Dynamic Response of Brittle Materials from Penetration and Split Hopkinson Pressure Bar Experiments

Three sets of penetration experiments into limestone targets were conducted with geometrically similar, steel rod projectiles that had length-to-diameter ratios of 10- and 7.1-, 12.7-, and 23A-mm-diameters. Results from these penetration experiments and previously developed penetration models suggested that the limestone target exhibited strain-rate sensitivity. A modified split Hopkinson pressure bar facility (SHPB) was used to study the dynamic material responses of brittle materials, such as limestone, under a state of one-dimensional stress. The conventional split Hopkinson pressure bar apparatus was modified by shaping the incident pulse such that the samples are in dynamic stress equilibrium and have nearly constant strain rate over most of the test duration. A thin disk of annealed or hard C11000 copper is placed on the impact surface of the incident bar in order to shape the incident pulse. An analytical model and data show that a wide variety of incident strain pulses can be produced by varying the geometry of the copper disks and the length and striking velocity of the striker bar. The pulse shaping model predictions are in good agreement with measurements. Thus, this work presents analytical models and experimental techniques that provide procedures to obtain dynamic, compressive stress-strain data for brittle materials.