Experimental methods at high rates of strain

The subject area covered by this review is a large and important one and to do it full justice would require a larger format than is available here. However, an attempt is made to outline the history of some of the more important developments and to include most major techniques. Taylor impact, Hopkinson bar testing in its various forms and 1-D plate impact are all considered in some detail. High-speed photography, particularly when used in association with optical techniques, is a key area and recent advances are discussed. 1. HISTORICAL FOUNDATIONS OF HIGH STRAIN-RATE TESTING As the 19th century progressed, there was an increasing awareness that the properties of materials under impact differed from those under static or 'dead' loading [1-5]. Experimental investigations were hampered by lack of suitable intrumentation, but considerable theoretical progress was made in the understanding of the propagation of stress waves in bounded media such as circular rods [6-9]. It is generally considered that J. Hopkinson in 1872 made the first experimental demonstration that metals can withstand a larger impulsive load in simple tension than they can under static loading [2, 3]. His son B. Hopkinson carried out further work in this area [10] and also invented an ingenious ballistic pendulum method for determining the shapes of pulses caused by the impact of bullets or the detonation of explosive charges at one end of a long rod [11]. By using momentum traps of different lengths, he was able to build up a picture of how much momentum passed down the rod as a function of time for different classes of event (assuming that these types of event are repeatable). Because he performed the pioneering work on determining the shapes of impulses travelling down rods, the device consisting of a long rod to convey a force pulse to a force transducer became known as the Hopkinson pressure bar. These achievements of the Hopkinsons were reviewed by G.I. Taylor in a lecture he gave in 1946 [12]. With the development of the continuous strip mill in the 1920s there was renewed interest in determining the dynamic properties of steels both in tension and in torsion, particularly at high temperatures [13-31]. Very little work seems to have been performed on dynamic compressive loading before the Second World War, though G.I. Taylor had been thinking of a method of estimating the dynamic strength of materials in compression towards the end of the 1930s [12]. His method consists of firing a solid cylinder of the material against a massive and rigid target. The dynamic flow stress of the cylinder material can be estimated by measuring the overall length of the impacted cylinder and the length of the undeformed (rear) section of the projectile [32,33] by means of the following simple formula: Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1994801 JOURNAL DE PHYSIQUE IV