The specimen in an SHPB system is a small disc or cylinder of material, with faces perpendicular to the cylindrical axis. A specimen must be large enough to adequately represent its bulk constituent material; a commonly used criterion is that the specimen contain at least 10 characteristic units of the material structure across all of its linear dimensions.³ The size of the transmitted pulse is another factor that governs the size of the specimen. As a result, soft materials may require a large radius.

The University of Cambridge scientists designed the MSHPB to measure the properties of materials at strain rates between 10⁴ s^-1 and 10⁵ s^-1. The device uses input/output bars with diameters ranging from 3.0 to 3.2 mm to test specimens with diameters between 0.5 and 1.5 mm. PTFE (polytetrafluoroethylene) bearings, carefully aligned and mounted on a single piece of steel, guide the bars at three points along their respective lengths. The input/output bars are each 300 mm long, and the striker bar is 100 mm long; the combined result produces a typical loading pulse of 40 μs. Technicians instrumented the bars with semiconductor strain gauges, which are available in the small size necessary for adhering to the bars. The short length of these gauges minimizes the inherent time averaging along the length of the gauge and, coupled with their high gauge factor, allows their use without additional amplification prior to recording the signal.

The MSHPB is part of the new mechanical properties laboratory located at AFRL’s HERD facility. Specializing in the testing of energetic materials, this laboratory contains not only the MSHPB, but also a full-size SHPB and equipment for quasi-static testing of materials. The addition of the MSHPB to the traditional suite of mechanical testing equipment provides an order of magnitude higher strain rate and allows scientists to test samples when only a small quantity of material is available. Currently, scientists are using these tools to study the effect of particle size on the mechanical properties of model energetic simulant composite systems. Future additions to these experimental capabilities will include temperature control, highspeed imaging, and in situ diameter measurement.

Dr. Jennifer L. Jordan and Mr. Clive R. Siviour (University of Cambridge), of the Air Force Research Laboratory’s Munitions Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn/index.htm. Reference document MN-H-05-02.

References

¹ Gray, G. T. III. “Classic Split Hopkinson Pressure Bar Technique.” ASM Handbook Vol 8: Mechanical Testing and Evaluation. eds. H. Kuhn and D. Medlin. Materials Park, OH: ASM International (2000): 462-476.
² Gorham, D. A. “Specimen Inertia in High- Strain-Rate Compression.” Journal of Physics D: Applied Physics, 22 (1989): 1888-1893.
³ Armstrong, R. W. Yield, Flow, and Fracture of Polycr ystals. ed. T. N. Barker. London: Applied Science Publ (1983).