SPECIMEN STRESS EQUILIBRIUM IN SPLIT HOPKINSON PRESSURE BAR TESTS OF CERAMICS AT HIGH STRAIN RATE

摘要

This paper provides an analytical method and guidelines for achieving valid Split Hopkinson Pressure Bar (SHPB) tests of ceramics at high strain rate. A one dimensional model of a SHPB test of an elastic specimen is expressed by the wave equation with the initial and boundary conditions. For any incident stress pulse, the analytical expression of the specimen stress is derived via Fourier transform analysis. Consequently, the general expressions for the bar stress at the two ends of the specimen, the specimen stress uniformity and the strain rate are established. For linear ramp loading, which is usually generated using pulse shaping technique in SHPB tests, the threshold time to achieve stress equilibrium and constant strain rate is determined. In addition, the strain rate when the specimen fractures is obtained. The effects of the bar-specimen mechanical impedance and the rising rate of the incident pulse on the stress uniformity and the strain rate are investigated. The analytical results are applied to determine the optimal test conditions for accurate and reliable SHPB experiments on ceramics at several thousand strain per-second.