High Strain Rate Servo-Hydraulic Testing of Magnesium and Other Materials

National Laboratory: 
Oak Ridge National Laboratory
Characterization Class: 
Extreme Environment Testing
Mechanical Behavior of Materials
Computational Tools Class: 
Structure-Properties
Processing/Manufacturing Class: 
Fabrication and synthesis
Shaping and forming
Description: 

The high rate test machine (HRTM) is built around a custom high-rate actuator designed and built by MTS Corp. Capacity is 9000 lbf at static conditions and 5500 lbf at maximum velocity of 700 in./sec. The stroke range is 15.5 inches of travel. The actuator has a single 400 gpm servo-valve supplied by two, five 5-gallon accumulators. The actuator is mounted in a MTS 100 kip, two-post load frame. A slack adapter that allows the machine to travel up to 7 inches before engaging and loading the specimen is attached to the actuator. This enables the machine to accelerate up to full speed and engage the specimen at constant velocity. Low-mass titanium alloy grips, specifically designed for high rate testing, are part of the load train components. The objective of reducing moving mass between the sample and load cell is to increase the Eigen frequency of the test setup, minimizing its effect on the stress-strain curves during high strain testing. Machine transducers include a DC strain gage load cell along with a charge amplified piezoelectric load cell (Kistler) with capacities up to 9000 lbf.

Capability Bounds: 

Specifications include:

  • Maximum Loading Velocity=700 in./s over an approximate 4-in. range.
  • Load Capacity is 9000 lbs. static and 5500 lbs. dynamic.
  • Total Stroke: 15.5 in.
  • Working Stroke is approximately 7.0 inches with slack adapter in the load train.
  • Control: MTS 407 servo-hydraulic controllers, with external command signal.
  • In-house developed synchronization and data acquisition (DAQ) systems.
  • High-speed digital imaging system (maximum Frame rate: 1,000,000 fps; three-dimensional imaging capability for full field-displacement map).
Unique Aspects: 

Special fixtures are used for conducting strain-limited material damage evolution studies at various strain rates. High-speed video and Digital Image Correlation are used to document the specimen response and measure displacement data for local strain measurements and fracture onset analysis. For the characteristic subset of loading states, material initial state with respect to porosity, grain size, and secondary particles is assessed via metallographic and x-ray tests. Evolving damage by void nucleation at secondary particles, void growth, and coalescence by micro-localizations also are assessed using metallographic and x-ray tests. Results are correlated with measurement of evolving effective material properties at the macro level. After correlation of micromechanics and macroscopic data from selected tests, the emphasis is on using measurements of effective material properties. The experimental scope is limited to tests necessary to develop parameters for models based on plasticity of voided solids, anisotropic plasticity continuum damage models, and models necessary for determining localization and failure parameters for finite element method (FEM) simulations.

Availability: 

Available for researchers through a user program.

Single Point of Contact: 

Dr. Srdjan Simunovic, Senior Staff Scientist, simunovics@ornl.gov, 865-241-3863

Supporting Document(s):