The conventional split Hopkinson pressure bar system for materials testing at high strain rates produces loading pulses that have a high level of oscillation superimposed on their approximately trapezoidal shape. These oscillations, which are a consequence of the short risetime of the loading pulse produced by the impact of a projectile, cause difficulty in the interpretation of the stress-strain results obtained by such a system. A modification to the conventional system is described, which virtually eliminates these oscillations, resulting in a smooth loading pulse and a significant improvement in the interpretation of the stress-strain behaviour. The modification involves placing a pre-loading bar before the two main bars of the conventional system. By suitable choice of pre-loading bar material with lower strength than the main bars, the high-frequency oscillations resulting from the projectile impact are attenuated as the pulse travels along the pre-loading bar; the loading pulse entering the main loading bar is then smooth with virtually no oscillations. Results for copper and steel test specimens are presented, which illustrate the significant advantages that the three-bar technique has over the conventional method in producing unambiguous stressstrain data.

To establish the best strategy for transmitting speech-derived information via a single tactile channel, measurements were made on the perception of frequency- and/or amplitude-modulated pulse-train stimuli, with a comparison of the electrotactile and vibrotactile modalities. In one experiment, vibrotactile perception of 2-oct step changes in stimulus frequency was found to be significantly better than electrotactile on a time-scale appropriate for the transmission of speech features (e.g., with practiced subjects, information transfer of 69% with 200-ms vibrotactile stimuli, 32% with 200-ms electrotactile stimuli). Perception of step changes in stimulus amplitude was similar in the two modalities when changes in amplitude were tailored to match the different dynamic ranges available. In a second experiment, vibrotactile-perception of voice fundamental frequency with various codings was investigated. Both experiments showed information transfer for vibrotactile stimuli to be greater when frequency and amplitude modulation were used together rather than with one or the other in isolation (sentence-stress identification scores: 66% for FM stimuli, 69% for AM stimuli, 80% for FM/AM stimuli). It is concluded that frequency- and amplitude-modulated vibratory stimulation is a good choice in a practical device for the profoundly hearing impaired.

Resume Dans le split Hopkinson barre technique pour compression, un echantillon en forme de disque serre entre deux fastes barres d'acier est compresse par une impulsion compressive. Les proprietes de la contrainte et deformation de l'echantillon peut h r e derive par la quantite d'impulsion compressive, renvoye et transmit par l'echantillon, en supposant que l'equilibre contrainte existe tout le long de l'echantillon. Neanmoins pendant la periode initiale de compression, les multiples reflexions generes aux deux barre/echantiBon interfaces cree une distribution compressive non-uniforme, qui anive a une evaluation peut-&tre incorrecte des proprietes initials de contrainte/deformation. Cette publication decrit une analyse de microordinateur afin d'etudier plusiers parametres importants qui influencent l'accumulation de l'equilibre contrainte d'ClasticitC et predit les formes d'impulsion refletes et transmit pour des impulsions de compression differents. Abstract In the compression testing version of the split Hopkinson pressure bar technique a small disc-shaped specimen, sandwiched between two high-strength steel bars, is compressed by a stress pulse. The stress/strain properties of the specimen can be derived from the amount of the stress pulse reflected and transmitted by the specimen, assuming that stress equilibrium exists throughout the specimen. However, during the initial loading phase the multiple reflections generated at the two specimenbar interfaces cause a non-uniform stress distribution, leading to possibly inaccurate estimates of the initial stresslstrain properties. The paper describes a microcomputer analysis to study many of the important parameters affecting the build-up to elastic stress equilibrium and to predict the reflected and transmitted pulse shapes for different loading pulses.

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