High‐Strain‐Rate Testing of Materials: The Split‐Hopkinson Pressure Bar

Abstract

Abstract Robust models that capture the physics of high‐rate material response are required for developing predictive capability for highly dynamic events. One develops a strength constitutive model for a particular material by measuring its mechanical properties over a range of temperatures, strain rates, and stress states. The samples of interest are loaded in compression, tension, or torsion over a range of loading rates and temperatures germane to the application of interest. Various mechanical testing frames are available that achieve nominally constant loading rates for limited plastic strains and, thereby, a constant strain rate. The standard screw‐driven or servohydraulic testing machines achieve strain rates of up to 5 s −1 . Specially designed testing machines, typically equipped with high‐capacity servohydraulic valves and high‐speed control and data acquisition instrumentation, can achieve strain rates as high as 200 s −1 during compression loading. To go even higher, we must employ projectile‐driven impacts that induce stress‐wave propagation in the sample materials. Chief among the techniques to measure the dynamic stress–strain response of materials is the split‐Hopkinson pressure bar. This technique is capable of achieving the highest uniform uniaxial stress loading of a specimen in compression at nominally constant strain rates of the order of 10 3 s −1 and true strains of 0.3. Stress in the Hopkinson bar is measured by using an elastic element in series with the specimen of interest. Stress waves are generated via an impact event and the elastic elements utilized are long bars such that the duration of the loading pulse is less than the wave transit time in the bar. In this article the principles of the split‐Hopkinson pressure bar, its data reduction, sample preparation, and limitations are detailed.