ABSTRACT--An experimental technique that modifies the conventional split Hopkinson pressure bar has been developed for measuring the compressive stress-strain responses of materials with low mechanical impedance and low compressive strengths such as elastomers at high strain rates. A high-strength aluminum alloy was used for the bar materials instead of steel, and the transmission bar was hollow. The lower Young's modulus of the aluminum alloy and the smaller cross-sectional area of the hollow bar increased the amplitude of the transmitted strain signal by an order of magnitude as compared to a conventional steel bar. In addition, a pulse shaper lengthened the rise time of the incident pulse to ensure stress equilibrium and homogeneous deformation in the low-impedance specimen. Experimental results show that the high strain rate, compressive stress-strain behavior of an elastomeric material can be determined accurately and reliably using this technique.

ABSTRACT--This paper presents a split Hopkinson pressure bar technique to obtain compressive stress-strain data for rock materials. This technique modifies the conventional split Hopkinson bar apparatus by placing a thin copper disk on the impact surface of the incident bar. When the striker bar impacts the copper disk, a nondispersive ramp pulse propagates in the incident bar and produces a nearly constant strain rate in a rock sample. Data from experiments with limestone show that the samples are in dynamic stress equilibrium and have constant strain rates over most of the test durations. In addition, the ramp pulse durations can be controlled such that samples are unloaded just prior to failure. Thus, intact samples that experience strains beyond the elastic region and postpeak stresses can be retrieved for microstructural evaluations. The paper also presents analytical models that predict the time durations for sample equilibrium and constant strain rate. Model predictions are in good agreement with measurements.KEY WORDS--Hopkinson bar, pulse shaping, rock materials, high strain rate The split Hopkinson pressure bar (SI-IPB) technique originally developed by Kolsky 1,2 has been used by many investigators to obtain dynamic compression properties of solid materials. The evolution of this experimental method and recent advances were discussed by Nicholas, 3 Follansbee, 4 Nemat-Nasser et al., 5 Ramesh and Narasimhan, 6 Gray 7 and Gray and Blumenthal. 8 This technique has mostly been used to study the plastic flow stress of metals that undergo large strains at strain rates between 10 2 -10 4 s -I. As discussed by Yadav et al.,9 data for the compressive flow stress of metals are typically obtained for strains larger than a few percentage points because the technique is not capable of measuring the elastic and early yield behavior. By contrast, most of the material behavior of interest for relatively brittle materials such as ceramics and rocks occurs at strains less than about 1.0 percent. In this study, we modified the conventional SHPB or Kolsky bar technique to obtain dynamic compressive stress-strain data for rock materials and conducted experiments with limestone samples that have failure strains less than 1.0 percent. The analytical and experimental work presented in this study for rock materials uses and extends recently published work on ceramic materials. We particularly cite the experimental and analytical work by Nemat-Nasser etal. 5 for pulse shaping and the sample equilibrium model published by Ravichandran and Subhash. 1~ D. J. Frew is a ResearchFor an ideal Kolsky compression bar experiment, the sample should be in dynamic stress equilibrium and should deform at a constant strain rate over most the duration of the test. To closely approximate these ideal conditions for experiments with brittle ceramic and rock materials, a properly designed, thin copper disk is placed on the impact surface of the incident bar so that a nondispersive ramp pulse propagates in the incident bar. Data from experiments pr...

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Low-strength and low-impedance materials pose significant challenges in the design of experiments to determine dynamic stress-strain responses. When these materials are tested with a conventional split Hopkinson pressure bar, the specimen will not deform homogeneously and the tests are not valid. To obtain valid data, the shape of the incident pulse and the specimen thickness must be designed such that the specimens are in dynamic equilibrium and deform homogeneously at constant strain rates. In addition, a sensitive transmission bar is required to detect the weak transmitted pulses. Experimental results show that homogeneous deformations at nearly constant strain rates can be achieved in materials with very low impedances, such as a silicone rubber and a polyurethane foam, with the experimental modifications presented in this study.

A dynamic experimental technique that is three orders of magnitude as sensitive in stress measurement as a conventional split Hopkinson pressure bar (SHPB) has been developed. Experimental results show that this new method is effective and reliable for determining the dynamic compressive stress-strain responses of materials with low mechanical impedance and low compressive strengths, such as elastomeric materials and foams at high strain rates. The technique is based on a conventional SHPB. Instead of a surface strain gage mounted on the transmission bar, a piezoelectric force transducer was embedded in the middle of the transmission bar of a high-strength aluminum alloy to directly measure the weakly transmitted force profile from a soft specimen. In addition, a pulse-shaper technique was used for increasing the rise time of the incident pulse to ensure stress equilibrium and homogeneous deformation in the low-impedance and low-strength specimen.KEY WORDS--Soft materials, Hopkinson bar, quartz crystal, dynamic behavior, high strain rate