Griggs apparatus

Griggs apparatus, also referred to as a Griggs rig, is a modified piston cylinder high pressure apparatus used to create an environment of high pressure, high temperature and to impart a deviatoric stress on a sample of material. It was conceived in the 1960s.

Sample sizes range depending on the specific Griggs apparatus but generally can be up to approximately 150 mm3, and temperatures of up to 1600 K along with pressures of approximately 3 GPa can be achieved.

The Griggs apparatus was conceived by David Griggs in the mid 1960s during his time at the University of California at Los Angeles (UCLA). Since the inception of the Griggs apparatus it has become the work horse of many rock deformation labs world-wide, and has also helped elucidate numerous facets of plastic deformation in crystalline materials including the hydrolytic weakening of quartz.

The Griggs machine utilizes the same principle that other high pressure apparatuses (such as the diamond anvil cell) use to create elevated pressure on a specimen.

By generating a nominal force, in the case of the Griggs machine through a hydraulic ram, a greater force can then be applied to the sample by decreasing the area of subsequent pistons in series with the ram and in contact with the sample.

The sample assembly is constructed of multiple cylindrical sleeves which are placed in the opening in the pressure vessel or “bomb”. The outermost sleeve is typically composed from NaCl which is used to transfer the vertical load applied from the steel piston into a confining pressure on the sample at the center of the assembly. NaCl is used since it is relatively weak and it assists in the transfer of stress. Directly inboard of the outer NaCl sleeve is a ceramic support sleeve with a graphite sleeve inside of it, which is used for resistive heating of the sample. The inner most sleeve which houses the sample along with the upper and lower alumina pistons, is commonly also composed of NaCl. In addition to this arrangement, the inner sleeve can also be composed of a ternary eutectic salt mixture which is called a molten salt cell. The advantage to the molten salt cell is the salt mixture melts at moderate temperatures which allows for a true hydrostatic pressure to be applied the sample. When using a molten salt cell it also becomes necessary to add an additional nickel capsule to contain the salt mixture in order to prevent damage to other parts of the sample assembly. Temperature is monitored by a side entry thermocouple(s) that penetrate the wall of the graphite furnace and are directly adjacent to the sample, and are typically threaded through protective mullite insulation. The deviatoric stress is transferred to the sample through the σ1 piston. This piston is orientated in series with the upper piston, sample and lower piston within the inner sleeve which all sits on top of a tungsten carbide bottom piston.

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