Improved targeting accuracy and the long-standing desire to minimize collateral damage are causing current and future munitions to become much smaller. As munitions size decreases, the explosive materials packed within bomb cases begin to carry a significant portion of the structural loads experienced by the warhead. In an ongoing program effort to determine the mechanical properties of explosives and other energetic materials, scientists at AFRL's High Explosives Research and Development (HERD) facility (Eglin Air Force Base, Florida) acquired a miniaturized split Hopkinson pressure bar (MSHPB) (see Figure 1). Designed and built by Mr. Clive Siviour under the guidance of Drs. John Field, Bill Proud, and Stephen Walley (of the United Kingdom's University of Cambridge, Physics and Chemistry of Solids Group), the MSHPB is capable of strain rates up to 105 s-1 in material samples. AFRL's European Office of Aerospace Research and Development sponsored the project.
The compressional split Hopkinson pressure bar (SHPB) is a common apparatus researchers use to measure material properties at strain rates between 500 s-1 and 104 s-1.1 Test engineers place the specimen between two metal rods, known as input and output bars. These bars are instrumented with strain gauges halfway down their respective lengths (see Figure 2). A light-gas gun accelerates the striker bar into the input bar, and the resulting impact produces the stress pulse, or incident wave, in the input bar. The stress pulse travels the length of the input bar in the form of an elastic wave that impacts the specimen, deforming it plastically under the load. The impedance change at the bar-specimen interface causes some of the incident wave energy to travel through the specimen and some to be reflected back into the input bar, forming transmitted and reflected waves, respectively. The strain gauges measure the magnitude and shape of these waves (see Figure 3), and test engineers then use this data in conjunction with a set of SHPB equations to calculate the stress and strain in the specimen as a function of time. By changing the velocity of the striker bar to vary the magnitude of the stress pulse, the engineers can control the strain rate in the specimen.
As the specimen compresses as a result of the input pulse, it expands radially. The inertial resistance to this radial expansion further increases the measured stress so that it becomes greater than the actual strength of the material. Scientists have demonstrated that (1) this inertial contribution to the stress is proportional to the square of the strain rate value; and (2) although scientists cannot correct for this inertia, they can minimize it through specimen design.2 To limit the effects of inertia, specimen size must decrease as strain rate increases. Unfortunately, reduced specimen size also reduces the amount of force transmitted by the input bar. Increasing the strain generated by the input force requires the cross-sectional area of the input/output bars to be decreased. Therefore, to measure the mechanical properties of materials at higher strain rates, engineers developed the MSHPB.
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.
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).