating the overall design, orthogonal stresses 1, 2 and 3 are components and construction of the cell. CAD-drawings for each component are provided in folder configured such that the total axial S1 of the supporting information. 1682 Butler et al. Mjo¨lnir: a miniature triaxial rock deformation apparatus J. Synchrotron Rad. (2020). 27, 1681–1687 research papers to resist the stresses imposed by the pressurized confining fluid 2.1. Modification of the basic design and axial load. We selected aluminium alloy 6061 for the pressure vessel which has a yield strength in T6 temper of Mjo¨lnir’s components are modular, and can be modified, 276 MPa at 24C (ASM Handbook, 1990). The material is exchanged and adapted to meet the requirements of indivi- corrosion resistant and contains no alloying components with dual experiments. The operational limits of any implementa- X-ray attenuation exceeding that of Al at concentrations of tion of Mjo¨lnir can be estimated from engineering calculations >1 wt%. Mjo¨lnir’s pressure vessel is designed with a 4 mm- but must be established, in practice, by testing the cell to thick wall (inner diameter 6 mm, outer diameter 14 mm). beyond the expected operating parameters of confining Guideline calculations of axial stress at full actuator load pressure, axial load and temperature. Each modification of (4.4 kN) and thick-walled cylinder hoop and radial stresses the cell design, especially where structural components are at 50 MPa confining pressure indicate that the axial stress altered, or operating conditions (pressure and/or tempera- (35 MPa) on the wall and combined hoop and radial stresses ture) are increased, requires reassessment by both calculation on the inner surface of the cell (122 MPa) are, in total, and testing under safe conditions. The cell is designed for use substantially less than the yield strength of the 6061 alumi- with liquid-pressure media only. Depending on the usage nium alloy, even at 150C. history, load-bearing components must be replaced at regular Pistons are either 1/8-inch HSS aircraft extension drill intervals. blanks for use with non-porous materials, or cold-drawn The design presented above can be modified in order to 1/8-inch 316 stainless steel high-pressure tubing for those enable experiments to be conducted at elevated temperature instances when pore fluid pressure and/or thermocouple in combination with elevated confining pressure and axial access to the sample are desired.

Load more

  7

Copy Link