How can solid ice be tested in laboratory environment (mechanical testing)?

Below is a blog post based on equipment that we have built for testing solid ice samples. We designed an ADMET biaxial testing system (eXpert 8600 series) in collaboration with the Rock and Ice Deformation Laboratory (RIDL) at Woods Hole Oceanograhic Institution (WHOI). 

Test Application

Scientists at Woods Hole Oceanographic Institution were looking for a testing system to simulate forces exerted on ice in the arctic. The data obtained would provide information on the mechanical properties of solid ice and be used in Rock and Ice Deformation Laboratory research and publications.

• Test types included axial compression, torque testing, creep testing, and attenuation (oscillatory load) testing,
• Specimens tested were solid ice samples, prepared and cut into a cylindrical shape with 1-in diameter and 2-in length,
• Tests would be performed at temperatures down to -30C and samples would be loaded at very slow rates to simulate forces exerted on ice in real-world scenarios.

Equipment Overview

ADMET Sales and Engineering teams recommended the eXpert 8603 axial-torsion table top testing system with engineered components and modified software.

The system would be engineered to perform torsion at 0.0000003 to 0.01 rpm and 0.01 to 40Nm torque and also axial compression at 0.000001 to 0.36 ipm and 5N to 5kN axial load.

 Components of the system include:

• eXpert 8603 testing machine with axial and torsion actuators 
• Biaxial load cell
• Custom fixture built for ice specimens
• Environmental chamber with front and back viewports
• Uniaxial backlash kit
• Torsion backlash kit
• Uniaxial position transducer
• Torque angle transducer
• MTESTQuattro Controller and Software

Features of the system include:

• Capable of both fixed rate and fixed load/torque tests
• Axial load capacity: 5kN
• Torque load capacity: 50Nm
• Full range of speeds:
  • 0.000001 to 0.36 ipm axial rate,
  • 0.0000003 to 0.01 rpm twist rate
  • Two different speed ranges at high gear/low gear:
    • Axial: 0.36 ipm at low gear and 0.03 ipm at high gear, 
    • Torsion: 3.6 degrees per min at low gear and  0.0001 degrees per min at high gear
  • Backlash in both axial and torsion
  • High accuracy and resolution:
    • Position Resolution: 0.00000004 in
    • Angle Resolution: 0.000009 deg
  • Environmental chamber full temperature range: -70C to 260C
  • Environmental chamber temperature tolerance: +/- 0.1 C

   ADMET Engineered Components

  ADMET eXpert 8603 built for WHOI was carefully designed to meet the customer’s requirements. The testing system was amongst the slowest moving machines built with speeds down to 0.8 degrees per minute in torsion. 

  Backlash kits with adjustable spring loaded fixtures were included to preload the axial actuator in the compressive direction and the torsion actuator in the clockwise and counterclockwise directions. The uniaxial compression backlash kit allowed up to 1-in stroke and the torsion backlash kit allowed up to 90-degree rotation during testing.

  The test application required testing ice at low temperatures. The ADMET F-280DT environmental chamber was customized per the user’s request and built with two viewports: one at the front as seen in our standard environmental chambers and another one at the back for researchers to observe and analyze ice deformation during testing. The test system was engineered such that the load/torque cell resided within the environmental chamber. The temperature tolerance of the chamber was +/- 0.1 C.

  To provide further accuracy in compressive strain and torsional strain measurements, an auxiliary transducer package with two transducers and their necessary mounting hardware was developed. A linear displacement transducer with position resolution down to 0.00000004 in was incorporated in the machine design and an angle transducer with unlimited clockwise and counterclockwise range and 0.000009 degree resolution was added to the back end of the motor. If, in the future, further uniaxial or angle displacement measurements on different points of the specimen are needed, additional transducers can also be added to the system.

  Controller & Software

  The testing system was equipped with ADMET’s MTESTQuattro PC-based controller and software for servo-control, data acquisition, reporting, and more. All readings, including input from the environmental chamber, would be recorded and accessed through the MTESTQuattro software. 

   

  

  A custom-designed fixture was built with hexagonal housing for ice specimens to allow very precise angular movements. The specimens would be placed in the fixture pocket and secured in place by sliding the grip locking door into the housing.

  MTESTQuattro channels were configured as below:

  1. Axial Load
  2. Axial Position
  3. Axial Strain
  4. Torque Input 
  5. Angle Measurement
  6. Auxiliary 1: Encoder Input (Fine Jog)
  7. Auxiliary 2: Angle Measurement (second axis)
  8. Temperature

MTESTQuattro axial-torsion controller standard capabilities include monotonic, cyclic, and segmented control profiles with software selectable control channels. The end-user informed ADMET that the laboratory would be using complex test profiles such as (1) hold in compression, (2) rotate back and forth or (2) load in torsion for several days at 5 in-lbf increments and (1) hold in torsion, (2) oscillate up and down at slow speeds. 

For WHOI testing application and custom speed requirements, the controller was validated with its fine jog feature to maintain low speeds at high and low gear and in uniaxial and torsion directions. 

In addition, MTESTQuattro features an autotuning feature for other readings such as temperature from the environmental chamber. This feature enabled accurate temperature control with tolerance +/- 0.1C.