Split Hopkinson Pressure Bar High Strain Rate Material Testing
SPLIT HOPKINSON PRESSURE BAR OPERATION The purpose of Split Hopkinson Pressure Bar testing is to obtain high strain rate material properties. ( A ) To initiate high strain rate testing of a material, a specimen is loaded between the incident and transmission bars. ( B ) Next, the striker bar launcher is pressurized with helium or nitrogen. When fired, the launcher releases the gas and propels a striker bar into the end of the incident bar. The collision creates a compression pulse, or stress wave, that propagates through the bar toward the specimen. (C) The incident wave is recorded by the incident strain gauge. Once the wave reaches the specimen, it splits into two smaller waves. ( D ) The transmitted wave travels through the specimen and into the transmission bar where the energy is recorded by the transmission strain gauge. The second wave is reflected away from the specimen and travels back down the incident bar. (E) At the end of the Split Hopkinson Pressure Bar, a stop bar absorbs the impact of the transmission bar to complete the test. (F) Both strain gauges measure the strain duration and amplification in the bars. The reflected (tensile pulse) recorded by the incidence strain gauge is used to calculate strain. The portion of the compression pulse that continues through the specimen is recorded by the transmission gauge and is used to calculate stress. Data from the strain gauges is routed through amplifiers and an oscilloscope to a laptop computer where it is stored.
OSCILLOSCOPE GRAPH Figure 1 shows how a typical oscilloscope output graph is displayed on a computer. The incident wave, transmitted wave and reflected wave are simultaneously collected and presented in a graph of Voltage (y-axis) versus Time (x-axis).
BAR SUPPORT The incident, transmission and stop bars are precision aligned using a series of bar supports. Each bar support moves laterally along an alignment key and is secured to the mounting rail using built-in screw clamps. To further assist with leveling, the supports glide on top of a blanchard ground surface. Proper bar alignment is critical for obtaining accurate test results and minimizes the amount of noise in your recorded data.
A Split Hopkinson Pressure Bar (SHPB) is used to obtain high strain rate material properties. The bar is used to impose a dynamic load on a material specimen akin to loads the material will experience in service. Determining how the material properties change under service loads can yield critical performance data. REL's precision-made Split Hopkinson Pressure Bar includes sophisticated yet intuitive technology to simplify the process for the operator. Use of the latest data recording devices and strain gauges ensure accurate measurements of material properties at a variety of strain rates.
The Split Hopkinson Pressure Bar by REL, Inc. can be found throughout the world and under the direction of research institutions, government labs, private businesses and a wide variety of material testing agencies. REL, Inc. has provided SHPB equipment or material testing to the following organizations: Johns Hopkins University Youngstown State University University of Florida Oak Ridge National Laboratory -
SAMPLE MATERIALS SPECIMEN i
Contact REL, Inc. at (906) 337-3018 to discuss your material analysis requirements.
t
INCIDENT BAR
TRANSMISSION BAR
Io r
i = initial pulse r = reflected pulse
Positive x Direction
t = transmitted pulse
I o = specimen length TENSILE SPECIMEN APPROXIMATELY:
0.375"W x 2.50"L x 0.125"H
REL, Inc. offers a tension Split Hopkinson Pressure Bar design that uses a machined tensile specimen for obtaining the most accurate test data.
REL, Inc. manufactures steel and polymer Split Hopkinson Pressure Bar systems that are configurable to test compression, tension and torsion samples. Sample sizes can range in diameter from 0.125" up to 3.000" and consist of a variety of materials, including: • Metal Alloys • Composite and Ceramics
• Foams and Plastics • Bio-tissue
* Deformed sample shown for comparative purposes.
Deformation of tested samples will vary depending on the material.
COMPRESSION SPLIT HOPKINSON PRESSURE BAR DESIGN
DYNAMIC STRAIN GAUGE REL uses high-precision bonded resistance strain gauges to capture energy from stress waves. The gauges are adhered to the surface of the incident and transmission bars. Signals transmitted from the bar to the gauge are captured and converted by REL’s proprietary data acquisition system.
OSCILLOSCOPE & BRIDGE AMPLIFIERS The strain gauges are routed through bridge amplifiers (shown in yellow) and an oscilloscope. Data from each gauge is conditioned and amplified for multi-channel, simultaneous dynamic recording and display. The oscillo-
scope converts the measured data from an analog signal to digital. REL’s proprietary software further converts the signal into a dynamic stress versus strain curve (Fig. 2).
STRIKER BARS Striker bars can be made from steel or polymers and supplied at different diameters (0.25" up to 3" or custom) and lengths (3" up to 24"). Various length striker bars will produce different loading durations.
STRIKER BAR LAUNCHER The velocity of the striker bar projectile is controlled by pressurized helium or nitrogen that is forced into the launch chamber. The type of material being tested and the construction of the striker bars in use generally determine operating pressure.
STRESS -VS- STRAIN GRAPH Data collected from the REL Split Hopkinson Pressure Bar (See Fig. 1) is processed through a series of formulas to obtain final data (shown above). REL includes an intuitive graphical user interface for the operator to quickly acquire dynamic material properties.
LAUNCHER CONTROLS The striker bar launcher controls include a gas fill valve (yellow handle), gas release valve (silver lever) and a breech load adjustment control (large black dial). Turning the dial moves a positioning rod inside of the launcher, which pushes the striker bar forward in the breech.
EQUIPMENT SPECIFICATIONS
*EXAMPLE COMPRESSION DESIGN
SYSTEM FLEXIBILITY - System Type (Compression, Tension or Torsion) - System Material Construction (Steel, Polymer, etc.) - Total Length of System - Incident, Compression and Stop Bar Diameter - Incident, Compression and Stop Bar Length - Striker Bar Diameter and Length - Striker Bar Launcher Pressure Range Requirement
- Strain Gauge Type and Construction - Momentum Trap Inclusion - Acquisition System Inclusion (Computer, Amplifiers & Oscilloscope)
* REL Split Hopkinson Pressure Bar dimensions shown for example compression design only. Complete system build requirements will determine final dimensions and equipment specifications. REL, Inc. reserves the right, under its Continuous Improvement Policy, to change construction or design details and furnish product when so altered without reference to illustrations or specifications used herein. Copyright Š2012 REL, Inc. All rights reserved.
Litho in U.S.A. Form No. 1000-06-12
MATERIAL TESTING AND ANALYSIS REL, Inc. offers comprehensive Split Hopkinson Pressure Bar testing with complete material analysis. For more information, contact an REL material specialist at (906) 337-3018 or visit the company web site at www.relinc.net
REL, Inc. 57640 North Eleventh Street | Calumet, MI 49913 906.337.3018 tel 906.337.2930 fax rel@relinc.net www.relinc.net