Appraisal of Pulse-Shaping Technique in Split Hopkinson Pressure Bar Tests for Brittle Materials

Split Hopkinson pressure bar (SHPB) technique has been frequently used to measure the uniaxial compressive stress–strain relation of brittle materials at intermediate strain-rates where pulse-shaping technique is employed to improve the stress uniformity and maintain a nearly constant strain-rate in the specimen during the effective loading period. This paper appraises the functions of the pulse-shaping technique in SHPB tests of brittle samples based on numerical simulations of SHPB tests. It is shown that a proper pulse-shaper can attenuate high frequency oscillations of the incident pulse and increase the rise-time of the pulse, resulting in the improvement of stress equilibrium and uniformity along the axial direction of an SHPB specimen. However, it is found that the inertia-induced confinement in the radial direction of a brittle specimen is still significant even though the shaped incident stress pulse can generate a nearly flat plateau in the reflected pulse in the SHPB test. It implies that the achievement of a nearly constant strain-rate represented by a nearly flat plateau in the reflected pulse in an SHPB test may not give a true nearly constant strain-rate in the SHPB specimen. It is concluded that the application of the pulse-shaping technique in SHPB tests on brittle materials may not change the nature of the observed transition strain-rate, which represents the transition of the stress state from a uniaxial-compressive-stress-dominated state to a confined compressive stress state, rather than the start of significant strain-rate effect. Therefore, inertia-induced radial confinement effect needs to be considered in the interpretation of any SHPB results for brittle materials even though a pulse-shaper is used.