Mechanical behavior of materials normally changes with strain rate. The behavior of materials under Quasi-static and at low loading rates are usually investigated using Universal testing devices. For high strain rate loading, however, there is not a universally accepted device and consequently, the results of high strain rate investigations are controversial and questionable. Most of material characteristics such as stress-strain curve, ductility, ultimate strength, fragmentation, damage and fracture mechanism may be quite different at high strain rates. Each of the dynamic test devices has some restrictions. For instance, the stress-strain curve of the materials can be captured at high strain rates only on Hopkinson bar. In this work, the restrictions of some devices such as drop hammer, Taylor test, Flying wedge, Shot impact test, dynamic tensile extrusion and Hopkinson bars which are used to characterize the material properties at high strain rates are discussed. The level of strain and strain rate and their restrictions are very important in examining the efficiency of each of the devices. For instance, necking or bulging in tensile and compressive Hopkinson bars, fragmentation in dynamic tensile extrusion and petaling in Taylor test are restricting issues in the level of strain rate attainable in the devices. A major restriction in most dynamic test apparatuses is the critical impact velocity. Critical speed is defined as the maximum impact velocity at which no damage occurs in the specimen. The critical speed in some techniques such as Taylor test, dynamic extrusion and shot impact test is investigated in this work. Finite element simulation, experiment and optimization techniques are used in the present study.
