What is a Split Hopkinson Pressure Bar?
A Split Hopkinson Pressure Bar system will contain a gas launcher; an incident bar, a transmission bar; and a stopping mechanism that are aligned and mounted on a flat base.
The gas launcher that is charged with pressurized gas to accelerate a striker bar at a measured velocity to create an impact incident. Striker bars of various lengths are used to produce a range of loading durations i.e. short striker bars impart short time duration loads. The impact of the striker bar on the incident bar sends a stress pulse traveling down the incident bar (at the speed of sound) that is detected by a strain gage mounted on the bar.
The pulse (a longitudinal wave) continues through the incident bar to the material sample to be tested. The test specimen is typically mounted between the incident and transmission bars. Due to impedance mismatches, part of the wave is reflected back into the incident bar (measured by strain gage) and part of the wave is transmitted into the transmission bar (measured by strain gage).The wave traveling through the system triggers data collection from the strain gages located on both the incident and transmission bars. Stress, strain, and strain rate are then calculated from the data collected by the strain gauges. This measured strain can be verify through the use of a high speed camera often used in high strain rate testing.
A Split Hopkinson Pressure Bar (SHPB) is used to test the high strain rate material properties of materials. The Hopkinson bar is used to impose a dynamic load on a material specimen akin to that which the material will experience in dynamic situations like vehicle crashes or other high-energy events.
Understanding Material Performance
From occupant protection components of a vehicle, ballistic and blast resistant armor, to high impact sports and volcanic eruptions, scientists need to understand the performance of a material under dynamic conditions. The Split Hopkinson Pressure Bar is the ideal tool because it provides a controlled impulse of energy.
REL offers testing of the client’s materials on an in-house Hopkinson bar, complete with specimen preparation, SEM analysis and evaluation. In addition, REL designs and builds custom Split Hopkinson pressure bars for a variety of companies and research institutions with tension, compression, sheer, and shock loading capabilities to be used in testing. Not only does REL produce a metallic SHPB for testing of high strength samples such as titanium and ceramics, but they also build a non-metallic SHPB for testing of low strength samples such as polymer foams and bio tissues.
REL’s 3rd Generation SURE-Test System
REL’s Gen-3 SHPB testing system has integrated what has been learned from the previous SURE-Test System platforms and has optimized the usability, the repeatability and the safety of the testing system.
Gen-3 is designed for better structural performance. The overall mass and stiffness of the testing system has been increased with thick wall steel box section tubing and laser cut base platforms for a rigid connection of the modular sections. The Gen-3 Sure-Test platform is designed to be modular in nature. This flexibility allows the system to be configured in different sizes, lengths and to integrate different testing modules. These variations give the system the ability to conduct a wide variety of test configurations in addition to classic SHPB testing.
The system also includes a linear rail system designed to mount and align accessory modules that are required to travel along the system length. These modules include hardware such as data acquisition (DAQ) workstations, environmental chambers and high-speed imaging and lighting equipment.
The SURE-Test System design has seen extensive use and validation in REL’s high strain rate test and development laboratory as well as in installations on various academic, military and commercial sites.
In addition to the safety training REL supplies with each system, the safety of REL’s system is improved through the remote firing of the SURE-Launch system. The firing control and wiring can be taken into an adjoining room to initiate a test. Locking out of the system can be easily completed through the disconnecting and locking up of the firing control in a location outside of the lab. This will insure that no un-trained users can operate the SURE-Launch Module.
The detailed specifications for a typical system is as follows:
Sure-Test Platform Base Equipment
- Three 10ft Sure-Flat Base Sections. These sections combine for a total of 30ft of double-wide mounting rails compatible with all Sure-Test system modules.
- Automated 1” Launcher assembly. This launcher allows striker bar diameters up to 1” diameter. Standard pressure units operate up to 300psi and launch striker bar projectiles up to 300 ft./sec. High velocity units are also available.
- Fifteen bar supports capable of carrying up to 1” diameter bars. These supports are 6” center height
- A hydraulic damper with stop stand designed to absorb excess energy from SHPB and dynamic impact test events. The stop stand also serves as a mounting base for load cells, sample platens and fixtures.
- A SURE-Speed chronograph module. The SURE-Speed is a highly accurate, compact chronograph designed for SHPB use. It records the striker speeds for dynamic testing and is easily configured with a keyed mounting base, adjustable height carrier and intuitive touch screen.
- Two sets of tension grips and collets built to REL’s latest specification for system bars. REL will also supply the setting tool required for installing a sample in these grips.
- These grips and collets have been developed and tested to provide a consistent and predicable dynamic response through the testing event.
- The setting tool supplied is used to set the grips into the sample at specified pressures. This issues consistent load response from test-test.
- A servo mechanical test system module for testing samples at low to medium strain rates. A typical system has a gear reduction option that will generate up to 5000lb push/pull up 6 in/sec. In direct drive it will generate 500lb push/pull up to 60 in/sec. The servo module mounts to the SURE-Flat base system and is compatible with all bar end grips, caps and samples from the SHPB hardware.
- The grips used in the SHPB testing are compatible with the servo mechanical test system to be able to obtain both quasi-static and high strain rate properties of materials.
Data Acquisition and Analysis
- A rail mounted workbench and drawer cabinet. This unit will attach to the accessory rail on the Sure-Flat system and roll to any position along the length of the system. It is typically used to mount computers, signal conditioners, oscilloscopes and store tools, samples and cables.
- Revolutionary Processing Software – SURE-Pulse is an open-source software that continues to be refined to not only produce SHPB testing results, but also process data from all generic load, strain date users may generate. The latest features in SURE-Pulse integrate image data captured by high-speed cameras. Using digital image correlation SURE-Pulse plots strain information directly on the processed sample images. This allows the stress-strain curve to be compared to the actual visual state of the material that the curve represents. The results file is portable and can allow for analysis to be shared across different users. The steps taken to transform the data are preserved, allowing back-tracing of test data.