Abstract
Direct impact Hopkinson pressure bar systems offer many potential advantages over split Hopkinson pressure bars, including access to higher strain rates, higher strains for equivalent striker velocity and system length, lower dispersion and faster achievement of force equilibrium. Currently advantages are gained at a significant cost: the fact that input bar data is unavailable removes all information about the striker impacted specimen face, preventing the determination of force equilibrium, and requiring approximations to be made on the sample deformation history. Recently photon Doppler velocimetry methods have been developed, which can replace strain gauges on Hopkinson bars. In this paper we discuss an experimental method and complementary data analysis for using Doppler velocimetry to measure surface velocities of the striker and output bars in a direct impact bar experiment, allowing similar data to be recorded as in a split bar system, with the same level of convenience. We discuss extracting velocity and force measurements, and improving the accuracy and convenience of Doppler velocimetry on Hopkinson bars. Results obtained using the technique are compared to equivalent split bar tests, showing improved stress measurements for the lowest and highest strains.
Highlights
Two general types of Hopkinson Bar systems exist for compression experiments: Split Hopkinson Pressure Bars (SHPBs, known as Kolsky Bars [1]) and Direct Impact systems (DIHBs) [2]
Sample disks of 1 mm thickness and 3 mm diameter made from a 99.9% pure aluminium and a commercial Dural alloy and were tested at equal striker velocities in both SHPB and DIHB arrangements
We have presented a method for directly measuring the velocities and forces at the impacted face of a specimen in a DIHB experiment
Summary
Two general types of Hopkinson Bar systems exist for compression experiments: Split Hopkinson Pressure Bars (SHPBs, known as Kolsky Bars [1]) and Direct Impact systems (DIHBs) [2]. In a typical SHPB test, a sample is placed between an input and output bar as depicted in. The bars remain elastic during the experiment, allowing the force and velocity histories of the end faces of the bars, and the sample, to be found from contact measurements at a distance. The input bar is removed and the sample is struck directly by the striker bar as shown in Vmax = Ybar Zbar (1) (a) A (b) E B C D F
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