THE FREELY EXPANDING RING TEST - A TEST TO DETERMINE MATERIAL STRENGTH AT HIGH STRAIN RATES

Le test d'expansion de cylindre (ERT) est un test de conception simple pour étudier le comportement des matériaux en grande déformation et pour de grandes vitesses de déformation. Le test est réalisé en plaçant un anneau mince du matériau à étudier dans un processus d'expansion radiale et en mesurant cette vitesse d'expansion. L'anneau est projeté par un explosif ; le test n'est pas devenu populaire à cause des problèmes posés par le lancement de l'anneau par l'explosif, notamment sur les modifications des propriétés par l'onde de choc. Pour déterminer l'aptitude de l'ERT à déterminer les propriétés des matériaux, une série d'expériences a été conçue sur un matériau sévèrement contrôlé (un cuivre revenu sans oxygène). Les anneaux récupérés ont été analysés et leur changement de dureté déterminé. La comparaison entre les données de l'ERT et celles obtenues avec des essais à [MATH] = 5x103 s-1 à la barre d'Hopkinson indique que la dureté induite par choc est approximativement équivalente à un écrouissage de 5%. Les données de l'ERT sur ce matériau pour des vitesses de déformation allant jusqu'à 2,3 x 104 s-1 sont présentées.The freely expanding ring test (ERT) is a conceptually simple test for determining the stress-strain behavior of materials at large strains and at high strain rates. This test is conducted by placing a thin ring of test material in a state of uniform radial expansion and then measuring its subsequent velocity- time history. The ring is usually propelled by a high explosive driving system. The test has not become popular in the materials property community, however, because there has been some concern about how the launching of the ring sample with an explosively generated shock wave might affect the properties to be measured. To determine the suitability of the ERT for these fundamental investigations, a series of experiments was performed on a carefully controlled material--oxygen-free electronic fully annealed copper. Recovered ring samples were analyzed and the change in hardness determined. Comparisons of the ERT data with that from Hopkinson bar tests at strain rates of about 5 x 103 s-1 indicate that the shock-induced hardness is approximately equivalent to a strain hardening of 5%. ERT data on this material at strain rates up to 2.3 x 104 s-1 are presented

Improved technique for determining dynamic material properties using the expanding ring

Since its introduction nearly two decades ago, the expanding ring test has shown considerable promise as a simple method of obtaining strain-rate-sensitive uniaxial material property data. The procedure is to monitor the kinematics of a uniformly expanding ring. The stress-strain-strain rate response of the ring material can then be calculated from the ring equation of motion and the recorded data. Efforts in the past have been based upon recording the transient displacement of the ring. Determination of the stress in the ring then required double differentiation with respect to time of the ring displacement. The work reported herein is an attempt to overcome this difficulty by directly measuring ring velocity as a function of time by means of a laser velocity interferometer; this method requires one less differentiation of the data. The procedure is illustrated by experiments performed on annealed and hardened copper rings. Results are compared to quasi-static material properties determined for the same materials using conventional techniques

Development of the freely expanding ring test for measuring dynamic material properties

Modifications to the freely expanding ring test for eliminating adverse two-dimensional effects are described and illustrated. The result is to substantially increase the strain-rate range over which dynamic material property data can be reliably obtained. Several different ring launching schemes are discussed, and data are presented that were taken with a particular shockless electromagnetic system. Results from initial attempts at measuring dynamic compressive properties with a contracting ring are presented

Dynamic response of containment vessels to blast loading

The dynamic response of steel, spherical containment vessels loaded by internal explosive blast was studied by experiments, computations, and analysis. Instrumentation used in the experiments consisted of strain and pressure gauges and a velocity interferometer. Data were used to rank the blast wave mitigating properties of several filler materials and to develop a scaling law relating strain, filler material, and explosive energy or explosive mass