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.
The measurement of the stress-strain properties of materials at high strain rates is principally carried out using the split Hopkinson pressure bar (SHPB) technique, in which high compressive strain rates, typically in the range 10 2 -10 4 s −1 , are achievable with small solid discs as the test materials. Torsional SHPB systems have also been developed for test specimens in the form of small hollow cylinders. Another version of the SHPB technique has enabled high-strain-rate tensile measurements to be made, but inertial effects tend to limit the strain rate to about 10 3 s −1 for the dogbone-shaped specimens. To achieve higher tensile strain rates, the authors have developed an expanding ring technique. This involves placing a thin ring of polymer as a sliding fit around a hollow, thick-walled, polymeric cylinder, the internal wall of which is loaded with a blast wave generated by an exploding wire. The tensile stress-strain properties of the ring material have been determined, at strain rates up to 1.6 × 10 4 s −1 , by observing the movement of the ring as it freely expands away from the cylinder. Several polymers have been studied and comparisons made with compressive stress-strain behaviour.
Trois polymères d'application ingénieure (HDPE, UHMWPE, et Nylatron GS) ont été étudiés à très grande vitesse de déformation (excédant 10 4 /s). On utilisait une technique d'anneau dilaté, qui nécessite le placement d'un fin anneau de polymère, comme bague glissante autour d'un cylindre/creux ayant les murs épais. Un chargement par onde engendré par un fil explosif, abouti au puise de pression qui s'exerce sur le mur interne du cylindre. Ce puise se propage à travers le mur du cylindre et est transféré partielement dans l'anneau. L'anneau se déplace a une grande vitesse du cylindre presque instantanément et en conséquence décélère sous une tension quasiment uniaxiale dû à la contrainte circonférentielle. L'anneau a été photographe avec un appareil de grande vitesse (des cadres de 10 5 à 10 6 /s) et on a pris des mesures duquels les vrais propriétés de déformation et de contrainte étaient déterminés. On a comparé les résultats obtenus du barre d'Hopkinson, en conditions quasistatiques avec les expériences de l'anneau dilaté.
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