Maximum attainable uniform strain rates with high-speed-crosshead testing machins: Theoretical considerations

Abstract

The effects of machine stiffness on the strain rates occurring in testpieces are discussed. The actual rate of strain can be markedly different from the simple expression of (crosshead velocity ÷ gauge length) for nominal strain rate. As the speed of testing machines is increased, an upper speed limit is set on the use of conventional machines by the effects of wave propagation in the testpiece: non-uniform stress and strain patterns are set up which make normal interpretation of mechanical property data very difficult. Some criteria determining the upper range of machine speeds are given in terms of the machine stiffness and specimen size. These bounds are further modified at high testing rates by the acceleration available on the machine-drive system. The overall restrictive limits are more noticeable the slower the propagation of waves in the testpiece, i.e. the limiting speed is lower for polymers than for metals.