page_title: Griggs apparatus
section_title: Design
hierarchical_section_title: Griggs apparatus / Design
caption_attribution_description: English: Expanded view of the outer (σ3) and inner (σ1) pistons.
context_page_description: 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.
context_section_description: A Griggs apparatus has the ability to create and maintain confining pressure on a sample while separately being able to strain the sample. Confining pressure is generated by advancing a hydraulic ram either by using a hand-operated lever pump or a servo-controlled syringe pump. The advancing ram then compresses the outer tungsten carbide piston (σ3) that in turn depresses the Pb plug at the top of the sample assembly and then in turn stresses the NaCl pressure medium. Deviatoric stress is created by a mechanical drive train that is powered by an electric motor that sits on top of the apparatus. When the electric motor is powered it engages with a set of gears that allow for variable strain rates to be chosen that range from 10⁻³sec-1 down to 10⁻⁸sec-1. 
  To eliminate the torque from the advancing drive train produced by the gear set, a recirculating ball screw is located in between the gear set and deformation piston (σ1) that provides the axial load. Connected in series with the deformation piston is also an external load cell which measures the load applied to the inner (σ1). The outer (σ3) and inner (σ1) pistons that sit above the sample are two separate pieces, this allows for the advancement of the deformation piston with the drive train without altering the confining pressure which is operated with the hydraulic ram.
  The design of the hydraulic ram produces an error in pressure measurements that are calculated from utilizing the oil pressure within the ram. Inside of the ram there is a large O-ring that seals oil in either the upper or lower portion of the ram. As the ram is pressurized and oil is transferred from the lower reservoir to the upper friction is produced by the motion of the O-ring along the inside wall of the ram. Since the ram needs to overcome the force of the friction to continue advancing, pressure measurements calculated using the oil pressure within the ram includes a contribution from the internal friction. The contribution of frictional force on pressure calculations can be described by the following relationship:
  Calculated pressure = Confining pressure + Internal friction within the ram.
  The total contribution of this friction to the value of measured pressure is different for each specific Griggs machine, but it has been shown that deviations between actual and measured pressure can be up to 10%.
caption: Expanded view of the outer (σ3) and inner (σ1) pistons.
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